PPADS and suramin as antagonists at cloned P2Y- and P2U-purinoceptors

Metabotropic receptors for ATP and UTP: exploring the correspondence between native and recombinant nucleotide receptors

In the past five years, an extended series (P2Y1-n) of metabotropic nucleotide (P2) receptors has been cloned from vertebrate tissues; these receptors are activated by either ATP or UTP, or both nucleotides. While certain cloned P2Y receptors appear to correspond functionally to particular native P2 receptor phenotypes, such pharmacological phenotypes could be explained by either a combination of several members of the P2Y1-n series being coexpressed in the same tissue or the existence of novel, uncloned P2Y subtypes. Here, Brian King, Andrea Townsend-Nicholson and Geoffrey Burnstock review recent findings on the matter of pharmacological relationships between native P2 and cloned P2Y receptors.

Characterization of the P2 receptors on the human umbilical vein endothelial cell line ECV304

1. To characterize the P2 receptors present on the human umbilical vein endothelial-derived cell line, ECV304, cytosolic Ca2+, ([Ca2+]c), responses were recorded in single cells and in cell suspensions to a series of nucleotides and nucleotide agonists. 2. Concentration response curves were obtained in fura-2-loaded ECV304 cell suspensions, with EC50 values of 4.2 microM for ATP, 2.5 microM for UTP and 14 microM for adenosine-5'-O-(3-thio)triphosphate (ATPgammaS). EC50 values for 2-methylthioATP, ADP, adenosine-5'-O-(2-thio)diphosphate (ADPbetaS) and AMP were 0.5 microM, 3.5 microM, 15 microM and 4.7 microM respectively, but maximal [Ca2+]c responses were less than those produced by a maximal addition of ATP/UTP. ECV304 cells were unresponsive to UDP and beta,gamma,methyleneATP. 3. Cross-desensitization studies on ECV304 cells suggested that ATP and UTP recognized the same receptor. However, ADP recognized a receptor distinct from the UTP-sensitive receptor and AMP recognized a third distinct receptor. 4. ECV304 [Ca2+]c responses to 2-methylthioATP were inhibited in the presence of 30 microM pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), whereas [Ca2+]c responses to UTP were unaffected by this treatment. 5. ECV304 cells responded to the diadenosine polyphosphate Ap3A with rises in [Ca2+]c. Apparent responses to Ap4A, Ap5A and Ap6A, were shown to be due to a minor nucleotide contaminant that could be removed by pre-treatment of the diadenosine samples with either alkaline phosphatase or apyrase. 6. ECV304 cells display a pharmacology consistent with the presence of at least two P2 receptors; a P2Y2 receptor insensitive to the diadenosine polyphosphates and a P2Y1 receptor sensitive to Ap3A. In addition, ECV304 cells respond to AMP with increases in [Ca2+]c via an as yet uncharacterized receptor.

Comparison of the actions of ATP and UTP and P(2X1) receptors in smooth muscle of the rat tail artery

The actions of ATP and uridine 5'-triphosphate (UTP) were compared at P2X1 receptors in acutely dissociated smooth muscles cells of the rat tail artery. ATP (30 nM-100 microM) and UTP (1 microM-1 mM) elicited concentration-dependent inward currents. ATP was approximately 100-fold more potent than UTP. In both cases, currents were activated within 3 ms of agonist application and had similar time-courses of activation and inactivation. The decay of responses for both agonists was concentration-dependent and in most cells could be fitted by two exponentials. The P2X receptor antagonists suramin (100 microM) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 5 microM) inhibited responses to both ATP and UTP. An action of UTP at P2X1 receptors has not previously been reported. However, since the responses to ATP and UTP had similar time-courses and as PPADS and suramin inhibited both agonists, it is concluded that ATP and UTP are acting at the same site in these cells, the P2X1 receptor.

Molecular cloning and characterization of rat P2Y4 nucleotide receptor

An intronless open reading frame encoding a protein (361aa in length) was isolated from a rat genomic library probed with a DNA fragment from rat heart. This protein showed 83% sequence identity with the human P2Y4 (hP2Y4) receptor and represents a homologue of the human pyrimidinoceptor. However, the rP2Y4 receptor is not selective for uridine nucleotides and, instead, shows an agonist potency order of ITP = ATP = ADP(pure) = UTP = ATPgammaS = 2-MeSATP = Ap4A > UDP(pure). ADP, ATPgammaS, 2-MeSATP and UDP are partial agonists. Thus, in terms of agonist profile, rP2Y4 is more like the P2U receptor subtype. The rP2Y4 receptor was reversibly antagonized by Reactive blue 2 but not by suramin which, otherwise, inhibits the hP2Y2 receptor (a known P2U receptor). Thus, rP2Y4 and the P2Y2 subtype appear to be structurally distinct forms of the P2U receptor (where ATP and UTP are equi-active) but can be distinguished as suramin-insensitive and suramin-sensitive P2U receptors, respectively.

Competitive and selective antagonism of P2Y1 receptors by N6-methyl 2'-deoxyadenosine 3',5'-bisphosphate

The antagonist activity of N6-methyl 2'-deoxyadenosine 3',5'-bisphosphate (N6MABP) has been examined at the phospholipase C-coupled P2Y1 receptor of turkey erythrocyte membranes. N6MABP antagonized 2MeSATP-stimulated inositol phosphate hydrolysis with a potency approximately 20 fold greater than the previously studied parent molecule, adenosine 3',5'-bisphosphate. The P2Y1 receptor antagonism observed with N6MABP was competitive as revealed by Schild analysis (pK(B) = 6.99 +/- 0.13). Whereas N6MABP was an antagonist at the human P2Y1 receptor, no antagonist effect of N6MABP was observed at the human P2Y2, human P2Y4 or rat P2Y6 receptors.

Evidence that ATP acts at two sites to evoke contraction in the rat isolated tail artery

1. The site(s) at which P2-receptor agonists act to evoke contractions of the rat isolated tail artery was studied by use of P2-receptor antagonists and the extracellular ATPase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156). 2. Suramin (1 microM(-1) mM) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (0.3-300 microM) inhibited contractions evoked by equi-effective concentrations of alpha,beta-methyleneATP (alpha,beta-meATP) (5 microM), 2-methylthioATP (2-meSATP) (100 microM) and adenosine 5'-triphosphate (ATP) (1 mM) in a concentration-dependent manner. Responses to alpha,beta-meATP and 2-meSATP were abolished, but approximately one third of the peak response to ATP was resistant to suramin and PPADS. 3. Contractions evoked by uridine 5'-triphosphate (UTP) (1 mM) were slightly inhibited by suramin (100 and 300 microM) and potentiated by PPADS (300 microM). 4. Desensitization of the P2X1-receptor by alpha,beta-meATP abolished contractions evoked by 2-meSATP (100 microM) and reduced those to ATP (1 mM) and UTP (1 mM) to 15+/-3% and 68+/-4% of control. 5. Responses to alpha,beta-meATP (5 microM) and 2-meSATP (100 microM) were abolished when tissues were bathed in nominally calcium-free solution, while the peak contractions to ATP (1 mM) and UTP (1 mM) were reduced to 24+/-6% and 61+/-13%, respectively, of their control response. 6. ARL 67156 (3-100 microM) potentiated contractions elicited by UTP (1 mM), but inhibited responses to alpha,beta-meATP (5 microM), 2-meSATP (100 microM) and ATP (1 mM) in a concentration-dependent manner. 7. These results suggest that two populations of P2-receptors are present in the rat tail artery; ligand-gated P2X1-receptors and G-protein-coupled P2Y-receptors.

Dependence of P2-nucleotide receptor agonist-mediated endothelium-independent relaxation on ectonucleotidase activity and A2A-receptors in rat portal vein

1. The mechanism of action of P2 nucleotide receptor agonists that produce endothelium-independent relaxation and the influence of ecto-ATPase activity on this relaxing effect have been investigated in rat portal vein smooth muscle. 2. At 25 degrees C, ATP, 2-methylthioATP (2-MeSATP) and 2-chloroATP (2-ClATP), dose-dependently inhibited spontaneous contractile activity of endothelium-denuded muscular strips from rat portal vein. The rank order of agonist potency defined from the half-inhibitory concentrations was 2-CIATP (2.7+/-0.5 microM, n=7) >ATP (12.9+/-1.1 microM, n=9) > or =2-MeSATP (21.9+/-4.8 M, n=4). In the presence of alphabeta-methylene ATP (alphabeta-MeATP, 200 microM) which itself produced a transient contractile effect, the relaxing action of ATP and 2-MeSATP was completely abolished and that of 2-ClATP strongly inhibited. 3. The non-selective P2-receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 100 microM) did not affect the relaxation induced by ATP, 2-MeSATP, and 2-ClATP. 4. The A2A-adenosine receptor antagonist ZM 241385 inhibited the ATP-induced relaxation in a concentration-dependent manner (1-100 nM). In the presence of 100 nM ZM 241385, the relaxing effects of 2-MeSATP and 2-ClATP were also inhibited. 5. ADP, AMP and adenosine also produced concentration-dependent inhibition of spontaneous contractions. The relaxing effects of AMP and adenosine were insensitive to alphabeta-MeATP (200 microM) but were inhibited by ZM 241385 (100 nM). 6. Simultaneous measurements of contraction and ecto-ATPase activity estimated by the degradation of [gamma-32P]-ATP showed that muscular strips rapidly (10-60 s) hydrolyzed ATP. This ecto-ATPase activity was abolished in the presence of EDTA and was inhibited by 57+/-11% (n=3) by 200 microM alphabeta-MeATP. 7. These results suggest that ATP and other P2-receptor agonists are relaxant in rat portal vein smooth muscle, because ectonucleotidase activity leads to the formation of adenosine which activates A2A-receptors.

Stretch activates Jun N-terminal kinase/stress-activated protein kinase in vascular smooth muscle cells through mechanisms involving autocrine ATP stimulation of purinoceptors

Mechanical strain has been implicated in phenotypic changes, including alteration of gene expression in vascular smooth muscle cells; however, the molecular basis for mechanotransduction leading to nuclear gene expression is largely unknown. We demonstrate in the present study that cyclic stretching of vascular smooth muscle cells dramatically activates Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) through an autocrine mechanism. Stretch causes time- and strength-dependent rise of the ATP concentration in media. The stretch-induced activation JNK/SAPK is attenuated by the addition of hexokinase or apyrase that scavenge ATP in media. Both the P2 receptor antagonist and the A1 subtype-selective P1 receptor antagonist partially inhibit stretch-induced activation of JNK/SAPK. The conditioned medium from stretched cells contains an activity to stimulate JNK/SAPK. The JNK-stimulating activity in the conditioned medium from stretched cells is attenuated by the addition of apyrase or P1 and P2 receptor antagonists. The addition of exogenous ATP or adenosine induces dose-dependent activation of JNK/SAPK. These results indicate that stretch activates JNK/SAPK in vascular smooth muscle cells through mechanisms involving autocrine stimulation of purinoceptors by ATP and its hydrolyzed product adenosine.

Pharmacological evidence for the existence of multiple P2 receptors in the circular muscle of guinea-pig colon

1. By using the sucrose gap technique, we have investigated the effect of the metabolically stable P2Y receptor agonist, adenosine 5'-O-2-thiodiphosphate (ADPbetaS), on the membrane potential and tension in the circular muscle of the guinea-pig proximal colon. All experiments were performed in the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM), nifedipine (1 microM), L-nitroarginine (L-NOARG, 100 microM) and of the tachykinin NK1 and NK2 receptor antagonists, SR 140333 (0.1 microM) and GR 94800 (0.1 microM), respectively. 2. ADPbetaS (100 microM for 15 s) evoked a tetrodotoxin- (1 microM) resistant hyperpolarization and contraction of the smooth muscle. In the presence of apamin (0.1 microM), the ADPbetaS-induced hyperpolarization was converted to depolarization and the contraction was potentiated while tetraethylammonium (TEA, 10 mM) did not affect significantly the response to ADPbetaS. The combined application of apamin and TEA reproduced the effect observed with apamin alone. 3. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acids (PPADS, 30 microM) slightly but significantly increased the ADPbetaS-induced hyperpolarization, while the contraction evoked by ADPbetaS was reduced by about 80%. Suramin (100 microM) did not affect the ADPbetaS-induced hyperpolarization but totally blocked the ADPbetaS-induced contraction. In the presence of suramin (100 microM), a small relaxation of the circular muscle was observed upon application of ADPbetaS. 4. The contraction and hyperpolarization evoked by ADPbetaS were abolished in Ca2+-free Krebs solution. The blocker of sarcoplasmic reticulum Ca2+ pump, cyclopiazonic acid (10 microM) reduced contraction and hyperpolarization induced by ADPbetaS by about 60 and 50%, respectively. 5. A comparison of our present and previous findings enables to conclude that at least 3 types of P2 receptors are present on the smooth muscle of the guinea-pig colon, as follows: (1) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by alpha,beta-methylene ATP (alpha,beta-meATP) and by endogenously released purines, sensitive to suramin and PPADS; (2) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by ADPbetaS and are resistant to suramin and PPADS; (3) excitatory P2 receptors, producing contraction, which are activated by ADPbetaS and are sensitive to suramin and PPADS. The data also support the idea of the existence of a restricted pool of specialized junctional P2 receptors producing the apamin-sensitive NANC inhibitory junction potential in response to endogenous ligand(s).

Nucleotide receptors in the nervous system. An abundant component using diverse transduction mechanisms

Extracellular nucleotides achieve their role as cell-to-cell communicators by acting at cell surface transmembrane receptors-the P2 receptors. Before molecular cloning led to the isolation of any P2-receptor sequence, a small number of receptor types had been proposed on the basis of pharmacological evidence. The application of molecular biology to this field of receptor research has indicated that a great underestimation of the number of receptor subtypes and of their abundance had occurred. There are now known to be seven characterized P2Y (G protein linked) receptors and the same number again of P2X receptors of the transmitter-gated ion channel type. In this review, we discuss the properties of these cloned receptors, their distribution within the nervous system, and their methods of signal transduction.

Direct demonstration of mechanically induced release of cellular UTP and its implication for uridine nucleotide receptor activation

ATP is released from most cell types and functions as an extracellular signaling molecule through activation of members of the two large families of P2X and P2Y receptors. Although three mammalian P2Y receptors have been cloned that are selectively activated by uridine nucleotides, direct demonstration of the release of cellular UTP has not been reported. Pharmacological studies of the P2Y4 receptor expressed in 1321N1 human astrocytoma cells indicated that this receptor is activated by UTP but not by ATP. Mechanical stimulation of 1321N1 cells also resulted in release of a molecule that markedly activated the expressed P2Y4 receptor. This nucleotide was shown to be UTP by two means. First, high performance liquid chromatography analysis of the medium from [33P]H3PO4-loaded 1321N1 cells illustrated that mechanical stimulation resulted in a large increase in a radioactive species that co-eluted with authentic UTP. This species was degraded by incubation with the nonspecific pyrophosphohydrolase apyrase or with hexokinase and was specifically lost by incubation with the UTP-specific enzyme UDP-glucose pyrophosphorylase. Second, a sensitive assay that quantitates UTP mass at low nanomolar concentrations was devised based on the nucleotide specificity of UDP-glucose pyrophosphorylase. Using this assay, mechanical stimulation of 1321N1 cells was shown to result in an increase of medium UTP levels from 2.6 to 36.4 pmol/10(6) cells within 2 min. This increase was paralleled by a similar augmentation of extracellular ATP levels. A calcein-based fluorescence quenching method was utilized to confirm that none of the increases in medium nucleotide levels could be accounted for by cell lysis. Taken together, these results directly demonstrate the mechanically induced release of UTP and illustrate the efficient coupling of this release to activation of P2Y4 receptors.

The interaction of diadenosine polyphosphates with P2x-receptors in the guinea-pig isolated vas deferens

1. The site(s) at which diadenosine 5',5"'-P1,P4-tetraphosphate (AP4A) and diadenosine 5', 5"'-P1,P5-pentaphosphate (AP5A) act to evoke contraction of the guinea-pig isolated vas deferens was studied by use of a series of P2-receptor antagonists and the ecto-ATPase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156). 2. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (300 nM - 30 microM), suramin (3-100 microM) and pyridoxal-5'-phosphate (P-5-P) (3-1000 microM) inhibited contractions evoked by equi-effective concentrations of AP5A (3 microM), AP4A (30 microM) and alpha,beta-methyleneATP (alpha,beta-meATP) (1 microM), in a concentration-dependent manner and abolished them at the highest concentrations used. 3. PPADS was more potent than suramin, which in turn was more potent than P-5-P. PPADS inhibited AP5A, AP4A and alpha,beta-meATP with similar IC50 values. No significant difference was found between IC50 values for suramin against alpha,beta-meATP and AP5A or alpha,beta-meATP and AP4A, but suramin was more than 2.5 times more potent against AP4A than AP5A. P-5-P showed the same pattern of antagonism. 4. Desensitization of the P2xi-receptor by alpha,beta-meATP abolished contractions evoked by AP5A (3 microM) and AP4A (30 microM), but had no effect on those elicited by noradrenaline (100 microM). 5. ARL 67156 (100 microM) reversibly potentiated contractions evoked by AP4A (30 microM) by 61%, but caused a small, significant decrease in the mean response to AP5A (3 microM). 6. It is concluded that AP4A and AP5A act at the P2xi-receptor, or a site similar to the P2xi-receptor, to evoke contraction of the guinea-pig isolated vas deferens. Furthermore, the potency of AP4A, but not AP5A, appears to be inhibited by an ecto-enzyme which is sensitive to ARL 67156.

Enhancement of the response to purinergic agonists in P2Y1 transfected 1321N1 cells by antagonists suramin and PPADS

1. We have previously shown that both suramin and pyridoxal-phosphate-6-azophenyl-2',4' disulphonic acid (PPADS) act as antagonists at transfected P2Y1 receptors. Here we show that under certain experimental conditions these two P2 antagonists can enhance the response to agonists acting at these receptors. 2. The expression of either P2Y1 or P2Y2 receptors in 1321N1 human astrocytoma cells results, on a change of medium, in an elevation of basal (no added agonist) accumulation of [3H]-inositol(poly)phosphates([3H]-InsPx) compared to cells not expressing these receptors. This elevation is much greater in P2Y1 transfectants than in P2Y, transfectants. 3. Both PPADS and suramin reduced this basal level of [3H]-InsPx accumulation in the P2Y1 expressing cells. 4. When a protocol was used which required changing the culture medium, antagonists were added at a concentration which reduced the basal accumulation by about 50%, there was a significant stimulation in response to increasing concentrations of 2-methylthioadenosine 5'-triphosphate (2MeSATP), in the absence of antagonists there was no significant effect of the agonist. 5. However, when 2MeSATP was added in the absence of a change of medium and with no antagonist present, there was a several fold increase in [3H]-InsPx accumulation. These results show that a release of endogenous agonist activity (possibly ATP/ADP) from the P2Y1 expressing cells can create conditions in which a response to an agonist such as 2MeSATP can only be seen in the presence of a competitive antagonist.

Purinergic facilitation of ATP-sensitive potassium current in rat ventricular myocytes

1. The effects of different purinergic agonists on the cardiac adenosine 5'-triphosphate (ATP)-sensitive potassium current (IK(ATP)), appearing during dialysis of rat isolated ventricular myocytes with a low-ATP (100 microM) internal solution under whole-cell patch-clamp conditions, were examined in the presence of a P1 purinoceptor antagonist. 2. The extracellular application of ATP in the micromolar range induced, besides known inward currents through cationic and chloride channels, the facilitation of IK(ATP) once IK(ATP) had already been partially activated during the low-ATP dialysis. 3. Analogues of ATP, alpha, beta-methyleneadenosine 5'-triphosphate (alpha, beta meATP), 2-methylthioadenosine triphosphate (2MeSATP), adenosine 5'-O-3-thiotriphosphate (ATP gamma S) similarly facilitated IK(ATP). UTP and ADP were very weak agonists while AMP and adenosine had no detectable effect. 4. The half-maximal stimulating concentration (C50) of alpha, beta meATP, an analogue that did not elicite the interfering inward cationic current was 1.5 microM. Similar apparent C50 (1-2 microM) were observed for ATP and analogues tested with somewhat less maximal effect of ATP gamma S. 5. Suramin, a nonselective P2-purinoceptor antagonist, altered IK(ATP) at the relatively high concentration required to inhibit purinoceptors. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a supposedly predominantly P2x-purinoceptor antagonist, at micromolar concentration inhibited the transient inward current but did not block the facilitation of IK(ATP). 6. Our results demonstrate that ATP and its analogues facilitate IK(ATP) in rat ventricular myocytes by stimulation of non-P1-, non-P2x-purinoceptors.

Cloned and transfected P2Y4 receptors: characterization of a suramin and PPADS-insensitive response to UTP

The P2Y family of receptors are G protein-coupled receptors for ATP, ADP, UTP and UDP. Recently several members of this family have been cloned, including the P2Y4, which is activated by UTP but not by ATP. In the present report, using receptors stably transfected into 1321N1 cells, we show that suramin acts as an antagonist at cloned P2Y1 and (less potently) P2Y2 receptors, but not at the cloned P2Y4 receptor. Furthermore, PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid), a potent antagonist at the P2Y1 receptor, is a relatively inneffective antagonist at the cloned P2Y4 receptor. This work moves us closer to the goal of classifying the native P2Y receptors on the basis of agonist and antagonist profiles.