P2 purine and pyrimidine receptors: emerging superfamilies of G-protein-coupled and ligand-gated ion channel receptors

Fluorescent epsilon-ATP analogues for probing physicochemical properties of proteins. Synthesis, biochemical evaluation, and sensitivity to properties of the medium

Despite the significance of the elucidation of proteins' physicochemical parameters to understand various molecular phenomena, direct methods for measuring these parameters are not readily available. Here, we propose the use of 8-[p-amino-Ph]-epsilon-ATP, 3b, as a fluorescent probe for the elucidation of physicochemical parameters of binding sites in certain proteins. We synthesized novel fluorescent nucleotide analogues based on an extension of the epsilon-ATP scaffold. These analogues bear a primary or tertiary p-amino-phenyl moiety on the etheno-bridge. We explored the recognition of the fluorescent analogues by the target proteins: P2Y(1)-receptor (P2Y(1)-R) and NTPDase1. Based on the high affinity to the P2Y(1)-R (EC(50) 100nM), 3b proved a suitable probe for the investigation of this receptor. Next, we elucidated the dependencies of the absorption and emission spectra of 3b on environmental parameters, for establishing correlation equations. These equations will help determine the properties of the ATP-binding site from the spectral data of the protein-bound 3b. For this purpose, the sensitivity of the probe to acidity, dielectricity, H-bonding, viscosity, and to correlation between these parameters was determined. Thus, the pH-dependence of 3b emission intensity is bell shaped. At pH2.8 the quantum yield (phi) is enhanced 150-fold, as compared to neutral pH. The basic nitrogen atoms of 3b were assigned and pK(a) values were determined. A linear relationship was found between log phi and log viscosity, however, emission maxima (lambda(max)) remained constant. A linear relationship was found between both phi and lambda(max) and dielectricity, as measured in protic or aprotic solvents of comparable viscosity. pK(a)-like values were measured in acid-titrated alcohols with varying dielectricity but comparable viscosity, or with varying viscosity but comparable dielectricity. An inverse relationship and a linear relationship were found between the pK(a) values of 3b and the medium dielectricity and viscosity, respectively. These correlations help the calibration of properties of a protein ATP-binding site.

Molecular recognition in purinergic receptors. 1. A comprehensive computational study of the h-P2Y1-receptor

P2Y receptors (P2Y-Rs) are attractive pharmaceutical targets due to their involvement in the modulation of many tissues and organs. The lack of experimental structural data on P2Y-Rs impedes structure-based drug design. The need to elucidate the receptor's molecular recognition, together with the limitations of previous receptor models, triggered the construction of a new molecular model for the h-P2Y1-R. Therefore, a h-P2Y1-R model was constructed by homology modeling using the 2.6 A crystal structure of bovine rhodopsin as a template and subsequently refined by constrained molecular dynamics (MD) simulations in a fully hydrated lipid bilayer environment. ATP was docked into the receptor binding site, followed by binding site refinement using Monte Carlo and MD simulations. Analysis of the h-P2Y1-R-ATP complex suggests that the triphosphate moiety is tightly bound by a multitude of interactions possibly including a Mg2+ ion, the ribose ring is not involved in specific interactions, and the adenine ring is bound via N1, N7, and N6. The molecular recognition of the h-P2Y1-R was further probed by ATP derivatives modified on the adenine ring, and correlated with EC50 values for these derivatives. Analysis of receptor:ligand complexes and quantum mechanical studies on model compounds support the role of both steric and electronic effects in improving H-bonding (via N1 and N6) and pi-stacking interactions. The computed h-P2Y1-R model was validated with respect to our previous biochemical results. We believe that this new model of the h-P2Y1-R provides the means for understanding phenomena such as the ligand's potency and receptor subtype selectivity.

Megazol combined with suramin improves a new diagnosis index of the early meningo-encephalitic phase of experimental African trypanosomiasis

In human African trypanosomiasis (HAT), the parasites invade the central nervous system (CNS), leading to the development of meningo-encephalitis and an irreversible demyelinating process, which kills the patient unless specific treatment is undertaken. Among the experimental trypanocides, the nitroimidazole derivative megazol alone at optimal doses does not cure late-stage disease tested in mouse models, however the combination of suramin and megazol is able to cure infected mice without CNS involvement. We recently developed an experimental model of HAT with a sharp decrease in both the food intake and the body weight which may constitute an effective index of the early meningo-encephalitic phase. Using this model, we tested this hypothesis by the exclusive effectiveness of a megazol and suramin combination treatment to eliminate CNS trypanosomes. Sprague-Dawley rats were infected with Trypanosoma brucei brucei AnTat 1.1E. Food intake and body weight were measured daily from the day of infection to death. Haematocrit was measured twice a week. Treatment consisted of 20 mg suramin per kg body weight administered intraperitoneally (i.p.) alone, or three daily doses (80 mg/kg) of megazol given per os, or suramin (20 mg/kg, i.p.) followed 24 h later by three daily doses (80 mg/kg) of megazol given per os. Treatment was followed by an increase in daily body weight and food intake similar to those of the control animals, 2 weeks after treatment. The anaemia developed after infection is also cleared as shown by the haematocrit measurements. The rats treated with megazol alone died about 29 days after treatment and those treated with suramin, after about 26 days. Seven months later, no signs of relapse were seen in 10 of 12 rats treated with the therapeutic combination, indicating that this chemotherapy regimen was curative. The results support our previous finding, i.e. the decrease in body weight may constitute a diagnosis index of the early meningo-encephalitic phase.

Adenosine 5'-O-(1-boranotriphosphate) derivatives as novel P2Y(1) receptor agonists

P2-receptors (P2-Rs) represent important targets for novel drug development. Most ATP analogues proposed as potential drug candidates have shortcomings such as limited receptor-selectivity and limited stability that justify the search for new P2-R agonists. Therefore, a novel series of nucleotides based on the adenosine 5'-O-(1-boranotriphosphate) (ATP-alpha-B) scaffold was developed and tested as P2Y(1)-R agonists. An efficient four-step one-pot synthesis of several ATP-alpha-B analogues from the corresponding nucleosides was developed, as well as a facile method for the separation of the diastereoisomers (A and B isomers) of the chiral products. The potency of the new analogues as P2Y(1)-R agonists was evaluated by the agonist-induced Ca2+ release of HEK 293 cells stably transfected with rat-brain P2Y(1)-R. ATP-alpha-B A isomer was equipotent with ATP (EC50 = 2 x 10(-7) M). However, 2-MeS- and 2-Cl- substitutions on ATP-alpha-B (A isomer) increased the potency of the agonist up to 100-fold, with EC50 values of 4.5 x 10(-9) and 3.6 x 10(-9) M, compared to that of the ATP-alpha-B (A isomer). Diastereoisomers A of all ATP-alpha-B analogues were more potent in inducing Ca2+ release than the corresponding B counterparts, with a 20-fold difference for 2-MeS-ATP-alpha-B analogues. The chemical stability of the new P2Y(1)-R agonists was evaluated by 31P NMR under physiological and gastric-juice pH values at 37 degrees C, with rates of hydrolysis of 2-MeS-ATP-alpha-B of 1.38 x 10(-7) s-1 (t1/2 of 1395 h) and 3.24 x 10(-5) s-1 (t1/2 = 5.9 h), respectively. The enzymatic stability of the new analogues toward spleen NTPDase was evaluated. Most of the new analogues were poor substrates for the NTPDase, with ATP-alpha-B (A isomer) hydrolysis being 5% of the hydrolysis rate of ATP. Diastereoisomers A and B exhibited different stability, with A isomers being significantly more stable, up to 9-fold. Furthermore, A isomers that are potent P2Y(1)-R agonists barely interact with NTPDase, thus exhibiting protein selectivity. Therefore, on the basis of our findings, the new, highly water-soluble, P2Y(1)-R agonists may be considered as potentially promising drug candidates.

Fast, efficient capillary electrophoresis method for measuring nucleotide degradation and metabolism

An easy and fast method for the quantitative analysis of nucleotides by capillary zone electrophoresis was developed. The method employing a neutral-bonded capillary and reversed polarity mode provided a good resolution and a short analysis time of less than 5 min. The samples were injected electrokinetically using -6 kV voltage for 30 s and detected by their UV absorbance at 254 nm. Constant current (-45 microA) was applied, and a phosphate buffer, pH 7.4, was used. The detection limits for ATP, UDP, and UTP ranged between 0.14 and 0.28 microM. This method was required for the investigation of the purity of the commercially available nucleotides used in pharmacological studies. In addition, the analytical method was applied to study the metabolism of nucleotides in a cell line, neuroblastoma x glioma hybrid cells (NG108-15), which is used in pharmacological studies with nucleotides, since it contains purine- and pyrimidine-sensitive nucleotide receptors. Furthermore, we used the new method for monitoring enzymatic studies using the enzyme hexokinase to convert nucleotide triphosphates to diphosphates.

Protonation studies of modified adenine and adenine nucleotides by theoretical calculations and (15)N NMR

The acid/base character of nucleobases affects phenomena such as self-association, interaction with metal ions, molecular recognition by proteins, and nucleic acid base-pairing. Therefore, the investigation of proton-transfer equilibria of natural and synthetic nucleos(t)ides is of great importance to obtain a deeper understanding of these phenomena. For this purpose, a set of ATP prototypes was investigated using (15)N NMR spectroscopy, and the corresponding adenine bases were investigated by theoretical calculations. (15)N NMR measurements provided not only acidity constants but also information on the protonation site(s) on the adenine ring and regarding the ratio of the singly protonated species in equilibrium. Substituents of different nature and position on the adenine ring did not change the preferred protonation site, which remained N1. However, for 2-thioether-ATP derivatives a mixed population of N1 and N7 singly protonated species was observed. Reduction of basicity of 0.4-1 pK(a) units relative to ATP was also observed for all evaluated ATP derivatives, except for 2-Cl-ATP, for which K(a) was ca. 10,000-fold lower. To explain the substitution-dependent variations in the experimental pK(a) values of the ATP analogues, gas-phase proton affinities (PA), Delta Delta G(hyd), and pK(a) values of the corresponding adenine bases were calculated using quantum mechanical methods. The computed PA and Delta Delta G(hyd) values successfully explained the experimental pK(a) values. A computational procedure for the prediction of accurate pK(a) values was developed using density functional theory and polarizable continuum model calculations. In this procedure, we developed a set of parameters for the polarizable continuum model that was fitted to reproduce experimental pK(a) values of nitrogen heterocycles. This method is proposed for the prediction of pK(a) values and protonation site(s) of purine analogues that have not been synthesized or analyzed.

Potent P2X7 Receptor Antagonists: Tyrosyl Derivatives Synthesized Using a Sequential Parallel Synthetic Approach

Novel analogs of 1-(N,O-bis[5-isoquinolinesulfonyl]-N-methyl-L-tyrosyl)-4-phenylpiperazine (KN-62,1) were synthesized and found to be potent antagonists in a functional assay, inhibition of ATP-induced K⁺ efflux in HEK293 cells expressing recombinant human P2X₇ receptors. Antagonism of murine P2X₇ receptors was also observed. The analogs consisted of L-tyrosine derivatives, of the general structure R₁-Tyr(OR₂)-piperazinyl-R₃, in which three positions were systematically varied in structure through facile acylation reactions. Each of the three positions was optimized in sequence through parallel synthesis alternating with biological evaluation, leading to the identification and optimization of potent P2X₇ antagonists. The optimal groups at R₁ were found to be large hydrophobic groups, linked to the α-amino position through carbamate, amide, or sulfonamide groups. The benzyloxycarbonyl (Cbz) group was preferred over most sulfonamides and other acyl groups examined, except for quinoline sulfonyl. At R₂, an arylsulfonate ester was preferred, and the order of potency was p-tolyl, p-methoxyphenyl, phenyl > α-naphthyl, β-naphthyl. A benzoyl ester was of intermediate potency. Aliphatic esters and carbonate derivatives at the tyrosyl phenol were inactive, while a tyrosyl O-benzyl ether was relatively potent. The most potent P2X₇ receptor antagonists identified in this study contained Cbz at the R₁ position, an aryl sulfonate at the R₂ position, and various acyl groups at the R₃ position. At R₃, t-butyloxycarbonyl- and benzoyl groups were preferred. The opening of the piperazinyl ring to an ethylene diamine moiety abolished antagonism. In concentration-response studies, a di-isoquinolinyl, Boc derivative, 4 (MRS2306), displayed an IC₅₀ value of 40 nM as an antagonist of P2X₇ receptor-mediated ion flux and was more potent than the reference compound 1. N^α-Cbz, Boc-piperazinyl derivatives, 11 (MRS2317), 22 (MRS2326), and 41 (MRS2409) were less potent than 1, with IC₅₀ values of 200-300 nM.

Adenosine-derived non-phosphate antagonists for P2Y(1) purinoceptors

Novel type antagonists for P2Y(1) adenine nucleotide receptors were synthesized by coupling of adenosine 5'-OH group with oligo-aspartate chain via a carbonyl linker. All these conjugates (AdoOC(O)Asp(n), n = 1-4) inhibited the 2MeSADP-stimulated synthesis of inositol phosphates in 1321N1 human astrocytoma cells stably expressing human P2Y(1) receptors. This inhibitory effect followed the rank order AdoOC(O)Asp(2)> AdoOC(O)Asp(3)> AdoOC(O)Asp(1)> AdoOC(O)Asp(4) with antagonistic constant pA(2) = 5.4 for AdoOC(O)Asp(2). Potency of this non-phosphate inhibitor was comparable with the previously known adenosine 3',5'- and 2', 5'-bisphosphates. Chemical and biological stabilities of these novel adenosine derived antagonists of the nucleotide receptor provide perspectives of their pharmacological implication.

Purinergic and pyrimidinergic receptors as potential drug targets

In the last decade, the field of purinergic pharmacology has continued to grow as the complexity of the receptor families and the various enzymes involved in purine metabolism have been defined in molecular terms. A major theme that has emerged from these studies is the functional complexity of the interactions between P1 and P2 receptors, based upon the dynamic interrelationship between ATP and adenosine as extracellular signaling molecules. It is now clear that ATP and its degradation products (particularly ADP and adenosine) form a complex cascade for the regulation of cell-to-cell communication that can function to attenuate the consequences of tissue trauma (e.g. ischemia) that involve alterations in cellular energy charge and depletion of ATP stores. In addition to the P2 receptor family, alterations in cellular ATP stores can also affect the function of other receptors, e.g. K(ATP) channels, and mitochondrial function. The discovery of pyrimidine-preferring (UTP/UDP) P2Y receptors has also raised the possibility that the corresponding nucleoside, uracil, may function as a signaling molecule.

Molecular recognition of modified adenine nucleotides by the P2Y(1)-receptor. 1. A synthetic, biochemical, and NMR approach

The remarkably high potencies of 2-thioether-adenine nucleotides regarding the activation of the P2Y(1)-receptor (P2Y(1)-R) in turkey erythrocyte membranes represent some of the largest substitution-promoted increases in potencies over that of a natural receptor ligand. This paper describes the investigation regarding the origin of the high potency of these P2Y(1)-R ligands over that of ATP. For this study, an integrated approach was employed combining the synthesis of new ATP analogues, their biochemical evaluation, and their SAR analysis involving NMR experiments and theoretical calculations. These experiments and calculations were performed to elucidate the conformation and to evaluate the electronic nature of the investigated P2Y(1)-R ligands. ATP analogues synthesized included derivatives where C2 or C8 positions were substituted with electron-donating groups such as ethers, thioethers, or amines. The compounds were tested for their potency to induce P2Y(1)-R-mediated activation of phospholipase C in turkey erythrocytes and Ca(2+) response in rat astrocytes. 8-Substituted ATP and AMP derivatives had little or no effect on phospholipase C or on calcium levels, whereas the corresponding 2-substituted ATP analogues potently increased the levels of inositol phosphates and ¿Ca(2+)(i). AMP analogues were ineffective except for 2-butylthio-AMP which induced a small Ca(2+) response. P2Y(1)-R activity of these compounds was demonstrated by testing these ligands also on NG108-15 neuroblastoma x glioma hybrid cells. NMR data together with theoretical calculations imply that steric, rather than electronic, effects play a major role in ligand binding to the P2Y(1)-R. Hydrophobic interactions and H-bonds of the C2 substituent appear to be important determinants of a P2Y(1)-R ligand affinity.

Novel ligands for P2 receptor subtypes in innervated tissues

Among suramin analogues, the properties of P2 receptor subtype blockade and ecto-nucleotidase inhibition appear to be controlled by different structural parameters (Fig. 1 and 2, Table 1; Van Rhee et al., 1994; Beukers et al., 1995; Bültmann et al., 1996; Damer et al., 1998a, 1998b; and this study): the molecular size of the compounds, the position of the sulfonic acid residues in the naphthalene rings, the substitution pattern of the benzoyl moieties and the 3'- or 4'-aminobenzoyl-linkages of the phenyl rings "1" and "2". As a result, compounds with different receptor selectivity profiles were obtained. A maximum in potency at and selectivity for P2X1 receptors is reached in NF279, which is a specific P2 receptor antagonist and the compound with the highest P2X1 vs. P2Y receptor and ecto-nucleotidase selectivity presently available.

ADP receptors and clinical bleeding disorders

ADP plays a key role in hemostasis and thrombosis. Despite its early identification in 1961 as the first known aggregating agent, the molecular basis of ADP-induced platelet activation is only beginning to be understood. The present review proposes a model of 3 purinergic receptors contributing separately to the complex process of ADP-induced platelet aggregation: the P2X(1) ionotropic receptor, responsible for rapid influx of ionized calcium into the cytosol; the P2Y(1) metabotropic receptor, responsible for mobilization of ionized calcium from internal stores, which initiates aggregation; and an as-yet-unidentified P2Y receptor coupled to G(alphai2), which is essential for the full aggregation response to ADP. It is probable that this as-yet-unidentified receptor is the molecular target of the ADP-selective antiaggregating drugs ticlopidine and clopidogrel. In addition, it is probably defective in patients with a bleeding diathesis that is characterized by selective impairment of platelet responses to ADP.

Control of apical membrane chloride permeability in the renal A6 cell line by nucleotides

1. The effect of extracellular nucleotides applied on the apical side of polarised A6 cells grown on permeant filters was investigated by measuring the changes in (i) the 36Cl efflux through the apical membranes, (ii) the intracellular chloride concentrations (aCli, measured with N-(6-methoxyquinolyl) acetoethyl ester, MQAE), (iii) ICl, the short-circuit current in the absence of Na+ transport and (iv) the characteristics of the apical chloride channels using a patch-clamp approach. 2. ATP or UTP (0.1-500 microM) transiently stimulated ICl. The sequence of purinergic agonist potencies was UTP = ATP > ADP >> the P2X-selective agonist beta,gamma-methylene ATP = the P2Y-selective agonist 2-methylthioATP. Suramin (100 microM) as the P2Y antagonist Reactive Blue 2 (10 microM) had no effect on the UTP (or ATP)-stimulated current. These findings are consistent with the presence of P2Y2-like receptors located on the apical membranes of A6 cells. Apical application of adenosine also transiently increased ICl. This effect was blocked by theophylline while the UTP-stimulated ICl was not. The existence of a second receptor, of the P1 type is proposed. 3. ATP (or UTP)-stimulated ICl was blocked by apical application of 200 microM N-phenylanthranilic acid (DPC) or 100 microM niflumic acid while 100 microM glibenclamide was ineffective. 4. Ionomycin and thapsigargin both transiently stimulated ICl; the nucleotide stimulation of ICl was not suppressed by pre-treatment with these agents. Chlorpromazin (50 microM), a Ca2+-calmodulin inhibitor strongly inhibited the stimulation of ICl induced either by apical UTP or by ionomycin application. BAPTA-AM pre-treatment of A6 cells blocked the UTP-stimulated ICl. Niflumic acid also blocked the ionomycin stimulated ICl. 5. A fourfold increase in 36Cl effluxes through the apical membranes was observed after ATP or UTP application. These increases of the apical chloride permeability could also be observed when following aCli changes. Apical application of DPC (1 mM) or 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB; 500 microM) produced an incomplete inhibition of 36Cl effluxes through the apical membranes in ATP-stimulated and in untreated monolayers. 6. In single channel patch-clamp experiments, an apical chloride channel with a unitary single channel conductance of 7.3 +/- 0.6 pS (n = 12) was usually observed. ATP application induced the activation of one or more of these channels within a few minutes. 7. These results indicate that multiple purinergic receptor subtypes are present in the apical membranes of A6 cells and that nucleotides can act as modulators of Cl- secretion in renal cells.

Two different ionotropic receptors are activated by ATP in rat microglia

1. Our aim was to assess whether ATP-induced inward currents in microglia are due to a single or more than one purinergic receptor. The ATP dose-response curve showed two components, whose presence might be due to the activation of high and low affinity receptors. 2. The P2Z/P2X7 specific receptor agonist benzoylbenzoyl-ATP (Bz-ATP) and some P2 receptor agonists were tested. The rank order of potency was Bz-ATP >> ATP = 2-methylthio-ATP (2-MeSATP) > alpha, beta-methylene ATP (alpha,beta-meATP) >= ADP. beta, gamma-MethyleneATP (beta,gamma-meATP), UTP and adenosine were ineffective. 3. The non-specific P2 receptor antagonist suramin antagonized by 92 +/- 2 % the inward current induced by 100 microM ATP, and by 51 +/- 8 and 68 +/- 6 % those induced by 3 mM ATP and 100 microM Bz-ATP, respectively. The P2Z/P2X7 antagonist oxidized ATP (oATP) almost abolished the inward current induced by 3 mM ATP or Bz-ATP, but was ineffective against 100 microM ATP. 4. Inward currents induced by low ATP concentrations (<= 100 microM) were generally followed by an almost complete and irreversible desensitization, while those elicited by ATP >= 1 mM showed only a partial decline. Interestingly, the inward current induced by 100 microM 2-MeSATP showed a large desensitization, while that induced by Bz-ATP did not. 5. In voltage-ramp experiments, the 100 microM ATP-induced current exhibited a slight inward rectification more visible at negative potentials, while the 3 mM ATP-induced current did not. 6. ATP induced a fast and large increase in [Ca2+] that promptly recovered in the continuous presence of low ATP doses, but did not recover in high ATP doses. As with desensitization, the response to Bz-ATP mimicked that of high doses of ATP. 7. When Ca2+ mobilization due to P2Y receptors was blocked by thapsigargin-induced Ca2+ depletion or by pertussis toxin treatment, 10 microM ATP was still able to induce a Ca2+ transient, which represented the contribution of the Ca2+ influx induced by P2X receptors 8. In conclusion, the inward currents and a fraction of the Ca2+ transients induced by ATP in microglia are due to at least two ATP-sensitive receptor channel types, whose different properties and sensitivity to ATP may be associated with different functional roles.

2-thioether 5'-O-(1-thiotriphosphate)adenosine derivatives as new insulin secretagogues acting through P2Y-Receptors

P2-Receptors (P2-Rs) represent significant targets for novel drug development. P2-Rs were identified also on pancreatic B cells and are involved in insulin secretion. Therefore, novel P2Y-R ligands, 2-thioether 5'-O-phosphorothioate adenosine derivatives (2-RS-ATP-alpha-S), were synthesized as potential insulin secretagogues. An efficient synthesis of these nucleotides and a facile method for separation of the chiral products are described. The enzymatic stability of the compounds toward pig pancreas type I ATPDase was evaluated. The rate of hydrolysis of 2-hexylthio-5'-O-(1-thiotriphosphate)adenosine (2-hexylthio-ATP-alpha-S) isomers by ATPDase was 28% of that of ATP. Some 2-thioether 5'-(monophosphorothioate)adenosine derivatives (2-RS-AMP-S) exerted an inhibitory effect on ATPDase. The apparent affinity of the compounds to P2Y(1)-R was determined by measurement of P2Y-R-promoted phospholipase C activity in turkey erythrocyte membranes. 2-RS-ATP-alpha-S derivatives were agonists, stimulating the production of inositol phosphates with K(0.5) values in the nanomolar range. 2-RS-AMP-S derivatives were full agonists, although 2 orders of magnitude less potent. All the compounds were more potent than ATP. The effect on insulin secretion and pancreatic flow rate was evaluated on isolated and perfused rat pancreas. A high increase, up to 500%, in glucose-induced insulin secretion was due to addition of 2-hexylthio-ATP-alpha-S in the nanomolar concentration range, which represents 100-fold enhancement of activity relative to ATP. 2-Hexylthio-AMP-S was 2.5 orders of magnitude less effective.

The P2Y1 receptor, necessary but not sufficient to support full ADP-induced platelet aggregation, is not the target of the drug clopidogrel

Recently we showed that the P2Y1 receptor coupled to calcium mobilization is necessary to initiate ADP-induced human platelet aggregation. Since the thienopyridine compound clopidogrel specifically inhibits ADP-induced platelet aggregation, it was of interest to determine whether the P2Y1 receptor was the target of this drug. Therefore we studied the effects of clopidogrel and of the two specific P2Y1 antagonists A2P5P and A3P5P on ADP-induced platelet events in rats. Although clopidogrel treatment (50 mg/kg) greatly reduced platelet aggregation in response to ADP as compared to untreated platelets, some residual aggregation was still detectable. In contrast, A2P5P and A3P5P totally abolished ADP-induced shape change and aggregation in platelets from both control and clopidogrel-treated rats. A2P5P and A3P5P (100 microM) totally inhibited the [Ca2+]i rise induced by ADP (0.1 microM) in control and clopidogrel-treated platelets, whereas clopidogrel treatment had no effect. Conversely, the inhibition of adenylyl cyclase induced by ADP (5 microM) was completely blocked by clopidogrel but not modified by A2P5P or A3P5P (100 microM). A3P5P (1 mM) reduced the number of [33P]2MeSADP binding sites on control rat platelets from 907 +/- 50 to 611 +/- 25 per platelet. After clopidogrel treatment, binding of [33P]2MeSADP decreased to 505 +/- 68 sites per platelet and further decreased to 55 +/- 12 sites in the presence of A3P5P (1 mM). In summary, these results demonstrate that the platelet P2Y1 receptor responsible for the initiation of aggregation in response to ADP is not the target of clopidogrel. Platelets may express another, as yet unidentified, P2Y receptor, specifically coupled to the inhibition of adenylyl cyclase and necessary to induce full platelet aggregation, which could be the target of this drug.

NF279: a novel potent and selective antagonist of P2X receptor-mediated responses

8,8'-(Carbonylbis(imino-4, 1 -phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis(1,3, 5-naphthalenetrisulfonic acid) (NF279) antagonized P2X receptor-mediated contractions in rat vas deferens, evoked by alpha,beta-methylene ATP (10 microM; pIC50=5.71) without affecting responses mediated via alpha1A-adrenoceptors, adenosine A1 and A2B receptors, histamine H1, muscarinic M3 and nicotinic receptors. The low inhibitory potency of NF279 on P2Y receptors in guinea-pig taenia coli (pA2=4.10) and at ecto-nucleotidases in folliculated Xenopus laevis oocytes (IC50 > 100 microM) indicates that NF279 is a novel specific and selective P2X receptor antagonist.

Extracellular ATP directly gates a cation-selective channel in rabbit airway ciliated epithelial cells

1. A membrane conductance activated by extracellular ATP was identified and characterized in freshly dissociated rabbit airway ciliated cells using the whole-cell and outside-out patch configurations of the patch-clamp technique. 2. In solutions designed to maximize currents through voltage-gated calcium channels, there were no indications of voltage-gated Ba2+ currents. 3. Extracellular ATP (but not UTP or ADP) activated a membrane conductance which remained activated for several minutes in the presence of ATP. The conductance was permeable to monovalent and divalent cations with approximate relative permeabilities (P) for PBa : PCs : PTEA of 4 : 1 : 0.1. Permeability to Cl- was negligible. 4. Including GDP-beta-S in the intracellular solution did not inhibit the effects of ATP, nor did GTP-gamma-S irreversibly activate the conductance. 5. In outside-out membrane patches, with GDP-beta-S in the pipette solution, ATP activated ion channels which had a chord conductance of approximately 6 pS in symmetrical 150 mM CsCl solutions at -120 mV. 6. Suramin (100 microM) inhibited the whole-cell currents activated by ATP (200 microM) by 93 +/- 3 %. Similar effects of suramin were observed on ATP-activated channels in outside-out membrane patches. 7. Extracellular ATP had a priming action on the response to subsequent exposure to ATP. At -40 mV, the time to half-maximal current activation (t1/2) was 46 +/- 9 s during the first exposure to 200 microM ATP and decreased to 5 +/- 3 s during a second exposure to the same concentration of ATP. The priming action of ATP was not inhibited by including GDP-beta-S in the intracellular solution. 8. The initial rate of activation increased with the concentration of ATP, and was voltage sensitive. During the first exposure to 200 microM ATP, t1/2 at +40 mV was 4-fold longer than t1/2 at -40 mV. 9. Half-maximal activation of the conductance shifted from 210 +/- 30 to 14 +/- 4 microM added ATP when CaCl2 in the extracellular solution was reduced from 1.58 to 0. 01 mM. The Hill coefficient for ATP was 1.2 in both solutions.10. These observations suggest that a form of ATP uncomplexed with divalent cations directly gates an ion channel (P2X receptor) in rabbit airway ciliated cells, which serves as a pathway for Ca2+ influx. This purinoceptor may contribute to sustained ciliary activation during prolonged exposures to ATP.