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Receptors for Purines and Pyrimidines VERA RALEVICa AND GEOFFREY BURNSTOCK School of Biomedical Sciences (V.R.), Queen’s Medical Centre, University of Nottingham, Nottingham, England; Autonomic Neuroscience Institute (G.B.), Royal Free Hospital School of Medicine, London, England This paper is available online at http://www.pharmrev.org
I. Introduction A. Overview Extracellular purines (adenosine, ADP, and ATP) and pyrimidines (UDP and UTP) are important signaling molecules that mediate diverse biological effects via cellsurface receptors termed purine receptors. In this review particular emphasis is placed on the discrepancy b
between the pharmacological properties of native and recombinant receptors for these agents. There are two main families of purine receptors, adenosine or P1 receptors, and P2 receptors, recognizing primarily ATP, ADP, UTP, and UDP. Adenosine/P1 receptors have been further subdivided, according to convergent molecular, biochemical, and pharmacological evidence into four subtypes, A1, A2A, A2B, and A3, all of which couple to G proteins. Based on differences in molecular structure and signal transduction mechanisms, P2 receptors divide naturally into two families of ligandgated ion channels and G protein-coupled receptors termed P2X and P2Y receptors, respectively; to date
seven mammalian P2X receptors (P2X1–7) and five mammalian P2Y receptors (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11) have been cloned, characterized, and accepted as valid members of the P2 receptor family. As correlates between cloned and endogenous receptors are established, the structural subdivision will replace an earlier system of subclassification identifying endogenous P2X, P2Y, P2U, P2T, and P2Z receptors principally according to their pharmacological profiles. A prominent issue addressed in this review is the apparent mismatch of pharmacological data in biological tissue relating to the P2 receptor subtypes classified on the basis of molecular structure. While it is logically satisfying to base receptor subclassification on amino acid sequencing where differences of 30 to 40% are generally regarded as justification for subtyping, it would seem that differences in sequence of less than 5% (even single point mutations) can result in substantial differences in pharmacological profile. Thus, receptor heterogeneity among species, together with receptor coexpression and the possible expression of new receptor subtypes that have not yet been cloned, complicates interpretation of pharmacological responses in some tissues. Thus, it will become apparent in the present review that, at least with the use of currently available, largely unselective agonists and antagonists, some response profiles do not fit those expected for the known P2 receptor subtypes. B. Historical Perspective Extracellular purines and pyrimidines have important and diverse effects on many biological processes including smooth muscle contraction, neurotransmission, exocrine and endocrine secretion, the immune response, inflammation, platelet aggregation, pain, and modulation of cardiac function. The concept of purines as extracellular signaling molecules was instigated by Drury and Szent-Gyo¨rgyi in 1929, in a comprehensive report showing that adenosine and adenosine 59-monophosphate (AMP), extracted from heart muscle, have pronounced biological effects, including heart block, arterial dilatation, lowering of blood pressure, and inhibition of intestinal contraction. Gillespie, in 1934, drew attention to the structure-activity relationships of adenine compounds, showing that deamination greatly reduces pharmacological activity, and that removal of the phosphates from the molecule influences not only potency, but also the type of response. Removal of phosphates was shown to increase the ability of adenine compounds to cause vasodilatation and hypotension, and ATP caused an increase in rabbit and cat blood pressure that was rarely or never observed with AMP or adenosine. Furthermore, ATP was shown to be more potent than AMP and adenosine in causing contraction of guinea-pig ileum and uterus (Gillespie, 1934). This was the first indication of different actions of adenosine and ATP and, by implication, the first indication of the existence of different purine receptors.
Early investigations into the effects of adenosine and ATP were made on a variety of tissues, but particularly the heart and vasculature (Gaddum and Holtz, 1933; Emmelin and Feldberg, 1948; Folkow, 1949; Green and Stoner, 1950). Initial studies on the effects of extracellular UTP also focused on its cardiovascular effects (Hashimoto et al., 1964; Boyd and Forrester, 1968; Urquilla, 1978; Sakai et al., 1979). Other early lines of purine research concerned the effects of purines on platelet aggregation (Born, 1962) and on mast cells (Cockcroft and Gomperts, 1980). Diverse responses to extracellular purines and pyrimidines have now been documented in a wide range of biological systems, from single cells to whole organisms, and include smooth muscle contraction, neurotransmission in the peripheral and central nervous system, exocrine and endocrine secretion, the immune response, inflammation, platelet aggregation, pain, and modulation of cardiac function (Burnstock and Kennedy, 1986; Gordon, 1986; Seifert and Schultz, 1989; Burnstock, 1990; Olsson and Pearson, 1990; Ralevic and Burnstock, 1991a; Jacobson et al., 1992b; Dubyak and el-Moatassim, 1993; Dalziel and Westfall, 1994; Fredholm, 1995; Burnstock and Wood, 1996; Ongini and Fredholm, 1996; Sebastiaˆo and Ribeiro, 1996). Insight into the physiological roles of extracellular purines and pyrimidines comes from studies of their biological sources and the stimuli for their release. In this respect, an important line of research stemmed from studies showing that adenosine is released from the heart during hypoxia to play an important role in reactive hyperemia (Berne, 1963; Gerlach et al., 1963). The general hypothesis of coupling of purine release to metabolic demands via local regulation of blood flow has been applied to other tissues and includes the release of adenine nucleotides, particularly ATP, from skeletal muscle during contraction (Boyd and Forrester, 1968; Forrester and Lind, 1969). Reports of ATP release from sensory nerves in the rabbit ear (Holton and Holton, 1953; Holton, 1959) were the first in a major line of research concerned with purines as neurotransmitters. ATP was detected in the venous perfusate following antidromic stimulation of the rabbit auricular nerve to elicit vasodilatation of the ear capillary bed, and both antidromic vasodilatation and vasodilatation to arterial infusion of ATP (but not that to other agents) were blocked by cholinesterase inhibitors (Holton and Holton, 1953; Holton, 1959). It is now known that ATP is an important neurotransmitter or cotransmitter and adenosine an important neuromodulator in both the peripheral and central nervous systems (see Stone, 1991; Burnstock, 1990; Edwards and Gibb, 1993; Fredholm, 1995). There are also hints that adenine dinucleotides may play neurotransmitter or neuromodulator roles in the central nervous system (Pintor and Miras-Portugal, 1995b).
RECEPTORS FOR PURINES AND PYRIMIDINES
Adrenal chromaffin granules (Cena and Rojas, 1990), platelets (Born and Kratzer, 1984; Gordon, 1986), mast cells (Osipchuk and Cahalan, 1992), erythrocytes (Forrester, 1990; Ellsworth et al., 1995), basophilic leukocytes (Osipchuk and Cahalan, 1992), cardiac myocytes (Forrester, 1990), fibroblasts (Grierson and Meldolesi, 1995b), and endothelial (Ralevic et al., 1991a, 1991c, 1995b; Bodin et al., 1992) and epithelial cells (Enomoto et al., 1994; Ferguson et al., 1997) are important sources of purines that can be released under physiological and pathophysiological conditions, which may act on the purine receptors associated with these or neighboring cells. Adenine dinucleotides are released from secretory ganules of thrombocytes, chromaffin cells and neurons, and may represent a novel class of signaling molecules (Hoyle, 1990; Ogilvie, 1992; Ogilvie et al., 1996). Not enough is known about the sources and release of pyrimidines, which limits our understanding of the role played by the widely distributed receptors that are activated by pyrimidines. However, steps toward resolving this are being made with the demonstration that UTP is released by physiologically relevant stimuli from cultured endothelial, epithelial, and astrocytoma cells (Enomoto et al., 1994; Saiag et al., 1995; Lazarowski et al., 1997a). Purines and pyrimidines mediate their effects by interactions with distinct cell-surface receptors. Early pharmacological evidence for the existence of adenosine receptors has been provided by specific antagonism by methylxanthines of adenosine-mediated accumulation of adenosine 39,59-cyclic monophosphate (cAMP) in rat brain slices (Sattin and Rall, 1970). “Purinergic” receptors were first formally recognized by Burnstock in 1978, when he proposed that these can be divided into two classes termed “P1-purinoceptors”, at which adenosine is the principal natural ligand, and “P2-purinoceptors”, recognizing ATP and ADP (Burnstock, 1978). This division was based on several criteria, namely the relative potencies of ATP, ADP, AMP, and adenosine, selective antagonism of the effects of adenosine by methylxanthines, activation of adenylate cyclase by adenosine, and stimulation of prostaglandin synthesis by ATP and ADP.
This major division remains a fundamental part of purine receptor classification, although adenosine/P1 and P2 receptors are now characterized primarily according to their distinct molecular structures, supported by evidence of distinct effector systems, pharmacological profiles, and tissue distributions. In addition, receptors for pyrimidines are now included within the P2 receptor family (table 1) (Fredholm et al., 1994, 1996). Note that it has been recommended that “P1 receptor” and “P2 receptor” replace the “P1/P2-purinoceptor” terminology (Fredholm et al., 1996). The terms “adenosine receptor” and “P1 receptor” are synonymous. Adenosine/P1 receptors are further divided into four subtypes, A1, A2A, A2B, and A3, on the basis of their distinct molecular structures and show distinct tissue distributions and pharmacological profiles. All couple to G proteins. P2 receptors were shown to be ligand-gated cation channels (Benham and Tsien, 1987) or involved G protein activation (Dubyak, 1991), which formed the basis of their subdivision into two main groups termed P2X receptors (ligand-gated cation channels) and P2Y receptors (G protein-coupled receptors) (Abbracchio and Burnstock, 1994; Fredholm et al., 1994). Subtypes are defined according to the molecular structure of cloned mammalian P2 receptors, discriminated by different numerical subscripts (P2Xn or P2Yn) (Burnstock and King, 1996; Fredholm et al., 1996). This forms the basis of a system of nomenclature that will replace the earlier subtype nomenclature (including P2X, P2Y, P2U, P2T, and P2Z receptors) as correlations between cloned and endogenous receptors are established. P3, P4, and P2YAp4A (or P2D) receptors have been proposed, but evidence for their existence is based solely on the distinct pharmacological profiles exhibited by some biological tissues. As this is no longer tenable for the identification and subclassification of receptors, it remains to be determined, preferably by molecular studies, how these correlate with cloned P2 receptors, and therefore exactly how they will fit within a unifying system of purine and pyrimidine receptor nomenclature.
TABLE 1 Families of receptors for purines and pyrimidines Adenosine/P1 receptors
— G protein-coupled
A1, A2A, A2B, A3
ATP ADP UTP UDP Adenine dinucleotides P2X Ion channel Nonselective porea P2X1–7, P2Xn
AMP does not activate P2 receptors, but may be an agonist at adenosine/P1 receptors. P2Xn, heteropolymeric receptors such as P2X2P2X3 and possibly others with subunit combinations currently unknown. a P2X7 (or P2Z) receptor only. b Endogenous uridine nucleotide-specific receptors, which may have counterparts in P2Y4 and P2Y6 receptors.
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The main aim of this review is to present the characteristics of receptors for purines and pyrimidines within a framework whereby comparisons can be made between cloned and endogenous receptors. For the P2 receptor family this is in order to promote the conversion from a system of nomenclature that is rapidly losing its value, to a unifying system of classification based on structurally distinct cloned receptors. However, pharmacological characterization of endogenous P2 receptors is often equivocal, largely because of the current lack of selective agonists and antagonists and because of complications introduced by the common and widespread coexpression of different types of P2 receptors. Thus, it will become apparent in the present review that in assigning names to endogenous P2 receptors we have needed to strike a balance between caution (against overinterpretation) and anticipation of the direction in which this field is heading. Signal transduction mechanisms, pharmacological response profiles, receptor desensitization, tissue distribution, and biological effects of receptors for purines and pyrimidines are considered. Because ATP and ADP are rapidly degraded to adenosine, and because most cells and tissues coexpress P1 and P2 receptors, we also consider the interactions that may occur between receptors belonging to these two families. Although modulation of ecto-nucleotidase expression and activity is an important means by which to regulate purine receptor function, this aspect of purinergic signaling is not dealt with in detail in this article; the reader is referred to other reviews on the subject (Ziganshin et al., 1994a; Zimmerman, 1996). II. Adenosine/P1 Receptors A. Introduction The adenosine/P1 receptor family comprises A1, A2A, A2B, and A3 adenosine receptors, identified by conver-
gent data from molecular, biochemical, and pharmacological studies (table 2). Receptors from each of these four distinct subtypes have been cloned from a variety of species and characterized following functional expression in mammalian cells or Xenopus oocytes (table 3). A1 and A2 receptors were described by Van Calker et al. in 1979, in studies showing that activation of these receptors by adenosine and its derivatives inhibited, via A1, or stimulated, via A2, adenylate cyclase activity in cultured mouse brain cells (Van Calker et al., 1979). The effects of adenosine could be antagonized by methylxanthines and the order of potency of adenosine analogs was different for the two receptors (Van Calker et al., 1979). Londos et al. (1980) independently drew similar conclusions using membrane preparations from rat adipocytes, hepatocytes, and Leydig tumor cells; the adenosine receptors coupled to inhibition of adenylate cyclase (those in adipocytes) they named Ri (corresponding to the A1 subtype) and the adenosine receptors coupled to stimulation of adenylate cyclase (those in hepatocytes and Leydig cells) they termed Ra (synonymous with the A2 subtype). This alternative system of nomenclature was based on “R” to designate the “ribose” moiety of the nucleoside and “i” and “a” to indicate inhibition and activation of adenylate cyclase respectively (Londos et al., 1980). The A1/A2 nomenclature is now used, which is fortunate because A1 receptors act through a variety of transduction mechanisms in addition to adenylate cyclase. A1a and A1b receptors have been proposed (Gustafsson et al., 1990), but this subdivision of the A1 receptor remains equivocal. A2 receptors are further subdivided into types A2A and A2B. The suggestion that A2 receptors could be divided into two classes was originally based on the recognition that adenosine-mediated stimulation of adenylate cyclase in rat brain was effected via distinct high affinity
TABLE 2 Classification of adenosine/P1 receptors A1
Human brain Canine thyroid Bovine brain Rabbit kidney Rat brain Mouse brain Guinea-pig brain Human hippocampus Canine thyroid Rat brain Guinea-pig brain Mouse bone marrow-derived mast cells Human hippocampus Rat brain Mouse bone marrow-derived mast cells Human striatum Human heart Sheep pars tuberalis Rabbit lung Rat brain Rat testis
binding sites (localized in striatal membranes) and low affinity binding sites (present throughout the brain) (Daly et al., 1983). This subdivision was supported in a study which compared the high affinity striatal A2 binding site with a low-affinity A2 binding site characterized in a human fibroblast cell line; the two sites were termed A2A and A2B, respectively (Bruns et al., 1986). Definitive evidence for the existence of these two subtypes comes from the cloning and sequencing of distinct A2A and A2B receptors which show distinct pharmacological profiles in transfected cells similar to those of the endogenous receptors. Consistent with the fact that these are distinct receptors, there is a considerable lack of amino acid sequence homology between cloned A1, A2A, A2B, and A3 receptors. For example, the homology between rat A1 and A2B receptors is only 45% (Stehle et al., 1992), and the human A3 receptor only shows approximately 50%, 43%, and 40% homology with human A1, A2A, and A2B receptors, respectively (Linden, 1994). The homology between A2A and A2B receptors is also slight, being approximately 46% when these subtypes are cloned from rat and 61% when cloned from human (Stehle et al., 1992; Pierce et al., 1992). An adenosine binding site with high affinity for 2-phenylaminoadenosine (CV 1808) (A2A-selective agonist) in rat striatal membranes has been suggested to be a novel “A4” adenosine receptor (Cornfield et al., 1992). The very low affinity of 2-[p-(2-carbonyl-ethyl)-phenylethylamino]59-N-ethylcarboxamidoadenosine (CGS 21680) and Nethylcarboxamidoadenosine (NECA) at this site were taken to indicate that this is not an A2 receptor. However, the binding studies were carried out at 4°C (Cornfield et al., 1992), and the existence of a distinct A4 receptor has been challenged by the demonstration that when similar binding studies are carried out at 21°C, the potency order
Libert et al., 1992; Townsend-Nicholson and Shine, 1992 Libert et al., 1989, 1991 Tucker et al., 1992; Olah et al., 1992 Bhattacharya et al., 1993 Reppert et al., 1991; Mahan et al., 1991 Marquardt et al., 1994 Meng et al., 1994a Furlong et al., 1992 Libert et al., 1989; Maenhaut et al., 1990 Chern et al., 1992; Fink et al., 1992 Meng et al., 1994b Marquardt et al., 1994 Pierce et al., 1992 Stehle et al., 1992; Rivkees and Reppert, 1992 Marquardt et al., 1994 Salvatore et al., 1993 Sajjadi et al., 1993 Linden et al., 1993 Hill et al., 1997 Zhou et al., 1992 Meyerhof et al., 1991; Zhou et al., 1992
of compounds at the striatal membrane site is characteristic of the A2A adenosine receptor (Luthin and Linden, 1995). Furthermore, in COS cells transfected with adenosine A2A receptors, binding studies carried out at 4°C produce an “A4” binding profile (Luthin and Linden, 1995). This justifies the more rigorous criteria now demanded for classification of receptors, whose identity must be proved by molecular as well as by biochemical or pharmacological means. There is a vast and rapidly growing literature on adenosine/P1 receptors; it has not been possible to do justice to this in the present review. Out of necessity, therefore, we focus on the more recent literature. B. Structure All adenosine receptors couple to G proteins. In common with other G protein-coupled receptors, they have seven putative transmembrane (TM) domains of hydrophobic amino acids, each believed to constitute an ahelix of approximately 21 to 28 amino acids. The Nterminal of the protein lies on the extracellular side and the C-terminal on the cytoplasmic side of the membrane. A pocket for the ligand binding site is formed by the three-dimensional arrangement of the a-helical TM domains, and the agonist is believed to bind within the upper half of this pore. The transmembrane domains are connected by three extracellular and three cytoplasmic hydrophilic loops of unequal size; typically the extracellular loop between TM4 and TM5 and the cytoplasmic loop between TM5 and TM6 is extended. These features are illustrated in a schematic of the A1 receptor in figure 1. N-linked glycosylation often occurs on the second extracellular loop; the roles of the carbohydrate moieties of the glycosylated receptor are not clear, although suggested functions include stabilization of protein conformation, protection of proteins from proteases, and mod-
RALEVIC AND BURNSTOCK
FIG. 1. Schematic of the A1 adenosine receptor. In common with other G protein-coupled receptors, the A1 receptor has seven putative transmembrane domains (I-VII) of hydrophobic amino acids, each believed to constitute an a-helix, which are connected by three extracellular and three intracellular hydrophilic loops. The number of amino acids comprising the extra- and intracellular loops and the extracellular N-terminal and intracellular C-terminal regions of the bovine A1 receptor are indicated in parentheses (Olah et al., 1992). The transmembrane regions comprise 23 to 25 amino acids in the bovine A1 receptor (Olah et al., 1992). The arrangement of the transmembrane regions forms a pocket for the ligand binding site. The location of histidine residues (H) in transmembrane regions VI (position 254) and VII (position 278) in the bovine A1 receptor, which are believed to be important in ligand binding (Olah et al., 1992), are indicated. Extracellular and transmembrane regions of the protein believed to be important in agonist and antagonist binding are indicated (Olah et al., 1994b,c). S-S denotes the presence of hypothetical disulfide bridges (Jacobson et al., 1993c). Glycosylation occurs on the second extracellular loop.
ulation of protein function. Current evidence suggests that glycosylation has no obvious influence on ligand binding (Piersen et al., 1994). The intracellular segment of the receptor interacts with the appropriate G protein with subsequent activation of the intracellular signal transduction mechanism. The third intracellular loop of the adenosine A2A receptor seems to be the main determinant of its G protein selectivity (Olah, 1997). Phosphorylation by protein kinases of amino acid residues on the cytoplasmic domains seems to be involved in desensitization of A2A and A3 receptors (Palmer and Stiles, 1997a, 1997b). The transmembrane regions are generally highly conserved, with particularly long stretches of amino acid homology being found in TM2, TM3, and TM5. Most sequence differences have been observed in a hypervariable region located at the N-terminal half of the second extracellular loop (Tucker and Linden, 1993). It is the residues within the transmembrane regions that are crucial for ligand binding and specificity and, with the exception of the distal (carboxyl) region of the second extracellular loop, the extracellular loops, the C-terminal and the N-terminal do not seem to be involved in ligand recognition (Olah et al., 1994b, 1995). A number of amino acid residues contribute, in different ways, to ligand specificity within the binding pocket. Site-
directed mutagenesis of the bovine A1 adenosine receptor suggests that conserved histidine residues in TM6 and TM7 are important in ligand binding. Histidine 278 in TM7 seems to be particularly important because mutation of this amino acid abolishes ligand binding (Olah et al., 1992). Mutagenesis of the human A1 adenosine receptor has shown that threonine 277 in TM7 is important in binding of the non-selective adenosine receptor agonist NECA, but has little effect on the affinity of binding of the A1 selective agonist (R)-N6-(2-phenyl-1-methylethyl)-adenosine (R-PIA), or of antagonists (TownsendNicholson and Schofield, 1994). Modification of Glu 16 in TM1 and Asp 55 in TM2 of the human A1 receptor alters the affinity of binding for [3H]CCPA (2-chloro-N6-cyclopentyladenosine) and other agonists, but does not affect antagonist binding (Barbhaiya et al., 1996). Site-directed mutagenesis of the human A2A adenosine receptor has identified several residues in TM5–7 that are involved in ligand binding (Kim et al., 1995). Glu 13 in TM1 of the human A2A receptor seems to be critically involved in agonist, but not antagonist recognition (Ijzerman et al., 1996). A potential problem inherent in the methodology of site-directed mutagenesis is that changes in primary structure may cause changes in tertiary structure of the molecule. This has been addressed by studies with chimeras constructed from structurally similar, but pharmacologically different receptors. The ligand binding properties of A1/A3 chimeric receptors support the concept of a crucial role for histidine residues in TM6 and TM7 in ligand binding (Olah et al., 1995). In addition, a critical role in ligand binding of the distal region of the second extracellular loop has been identified, although its specific interactions are not yet clear (Olah et al., 1994b). Possible roles include direct interaction of an amino acid residue(s) within this region with the ligand, an influence on the conformation of the receptor and/or steric hindrance. Construction of chimeric human A1 and rat A2A adenosine receptors was used to show that TM1– 4 are important in A1 receptor agonist and antagonist binding and ligand specificity (Rivkees et al., 1995a). C. Agonists Analogs with greater stability than adenosine are produced by modification of the N6 and C2 positions of the adenine ring and the 59-position of the ribose moiety of adenosine, and have been used extensively in the characterization of adenosine/P1 receptors. NECA (Williams, 1989), N-[2-(4-aminophenyl)ethyl] adenosine (APNEA) (Fozard and Carruthers, 1993), and N6-(3-[125I]iodo-4aminobenzyl)-59-N-methylcarboxamidoadenosine (125IAB-MECA) (Olah et al., 1994a) do not discriminate between adenosine receptor subtypes. Agonists with subtype selectivity are detailed in the sections on individual adenosine receptor subtypes and the chemical structure of some of these are illustrated in figure 2.
RECEPTORS FOR PURINES AND PYRIMIDINES
chronotropic effects without prior conversion to adenosine. These effects are not consistent with the pharmacological profile of any of the established subtypes of adenosine/P1 receptor, and in some respects are similar to the profile described for the P3 receptor. D. Antagonists
FIG. 2. The chemical structure of some agonists at adenosine/P1 receptors.
ATP and metabolically stable ATP derivatives, i.e., adenosine 59-O-(3-thiotriphosphate)(ATPgS) and b,gmethylene ATP (b,g-meATP), can act directly as agonists at adenosine/P1 receptors in some tissues where responses are blocked by methylxanthines, but are not affected by adenosine deaminase or by blockade of 59nucleotidase. b,g-MeATP is approximately equipotent with adenosine at mediating contraction of smooth muscle adenosine/P1 receptors of rat colon (Bailey and Hourani, 1990), and relaxation via adenosine/P1 receptors of rat duodenum (Hourani et al., 1991), and guinea-pig trachealis muscle (Piper and Hollingsworth, 1996). ATP, ATPgS, and b,g-meATP inhibit [3H]-NA release in a variety of tissues via receptors that are blocked by the A1 selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) as well as by the P2 receptor antagonist cibacron blue (Von Ku¨gelgen et al., 1992, 1995b, 1996). ATP (Collis and Pettinger, 1982) and diadenosine polyphosphates (Hoyle et al., 1996; Vahlensieck et al., 1996) have been reported to stimulate directly adenosine/P1 receptors in guinea-pig atria, eliciting negative inotropic and
Xanthines and xanthine derivatives, including the natural derivatives theophylline and caffeine, are nonselective adenosine/P1 receptor antagonists. They are not universal adenosine/P1 receptor antagonists; xanthine-resistant relaxations to adenosine and its analogs were observed in guinea-pig aorta (Collis and Brown, 1983; Martin, 1992), rat aorta (Prentice and Hourani, 1996), guinea-pig trachea (Brackett and Daly, 1991), porcine coronary artery (Abebe et al., 1994), and guineapig taenia cecum (Prentice et al., 1995). Some A3 receptors, namely those of rat, rabbit, and gerbil, are characteristically insensitive to methylxanthines, thus it is possible that the xanthine-resistant responses to adenosine described in some tissues occur following actions of adenosine at mast cell A3 receptors and the subsequent release of vasoactive mediators. This hypothesis would predict that guinea-pig and pig A3 receptors are also xanthine-insensitive, because xanthine-resistant responses to adenosine have been reported in these species. It would be interesting to see if these responses can be blocked by inhibitors of mast cell degranulation. 8-Phenyltheophylline and the more water soluble 8-(p-sulfophenyl)theophylline (8-SPT) (Daly et al., 1985) are more potent than theophylline at adenosine/P1 receptors, but are not subtype-selective. 8-SPT and its derivative 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) do not cross the blood-brain barrier, being purely peripherally acting adenosine/P1 receptor antagonists (Daly et al., 1985) and thus can be used to discriminate between central and peripheral adenosine receptors. A number of xanthines and non-xanthines identified as adenosine receptor antagonists with reasonable subtype selectivity are described below (see Sections III.F., IV.F., and VI.F.) and their chemical structures illustrated in figure 3. III. A1 Receptor Subdivision of A1 receptors into high affinity A1a receptors and low affinity A1b receptors has been proposed (Gustafsson et al., 1990). This was based on the description of high-affinity binding sites for adenosine agonists and antagonists in rat and guinea-pig brain (A1a) and low-affinity binding sites in rat vas deferens and guineapig ileum (A1b) (Gustafsson et al., 1990). However, there are no cloned equivalents for these putative subtypes and their existence remains equivocal. It is possible that these reflect high and low affinity states of the same A1 receptor.
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FIG. 3. The chemical structure of some antagonists at adenosine/P1 receptors.
A. Cloned A1 Receptors A1 receptors have been cloned from several species (table 3). The human adenosine A1 receptor subtype gene (ADORA1) has been localized to chromosome 1q32.1 (Townsend-Nicholson et al., 1995a). The variability in the primary sequence of the A1 receptor between species is less than 10% for A1 receptors from dog, rat, and cow, and less than 5% between bovine and human A1 receptors, but this seems to be sufficient to cause considerable interspecies differences in ligand binding (Tucker and Linden, 1993) and subtle differences in the mechanisms underlying receptor desensitization (Ramkumar et al., 1991; Nie et al., 1997; Palmer and Stiles, 1997b). Species homologs of A1 receptors have been suggested to differ in their ability to discriminate among the related Go/Gi protein alpha subunits (Jockers et al., 1994). B. Signal Transduction Mechanisms The A1 receptor mediates a broad range of signaling responses, which may be caused by its coupling to different G proteins within the Gi/o family (Freissmuth et al., 1991; Munshi et al., 1991). The G proteins Gi and Go are substrates for pertussis toxin that ADP-ribosylates the a-subunit of Gi/o/t family members, uncoupling them from receptors. Accordingly, effects mediated by A1 receptors are generally blocked by pertussis toxin. However, presynaptic A1 receptors seem to be at least partly resistant to pertussis toxin (Fredholm et al., 1989; Ha-
suo et al., 1992); the reason for this could be the very tight coupling of the presynaptic A1 receptors to potentially pertussis toxin-sensitive G proteins, rather than coupling to pertussis toxin-insensitive G proteins (Van der Ploeg et al., 1992). A partially-purified protein with selectivity for G protein a subunits has been shown to stabilize the rat brain A1 receptor-G protein complex, thereby promoting tight coupling of the A1 receptor with its G protein (Nanoff et al., 1997). Interestingly, this is a feature of the rat brain but not the human brain A1 receptor; the latter is not under the control of a coupling cofactor, but operates according to the classic ternary complex model of receptor-G protein coupling (Nanoff et al., 1997). The most widely recognized signaling pathway of A1 receptors is inhibition of adenylate cyclase causing a decrease in the second-messenger cAMP (Van Calker et al., 1978; Londos et al., 1980). This in turn modulates the activity of cAMP-dependent protein kinase, which phosphorylates diverse protein targets. A1 coupling to adenylate cyclase has been described in a number of tissues including brain, adipose tissue, and testes. In addition to direct modulation of signaling pathways downstream to cAMP, inhibition of adenylate cyclase via A1 receptors blocks the effects of other agents which act by stimulating adenylate cyclase activity in cells. Another signaling mechanism of A1 receptors is activation of phospholipase C (PLC) leading to membrane phosphoinositide metabolism and increased production of inositol 1,4,5-triphosphate (IP3) [and diacylglycerol (DAG)] and Ca21 mobilization. This has been described in chinese hamster ovary (CHO)-K1 cells expressing the cloned human A1 receptor (Iredale et al., 1994; Megson et al., 1995) as well as at endogenous A1 receptors in a number of tissues including DDT1 MF-2 smooth muscle cells (Gerwins and Fredholm, 1992a,b; White et al., 1992), heart (Scholz et al., 1993), myometrium (Schiemann et al., 1991a,b), rabbit cortical collecting tubule cells (Arend et al., 1989), renal cells (Weinberg et al., 1989), tracheal epithelial cells (Galietta et al., 1992), cultured mesangial cells (Olivera et al., 1992), and primary astrocytes (Peakman and Hill, 1995). IP3 stimulates the release of Ca21 from intracellular stores via interactions with specific receptors located on the sarcoplasmic reticulum. Elevation of cytosolic Ca21 by IP3 can stimulate a variety of signaling pathways, including a family of phosphatidyl serine-dependent serine/threonine-directed kinases collectively called protein kinase C (PKC) (of which there are at least 11 different isoforms), phospholipase A2 (PLA2), Ca21-dependent K1 channels, and nitric oxide synthase (NOS). Depletion of Ca21 from IP3-sensitive pools may promote Ca21 influx from extracellular sources. Activation of phospholipase D (PLD) via A1 adenosine receptors in DDT1 MF-2 smooth muscle cells has been described (Gerwins and Fredholm, 1995a, 1995b), although as in the majority of cell systems this may be
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downstream of phosphoinositide hydrolysis and may require the intermediate activation of PKC or Ca21. Stimulation of A1 receptors can activate several types of K1 channel, described principally in cardiac muscle and neurons. In supraventricular tissues (sino-atrial and atrioventricular node, and atrium), the A1 receptor couples directly via pertussis toxin-sensitive G proteins to K1 channels (the same K1 channels are activated by both adenosine and acetylcholine), and activation causes bradycardia (Belardinelli et al., 1995a; Bu¨nemann and Pott, 1995; Ito et al., 1995). A1 adenosine receptors also couple to ATP-sensitive K1 channels (KATP channel); the activity is additionally regulated by metabolic demand (they close when intracellular ATP levels are high). Coupling seems to occur through the G protein in a membrane-delimited manner (Kirsch et al., 1990; Dart and Standen, 1993), although coupling via cytosolic factors is possible given the strong evidence that A1 receptors, KATP channels, and PKC all have a role in ischemic preconditioning. A1 receptor coupling to KATP channels has been described in rat and guinea-pig ventricular myocytes (Kirsch et al., 1990; Ito et al., 1994), porcine coronary arteries (Merkel et al., 1992; Dart and Standen, 1993), rabbit heart (Nakhostine and Lamontagne, 1993), and rat cerebral cells (Heurteaux et al., 1995). Activation of KATP channels mediates a reduction in action potential duration, vasodilatation and an increase in blood flow, which is consistent with their having a pivotal role in the coupling of vascular tone to metabolic demand determined both by intracellular purines (ATP/ ADP levels) and by the extracellular actions of adenosine (released, for instance, during hypoxia or ischemia). Neurons express multiple K1 channels that A1 receptors may couple to regulate membrane potential and determine action potential frequency and duration. A1 receptors reduce neuronal excitability and decrease firing rate by a hyperpolarizing effect mediated mainly by an increase in K1 conductance (Trussell and Jackson, 1985; Greene and Haas, 1991; Pan et al., 1995). A1 receptors also couple to inhibition of Ca21 currents, which may account for inhibition of neurotransmitter release, although other or multiple mechanisms may be involved in this process (see Fredholm, 1995). Inhibition of Ca21 currents by A1 receptors has been described in dorsal root ganglion neurons (Dolphin et al., 1986), rat hippocampal pyramidal neurons (Scholz and Miller, 1991), rat sympathetic neurons (N-type Ca21 channels, plus an unidentified Ca21 channel) (Zhu and Ikeda, 1993), rat brainstem (predominantly N-type, but also P-type Ca21 channels) (Umemiya and Berger, 1994), hippocampal CA1 neurons (N-type, plus some unidentified Ca21 channels) (Wu and Saggau, 1994), hippocampal CA3 neurons (N-type Ca21 channel) (Mogul et al., 1993), and mouse motoneurons (N-type Ca21 channel) (Mynlieff and Beam, 1994). In atrial myocytes adenosine has an inhibitory effect on basal L-type Ca21 current,
although this is small and may be secondary to a reduction in basal cAMP (Belardinelli et al., 1995a). C. Desensitization Several mechanisms, operational at different levels of the signal transduction cascade, contribute to differential desensitization of G protein-coupled receptors. Rapid desensitization (occuring within a few minutes of agonist exposure) seems to involve phosphorylation of specific residues on the receptor C-terminal or the cytoplasmic loops by G protein-coupled receptor-specific kinases (GRKs) and/or kinases regulated by levels of intracellular second-messengers such as cAMP-dependent protein kinase. The phosphorylated receptor may bind to members of a family of proteins called arrestins, which cause uncoupling of the receptor from its G proteins. Desensitization occuring over a longer time course also involves uncoupling of the receptor-G proteins complex, but phosphorylation does not seem to be a prerequisite. Sequestration of receptors into an intracellular compartment may occur, as described for the increase in A1 receptors in light vesicle membrane fractions prepared from the hamster vas deferens smooth muscle cell line, DDT1 MF-2 cells, after chronic exposure to R-PIA (Ramkumar et al., 1991). Prolonged exposure to agonist may additionally lead to down-regulation of receptors and/or of the associated G proteins. Desensitization of A1 receptors by exposure to adenosine analogs has consistently been described both in vitro and in vivo, but this usually requires prolonged exposure to agonist (from 15 minutes to several hours or even days) (Parsons and Stiles, 1987; Ramkumar et al., 1991; Abbracchio et al., 1992; Green et al., 1992; Lee et al., 1993; Longabaugh et al., 1989; Casati et al., 1994). This is considerably longer than the time to desensitization of A3 receptors which typically undergo significant desensitization within several minutes. Interestingly, while an agonist-stimulated increase in phosphorylation has been described for A1 receptors in hamster DDT1 MF-2 cells in association with receptor uncoupling from G proteins and desensitization, presumably by GRKs (Ramkumar et al., 1991; Nie et al., 1997), phosphorylation does not occur for the human A1 receptor expressed in CHO cells at a time when receptor down-regulation is observed (Palmer and Stiles, 1997b). Down-regulation of A1 receptors and/or of the associated G proteins after prolonged exposure to agonist has been reported in most of the cells and tissues in which this has been studied (Parsons and Stiles, 1987; Longabaugh et al., 1989; Green et al., 1992; Ramkumar et al., 1991, 1993a; Abbracchio et al., 1992). Down-regulation of G proteins following A1 receptor activation may lead to heterologous receptor desensitization. Chronic stimulation of A1 receptors in adipocytes in vivo (Longabaugh et al., 1989) and in isolated adipocytes (Green et al., 1992) with (-)N6-phenylisolpropyl adenosine (PIA) for up to 6 and 7 days, respectively,
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causes down-regulation of A1 receptors, non-uniform down-regulation of Gi proteins, and heterologous desensitization of other lipolytic hormone responses. In contrast, chronic (7 days) infusion of (R)N6-phenylisopropyl adenosine (R-PIA) in guinea-pigs homologously desensitizes the atrioventricular nodal response to adenosine: there is down-regulation of A1 adenosine receptors, a decrease in high affinity A1 receptors, and a decrease in Gi and Go proteins, but no change in responses mediated by muscarinic receptors (Dennis et al., 1995). D. Sensitization/Up-Regulation Long-term treatment with adenosine/P1 receptor antagonists generally leads to an increase in the effects of adenosine via a selective increase in the number of A1 receptors, receptor sensitization and/or altered interaction between the receptor and the associated G proteins (Fredholm, 1982; Murray, 1982; Fredholm et al., 1984; Green and Stiles, 1986; Ramkumar et al., 1991; Fastbom and Fredholm, 1990; Zhang and Wells, 1990; Lupica et al., 1991a, 1991b; Shi et al., 1994). Long-term (12 day) caffeine treatment of rats increases the number of hippocampal A1 (but not A2A) receptors, without any changes in A1 messenger ribonucleic acid (mRNA), suggesting that the adaptive changes are at the posttranslational level (Johansson et al., 1993a). An increase in the density of cortical A1 receptors has been described after chronic caffeine injestion in mice, but surprisingly, given that striatal adrenergic, cholinergic, GABA, and serotonin receptors and Ca21 channels are also affected by this treatment, there is no change in the density of striatal A2A receptors (Shi et al., 1993). E. Agonists Certain N6-substituted adenosine derivatives, such as N -cyclopentyladenosine (CPA), N6-cyclohexyladenosine (CHA), and R-PIA, are selective agonists at A1 receptors with Ki values in the range of 0.6 to 1.3 nM (see Jacobson et al., 1992b) (table 2). Substitutions at both the N6- and C2-positions have produced 2-chloro-CPA (CCPA) which is A1 selective, 1500-fold versus A2 receptors in binding studies in rat brain, with a Ki of 0.6 nM (Lohse et al., 1988; Thompson et al., 1991; Jacobson et al., 1992b). N-[1S, trans,2hydroxycyclopentyl] adenosine (GR79236) has been reported to be an A1 selective agonist, which is approximately equipotent with CPA in a variety of isolated tissues and cell types (Reeves et al., 1993; Gurden et al., 1993). 6
(PACPX), DPCPX, and xanthine amine congener (XAC) (Bruns et al., 1987; Martinson et al., 1987; Shimada et al., 1991) (fig. 3). Of these, DPCPX has the greatest affinity (Ki 1.5 nM) for A1 receptors and the greatest A1-subtype selectivity (A2/A1 affinity ratio 740), as shown in rat brain membranes (Bruns et al., 1987; Lohse et al., 1987). The human A1 receptor has an approximately lower affinity for DPCPX (Libert et al., 1992; Klotz et al., 1998). A number of other 8-substituted xanthines, including (6)-8-(3-oxocyclopentyl)-1,3-dipropylxanthine (KFM 19) and KW-3902 (8-noradamant-3-yl1,3-dipropylxanthine), have been shown to be selective antagonists at A1 receptors (see Williams, 1989; Jacobson et al., 1992b). The alkylxanthine 1,3-dipropyl-8-[2-(5,6-epoxy)norbornyl]xanthine (ENX) is a potent (KB 3.6 nM) and selective antagonist at A1 receptors in the guinea- pig heart and brain and in DDT1 MF-2 cells, with 400-fold greater affinity of binding versus A2A receptors in guineapig brain (Belardinelli et al., 1995b). Several classes of non-xanthine antagonists have been described, some showing reasonable affinity and selectivity for the A1 receptor (see Jacobson et al., 1992b; Daly et al., 1993). Some of the more active of these are the tricyclic non-xanthine antagonists, including the triazoloquinazolines (Francis et al., 1988), the triazoloquinoxalines (Trivedi and Bruns, 1988; Sarges et al., 1990), and the imidazoquinolines (Van Galen et al., 1991). The adenine derivative 1,3-dipropyl-8-[2,(5,6-epoxy)norbornyl]xanthine (N 0861) is reasonably selective (10- to 47-fold versus A2A receptors) and potent at A1 receptors in a number of tissues (May et al., 1991; Martin et al., 1993a; Belardinelli et al., 1995b). This compound has been superceded by the S-enantiomer 12 (CVT-124) with nanomolar selectivity and 1800- and 2400-fold selectivity at rat and cloned human A1 receptors, respectively (Pfister et al., 1997), and by 8-(N-methylisopropyl)amino-N6-(59-endohydroxy-endonorbornyl-)9-methyl adenine (WRC 0571) with 62-fold selectivity versus the A2A receptor and 4670-selectivity versus the A3 receptor (Martin et al., 1996). (1)-(R)-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)acryloyl]-2-piperidine ethanol, FK 453, has been reported to be a potent and selective A1 receptor antagonist with IC50 values of approximately 17 nM at rat cortical A1 receptors and 11 mM at striatal A2 receptors (Terai et al., 1995). Chiral pyrolo[2,3-d]pyrimidine and pyrimido[4,5-b]indole derivatives have been shown to be potent and highly stereoselective A1 adenosine receptor antagonists (Mu¨ller et al., 1996a).
F. Antagonists Most of the selective A1 receptor antagonists described to date are xanthine-based derivatives. The introduction of hydrophobic (particularly phenyl or cycloalkyl) substituents into position 8 of the xanthine ring has yielded potent and A1-selective antagonists, including 1,3-dipropyl-8-phenyl(2-amino-4-chloro)xanthine
G. Distribution and Biological Effects A1 receptors are widely distributed in most species and mediate diverse biological effects. There is considerable literature in this area. Thus, this section is intended to give an indication of the ubiquity and diversity of actions mediated by adenosine at A1 receptors, rather
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FIG. 4. Tissue distribution of adenosine receptor mRNA expression as examined by RT-PCR. Sizes of PCR products are given in base pairs. (From Dixon et al., 1996, Br J Pharmacol 118:1461–1468; with permission from McMillan Press Limited.)
than to provide a comprehensive account of A1 receptor distribution and effects. A1 receptors are particularly ubiquitous within the central nervous system (CNS), with high levels being expressed in the cerebral cortex, hippocampus, cerebellum, thalamus, brain stem, and spinal cord (Reppert et al., 1991; Dixon et al., 1996) (fig. 4). Immunohistochemical analysis using polyclonal antisera generated against rat and human A1 adenosine receptors has identified different labeling densities of individual cells and their processes in selected regions of the brain (Rivkees et al., 1995b). A1 receptor mRNA is widely distributed in peripheral tissues having been localized in vas deferens, testis, white adipose tissue, stomach, spleen, pituitary, adrenal, heart, aorta, liver, eye, and bladder (Reppert et al., 1991; Dixon et al., 1996). Only very low levels of A1 mRNA are present in lung, kidney, and small intestine (Reppert et al., 1991; Stehle et al., 1992; Dixon et al., 1996) (fig. 4). It is now well established that adenosine is released from biological tissues during hypoxia and ischemic conditions. One of its effects is to reduce neuronal activity and thereby oxygen consumption; thus it acts as a neuroprotective agent. A significant part of these effects seem to be mediated by the A1 receptor. A1 receptors are located pre and postsynaptically on cell bodies, and on axons, where they mediate inhibition of neurotransmission by decreasing transmitter release, hyperpolarizing neuronal membranes, reducing excitability and firing rate, and altering axonal transmission. Adenosine can also exert behavioral effects: adenosine actions at A1 receptors have been implicated in sedative, anticonvulsant, anxiolytic, and locomotor depressant effects (Nikodijevic et al., 1991; Stone, 1991; Jain et al., 1995; Malhotra and Gupta, 1997). Conversely, xanthine antagonists such as caffeine and theophylline have central stimulatory properties ascribed, at least in part, to inhibition of endogenous adenosine, although inhibition of
cyclic nucleotide phosphodiesterases may contribute to this effect. A1 receptors mediate cardiac depression through negative chronotropic, dromotropic, and inotropic effects (see Olsson and Pearson, 1990). Slowing of the heart rate occurs via A1 receptors on sinoatrial and atrioventricular nodes causing bradycardia and heart block, respectively, while the inotropic effects include a decrease in atrial contractility and action potential duration (Olsson and Pearson, 1990). This aspect of A1 receptormediated effects has found application in the clinical use of adenosine to treat supraventricular tachycardia, and in the use of adenosine receptor antagonists in the treatment of bradyarrhythmias. In the kidney, activation of A1 receptors mediates diverse effects including vasoconstriction (principally of the afferent arteriole), a decrease in glomerular filtration rate, mesangial cell contraction, inhibition of renin secretion, and inhibition of neurotransmitter release (Olivera et al., 1989; Agmon et al., 1993; Barrett and Droppleman, 1993; Munger and Jackson, 1994). Intravenous and intra-aortic administration of adenosine in rats decrease water and sodium excretion via A1 receptors, while selective antagonism of A1 receptors causes diuresis and natriuresis (see Mizumoto et al., 1993; Van Beuren et al., 1993). Intrarenal administration of adenosine, but not of the A2A selective agonist CGS 21680, in dogs also decreases water and sodium excretion (Levens et al., 1991a,b). Furthermore, A1 receptors increase transepithelial resistance and reduce Na1 uptake in inner medullary collecting duct cells in culture (Yagil et al., 1994). On the other hand, intrarenal administration of adenosine and the A1-selective agonist CHA in rats has been shown to induce marked diuresis and natriuresis which can be inhibited by the A1-selective antagonist DPCPX (Yagil, 1994). Direct effects on blood vessel tone via adenosine actions on A1 receptors are rare. A more significant role of A1 receptors with regard to regulation of blood vessel tone appears to be prejunctional modulation of neurotransmitter release. Prejunctional inhibition of neurotransmission via A1 receptors on perivascular sympathetic (Gonc¸alves and Queiroz, 1996) and capsaicinsensitive sensory afferents (Rubino et al., 1993) has been shown. However, A1 receptors have been observed to mediate relaxation of porcine coronary artery (Merkel et al., 1992), and contraction of guinea-pig aorta (Stoggall and Shaw, 1990) and pulmonary artery (Szentmiklo´si et al., 1995). A1 receptors have also been reported to mediate contraction of rat isolated spleen (Fozard and Milavec-Krizman, 1993) and rat vas deferens (Hourani and Jones, 1994), as well as bronchoconstriction and bronchial hyperresponsiveness (Ali et al., 1994a, 1994b; Pauwels and Joos, 1995; el-Hashim et al., 1996). Diverse A1-mediated effects in the gut have been described, including inhibition of peristalsis of rat jejunum (Hancock and Coupar, 1995b), relaxation of longitudinal muscle of
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rat duodenum (Nicholls et al., 1992, 1996), and contraction of rat colonic muscularis mucosa (Bailey et al., 1992; Reeves et al., 1993). Interestingly, adenosine mediates contraction of guinea-pig myometrial smooth muscle via A1 receptors that in non-pregnant animals are coupled to the formation of IP3, but in pregnant animals are coupled both to IP3 and negatively to adenylate cyclase (Schiemann and Buxton, 1991; Schiemann et al., 1991a,b). Selective inhibition of the synthesis of A1 receptors with antisense oligonucleotides confirmed that these receptors are involved in an animal model of asthma (Nyce and Metzger, 1997). There was a marked reduction in the number of A1 receptors in the lung and attenuation of airway constriction to adenosine, histamine, and dustmite allergen (Nyce and Metzger, 1997). Although the site of action remains to be determined, selective antagonism of A1 receptors offers a possible new approach in asthma therapy. A1 receptors on bovine pulmonary artery endothelial cells have been shown to mediate Cl2 efflux (Arima et al., 1994). In human airway epithelial cells, A1 receptors have been reported to mobilize intracellular Ca21 and activate K1 and Cl2 conductance (Rugolo et al., 1993), while selective inhibition of A1 receptors with DPCPX increases cAMP-activated Cl2 conductance (McCoy et al., 1995). A1 adenosine receptors on rat cochleal membranes (Ramkumar et al., 1994), astrocytes (Peakman and Hill, 1994), and epididymal spermatozoa (Minelli et al., 1995) have been described. Release of Ca21 from internal stores in perisynaptic glial cells of the frog neuromuscular junction via A1 receptors has been described (Robitaille, 1995). Adenosine acts via A1 receptors and inhibition of cAMP to inhibit lipolysis and increase insulin sensitivity in adipose tissue (Londos et al., 1985; Green, 1987). Abnormal A1 receptor function in genetic obesity has been proposed, showing that lipolysis is less active and A1 receptor signaling more active, which may be caused by changes in receptor phosphorylation, but also possibly by adenylate cyclase activity (LaNoue and Martin, 1994; Berkich et al., 1995). In contrast, insulin sensitivity is decreased by activation of A1 receptors in skeletal muscle (Challis et al., 1992). A1 receptors on pancreatic b cells mediate inhibition of insulin secretion (HillaireBuys et al., 1989). A1 receptors have been widely reported to mediate the protective effects of adenosine in preconditioning and during ischemia or during reperfusion injury in the heart (Tsuchida et al., 1993, 1994; Yao and Gross, 1993; Lee et al., 1995; Lasley and Mentzer, 1995; Strickler et al., 1996; Grover et al., 1992; van Winkle et al., 1994; Sakamoto et al., 1995; Mizumura et al., 1996; Stambaugh et al., 1997), lung (Neely and Keith, 1995), and brain (Heurteaux et al., 1995). Strong evidence for a protective role of A1 adenosine receptors comes from
studies with transgenic mice over expressing the A1 receptor. Mice over expressing the A1 receptor have been shown to have an increased myocardial resistance to ischemia (Matherne et al., 1997). The mechanism involved is not yet clear; it may involve A1 receptor activation of KATP channels as infarct size reduction after activation of A1 receptors has been reported to be completely abolished by the blockade of KATP channels (Grover et al., 1992; van Winkle et al., 1994; Mizumura et al., 1996). On the other hand, there seems to be a general consensus that PKC is involved in ischemic preconditioning, and activation of PKC was shown to be the critical factor involved in limitation of myocardial infarct size by A1 receptors in anaesthetized rabbits (Sakamoto et al., 1995). However, not all researchers are in agreement that adenosine is cardioprotective, or that A1 receptors mediate ischemic preconditioning (Asimakis et al., 1993; Ganote et al., 1993; Hendrikx et al., 1993; Lasley et al., 1993; Liu et al., 1994). In addition, a protective role for adenosine A3 receptors has been suggested (see Section VI.G.). Reperfusion of ischemic tissue results in locally increased permeability and pulmonary edema that is associated with neutrophil accumulation in the microvasculature; neutrophil-endothelial cell interactions are known to be a prerequisite for the associated microvascular injury. Paradoxically, given the protective role of A1 receptors in ischemia-reperfusion injury, adenosine contributes to inflammatory reactions via effects on neutrophil and/or endothelial A1 receptors. This is done by augmenting responses to microbial stimuli, promoting chemotaxis, adhesion to endothelium, phagocytosis, and release of reactive oxygen intermediates (Cronstein et al., 1990; Cronstein, 1994; Zahler et al., 1994; Bullough et al., 1995; Felsch et al., 1995). It is possible that the local concentration of adenosine is crucial in determining which type of response predominates. A concentration-dependent dual protective-destructive role has also been described for the A3 adenosine receptor, but what is even more intriguing is that it involves high and low levels of activation of A3 receptors on the same cell (in both HL-60 and U 937 cells) (Yao et al., 1997). A1 adenosine receptors have been implicated in modulation of nociception in the spinal cord (Reeve and Dickenson, 1995) and in the periphery (Karlsten et al., 1992; Ocana and Baeyens, 1994). This may involve inhibition of sensory neurotransmitter release, because A1 receptors have been shown to mediate inhibition of calcitonin gene-related peptide (CGRP) release from capsaicin-sensitive sensory neurons in the spinal cord (Santicoli et al., 1993) and in the periphery (Rubino et al., 1993), as well as inhibit GABA currents in dorsal root ganglion neurons (Hu and Li, 1997). Analgesic effects of caffeine have also been described. These effects have been attributed to caffeine’s effects on supraspinal A1 receptors because caffeine’s effect is mimicked by the A1-selective agonist 8-cyclopentyltheophylline (CPT);
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spinally or peripherally administered caffeine lacks antinociceptive effects (Sawynok and Reid, 1996). Synergistic interactions between A1 adenosine receptors and receptors coupled to a different class of G protein, typically pertussis toxin insensitive Gq/11 proteins, have been described, whereby coactivation of the receptors results in an augmented increase in effectors/second-messengers derived from the Gq/11 protein coupled pathway. The intracellular mechanisms underlying this potentiation are not well understood and have been suggested variously to involve intra- and extracellular calcium, second-messengers, and Gi protein bg subunits. Early evidence for this kind of interaction came with the observation that adenosine enhances a1-adrenoceptorinduced accumulation of cAMP in rat vas deferens (Ha¨ggblad and Fredholm, 1987). Synergistic interactions have since been shown in DDT1 MF-2 cells for A1 receptors and ATP receptors (Gerwins and Fredholm, 1992a), histamine H1 receptors (Dickenson and Hill, 1994), and bradykinin receptors (Gerwins and Fredholm, 1992b). A1 receptors transfected into CHO cells act synergistically with receptors for thrombin (Dickenson and Hill, 1997), cholecystokinin A (Dickenson and Hill, 1996), and ATP (Megson et al., 1995). A1 receptors in astrocytes interact synergistically with histamine H1 receptors (Peakman and Hill, 1995) and glutamate receptors (Ogata et al., 1994) to raise levels of [Ca21]i. Synergistic interactions between A1 and a1-adrenoceptor mediated increases in inositol phosphate accumulation has been shown in mouse striatal astrocytes (el-Etr et al., 1992a,b; Marin et al., 1993). In hippocampal neurons, positive interactions have been described between adenosine A1 and GABAA receptors (Akhondzadeh and Stone, 1994), as well as negative interactions between A1 and metabotropic glutamate receptors (de Mendonc¸a and Ribeiro, 1997). Cross-talk between A1 and other receptors is clearly widespread; its physiological significance is an important area for future research. IV. A2A Receptor A. Cloned A2A Receptors The A2A receptor has been cloned from several species (table 3) and has a characteristic pharmacological profile in transfected cells consistent with that of the endogenous receptor. The first cloned adenosine receptor, RDC8, cloned from a canine thyroid cDNA library (Libert et al., 1989), was subsequently identified as an A2A receptor based on the binding of [3H]NECA and [3H]CGS 21680, and by activation of adenylate cyclase in cells transfected with the receptor (Maenhaut et al., 1990). The exogenous A2A receptor was shown to have a tissue distribution similar to endogenous A2A binding sites in brain, that is, limited to the striatum, nucleus accumbens and olfactory tubercule (Schiffmann et al., 1990). Subsequently, A2A receptors were cloned from rat brain (Chern et al., 1992; Fink et al., 1992), human hippocampus (Furlong et al., 1992), and guinea-pig
brain (Meng et al., 1994b). Both A2A and A2B receptors have been cloned from mouse bone marrow-derived mast cells (Marquardt et al., 1994). The gene for the A2A receptor has been mapped to human chromosome 22 (MacCollin et al., 1994; Peterfreund et al., 1996) with reported chromosomal localizations of 22q11.2 (Le et al., 1996) and 22q11.2-q13.1 (Libert et al., 1994). In common with the other adenosine receptor subtypes, there is significant interspecies differences in the amino acid sequences of cloned A2A receptors; for example, between rat and human A2A receptors there is approximately 84% amino acid homology (Chern et al., 1992; Fink et al., 1992; Furlong et al., 1992; Linden, 1994), and between rat and dog A2A receptors 82% homology (Chern et al., 1992; Fink et al., 1992). The significantly greater molecular weight of the A2A receptor (45 kDa) compared with the other adenosine receptor subtypes (36 to 37 kDa) can largely be attributed to its substantially longer carboxy terminal domain. This region is not involved in tight coupling to Gs proteins because this is a function predominantly of the N-terminal segment of the third intracellular loop (Olah, 1997). A truncated mutant of the canine A2A adenosine receptor was used to show that neither the long carboxyterminus nor the glycosidic moieties are required for ligand binding (Piersen et al., 1994). Site-directed mutagenesis of the human A2A adenosine receptor has been used to identify the various residues involved in agonist and antagonist binding (Kim et al., 1995; Ijzerman et al., 1996). B. Signal Transduction Mechanisms The most commonly recognized signal transduction mechanism for A2A receptors is activation of adenylate cyclase. This implies coupling with the G protein Gs, although other G proteins may also be involved. Vibrio cholerae (cholera toxin) ADP-ribosylates the a-subunit of Gs family members, inhibiting the intrinsic GTPase activity of Gas and thus has been useful in characterizing members of this family. Coupling of the A2A receptor to its G protein is tight (see Palmer and Stiles, 1995). Hence, there is only slow dissociation of agonist from the receptor and stabilization of the receptor-G protein complex. cAMP-independent signaling has been suggested for A2A receptors on striatal GABA nerve terminals (Kirk and Richardson, 1995) and striatal cholinergic nerve terminals (Gubitz et al., 1996). In striatal nerve terminals, A2A receptors are suggested to mediate dual signaling via P- and N-type Ca21 channels linked to Gs/ adenylate cyclase/PKA and cholera toxin-insensitive G protein/PKC, respectively (Gubitz et al., 1996). It has been suggested that A2A receptor-mediated inhibition of superoxide anion generation in neutrophils may be mediated via cAMP-independent activation of a serine/ threonine protein phosphatase (Revan et al., 1996).
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A2A receptor-mediated facilitation of synaptic transmission and transmitter release seems to occur through potentiation of presynaptic P-type Ca21 channels, and probably involves adenylate cyclase and activation of a cAMP-dependent protein kinase (Mogul et al., 1993; Correia-de-Sa´ and Ribeiro, 1994a; Umemiya and Berger, 1994; Gubitz et al., 1996). KATP channels are suggested to be involved in coronary vasodilatation mediated by A2 receptors in the dog (Akatsuka et al., 1994). Activation of KATP channels by A2 receptors in arterial myocytes is suggested to involve a cAMP-dependent protein kinase (Kleppisch and Nelson, 1995). C. Desensitization Desensitization of A2A receptors has been reported, which may be more rapid, similar to, or less rapid than that of A1 receptors. In DDT1 MF-2 cells, the t1/2 for desensitization of A2A receptors (45 min) is more rapid than that for A1 receptors, and in contrast to A1 receptors, there is no change in A2A receptor number or affinity (Ramkumar et al., 1991). A2A receptor desensitization after exposure to A2- or A2A-selective agonists for up to several minutes to 4h has been observed in a number of tissues including porcine coronary artery (Makujina and Mustafa, 1993), rat aortic vascular smooth muscle cells (Anand-Srivastava et al., 1989), DDT1 MF-2 smooth muscle cells (Ramkumar et al., 1991), rat pheochromocytoma PC12 cells (Chern et al., 1993), and in canine A2A receptors expressed in CHO cells (Palmer et al., 1994). On the other hand, guinea-pig coronary artery A2A receptors do not desensitize after more than 2h exposure to 2-[(2-aminoethylamino) carbonylethylphenylethylamino]-59-N-ethylcarboxamido adenosine (APEC) or 1,4-phenylene-diisothiocyanate, 4-isothiocyanatophenyl aminothiocarbonyl-APEC (DITC-APEC) (Niiya et al., 1993). Furthermore, A2A receptors seem to be relatively resistant compared with A1 receptors to desensitization in rat brain slices (Abbracchio et al., 1992) and in spontaneously hypertensive rats after chronic treatment with A1 and A2 selective agonists in vivo (Casati et al., 1994). In rat striatum slices, A2 receptors do not desensitize following exposure to NECA for up to 1h, whereas A1 receptors desensitize rapidly (Abbracchio et al., 1992). The mechanism underlying desensitization of A2A receptors has been studied in some detail in transfected CHO cells, where it has been shown that exposure to agonist causes rapid desensitization and phosphorylation (Palmer et al., 1994; Palmer and Stiles, 1997b). The threonine 298 residue of the carboxy terminal of the A2A receptor seems to be essential for agonist-stimulated rapid receptor phosphorylation and short-term, but not long-term, desensitization (Palmer and Stiles, 1997a). The majority of the C terminal seems not to be involved in desensitization, because desensitization of a truncated mutant lacking the majority of the A2A carboxylterminal (the last 95 residues) is unchanged (Palmer
and Stiles, 1997a). Evidence that desensitization may involve GRKs, implying uncoupling of the receptor-G protein complexes, has been provided by a study in NG108 –15 mouse neuroblastoma 3 rat glioma cells mutants overexpressing GRK2, where the rate of desensitization of endogenous A2A and A2B receptors was markedly slowed (Mundell et al., 1997). This effect was selective in that agonist-induced desensitization of secretin and IP-prostanoid receptor stimulated adenylate cyclase were not affected by dominant negative mutant GRK2 overexpression (Mundell et al., 1997). Receptor sequestration, whereby a receptor translocates to a ”light membrane” fraction, has been described for A2A receptors expressed in CHO cells, but this seems to be involved in the recovery of the response of the receptor rather than in desensitization (Palmer et al., 1994). Studies of long-term desensitization of endogenous A2A receptors in rat pheochromocytoma PC12 cells showed that whereas a 30 min exposure of A2A receptors to CGS 21680 is associated with inhibition of adenylate cyclase activity, long-term agonist exposure (12–20h) is associated additionally with down regulation of Gs a proteins and activation of phosphodiesterase (Chern et al., 1993). Long-term (24h) exposure to agonist may additionally lead to down-regulation of receptor number and up-regulation of inhibitory G proteins (Palmer et al., 1994; Palmer and Stiles, 1997a). Approximately 2 weeks of continuous infusion of either NECA or CGS 21680 causes a decrease in the number of A2A receptor binding sites in rat striatum (Porter et al., 1988; Webb et al., 1993a). A calcium-independent PKC isoenzyme seems to be involved in phosphorylation and inhibition of adenylate cyclase type VI activity after prolonged stimulation and desensitization of the A2A receptor, at least in rat pheochromocytoma PC12 cells (Lai et al., 1997), providing an additional mechanism by which to regulate A2A receptor signal transduction. D. Sensitization/Up-Regulation Striatal A2A adenosine receptors in rats and mice are up-regulated after chronic caffeine ingestion (Hawkins et al., 1988; Traversa et al., 1994). A2A receptors seem to be less prone to up-regulation after chronic blockade with non-selective antagonists than are A1 receptors (Lupica et al., 1991a; Johansson et al., 1993a). E. Agonists A2A receptors do not generally bind N6-substituted adenosine derivatives and show a preference for derivatives with modifications of the 2nd position of the adenine ring; bulky substituents in this position can selectively enhance A2A receptor affinity (Jacobson et al., 1992b; Cristalli et al., 1994; Siddiqi et al., 1995). Several synthetic A2A-selective agonists are modeled according to this structural modification. It should be noted that the agonist studies detailed below have been carried out in species other than humans, and that the human A2A
RECEPTORS FOR PURINES AND PYRIMIDINES
receptor has a comparatively lower affinity of binding for CGS 21680 and other adenosine receptor agonists (Dionisotti et al., 1997; Klotz et al., 1998). The C2-substituted NECA derivative, CGS 21680, is 140-fold selective for the A2A versus the A1 receptor (Hutchison et al., 1990) (fig. 2). CGS 21680 has only very low affinity at the A2B receptor, and thus has been used extensively to discriminate between A2A and A2B subtypes (Jarvis et al., 1989; Lupica et al., 1990). [3H]CGS 21680 has been reported to bind in rat cortex and hippocampus to adenosine binding sites different to the classic striatal A2A receptors, which does not seem to be caused by high and low affinity states of the same A2A receptor, or to binding at A3 or A4 receptors (Johansson et al., 1993b; Cunha et al., 1996; Lindstro¨m et al., 1996). Amine derivatives of CGS 21680, namely APEC (fig. 2), DITC-APEC and 2-[4-(2-([4-aminophenyl]methylcarbonyl)-ethyl)-phenyl]ethylamino-59-N-ethylcarboxamidoadenosine (PAPA-APEC), are A2A-selective agonists (Barrington et al., 1989; Ramkumar et al., 1991; Jacobson et al., 1992a; Niiya et al., 1993). DITC-APEC binds covalently, causing irreversible activation of the A2A receptor (Niiya et al., 1993). The C2-substituted adenosine derivative CV 1808 displays poor selectivity (approximately 5-fold) for the A2A versus the A1 receptor (Kawazoe et al., 1980; Bruns et al., 1986), but is a valuable precursor for the synthesis of more selective A2A receptor agonists. N6-(2(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl)-adenosine (DPMA) is a selective A2A receptor agonist (Merkel et al., 1992; Alexander et al., 1994). A series of 2-aralkynyl and 2-heteroalkynyl derivatives of NECA have been studied for their selectivity at the A2A receptor (Cristalli et al., 1995). Of these, the 4-formylphenylethynyl derivative shows affinity in the low nanomolar range and approximately 160-fold selectivity. 2-Hexyl-59-N-ethylcarboxamidoadenosine (2HENECA) has been suggested to be selective at A2A receptors with 60- and 160-fold selectivity in binding studies for A2A versus A1 receptors in rat and bovine brain, respectively (Monopoli et al., 1994). Although NECA itself is approximately equipotent at A1 and A2A receptors, it can be useful in A2A receptor characterization provided that A1-selective ligands are shown not to have equivalent effects. The 2-hydrazinoadenosine, WRC-0470 (2-cyclohexylmethylidenehydrazinoadenosine) has been shown to be a potent and selective A2A agonist, with low nanomolar affinity at recombinant A2A receptors transfected in mammalian cells and in functional assays in a variety of tissues (Martin et al., 1997b). F. Antagonists Several antagonists selective for the A2A receptor have been synthesized. 8-(3-chlorostyryl)caffeine (CSC) is a potent (Ki 54 nM) and selective A2A antagonist in radioligand binding assays in rat brain (520-fold selec-
tive versus A1 receptors), in reversing agonist effects on adenylate cyclase in PC12 cells (22-fold selective), and in blocking locomotor depression elicited by the A2A-selective agonist APEC in vivo (Jacobson et al., 1993a) (fig. 3). 1,3-dialkyl-7-methyl-8-(3,4,5-trimethoxystyryl)xanthine (KF-17837) has been described as a potent and selective A2A antagonist with 62-fold selectivity for A2A over A1 receptors in binding studies in rat brain, and 30-fold selectivity for the A2A over the A2B receptor in inhibition of cAMP accumulation (A2A IC50 5 53 nM; A2B IC50 5 1500 nM) (Shimada et al., 1992; Kanda et al., 1994; Nonaka et al., 1994). DMPX (3,7-dimethyl-1-propargylxanthine) derivatives have been shown to be potent and selective A2A antagonists; 8-(m-bromostyryl)-DMPX has a Ki value of 8.2 nM and is 146-fold selective versus A1 receptors (Mu¨ller et al., 1996b). ZM 241385, (4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo [2,3-a] [1,3,5]triazin-5-yl amino]ethyl)phenol) is a potent and selective non-xanthine A2A adenosine receptor antagonist (Poucher et al., 1995) (fig. 3). It has high affinity for the A2A receptor (pA2 value approximately 9), is 1000- and 91-fold selective versus A1 and A2B receptors, respectively, and has virtually no effects at A3 receptors (Poucher et al., 1995). [3H]SCH 58261 ([3H-5-amino-7-(2-phenylethyl)-2-(2furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine) is a novel potent and selective A2A antagonist radioligand which binds with low nanomolar affinity to A2A receptors in human platelet and rat striatal membranes, and at A2A receptors transfected into CHO cells (Zocchi et al., 1996; Dionisotti et al., 1997). The analog SCH 63390 (5-amino-7-(3-phenylpropyl)-2-(2-furyl)pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine) has similar potency at A2A receptors, but greater selectivity (210-fold) (Baraldi et al., 1996). G. Distribution and Biological Effects A2A receptors have a wide-ranging but restricted distribution that includes immune tissues, platelets, the CNS, and vascular smooth muscle and endothelium. Functional studies concerned with A2A receptors in isolated cells and tissues, in the central and peripheral nervous systems, and in isolated blood vessels and vascular beds, are listed in tables 4, 5 and 6, and illustrate the wide distribution and diverse biological effects mediated by this receptor. Within the brain, the highest levels of A2A receptors are in the striatum, nucleus accumbens, and olfactory tubercle (regions which are rich in dopamine) (Ongini and Fredholm, 1996). Low levels of A2A receptor also seem to be expressed in most other brain regions, although for striatal cholinergic neurons this is controversial (Dixon et al., 1996; Peterfreund et al., 1996; Jin and Fredholm, 1997; Svenningsson et al., 1997). Striatal neurons express A2A receptors in close association with dopamine D2 receptors and specific negative interactions have been described (Fe´rre et al., 1991, 1992, 1997;
RALEVIC AND BURNSTOCK TABLE 4 Distribution and effects mediated by endogenously expressed A2 adenosine receptors Tissue
Astrocytes Astrocytes, type 1 Astrocytes, type 2 Astroglioma cell line D384 Astrocytoma cell line U373 Neutrophils
— — Degranulation Interleukin-8 secretion by A2B — 2 Aggregation
Fink et al., 1992; Schiffmann and Vanderhaeghen, 1993). Outside the brain, the most abundant expression of human A2A mRNA is in immune tissues, eye and skeletal muscle; heart, lung, bladder, and uterus also show strong expression, with less abundant expression in small intestine, kidney, spleen, stomach, testis, skin, kidney, and liver (Dixon et al., 1996; Peterfreund et al., 1996). A2A receptors in the CNS and particularly in the peripheral nervous system (PNS) generally facilitate neurotransmitter release (table 5). The negative interactions that have been observed between A2A and dopamine D2 receptors involve a reduced affinity of agonist binding to dopamine D2 receptors upon stimulation of A2A receptors in rat striatal membranes (Ferre´ et al., 1991, 1992, 1997). This raises the possibility of using A2A receptor antagonists as a novel therapeutic approach in the treatment of Parkinsons disease, to reduce the profound disabling effects arising from degeneration of dopaminergic nigrostriatal neurons of the basal ganglia in this disease (Richardson
Hindley et al., 1994 Peakman and Hill, 1994, 1996 Peakman and Hill, 1996 Altiok et al., 1992; Fredholm and Altiok, 1994 Fiebich et al., 1996 Zhang et al., 1996; Walker et al., 1996 Cronstein et al., 1990, 1992; Salmon and Cronstein, 1990; Gurden et al., 1993; Cronstein, 1994; Bullough et al., 1995; Felsch et al., 1995 Nonaka et al., 1994; van der Ploeg et al., 1996 Marquardt et al., 1994 Auchampach et al., 1997a Feoktistov and Biaggioni, 1995 Bruns et al., 1986; Brackett and Daly, 1994 Huttemann et al., 1984; Gurden et al., 1993; Monopoli et al., 1994; Cristalli et al., 1995 Casado et al., 1992; Mateo et al., 1995 Koizumi et al., 1994 Hide et al., 1992; Chern et al., 1993; Nonaka et al., 1994; van der Ploeg et al., 1996 Gharib et al., 1992 Blazynski and McIntosh, 1993 Blazynski, 1993 McIntosh and Blazynski, 1994 Pauwels and Joos, 1995 Losinski and Alexander, 1995 Burnstock et al., 1984 Nicholls et al., 1992 Nicholls et al., 1996 Stehle et al., 1992 Stehle et al., 1992 Hancock and Coupar, 1995a Strohmeier et al., 1995 Ainz et al., 1993 Buxton et al., 1987 Stanley et al., 1987 Nakashima et al., 1993 Churchill and Churchill, 1985; Churchill and Bidani, 1987 Freissmuth et al., 1987 Chapal et al., 1985 Nicholls et al., 1992; Stehle et al., 1992 Shen et al., 1993
et al., 1997). Interactions are not observed between A2A and D2 receptors transfected into COS-7 cells; it was suggested that the receptors do not interact directly to influence agonist binding (Snaprud et al., 1994). Interestingly, activation of A2A receptors on rat striatal nerve terminals causes desensitization of coexpressed A1 receptors by a mechanism which seems to involve PKC (Dixon et al., 1997a). It is noteworthy that both D2 dopamine and A1 adenosine receptors couple to Gi proteins to cause inhibition of adenylate cyclase. Thus, with respect to the actions of adenosine at A2A receptors, negative A2A-A1 and A2A-D2 interactions will shift the balance of intracellular signaling further toward stimulation of cAMP. Interactions between A2A receptors and dopamine D1 receptors, and receptors for CGRP, glutamate, and acetylcholine have also been reported (see Sebastia`o and Ribeiro, 1996). Negative interactions whereby activation of the A2A receptor blocks the protective effects of preconditioning hypoxia, believed to be via A1 and A3 receptors, have been described (Strickler et al., 1996).
RECEPTORS FOR PURINES AND PYRIMIDINES TABLE 5 Functional distribution of endogenously expressed A2 adenosine receptors in central and peripheral nervous systems Location
Barraco et al., 1995 Castillo-Mele´ndez et al., 1994 Hauber and Munkle, 1995 Zetterstro¨m and Fillenz, 1990 Brown et al., 1990; Kurokawa et al., 1994 Kirkpatrick and Richardson, 1993 No¨renberg et al., 1997b Kirk and Richardson, 1995 Ishikawa et al., 1997 DeLander and Hopkins, 1987
1 Electrically and CGRP-evoked [3H]ACh release 1 Excitability 2 Capsaicin-evoked substance P release Depolarization 1 Electrically evoked NA release 1 Electrically evoked NA release
Correia-de-Sa´ and Ribeiro, 1994a,b; Correia-deSa´ et al., 1996 Christofi et al., 1994 Morimoto et al., 1993 Castillo-Melendez et al., 1994 Gonc¸alves and Queiroz, 1993 Gonc¸alves and Queiroz, 1996
2 2 1 2 2
Behavioral effects of A2A receptors are evidenced by A2A-mediated cataleptic activity and antagonism of apomorphine-induced climbing (an animal model of schizophrenia) (Kanda et al., 1994; Kafka and Corbett, 1996). In the vasculature, A2A receptors have been described on both the smooth muscle and endothelium, where they are associated with vasodilatation (table 6). There seems to be considerable variation in A2A receptor expression between blood vessels, although it is possible that vessels unresponsive to A2A-selective agonists do express the receptor but at very low levels, or that the receptor is not coupled to a functional response. This functional diversity is exemplified by the fact that A2A receptors mediate relaxation of rat aorta and bovine coronary artery (Conti et al., 1993), whereas in guinea-pig pulmonary artery (Szentmiklo´si et al., 1995) and rat mesenteric arterial bed (Rubino et al., 1995), adenosine-mediated relaxation is mediated via the A2B receptor, and relaxation via A2A receptors is weak or non existent (fig. 5). Adenosine has a mitogenic effect on endothelial cells, which in human endothelial cells is mediated via the A2A receptor and subsequent activation of mitogen-activated protein kinase (MAPK) (Sexl et al., 1997). The mitogenic activation seems to be independent of Gs, Gi and typical PKC isoforms, but is associated with activation of p21ras (Sexl et al., 1997). An interesting development in this field is provided by a study of A2A receptor knockout mice (Ledent et al.,
Kurokawa et al., 1994 Phillis, 1990; Lin and Phillis, 1991 Phillis et al., 1993a,b O’Regan et al., 1992a O’Regan et al., 1992b Mayfield et al., 1993 Kurokawa et al., 1994 Jin and Fredholm, 1997 Cunha et al., 1994 Mogul et al., 1993 Cunha et al., 1995 Barraco et al., 1993, 1994 Barraco et al., 1993; Ergene et al., 1994
1997). These mice showed reduced exploratory activity. Caffeine, which normally stimulates locomotor activity, substantially depressed activity. The A2A knockout mice also showed increased aggresiveness, hypoalgesia, an increase in blood pressure and heart rate, and an increase in platelet aggregation (Ledent et al., 1997). It is satisfying that these findings are broadly consistent with those predicted from studies of the endogenous A2A receptor in isolated cells and tissues, and in whole animals. V. A2B Receptor A. Cloned A2B Receptors A2B receptors have been cloned from human hippocampus (Pierce et al., 1992), rat brain (Rivkees and Reppert, 1992; Stehle et al., 1992), and mouse bone marrowderived mast cells (Marquardt et al., 1994) (table 3). The human A2B adenosine receptor gene (ADORA2B) has been localized to chromosome 17p11.2-p12 (Townsend-Nicholson et al., 1995b) and 17p12 (Jacobson et al., 1995a). A human A2B receptor pseudogene has been cloned and localized to chromosome 1q32 (Jacobson et al., 1995a). Although the pseudogene is unable to encode a functional receptor, it is 79% identical with the functional A2B receptor. Thus, it was noted that the existence of the transcript in tissues could lead to misinterpretation of in situ hybridization and northern blot analysis when probes are used to recognize sequences common to these receptors (Jacobson
RALEVIC AND BURNSTOCK TABLE 6 Functional distribution of endogenously expressed vascular A2 adenosine receptors Vessel and species
Aorta; rabbit Aorta; rat
A2A A2A, A2B
N.D. EC, SMa
Aortic EC; human Aortic SM cells; rat Coeliac artery; rabbit Coronary artery; bovine Coronary artery; canine Coronary artery; human Coronary artery; porcine Coronary artery EC; guinea-pig Coronary bed/vessels; guinea-pig Corpus cavernosum; rabbit DDT1 MF-2 cells (SM cells) Hepatic arterial bed; rabbit Mammary artery; human Mesenteric arterial bed; rat Mesenteric arterial bed; rat Mesenteric artery; rabbit Placental arterial bed; human Pulmonary artery; guinea pig Pulmonary arterial bed; rat Pulmonary artery and vein; rabbit Pulmonary arterial bed; rabbit Renal artery; rat Renal bed; rat Saphenous vein; canine Saphenous vein; human Umbilical vein EC; human
EC SM N.D. N.D. N.D. N.D. EC, SM EC EC, SM EC, SM SM N.D. N.D. EC, SM SM N.D. N.D. SM SM EC, SM N.D. EC SM N.D. N.D. EC
Hargreaves et al., 1991; Martin, 1992; Martin et al., 1993b; Gurden et al., 1993; Alexander et al., 1994 Balwierczak et al., 1991 Conti et al., 1993; Lewis et al., 1994; Monopoli et al., 1994; Prentice and Hourani, 1996 Iwamoto et al., 1994 Dubey et al., 1996 Balwierczak et al., 1991 Conti et al., 1993; Monopoli et al., 1994 Balwierczak et al., 1991; Gurden et al., 1993 Makujina et al., 1992 Balwierczak et al., 1991; Abebe et al., 1994; Monopoli et al., 1994 Schiele and Schwabe, 1994 Martin et al., 1993b; Vials and Burnstock, 1993 Chiang et al., 1994 Ramkumar et al., 1991 Mathie et al., 1991a,b Makujina et al., 1992 Hiley et al., 1995 Rubino et al., 1995 Balwierczak et al., 1991 Read et al., 1993 Szentmiklo´si et al., 1995 Haynes et al., 1995 Steinhorn et al., 1994 Pearl, 1994 Martin and Potts, 1994 Levens et al., 1991a,b; Agmon et al., 1993 Hargreaves et al., 1991 Makujina et al., 1992 Sobrevia et al., 1997
EC, endothelium; SM, smooth muscle; N.D., not determined. a A2A adenosine receptor only.
et al., 1995a). As with the other adenosine receptor subtypes, there is considerable species differences in the sequence of the A2B receptor; for example, 86% amino acid sequence homology between rat and human A2B receptors (Stehle et al., 1992; Pierce et al., 1992; Linden, 1994).
finity of A2B receptors for adenosine raises the possibility that they may still be fully operational, and thus may act as a backup for adenosine responses, when the higher affinity coexpressed A2A receptors have been activated and desensitized.
B. Signal Transduction Mechanisms
D. Agonists and Antagonists
A2B receptor coupling to different signaling pathways has been reported, including activation of adenylate cyclase, Gq/G11-mediated coupling to PLC and IP3-dependent increase in [Ca21]i (in human mast cells) (Feoktistov and Biaggioni, 1995), and coupling to PLC when expressed in Xenopus oocytes (Yakel et al., 1993).
Despite intensive efforts in this area, there are no A2B-selective agonists. Thus, at present, activation of adenylate cyclase in membranes and accumulation of cAMP in cells is used to characterize A2B receptors, provided a lack of activity/binding of A1-, A2A-, and A3selective agonists is confirmed. As with A2A receptors, A2B receptors show a preference for adenosine derivatives with modifications of the C2 position of the adenine ring. NECA is currently the most potent agonist at A2B receptors, having low micromolar affinity (Brackett and Daly, 1994; Alexander et al., 1996; Klotz et al., 1998), but is less useful in characterization of A2B receptors in cells or tissues in which A2A receptors are coexpressed because it is non-selective. 2-ClADO, N6-(3-iodobenzyl)-59(N-methylcarbamoyl)adenosine (IB-MECA), and R-PIA are among the more potent of other conventional adenosine-receptor agonists that act also at A2B receptors, but their affinity for the A2B receptor is relatively low (EC50 values 9 to 11 mM) (Brackett and Daly, 1994; Klotz et al., 1998). Enprofylline blocks A2B receptors in human mast cells HMC-1 (Ki 7 mM) and canine BR mastocytoma cells and
C. Desensitization The lack of A2B receptor-selective agonists has undoubtedly contributed to the general lack of information on A2B receptor desensitization. In rat PC12 cells, the A2B response has been shown to be reduced in A2Adesensitized cells, possibly through common inhibition of adenylate cyclase (Chern et al., 1993). In mutant NG108 –15 cells overexpressing GRK2, desensitization of endogenous A2B receptors was markedly less than that in normal cells (t1/2 15–20 min), indicating that receptor phosphorylation and uncoupling from G proteins may be involved in desensitization of A2B receptors (Mundell et al., 1997). Although it is not yet clear whether there are inherent differences in the rates of desensitization of A2A and A2B receptors, the lower af-
RECEPTORS FOR PURINES AND PYRIMIDINES
FIG. 5. Species variation in functional expression of vasodilator A2A and A2B receptors. Note that the agonist potencies suggest the presence of A2A receptors in rat aorta (a) and bovine coronary artery (b), and A2B receptors in rat mesenteric arterial bed, (c) and guinea-pig pulmonary arteries (d). a., b. Mean dose-response curves for the vasorelaxant activity induced by some adenosine agonists in isolated rat aorta (a) and bovine coronary artery (b). Each response is expressed as the percentage of the maximum contraction induced by PGF2a (3 mM). Vertical bars represent 95% confidence limits. (From Conti et al., 1993). c. Dose-response curves showing vasodilator responses of the rat mesenteric vascular bed to ATP (Œ), 2-meSATP (n), adenosine (L), 2-CADO (,), NECA (), CPA (E) and CGS 21680 (F). Vasodilator response are shown as percent vasodilatation of the methoxamine sustained tone taken as 100% and are the mean of 4 to 7 preparations. Response are to bolus injections of drugs. Symbols show means 6 SEM (From Rubino et al., 1995, Br J Pharmacol 115: 648 – 652; with permission from McMillam Press Limited). d. Concentration-dependent relaxation of guinea pig pulmonary arteries by NECA (Œ; n 5 5), CADO (r; n 5 5), adenosine (; n 5 16), CGS 21680 (n; n 5 5), R-PIA (l; n 5 5) or CPA (F; n 5 15). Relaxant responses are expressed as a percentage of the noradrenaline-contraction (mean 6 SEM). (From Szentmiklo´si et al., 1995).
is inactive at A1, A2A, and A3 receptors. It may, therefore, be a valuable starting compound from which to develop more potent selective A2B receptor antagonists (Feoktistov and Biagionni, 1996). The non-xanthine alloxazine has been reported as having approximately 9-fold selectivity for the A2B compared with the A2A receptor (Brackett and Daly, 1994). XAC and CGS 15943 are antagonists with low nanomolar affinity at A2B receptors, but are non-selective versus other subtypes of adenosine receptor (Alexander et al., 1996; Klotz et al., 1998). E. Distribution and Biological Effects A2B receptors are found on practically every cell in most species; however, the number of receptors is small and relatively high concentrations of adenosine are generally needed to evoke a response. The sensitive technique of reverse transcription-polymerase chain reaction (RT-PCR) showed low levels of A2B receptors in all rat brain regions tested (Dixon et al., 1996). Northern blot analysis showed relatively high expression of A2B receptors in the caecum, large intestine, and urinary
bladder, with lower levels in the brain, spinal cord, lung, vas deferens, and pituitary (Stehle et al., 1992). RT-PCR revealed the highest expression of A2B receptors in the proximal colon, with lower levels in the eye, lung, uterus, and bladder; still lower levels in the aorta, stomach, testis, and skeletal muscle; and the lowest levels in the jejunum, kidney, heart, skin, spleen, and liver (Dixon et al., 1996). Selected distributions and biological effects mediated by A2B receptors in isolated cells and tissues are listed in tables 4 and 6. Functional studies have identified A2B receptors in airway smooth muscle, fibroblasts, glial cells, the gastrointestinal tract, and the vasculature. A2B receptors have been cloned from, and immunolocalized on, mouse bone marrow-derived mast cells (Marquardt et al., 1994), and shown to mediate degranulation of canine BR mastocytoma cells (Auchampach et al., 1997a). They have also immunolocalized and been shown to activate human mast cells (Feoktistov and Biagionni, 1996). This implies a possible role in allergic and inflammatory disorders. The antiasthmatic effects of enprofylline, a potential A2B receptor antagonist, are consistent with this hypothesis (Feoktistov and Biaggioni, 1996). Vascular A2B receptors identified by pharmacological and biochemical studies are listed in table 6, which shows that these receptors may couple to a functional response (vasodilatation) in both smooth muscle and endothelium. Interestingly, A2B receptors seem to be important in mediating vasodilatation in some vessels, including the rat mesenteric arterial bed (Rubino et al., 1995) and guinea-pig pulmonary arteries (Szentmiklo´si et al., 1995), but not in others where the A2A subtype predominates (table 6, fig. 5). Rat aortic smooth muscle A2B receptors have been implicated in inhibition of growth (Dubey et al., 1996), identifying a possible longterm trophic role for these receptors. VI. A3 Receptor A. Cloned A3 Receptors A3, the fourth distinct adenosine receptor, was identified relatively late in the history of adenosine/P1 receptors with the cloning, expression, and functional characterization of a novel adenosine receptor from rat striatum (Zhou et al., 1992). This was identical with a clone previously isolated from a rat testis cDNA library encoding a G protein-coupled receptor with greater than 40% sequence homology with canine A1 and A2A adenosine receptors, although its ligand had not then been identified (Meyerhof et al., 1991). The recombinant striatal A3 receptor does not resemble any other adenosine/P1 subtypes in agonist or antagonist binding; it binds ligands with a potency order of R-PIA 5 NECA . S-PIA and is coupled to inhibition of adenylate cyclase activity in a pertussis toxin-sensitive manner; it binds with high affinity to the radioligand N6-2-(3-iodo-4-
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aminophenyl)ethyladenosine but not to the A2A-selective adenosine ligand [3H]CGS 21680 or the alkylxanthine antagonists XAC, IBMX, or the A1-selective antagonist DPCPX. Homologs of the rat striatal A3 receptor have been cloned from sheep pars tuberalis (pituitary tissue) (Linden et al., 1993), human heart (Sajjadi and Firestein, 1993, and striatum (Salvatore et al., 1993) (see also Linden, 1994) (table 3). Interspecies differences in A3 receptor structure are large; the rat A3 receptor shows only approximately 74% sequence homology with sheep and human A3 receptors each, although there is 85% homology of sheep and human A3 receptors. This is reflected in the very different pharmacological profiles of the species homologs, particularly with respect to antagonist binding, and this has caused considerable complications in the characterization of this receptor. The human A3 receptor has been localized to chromosome 1 p13.3 (Monitto et al., 1995). The rat, but not the human, A3 receptor transcript may be subject to extensive alternative splicing, further evidence of the profound interspecies differences involving the A3 receptor. A splice variant of the rat A3 receptor (A3i), having a 17 amino acid insertion within the second intracellular loop, has been cloned and characterized (Sajjadi et al., 1996). There was no evidence for alternative splicing of the human A3 receptor transcript (Sajjadi et al., 1996). This A3 receptor has taken precedence over the controversial A3 receptor defined principally according to its pharmacological profile by Ribeiro and Sebastia`o (1986), which probably represents an A1 receptor (Carruthers and Fozard, 1993; Ribeiro and Sebastia`o, 1994). B. Signal Transduction Mechanisms The A3 receptor is G protein-linked, coupling to Gia2-, Gia3- and, to a lesser extent, to Gq/11 proteins (Palmer et al., 1995b). In rat basophilic leukemia cells (RBL-2H3; a cultured mast cell line) (Ali et al., 1990; Ramkumar et al., 1993b) and in rat brain (Abbracchio et al., 1995a), the A3 receptor stimulates PLC and elevates IP3 levels and intracellular Ca21. PKC has been suggested to be involved in A3 receptor-mediated preconditioning in rabbit cardiomyocytes (Armstrong and Ganote, 1994). The A3 receptor has also been shown to inhibit adenylate cyclase activity (Zhou et al., 1992; Abbracchio et al., 1995b). C. Desensitization Recombinant rat and human A3 receptors have been shown to desensitize within minutes in response to agonist exposure; this is associated with uncoupling of the receptor-G protein complex, as indicated by a reduction in the number of high affinity binding sites (Palmer et al., 1995a; Palmer et al., 1997). Desensitization of the rat A3 receptor is rapid (within a few minutes), homologous, and is associated with rapid phosphorylation by a
G protein-coupled receptor kinase similar to, or identical with, GRK2 (Palmer et al., 1995a; Palmer and Stiles, 1997b). Rapid, homologous functional desensitization of A3 receptors has also been described in RBL-2H3 cells (Ali et al., 1990; Ramkumar et al., 1993b). A chimeric A1-A3 receptor constructed from an A1 receptor (nondesensitizing under the conditions of the study) and the C-terminal domain of an A3 receptor was expressed in CHO cells and shown to undergo rapid desensitization. This indicates that the C-terminal domain of the A3 receptor is the site for phosphorylation by the G proteincoupled receptor kinases involved in desensitization (Palmer et al., 1996). The effects of long-term agonist exposure on interaction of the rat A3 receptor with G proteins was assessed using a transfected CHO cell system (Palmer et al., 1995b). Chronic exposure of A3 receptors to the nonselective agonist NECA (for up to 24h) causes selective down-regulation of Gia3- and b-subunits, without changing levels of Gia2 or Gq-like proteins (Palmer et al., 1995b). D. Up-Regulation In situ hybridization identified the A3 receptor in mesenchymal cells and eosinophils within the lamina propria of the airways and the adventitia of blood vessels in the lung, as well as in peripheral eosinophils, but interestingly, not in mast cells (Walker et al., 1997). It was found that the A3 receptor transcript was greater in lung tissue from subjects with airway inflammation than in normal lung. This is consistent with the hypothesis that there is a distinct distribution of the A3 receptor in inflammatory cells and that this is up-regulated in airway inflammation (Walker et al., 1997). E. Agonists The main class of selective A3 receptor agonists is the N6-substituted adenosine-59-uronamides. N6-benzylNECA is potent (Ki 6.8 nM) and moderately selective (13and 14-fold versus A1 and A2A) at rat A3 receptors transfected into CHO cells (van Galen et al., 1994). N6-(3-iodobenzyl)-59-(N-methylcarbamoyl)adenosine (IB-MECA) (Ki 1.1 nM) is 50-fold selective for rat brain A3 receptors versus A2A or A1 receptors (Gallo-Rodriguez et al., 1994) (fig. 2). The iodinated radioligand [125I]AB-MECA binds with approximately nanomolar affinity to rat brain A3 adenosine receptors expressed in CHO cells, but also binds to native A1 receptors. Selectivity is increased by 2-substitution of N6-benzyladenosine-59-uronamides; 2-chloro-IB-MECA (2Cl-IB-MECA, Ki 5 0.33 nM) is highly selective for A3 versus A1 and A2A receptors, by 2500- and 1400-fold, respectively (Kim et al., 1994) (fig. 2). There is pronounced interspecies differences in the relative affinities of agonist binding at A3 receptors (Ji et al., 1994; Linden, 1994).
RECEPTORS FOR PURINES AND PYRIMIDINES
F. Antagonists Several classes of compounds have been developed as A3 antagonists. One class comprises xanthines and their derivatives. Rat, rabbit, and gerbil brain A3 receptors bind only weakly to xanthine derivatives compared with human and sheep A3 receptors, which exhibit high affinity (Zhou et al., 1992; Linden et al., 1993; Salvatore et al., 1993; Ji et al., 1994). The most potent of the 8-phenyl-substituted xanthines, I-ABOPX (3-(3-iodo-4aminobenzyl)-8-(4-oxyacetate)phenyl-1-propylxanthine, or BW-A522) binds with nanomolar affinity to human and sheep A3 receptors (Linden et al., 1993; Salvatore et al., 1993), but by contrast with micromolar affinity at rabbit, gerbil, and rat A3 receptors (Ji et al., 1994). Five chemical classes of non-xanthine antagonists have been reported. L-268605 (3-(4-methoxyphenyl)-5amino-7-oxo-thiazolo [3, 2]pyrimidine) is a potent and selective A3 antagonist with a Ki value of 18 nM and no appreciable affinity for human A1 and A2A receptors (Jacobson et al., 1996) (fig. 3). Another class is represented by L-249313 (6-carboxymethyl-5,9-dihydro-9methyl-2-phenyl-[1, 2, 4]-triazolo[5,1-a][2, 7]naphthyridine) with high affinity at cloned human A3 receptors, Ki value of 13 nM, but low affinity at native rat brain A3 receptors, Ki 58 mM, and selectivity of approximately 300- and 1460-fold over A1 and A2A receptors, respectively (Jacobson et al., 1996) (fig. 3). The three other categories of molecules with promise as A3 receptor antagonists are the flavonoid MRS 1067 (3,6-dichloro-29isopropyloxy-49-methyl-flavone), the 6-phenyl-1,4-dihydropyridines MRS 1097 (3,5-diethyl[2-methyl6-phenyl-4-(2-phenyl-(E)-vinyl]-1,4-(6)-dihydropyridine3,5-dicarboxylate) and MRS 1191 (3-ethyl 5-benzyl 2-methyl-6-phenyl-4-phenylethynyl-1,4-(6)-dihydropyridine-3,5-dicarboxylate) and the triazoloquinazolene MRS 1220 (9-chloro-2-(2-furyl)-5-phenylacetylamino[1, 2, 4]triazolo[1,5-c]quinazoline). Of these, MRS 1220 and MRS 1197 show promise as potent and selective competitive antagonists, with Ki values of 0.6 and 31 nM, respectively, for inhibition of [125I]AB-MECA binding and KB values of 1.7 and 92 nM at human recombinant A3 receptors (Jacobson et al., 1997). A much lower affinity was observed at the rat A3 receptor: .2000-fold for MRS1220 and 112-fold for MRS 1197 (Jacobson et al., 1997) as has been noted with xanthine-based antagonists. G. Distribution and Biological Effects The A3 receptor is widely distributed, but its physiological role is still largely unknown. A3 mRNA is expressed in testis, lung, kidneys, placenta, heart, brain, spleen, liver, uterus, bladder, jejunum, proximal colon, and eye of rat, sheep, and humans (Zhou et al., 1992; Linden et al., 1993; Salvatore et al., 1993; Linden, 1994; Rivkees, 1994; Dixon et al., 1996) (fig. 4). A3 mRNA was not detected in rat skin or skeletal muscle (Dixon et al., 1996) (fig. 4). Rat testis seems to have particularly high
concentrations of A3 mRNA (in spermatocytes and spermatids), compared with rather lower levels in most other rat tissues (Linden et al., 1993; Salvatore et al., 1993). The highest levels of human A3 mRNA are found in lung and liver, with lower levels in aorta and brain (Salvatore et al., 1993). In sheep, the highest levels of A3 mRNA are found in lung, spleen, pars tuberalis, and pineal gland (Linden et al., 1993). PCR was used to establish the presence of A3 receptors in rabbit cardiac myocytes (Wang et al., 1997). The A3 receptor on mast cells facilitates the release of allergic mediators including histamine, suggesting a role in inflammation (Ramkumar et al., 1993b). Systemic administration of 3-IB-MECA causes scratching in mice that is prevented by coadministration of a histamine antagonist (Jacobson et al., 1993b). APNEA has been shown to be a bronchoconstrictor in rats in vivo, an effect that may be mediated by mast cells (Pauwels and Joos, 1995), but it does not elicit bronchoconstriction in rabbits (el-Hashim et al., 1996). Constriction mediated by adenosine in isolated arterioles of golden hamster cheek pouches is blocked by an inhibitor of mast cell degranulation, which suggests a role for A3 receptors on mast cells in this response (Doyle et al., 1994). The A3 receptor has been implicated in the 8-SPTresistant hypotensive response to APNEA in the pithed rat (Fozard and Carruthers, 1993). The response is pertussis toxin-sensitive and is blocked by the A3 receptor antagonist BW-A522 (Fozard and Hannon, 1994). However, it seems that the hypotensive response may be caused by the secondary action of histamine released after activation of mast cell A3 receptors (Hannon et al., 1995). Systemic administration of 3-IB-MECA depresses locomotor activity in mice, which may suggest a role for brain A3 adenosine receptors in modulation of behavior (Jacobson et al., 1993b). Interestingly, activation of rat hippocampal A3 receptors has been shown to desensitize A1 receptor-mediated inhibition of excitatory neurotransmission in this brain region, indicating cross-talk between these two receptors (Dunwiddie et al., 1997). A3 receptors on human eosinophils (Kohno et al., 1996a) and human promyelocytic HL-60 cells (Kohno et al., 1996b; Yao et al., 1997) seem to be involved in apoptosis, an active self-destructive process caused by a genetically programmed cascade of molecular events involving DNA degradation and death of the cell by nuclear and cytoplasmic breakup. This seems to require high concentrations of agonist or chronic activation of the A3 receptor in a manner that mimicks the requirement of high levels of ATP to activate the non-specific pore-formation of the P2X7 receptor and apoptosis, and suggests that this potentially autocatalytic process may occur during pathological conditions resulting in cell damage and release of high levels of purines. Apoptotic effects are caused by high concentrations (micromolar) of A3 receptor agonist in HL-60 leukemia and U-937
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lymphoma cells, but paradoxically, A3 receptor antagonists also induce apoptotic cell death, and this is opposed by low (nanomolar) concentrations of Cl-IB-MECA (Yao et al., 1997). This indicates that low-level activation of A3 receptors may result in cell protection, and furthermore that this may occur as a consequence of endogenously released adenosine (Yao et al., 1997). Acute stimulation of A3 receptors with micromolar concentrations of Cl-IB-MECA has also been shown to cause lysis of granular hippocampal neurons in culture (Von Lubitz et al., 1996). A3 receptors may be involved in the cardioprotective effect of adenosine in ischemia and preconditioning during ischemia reperfusion injury (Liu et al., 1994; Armstrong and Ganote, 1994, 1995; Auchampach et al., 1997b; Stambaugh et al., 1997). Preconditioning is blocked by A3 receptor antagonists, whereas APNEA (A1/A3 selective), but not R-PIA (A1 selective), protect against ischemia in rabbit cardiomyocytes (Armstrong and Ganote, 1995). A3 receptors have been shown to mediate preconditioning and to reduce myocardial injury (Strickler et al., 1996; Tracey et al., 1997). In isolated cardiac myocytes, maximal preconditioning-induced cardioprotection was shown to require activation of both A1 and A3 receptors (Wang et al., 1997). Acute IB-MECA has a detrimental effect on ischemic brain injury, whereas chronic IB-MECA has a protective effect (Von Lubitz et al., 1994). This dual effect mimicks the effects of Cl-IB-MECA on leukemia and lymphoma cell lines (Yao et al., 1997). Activation of an A3 receptor in basophilic leukemia cells (RBL-2H3), endothelial cells, cardiac myocytes, and smooth muscle cells activates the cellular antioxidant defense system by increasing the activity of superoxide dismutase, catalase, and glutathione reductase, thereby providing a means by which adenosine may have a cytoprotective action in ischemia (Maggirwar et al., 1994). VII. Integrated Effects of Adenosine/P1 Receptors A1, A2A, A2B, and A3 adenosine receptors have distinct but frequently overlapping tissue distributions. The fact that more than one adenosine/P1 receptor subtype may be expressed by the same cell raises questions about the functional significance of this colocalization. Because the different adenosine/P1 receptor subtypes have quite different affinities for the endogenous agonist, the local concentration of adenosine in physiological and pathophysiological conditions is likely to be extremely important. EC50 values for adenosine at rat A1, A2A, A2B, and A3 receptors of 73 (Daly and Padgett, 1992), 150 (Daly and Padgett, 1992), 5100 (Peakman and Hill, 1994), and 6500 (Zhou et al., 1992), respectively, have been reported. At rat phrenic motor nerve terminals (Correiade-Sa´ et al., 1996) and prejunctional receptors in rat vas deferens (Gonc¸alves and Queiroz, 1993), the concentration of adenosine needed to increase transmitter release
via activation of A2A receptors seems to be higher than that required to inhibit transmitter release via A1 receptors. Because adenosine is formed as a breakdown product of ATP released from nerves, this implies that the adenosine concentration is crucially linked to the ongoing neuronal activity, which therefore may be an important determinant of the subtype of autoregulatory adenosine receptor that is activated. In rat hemidiaphragm, the frequency and intensity of stimulation of motor nerves and subsequent formation of endogenous adenosine was shown to be critical, with high-intensity, highfrequency nerve stimulation favoring A2A receptor-mediated facilitation of [3H]acetylcholine (ACh) release (Correia-de-Sa´ et al., 1996). Thus, adenosine concentration and receptor affinity may determine the pattern of differential activation of coexpressed A1 and A2A receptors (and other adenosine receptors). Expression of more than one type of adenosine/P1 receptor on the same cell may allow the common agonist adenosine to activate multiple signaling pathways. Adenylate cyclase is a common effector, which is negatively coupled to A1 and A3 receptors and positively coupled to A2 receptors, affording the opportunity for reciprocal control and, therefore, fine tuning of this signaling pathway. Coexisting A1 and A2 adenosine receptors with opposite actions on adenylate cyclase activity have been described in a number of cells, including the smooth muscle cell line DDT1 MF-2 (Ramkumar et al., 1991), cultured porcine coronary artery smooth muscle cells (Mills and Gewirtz, 1990), and glomeruli and mesangial cells (Olivera and Lopez-Novoa, 1992). A1 and A2B receptors on primary rat astrocytes each regulate adenylate cyclase activity, but independently (Peakman and Hill, 1994). The extracellular adenosine concentration may be a crucial determinant of the differential activation of coexisting adenosine/P1 receptors under pathophysiological as well as physiological conditions. Induction and inhibition of the inflammatory response by neutrophil A1 and A2 receptors, respectively, has been reported (Cronstein, 1994; Bullough et al., 1995). Low concentrations of adenosine caused activation of the A1 receptor and induced superoxide anion generation, phagocytosis via Fc receptors, and adhesion to endothelial cells, whereas higher concentrations of adenosine (.500 nM) required to saturate A2 receptors lead to inhibition of these effects. A2A and A2B receptors coexist on fetal chick heart cells; the high affinity A2A receptor has been suggested to be an important modulator of myocyte contractility under physiological conditions, whereas under pathophysiological conditions, such as cardiac ischemia resulting in release of large amounts of adenosine, the low affinity A2B receptor may assume functional significance (Liang and Haltiwanger, 1995). Such studies are helping to expand on the established link between adenosine release and the metabolic demands of tissues by
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building in specific actions on identified cell-surface adenosine/P1 receptors. Stimulation of the A2A receptor on rat striatal synaptosomes causes desensitization of coexpressed A1 receptors, favoring A2A receptor-mediated signaling (Dixon et al., 1997a). This has important implications for other coexpressed adenosine receptors, and it would be interesting to see if this is a general phenomenon for these subtypes. There is an interesting sidedness to the opposite responses evoked by A1-like and A2A-like adenosine receptors colocalized on monolayers of renal epithelial cells (Casavola et al., 1997). The A1-like receptors are located on the apical surface and mediate inhibition of transepithelial Na1 transport by (a) inhibition of the basolaterally located Na1/H1 exchanger and (b) an increase in intracellular H1, probably via Ca21/PKC. The A2A-like receptors are located on the basolateral side and stimulate transepithelial Na1 transport, suggested to be via stimulation of Na1/H1 exchange and thereby cellular alkalinization, probably via an increase in cAMP/PKA (Casavola et al., 1997). The same adenosine receptor can elicit a different functional response in different tissues. In rat duodenum, A2B (and A1) adenosine receptors on the longitudinal muscle mediate relaxation, whereas A2B receptors on the muscularis mucosae mediate contraction (Nicholls et al., 1996). Integrated effects of adenosine/P1 receptors in whole tissue responses are considered, together with P2 receptors, in Section XXII. VIII. P2 Receptors A. Introduction P2 receptors are divided into two main classes based on whether they are ligand-gated ion channels (P2X receptors) or are coupled to G proteins (P2Y receptors) (Abbracchio and Burnstock, 1994; Fredholm et al., 1994) (table 7). The P2X/P2Y nomenclature was adopted from that originally used in a subdivision of P2 receptors proposed in 1985 by Burnstock and Kennedy, who described “P2X-” and “P2Y-purinoceptors” with distinct pharmacological profiles and tissue distributions: the “P2X purinoceptor” was shown to be most potently activated by the stable analogs of ATP, a,b-methylene ATP (a,bmeATP), and b,g-meATP. At the “P2Y-purinoceptor” 2-methylthio ATP (2MeSATP) was the most potent agonist and a,b-meATP and b,g-meATP were weak or inactive. Furthermore, the “P2X-purinoceptor” was shown to be selectively desensitized by a,b-meATP and to be antagonized by 39-O-(3-[N-(4-azido-2-nitrophenyl)amino]-propionyl)ATP (ANAPP3) (Burnstock and Kennedy, 1985). Distinct tissue distributions and functions reinforced this subdivision: “P2X-purinoceptors” were shown to be present in vas deferens, urinary bladder, and vascular smooth muscle, and to mediate contraction; “P2Y-purinoceptors” were
shown to be present in guinea-pig taenia coli and on vascular endothelial cells, as well as to mediate relaxation. P2 receptors have since been cloned from smooth muscle and endothelium; the pharmacological profiles originally attributed to “P2X-” and “P2Y-purinoceptors” seem to correspond most closely to activation of P2X1-like and P2Y1–like receptors, respectively. However, it is now apparent that there is heterogeneity of P2X responses among different smooth muscles, and of P2Y responses between taenia coli and endothelium, which may be caused by different receptor subtypes or small differences in structure of the same receptor. Other P2 receptors that have been identified in biological tissue principally according to their different pharmacological profiles are the P2U receptor (activated equally by ATP and UTP; widely distributed), the P2T receptor (platelet ADP receptor; mediates aggregation), and the P2Z receptor (found on mast cells and lyphocytes; mediates cytotoxicity and degranulation) (Gordon, 1986; O’Connor et al., 1991). P2S (Wiklund and Gustafsson, 1988), P2R (Von Ku¨gelgen and Starke, 1990), P2D (Pintor et al., 1993), uridine nucleotide-specific receptors (“pyrimidinoceptors”) (Seifert and Schultz, 1989; Von Ku¨gelgen and Starke, 1990), P3 (Shinozuka et al., 1988; Forsythe et al., 1991), and P4 (Pintor and Miras-Portugal, 1995a) receptors have also been proposed. Of these the P2U, P2Z, and uridine nucleotide-specific receptors have been cloned. Because receptor subclassification based on pharmacological criteria alone is no longer tenable, the separate identity of the other proposed subtypes remains to be proved. The revision of P2 receptor nomenclature was prompted by evidence that extracellular ATP works through two different transduction mechanisms, namely intrinsic ion channels and G protein-coupled receptors (Benhan and Tsien, 1987; Dubyak, 1991), and by the cloning of the first two P2 receptors, P2Y1 (a “P2Y-purinoceptor”) (Webb et al., 1993b) and P2Y2 (a “P2U-purinoceptor”) (Lustig et al., 1993). It was also becoming increasingly apparent that there was significant heterogeneity among native P2 receptors, reflected in an increasing diversity of pharmacological response profiles that could not easily be accommodated within the existing system of receptor subclassification. Thus, in 1994 it was formally suggested that P2 receptors should be divided into two broad groups termed P2X and P2Y according to whether they are ligand-gated ion channels or are coupled to G proteins, respectively, with subtypes defined by the different structure of mammalian P2 receptors (Abbracchio and Burnstock, 1994; Barnard et al., 1994; Fredholm et al., 1994). To date seven mammalian P2X receptors, P2X1–7, and five P2Y receptors, P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 have been cloned, characterized pharmacologically and accepted as valid members of the P2 receptor family. The use of lower case to define the cloned p2y3 receptor reflects the possibility that this may be the avian ho-
RALEVIC AND BURNSTOCK TABLE 7 P2 receptor signal transduction mechanisms, agonists, and antagonists Family
Receptor type Signaling pathway
Ion channel Nonselective porea N.A.
Ca21 .. Na1 . K1
ATP ATPgS 2MeSATP Ap4A a,b-meATPl b,g-meATPl BzATPa
N.A., not applicable. a P2X7 and endogenous P2X7-like receptor. b P2Y1 and endogenous P2Y1-like receptors acting through PLC couple to Gq/11 proteins; P2Y1 and endogenous P2Y1-like receptors acting through adenylate cyclase couple to Gi proteins; P2Y2 and endogenous P2Y2-like receptors, P2Y4 and P2YADP receptors couple to Gq/11 and Gi proteins; p2y3 and P2Y6 receptors couple to Gq/11 proteins. c P2Y1 and endogenous P2Y1-like receptors and P2YADP receptors. d Some endogenous P2Y1-like receptors activate K1 channels via interactions with their G protein subunits. e P2Y1 and endogenous P2Y1-like receptor signaling; possibly downstream of PKC. f P2Y1 and P2Y2 receptors and their endogenous counterparts; signaling possibly downstream of PKC. g P2Y1 and P2Y2 receptors and their endogenous counterparts; signaling downstream of PKC. h Secondary to activation of PLC, although activation of K1 currents by some endogenous P2Y1-like receptors is via direct interactions with G protein subunits. i P2Y1 and P2Y2 receptors and their endogenous counterparts; ATP is an antagonist at P2YADP receptors. j P2Y2 and endogenous P2Y2-like receptors. k P2Y1 and endogenous P2Y1-like receptors. l P2X1, P2X3 and heteromeric P2X2P2X3 receptors. m P2Y2 and endogenous P2Y2-like receptors and P2Y4 receptors. n P2Y6 receptor. o P2YADP. p P2Y1 and endogenous P2Y1-like receptors coupled to AC. Abbreviations: AC, adenylate cyclase; ADPbF, adenosine 59-O-(2-fluoro)-diphosphate; ADPbS, adenosine 59-O-(2-thio-diphosphate; cAMP, adenosine 39,59-cyclic monophosphate; A3P5PS, adenosine 39-phosphate 59-phosphosulfate; ARL 67085, 6-N,N-diethyl-D-b,g-dibromomethylene ATP; ATPgS, adenosine 59-O-(3-thiotriphosphate); BzATP, 39-O-(4-benzoyl)benzoyl ATP; DAG, diacylglycerol; FPL 66096, 2-propylthio-D-b,g-difluoromethylene ATP; IP3, inositol 1,4,5-trisphosphate; KN-62, 1-[N,O-bis(5isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine; MAPK, mitogen-activated protein kinase; a,b-meATP, a,b-methylene ATP; b,g-meATP, b,g-methylene ATP; 2MeSADP, 2-methylthio ADP; 2MeSATP, 2-methylthio ATP; MRS 2179, N6-methyl modification of 29-deoxyadenosine 39,59-bisphosphate; NF023, symmetrical 39-urea of 8-(benzamido)naphthalene-1,3,5-trisulfonic acid; NF279, 8,89-(carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis(1,3,5-naphthalenetrisulfonic acid); PLCPC, phosphatidylcholine-specific phospholipase C; PKC, protein kinase C; PLA2, phospholipase A2; PLC, phospholipase C; PLD, phospholipase D; PPADS, pyridoxal phosphate-6-azophenyl-29,49-disulfonic acid; suramin, 8-(3-benzamido-4-methylbenzamido)-naphthalene-1,3,5-trisulfonic acid; UTPgS, uridine 59-O-(3-thiotriphosphate).
molog of the human P2Y6 receptor, although this has not yet been confirmed. The jump in sequence in the numbering of the P2Y receptor family is caused by the recognition that certain receptors had been erroneously identified as belonging to this family, leading to the subsequent withdrawal of P2Y5 (Webb et al., 1996b) and P2Y7 (Akbar et al., 1996). The cloned receptors P2Y9 and P2Y10 are also not nucleotide receptors. A P2Y receptor cloned from Xenopus neural plate (provisionally called P2Y8) is not included in the definitive P2Y receptor family recognized by the IUPHAR committee, based largely on the rationale that this is a non-mammalian receptor. The platelet ADP receptor P2YADP (or P2T receptor) has not yet been cloned and, therefore, as recom-
mended by the IUPHAR committee, the name of this receptor is given in italics. P2Y4 (human but not rat receptor) and P2Y6 are uridine nucleotide-specific receptors (receptors not activated or only weakly activated by purines) that have been cloned and shown to be sensitive preferentially to UTP and UDP, respectively (the rat P2Y4 receptor is also activated potently by ATP; see Section XV). Their identification complements earlier suggestions of the existence of endogenous uridine nucleotide-specific receptors based on distinct pharmacology of some biological tissue. Before the cloning of these receptors, the possibility that there were subtypes of endogenous uridine nucleotide-specific receptors was not considered,
RECEPTORS FOR PURINES AND PYRIMIDINES
and by implication the possibility of different UTP/UDP selectivities for members of this family was not appreciated. Thus, in much of the literature to date, the agonist potency profiles documented for endogenous uridine nucleotide-specific receptors are incomplete, leaving open the possibility that these are P2Y4 or P2Y6 receptors, or some other subtype not yet cloned. The lack of selective agonists and antagonists, and complications introduced by receptor coexpression and agonist interconversion, means that the subtype identity of most endogenous uridine nucleotide-specific receptors is currently unclear. Because of this, a separate section in this review is devoted to endogenous uridine nucleotide-specific receptors (see Section XVIII.). Interestingly, the P2Y11 receptor is so far the only P2Y receptor selective for ATP versus other purine and pyrimidine nucleotides. For researchers in this field, important discoveries made in the last 10 years have been the source of insight, and in some cases frustration, because these demand a reevaluation of conclusions drawn from earlier studies on P2 receptors. These include the discovery that: (a) multiple P2X receptor proteins are often coexpressed in different proportions in different tissues; (b) P2X receptors are multisubunit receptors that may exist as heteromers with different pharmacology compared with the homomers; (c) cations can profoundly affect P2X channel activity; (d) 2MeSATP, previously widely regarded as a selective “P2Y-purinoceptor” agonist, is also a potent agonist at P2X receptors; (e) ecto-nucleotidases can profoundly alter agonist potencies; and (f) antagonists previously used with some confidence as P2 receptor blockers are non-selective, can modulate ectonucleotidase activity and may have allosteric effects on P2 receptors. The general lack of selective agonists and antagonists, together with complications introduced by coexistence of different P2 receptors and impure solutions caused by purine and pyrimidine degradation and interconversion, also has significantly hindered advances in P2 receptor characterization. Although much valuable information can be derived from studies of populations of cells in culture, there are potential pitfalls associated with this technique. Thus, emerging evidence that the expression of P2 receptors may alter in culture conditions adds another potential complication to the study of purine receptors. For example, astrocytes studied in situ, or after acute isolation from rat brain, are insensitive or only a few cells respond to ATP, whereas in primary cultures, there is a profound increase in the number of cells responding to ATP (Jabs et al., 1997; Kimelberg et al., 1997). Similarly, up-regulation of the P2Y2 receptor in rat salivary gland cells in culture compared with acutely isolated cells has been reported (Turner et al., 1997). Thus, caution is needed in the interpretation of studies of P2 receptors on cells in culture. Autocatalytic release of ATP has been shown from endothelial cells (Yang et al., 1994) and it is possible that
this phenomenon will be described for other cell types as well as for other purines and pyrimidines. In addition, ATP is released from many different cells in response to stimuli such as shear stress and hypoxia, which may be relevant for the ongoing level of activation of purine receptors expressed by the same or neighboring cells. This may be particularly important with respect to the activity of P2X1 and P2X3 receptors, as these receptors desensitize rapidly. Because of the diverse reasons discussed above, it is currently a considerable challenge to dissect out and characterize endogenous receptors for purines and pyrimidines in different biological systems, and even more of a challenge to identify for each of these a physiological or pathophysiological role. However, endogenous receptor counterparts have been shown for some cloned P2 receptors, matching both in terms of receptor distribution, signaling mechanisms, and pharmacology. In this review, we use the name of the clone in preference to the classical nomenclature where possible to promote the conversion from the older system to the newer terminology. However, because for the majority of cases this characterization is currently equivocal, we qualify this with the term “-like”. Thus, “P2X1-like receptor” replaces the classical “P2X-purinoceptor” of smooth muscle, “P2X7-like receptor” is used for the “P2Z-purinoceptor”, “P2Y1-like receptor” is used in preference to the classical “P2Y-purinoceptor,” and “P2Y2-like receptor” replaces “P2U-purinoceptor”. Unequivocal characterization of endogenous P2 receptors awaits the development and use of subtype-selective agonists and antagonists. B. Agonists P2 receptors have broad natural ligand specificity, recognizing ATP, ADP, UTP, UDP, and the diadenosine polyphosphates (table 7). The chemical structures of some principal P2 receptor agonists are illustrated in figure 6. At present there are no agonists or antagonists that discriminate adequately between families of P2X and P2Y receptors, or between subtypes of receptors within each of these groups (table 7). Some of the most useful agonists are the stable ATP analogs a,b-meATP and b,g-meATP, which if effective, strongly imply actions at P2X receptors (specifically at P2X1 and P2X3 subtypes) and are generally inactive at P2Y receptors. Also useful are ADP, adenosine 59-O-(2-thiodiphosphate)(ADPbS,) and UTP, as these are agonists at some P2Y receptors, but are weak or inactive at P2X receptors. Agonist potency orders, important in the characterization of cloned and native P2 receptors, are profoundly influenced by the different stabilities of P2 receptor ligands in the presence of ecto-nucleotidases. a,b-MeATP is considerably more stable than ATP and 2MeSATP when ecto-nucleotidase activity is not suppressed, which contributes significantly to its greater potency (up to 100-fold more potent) at native P2X1 receptors in vascular smooth muscle, bladder, and vas deferens. However,
RALEVIC AND BURNSTOCK
is a particularly important consideration in the pharmacology of P2X receptors because of the wide range of stabilities of commonly-used P2X agonists, but seems to have had less of an impact on P2Y receptor profiles, probably because many of the commonly used P2Y agonists are similarly unstable. An additional consideration is that many P2 receptor antagonists inhibit ecto-nucleotidase activity, which may reduce their effectiveness against biologically unstable P2 agonists. C. Antagonists
FIG. 6. The chemical structure of some key agonists at P2 receptors. (Adapted from Windscheif, 1996).
when ecto-ATPase effects are controlled by use of single cells and rapid concentration-clamp applications of agonist, or by inhibition of ecto-ATPase activity [for instance using 6-N,N-diethyl-D-b,g-dibromomethylene ATP (ARL 67156) or removal of divalent cations], a,bmeATP is less potent than 2MeSATP and ATP at native and cloned P2X1 receptors (Crack et al., 1994; Evans and Kennedy, 1994; Humphrey et al., 1995; Khakh et al., 1995b). Thus, greater caution is now advised in the interpretation of the order of agonist potency where ecto-nucleotidase activity has not been suppressed. This
Antagonists selective for subtypes of P2X and P2Y receptors are considered in later sections on individual receptors (see Sections X.F., XII.E., XVIV.D.). This section considers other established and putative P2 receptor antagonists, which, unfortunately, do not discriminate well, if at all, between P2X or P2Y receptors, let alone for subtypes within these families (table 7). Many of these also inhibit ecto-nucleotidases and may have allosteric effects on the receptor (Michel et al., 1997). Table 8 summarizes the potencies of some of the most commonly used antagonists at recombinant and endogenous P2 receptors. The general lack of selective antagonists highlights the problems currently encountered in subtype-identification of P2 receptors using ligand binding. The chemical structures of some P2 receptor antagonists are illustrated in figure 7. In principle, any P2 receptor antagonist should be tested for its selectivity against all known subtypes of this family. Evaluation of antagonist selectivity at heteromeric P2X receptors is also important because of its relevance for biological tissue where P2X receptor proteins are typically coexpressed; such studies might additionally provide useful information about the specific contribution of the different subunits to the pharmacology of the receptor heteromer. A commonly used biological assay is antagonism of constriction by a,b-meATP of vas deferens and vascular smooth muscle. This is generally taken as an indication of actions at endogenous P2X1-like receptors for a number of reasons: (a) the P2X1 receptor has been cloned from smooth muscle; (b) immunohistochemical studies have shown that it is the predominant P2X receptor protein expressed by smooth muscle; (c) a,b-meATP is selective for P2X1 and P2X3 receptors, but the latter is not expressed by smooth muscle; and (d) the smooth muscle P2X response shows a similar pharmacology to the recombinant P2X1 receptor, and as with the P2X1 receptor, undergoes rapid desensitization. Relaxant effects of 2MeSATP or ADPbS at guinea-pig taenia coli and via the vascular endothelium have been used to examine antagonist potencies at endogenous P2Y1-like receptors. The potencies of antagonists at endogenous P2 receptors in these and other biological assays are reported in table 8b. 1. Suramin. The trypanoside suramin (8-(3-benzamido-4-methylbenzamido)-naphthalene-1,3,5-trisulfonic acid) is generally selective as an antagonist at P2
RECEPTORS FOR PURINES AND PYRIMIDINES TABLE 8a Antagonist selectivities at cloned P2 receptors Receptor
IC50 and pA2 values are mM; N.D., not determined. a RB2. b Basilen blue (isomer of RB2). c Cibacron blue (isomer of RB2). d 33% inhibition of the UTP response. e Less potent than RB2 and PPADS. f Less potent than RB2. g References: 1 Collo et al., 1996; 2 Evans et al., 1995; 3 Bo et al., 1995; 4 Soto et al., 1996a; 5 Buell et al., 1996b; 6 Garcia-Guzman et al., 1997a; 7 Surprenant et al., 1996; 8 Rassendren et al., 1997; 9 Brown et al., 1995; 10 Charlton et al., 1996a; 11 Schachter et al., 1996; 12 Webb et al., 1996a; 13 Charlton et al., 1996b; 14 Communi et al., 1996a; 15 Chang et al., 1995; 16 Bogdanov et al., 1998; 17 Robaye et al., 1997; 18 Communi et al., 1997.
receptors versus other types of receptors (Dunn and Blakeley, 1988) (but see later this section), but is not a universal P2 receptor antagonist, and does not discriminate between P2X and P2Y receptors (table 8). Furthermore, suramin inhibits ecto-nucleotidase (Crack et al., 1994; Beukers et al., 1995; Ziganshin et al., 1995; Bu¨ltmann et al., 1996b; Chen et al., 1996c) and neural ectodiadenosine polyphosphate hydrolase (Mateo et al., 1996) activity, which may complicate interpretation of antagonist activity when it is used against ligands which are biologically unstable. Antagonism by suramin of recombinant and endogenous P2X and P2Y receptors occurs with relatively low potency (pA2 values approximately 5) (table 8). Antagonism is frequently non-competitive. Suramin is weak or inactive at recombinant P2X6 and P2X4 receptors (Buell et al., 1996b) and at P2Y6 and human P2Y4 receptors (Chang et al., 1995; Communi et al., 1996a; Robaye et al., 1997). Suramin is an antagonist at a subpopulation of endogenous P2Y2-like receptors (Hoiting et al., 1990; Murrin and Boarder, 1992; Henning et al., 1992, 1993; Carew et al., 1994; Chen et al., 1994b; Sipma et al., 1994; Ho et al., 1995; Paulais et al., 1995; Ziyal, 1997), and blocks native P2X7 (or P2Z) receptors in human lymphocytes (Wiley et al., 1993). Inhibition by suramin of nicotinic receptors in chick cultured sympathetic neurons (Allgaier et al., 1995b), GABA and glutamate receptors in rat hippocampal neurons (Nakazawa et al., 1995), and vasoactive intestinal polypeptide (VIP)- and 5-hydroxytryptamine (5-HT)-mediated relaxations of the guinea-pig proximal colon
(Briejer et al., 1995) have been described, at concentrations within the range used for block of P2 receptors. Suramin at 100 mM inhibits, by approximately 40%, GABA and glutamate receptor currents in rat hippocampal neurons (Nakazawa et al., 1995), and 300 mM suramin produces approximately 40% block of 1,1-dimethyl-4-phenylpiperazinium (DMPP; nicotinic receptor agonist)-induced overflow of [3H]NA in cultured chick sympathetic neurons (Allagaier et al., 1995b). Inhibition by suramin of NMDA-gated ion channels (IC50 68 mM) was described in mouse hippocampal neurons (Peoples and Li, 1998). In guinea-pig proximal colon, 300 mM suramin is a more potent inhibitor of relaxant responses to VIP (virtually abolishing responses) than of responses to ATP, and also produces a modest block of 5-HT-induced relaxation (Briejer et al., 1995). Other diverse effects of suramin include inhibition of the binding of growth factors, inhibition of the GTPase activity of certain G proteins, and inhibition of DNA and RNA polymerases (see Voogd et al., 1993). Suramin and its analogs have been shown to block responses at A1 adenosine and D2 dopamine receptors in brain membranes by inhibiting the formation of the agonist/receptor/G protein complex (Beindl et al., 1996). Although this should be borne in mind when interpreting the effects of suramin in biological systems, it should be noted that these studies were carried out on brain membrane preparations and that because of its highly polar nature, suramin does not readily cross cell membranes. 2. NF023. NF023 (symmetrical 39-urea of 8-(benzamido)naphthalene-1,3,5-trisulfonic acid) is a suramin-
RALEVIC AND BURNSTOCK TABLE 8b Antagonist selectivities at endogenous P2 receptors Receptora
Rat vas deferens
Rabbit vas deferens Guinea-pig vas deferens Rat mesenteric bed Hamster mesenteric bed Rabbit ear artery Rabbit saphenous artery Rat renal vascular bed Guinea-pig ileum submucosal arterioles Rabbit urinary bladder Guinea-pig urinary bladder Human urinary bladder Human saphenous vein Rat vagus nerve Guinea-pig taenia coli
Rat duodenum Rat mesenteric bed Bovine aorta Rat aorta Turkey erythrocytes Bovine pulmonary artery EC Rabbit thoracic aorta 1EC(ATP) 2EC(ATP) C6 glioma cells 1IP3 2cAMP
P2Y P2Y P2Y P2Y P2Y P2Y
(P2Y1) (P2Y1) (P2Y1) (P2Y1) (P2Y1) (P2Y1)
Rat astrocytes Mouse vas deferens Rat atria Rat mesenteric bed Hamster mesenteric bed Bovine aorta Rat aorta Bovine pulmonary artery EC C6 glioma C2C12 myotubes Rat astrocytes Rat neuroblastoma 3 glioma cells RAW 264.7 macrophages
P2X1-like receptor-mediated responses were determined against the effects of a,b-meATP; P2Y1-like receptor-mediated responses were determined against the effects of ADPbS or 2MeSATP; P2Y2-like receptor-mediated responses were determined against the effects of UTP (in tissues in which ATP is approximately equipotent with UTP). NC, noncompetitive; 1EC, with endothelium; -EC, without endothelium. a The likely cloned receptor counterparts of endogenous responses are indicated in parentheses. b pKB 6.6 for iso-PPADS (Khakh et al., 1994). c Cibacron blue. d Suramin antagonized only the lower part of the a,b-meATP response curve (Palea et al., 1995). e pKB 6.0 for iso-PPADS (Trezise et al., 1994c). f Antagonism of ATPgS inhibition of [3H]NA overflow. g Antagonism of ATP- and ATPgS-mediated inhibition of evoked [3H]NA overflow. h Tested against contractions to UTP. i Tested against depolarizations to UDP and UTP; responses to a,b-meATP and ATP were blocked. j At high concentrations (.100 mM). k References: 1, Khakh et al., 1994; 2, Trezise et al., 1994b; 3, Bu¨ltmann et al., 1996b; 4, Lambrecht et al., 1992; 5, Lambrecht et al., 1996; 6, McLaren et al., 1994; 7, Bailey and Hourani, 1995; 8, Windscheif et al., 1994; 9, Ziyal, 1997; 10, Leff et al., 1990; 11, Lambrecht, 1996; 12, Ziganshin et al., 1994b; 13, Eltze and Ullrich, 1996; 14, Galligan et al., 1995; 15, Ziganshin et al., 1993; 16, Usune et al., 1996; 17, Palea et al., 1995; 18, von Ku¨gelgen et al., 1995; 19, Trezise et al., 1994b; 20, Trezise et al., 1994c; 21, Hoyle et al., 1990; 22, Windscheif et al., 1995a; 23, Ralevic and Burnstock, 1996a; 24, Wilkinson et al., 1994; 25, Hansmann et al., 1997; 26, Boyer et al., 1994; 27, Chen et al., 1996a; 28, Lin and Chuang, 1994; 29, Ho et al., 1995; 30, von Ku¨gelgen et al., 1994; 31, von Ku¨gelgen et al., 1995b; 32, Henning et al., 1993; 33, Reiser, 1995; 34, Chen et al., 1996c; 35, Lagaud et al., 1996; 36, Connolly and Harrison, 1995; 37, Hourani et al., 1992; 38, Windscheif et al., 1995b.
based compound which is moderately selective as an antagonist of P2X receptors. NF023 is about 30-fold selective for P2X1-like receptors in the rat vas deferens versus P2Y1-like receptors in the guinea-pig taenia coli
(Bu¨ltmann et al., 1996b). It has 79-fold selectivity for endogenous P2X1-like receptors in rabbit vas deferens versus P2Y1-like receptors in turkey erythrocytes; pA2 values of 5.5 to 5.7 at P2X1-like receptors in rabbit
RECEPTORS FOR PURINES AND PYRIMIDINES
FIG. 7. The chemical structures of some P2 receptor antagonists. (Adapted from Windscheif, 1996).
isolated blood vessels, rabbit vas deferens, and rat and hamster mesenteric arterial beds; and pA2 values of 4.6 to 5.5 at vascular and nonvascular smooth muscle P2Y1like receptors (Lambrecht et al., 1996; Ziyal, 1997; Ziyal et al., 1997). Its effects at the other P2X (and P2Y) receptor subtypes have not been reported. Antagonism is competitive and reversible. Like the parent compound suramin, NF023 inhibits ecto-nucleotidase activity, but unlike suramin, it has high P2X1-like versus ecto-nucleotidase-selectivity (Beukers et al., 1995; Bu¨ltmann et al., 1996b). 3. NF279. NF279 (8, 89-(carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis(1,3,5naphthalenetrisulfonic acid) is a suramin analog that is about 10-fold more potent than NF023 in blocking a,bmeATP-mediated contractions at P2X1-like receptors in rat vas deferens, pIC50 5.7 (Damer et al., 1998). With a pA2 value of 4.3 at P2Y1-like receptors in the guinea-pig taenia coli, it has the highest P2X- versus P2Y- and ecto-nucleotidaseselectivity so far reported (Damer et al., 1998). 4. Pyridoxal-5-phosphate (P5P). P5P is a non-selective P2 receptor antagonist, but has proved useful as a starting compound for the synthesis of more P2X-selective antagonists (Lambrecht et al., 1996). Antagonism by P5P is, however, selective versus non-purine receptors and seems to be competitive at P2X1-like receptors in vas deferens of rabbit (Lambrecht et al., 1996) and rat (Trezise et al., 1994b), and at a,b-meATP-mediated depolarization of rat vagus nerve (Trezise et al., 1994b). P5P non-competitively inhibits responses mediated by recombinant receptors P2X1 and P2X2 but is less potent than its derivative pyridoxalphosphate-6-azophenyl29,49-disulfonic acid (PPADS) (Evans et al., 1995). P5P inhibits a,b-meATP-induced depolarization of rat superior cervical ganglion (Connolly, 1995). 5. PPADS. Although originally put forward as a P2Xselective antagonist, unfortunately it must now be accepted that PPADS is a non-selective (but non-universal) P2 receptor antagonist. PPADS is a slowlyequilibrating and slowly-reversible antagonist with pA2 values of approximately 6 to 6.7 at endogenous P2X1-like receptors in a variety of smooth muscle preparations (table 8; Lambrecht et al., 1996; Ziganshin et al., 1993, 1994b; Bu¨ltmann and Starke, 1994a; McLaren et al., 1994; Windscheif et al., 1994; Galligan et al., 1995; Von Ku¨gelgen et al., 1995a; Eltze and Ullrich, 1996; Ralevic and Burnstock, 1996b; Usune et al., 1996). It also blocks recombinant P2X1, P2X2, P2X3, and P2X5 receptors with IC50 values of 1 to 2.6 mM (Collo et al., 1996). A lysine residue in receptors P2X1, P2X2, and P2X5 (amino acid 249 in P2X1) seems to be involved in the slowly reversible component of block by PPADS, probably involving formation of a Schiff’s base (Buell et al., 1996b). Rat recombinant P2X4 and P2X6 receptors are not blocked by PPADS (Buell et al., 1996b; Collo et al., 1996; Soto et al., 1996a,b; Garcia-Guzman et al., 1997a), but interestingly, the human homolog of the P2X4 receptor is
RALEVIC AND BURNSTOCK
blocked by PPADS with an IC50 of 28 mM (Garcia-Guzman et al., 1997a). PPADS antagonizes depolarizations induced by a,b-meATP in rat superior cervical ganglion (Connolly, 1995). PPADS generally blocks endogenous P2Y1-like and recombinant P2Y1 receptors coupled to PLC (Boyer et al., 1994; Brown et al., 1995; Charlton et al., 1996a; Schachter et al., 1996) but not those coupled to inhibition of adenylate cyclase (Boyer et al., 1994; Webb et al., 1996c). PPADS has been reported to be inactive at P2Y1like receptors in smooth muscle of rabbit mesenteric artery and endothelium of rabbit aorta (Ziganshin et al., 1994b), but blocks those in rat duodenum, guinea-pig taenia coli (pA2 values 5.1 and 5.3, respectively) (Windscheif et al., 1995a), and rat mesenteric arterial bed (pA2 value 6.0) (Ralevic and Burnstock, 1996b). PPADS blocks P2Y2-like receptors in astrocytes from the dorsal horn of the spinal cord (IC50 approximately 0.9 mM) (Ho et al., 1995) but not P2Y2-like receptors on rat mesenteric arterial endothelium (Windscheif et al., 1994; Ralevic and Burnstock, 1996a), or those on cultured bovine aortic endothelial cells (Brown et al., 1995). PPADS antagonizes responses to UTP at the recombinant P2Y4 receptor (IC50 value approximately 15 mM) (Communi et al., 1996a). At high concentrations PPADS blocks P2YADP receptor-mediated ADP-induced platelet aggregation and inhibits ecto-nucleotidase activity (Windscheif et al., 1995b; Chen et al., 1996c). At concentrations greater than 10 mM, non-specific effects of PPADS have been reported involving inhibition of IP3-induced [Ca21]i mobilization (Vigne et al., 1996). 6. Iso-PPADS. An isomer of PPADS, pyridoxalphosphate-6-azophenyl-29,59-disulfonic acid (iso-PPADS) is a slowly-equilibrating and slowly-reversible antagonist of responses at P2X receptors with similar potency to PPADS (Trezise et al., 1994c) and competes for [3H]a,bmeATP binding sites in the rat vas deferens (Khakh et al., 1994). Iso-PPADS blocks depolarizations evoked by a,b-meATP, but not those to UTP in rat superior cervical ganglion, but in contrast to PPADS also blocks depolarizations to muscarine (Connolly, 1995). 7. Reactive blue 2. The anthraquinone-sulfonic acid derivative reactive blue 2 (synonymous with cibacron blue) is a non-competitive P2 receptor antagonist which does not discriminate adequately between P2X and P2Y subtypes. In the vasculature, it has micromolar affinity and some selectivity for endothelial P2Y1 and smooth muscle P2Y1-like receptors versus other vascular P2X and P2Y receptors; however, selectivity versus the smooth muscle P2X1-like receptor is low, and its use is limited by a narrow effective concentration range and time of exposure (Burnstock and Warland, 1987a; Hopwood and Burnstock, 1987; Houston et al., 1987). Reactive blue 2 antagonism of P2Y receptors includes block of the recombinant P2Y6 receptor (Chang et al., 1995) and some endogenous P2Y2-like and uridine nucleotide-specific receptors (Nakaoka and Yamashita, 1995; Chen et
al., 1996c). Reactive blue 2 blocks selectively contractile responses to ADPbS at a P2Y-like receptor, but enhances P2X receptor-mediated contractions to a,bmeATP and ATP in rat anococcygeus smooth muscle (Najbar et al., 1996) Reactive blue 2 also has been shown to block responses mediated by endogenous P2X receptors in adult rat superior cervical and nodose ganglia, and guinea-pig coeliac ganglion (Silinsky and Gerzanich, 1993; Connolly and Harrison, 1994; Khakh et al., 1995a), rat vagus nerve (Trezise et al., 1994c), urinary bladder and vas deferens (Choo, 1981; Bo et al., 1994; Bu¨ltmann and Starke, 1994a; Suzuki and Kokubun, 1994), endogenous P2X7-like receptors (McMillian et al., 1993; Wiley et al., 1993), and recombinant P2X2 (Brake et al., 1994) and P2X4 (Bo et al., 1995; Se´gue´la et al., 1996) receptors. Thus, this compound does not discriminate adequately between P2X and P2Y receptors, although it may be useful in discriminating between subtypes of coexisting P2 receptors. Inhibition by reactive blue 2 of GABA and glutamate receptors (Motin and Bennett, 1995; Nakazawa et al., 1995), and NMDA-gated ion channels (Peoples and Li, 1998) further advises caution in the use of this compound. Inhibition of ectoATPase activity by reactive blue 2 also has been reported (Stout and Kirley, 1995). 8. Reactive red. Reactive red is at least 350 times more potent than reactive blue 2 as a competitive antagonist at the P2Y1-like receptor of guinea-pig taenia coli (Kd, 28 nM); however, it is only 15-fold selective versus the P2X1like receptor in rat vas deferens, and has ecto-nucleotidase activity (Bu¨ltmann and Starke, 1995). Its effects at other P2X and P2Y subtypes are largely unknown. 9. Trypan blue. Trypan blue blocks selectively (versus K1 and noradrenaline) a,b-meATP-mediated contractions at the P2X1-like receptor in rat vas deferens but is also an inhibitor of ADPbS-mediated relaxations via P2Y1-like receptors in guinea-pig taenia coli and an inhibitor of ecto-nucleotidase activity (Bu¨ltmann et al., 1994; Wittenburg et al., 1996). 10. Evans blue. Evans blue blocks selectively responses to a,b-meATP in the rat vas deferens versus those mediated by ADPbS in the guinea-pig taenia coli, but potentiates contraction to ATP, ADP, and 2MeSATP in a manner attributable in part to ecto-nucleotidase inhibition; it also has non-specific potentiating effects (Bu¨ltmann and Starke, 1993; Bu¨ltmann et al., 1995; Wittenburg et al., 1996). The desmethyl derivative of Evans blue, NH01, is highly selective for the P2X1-like receptor in vas deferens versus the P2Y1-like receptor in guinea-pig taenia coli (Kd values 0.8 and . 100 mM, respectively), but is only moderately selective for the P2X1 receptor versus inhibition of ecto-nucleotidase activity (Wittenburg et al., 1996). 11. DIDS. The Cl2 transport blocker 4,49-diisothiocyanatostilbene-2,29-disulfonate (DIDS) is a noncompetitive, pseudo-irreversible antagonist of P2X1-like recep-
RECEPTORS FOR PURINES AND PYRIMIDINES
tor-mediated contractions to a,b-meATP and of the purinergic component of the neurogenic contractile response in guinea-pig and rat vas deferens, and is selective versus the P2Y1-like receptor of guinea-pig taenia coli (Fedan and Lamport, 1990; Bu¨ltmann and Starke, 1994b; Bu¨ltmann et al., 1996a). However, it is nonselective versus inhibition of ecto-nucleotidase activity (Bu¨ltmann et al., 1996a). DIDS discriminates between subtypes of P2X receptors, being a potent inhibitor of responses mediated at the P2X1 receptor cloned from human bladder (IC50 3 mM), but less than 40% effective at recombinant P2X2 receptors from PC12 cells at concentrations of up to 300 mM (Evans et al., 1995). DIDS blocks depolarization to a,b-meATP in rat superior cervical ganglia, but has no effect on depolarization to UTP or potassium, or hyperpolarization to adenosine (Connolly and Harrison, 1995a). DIDS and some analogs of DIDS also block endogenous P2X7-like receptors (elMoatassim and Dubyak, 1993; McMillian et al., 1993; Soltoff et al., 1993). DIDS, PPADS, and dextran sulfate discriminate between recombinant human P2X1 and rat P2X2 receptors in displacement of binding studies, having 7- to 33-fold higher affinity for P2X1 receptors (Michel et al., 1996). 12. Arylazidoaminopropionyl ATP (ANAPP3). ANAPP3, a photo-affinity analog of ATP, activates and desensitizes endogenous smooth muscle P2X1-like receptors, irreversibly so after exposure to light, and selectively versus non-purine receptors (Hogaboom et al., 1980; Fedan et al., 1985; Venkova and Krier, 1993). Its effects at other P2X receptor subtypes have not been determined. However, ANAPP3 also weakly antagonizes relaxations to ATP, ADP, and adenosine in the guineapig taenia coli (Westfall et al., 1982). 13. 2-Alkylthio derivatives of ATP. 2-Alkylthio derivatives of ATP are potent P2Y1 receptor antagonists: both base modifications, leading to 8-(6-aminohexylamino)ATP and N-oxide ATP, and ribose modifications, leading to 29,39-isopropylidene-AMP, result in derivatives that display selectivity for endothelial P2Y1-like receptors and are virtually inactive at smooth muscle P2Y1-like and P2X1-like receptors (Burnstock et al., 1994). 14. 59-p-Fluorosulfonyl benzoyladenosine. This is an irreversible inhibitor of ATP-induced Ba21 influx via the P2X7 receptor in human lymphocytes, although maximal inhibition does not exceed 90% (Wiley et al., 1994). IX. P2X Receptors P2X receptors are ATP-gated ion channels which mediate rapid (within 10 ms) and selective permeability to cations (Na1, K1 and Ca21)(Bean, 1992; Dubyak and el-Moatassim, 1993; North, 1996). This is appropriate given their distribution on excitable cells (smooth muscle cells, neurons, and glial cells) and role as mediators of fast excitatory neurotransmission to ATP in both the central and peripheral nervous systems. This contrasts with the slower onset of response (less than 100 ms) to
ATP acting at metabotropic P2Y receptors, which involves coupling to G proteins and second-messenger systems. Seven P2X receptor proteins (P2X1 to P2X7) have been cloned and the ion channels formed from homomeric association of the subunits when expressed in Xenopus oocytes or in mammalian cells have been functionally characterized and show distinct pharmacological profiles (table 9). The P2X7 receptor is considered separately below (see Section X.) because it is functionally unique among P2X receptors in being able to act as a non-selective pore. A. Structure Structural features of P2X receptors have been predicted from the amino acid sequences of cloned P2X receptor subunits. It is important to bear in mind that the P2X proteins that have been cloned are receptor subunits, not actual receptors; a single 2 transmembrane subunit alone cannot form an ion channel. The proteins have 379 to 472 amino acids and are believed to insert into the cell membrane to form a pore comprising two hydrophobic transmembrane domains, with much of the protein occuring extracellularly as an intervening hydrophilic loop (fig. 8). The overall structure of the receptor most closely resembles that of amiloride-sensitive epithelial Na1 channels. The putative extracellular loop of cloned receptors P2X1 to P2X7 has 10 conserved cysteine residues, 14 conserved glycine residues and 2 to 6 potential N-linked glycosylation sites. It is believed that disulfide bridges may form the structural constraints needed to couple the ATP-binding site to the ion pore. Most of the conserved regions are in the extracellular loop, with the transmembrane domains being less well-conserved. The quaternary structures of classical ligand-gated channels, for example, those of the nicotinic ACh receptor and the epithelial Na1 channel, generally take the form of heteromeric complexes of structurally related subunits. P2X receptors are believed to complex in a similar way in biological tissues. Their subunit stoichiometry is unknown, but may involve three subunits (or multiples of three subunits) based on SDS polyacrylamide gel electrophoresis estimates of the relative molecular mass of the recombinant P2X1 and P2X3 receptors determined under non-denaturing conditions (Nicke et al., 1998). The pharmacological properties of endogenous P2X receptors in smooth muscle and PC12 cells correlate well with those of the recombinant receptors cloned from these tissues, P2X1 and P2X2 receptors, respectively; both native and recombinant P2X1 receptors are sensitive to a,b-meATP and rapidly desensitize, whereas P2X2 receptors are insensitive to a,b-meATP and do not desensitize. A good correlation is also seen between the properties of endogenous P2X receptors in neonatal dorsal root ganglion and the recombinant P2X3 receptor (cloned from and expressed predominantly or exclu-
RALEVIC AND BURNSTOCK TABLE 9 Cloned P2X receptors
Number of amino acids
cDNA library source
Human urinary bladder Rat vas deferens Mouse urinary bladder
ATP . a,b-meATP 2MeSATP . ATP . a,b-meATP —
Valera et al., 1995; Longhurst et al., 1996 Valera et al., 1994 Valera et al., 1996
Rat PC12 cells Rat cerebellum
2MeSATP . ATP; a,b-meATP inactive 2MeSATP 5 ATP 5 a,b-meATP
Brake et al., 1994 Bra¨ndle et al., 1997; Simon et al., 1997
Human heart, spinal cord Rat DRG cells Rat DRG cells
2MeSATP . ATP . a,b-meATP 2MeSATP . ATP . a,b-meATP . UTP ATP . 2MeSATP . a,b-meATP
Garcia-Guzman et al., 1997b Chen et al., 1995a Lewis et al., 1995
Human brain Rat brain Rat brain Rat hippocampus Rat SCG Rat pancreatic islet
ATP .. 2MeSATP $ CTP . a,b-meATP ATP .. 2MeSATP $ CTP . a,b-meATP ATP . 2MeSATP .. a,b-meATP ATP . 2MeSATP .. a,b-meATP ATP; a,b-meATP inactive ATP, ADP, 2MeSATP .. a,b-meATP
Garcia-Guzman et al., 1997a Soto et al., 1996a Se´gue´la et al., 1996 Bo et al., 1995 Buell et al., 1996b Wang et al., 1996
Collo et al., 1996
ATP . 2MeSATP . ADP a,b-meATP inactive ATP . 2MeSATP . ADP
Garcia-Guzman et al., 1996
Rat superior cervical ganglion Rat brain
ATP . 2MeSATP . ADP; a,b-meATP inactive
Collo et al., 1996
Rat macrophage and brain Human monocytes
BzATP . ATP . UTP ATP . UTP . BzATP BzATP . ATP . 2MeSATP . ADP; UTP inactive BzATP . ATP
Soto et al., 1996b Nuttle et al., 1993 Surprenant et al., 1996 Rassendren et al., 1997
Splice variant, also termed P2X2-2.
sively in sensory neurons); both are sensitive to a,bmeATP and rapidly desensitize (Evans and Surprenant, 1996). Thus, there is good reason to believe that the native P2X receptors in these tissues are predominantly homomers formed by the association of a single type of subunit. However, this is not always the case. ATP-gated currents at endogenous P2X receptors in rat nodose neurons are mimicked by a,b-meATP and do not desensitize (Lewis et al., 1995), a pharmacological profile that does not correspond to any of the homomeric P2X receptors cloned to date; all are expressed in sensory ganglia except P2X7. Although P2X3 is expressed preferentially in sensory neurons, currents evoked by ATP and a,bmeATP at the recombinant P2X3 receptor rapidly desensitize. However, when P2X3 is coexpressed in HEK293 cells with P2X2 (but not with other subtypes), a nondesensitizing response to ATP is observed which mimicks that seen in rat nodose neurons and which cannot be explained by additive effects of the two homomeric channels (Lewis et al., 1995). It was suggested that a new heteromeric receptor, P2X2P2X3, is formed from the P2X3 and P2X2 subunits (Lewis et al., 1995). This hypothesis is supported by the observation of a high level of colocalization of P2X2- and P2X3-immunoreactivity in rat nodose and dorsal root ganglia (Vulchanova et al., 1997). Direct evidence for the formation of a P2X2P2X3 heteromer comes from a study showing that in cells
coinfected with P2X2 and P2X3 receptors, the two proteins can be cross-immunoprecipitated with antibodies specific for either of the epitope tags introduced at the C terminal of the proteins (Radford et al., 1997). Electrophysiological studies showing sensitivity to a,b-meATP and a slowly desensitizing response is consistent with formation of heteromeric receptors because this is distinct from responses mediated by homomeric P2X2 and P2X3 receptors (Radford et al., 1997). Further evidence for the existence of P2X2P2X3 heteromers in sensory neurons is suggested by electrophysiological studies in cultured neurons of adult rat dorsal root (Robertson et al., 1996) and trigeminal ganglion neurons (Cook et al., 1997). However, heterogeneity within populations of sensory neurons has been identified in the form of single labeling for P2X2 or P2X3 of some rat nodose and dorsal root neurons (possibly coexisting with other P2X proteins) (Vulchanova et al., 1997), and by the demonstration of two types of inward current to ATP (transient and slowly desensitizing) in tooth-pulp nociceptors (Cook and McCleskey, 1997). This raises interesting questions about the patterns of P2X receptor subtype expression and the physiological properties of different neurons. The likely formation of P2X2P2X3 heteromers in sensory neurons has important implications for the subunit organization of P2X receptors in other biological tissues, because the different P2X proteins have widespread and
RECEPTORS FOR PURINES AND PYRIMIDINES
FIG. 8. Diagram depicting a proposed transmembrane topology for P2X2 protein showing both N- and C-terminals in the cytoplasm. Two putative membrane spanning segments (M1 and M2) traverse the lipid bilayer of the plasma membrane and are connected by a hydrophilic segment of 270 amino acids. This putative extracellular domain is shown containing two disulfide-bonded loops (S-S) and three N-linked glycosyl chains (triangles). The P2X2 cDNA was sequenced on both strands using Sequanase. (From Brake et al., 1994).
overlapping distributions. However, it seems that not all combinations are possible; for example, cotransfected P2X1 and P2X2 subunits do not combine to form heteromeric receptors (Surprenant, 1996). Figure 9 shows examples of ATP-gated currents in native cells and how these correlate with recombinant P2X receptors. Alternative splicing of P2X pre-messenger RNA has been shown for the P2X2 receptor (Bra¨ndle et al., 1997; Simon et al., 1997). The splice variant exhibits a different pharmacology to the native receptor, suggesting that there may be heterogeneity in responses of tissues expressing the different proteins. B. Cloned P2X Receptors 1. P2X1 receptor. The P2X1 receptor has been cloned from rat vas deferens and human and mouse urinary bladder (Valera et al., 1994, 1995, 1996) (table 9). The recombinant receptor is activated by 2MeSATP $ ATP . a,b-meATP .. ADP, and inward currents evoked by these compounds are reversibly blocked by suramin and PPADS (Valera et al., 1994). The receptor desensitizes very rapidly (in hundreds of milliseconds). P2X1 receptor mRNA is expressed in urinary bladder, smooth muscle layers of small arteries and arterioles, and vas deferens, with lower levels in lung and spleen (Valera et al., 1994; Collo et al., 1996). P2X1 receptor mRNA is also expressed in dorsal root ganglia, trigeminal ganglia, coeliac ganglia, spinal cord, and rat brain (Valera et al., 1994; Webb et al., 1995; Collo et al., 1996).
FIG. 9. Examples of ATP-gated currents evoked in native cells (A-D) and in HEK293 cells expressing homomeric (E-G) or heteromeric (H) P2X receptors. Bars above each trace refer to the duration of agonist application. All recordings are at holding potential of 270 mV. Traces shown in C from neonatal dorsal root ganglion neurons are unpublished records kindly supplied by M. Rae, S. Robertson, E. Rowan, and C. Kennedy, University of Strathclyde; all other traces from authors unpublished records. (From Evans and Surprenant, 1996.)
The P2X1 receptor seems to be the most significant P2X subtype in vascular smooth muscle, although P2X4 receptors may also be expressed (Soto et al., 1996a). The similar pharmacological profiles and desensitization of the recombinant P2X1 receptor and its native counterpart is consistent with the concept that the vascular smooth muscle P2X receptor is a P2X1 receptor homomer. ATP-gated ion channels in platelets and megakaryocytes have a similar pharmacology to the recombinant P2X1 receptor, which has led to the suggestion that these ion channels are P2X1 receptors (Somasundaram and Mahaut-Smith, 1994; MacKenzie et al., 1996). 2. P2X2 receptor. The P2X2 receptor first cloned from rat pheochromocytoma PC12 cells (originally called P2XR1) (Brake et al., 1994) displays only 41% amino acid homology with the rat vas deferens P2X1 receptor. At the recombinant P2X2 receptor ATP, adenosine 59-O(3-thiotriphosphate) (ATPgS) and 2MeSATP are approximately equipotent at eliciting non-selective inward cation currents, whereas a,b-meATP and b,g-meATP are inactive as agonists or antagonists (Brake et al., 1994). This receptor undergoes little or no desensitization. It also differs from the P2X1 receptor in that it is less permeable to Ca21 and shows much higher sensitivity to inhibition by extracellular Ca21 (Evans et al., 1996). P2X2 receptor mRNA is distributed in bladder, brain, spinal cord, superior cervical ganglia, adrenal medulla, intestine, and vas deferens, with highest levels found in the pituitary gland and vas deferens (Brake et al., 1994). Distinct but restricted patterns of distribution of P2X2 mRNA have been described within rat brain (Collo et al., 1996). P2X2 receptor mRNA is the only P2X mRNA
RALEVIC AND BURNSTOCK
observed in the adrenal medulla (Collo et al., 1996). P2X2 mRNA is absent from skeletal muscle, and several organs including heart, liver, kidney, lung, and spleen. Immunohistochemical detection shows a widespread distribution of the P2X2 receptor in brain and spinal cord (Vulchanova et al., 1996). The pharmacological profile of the P2X response in PC12 cells, namely insensitivity to a,b-meATP and lack of desensitization, is consistent with the concept that this is an endogenous counterpart of the P2X2 receptor. Sequence homology (about 40%) between P2X2 and a partial cDNA called RP-2 encoding for a protein activated in thymocytes undergoing programmed cell death, has led to the suggestion that RP-2 may encode an ion channel subunit activated by ATP released during apoptosis (Brake et al., 1994). A splice variant of a P2X2 receptor has been isolated from rat cerebellum and characterized pharmacologically (Bra¨ndle et al., 1997; Simon et al., 1997). The protein, termed P2X2(b) or P2X2–2, has a 69 amino acid deletion of the carboxyl-terminal, shows a similar distribution in the rat central and peripheral nervous system as the original P2X2 receptor (distinguished by the terminology P2X2(a)), and forms a homomeric receptor mediating inward currents to ATP (Bra¨ndle et al., 1997; Simon et al., 1997). Although the P2X2(b) receptor was equally sensitive to agonists as the P2X2(a) receptor, it showed significantly lower antagonist sensitivity and a faster rate of desensitization. Two other splice variants were also identified, and designated p2X2(c) and p2X2(d) to indicate that their functional significance remains to be determined (Simon et al., 1997). A truncated form of the P2X2 receptor (360 amino acids compared with the 472 of P2X2), P2X2–1 (originally called P2xR1), has been isolated from the pituitary gland and secretory epithelial tissue of rat cochlea (Housley et al., 1995). 3. P2X3 receptor. The P2X3 receptor cloned from rat dorsal root ganglion (Chen et al., 1995a; Lewis et al., 1995) shows only 43% amino acid sequence homology with the P2X1 receptor and 47% identity to the P2X2 receptor. The P2X3 receptor is activated by agonists with a potency order of 2MeSATP .. ATP . a,b-meATP and undergoes rapid desensitization (in less than 100 ms). The P2X3 receptor has a very restricted distribution; it is expressed only by a subset of sensory neurons (trigeminal, nodose, and dorsal root ganglia), and is absent from sympathetic, enteric and central nervous system neurons, and smooth muscle (Chen et al., 1995a; Lewis et al., 1995; Collo et al., 1996). All of the other cloned P2X receptors also have been localized in sensory neurons. The human P2X3 receptor transcript is limited to spinal cord and heart (Garcia-Guzman et al., 1997b). Interestingly, whereas the homomeric P2X3 receptor accounts for rapidly desensitizing currents to ATP and a,bmeATP in neonatal sensory neurons (Krishtal et al., 1988a, 1988b; Li et al., 1993; Robertson et al., 1996), a
heteromeric P2X2P2X3 receptor seems to account for the nondesensitizing response in adult sensory neurons (Lewis et al., 1995), suggesting that there may be differential expression of P2X subunits in sensory neurons in development. 4. P2X4 receptor. The P2X4 receptor protein has been cloned from rat hippocampus (Bo et al., 1995), DRG cells (Buell et al., 1996b), rat (Se´gue´la et al., 1996; GarciaGuzman et al., 1997a) and human brain (Soto et al., 1996a; Garcia-Guzman et al., 1997a), as well as rat endocrine tissue (Wang et al., 1996). The P2X receptor cloned from rat brain by Se´gue´la et al. (1996) was refered to as P2x3 in their paper, but a comparison of the receptor sequence with known subtypes identifies it as P2X4. A sequence homology of 87% between the human and rat P2X4 receptors is sufficiently different to produce subtle differences in antagonist binding and desensitization. The recombinant P2X4 receptor is most potently activated by 2MeSATP, but a,b-meATP is weak or inactive (Bo et al., 1995; Se´gue´la et al., 1996). P2X4 is relatively insensitive to the antagonists suramin and PPADS; high concentrations (.100 mM) are required to block ATP-evoked currents (Bo et al., 1995; Se´gue´la et al., 1996), although the human receptor shows a higher sensitivity for suramin and PPADS (Garcia-Guzman et al., 1997a). A lysine residue present in the P2X1 and P2X2 receptors, but absent in the P2X4 receptor, is critical for the binding of antagonists but not agonists (Buell et al., 1996a). The P2X4 receptor does not desensitize rapidly, although reversible rundown of the current occurs during prolonged exposure to ATP (Se´gue´la et al., 1996). More rapid desensitization of the human P2X4 receptor (Garcia-Guzman et al., 1997a) compared with the rat P2X4 receptor (Buell et al., 1996a) has been described. P2X4 ATP-gated currents are potentiated by coapplication of Zn21 (Se´gue´la et al., 1996; Garcia-Guzman et al., 1997a). P2X4 receptor mRNA is expressed in brain, spinal cord, sensory ganglia, superior cervical ganglion, lung, bronchial epithelium, thymus, bladder, acinar cells of the salivary gland, adrenal gland, testis, and vas deferens (Bo et al., 1995; Buell et al., 1996b; Collo et al., 1996; Se´gue´la et al., 1996). Within the brain and spinal cord, the distribution of P2X4 mRNA is very similar to, but not identical with, that of the P2X6 receptor (Collo et al., 1996). P2X4 receptor mRNA is unique in that it is the only type expressed by acinar cells of the salivary gland (Collo et al., 1996). 5. P2X5 receptor. This P2X receptor was first cloned from rat coeliac ganglia (Collo et al., 1996). Human homologs of the P2X5 receptor have tentatively been identified (Tokuyama et al., 1996a, 1996b). Rapid inward currents are activated by ATP . 2MeSATP . ADP, whereas a,b-meATP is ineffective as an agonist. The receptor does not readily desensitize. Currents are readily inhibited by suramin and PPADS. In situ hybridization shows P2X5 mRNA in motoneurons of the ven-
RECEPTORS FOR PURINES AND PYRIMIDINES
tral horn of the cervical spinal cord, and in neurons in the trigeminal and dorsal root ganglia. With the exception of the mesencephalic nucleus of the trigeminal nerve, the brain does not express P2X5 mRNA (Collo et al., 1996). Appropriately, functional studies have identified P2X receptors in rat trigeminal mesencephalic nucleus neurons with a profile most similar to that of P2X5 receptors (Khakh et al., 1997) 6. P2X6 receptor. This clone was isolated from a rat superior cervical ganglion cDNA library (Collo et al., 1996). Rapid currents are mediated by ATP . 2MeSATP . ADP, but a,b-meATP has no effect. Currents are only partially inhibited by suramin or PPADS. P2X6 mRNA is heavily expressed in the CNS, with heaviest staining in cerebellar Purkinje cells and ependyma (Collo et al., 1996). Staining is also detected in the cervical spinal cord, notably in spinal motoneurons of lamina IX, and the superficial dorsal horn neurons of lamina II. P2X6 mRNA is also present in trigeminal, dorsal root, and coeliac ganglia; and in gland cells of the uterus, granulosa cells of the ovary, and bronchial epithelia, but is absent from salivary epithelia, adrenal medulla, and bladder smooth muscle (Collo et al., 1996). 7. P2X7 receptor. This receptor is considered in detail in Section X. C. Signal Transduction Mechanisms P2X receptors mediate the rapid (onset within 10 ms) non-selective passage of cations (Na1, K1, Ca21) across the cell membrane resulting in an increase in intracellular Ca21 and depolarization (Bean, 1992; Dubyak and el-Moatassim, 1993). The direct flux of extracellular Ca21 through the channel constitutes a significant source of the increase in intracellular Ca21. However, membrane depolarization leads to the secondary activation of voltage-dependent Ca21 channels, which probably make the primary contribution to Ca21 influx and to the increase in intracellular Ca21. Because this transduction mechanism does not depend on the production and diffusion of second-messengers within the cytosol or cell membrane, the response time is very rapid, and appropriately plays an important role in fast neuronal signaling and regulation of muscle contractility. P2X channels often show considerable current fluctuation, or “flickery bursts,” in the open state that may represent unresolved closures or rapid transition between states (Evans and Surprenant, 1996). Selectivity for Ca21 permeability between P2X receptors on sensory versus autonomic nerves and smooth muscle has been suggested, but the patterns are not entirely clear (see Evans and Surprenant, 1996). The kinetics of ATP-gated currents have been reviewed (Surprenant, 1996). Cations can modulate ATP-activated currents in native and endogenous P2X receptors. Mg21 and Ca21 generally inhibit P2X receptor currents, probably by decreasing the affinity of the ATP binding site by an allosteric change in the receptor (Honore´ et al., 1989;
Nakazawa et al., 1990; Li et al., 1997a). However, an increase in the transient ATP response (but not the slowly-desensitizing ATP response) has been observed when Ca21 replaces Na1 in the extracellular solution in rat trigeminal sensory neurons (Cook and McCleskey, 1997). Interestingly, the recombinant P2X2 receptor seems to be more susceptible than the P2X1 receptor to inhibition by increases in extracellular Ca21 (Evans et al., 1996). Allosteric interactions may also be responsible for the ability of monovalent cations to negatively modulate binding to recombinant P2X4 receptors (Michel et al., 1997), and trivalent cations to negatively modulate the binding site of recombinant P2X1 and P2X2 receptors and the endogenous receptor of PC12 cells (Nakazawa et al., 1997). Zn21 potentiates the cation conductance induced by ATP at most P2X receptors, including those in rat superior cervical ganglion (Cloues et al., 1993; Cloues, 1995), nodose and coeliac ganglion neurons (Li et al., 1993, 1996), PC12 cells (Koizumi et al., 1995a), and recombinant P2X1 (Brake et al., 1994) and P2X4 receptors (Se´gue´la et al., 1996). The P2X7 receptor is an exception in this respect because it is inhibited by Zn21 and Cu21 (Virginio et al., 1997). Ni21 enhances ATP-activated currents in rat superior cervical ganglia (Cloues et al., 1993) and Cd21 potentiates ATP-evoked inward currents and dopamine release in rat phaeochromocytoma cells (Ikeda et al., 1996). Modulation of the affinity of the ATP-binding site occurs by extracellular protons; acid pH causes an increase, and alkaline pH causes a decrease in currents, as shown for the recombinant P2X2 receptor and endogenous P2X receptors in rat dorsal root and nodose ganglion cells (King et al., 1996b; Li et al., 1996, 1997b; Wildman et al., 1997). This may be particularly significant for P2X receptor-mediated signaling in pathophysiological conditions where injury or inflammation can profoundly alter extracellular pH. D. Desensitization P2X receptors can be divided into two broad groups according to whether they desensitize rapidly, that is, within 100 to 300 ms, or slowly if at all (table 10). This subdivision hinges critically on the time to desensitization; “rapid” desensitization should not be confused with desensitization which occurs over a few seconds, and thus is a phenomenon which is difficult to identify in other than studies of single channel activity. As a general rule, all rapidly desensitizing P2X receptors are activated by a,b-meATP as well as by 2MeSATP and ATP. These include: recombinant P2X1 and P2X3 receptors; their endogenous counterparts, namely P2X1-like receptors of smooth muscle (with some exceptions, indicated below); P2X1-like receptors of promyelocyte HL60 cells (Buell et al., 1996b); and platelets (MacKenzie et al., 1996) and P2X3-like receptors of neonatal sensory neurons (dorsal root ganglion and nodose ganglion)
RALEVIC AND BURNSTOCK TABLE 10 Distinguishing pharmacological characteristics of P2 receptors P2X receptors
—, weak or inactive; N.D., not determined. a Rat, but not human. P2Y4 receptor is sensitive to ATP 5 UTP. b ATP is a competitive antagonist. Lower case is used to designate the p2y3 receptor in recognition that it is a nonmammalian (chick) receptor and may be the homolog of the mammalian P2Y6 receptor.
(Krishtal et al., 1988a,b; Li et al., 1993; Robertson et al., 1996). Desensitization of P2X3-like receptors of neonatal sensory neurons, but not P2X1-like receptors of smooth muscle, is concentration-dependent (Evans and Surprenant, 1996; Robertson et al., 1996). Desensitization will clearly serve to terminate the purinergic response even though ATP release may still be ongoing, but exactly why this is more important in some tissues remains to be determined. P2X receptors which do not desensitize rapidly, desensitize slowly or not at all. These “non-desensitizing” P2X receptors are defined as receptors for which the currents are maintained for at least a few seconds in the continuous presence of agonist. Non-desensitizing P2X receptors can be further subdivided into two groups: 1) those that are sensitive to a,b-meATP, and 2) those that are insensitive or only weakly sensitive to a,b-meATP (Evans and Surprenant, 1996). Non-desensitizing a,bmeATP-sensitive P2X receptors are those in adult sensory ganglia (nodose and dorsal root ganglion) (Krishtal et al., 1988a, 1988b; Li et al., 1993; Khakh et al., 1995a; Wright and Li, 1995), and guinea-pig coeliac ganglion (Evans et al., 1992; Khakh et al., 1995a). It has been suggested that these receptors may be heteromers of P2X2 and P2X3 subunits (P2X2P2X3 receptors) (Lewis et al., 1995) (fig. 9). Non-desensitizing a,b-meATP-sensitive responses have also been shown in some smooth muscle, namely in the arterial vasculature of human placenta (Dobronyi et al., 1997; Ralevic et al., 1997), and intestine of the three-spined stickleback Gasterosteus aculeatus L (Knight and Burnstock, 1993), and similarly may be caused by actions at P2X heteromers. Non-desensitizing a,b-meATP-sensitive P2X receptors have also been described in the CNS, on rat locus coeruleus neurons (Tscho¨pl et al., 1992; Shen and North, 1993),
and some rostral ventrolateral medulla neurons (Ralevic et al., 1996). Non-desensitizing a,b-meATP-insensitive P2X receptors are cloned P2X2, P2X4, P2X5, and P2X6 receptors (table 10a), as well as native P2X receptors on most autonomic neurons, including rat superior cervical ganglia (Cloues et al., 1993; Nakazawa and Inoue, 1993; Khakh et al., 1995a), guinea-pig submucosal enteric neurons (Barajas-Lopez et al., 1994), PC12 cells (Nakazawa et al., 1990; Nakazawa and Hess, 1993; Kim and Rabin, 1994), rat cardiac parasympathetic ganglia (Fieber and Adams, 1991), and chick ciliary ganglion neurons (Abe et al., 1995). Non-desensitizing a,bmeATP-insensitive receptors have also been described in the CNS in nucleus tractus solitarius neurons (Ueno et al., 1992; Nabekura et al., 1995) and trigeminal mesencephalic nucleus neurons (Khakh et al., 1997); these may correspond to P2X4, P2X5, or P2X6 receptors, or to combinations of these subunits, given the rich expression of these proteins in the brain. ATP-gated a,bmeATP-insensitive currents in myometrial smooth muscle cells from pregnant rats have been reported to be resistant to desensitization (Honore´ et al., 1989). The mechanism of P2X receptor desensitization is not well understood. For the rapidly desensitizing P2X1 receptor, this may involve the hydrophobic domains of the receptor because transfer to the P2X2 receptor of both of the hydrophobic domains, but not the extracellular loop, of the P2X1 receptor changes the phenotype of the P2X2 receptor from non-desensitizing to rapidly-desensitizing (Werner et al., 1996). Amino acid deletions of the carboxyl terminal of the P2X2 receptor produces splice variants that desensitize more rapidly than the original receptor (Bra¨ndle et al., 1997; Simon et al., 1997). On the other hand, the N-terminal region of the receptor has
RECEPTORS FOR PURINES AND PYRIMIDINES
been suggested to be important in desensitization of the P2X3 receptor (King et al., 1997). Desensitization of the P2X3 receptor seems to involve the activation of calcineurin through the entry of extracellular calcium (King et al., 1997). E. Agonists and Antagonists There are no universal or subtype-selective P2X receptor agonists. ATP and diadenosine polyphosphates with a phosphate chain length greater than or equal to three are naturally-occuring agonists at P2X receptors (Hoyle et al., 1989; Hoyle, 1990; Bo et al., 1994; Schlu¨ter et al., 1994; Bailey and Hourani, 1995; Ralevic et al., 1995a; Usune et al., 1996). The greater potency of the longer chain diadenosine polyphosphates (Ap4A-Ap6A) compared with ATP at endogenous P2X1-like receptors may be caused by their greater resistance to breakdown (Hoyle, 1990; Ogilvie, 1992; Ralevic et al., 1995a). UTP is a weak agonist of P2X3 receptors (Chen et al., 1995a; Robertson et al., 1996) and may interact with P2X1-like receptors in rat urinary bladder (Hashimoto and Kokubun, 1995) as well as mouse vas deferens (Von Ku¨gelgen et al., 1990). In physiological solution, Ca21 and Mg21 ions form complexes with the free acid ATP42, such that the solution contains a mixture of ATP42, MgATP22, and CaATP22 (together with lower concentrations of the species variants MgHATP2, CaHATP2, and Ca2ATP). Under physiological conditions, ATP42 is a minor component of the total ATP concentration (approximately 1 to 10% depending on temperature, pH, and divalent cation concentration). The concentration of ATP42 decreases with increasing cation concentration and with acidic pH (that results in conversion of ATP42 to HATP32, which has proved useful in studies aimed at investigating the identity of the active form of ATP). Cockroft and Gomperts (1980) raised the question of which was the active form of ATP with their suggestion that ATP42 causes an increase in mast cell plasma membrane permeability. It has since been shown that this form of the ligand is likely to be responsible for pore-forming actions in mast cells, macrophages, and lymphocytes as well as a number of other cell types expressing a receptor termed the P2Z or P2X7 receptor. Addition of Mg21 forms the inactive species MaATP22 and thereby reduces the concentration of ATP42, rapidly closing the cation channel (Greenberg et al., 1988; el-Moatassim and Dubyak, 1993; Gargett et al., 1996; Lin and Lee, 1996). Similarly, 39-O-(4-benzoyl)benzoyl ATP (BzATP42), and not the complex MgBzATP22, seems to be the active species in P2Z or P2X7-mediated pore formation. The idea that ATP42 is the active form of ATP has been extended to P2X receptors other than the P2Z or P2X7 receptor. Hence, ATP42 has been suggested to be the ligand that activates P2X receptors in guinea-pig vas deferens smooth muscle (Fedan et al., 1990), rat parotid acinar cells (McMillian et al., 1993), and PC12 cells (Kim
and Rabin, 1994; Choi and Kim, 1996); it also mediates ATP-gated currents in pregnant rat myometrial smooth muscle cells (Honore´ et al., 1989). The P2X receptors expressed by these tissues do not form nonspecific membrane pores. In these studies, suggestion of a role for ATP42 as the active ligand is based primarily on the fact that responses are inhibited by elevation of extracellular Mg21 or other cations which chelate with ATP, and because responses correlate well with the calculated ATP42 concentration and not with the total ATP concentration or with the concentration of Mg21 in solution. However, this alone does not seem to be sufficient evidence in light of more recent studies which show that divalent cations can influence agonist potency by effects other than by changes in the relative concentrations of the ATP species in solution. It is now apparent that interpretation of the effects of removal of Mg21 and Ca21 from solution on agonist potency is complicated by additional inhibition of ectonucleotidase activity, disinhibition of single channel conductance of P2X receptors, and possibly membrane depolarization. These effects seem to have a greater influence on the end response than does a shift in the concentration of the active species of ATP. Inhibition of ecto-nucleotidase activity seems to be the overriding effect of Ca21 and Mg21 removal on agonist potency in the rat isolated vagus nerve, where the potency of responses to ATP and 2MeSATP was increased, but that of the stable analog a,b-meATP was unchanged (Trezise et al., 1994a). Studies on single channel conductance of native P2 receptors in rat nodose ganglion, PC12 cells, and recombinant P2X1 and P2X2 receptors, in which consideration of ecto-nucleotidase activity is effectively bypassed in conditions of concentration clamp, have confirmed that raising Ca21 or Mg21 decreases the potency of ATP (Nakazawa and Hess, 1993; Evans et al., 1996; Li et al., 1997a; Virginio et al., 1997). However, the mechanism seems to involve a decrease in the affinity of the agonist binding site by allosteric effects on the receptor (although direct cation block of the channels is also possible) (Nakazawa and Hess, 1993; Evans et al., 1996; Li et al., 1997a). The fact that recombinant P2X2 receptors show a higher sensitivity than P2X1 receptors to inhibition by extracellular Ca21 (Evans et al., 1996) is further consistent with the hypothesis that cation modulation of P2X receptors is due to changes occuring at the level of the receptor, and can be influenced by the intrinsic properties of that receptor, rather than a change in the relative concentrations of ATP species in the extracellular solution. Because of these complicating factors, the identity of the active species of ATP acting at P2X receptors is currently unclear. a,b-MeATP is an agonist at recombinant P2X1, P2X3, and heteromeric P2X2P2X3 receptors; endogenous P2X1like receptors in smooth muscle, platelets, and HL60 cells; P2X3-like receptors in neonatal nodose and dorsal root ganglia; and P2X receptors in guinea-pig coeliac
1[Ca21]i via PKC
2MeSATP . ATP $ a,b-meATP ATP .. 2MeSATP $ ADP . ADO . AMP
Excitation of hypoglossal nerve No inward currents Rapid 1 [Ca21]i
Slow inward currents Inward currents
K1 channel Rapid inward current
TABLE 11 P2 receptors in the central nervous system
Ikeuchi and Nishizaki, 1995a
Khakh et al., 1997 Ueno et al., 1992; Nabekura et al., 1995 Shen and North, 1993 Sun et al., 1992 Ralevic et al., 1996 Zhang et al., 1995
Shen and North, 1993
Ikeuchi et al., 1995a
Chessell et al., 1997
Edwards et al., 1992
Nieber et al., 1997
Harms et al., 1992 Tscho¨pl et al., 1992 Shen and North, 1993
Nabekura et al., 1995 Chen et al., 1994a
Funk et al., 1997
Ikeuchi et al., 1996a,b
Inoue et al., 1992 Balachandran and Bennett, 1996
Nabekura et al., 1995
Ikeuchi et al., 1995b
Ikeuchi and Nishizaki, 1995b
Ikeuchi and Nishizaki, 1996a
452 RALEVIC AND BURNSTOCK
Ikeuchi and Nishizaki, 1996b —
Jahr and Jessel, 1983 Fyffe and Perl, 1984 Li and Perl, 1995 Yes
A rapid inward current is also observed, but is blocked by a non-NMDA receptor antagonist. Desensitization observed in a subpopulation of a,b-meATP-sensitive neurones. Reproducible responses to ATP on rapid application at ,1 min intervals. GDPbS, guanosine-59-O-(2-thiodiphosphate); G protein inhibitor; PTX, pertussis toxin; RB2, reactive blue 2. c
a,b-meATP . ATP . UTP . 2MeSATP . bg-meATP ATP $ 2MeSATP .. a,b-meATP $ ADP ATP ATP ATP, ATPgS Supraoptic magnocellular neurosecretory cells (hypothalamic) Tuberomammillary nucleus Dorsal horn of spinal cord
Furukawa et al., 1994
Hiruma and Bourque, 1995
Day et al., 1993 ATP 5 a,b-meATP — Supraoptic vasopressin neurones
P2X P2 Suramin Suramin
ATP Substantia nigra
Inward currents at 1 in 12 neurones 1 Firing Block of excitation to vagus nerve stimulation Depolarization, 1 input conductance
Nabekura et al., 1995
RECEPTORS FOR PURINES AND PYRIMIDINES
ganglion. a,b-meATP generally does not bind to P2Y receptors; it is weak or inactive (EC50 values 100 mM) at recombinant receptors P2X2 and P2X4 –7 and at the likely endogenous P2X receptor couterparts (Collo et al., 1996; Evans and Surprenant, 1996). a,b-meATP-sensitive P2X receptors are sensitive to ATP, 2MeSATP, and a,b-meATP with EC50 values of approximately 0.5 to 5 mM, whereas a,b-meATP-insensitive P2X receptors are generally less sensitive to ATP and 2MeSATP (EC50 values 8 to 50 mM) (Collo et al., 1996; Evans and Surprenant, 1996). P2X receptors that are sensitive to a,b-meATP can be divided into two groups according to whether they are (rapidly) desensitizing or are non-desensitizing (see also Section IX.D., Desensitization). a,b-MeATP-sensitive desensitizing P2X receptors are cloned P2X1 and P2X3 receptors and their likely endogenous counterparts. a,bMeATP-sensitive non-desensitizing P2X receptors include some smooth muscle P2X receptors (Knight and Burnstock, 1993; Dobronyi et al., 1997; Relevic et al., 1997), P2X receptors on adult dorsal root ganglion and nodose ganglion, and guinea-pig coeliac neurons as well as heteromeric P2X2P2X3 receptors (Krishtal et al., 1988a,b; Evans et al., 1992; Li et al., 1993; Khakh et al., 1995a; Lewis et al., 1995; Wright and Li, 1995). Notably, L-b,g-meATP is active at P2X but not at P2Y receptors. It can discriminate between a,b-meATP-sensitive P2X receptors on smooth muscle of vas deferens and those on neurons. It is approximately equipotent with a,b-meATP and ATP at vas deferens and at the recombinant P2X1 receptor when ecto-nucleotidase activity is supressed, but ineffective at P2X receptors of rat vagal neurons, rat nodose ganglion neurons, and guineapig coeliac neurons (Trezise et al., 1995; Surprenant, 1996). ATPgS is an agonist at recombinant P2X2 and P2X4 receptors (Brake et al., 1994; Bo et al., 1995). It is a partial agonist at recombinant P2X1 and P2X2 receptors, as well as at endogenous receptors in vas deferens, PC12 cells, and nodose and coeliac ganglia (Surprenant, 1996) with potency generally less than that of ATP. PPADS, NF023, and NF279 show selectivity as antagonists at P2X versus P2Y receptors (see Section VIII.C.). F. Distribution and Biological Effects Tissue distributions of the different cloned P2X receptor proteins are detailed in the section on cloned receptors (see Section IX.B.). Most of the receptor proteins have widespread distributions and most tissues express more than one subtype of P2X receptor, which may lead to heteropolymerization. Exceptions are P2X3, which is only expressed in sensory ganglia (Chen et al., 1995a; Lewis et al., 1995), P2X1, which is the principal subtype expressed in smooth muscle (Valera et al., 1994; Collo et al., 1996), and P2X4, which is the only subtype expressed by acinar cells of salivary glands (Buell et al., 1996b). The principal distribution of P2X receptors is on excit-
RALEVIC AND BURNSTOCK
able tissue such as smooth muscle and nerves, although they have also been cloned from, or have been shown to be expressed by, endocrine tissues (P2X4; Wang et al., 1996), platelets (P2X1-like; MacKenzie et al., 1996), and promyelocyte HL60 cells (P2X1-like; Buell et al., 1996a). Autoradiography using [3H]-a,b-meATP, which labels P2X1 and P2X3 receptors, has shown high and low affinity binding sites in vascular smooth muscle, urinary bladder, brain, spinal cord, heart, liver, spleen, and cochlea (Bo and Burnstock, 1990, 1993, 1994; Michel and Humphrey, 1993; Balcar et al., 1995; Mockett et al., 1995). The significance of the two binding sites is not clear, and may represent distinct P2X subtypes, although [3H]a,b-meATP binding to nucleotide-binding proteins cannot be excluded. At least two high affinity binding sites for [3H]a,b-meATP were described in a rat aortic endothelial cell line, one of which was suggested to correspond to labeling of 59-nucleotidase, advising caution in the use of this radioligand (Michel et al., 1995). 1. CNS. P2X receptors are widely distributed in the CNS; excitation and activation of cation channels by ATP and/or a,b-meATP have been described throughout the brain and spinal cord (table 11). However, despite the widespread distribution of P2X receptors, evidence that ATP acts as a fast excitatory transmitter in the brain has so far been convincingly provided only for the medial habenulla (Edwards et al., 1992; Edwards and Gibb, 1993) and locus coeruleus (Nieber et al., 1997). In these regions, synaptic currents are blocked by suramin and by desensitization with a,b-meATP, and are mimicked by ATP and a,b-meATP. Interestingly, the nondesensitizing receptors P2X2, P2X4, and P2X6 are the most abundantly expressed P2X receptors in the brain (Kidd et al., 1995; Collo et al., 1996), which correlates well with the majority of functional studies that show a lack of desensitization of P2X receptors in the CNS (table 11). Activation of P2X receptors increases the activity of neurons in the rostral ventrolateral medulla and the pre-Bo¨tzinger region, areas within the brainstem that contribute specifically to central regulation of the cardiovascular system and respiratory drive (Sun et al., 1992; Ralevic et al., 1996, 1998). Pronounced effects on blood pressure and respiratory drive observed on microinjection of ATP and a,b-meATP into these regions indicates a potential role for P2X receptors in central modulation of the cardiovascular and respiratory systems (Sun et al., 1992; Ralevic et al., 1996, 1998). Clarification of the physiological significance of these findings awaits identification of the specific pathways and release of endogenous ATP acting as a mediator of these effects. There are marked regional differences in excitation by ATP of neurons throughout the brain. For instance, in rat brain, responses to ATP are elicited in 100% of neurons in the locus coeruleus, 96% of neurons in the dorsal motor nucleus, and 25% of neurons in the nucleus trac-
tus solitarius, while neurons in the mesencephalic and parabrachial nucleii are insensitive to ATP (Shen and North, 1993; Nabekura et al., 1995). The functional significance of this is not clear. These values correlate poorly with the reported densities of [3H]a,b-meATP binding in rat brain (Bo and Burnstock, 1994), probably because [3H]a,b-meATP binds most strongly to P2X1 and P2X3 receptors and does not reflect adequately the distribution of other P2X subtypes. A strong correlation between the percentage of cells responding to ATP and ACh/nicotine suggests colocalization of P2X and nicotinic ACh receptors (Nabekura et al., 1995). 2. Sensory nerves. Rapid inward currents are mediated by ATP in the dorsal horn of the spinal cord (Li and Perl, 1995; Li et al., 1997b), and there is evidence for P2X receptor-mediated fast synaptic transmission via ATP in a small subset of dorsal horn neurons (Bardoni et al., 1997). Glutamate evoked release after activation of P2X receptors on dorsal root ganglion neurons indicates a role for presynaptic P2X receptors (Gu and MacDermott, 1997). ATP-gated currents have also been shown on many sensory ganglion neurons (Krishtal et al., 1988a,b; Khakh et al., 1995a; Wright and Li, 1995; Robertson et al., 1996; Li et al., 1993, 1997a,b). P2X2P2X3 heteropolymeric receptors have been suggested to account for non-desensitizing ATP-gated currents in adult sensory ganglia (Lewis et al., 1995). P2X receptors also been shown in peripheral sensory nerve terminals, on capsaicin-sensitive sensory nerve terminals in canine lung (Pelleg and Hurt, 1996) and rat hindpaw (BlandWard and Humphrey, 1997), and in rat tooth pulp sensory neurons (Cook et al., 1997), where they may be involved in nociception. Immunohistochemical studies indicate the involvement of P2X3-like receptors in ATP responses in sensory nerves of tooth pulp (Cook et al., 1997). Together, these findings are consistent with the concept that ATP may be involved in the generation of pain signals via P2X receptors 3. PNS. ATP may act via P2X receptors to mediate transmission between neurons, as first shown by suramin-mediated block of synaptic currents between cultured coeliac ganglion cells (Evans et al., 1992; Silinsky et al., 1992). ATP-gated currents also have been shown on many sympathetic (Cloues et al., 1993; Cloues, 1995; Khakh et al., 1995a) and parasympathetic ganglia (Fieber and Adams, 1991; Abe et al., 1995; Sun and Stanley, 1996) The presynaptic P2 receptors on postganglionic sympathetic neurons may belong to the P2X receptor family. These include P2 receptors on cultured rat sympathetic neurons that mediate NA release (Boehm, 1994; Boehm et al., 1995), P2 receptors in chick cultured sympathetic neurons that facilitate electrically-evoked [3H]NA release (Allgaier et al., 1994a,b, 1995a,b), and P2X (P2X2like) receptors in pheochromocytoma cells that mediate NA and dopamine release (Inoue et al., 1991; Majid et al., 1992, 1993; Nakazawa and Inoue, 1992; Ikeda et al.,
RECEPTORS FOR PURINES AND PYRIMIDINES
1996). a,b-MeATP acts at presynaptic P2X-like receptors on cholinergic and nonadrenergic axons of guineapig ileum to enhance electrically-evoked release of [3H]choline and [3H]NA, respectively (Sperlagh and Vizi, 1991). Activation of cholinergic nerves in guineapig ileum via P2X-like receptors has been proposed (Kennedy and Humphrey, 1994). Multiple P2X receptors, predominantly P2X2-like receptors and rapidly desensitizing P2X receptors (P2X1- or P2X3-like), have been described on guinea-pig myenteric neurons (Zhou and Galligan, 1996). In rat isolated vagus nerve, responses to high, but not low, concentrations of a,bmeATP are resistant to antagonism by suramin and reactive blue 2, but are attenuated by iso-PPADS, suggesting heterogeneity of endogenous P2X receptors (Trezise et al., 1994c). An ATP-gated channel sensitive to suramin and insensitive to UTP mediates NA release from a subpopulation of adrenal chromaffin cells (Castro et al., 1995). 4. Smooth muscle. ATP neurotransmission in the PNS identifies a physiological role for P2X receptors on smooth muscle, and as mediators of excitatory junction potentials (EJPs), depolarization, and constriction (Burnstock, 1990; Burnstock and Ralevic, 1996). The postjunctional response of the vas deferens, and most blood vessels to sympathetic nerve stimulation, is a rapid EJP that is blocked by tetrodotoxin, guanethidine, P2 receptor antagonists, and by desensitization of the P2X1-like receptor with a,b-meATP, but is resistant to a-adrenoceptor blockade (Burnstock, 1990; Von Ku¨gelgen and Starke, 1991). Longer periods of stimulation result in summation of the EJPs and the membrane depolarizes allowing the opening of voltage-dependent Ca21 channels, Ca21 entry, and contraction. The P2X1 protein is the predominant subtype expressed in vascular smooth muscle, although P2X4 transcripts have been shown to be expressed in rat aorta and vena cava (Soto et al., 1996a). This correlates well with the rapid desensitization of ATP and a,b-meATP-mediated contractile responses observed in most smooth muscle preparations (Burnstock and Kennedy, 1985; Ralevic and Burnstock, 1988, 1991a,b). The rabbit saphenous artery provides a classic example of a vessel in which pharmacological manipulations have been used to identify the relative contributions of NA and ATP to sympathetic neurotransmission (Burnstock and Warland, 1987b; Warland and Burnstock, 1987). In this vessel, sympathetic nerve stimulation produces a contractile response of which less than 30% is blocked by the a1-adrenoceptor antagonist prazosin, whereas the remainder, the purinergic component, is abolished by a,b-meATP (Burnstock and Warland, 1987b) (fig. 10). The sympathetic origin of the purinergic response is confirmed by the fact that reserpine treatment, which depletes sympathetic nerves of their catecholamine content, fails to abolish nerve-mediated con-
FIG. 10. Contractions produced in the isolated saphenous artery of the rabbit on neurogenic transmural stimulation (0.08 – 0.1 msec; supramaximal voltage) for 1 sec (a,b) at the frequencies (hz) indicated (Œ). Nerve stimulations were repeated in the presence of 10 mM prazosin added before (a) or after (b) desensitization of the P2-purinoceptor with a,bmethylene ATP (a,b-meATP) as indicated on the figure by the arrowed lines. The horizontal bar signifies 4 min and the vertical bar 1 g. (From Burnstock and Warland, 1987b, Br J Pharmacol 90:111–120; with permission from McMillan Press Limited.)
tractions despite a greater than 95% reduction in tissue NA content. It can be envisaged that rapid desensitization of the P2X response in smooth muscle may result in attenuation of sympathetic contraction both by effectively eliminating the purinergic component of the response, as well as by removing the potential for synergistic augmentation of the response by postjunctional interactions involving P2X receptors and adrenoceptors (see Ralevic and Burnstock, 1990, 1991a). The physiological significance of rapid desensitization of the smooth muscle P2X receptor is currently unclear, although a role in negative modulation of the sympathetic response during repetitive or prolonged neurogenic stimulation seems to be indicated. The contractile response mediated by P2X receptors in the perfused arterial vasculature of human placental cotyledons is a rare example of a vascular smooth muscle P2X response that does not desensitize (Dobronyi et al., 1997; Ralevic et al., 1997); it may be significant that placental blood vessels are also unique in that they are not innervated. The expression of more than one functionally-coupled P2X receptor in a single tissue is suggested in the rat vas deferens where three distinct contraction-mediating receptors for ATP were proposed based on differential functional antagonism by PPADS, suramin and reactive blue 2, and different susceptibility to desensitization (Bu¨ltmann and Starke, 1994a). Suramin-resistant components of the contractile response to ATP, which may be caused by actions at suramin-insensitive P2X4 and P2X6 receptors, have been described in vas deferens of mouse (Von Ku¨gelgen et al., 1990), rat (Bu¨ltmann and Starke, 1994a), and guinea pig (Bailey and Hourani,
RALEVIC AND BURNSTOCK
1994, 1995), and in frog aorta (Knight and Burnstock, 1996), as well as human urinary bladder (Palea et al., 1995). Where this was examined, the suramin-resistant contractile response to ATP does not appear to be caused by actions at a P2Y2-like receptor, or to ecto-nucleotidase inhibition by suramin (Von Ku¨gelgen et al., 1990; Bailey and Hourani, 1994, 1995; Knight and Burnstock, 1996). A suramin-resistant component of constriction to ATP in cat colon circular muscle also cannot be explained by the ectoATPase activity of suramin (Venkova and Krier, 1993). Differences in pharmacological profiles have been reported for smooth muscle P2X1-like receptors of urinary bladder, vas deferens, and blood vessels (Abbracchio and Burnstock, 1994; Burnstock et al., 1994). Notably, 2MeSATP and derivatives of ATP are inactive in rabbit saphenous artery but are agonists at P2X1-like receptors in guinea-pig vas deferens and bladder (Burnstock et al., 1994). Non-desensitizing responses of smooth muscle to a,b-meATP have been described in human placental arteries (Dobronyi et al., 1997; Ralevic et al., 1997), and stickleback intestine (Knight and Burnstock, 1993), which is different from the rapidly desensitizing P2X1like response to a,b-meATP typical of other smooth muscle preparations. It is possible that the non-desensitizing response is mediated by heteromeric P2X receptors with subunits confering both sensitivity to a,b-meATP and resistance to desensitization. In rat and human urinary bladder, but not in dog bladder, a,b-meATP mediates contraction, suggesting species heterogeneity with respect to expression of P2X receptors in this issue (Palea et al., 1994; Suzuki and Kokubun, 1994). b,g-MeATP is a potent constrictor of human saphenous vein, but is weak or inactive in human extrarenal veins and arteries (Von Ku¨gelgen et al., 1995a), suggesting that P2X receptor proteins are differentially distributed among vessels. 5. Blood cells. ATP and a,b-meATP activate cation channels in human platelets that have been suggested to be P2X1 receptors (MacKenzie et al., 1996). The currents are mimicked by the spontaneous activation of single channel currents in platelets, suggested to be caused by autocrine activation following release of endogenous ADP and ATP from the platelets. In rat megakaryocytes, ATP and ATPgS activate a rapid (100 ms) nonselective cation channel that rapidly desensitizes (Somasundaram and Mahaut-Smith, 1994), and may also be mediated by a P2X1 receptor. Currents elicited by exogenous ATP or a,b-meATP at P2X1-like receptors in HL60 cells can only be observed when the ongoing desensitization by ATP released from these cells is removed (Buell et al., 1996a), suggesting that P2X1 receptors may be more widely distributed than currently anticipated. Interactions between P2X and nicotinic ACh receptors, or possibly direct activation by ATP of ACh receptors (possibly by actions on different subunits), have
been described in PC12 cells (Nakazawa et al., 1990; Nakazawa, 1994), cultured Xenopus myotomal muscle cells (Igusa, 1988), membranes of rat superior cervical ganglion (SCG) cells (Nakazawa and Inoue, 1993; Nakazawa, 1994), and postjunctional ACh receptors in rat cultured flexor digitorum brevis muscle fibers (Mozrzymas and Ruzzier, 1992). ATP-induced [3H]NA release from chick sympathetic neurons is blocked by nicotinic receptor antagonists (Allgaier et al., 1995b). However, ATP does not act at nicotinic receptors in guinea-pig coeliac ganglion (Evans et al., 1992), rat intracardiac neurons (Fieber and Adams, 1991), or, controversially, rat SCG neurons (Cloues et al., 1993; Boehm, 1994). X. P2X7 and Endogenous P2X7-Like (or P2Z) Receptors The P2X7 receptor cloned from rat macrophages and brain by Surprenant et al. in 1996 is the cytolytic “P2Z receptor” previously described in mast cells, macrophages, fibroblasts, lymphocytes, erythrocytes, and erythroleukemia cells. In line with the main aim of this review, “P2X7-like receptor” is used for the endogenous receptor counterpart of the P2X7 receptor in preference to “P2Z receptor”. A unique feature of cloned P2X7 and endogenous P2X7-like receptors is that, whereas under physiological conditions these function like other P2X receptors in that they are selectively permeable to small cations only, in the continued presence of ATP and when divalent cation levels are low, the cation channel can convert to a pore, permeable to small molecules as well as ions. A. Structure The P2X7 receptor and its endogenous counterpart is structurally similar to other P2X receptors (see Section IX A), except for the fact that it has a significantly longer intracellular C-terminal (240 amino acids) than other P2X receptors, of which at least the last 177 amino acids are crucial for the induction of the non-selective pore (Surprenant et al., 1996). B. Cloned P2X7 Receptors The P2X7 receptor was first cloned from rat brain and macrophages (Surprenant et al., 1996). The recombinant receptor has an agonist potency order for eliciting inward currents of 39-O-(4-benzoyl)benzoyl ATP (BzATP) .. ATP .. 2MeSATP . ATPgS . ADP (Surprenant et al., 1996) (table 9). The human homolog has been cloned and shows a lower sensitivity to agonists (Rassendren et al., 1997). In low divalent cation solution, agonists induce sustained currents and the channel becomes permeable to molecules of up to 900 daltons, although in normal solution selectivity for small cations is observed (Surprenant et al., 1996). As with other P2X receptors, this receptor is inhibited by divalent cations (Rassendren et al., 1997; Virginio et al., 1997).
RECEPTORS FOR PURINES AND PYRIMIDINES
C. Signal Transduction Mechanisms Brief activation of the recombinant P2X7 receptor and its endogenous counterpart causes rapid membrane depolarization and cation influx and is a reversible process. However, sustained activation causes an increase in permeability by allowing bidirectional transport of a variety of ions including Na1, K1, and Ca21 and small molecules with a molecular weight of less than or equal to 900 daltons, except in lymphocytes where the limit is 200 –300 daltons. This effect is associated with cytotoxicity. Permeabilization involves the cytoplasmic C terminus of the protein because it does not occur with a truncated P2X7 receptor lacking the last 177 residues, although cation function of the receptor is retained. The different upper size limit of the pore for P2X7-like receptors in different cells may represent isoforms of the receptor or different conductance states. In murine and human macrophages (el-Moatassim and Dubyak, 1992, 1993; Humphreys and Dubyak, 1996) and human leukaemic lymphocytes (Gargett et al., 1996; Gargett and Wiley, 1997), activation of P2X7-like receptors causes activation of phospholipase D, although the mechanism is unknown. In lymphocytes this has been suggested to be coupled to the influx of bivalent cations (Gargett et al., 1996), whereas in murine macrophages it is suggested to occur distinct from P2X7-like pore formation (el-Moatassim and Dubyak, 1993). In murine macrophages BzATP-induced activation of phospholipase D is not mimicked by Ca21-mobilizing agonists or by activators of protein kinase C (el-Moatassim and Dubyak, 1992), and in a human monocyte cell line it is blocked by calcium-calmodulin kinase II inhibition (Humphreys and Dubyak 1996). Activation of the P2X7-like receptor of human macrophages triggers the release of the inflammatory cytokine IL-1b, which may provide a clue to the physiological and/or pathophysiological role of this receptor (Griffiths et al., 1995; Ferrari et al., 1997). D. Desensitization Currents evoked at recombinant P2X7 and endogenous P2X7-like receptors do not readily desensitize. However, species differences in the time for which the current flows caused by brief application of agonist have been described. Currents elicited by BzATP at the recombinant rat P2X7 receptor decline slowly, particularly in low divalent cation solution, leading to sustained currents (10 –20 min) even by very brief agonist application (1–3s) (Surprenant et al., 1996). By contrast, currents evoked at the human P2X7 receptor decline to baseline within 10 –20 sec of discontinuing agonist application (Rassendren et al., 1997). E. Agonists The recombinant P2X7 receptors and its endogenous counterpart have high selectivity for ATP, with most
other purine compounds having little or no activity. The active ligand is suggested to be the tetrabasic acid ATP42 (Cockcroft and Gomperts, 1980), which is present as approximately 1% of the relatively high concentration (100 mM) of ATP that is required to activate this receptor. Thus, reducing the extracellular cation concentration increases agonist potency. Increasing the concentration of Mg21 rapidly closes the cation channel, although it is not clear to what extent this is due to the formation of the inactive MgATP22 complex, caused by direct block of the ion channel, or caused by a decrease in affinity caused by allosteric modulation of the receptor (Virginio et al., 1997). By contrast with other P2X receptors, the P2X7-like receptor is inhibited by Cu21 and Zn21 (Virginio et al., 1997). P1,P4-diadenosine tetraphosphate (Ap4A) can activate the P2X7-like receptor of mast cells, possibly because of its quadruple negative charge (Tatham et al., 1988). BzATP is currently the most potent agonist at the endogenous P2X7-like receptor; it is 10 to 100 times more potent than ATP in activating P2X7-like receptors in a number of cells (Gonzalez et al., 1989a; Erb et al., 1990; el-Moatassim and Dubyak, 1992; Soltoff et al., 1992; McMillian et al., 1993; Nuttle et al., 1993), although it is only twice as potent as ATP in eliciting cytolysis of hepatocytes (Zoetewij et al., 1996). Species differences between human and murine macrophage P2X7-like receptors have been suggested, based on different sensitivities to permeabilization by ATP, BzATP, and ATPgS (Hickman et al., 1994). F. Antagonists KN-62 (1-[N,O-bis(5-isoquinolinesulfonyl)-N-methylL-tyrosyl]-4-phenylpiperazine) has been described as a potent antagonist at the P2X7-like receptor of human lymphocytes with an IC50 of approximately 12 nM (Gargett and Wiley, 1997). 29,39-Dialdehyde ATP (oxidized ATP) is an antagonist at the P2X7-like receptor, but is irreversible and requires prolonged exposure of cells to high concentrations of inhibitor (Murgia et al., 1993; Wiley et al., 1994; Falzoni et al., 1995; Humphreys and Dubyak, 1996; Zoetewij et al., 1996; Surprenant et al., 1996). G. Distribution and Biological Effects P2X7 mRNA and protein are distributed in bone marrow cells, including granulocytes, monocytes/macrophages and B lymphocytes, and in macrophages in brain, as shown by evidence from functional studies on these cell types (Collo et al., 1997). Functional studies have shown that P2X7-like receptor distribution is generally limited to cells of hemopoietic origin including mast cells (Cockcroft and Gomperts, 1980; Tatham et al., 1988; Tatham and Lindau, 1990), macrophages (Steinberg et al., 1987; Greenberg et al., 1988; el-Moatassim and Dubyak, 1992, 1993; Murgia et al., 1992, 1993; Hickman et al., 1994; Falzoni et al.,
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1995), the human monocyte cell line THP-1 (Humphreys and Dubyak, 1996), fibroblasts (Weisman et al., 1989; Erb et al., 1990; Pizzo et al., 1992), erythrocytes (Parker and Snow, 1972), erythroleukaemia cells (Chahwala and Cantley, 1984), and lymphocytes (Wiley et al., 1994; Gargett et al., 1996; Jamieson et al., 1996; Markwardt et al., 1997). P2X7-like receptors are also present on hepatocytes (Zoetewij et al., 1996) and parotid and salivary gland acinar cells (Sasaki and Gallacher, 1990; McMillian et al., 1993; Soltoff et al., 1992, 1993). Although several roles for the P2X7 receptor have been proposed, its physiological significance is largely unknown. The increased permeability caused by activation of the P2X7-like receptor results in large ion fluxes and leakage of small metabolites. On prolonged stimulation it may cause cell swelling, vacuolization, and cell death by necrosis or apoptosis (Dubyak and elMoatassim, 1993). The biological significance of this cytotoxic effect of ATP is not clear, but may have a role in the elimination of unwanted cells during physiological or pathological cell and tissue turnover. There is increasing evidence to support suggestions that the P2X7 receptor is involved in signaling between macrophages or other cells involved in the immune response and target cells (Steinberg and Di Virgilio, 1991; Dubyak and elMoatassim, 1993); the P2X7-like receptor is involved in fusion of macrophages to form multinucleated giant cells that die shortly after fusion, a process that is inhibited by oxidized ATP (Chiozzi et al., 1997). Furthermore, ATP causes the release of the inflammatory cytokine IL-1b via the P2Y7-like receptor of human macrophages (Griffiths et al., 1995; Ferrari et al., 1997). Loss of the adhesion molecule L-selectin from leukocytes after activation of P2X7-like receptors implicates a role for these receptors in modulation of leukocyte binding to endothelial cells and migration through the vascular wall (Jamieson et al., 1996; Wiley et al., 1996). XI. P2Y Receptors P2Y receptors are purine and pyrimidine nucleotide receptors that are coupled to G proteins. Currently this includes the cloned mammalian receptors P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11, and the P2YADP (or P2T) receptor (that has not yet been cloned), and endogenous uridine nucleotide-specific receptors (that show some pharmacological similarities with cloned P2Y4 and P2Y6 receptors) (tables 10 and 12). The chick p2y3 receptor may be the homolog of the human P2Y6 receptor (hence lower case lettering). Putative P2Y5, P2Y7, P2Y9, and P2Y10 receptors are not included in the definitive P2Y receptor family after convincing evidence that these are not P2Y receptors. A receptor claimed as P2YAp4A (or P2D) has not yet been cloned, but may belong to the P2Y receptor family. A P2Y receptor has been cloned from Xenopus neural plate (Bogdanov et al., 1997). Receptors for pyrimidines that are activated specifically by uridine nucleotides, but not by adenine nucleo-
sides or nucleotides, were first proposed by Seifert and Schultz in 1989. This proposal has been confirmed by the cloning of two uridine nucleotide-specific receptors, P2Y4 (human) and P2Y6, showing preference for UTP and UDP, respectively (Communi et al., 1996b, c) (but see Section XV). Subsequent to Seifert and Schultz’s proposal, but before the cloning of P2Y4 and P2Y6 receptors, some confusion in the literature was caused by the identification of “P2U-purinoceptors”, activated equipotently by UTP and ATP (O’Connor et al., 1991), because P2U receptors were often loosely termed “pyrimidinoceptors” and separate identity of these and receptors activated preferentially by UTP or UDP (but weakly or not at all by ATP) was often indistinct. The cloning of the P2Y2 receptor and its characterization as a receptor activated by ATP, as well as UTP, helped to reinforce the concept that this receptor is distinct from receptors that are activated selectively by pyrimidines. A. Structure P2Y receptors are 308 to 377 amino acid proteins with a mass of 41 to 53 kDa after glycosylation. The seven transmembrane domain tertiary structure of P2Y receptors is common to that of other G protein-coupled receptors, general features of which have been described for adenosine P1 receptors (see Section.II.B.). A model of the P2Y receptor, based on the primary sequence of the P2Y1 receptor and using the structural homolog rhodopsin as a G protein-coupled receptor template, has identified positively charged amino acid residues in transmembrane regions 3, 6, and 7 that may be involved in ligand binding by electrostatic interactions with the phosphates of ATP (Van Rhee et al., 1995). Several of these amino acids are conserved in other G proteincoupled receptors. Site-directed mutagenesis of the P2Y2 receptor to convert positively charged amino acids in transmembrane regions 6 and 7 to neutral amino acids causes a 100- to 850-fold decrease in the potency of ATP and UTP, which suggests a role for these amino acids in binding purines and pyrimidines (Erb et al., 1995). By contrast, the most critical residues for ATP binding at the human P2Y1 receptor are in transmembrane regions 3 and 7 on the exofacial side of the receptor (Jiang et al., 1997). Most P2Y receptors act via G protein coupling to activate PLC leading to the formation of IP3 and mobilization of intracellular Ca21. Coupling to adenylate cyclase by some P2Y receptors has also been described. The response time of P2Y receptors is longer than that of the rapid responses mediated by P2X receptors because it involves second-messenger systems and/or ionic conductances mediated by G protein coupling. Signaling pathways for the P2Y receptor subtypes are considered in detail in the sections for each of these receptors.
RECEPTORS FOR PURINES AND PYRIMIDINES
XII. P2Y1 and Endogenous P2Y1-Like Receptors The P2Y1 receptor, and its endogenous counterpart termed P2Y1-like, is a receptor for the endogenous ligands ADP, ATP, and certain diadenosine polyphosphates; it is not activated by UDP and UTP. It seems to be more sensitive to adenine nucleotide diphosphates than to triphosphates. Sensitivity to ATP seems to be variable; many P2Y1 and P2Y1-like receptors are relatively insensitive to ATP (ATP may act as a partial agonist), but are strongly activated by ADP (see Heterogeneity of P2Y1-like receptors, Section XII.F.). Characteristically, among all other P2Y subtypes, the P2Y1 receptor and its endogenous counterpart are strongly activated by 2MeSATP, ADP, ADPbS, and adenosine-59O-(2-fluoro)-diphosphate (ADPbF) (table 10b). In the present review, evidence for G protein coupling, and evidence that 2MeSATP and ADP or ADPbS or ADPbF are full and potent agonists, is taken as provisional evidence for an endogenous P2Y1-like receptor, although this remains to be confirmed with the development and use of selective agonists and antagonists. A. Cloned P2Y1 Receptors The first cloned P2Y1 receptor was from chick brain (Webb et al., 1993b) (table 12). The recombinant receptor
is activated by agonists with a potency order of 2MeSATP $ ATP .. ADP, although a,b-meATP, b,gmeATP, and UTP are inactive (Webb et al., 1993b). Responses to ATP and 2MeSATP are antagonized by suramin and reactive blue 2. Activation of the recombinant P2Y1 receptor mediates IP3 formation and an increase in intracellular Ca21, but no change in cAMP levels (Simon et al., 1995). Homologs of the chick brain P2Y1 receptor have been cloned from a variety of species (table 12). Notably, the relative potency of ATP and ADP differs widely between recombinant P2Y1 and endogenous P2Y1-like receptors. Although it is possible that for recombinant receptors this is because of differences in assay conditions, the unequivocal insensitivity to ATP of some endogenous P2Y1-like receptors (Dixon et al., 1995; Ralevic and Burnstock, 1996a; Webb et al., 1996b) suggests that this is likely to be due to inherent differences in receptor structure. B. Signal Transduction Mechanisms The main signal transduction pathway of recombinant P2Y1 and endogenous P2Y1-like receptors is activation of PLC. From studies of the P2Y1-like receptor in turkey erythrocytes, the G protein has been identified as a Gq protein, G11, and is insensitive to pertussis and cholera
TABLE 12 Cloned P2Y receptors Receptor
Number of amino acids
cDNA library source
Human brain Human prostate and ovary Human placenta Human HEL cells Bovine endothelium Rat insulinoma cells Rat ileal myocytes Mouse insulinoma cells Turkey brain Chick brain
2MeSATP . ATP .. UTP 2MeSATP . ATP 5 ADP — — 2MeSATP 5 ADP . ATP .. UTP 2MeSATP . 2Cl-ATP . ATP (a,b-meATP inactive) 2MeSATP 5 2ClATP . ADP . ATP (UTP inactive) — 2MeSATP . ADP . ATP (UTP inactive) 2MeSATP . ATP . ADP (UTP inactive)
Schachter et al., 1996 Janssens et al., 1996 Le´on et al., 1995, 1997 Ayyanathan et al., 1996 Henderson et al., 1995 Tokuyama et al., 1995 Pacaud et al., 1996 Tokuyama et al., 1995 Filtz et al., 1994 Webb et al., 1993b
Human CF/T43 epithelial cells Human bone Rat microvascular coronary EC Rat alveolar type II cells Rat pituitary Wistar Kyoto rata Mouse NG108-15 neuroblastoma cells
ATP 5 UTP .. 2MeSATP — — ATP 5 UTP ATP 5 UTP . ADP 5 UDP . GTP — ATP 5 UTP . ATPgS .. 2MeSATP
Parr et al., 1995 Bowler et al., 1995 Go¨decke et al., 1996 Rice et al., 1995 Chen et al., 1996b Seye et al., 1996 Lustig et al., 1993
UDP . UTP . ADP . 2MeSATP . ATP
Webb et al., 1995, 1996a
Human placenta Human placenta Human chromosome X Rat heart
UTP . ATP 5 ADPc
Communi et al., 1996b Stam et al., 1996 Nguyen et al., 1996 Bogdanov et al., 1998
Human placenta and spleen Rat aortic smooth muscle Activated T-cells
UDP . UTP . ADP . 2MeSATP .. ATP UTP . ADP 5 2MeSATP . ATP —
Communi et al., 1996b Chang et al., 1995 Southey et al., 1996
ATP . 2MeSATP ... ADP (UTP, UDP inactive)
Communi et al., 1997
Tissue not specified. b p2y3 may be the chick homologue of the mammalian P2Y6 receptor. c The reported activity of UDP at the P2Y4 receptor has been shown to be caused by UTP present as a contaminant.
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toxin, which activates PLCb isoenzymes via its a subunit (Waldo et al., 1991a, 1991b; Maurice et al., 1993). Insensitivity or partial sensitivity to pertussis toxin is characteristic of most endogenous P2Y1-like receptors coupled to PLC, indicating the involvement of Gq/11 proteins. In contrast, P2Y1-like receptors coupled to inhibition of adenylate cyclase are typically blocked by pertussis toxin, indicating an involvement of Gi proteins (Boyer et al., 1995; Berti-Mattera et al., 1996; Webb et al., 1996c). IP3 formation and Ca21 mobilization can stimulate a variety of signaling pathways including PKC, PLA2, Ca21-dependent K1 channels, NOS and subsequent endothelium-derived relaxing factor (EDRF) formation, and can generate endothelium-derived hyperpolarizing factor (EDHF). The main physiological target of DAG is stimulation of PKC, which in turn may stimulate phosphatidyl choline-specific PLC, PLD, the MAPK pathway, and Ca21 influx via voltage-operated Ca21 channels. Generation of PKC (with no detectable elevations in IP3 or cytosolic Ca21) and subsequent rapid tyrosine phosphorylation of MAPK seems to be the pathway by which P2Y1-like (and P2Y2-like) receptors on endothelial cells mediate prostacyclin production (Bowden et al., 1995; Patel et al., 1996). This pathway is involved in cell metabolism, secretion, gene expression, and growth. P2Y1like receptor activation of a phosphatidyl choline-specific PLC, and of PLD, has been reported (Martin and Michaelis, 1989; Pirotton et al., 1990; Purkiss and Boarder, 1992), although activation may occur downstream of PKC. A second signaling pathway of endogenous P2Y1-like receptors may be inhibition of adenylate cyclase. This has been described for P2Y1-like receptors in a clonal population of rat brain capillary endothelial cells (B10 cells) (Webb et al., 1996c). The two pathways are expressed independently, that is, P2Y1-like activation of PLC does not coincide with P2Y1-like inhibition of adenylate cyclase. It is not yet clear whether this involves differential G protein-coupling or is caused by heterogeneity of P2Y1-like receptors (Webb et al., 1996c). P2Y receptor-mediated adenylate cyclase inhibition was originally described for P2Y1-like receptors in rat C6 glioma cells and the clonal cell line C6 –2B (Pianet et al., 1989; Valeins et al., 1992; Lin and Chuang, 1993; Boyer et al., 1993, 1994, 1995). However, the decrease in cAMP in C6 cells is not blocked by selective antagonists of the P2Y1 receptor,which suggests that these receptors are distinct from P2Y1 receptors coupled to activation of PLC (Boyer et al., 1996). P2Y1-like receptor-mediated inhibition of adenylate cyclase activity has also been described in Schwann cells (Berti-Mattera et al., 1996). Inhibition of adenylate cyclase is pertussis toxin-sensitive, indicating an involvement of Gi proteins, but it is unclear whether activation is mediated by a, b, or g subunits (Boyer et al., 1995; Harden et al., 1995; Webb et al., 1996c).
P2Y1-like receptors may mediate membrane-delimited G protein regulation of ion channels, that is, lack the involvement of cytosolic second-messenger systems. Although membrane-delimited regulation is frequently assumed to imply a direct physical interaction between the active G protein subunit and the ion channel, some ion channels may be regulated by lipid-soluble secondmessengers such as arachidonic acid and metabolites (Wickman and Clapham, 1995). In rat cerebellar neurons, the opening of an outwardly rectifying, pertussis toxin-insensitive GDPbS-sensitive K1 current by 2MeSATP . ADP . ATP activation of a P2Y1-like receptor was suggested via coupling of the b,g subunits of the G protein to a K1 channel (Ikeuchi and Nishizaki, 1996a). The single channel currents induced by 2MeSATP were without latency, suggesting that the channel was activated only by plasma membrane factors without the involvement of intracellular components (Ikeuchi and Nishizaki, 1996a). An ADP-sensitive K1 channel in inferior colliculus (Ikeuchi and Nishizaki, 1995b) and medullar (Ikeuchi et al., 1995a) neurons was also suggested to be activated by direct action of the bg subunits of the G protein. In contrast, 2MeSATP and ATP activation of a K1 channel in striatal neurons seems to be mediated via PKC (Ikeuchi and Nishizaki, 1995a). In some cells, P2Y1-like receptors are colocalized with P2Y2-like receptors. The biological significance of this is not clear, particularly where ATP is a common agonist, but makes more sense where the P2Y1-like receptor is selective for ADP, and ATP acts only at the P2Y2-like receptor (as has shown to be the case for coexisting P2Y1- and P2Y2-like receptors on some endothelial cells). The receptors have similar signaling pathways, although the P2Y1-like receptor seems to be more sensitive than the P2Y2-like receptor to manipulations of PKC activity. This is likely to be related to the important role of PKC as a negative feedback regulator of PLC activity to allow finely tuned regulation of this signaling pathway. Thus, stimulation of PKC with 12-O-tetradecanoyl-b-phorbol 13-acetate (TPA) causes a greater inhibition of P2Y1- than of P2Y2-like receptor mediated responses in rat osteoblastic cells (Gallinaro et al., 1995). The IP3 response of the endothelial P2Y1-like receptor is attenuated by stimulation of PKC with phorbol 12-myristate 13-acetate and enhanced by PKC inhibition with Ro 31– 8220, but the P2Y2-like response is less affected or is unaffected (Purkiss et al., 1994; Communi et al., 1995; Chen et al., 1996a). Discrimination between the signaling pathways of P2Y1- and P2Y2-like receptors, and the ways in which these may be differentially modulated, might provide some clues about the biological significance of their colocalization. C. Desensitization In general, P2Y1 and P2Y1-like receptors do not readily desensitize. When this does occur, as with other G protein-coupled receptors, desensitization may in-
RECEPTORS FOR PURINES AND PYRIMIDINES
volve receptor phosphorylation by protein kinases and uncoupling from the associated G protein. Studies of the P2Y1-like receptor in turkey erythrocyte membranes showed that desensitization (t1/2 15 min) is heterologous, involves multiple mechanisms, and does not involve PKC or intracellular Ca21 (Galas and Harden, 1995). In cultured bovine aortic endothelial cells, preexposure to 2MeSATP or UTP causes homologous partial desensitization of IP3 formation by P2Y1- and P2Y2-like receptors, respectively, and heterologous partial desensitization of the 2MeSATP response by UTP (Wilkinson et al., 1994). P2Y1-like receptor desensitization has also been observed in rat colon muscularis mucosae (Hourani et al., 1993) and rabbit mesenteric arterial smooth muscle (Ziganshin et al., 1994b). D. Agonists The P2Y1 and P2Y1-like receptor is generally more sensitive to adenine nucleotide diphosphates than to triphosphates. ADPbS, ADPbF, and 39-deoxyATPaS (dATPaS) are potent agonists at P2Y1 receptors. 2MeSATP is a potent and selective agonist at the P2Y1 and P2Y1-like receptor versus other cloned P2Y receptors (but see P2Y11 receptor, Section XVII.), but is also a potent agonist at most P2X receptors. a,b-meATP, b,gmeATP, and UTP are inactive and thus are useful as negative evidence in the characterization of this receptor. Certain of the diadenosine polyphosphates (particularly those with a phosphate chain of three phosphates or less) may be natural, albeit non-selective, agonists at P2Y1-like receptors (Ralevic et al., 1995a; Pintor et al., 1996). The potency of ATP differs widely among endogenous P2Y1-like receptors, and the lack of effect of ATP at some endogenous P2Y1-like receptors is unequivocal (Dixon et al., 1995; Ralevic and Burnstock, 1996a; Webb et al., 1996b). This would tend to rule out the possibility that this heterogeneity is caused by contamination of solutions of ADP and ATP caused by purine interconversion and metabolism. However, molecular evidence does not support a subdivision of the P2Y1 receptor, and heterogeneity of ADP/ATP relative potencies is also apparent for recombinant P2Y1 receptors (table 12). The charge carried by the molecule may influence agonist potency; it has been suggested that ATP uncomplexed with divalent cations, ATP42, is the preferred agonist of the P2Y1-like receptor expressed on bovine aortic endothelial cells (Motte et al., 1993b). In the guinea-pig taenia coli, the order of potency for relaxation at the P2Y1-like receptor by non-hydrolysable analogs of b,g-meATP reflects the order of electronegativity, with the more acidic analogs being more potent: AMPPCF2P . AMP-CCl2P . b,g-meATP (Cusack et al., 1987). 2-Thioether derivatives of adenine nucleotides, including 2-hexylthio ATP and 2-cyclohexylthio ATP, are potent agonists at P2Y1-like receptors coupled to adenylate cyclase (EC50 values 28 and 58 pM respectively),
but are significantly less potent at PLC-coupled P2Y1 receptors (Boyer et al., 1995). N6-Methyl ATP is selective for P2Y1-like receptors in the taenia coli versus vascular P2Y1-like receptors (Fischer et al., 1993; Burnstock et al., 1994). E. Antagonists Adenosine 39,59- and 29,59-bisphosphates act as competitive antagonists at the P2Y1 receptor coupled to PLC; adenosine-39-phosphate-59-phosphosulfate (A3P5PS) and adenosine-39-phosphate-59-phosphate (A3P5P) block responses at the recombinant P2Y1 receptor with pKB values of 6.5 and 5.7, respectively (Boyer et al., 1996). These compounds are inactive at the adenylate cyclase-coupled P2Y1-like receptor of C6 glioma cells and at recombinant P2Y2, P2Y4, or P2Y6 receptors (Boyer et al., 1996). Interestingly, A3P5PS and A3P5P are partial agonists at the turkey but not the human recombinant P2Y1 receptor. N6-methyl modification of 29-deoxyadenosine 3959-bisphosphate, to produce the compound MRS 2179, enhanced antagonist potency (IC50 value 330 nM) by 17-fold and eliminated the partial agonist properties observed with the lead compound, resulting in the most potent P2Y1 receptor antagonist reported to date (Camaioni et al., 1998). F. Heterogeneity of P2Y1 and Endogenous P2Y1-Like Receptors Although endogenous P2Y1-like receptors couple to different signal transduction pathways and there may be profound differences in their ligand binding profiles, molecular evidence does not support the subdivision of this receptor. It seems most likely that this heterogeneity may arise from small differences in structure. Sequence homology of only 84% between turkey and human P2Y1 receptors may explain why A3P5PS and A3P5P are partial agonists at the turkey P2Y1 receptor but not its human homolog (Boyer et al., 1996). These receptors were expressed in the same cell type and assayed under the same conditions. Heterogeneity in ligand binding at P2Y1 receptors includes both agonist and antagonist binding profiles. Recombinant P2Y1 receptors cloned from different species and tissues show different relative potencies to ATP and ADP (table 12), as do their endogenous counterparts. Although the true potency of ATP at endogenous P2Y1-like receptors is difficult to assess because of actions at coexisting receptors and rapid breakdown by ecto-nucleotidases, ADP-specific P2Y1-like receptors that are activated potently by ADP and 2MeSATP, but weakly or not at all by ATP, have been described in a number of isolated cells and tissues, including rat hepatocytes (Keppens and deWulf, 1991; Keppens et al., 1992; Dixon et al., 1995), endothelium of rat mesenteric arteries (Ralevic and Burnstock, 1996a,) and rat brain capillary endothelial cells (Feolde et al., 1995; Webb et al., 1996c). The P2 receptor antagonist PPADS has been shown to block vasodilatation mediated by ADP and
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2MeSATP (at a P2Y1-like receptor) but not to ATP and UTP (at a P2Y2-like receptor), which implies that at least in rat mesenteric arteries, ATP does not act at P2Y1-like receptors, although it does act at P2Y2-like receptors (Ralevic and Burnstock, 1996a). This has important implications for the agonist selectivity of P2Y1 receptors in other tissues. ADP-specific P2Y1-like receptors may account for some of the ambiguities in the literature concerning classification of P2Y receptors. Thus, ADP-activated P2Y receptors identified as “P2T” (P2YADP) receptors in osteoblasts (Sistare et al., 1994, 1995) are likely to be ADP-specific P2Y1 receptors because 2MeSATP and ADP are equipotent agonists (Reimer and Dixon, 1992; Sistare et al., 1994, 1995; Dixon et al., 1997b). A “P2T” receptor coexisting with the P2Y2 receptor in porcine ovarian granulosa cells may also be an ADP-specific P2Y1 receptor (Kamada et al., 1994). PPADS is able to discriminate between some P2Y1 receptors; it generally blocks recombinant P2Y1 receptors and endogenous P2Y1-like receptors coupled to PLC (Boyer et al., 1994; Brown et al., 1995; Charlton et al., 1996a; Schachter et al., 1996) but has no effect at P2Y1like receptors coupled to inhibition of adenylate cyclase (Boyer et al., 1994; Webb et al., 1996c). On the other hand, PPADS is ineffective at rabbit aortic endothelial P2Y1-like receptors, where PLC coupling might be expected (Ziganshin et al., 1994b). Block of P2Y1-like receptors with different pA2 values also implies receptor heterogeneity: pA2 values 5.1 and 5.3 in rat duodenum and guinea-pig taenia coli, respectively, (Windscheif et al., 1995a); pA2 values 6.0 in rat mesenteric arterial endothelium (Ralevic and Burnstock, 1996a) and at recombinant turkey brain (Charlton et al., 1996a) P2Y1 receptors. PPADS is ineffective as an antagonist at rabbit mesenteric arterial smooth muscle P2Y1-like receptors (Ziganshin et al., 1994b). Different sensitivities to ATP and analogs of ATP have been shown for P2Y1-like receptors in guinea-pig taenia coli, and in vascular endothelium and smooth muscle (Fischer et al., 1993; Burnstock et al., 1994; Abbracchio and Burnstock, 1994). Among other differences, N6-methylATP is a selective agonist at guinea-pig taenia coli P2Y1-like receptors, but is inactive at vascular P2Y1like receptors (Fischer et al., 1993; Burnstock et al., 1994). Relaxation by a,b-meATP of the guinea-pig taenia coli seems to be via a P2Y receptor of undetermined subtype as this response is not blocked by the P2Xselective antagonist Evans blue (Bu¨ltmann et al., 1996). 2-Thioether derivatives of adenine nucleotides are potent agonists at adenylyl cyclase-linked P2Y1-like receptors in C6 rat glioma cells, but not at PLC-linked P2Y1like receptors of turkey erythroctyes (Boyer et al., 1995). Interestingly, ATP seems to be a partial agonist at adenylate cyclase-coupled P2Y receptors. At the endothelial P2Y1-like receptor, P1,P3-diadenosine triphosphate (Ap3A) is the most potent ligand and P1,P5-diadenosine
pentaphosphate (Ap5A) is inactive (Ralevic et al., 1995a). G. Distribution and Biological Effects P2Y1 and P2Y1-like receptors are widely distributed having been described in heart, vascular, connective, immune, and neural tissues. The transcript for chick brain P2Y1 mRNA is distributed in brain, spinal cord, gastrointestinal tract, spleen, and skeletal muscle, but not in heart, liver, stomach, lung, or kidney (Webb et al., 1993b). In the rat, P2Y1 receptor mRNA is expressed at variable levels in many tissues including heart, brain, spleen, lung, liver, skeletal muscle, and kidney, but is not detected in testis (Tokuyama et al., 1995). Within the brain, P2Y1 mRNA has a widespread but specific distribution, being particularly rich in various nuclei of the telencephalon, diencephalon, and mesencephalon as well as in the external granule, Purkinje, and internal granule cells of the cerebellum (Webb et al., 1994). Receptors with the pharmacological profile of a P2Y1 receptor have been identified in functional studies in a wide variety of cells including rat astrocytes (Pearce et al., 1989; Pearce and Langley, 1994), frog glial cells (Robitaille, 1995), avian erythrocytes (Berrie et al., 1989; Boyer et al., 1989), rat osteoblasts (Reimer and Dixon, 1992; Gallinaro et al., 1995), pancreatic b cells (Petit et al., 1988), rat mast cells (Osipchuk and Cahalan, 1992), rat alveolar type II cells (Rice and Singleton, 1987), human T-leukemia cells (Biffen and Alexander, 1994), rat cochlear lateral wall (Ogawa and Schacht, 1995), and rat cochlear lateral wall epithelial cells (Ikeda et al., 1995). The physiological significance of these receptors is still largely undetermined. Diverse P2Y1-like receptor-mediated metabolic effects include insulin secretion from pancreatic b-cells (Bertrand et al., 1987; HillaireBuys et al., 1991, 1993, 1994), renin secretion in renal cortical slices (Churchill and Ellis, 1993a, 1993b), gluconeogenesis in renal cortical tubules (Cha et al., 1995), and glycogenolysis in rat hepatocytes (Keppens and De Wulf, 1991). The distribution of P2Y1-like receptors on vascular endothelium and smooth muscle cells implies a role in the regulation of vascular tone. In most blood vessels, P2Y1-like receptors are present on the endothelium and mediate vasodilatation by Ca21-dependent activation of endothelial NOS and generation of EDRF and by generation of EDHF. Endothelial prostacyclin production is also stimulated by the P2Y1-like receptor, but this seems to play a minimal role in vasodilatation, at least under physiological conditions. The fact that ATP and ADP are released locally from endothelial cells during shear stress and hypoxia and from platelets during aggregation, identifies a possible role for endothelial P2Y1-like receptors in modulation of vascular tone under normal conditions and during thrombosis. P2Y1-like receptors on pulmonary artery endothelium may be involved in stimulation of leukocyte adhesion (Dawicki et al., 1995).
RECEPTORS FOR PURINES AND PYRIMIDINES
P2Y1-like receptors are present on the smooth muscle of a number of blood vessels and, like their endothelial counterparts, mediate vasodilatation (Kennedy and Burnstock, 1985; Mathieson and Burnstock, 1985; Burnstock and Warland, 1987a; Liu et al., 1989; Brizzolara and Burnstock, 1991; Keefe et al., 1992; Corr and Burnstock, 1994; Qasabian et al., 1997; Simonsen et al., 1997). P2Y1-like receptors (and P2Y2-like receptors) are expressed by human coronary artery smooth muscle cells in culture (Strøbæk et al., 1996). The mechanism underlying relaxation by smooth muscle P2Y1-like receptors is not known but may involve activation of K1 channels. In rabbit mesenteric arteries and skeletal muscle-resistance arteries, glibenclamide partially blocks smooth muscle hyperpolarization and relaxation to ADP, indicating a role for KATP channels (Brayden, 1991). The smooth muscle P2Y1-like receptor of rabbit pulmonary artery mediates relaxation independently of mobilization of intracellular Ca21 (in contrast with that mediated by coexisting P2Y2-like receptors) implying lack of involvement of the PLC pathway (Qasabian et al., 1997). The biological significance of P2Y1-like receptors expressed by the smooth muscle of rabbit portal vein (Brizzolara et al., 1993) (fig. 11), guinea-pig pulmonary artery (Liu et al., 1992), and lamb small coronary arteries (Simonsen et al., 1997) may be in mediation of the neurogenic, purinergic (non-adrenergic non-cholinergic) relaxation shown in these vessels. It is possible that vascular smooth muscle P2Y1-like receptors mediate relaxation to ATP released as a neurotransmitter from sensory-motor nerves. A P2Y1-like receptor on cultured aortic smooth muscle cells has been reported to mediate the mitogenic effect of ATP via activation of PKC, and then Raf-1 and MAPK (Yu et al., 1996); it has also been reported to cause induction of immediate early genes (Malam-Souley et al., 1996), which indicates a role in vascular smooth muscle proliferation. Interestingly, autocatalytic release of ATP (ATP-mediated release of ATP) has been described in guinea-pig cardiac endothelial cells, which may involve P2Y1-like receptors (Yang et al., 1994). A P2Y1-like receptor on rat basophilic leukocyte cells is suggested to amplify intracellular Ca21 signaling and secretory responses to antigen stimulation, and to propagate the response to neighboring cells partly by the release of additional stores of ATP from secretory granules (Osipchuk and Cahalan, 1992). Activation of the P2Y1-like receptor expressed on platelets leads to platelet shape change, aggregation, and intracellular calcium rise, with no effect on adenylate cyclase (Daniel et al., 1998; Hechler et al., 1998; Jin et al., 1998). This effect is blocked by the selective P2Y1 receptor antagonists A2P5P and A3P5P. The P2Y1 receptor seems to be crucial for triggering the ADP-induced shape change, whereas aggregation is mediated by cooperative effects with platelet P2YADP (or P2T) re-
FIG. 11. Relaxations of the rabbit portal vein to neurogenic transmural stimulation for 10 sec (2 to 64 Hz, 0.7 ms, 100 V) at 5 min intervals. Guanethidine (3.4 mM) and atropine (0.114 mM) were present throughout to block adrenergic and cholinergic neurotransmission respectively. Tone was induced with ergotamine (8.6 mM). Panel (a) shows that preincubation with suramin (30 mM) for 20 min reduced the nerve-mediated relaxations compared with controls and that suramin-resistant neurogenic relaxations were abolished 20 min after the addition of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM). Panel (b) shows that neurogenic relaxations remaining after 20 min pretreatment of the tissue with L-NAME (0.1 mM) were abolished 20 min after the addition of suramin (30 mM). In (c), the effect of adding L-NAME (0.1 mM) to the tissue is shown; there was an additional rise in tone and inhibition of the response to nerve stimulation after a 20 min incubation period. The subsequent treatment of tissues with L-arginine (10 mM) for 20 min reversed this effect. Each of the traces in (a), (b), and (c) is representative of similar results in six separate experiments. (From Brizzolara et al., 1993, Br J Pharmacol 109:606 – 608; with permission from McMillan Press Limited).
ceptor-mediated inhibition of adenylate cyclase (Daniel et al., 1998; Hechler et al., 1998; Jin et al., 1998). P2Y1 receptor mRNA is selectively expressed by large diameter sensory neurons and when expressed in oocytes was shown to be mechano-sensitive and to exhibit inward currents (Nakamura and Strittmatter, 1996). A functional correlate may be ATP-triggered Ca21 release from IP3-sensitive Ca21 stores in large DGR neurons; [Ca21]i transients were not elicited by small neurons (Svichar et al., 1997). ATP inhibits the light-evoked release of ACh from rabbit retinal cholinergic neurons in a DPCPX-insensitive manner, although the receptor subtype is not clear (Neal and Cunningham, 1994). A P2Y1-like receptor may mediate inhibition by ATP and 2MeSATP (but not a,b-meATP) of excitatory postsynaptic potentials in guinea-pig submucosal neurons, and although it is suggested that it is a P3-like receptor, it is not activated by adenosine (Barajas-Lo´pez et al., 1995). P2Y1-like receptors mediate the opening of K1 channels in rat cultured cerebellar neurons, striatal neurons, superior and inferior colliculus neurons, medullar neu-
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rons, hippocampal neurons, and spinal neurons (Ikeuchi et al., 1995a,b; 1996a,b; Ikeuchi and Nishizaki, 1995b; 1996a,b). The transduction mechanism seems to be a pertussis toxin-insensitive G protein which directly opens the potassium channels via its bg subunit. Adenosine seems to be an agonist at P2Y1-like receptors in hippocampal neurons (Ikeuchi et al., 1996a) and neurons of the superior colliculus (Ikeuchi et al., 1995b), raising the possibility that these are P1 or P3 receptors. A P2Y1-like receptor mediates dopamine release in rat striatum (Zhang et al., 1995). An increase in the firing rate of rat medial vestibular nucleus neurons by ADPbS has been attributed to activation of P2Y receptors (Chessell et al., 1997). XIII. P2Y2 and Endogenous P2Y2-Like Receptors The P2Y2 receptor (and its endogenous counterpart, formerly called the P2U receptor) is activated by ATP and UTP with approximately equal potency and is insensitive or is only weakly activated by ADP and other nucleoside diphosphates, 2MeSATP and a,b-meATP (table 10b). In this review, endogenous receptors exhibiting this pharmacological profile have provisionally been termed P2Y2-like (but see Section XV.). A. Cloned P2Y2 Receptors The first cloned P2Y2 receptor was from mouse NG108 –15 neuroblastoma cells (Lustig et al., 1993). Species homologs have been cloned from rat, cat, and human (table 12). B. Signal Transduction Mechanisms Cloned P2Y2 and endogenous P2Y2-like receptors couple via both Gi/o and Gq/11 proteins to mediate phospholipid breakdown and phosphoinositides as well as Ca21 mobilization via PLCb, an effect which may accordingly be pertussis toxin-sensitive, -partially sensitive, or -insensitive (see Dubyak and el-Moatassim, 1993). P2Y2like receptor coupling to Gi proteins involves the bg Gi protein subunits, which stimulate phospholipase C-b2. IP3 formation, Ca21 mobilization, and a variety of signaling pathways including PKC, PLA2, Ca21-dependent K1 channels, and EDRF and EDHF formation. The specific downstream involvement of a given signaling pathway seems to be partially dependent on the cell type in which the P2Y2-like receptor is expressed. Activation of PLD and stimulation of phosphatidylcholine breakdown by P2Y2-like receptors has been reported (Purkiss and Boarder, 1992; Pfeilschifter and Merriweather, 1993; Balboa et al., 1994; Gerwins and Fredholm, 1995a,b). The mechanism of activation of PLD is unclear but may involve the combined actions of PKC, Ca21, and G proteins, as suggested for P2Y2-mediated pertussis toxin-insensitive activation of PLD in DDT1 MF2 cells (Gerwins and Fredholm, 1995b). As with the P2Y1-like receptor, protein tyrosine phosphorylation
and MAPK activation seems to be the major route for P2Y2-like receptor-mediated prostacyclin production in endothelial cells (Bowden et al., 1995; Patel et al., 1996). This occurs subsequent to activation of PKC and does not involve IP3 or cytosolic Ca21 (Patel et al., 1996). Stress-activated protein kinases, independent of PKC activation, have been shown to be activated by ATP and UTP in rat renal mesangial cells (Huwiler et al., 1997). Secondary to activation of PLC and mobilization of Ca21, the P2Y2-like receptor mediates the opening of Ca21-sensitive Cl2 channels in airway epithelia (Clarke and Boucher, 1992; Stutts et al., 1992), intrahepatic biliary epithelial cell lines (Wolkoff et al., 1995), and avian exocrine salt gland cells (Martin and Shuttleworth, 1995), which drives fluid secretion. Activation of P2Y2-like receptors stimulates cation and K1 currents via Ca21-dependent signaling mechanisms in HTC cells from a rat liver tumor cell line (Fitz and Sostman, 1994). UTP and ATP mediate depolarization of supraoptic neurosecretory cells in rat hypothalamus by the opening of a non-selective cation channel (Hiruma and Bourque, 1995). A P2Y2-like receptor has been shown to mediate inhibition of adenylate cyclase in some cells, although as shown in C6 –2B rat glioma cells, this may occur secondary to an increase in cytosolic free Ca21 (Munshi et al., 1993). Inhibition of cAMP accumulation by UTP and ATP at a P2Y2-like receptor in NCB-20 cells is accompanied by an elevation in intracellular Ca21 (Garritsen et al., 1992). A pertussis toxin-sensitive G protein mediates P2Y2-like inhibition of cAMP accumulation in cultured renal mesangial cells (Schulze-Lohoff et al., 1995). In the renal epithelial cell line, MDCK-D1 cells UTP and ATP mediate an increase in cAMP that is blocked by indomethacin identifying a cyclooxygenase-dependent mechanism; this suggests the involvement of PGE2 (Post et al., 1996). An increase in cGMP levels mediated by P2Y2-like receptors in mouse neuroblastoma 3 rat glioma hybrid cells occurs secondary to mobilization of intracellular Ca21 (Reiser, 1995). Inhibition of N-type calcium currents by P2Y2-like receptors expressed in sympathetic neurons has been reported (Filippov et al., 1997). P2Y2-like receptors are colocalized with P2Y1-like receptors on many cells and have a common signaling pathway in PLC. P2Y2-like responses are less sensitive to manipulations of the PKC pathway (Purkiss et al., 1994; Communi et al., 1995; Gallinaro et al., 1995; Chen et al., 1996a) (see also Section XII.B., on P2Y1 and P2Y1like receptor signal transduction mechanisms). C. Desensitization P2Y2 and endogenous P2Y2-like receptors do not readily desensitize. However, tachyphylaxis of a P2Y2like response has been reported in UMR-106 rat osteoblasts (Sistare et al., 1994), human term placental (trophoblastic) cells (Petit and Belisle, 1995), rat cultured
RECEPTORS FOR PURINES AND PYRIMIDINES
pituitary cells (gonadotropes) (Chen et al., 1994b, 1995b), C6 –2B rat glioma cells (Munshi et al., 1993), and in cultured endothelial cells (Motte et al., 1993a; Wilkinson et al., 1994; Nobles et al., 1995). Maximum desensitization of the P2Y2 receptor in mouse epithelial cells was observed at 5 to 10 min after UTP exposure, and full receptor responsiveness recovered at the same time after removal of agonist (Garrard et al., 1998). The mechanism of desensitization is not well understood, but as with many G protein-coupled receptors may involve phosphorylation of the intracellular regions of the receptor. The C terminal may be important because progressively larger truncations of this region of the P2Y2 receptor decreased the rate and magnitude of desensitization (Garrad et al., 1998). Plasticity of expression of the P2Y2 receptor during in vitro differentiation and inflammatory activation of HL-60 human promyelocytic leukocytes has been described (Martin et al., 1997a). When HL-60 cells differentiate into neutrophils, P2Y2 receptor mRNA levels and receptor function are largely preserved. In contrast, differentiation of HL-60 cells into monocytes/macrophages is associated with a complete loss of P2Y2 receptor-mediated function and a 10-fold reduction of P2Y2 mRNA levels; this suggests receptor down-regulation (Martin et al., 1997a). It was suggested that downregulation of the P2Y2-like receptor might be related to inflammatory activation rather than differentiation. D. Up-Regulation P2Y2-like receptor activity and P2Y2 receptor mRNA levels were increased in rat submandibular gland after ligation of the main excretory duct but not in the contralateral nonligated gland, indicating that changes in expression of the P2Y2 receptor may occur during pathological conditions (Turner et al., 1997). E. Agonists and Antagonists UTP and ATP are natural ligands at P2Y2 and P2Y2like receptors, and are approximately equipotent. 2MeSATP and a,b-meATP are weak or inactive, which provides useful negative evidence in the characterization of this receptor. UTPgS is equipotent with UTP and ATP at recombinant P2Y2 and endogenous P2Y2-like receptors, but has the advantage of being resistant to hydrolysis (Lazarowski et al., 1996). ATPgS has been shown to be an agonist at recombinant P2Y2 receptors, but is less potent than UTP and ATP (Lustig et al., 1993; Lazarowski et al., 1995). Ap4A is a potent agonist at recombinant P2Y2 receptors with a potency greater than ATPgS and is within the same range as UTP and ATP, raising the possibility that it is an endogenous regulator of these receptors (Lazarowski et al., 1995). It has been suggested that endogenous P2Y2-like receptors are preferentially activated by the fully ionized forms of ATP and UTP, ATP42, and UTP42 in bovine aortic endothelial cells (Lustig et al., 1992; Motte et al.,
1993b), human neutrophils (Walker et al., 1991), a cultured neuroblastoma-glioma hybrid cell line (NG108–15 cells) (Lin et al., 1993), rat lactotrophs (Carew et al., 1994), mouse pineal gland tumor cells (Suh et al., 1997), and MDCK cells (Yang et al., 1997). The UTP and ATP responses were shown to correlate with the concentration of the fully ionized form of these agonists and not with the concentration of their cation complexes or other ionized forms. Although both UTP and ATP are rapidly degraded and augmentation of responses in Mg21-free medium by ecto-nucleotidases must be considered, this seems not to be involved because potentiation of responses was also observed for the stable agonist ATPgS (Yang et al., 1997). Direct effects of cations on the receptor are also possible. There are no selective antagonists at P2Y2 and P2Y2like receptors. Suramin and PPADS are nonselective antagonists at subpopulations of P2Y2-like receptors (see Section XIII.F., Heterogeneity of P2Y2 and Endogenous P2Y2-Like Receptors). F. Heterogeneity of P2Y2 and Endogenous P2Y2-Like Receptors Endogenous P2Y2-like receptors show two phenotypes of response with respect to antagonism by suramin and PPADS. However, there is no molecular evidence to support a subdivision of P2Y2 receptors. The differences in sensitivities to antagonists do not correspond to species differences or to the apparent division according to differences in G protein coupling. Suramin-insensitive P2Y2-like receptors are those on bovine aortic endothelial cells (Wilkinson et al., 1994), rat duodenum muscularis mucosae (Johnson et al., 1996), rabbit aortic endothelium (Chinellato et al., 1994), and rat mesenteric arterial endothelium (Ziyal, 1997). PPADS-insensitivity is also reported for P2Y2-like receptors on rat mesenteric arterial endothelium (Ralevic and Burnstock, 1996a), as well as for P2Y2-like receptors on rat renal artery smooth muscle (Eltze and Ullrich, 1996) and bovine aortic endothelial cells (Brown et al., 1995). Suramin-sensitive endogenous P2Y2-like receptors include those on mouse C2C12 myotubes (Henning et al., 1992, 1993), rat pituitary gonadotrophs (Chen et al., 1994b), mouse cortical thick ascending limb segments (Paulais et al., 1995), rat lactotrophs (Carew et al., 1994), hamster mesenteric endothelium (Ziyal, 1997), rat PC12 cells (Murrin and Boarder, 1992), DDT MF-2 cells (Hoiting et al., 1990; Sipma et al., 1994), rat astrocytes (Ho et al., 1995), early embryonic chick neural retina (Sugioka et al., 1996; but also see Section XVII. on Endogenous Uridine Nucleotide-Specific Receptors), rat brain endothelial cells (Nobles et al., 1995), rabbit pulmonary artery endothelium and cultured smooth muscle cells (Qasabian et al., 1997), bovine pulmonary artery endothelium (Chen et al., 1996c), mouse mammary tumor epithelial cells (Enomoto et al., 1994), and mouse neuroblastoma and rat glioma hybrid cells (Reiser, 1995). PPADS is also an inhibitor of P2Y2-like receptors
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in mouse neuroblastoma and rat glioma hybrid cells (Reiser, 1995), as well as of P2Y2-like receptors in rat astrocytes (Ho et al., 1995). G. Distribution and Biological Effects P2Y2 and endogenous P2Y2-like receptors are widely distributed, but relatively little is known about their physiological significance. Particularly intriguing is the functional significance of a receptor that can be activated equally by purines and pyrimidines; to establish the physiological relevance of this it is important to know more about whether there are different sources or differential release of UTP and ATP. Some of these questions may be answered in the not too distant future as a result of the recent development of a radiometric assay based on the nucleotide specificity of UDP-glucose pyrophosphohydrolase, which is capable of detecting nanomolar concentrations of UTP (Lazarowski et al., 1997a). UTP has been shown to be released from endothelial cells by increased flow (Saiag et al., 1995) and is released from epithelial and astrocytoma cells by perturbation of the bathing medium (mechanical stimulation) (Enomoto et al., 1994; Lazarowski et al., 1997a). ATP is also released from these cells under these conditions, although whether its release is independent of that of UTP is unclear. UTP is stored in platelets (Goetz et al., 1971), which may be significant in modulation of vascular contractility during platelet aggregation in pathophysiological conditions. Northern blot analysis revealed distribution of P2Y2 receptor mRNA in spleen, testes, kidney, liver, lung, heart, and brain (Lustig et al., 1993; Parr et al., 1995). Alveolar type II cell P2Y2 receptor mRNA is expressed in rat heart, kidney, lung, spleen, and testis, but not in brain or liver (Rice et al., 1995). The P2Y2 receptor cloned from human osteoclastoma is expressed in osteoclastoma, bone, and osteoblasts (Bowler et al., 1995). P2Y2 receptor mRNA has been localized in primary cultures of rat aortic smooth muscle cells (Chang et al., 1995) and in cardiac myocytes and fibroblasts (Webb et al., 1996d). As shown in functional studies, receptors exhibiting the pharmacological properties of the P2Y2 receptor are present in a wide variety of cells and tissues including astrocytes, different types of blood cells, chromaffin cells, endothelial cells, epithelial cells, fibroblasts, glial cells, hepatocytes, keratinocytes, myocytes, osteoblasts, pancreatic b-cells, pheochromocytoma PC12 cells, pituitary cells, thyrocytes, and tumor cells (table 13). In the vasculature, P2Y2-like receptors are generally present on the endothelium where they stimulate the synthesis and release of prostacyclin and NO, leading to vasodilatation (Ralevic and Burnstock, 1991a, 1991b; 1996a, 1996b). Smooth muscle contraction mediated equipotently by UTP and ATP may indicate P2Y2-like receptors, although the G protein coupling of these receptors remains to be confirmed. These receptors have
been described in rat pulmonary vasculature (Rubino and Burnstock, 1996), rat renal vasculature (Eltze and Ullrich, 1996), bovine middle cerebral artery (Miyagi et al., 1996a), and rat duodenum (Johnson et al., 1996). Interestingly, Ca21-mobilizing P2Y2-like receptors described on cultured smooth muscle cells of rabbit pulmonary artery are not coupled to a functional response (Qasabian et al., 1997). A clue to their role may lie in the demonstration that P2Y2-like receptors mediate an increase in expression of immediate-early and delayedearly cell cycle-dependent genes in cultured aortic smooth muscle cells, in contrast with the induction only of immediate-early genes by 2MeSATP in the same cells (Malam-Souley et al., 1996). Enhanced leukocyte adherence to cultured pulmonary artery endothelial cells by P2Y2-like receptors has been shown (Dawicki et al., 1995). P2Y2 receptors on neutrophils stimulate degranulation, potentiate N-formyl-methionyl-leucyl-phenylalanine (FMLP)-induced superoxide formation, and induce aggregation (Kuroki et al., 1989; Seifert et al., 1989a,b; Walker et al., 1991). P2Y2like receptors on HL-60 cells mediate activation of NADPH oxidase and superoxide generation and mediate potentiation of FMLP-induced superoxide formation (Seifert et al., 1989a), while those on neutrophils and HL-60 cells induce chemotaxis and actin polymerization (Verghese et al., 1996). P2Y2-like receptors on gonadotrophs mediate the release of luteinizing hormone (Chen et al., 1995b). P2Y2-like receptors are Cl2 secretagogues in human nasal mucosa, probably via activation of Ca21dependent Cl2 channels (Mason et al., 1991; Stutts et al., 1992); this is an effect which has been explored for its potential in the pharmacological control of cystic fibrosis, a disease characterized by a failure to secrete Cl2 ions into the airway lumen leading to dehydration of airway secretions. Coupling of P2Y2-like receptors to catecholamine secretion in PC12 cells is controversial, having been reported by some researchers (Majid et al., 1993; Koizumi et al., 1995b), but not by others (Barry and Cheek, 1994; Nikodijevic et al., 1994; de Souza et al., 1995). It is intriguing that while there is no good evidence for UTP release as a neurotransmitter, it is able to modulate the release of other substance from nerves. It has been shown recently (Bogdanov et al., 1998) that, unlike the human P2Y4 receptor (see Section XV.), which is selective for UTP, the rat P2Y4 homolog is equisensitive to ATP and UTP; that is, in agonist profile it is identical with rat P2Y2. Therefore, it seems likely that the endogenous receptor called P2Y2-like in this section may be a P2Y2 or a P2Y4 receptor, at least where rat tissue is concerned. However, since there is a differential sensitivity to widely used antagonists, it should be possible to distinguish which receptor is operating in a particular tissue. In view of this new data, it is now clear that the former P2U receptor cannot be equated with a single P2Y subtype.
RECEPTORS FOR PURINES AND PYRIMIDINES
XIV. p2y3 Receptor
XVI. P2Y6 Receptor
This receptor has been cloned from chick brain and has nucleotide selectivity with a potency order of UDP . UTP . ADP . 2MeSATP . ATP (Webb et al., 1995, 1996a). The designation p2y3 reflects the current reservations expressed by the IUPHAR nomenclature committee about its inclusion as a distinct subtype within the P2Y receptor family because no mammalian homolog has yet been identified. It has been suggested that this may be the chick homolog of the mammalian P2Y6 receptor, with which it has 62% sequence homology, although this has not yet been confirmed. This receptor is activated by UDP, and to a lesser extent UTP and ADP, and couples to PLC. Its expression is rather restricted, being detected in spleen, spinal cord, kidney, and lung.
This uridine nucleotide-specific receptor has been cloned from rat aortic smooth muscle (Chang et al., 1995) and human placenta and spleen (Communi et al., 1996b). The receptor is activated most potently by UDP but weakly or not at all by UTP, ATP, ADP, or 2MeSATP (Communi et al., 1996b; Nicholas et al., 1996). Other diphosphonucleotides are full agonists at the receptor but have lower affinities. The response is pertussis toxin insensitive, indicating the involvement of Gq/11 proteins in stimulation of PLC and in the formation of IP3. Interestingly, the IP3 response of the human cloned P2Y6 receptor decays only slowly after stimulation, remaining above baseline for more than an hour after stimulation; this is a response that is fully reproducible without the need for a long recovery period (Robaye et al., 1997). P2Y6 mRNA is found abundantly in various rat tissues including placenta, thymus, lung, stomach, intestine, spleen, mesentery, heart, and aorta (Chang et al., 1995; Communi et al., 1996b). P2Y6, along with P2Y1 and P2Y2, but not P2Y4 mRNA, has been detected in adult rat cardiac myocytes (Webb et al., 1996d). It has been suggested that the P2Y6 receptor accounts for uridine nucleotide-specific responses in C6 –2B cells (Nicholas et al., 1996). A receptor activated by UDP in human nasal epithelial cells that is distinct from the P2Y2 receptor may be an endogenous P2Y6 receptor (Lazarowski et al., 1997b). The receptor promotes [3H]inositol phosphate accumulation and an increase in [Ca21]i and Cl2 secretion, is present on the mucosal but not on the serosal surface, and desensitizes more readily than responses to UTP (Lazarowski et al., 1997b). Interestingly, a uridine nucleotide-specific receptor responding to UDP in Caco-2 human intestinal epithelial cells seems to be located on the apical but not on the basolateral membrane (Inoue et al., 1997). The more widespread distribution of the P2Y6 receptor, compared with the P2Y4 receptor, suggests that this receptor is more likely to account for endogenous uridine nucleotide-specific responses.
XV. P2Y4 Receptor This uridine nucleotide-specific receptor has been cloned from human placenta (Communi et al., 1996c), human chromosome X (Nguyen et al., 1996), and rat heart (Bogdanov et al., 1998). The human P2Y4 receptor is highly selective for UTP over ATP and is not activated by nucleoside diphosphates. ATP can act as an antagonist and partial agonist. The human P2Y4 receptor seems to couple to two distinct G proteins: a Gi protein at the early stage and a Gq/11 protein at a later stage of signaling to activate PLC and IP3 formation (Communi et al., 1996a). The IP3 response declines within minutes of stimulation of the receptor and is not readily reproducible, indicating desensitization (Robaye et al., 1997). The human P2Y4 receptor is not blocked by suramin, but has been reported to be both blocked by PPADS (IC50 approximately 15 mM) (Communi et al., 1996a) and to be relatively insensitive to block by PPADS (used at 30 mM) (Charlton et al., 1996b). P2Y4 has a restricted distribution; it is expressed almost exclusively in placenta with low levels of expression in lung, and absent in most other tissues. A P2Y4 receptor (initially termed P2P) has been described in rat pancreas (Stam et al., 1996). P2Y4 mRNA (and P2Y2 mRNA, as well as barely detectable levels of P2Y6 mRNA) has been detected in vascular smooth muscle (Erlinge et al., 1998). The recent cloning of a rat P2Y4 receptor has shown that the recombinant receptor is activated equipotently by ATP and UTP (ADP, ATPgS, 2MeSATP, and Ap4A are also equipotent, but are partial agonists) (Bogdanov et al., 1998). Clearly, with respect to ATP and UTP sensitivity, this is identical with the profile described for the P2Y2 receptor. Important implications arising from this are that some P2Y2-like responses may be mediated by a P2Y4 receptor, at least in rat tissues, and that the P2U receptor cannot be equated with a single P2Y subtype.
XVII. P2Y11 Receptor The P2Y11 receptor was cloned from human placenta (Communi et al., 1997). The receptor has 33% amino acid identity with the P2Y1 receptor, its closest homolog, and 28% homology with the P2Y2 receptor. The receptor couples to the stimulation of both the phosphoinositide and the adenylyl cyclase pathways; in this respect, it is unique among the P2Y family. Interestingly, this receptor seems to be the only P2Y receptor selective for ATP because it is stimulated by agonists with a rank order of potency of ATP . 2MeSATP ... ADP, with UTP and UDP inactive (Communi et al., 1997). Northern blot analysis detected mRNA corresponding to the P2Y11 receptor in spleen and HL-60 cells (Communi et al., 1997).
RALEVIC AND BURNSTOCK TABLE 13 Functional distribution of P2Y receptors
Rice and Singleton, 1987; Rice et al., 1995 Pearce and Langley, 1994; Salter and Hicks, 1994; Ho et al., 1995; Chen and Chen, 1996
Boyer et al., 1989, 1994 Shi et al., 1995
Osipchuk and Cahalan, 1992; Qian and McCloskey, 1993 Biffen and Alexander, 1994 Greenberg et al., 1988; Nuttle et al., 1993; Lin and Lee, 1996 Vittet et al., 1992; Uneyama et al., 1994 Fan and McCloskey, 1994 Yamaguchi et al., 1994 Zhang et al., 1996 Hourani et al., 1992; Hall and Hourani, 1993; Hechler et al., 1998; Fagura et al., 1998; Daniel et al., 1998; Jin, et al., 1998 Iredale and Hill, 1993 Kaplan et al., 1996 Reichsman et al., 1995
— Yes Yes — Yes
— — — Yes —
Yes — — — Yes
— — — — —
Yes — Yes
Yes Yes Yes
— — —
— — —
— — Yesd
Yes Yes Yes
— — —
— — —
Chan et al., 1996 Yu and Turner, 1991 Motte et al., 1993a,b; Briner and Kern, 1994; Purkiss et al., 1993, 1994; Wilkinson et al., 1994; Communi et al., 1995; Nobles et al., 1995; Miyagi et al., 1996b; Ralevic and Burnstock, 1996a,b; Ralevic et al., 1991b, 1997; Simonsen et al., 1997 Yang et al., 1996
Epithelium Intestinal, apical; human Intestinal, basolateral; human Intrahepatic biliary; human Mammary tumour; mouse Mammary tumour; human MDCK cells; canine
— Yes — — Yes Yes
Yes Yes Yes Yes Yes Yes
— — — — — —
Yes — —
Nasal mucosa; human Ocular ciliary; human Otocyst; embryonic chick Pancreatic; human cystic fibrosis Retinal pigment epithelium Tracheal; hamster Tracheal; rabbit Thymic; rat Submandibular salivary; mouse Sweat gland; equine Fibroblasts
— — Yes — — — Yes ?h Yes Yes —
?f Yes — Yes Yesg Yes Yes ?h Yes Yes Yes
— — — — — — — — — — —
Yes — — — — — — ?h — — —
— Yes Yes Yes —
Yes — — — Yes
— — — — —
— — Yes — —
Kimball and Mulholland, 1996 Kirischuk et al., 1995b No¨renberg et al., 1997 Kirischuk et al., 1995a Kirischuk et al., 1995a
Boyer et al., 1994, 1995, 1996; Munshi et al., 1993; Lin and Chuang, 1994; Nicholas et al., 1996; Schachter et al., 1996 Lin et al., 1993; Filippov et al., 1994; Reiser et al., 1995 Abdullah et al., 1996 Charest et al., 1985; Keppens and DeWulf, 1991; Keppens et al., 1992; Dixon et al., 1995 Pillai and Bikle, 1992
— Yes —
Yes — Yes
— — —
— — —
Paulais et al., 1995 Cha et al., 1995 Ecelbarger et al., 1994
Huwiler and Pfeilschifter, 1994; Schulze-Lohoff et al., 1992, 1995; Takeda et al., 1996
Inoue et al., 1997 Inoue et al., 1997 Wolkoff et al., 1995 Enomoto et al., 1994 Flezar and Heisler, 1993 Zegarra-Moran et al., 1995; Firestein et al., 1996; Yang et al., 1997 Lazarowski et al., 1997b Wax et al., 1993 Nakaoka and Yamashita, 1995 Chan et al., 1996; Montserrat et al., 1996 Peterson et al., 1997 Abdullah et al., 1996; Kim et al., 1996 Aksoy et al., 1995 Liu et al., 1995 Gibb et al., 1994 Ko et al., 1994 Fine et al., 1989; Gonzalez et al., 1989b,c; Marsault et al., 1992; Grierson and Meldolesi, 1995a,b
RECEPTORS FOR PURINES AND PYRIMIDINES TABLE 13 (Continued) P2Y1-likea
Myocytes Cardiac Gastrointestinal Vascular
Yes Yes —
— Yes Yes
— — —
— — —
Ovarian granulosa cells Human Porcine Ovarian CHO cells Pancreatic b cells Pheochromocytoma PC12 cells
— Yesd Yes Yes Yes
Yes Yes Yes — Yes
— —j — — —
— — — — —
Kamada et al., 1994; Lee et al., 1996 Kamada et al., 1994 Iredale and Hill, 1993 Bertrand et al., 1987; Hillaire-Buys et al., 1994 Murrin and Boarder, 1992; Majid et al., 1992, 1993; Barry and Cheek, 1994; Nikodijevic et al., 1994; de Souza et al., 1995; Koizumi et al., 1995b
Pituitary cells Gonadotrophs Lactotrophs Salt gland cells Schwann cells
— — Yes Yes
Yes Yes Yes Yes
— — — —
— — — —
Chen et al., 1994b, 1995b Carew et al., 1994 Martin and Shuttleworth, 1995 Berti Mattera et al., 1996; Ansselin et al., 1997; Green et al., 1997
Smooth muscle Gastrointestinal Vascular
Johnson et al., 1996 Kennedy and Burnstock, 1985; Mathieson and Burnstock, 1985; Burnstock and Warland, 1987; Liu et al., 1989; Brizzolara and Burnstock, 1991; Keef et al., 1992; Corr and Burnstock, 1994; Simonsen et al., 1997 Eltze and Ullrich, 1996; Miyagi et al., 1996a; MalamSouley et al., 1996; Rubino and Burnstock, 1996; Qasabian et al., 1997 Von Ku¨gelgen et al., 1987, 1990; Saiag et al., 1990, 1992; Ralevic and Burnstock, 1991b; Juul et al., 1992; Lagaud et al., 1996; Matsumoto et al., 1997 Scho¨fl et al., 1995 Petit and Belisle, 1995
— — Yes
Yes Yes —
— — —
— — —
Dubyak and De Young, 1985 Fitz and Sostman, 1994 Kumagai et al., 1991
Qu et al., 1993; Scamps and Vassort, 1994 Blottie`re et al., 1996; Pacaud et al., 1996 Erlinge et al., 1995; Pacaud et al., 1995; Guibert et al., 1996; Malam-Souley et al., 1996; Strøbæk et al., 1996; Qasabian et al., 1997 Bowler et al., 1992; Sistare et al., 1994, 1995; Reimer and Dixon, 1992; Gallinaro et al., 1995; Dixon et al., 1997b
a P2Y1-like, P2Y receptors other than P2Y2, P2Y4, P2Y6, P2YADP, and endogenous uridine nucleotide-specific receptors; probably P2Y1 receptors (based on sensitivities to 2MeSATP and/or ADP, and signalling pathways), although other P2Y subtypes cannot be excluded. b P2Y2-like, activated by ATP 5 UTP suggesting a possible identity as P2Y2 receptors, although at least in rat tissues a P2Y4 subtype identity cannot be excluded (as rat P2Y4 receptors are activated by ATP 5 UTP). The possible presence of uridine nucleotide-specific receptors cannot be excluded in tissues responding to UTP. c ADP-specific P2Y receptors, activated by ADP but not by ATP. d Denotes ADP-specific P2Y receptors (ATP weak or inactive); note that this is also the agonist profile of P2YADP receptors. e These may be the same P2Y1-like receptor. f The response to UTP was distinct from that to UDP, but it is not clear whether this is via actions at a P2Y2- or P2Y4-like receptor. g UTP was five-fold more potent than ATP, thus uridine-nucleotide-specific receptors are possible. h Subtype(s) not clear: stimulation of PGE2 production by ATPgS $ UTP . ATP. i P2Y6 (Nicholas et al., 1996). j P2YADP receptors have been described; however, it is likely that these are ADP-specific P2Y receptors.
XVIII. Endogenous Uridine Nucleotide-Specific Receptors The inclusion of this as a separate section is a reflection of the current lack of information about the correlation between cloned (P2Y4 and P2Y6) and endogenous uridine nucleotide-specific receptors. It is not intended to imply that these receptors are different, although this is a possibility. The existence of P2Y2, P2Y4, and P2Y6 receptors identifies two receptors that can be activated by UTP (P2Y2, P2Y4) and one that can be activated by UDP (P2Y6). Thus, it is not always clear which of these receptors mediates uridine nucleotide-mediated responses in cells and tissues. Additional complications are introduced by the coexistence of P2 receptors, the lack of selective agonists and antagonists, and the inter-
conversion and degradation of agonists leading to contamination of solutions and to the possibility of obtaining false positive as well as negative results. With hindsight, some characterization of endogenous uridine nucleotide-specific responses in many tissues might have been achieved by more complete information on agonist activity profiles, specifically giving information about their UTP/UDP selectivity. It would be worthwhile to re-evaluate the pharmacological profile of biological tissues in light of new information on these P2Y receptors. A. Signal Transduction Mechanisms A uridine nucleotide-specific receptor in C6 –2B rat glioma cells mediates pertussis toxin-sensitive activa-
RALEVIC AND BURNSTOCK
tion of PLC and an increase in IP3 by UTP and UDP, but is not activated by ATP and ADP (Lazarowski and Harden, 1994). The uridine nucleotide-specific receptor in RAW 264.7 macrophages is coupled to pertussis toxinsensitive and -insensitive G proteins that mediate activation of phospholipase A2 (PLA2) and PLC, respectively (Lin and Lee, 1996). B. Agonists and Antagonists Uridine nucleotide-specific receptors are activated by UTP and/or UDP, but are not activated or only weakly activated by ATP, ADP, 2MeSATP, and a,b-meATP. There are no selective antagonists at uridine nucleotidespecific receptors. In general, responses are insensitive to P2 receptor antagonists. However, suramin and reactive blue 2 have been reported to block the UTP-specific inositol phosphate response of RAW 264.7 macrophages (Lin and Lee, 1996). C. Distribution and Biological Effects Uridine nucleotide-specific receptors, suggested to be P2Y6 receptors, have been described on C6 –2B cells where they coexist with P2Y1-like and P2Y2-like receptors (Boyer et al., 1993). Uridine nucleotide-specific receptors are also found on macrophages (Lin and Lee, 1996) and microglial cells (No¨renberg et al., 1997a). They have been shown to mediate metabolic effects, membrane ion fluxes, and hemodynamic effects in perfused rat liver (Haussinger et al., 1987). Uridine nucleotide-specific receptors mediating Cl2 secretion on human nasal mucosal (Lazarowski et al., 1997b) and intestinal epithelial cells (Inoue et al., 1997) are activated by UDP, perhaps indicating that these are P2Y6 receptors. Uridine nucleotide-specific receptors are found on vascular endothelium and smooth muscle. A pertussis toxin-sensitive uridine nucleotide-specific receptor coexists with P2Y2-like and P2Y1-like receptors on guinea-pig cardiac endothelial cells (Yang et al., 1996). Uridine nucleotide-specific receptors mediating contractile responses to UTP (but not to ATP) have been described on vascular smooth muscle (Von Ku¨gelgen et al., 1987, 1990; Saiag et al., 1990, 1992; Ralevic and Burnstock, 1991b; Juul et al., 1992; Lagaud et al., 1996). These receptors are resistant to desensitization by a,b-meATP and/or do not show cross-tachyphylaxis with responses to ATP and/or are unaffected by antagonists including PPADS and suramin. It is possible that these correspond to human P2Y4 receptors. In canine epicardial coronary arteries, vasoconstriction mediated by UTP and UDP at P2Y receptors does not cross-desensitize and is distinct from vasoconstriction mediated by ATP (Matsumoto et al., 1997); this suggests effects mediated at uridine nucleotide-specific receptors similar or identical with human P2Y4 and P2Y6 receptors, respectively. A uridine nucleotide-specific receptor has been described in neurons of the rat superior cervical ganglion
(SCG) (Boehm et al., 1995; Connolly, 1995; Connolly and Harrison, 1995a, b). This receptor is activated by UTP and UDP but not by ATP, causing depolarization and transmitter release. Suramin does not block this SCG receptor (Connolly and Harrison, 1995b). The approximately 5-fold greater potency of UTP, compared with ATP in elevating intracellular Ca21 in early embryonic chick neural retina, may suggest the involvement of a uridine nucleotide-specific receptor, although the authors of this study conclude that a P2Y2like (P2U) receptor is involved (Sugioka et al., 1996). It is also possible that a combination of coexpressed P2Y receptors mediate this response. The biological significance of uridine nucleotide-specific receptors is unknown, but may imply differential release of purines and pyrimidines. XVIV. P2YADP (or P2T) Receptor The P2YADP (or P2T) receptor is activated by ADP, whereas ATP is a competitive antagonist. Because this receptor has not yet been cloned from the platelets or megakaryoblastic cells in which it is expressed, the recommendation of the IUPHAR committee is that the name of this receptor is written in italics. It has been suggested that the P2YADP receptor is equivalent to the P2Y1 receptor based on their similar pharmacological profiles and the fact that P2Y1 receptor mRNA is present in platelets and megakaryoblastic cells lines (Le´on et al., 1997). Although this seemed an attractive hypothesis with which to explain the enigma of the P2YADP (or P2T) receptor, there is now convincing pharmacological evidence that the P2YADP (or P2T receptor) is not equivalent to the P2Y1 receptor; both of these receptors are expressed on platelets and cooperate to mediate platelet shape change and aggregation (Daniel et al., 1998; Fagura et al., 1998; Hechler et al., 1998; Jin et al., 1998). Notably, 2MeSATP is a full and potent agonist at the recombinant P2Y1 receptor, whereas it is a noncompetitive antagonist at the P2YADP (or P2T) receptor, and selective antagonists of the P2Y1 receptor do not block ADP-induced inhibition of adenylate cyclase in platelets. A. Signal Transduction Mechanisms The P2YADP (or P2T) receptor couples to a Gi2 protein to mediate inhibition of adenylate cyclase activity (Hall and Hourani, 1993; Hourani and Hall, 1996). Conflicting reports that the P2YADP (or P2T) receptor may or may not also activate PLC, generating IP3 and elevating levels of intracellular Ca21, most likely came from observed effects of ADP at coexisting platelet P2Y1 receptors. Platelet P2Y1 receptors coupled to activation of PLC are now known to play a significant role in platelet shape change and cooperative aggregation with P2YADP (or P2T) receptors (Daniel et al., 1998; Hechler et al., 1998; Jin et al., 1998).
RECEPTORS FOR PURINES AND PYRIMIDINES
In platelets activated by ADP, rapid influx of extracellular Ca21 forms a significant component of the increase in intracellular Ca21. A component of this Ca21 influx seems to be caused by ADP actions on platelet P2X1-like receptors (coexisting with P2YADP and P2Y1 receptors) causing the opening of these nonselective cation channels (Soslau et al., 1995; MacKenzie et al., 1996) (also see Section IX.F.). Platelet aggregation seems to be mediated by a combination of the above pathways stimulated by P2YADP (or P2T receptor), P2Y1-like, and P2X1like receptor activation. B. Desensitization Homologous desensitization of the P2YADP (or P2T) response has been observed in human erythroleukemic cells (Shi et al., 1995). C. Agonists ADP is the archetypal agonist at P2YADP receptors. The analogs 2-chloroADP and 2-MeSADP are more potent agonists at P2YADP receptors than ADP, and ADPaS and ADPbS are partial agonists (Hall and Hourani, 1993; Hourani and Hall, 1996). D. Antagonists FPL 66096 (2-propylthio-d-b,g-difluoromethylene ATP) (pA2 8.7) (Humphries et al., 1994) and ARL 67085 (formerly FPL 67085) (2-propylthio-b,g-dichloromethylene-d-ATP) (Humphries et al., 1995) are potent and selective competitive antagonists at platelet P2YADP receptors. ATP is a competitive antagonist, with the preferred form being ATP42. The competitive effects of ATP at the P2YADP receptor may be physiologically meaningful because degradation to ADP by platelet ecto-ATPase is slow (Beukers et al., 1993). 2Cl-ATP, b,g-meATP, Ap4A, Ap5A, and P1,P6-diadenosine hexaphosphate (Ap6A) are also competitive antagonists; 2MeSATP and adenosine are non-competitive antagonists at platelet P2YADP receptors (Harrison et al., 1975; Ogilvie, 1992; Hall and Hourani, 1993). At high concentrations, Ap3A has antithrombotic effects at the P2YADP receptor. This is in contrast with its pro-thrombotic effects at low concentrations (Ogilvie, 1992), although breakdown to ADP and adenosine may be involved. Ap4A, Ap5A, and Ap6A also inhibit ADP-induced platelet aggregation, probably by competitive interaction with the P2YADP receptor (Ogilvie et al., 1996). a,b-meATP and UTP are weak inhibitors of platelet aggregation (Hall and Hourani, 1993). Suramin is a non-selective antagonist at the P2YADP receptor (Hourani et al., 1992; Hall and Hourani, 1993). E. Distribution and Biological Effects The distribution of the P2YADP receptor seems to be limited to platelets and megakaryoblastic cell lines (Vittet et al., 1992; Shi et al., 1995). The lack of subtype-
specific agonists and antagonists apparently has led to erroneous descriptions of P2T (P2YADP) receptors on a number of other cell types including osteoblasts (Sistare et al., 1994, 1995) and porcine ovarian granulosa cells (Kamada et al., 1994); it is likely that these are in fact ADP-specific P2Y1-like receptors. The P2 receptor described in porcine ovarian granulosa cells, where ATP is a competitive antagonist of ADP-induced [Ca21]i mobilization (Kamada et al., 1994), may be an ADP-specific P2Y1-like receptor, where ATP is a partial agonist. A role for the platelet P2YADP receptor has been clearly defined; it mediates the aggregation of platelets to ADP during thrombosis (Born, 1962; Born and Kratzer, 1984). One source of ADP activating the P2YADP receptor may be that derived from ATP released from damaged cells in the vessel wall. The dense granules of platelets are themselves sources of high concentrations of ATP and ADP (approximately 1 M) such that platelet aggregation and degranulation leading to the release of these nucleotides is an autocatalytic process. The adenine dinucleotides Ap3A and Ap4A are co-stored with ADP and ATP in platelets and comprise up to 5% of the total adenine nucleotide content of the dense granules (micromolar to millimolar concentrations) (Flodgaard and Klenow, 1982; Luthje and Ogilvie, 1983; Schluter et al., 1994); they are less rapidly metabolized than ATP and may have a role in the platelet aggregatory response. Complex and cooperative signaling pathways mediated by coexisting P2YADP, P2Y1, and P2X1 receptors seem to underlie the change in platelet shape, platelet aggregation, and secretion of dense granules to ADP. The P2Y1 receptor seems to be necessary to trigger platelet shape change and aggregation (Daniel et al., 1998; Hechler et al., 1998; Jin et al., 1998). The P2X1like receptor mediates an initial rapid influx of Ca21 in platelets (MacKenzie et al., 1996), which may also contribute to initiate the change in platelet shape. This Ca21 influx precedes, but is independent of, the mobilization of intracellular Ca21 by the P2YADP receptor (Hallam and Rink, 1985; Sage et al., 1990). Mobilization of intracellular Ca21 and adenylate cyclase by the P2YADP receptor seems to be linked to platelet aggregation and cooperates with effects mediated by the P2Y1 receptor, such that antagonism of either receptor is sufficient to block the response. Oscillations in [Ca21]i have been described, which seem to involve the repetitive emptying and refilling of intracellular calcium stores. The mobilization of [Ca21]i seems to be required for activation of a secondary phase of Ca21 influx (Sage et al., 1990). XX. Other P2Y Receptors The following G protein-coupled receptors have been cloned and proposed as members of the P2Y receptor family. Of these, the p2y5, p2y7, p2y9, and p2y10 receptors have now been shown unequivocally not to belong to the P2Y receptor family, and the inclusion of the Xeno-
RALEVIC AND BURNSTOCK
pus P2Y receptor (P2Y8) does not seem likely as it lacks a mammalian homologue. A. p2y5 Receptor A receptor expressed in activated chicken T lymphocytes was proposed as a P2Y receptor based on nucleotide binding assays (Webb et al., 1996b). No functional evaluation was provided. When the turkey homolog was expressed in 1321N1 human astrocytoma cells, it was shown that no signaling responses were evoked by nucleotides; this indicates that the receptor is not a member of the P2Y receptor family (Li et al., 1997c). It was noted that caution should be used when interpreting the results of binding assays in the absence of robust ligands and that a prerequisite for the identification of additional P2Y receptors should be a functional demonstration of signaling responses in an appropriate cell line (Li et al., 1997c). B. p2y7/Leukotriene B4 Receptor It was suggested that a receptor cloned from human HEL cells was a P2Y7 receptor based on binding and activation by purine nucleotides when transfected in COS-7 cells (Akbar et al., 1996). However, its structure, which was noted to share 30% or less homology with other cloned P2Y receptors, has been found to be identical with that of the leukotriene B4 receptor cloned from HL-60 cells, and sensitivity to purines can be explained by intrinsic purinoceptors (P2Y2) in COS-7 cells (Yokomizo et al., 1997). Expression of the putative P2Y7 receptor in 1321N1 human astrocytoma cells has confirmed that this receptor is not activated by nucleotides and is not a member of the P2Y receptor family (Herold et al., 1997). C. Xenopus P2Y Receptor (P2Y8) A P2Y receptor cloned from Xenopus neural plate is activated equipotently by purine and pyrimidine compounds with three phosphates; ATP 5 UTP 5 ITP 5 CTP 5 GTP (Bogdanov et al., 1997). The cloned receptor has a particularly long C terminal of 216 amino acids (compared with approximately 16 to 67 amino acids of other P2Y receptors) that contributes to the greater length of this protein. It has been suggested that this receptor may have a role in early development of the nervous system. The receptor was tentatively named P2Y8. As a mammalian homolog of this receptor has not been identified, its inclusion as a distinct subtype of the P2Y receptor family does not seem likely. D. P2Y9 and P2Y10 Receptors These cloned receptors, submitted to Genbank, are not nucleotide receptors. E. P2YAp4A (or P2D) Receptor It has been proposed that there is a distinct class of purine receptor, originally termed P2D (“D” for dinucle-
otide), which has high affinity for the diadenosine polyphosphates (Pintor et al., 1993). This receptor has not yet been cloned and thus has been given the tentative name P2YAp4A. It is possible that this receptor belongs to the P2Y receptor superfamily because it seems to couple to G proteins. In rat brain synaptosomes, [3H]Ap4A and [3H]ADPbS bind to high and low affinity binding sites (Pintor et al., 1993). The high affinity binding sites display an agonist potency profile that is inconsistent with that of any known subtype of P2 receptor: Ap4A . ADPbS . b,gmeATP . a,b-meATP .. 2MeSATP. In rat hippocampal slices, Ap4A and Ap5A activate PKC (Klishin et al., 1994), which suggests the coupling of the putative P2YAp4A (or P2D) receptor to G proteins. However, inhibition of synaptic transmission by diadenosine polyphosphates in hippocampal slices could be inhibited by adenosine receptor antagonists (Klishin et al., 1994). So far, this receptor has been described only in the CNS (Pintor et al., 1993; Klishin et al., 1994). F. P3 Receptor A distinct P3 receptor that is activated by both nucleosides and nucleotides, and is antagonized by both xanthines and a,b-meATP, has been proposed (Shinozuka et al., 1988; Forsythe et al., 1991). In aiming toward a unifying system of purine and pyrimidine receptor nomenclature, this receptor may need to be renamed according to the new system of purine receptor classification when further information on its structure, signal transduction mechanisms, and pharmacological profile become available. Responses mediated by ATP at the P3 receptor are independent of its breakdown to adenosine, and stable analogs of ATP are also agonists. In some respects this receptor is similar to those P1 receptors which bind ATP and its analogs (Bailey and Hourani, 1990; Hourani et al., 1991; Von Ku¨gelgen et al., 1992; King et al., 1996a; Piper and Hollingsworth, 1996). In general, the P3 receptor is prejunctional. It is activated by agonists with a potency order of 2Cl-adenosine . b,g-meATP . ATP 5 adenosine, as determined for inhibition of evoked release of NA from sympathetic nerves in rat tail artery (Shinozuka et al., 1988). This receptor has also been described in rat vas deferens, and UTP was additionally shown to inhibit NA overflow (Forsythe et al., 1991). A receptor activated by adenosine and ATP, which is blocked by a,b-meATP, mediates outward K1 currents, and has been identified as a novel P1 receptor, may be equivalent to the P3 receptor (King et al., 1996a). Facilitation by ATP and adenosine of evoked NA release has been shown in some vascular smooth muscle. These effects are blocked by a,b-meATP and 8-SPT, but a,b-meATP is ineffective as an agonist (Miyahara and Suzuki, 1987; Zhang et al., 1989; Todorov et al., 1994; Ishii et al., 1995); it has been suggested that this may
RECEPTORS FOR PURINES AND PYRIMIDINES
represent a subtype of the P3 receptor (Dalziel and Westfall, 1994). Another distinct P3 receptor has been proposed in smooth muscle of rabbit thoracic aorta; it is activated by both adenosine and ATP, but is xanthine- and suramininsensitive (Chinellato et al., 1994). G. P4/Diadenosine Polyphosphate-Specific Receptor A novel receptor for diadenosine polyphosphates, distinct from the P2YAp4A (or P2D) receptor, has been proposed based on a study in rat brain synaptosomes (Pintor and Miras-Portugal, 1995a). Because this receptor is not activated by ATP, the term P4 has been suggested. Increases in synaptosomal Ca21 elicited by Ap4A and Ap5A were not blocked with suramin and methylxanthines, in contrast with the increases in Ca21 evoked by ATP, a,b-meATP, and ADPbS. Furthermore, the actions of Ap4A and ATP did not cross-desensitize, although there was homologous desensitization to Ap5A. It has been suggested that this receptor may be an ion channel, or is coupled to a Ca21 channel (Pintor and Miras-Portugal, 1995a). This receptor has not been cloned and its existence as a distinct subtype is controversial. The synthesis of diinosine polyphosphates as antagonists with some selectivity for the effects of Ap5A in rat brain synaptosomes versus the effects mediated by ATP may prove useful in the characterization of dinucleotide receptors (Pintor et al., 1998). XXI. Integrated Effects of P2 Receptors Many cells express more than one type of P2 receptor. The biological significance of this is not entirely clear but allows potential regulation of multiple effectors, fine tuning of agonist-evoked responses, and/or synergy. The quite different specificities of many P2 receptors for endogenous agonists suggest that the source and local concentration of ADP, ATP, UDP, UTP, and adenine dinucleotides may be important; more detailed information on this might provide some insight into the biological significance of P2 receptor coexistence. A number of cells seem to express more than one type of P2Y receptor: for example, P2Y1- and P2Y2-like receptors are expressed on cortical astrocytes, osteoblasts, hepatocytes, endothelial, and epithelial cells; P2Y1- and P2Y2-like and uridine nucleotide-specific receptors are expressed on cardiac endothelial cells (Yang et al., 1996) (also see table 13). These receptors typically have a common signaling pathway in PLC, and downstream divergence at subsequent steps of this pathway may be important. Synergism does not seem to occur. Differential expression and coexpression of receptors among similar cells has been shown for P2Y1-like and P2Y2-like receptors on individual cultured human osteoblasts (Dixon et al., 1997b) and for astrocytes from the dorsal spinal cord of the rat (Ho et al., 1995). Coexpression may also differ among tissues: functional studies suggest that hamster mesenteric arteries have predom-
inantly P2Y2-like receptors and few P2Y1-like receptors (Ralevic and Burnstock, 1996b), whereas the converse seems to be true for piglet aorta (Martin et al., 1985) and lamb small coronary arteries (Simonsen et al., 1997) where UTP is a very weak agonist. However, it is possible that these receptors are expressed but are not coupled to a vasomotor response. The physiological significance of the differential expression of P2Y receptors at the level of single cells and tissues remains to be determined. P2X1-like and P2Y1-like receptors coexist on the smooth muscle in some vessels; they may reciprocally control vascular tone by acting as mediators of vasoconstriction and vasodilatation, respectively. This may occur following release of ATP from the terminals of perivascular sympathetic and sensory nerves, respectively. Cooperative effects have been shown for coexisting P2X1-like, P2Y1–like, and P2YADP (or P2T) receptors on platelets, which mediate ionotropic Ca21 influx and mobilization of intracellular Ca21, respectively, to bring about changes in platelet shape and aggregation (Hourani and Hall, 1996; MacKenzie et al., 1996; Daniel et al., 1998; Hechler et al., 1998; Jin et al., 1998). P2Y2- and P2X7-like receptors coexist on macrophages, although the functional significance of this, if any, remains to be determined. Receptor expression may be regulated differently under different physiological and pathophysiological conditions, thereby altering patterns of coexpression. Expression of P2 receptors on mononuclear phagocytes is regulated differently by proinflammatory cytokines, which cause rapid down-regulation of P2X1-like and P2Y2-like receptors, but concomittant massive up-regulation of P2X7-like receptors (Dubyak et al., 1996). There also is differential functional expression during development; P2Y1-like receptors are expressed only in early myeloid progenitor cells, whereas P2Y2-like receptors are expressed in late stage progenitor cells, and mature monocytes and neutrophils (Dubyak et al., 1996; Martin et al., 1997a). Integrated effects of P2 receptors in whole tissues are considered in the next section. XXII. Integrated Effects of Adenosine/P1 and P2 Receptors P1/P2 receptor coexistence has been identified for many cell types; these include P2X7-, A2A-, A2B-, and A3-like receptors on mast cells; P2Y1, P2Y2, A2A, and A2B receptors on endothelial cells; A1 and P2X1-like receptors on smooth muscle cells; and A2A, A2B, P2Y2-, and P2X2-like receptors on PC12 cells. The functional significance of this is not entirely clear. Among other possible interactions there may be reciprocal effects, as shown for A1 receptor-mediated inhibition and P2Y1-like receptormediated stimulation of insulin secretion in pancreatic b-cells (Hillaire-Buys et al., 1989, 1993, 1994). Activation of A2A receptors inhibits ATP-induced Ca21 influx
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via P2X receptors in PC12 cells (Park et al., 1997), indicating antagonistic interplay between these systems. Integration of purine receptor-mediated responses at the level of whole tissues is illustrated by purinergic control of blood vessel tone, which involves vasoconstrictor P2X1-like and uridine nucleotide-specific receptors on vascular smooth muscle, vasodilator P2Y1-like, P2Y2like, A2A, and A2B receptors found on smooth muscle and endothelium, and prejunctional A1 receptors that modulate the release of neurotransmitter from perivascular nerves (fig. 12). Normal patterns of purinergic signaling may alter dramatically under pathophysiological conditions. The net effect of purine receptors may be vasodilatation if endothelial cells are intact, but vasoconstriction will predominate if the endothelium is damaged. When endothelial cells are damaged, collagen is exposed. Platelets adhere to the collagen and release ADP, ATP, UTP, and adenine dinucleotides, together with other substances such as 5-HT. Several substances promote further aggregation via activation of platelet P2X1-like, P2Y1-like, and P2YADP receptors. Purines and pyrimidines released from platelets can also act on endothelial and/or vascular smooth muscle cell P2 receptors. In an inflammatory reaction, ATP may be released from sensory nerves to have effects on mast cell P2X7-like receptors, although its breakdown product adenosine may activate coexisting mast cell A3 receptors, leading to further effects on vascular tone after release of mast cell mediators. Understanding how responses mediated by purine receptors are integrated in biological systems depends on information on the sources of the natural agonists, as well as on the receptor signaling pathways. In addition, the metabolic relationship between purines, whereby extracellular ATP is rapidly catabolized to ADP and adenosine has important implications for colocalized adenosine/P1 and P2 receptors as there may be an interplay between these receptors. Notably, many of the above studies are concerned with short-term interactions between coexisting purine receptors, which represents only one aspect of purine and pyrimidine receptor signaling. Particularly for metabotropic G protein-coupled receptors, long-term trophic interactions are likely to be important (Cowen et al., 1991; Abbracchio et al., 1995b; Neary, 1996) and may lead to further insights into the significance of P1/P2 receptor coexistence and the cross-talk that may occur between these receptors. Further information awaits the development of selective agonists and antagonists and studies with genetic “knockout” animals. XXIII. Conclusions In this review we have considered in detail the pharmacological actions and interactions of purines and pyrimidines in different cells and tisssues. These are presented within a framework intended to facilitate
FIG. 12. Schematic of integrated effects of P1 and P2 purine receptors in the local control of vascular tone. Noradrenaline (NA), ATP, calcitonin gene-related peptide (CGRP), and substance P (SP) can be released from nerves in the adventitia to act on their respective receptors in the smooth muscle, causing vasoconstriction or vasodilatation. Prejunctional A1 receptors modulate the release of neurotransmitter from sympathetic and sensory afferents. P2X2/3 heteromers, possibly together with the corresponding homomeric P2X receptors, may be present on the peripheral terminals of sensory nerves where they may modulate sensory neurotransmission. Vasoconstriction following ATP release from perivascular nerves is mediated predominantly by P2X1 receptors on the smooth muscle, while vasodilatation is mediated by smooth muscle P2Y receptors (P2Y1-like). P2Y receptors (possibly P2Y2, P2Y4, or P2Y6) are also present on some vascular smooth muscle and mediate vasoconstriction to purines and pyrimidines of currently undetermined source. Vasodilatation may also be mediated by smooth muscle A2A and A2B adenosine receptors. ATP and its breakdown product ADP, and UTP, can be released from endothelial cells by shear stress or hypoxia, to act on endothelial P2Y1 and P2Y2 receptors to mediate relaxation mainly via endothelium-derived relaxing factor (EDRF, or nitric oxide) or endothelium-derived hyperpolarizing factor (EDHF). ATP can be broken down rapidly to adenosine, which may act on endothelial and smooth muscle A2A and A2B receptors to mediate vasodilatation. (Adapted from Burnstock, 1990).
comparison between cloned and endogenous receptors and, thereby, to promote the development of the unifying system of nomenclature based on cloned receptors. For adenosine/P1 receptors, the availability of potent and selective pharmacological ligands has been crucial in the subclassification of this family into four subtypes. For P2 receptors, responses of biological tissue have been described that do not correspond well with those of any cloned P2 receptors; there are diverse reasons, including the fact that small differences in molecular structure of a receptor are commonly found between
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species and tissues and may profoundly influence its properties. Other reasons include differences in assay conditions and because coexpression of different subtypes of receptors for purines and pyrimidines is common, which leads to complex pharmacological profiles. The lack of subtype-selective agonists and antagonists with which to adequately discriminate between responses is a significant handicap. Furthermore, while we have a reasonably good idea of the properties of homomeric recombinant P2X receptors, the relative contribution of individual subunits to responses mediated by heteromeric receptors is less clear. Although G protein-coupled P2Y receptors are single membrane-spanning proteins, diversity may be introduced by alternate G protein and/or second-messenger coupling. Major advances in adenosine/P1 receptor research in the last few years include an increased understanding of the mechanisms underlying desensitization and neuroand cardiac-protection, therefore offering novel approaches for pharmacological manipulation of receptor activity in disease. Much still needs to be learned about the A2B receptor, and the development of selective agonists and antagonists is urgently needed. As A2A and A2B receptors are often coexpressed by the same cell, this would promote investigations into short-term crosstalk and the long-term functional relationship between these subtypes. Newly developed ligands at the A3 receptor will provide insights into the significance of its relatively restricted distribution and will increase our understanding of its dual protective and toxic effects. While it has long been appreciated that the different adenosine/P1 receptor subtypes have different affinities for adenosine, the fact that a single subtype can mediate opposite effects depending on its level of activation is a relatively new concept and an exciting area for further investigation. Little is known about the integrated patterns of events arising from differential activation and desensitization or up-regulation of coexisting receptors under conditions of different concentrations of adenosine, and this may be an important area for future research. There has been a tremendous interest in the P2 receptor research in the last decade and many exciting issues have been raised. Specific questions of interest include the physiological significance of cation and pH modulation of P2X receptor activity, the true species of ATP that is the active ligand at P2 receptors, the mechanism of desensitization of P2X receptors, and the biological significance of a receptor that is activated equipotently by ATP and UTP (P2Y2 and some P2Y4 receptors). We expect the future will see important developments in research on receptors for pyrimidine nucleotides and investigations into the role of diadenosine polyphosphates as extracellular signaling molecules. Questions raised about the separate identity of the putative P3 receptor, and the P2D and P4 receptors claimed for adenine dinucleotides, currently identified solely by
their distinct pharmacology, are also likely to be resolved. Identification of novel splice variants may add significantly to the repertoire of P2 receptor-mediated responses. It is interesting that no receptors acting as ion channels, selective for extracellular pyrimidines, have been described, which is perhaps surprising given that some parallels exist for the putative extracellular roles of purines and pyrimidines. Given the widespread distribution of receptors responsive to UTP, characterization of the sources and conditions which mediate UTP release is important; there is no evidence for UTP release as a neurotransmitter to date, but it has been shown to modulate neurotransmission. The development of an assay for detection of nanomolar quantities of UTP is an exciting and important development in this field (Lazarowski et al., 1997a). Clearly, potent and selective agonists and antagonists are needed in purine and pyrimidine receptor research. Fortunately, groups in many universities and pharmaceutical industries are seeking to identify such ligands and, with the aid of high throughput screening, there is a good possibility that these and other questions will be answered in the not too distant future. The possibility of developing a transgenic animal model in which the animal P1 or P2 receptor subtype is replaced with the human homologue has been raised as a possible means of examining the function and pharmacology of the human receptor in biological tissue, with the intent of developing therapeutic strategies for human disease. Acknowledgments. The support of the Royal Society is gratefully acknowledged. Drs. C.H.V. Hoyle and A. Townsend-Nicholson are thanked for helpful comments on the manuscript. Mr. R. Jordan is thanked for help in the preparation of the manuscript. REFERENCES Abbracchio MP, Brambilla R, Ceruti S, Kim HO, Von Lubitz DK, Jacobson KA and Cattabeni F (1995a) G protein-dependent activation of phospholipase C by adenosine A3 receptors in rat brain. Mol Pharmacol 48:1038 –1045. Abbracchio MP and Burnstock G (1994) Purinoceptors: Are there families of P2X and P2Y purinoceptors? Pharmacol Ther 64:445– 475. Abbracchio MP, Ceruti S, Burnstock G and Cattabeni F (1995b) Purinoceptors on glial cells of the central nervous system: Functional and pathological implications, in Adenosine and Adenine Nucleotides: From Molecular Biology to Integrative Physiology (Belardinelli L and Pelleg A eds) pp 271–280, Kluwer Academic Press, Norwell. Abbracchio MP, Fogliatto G, Paoletti AM, Rovati GE and Cattabeni F (1992) Prolonged exposure of rat brain slices to adenosine analogs: Selective desensitization of adenosine A1 but not A2 receptors. Eur J Pharmacol 227:317–324. Abdullah LH, Davis SW, Burch L, Yamauchi M, Randell SH, Nettesheim P and Davis CW (1996) P2U purinoceptor regulation of mucin secretion in SPOC1 cells, a goblet cell line from the airways. Biochem J 316:943–951. Abe Y, Sorimachi M, Itoyama Y, Furukawa K and Akaike N (1995) ATP responses in the embryo chick ciliary ganglion cells. Neuroscience 64:547–551. Abebe W, Makujina SR and Mustafa SJ (1994) Adenosine receptor-mediated relaxation of porcine coronary artery in presence and absence of endothelium. Am J Physiol 266:H2018 –H2025. Agmon Y, Dinour D and Brezis M (1993) Disparate effects of A1- and A2-receptor agonists on intrarenal blood flow. Am J Physiol 265:F802–F806. Ainz LF, Salgado C, Gandarias JM, Gomez R, Vallejo A and Gil-Rodrigo CE (1993) P1(A2/Ra)-purinoceptors may mediate the stimulatory effect of adenosine and adenosine analogs on acid formation in isolated rabbit parietal cells. Pharmacol Res 27:319 –334. Akatsuka Y, Egashira K, Katsuda Y, Narishige T, Ueno H, Shimokawa H and Takeshita A (1994) ATP sensitive potassium channels are involved in adenosine A2 receptor mediated coronary vasodilatation in the dog. Cardiovasc Res 28:906 – 911. Akbar GKM, Dasari VR, Webb TE, Ayyanathan K, Pillarisetti K, Sandhu AK, Athwal RS, Daniel JL, Ashby B, Barnard EA and Kunapuli SP (1996) Molecular cloning of a novel P2 purinoceptor from human erythroleukaemic cells. J Biol Chem 271:18363–18367.
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