Biomedical subjects

P Savi

Publications and source records attributed to P Savi.

At least 55 records · Page 3Linked to original sources

PubMed1996

cAMP is not an important messenger for ADP-induced platelet aggregation.

In rat platelets, basal cAMP level did not vary significantly during ADP-induced aggregation. In the same conditions, no variation in the cAMP content was observed in platelets from rats treated with clopidogrel, whereas ADP-induced aggregation was totally inhibited. ADP decreased cAMP level in control prostacyclin- or forskolin-stimulated platelets whereas, in treated platelets, adenylyl cyclase down-regulation was strongly inhibited. SQ 22536 (500 microM), an inhibitor of adenylyl cyclase, strongly reduced the cAMP content of both control and treated platelets but did not reverse the anti-aggregating activity of clopidogrel, showing that inhibition of ADP-induced adenylyl cyclase down-regulation in treated platelets was not responsible for the anti-aggregating effect of clopidogrel. Similar results were obtained in rabbit platelets. These results therefore demonstrate that cAMP is not an important second messenger for ADP-induced platelet aggregation and suggest that another activating pathway, linked to the low affinity ADP receptor present on the platelet surface might be involved in the aggregation process.

Adenine↗

PubMed1996

Ultrastructural studies of platelet aggregates from human subjects receiving clopidogrel and from a patient with an inherited defect of an ADP-dependent pathway of platelet activation.

Our study investigated the effect of the antithrombotic drug clopidogrel (75 mg/d for 7 days) on the ultrastructure of platelet aggregates induced by ADP or 2-methylthio-ADP (2-MeS-ADP) in citrated platelet-rich plasma and examined the activation state of the GP IIb/IIIa complexes. Results were compared with those obtained for patient M.L., who has a congenital disorder characterized by a reduced and reversible platelet response to ADP. When untreated normal platelets were stimulated with high-dose ADP, electron microscopy revealed large and stable aggregates often surrounded by a layer of what appeared to be degranulated platelets. The reversible aggregates of platelets from subjects receiving clopidogrel or from patient M.L. did not show this layer. Electron microscopy showed that in both situations, the aggregates were composed of loosely bound platelets with few contact points. Immunogold labeling of ultrathin sections of Lowicryl-embedded aggregates formed by ADP or 2-MeS-ADP showed a much decreased platelet surface staining by (1) a polyclonal anti-fibrinogen antibody and (2) AP-6, a murine anti-ligand-induced binding site monoclonal antibody specific for GP IIb/IIIa complexes occupied with fibrinogen. Similar findings were seen after disaggregation, when many single platelets were present that showed no signs of secretion. Flow cytometry confirmed that the number of ligand-occupied GP IIb/IIIa complexes was much lower on platelets stimulated with ADP or 2-MeS-ADP after clopidogrel treatment. As expected from previous studies, ADP-induced platelet shape change and Ca2+ influx were unaffected by clopidogrel. These results agree with the hypothesis that platelet activation by ADP is biphasic and highlight a receptor-induced activation pathway affected by clopidogrel (or congenitally impaired in patient M.L.) that is necessary for the full activation of GP IIb/IIIa and the formation of stable macroaggregates.

Adenosine Diphosphate↗

PubMed1995

Human umbilical vein endothelial cells express high affinity neurotensin receptors coupled to intracellular calcium release.

The binding of 125I-neurotensin (NT) to human umbilical vein endothelial cell monolayers was studied. At 20 degrees C, 125I-NT bound to a single class of binding sites with a dissociation constant of 0.23 +/- 0.08 nM and a binding site density of 5500 +/- 1300 sites/cell (n = 3). 125I-NT also bound to human aortic endothelial cells with a dissociation constant of 0.6 +/- 0.26 nM and a binding site density of 32000 +/- 1700 sites/cell. Association and dissociation kinetics were of a pseudo-first order and gave association and dissociation rate constant values of 1.6 x 10(6) M-1 s-1 and 3.5 x 10(-4) s-1, respectively. 125I-NT binding was inhibited by NT analogues with a rank order of potency similar to that characterizing brain high affinity NT binding sites (K0.5, nM): NT8-13 (0.11) > NT (0.35) > acetyl-NT8-13 (1.5) > [Phe11]NT (12) > [D-Tyr11]NT (> 1000). 125I-NT binding was also inhibited by the non-peptide NT antagonist SR 48692 (Ki = 16 nM) but was not affected by levocabastine, an inhibitor of low affinity brain NT binding sites. NT had no effect on cGMP levels in endothelial cells but NT and its analogues increased 45Ca2+ efflux from endothelial cells at nanomolar concentrations with a rank order of potency which was identical to that observed in binding experiments. This effect was inhibited by SR 48692 (IC50 = 8 nM). NT was able to increase phosphoinositide turnover in these cells, and this effect was blocked by SR 48692. The correlation between dissociation constants of NT analogues in binding experiments and IC50 values in 45Ca2+ efflux experiments was very high (r = 0.997) with a slope near unity, indicating that 125I-NT binding sites are functional NT receptors coupled to phosphoinositide hydrolysis and Ca2+ release in human umbilical vein endothelial cells.

Binding Sites↗

PubMed1995

An inherited bleeding disorder linked to a defective interaction between ADP and its receptor on platelets. Its influence on glycoprotein IIb-IIIa complex function.

Much discussion has concerned the central role of ADP in platelet aggregation. We now describe a patient (M.L.) with an inherited bleeding disorder whose specific feature is that ADP induces a limited and rapidly reversible platelet aggregation even at high doses. Platelet shape change and other hemostatic parameters were unmodified. A receptor defect was indicated, for, while epinephrine normally lowered cAMP levels of PGE1-treated (M.L.) platelets, ADP was without effect. The binding of [3H]2-methylthio-ADP decreased from 836 +/- 126 molecules/platelet for normals to 30 +/- 17 molecules/platelet for the patient. Flow cytometry confirmed that ADP induced a much lower fibrinogen binding to (M.L.) platelets. Nonetheless, the binding in whole blood of activation-dependent monoclonal antibodies showed that some activation of GP IIb-IIIa complexes by ADP was occurring. Platelets of a patient with type I Glanzmann's thrombasthenia bound [3H]2-methylthio-ADP and responded normally to ADP in the presence of PGE1. Electron microscopy showed that ADP-induced aggregates of (M. L.) platelets were composed of loosely bound shape-changed platelets with few contact points. Thus this receptor defect has a direct influence on the capacity of platelets to bind to each other in response to ADP.

Adenosine Diphosphate↗

PubMed1994

Evidence for the existence of two different ADP-binding sites on rat platelets.

[3H]-2-Methylthio-ADP ([3H]-2-MeS-ADP), a stable analogue of ADP bound to one type of specific binding sites on rat platelets (KD = 0.77 +/- 0.07 nM, Bmax = 160 +/- 11 fmol/10(8) cells). 2-MeS-ADP and ADP antagonized [3H]-2-MeS-ADP binding, showing respective Ki values of 1.4 +/- 0.1 nM and 486 +/- 78 nM. Clopidogrel, a potent and specific inhibitor of ADP-induced platelet aggregation partially inhibited (approximately 70% inhibition) the binding of [3H]-2-MeS-ADP at the same time it abrogated 2-MeS-ADP- and ADP-induced adenylyl cyclase inhibition and aggregation. A population of clopidogrel-resistant [3H]-2-MeS-ADP binding sites was detected on platelets from treated animals. These receptor sites (KD = 0.9 +/- 0.2 nM, Bmax = 47 +/- 5 fmol/10(8) platelets) which showed high affinity for both ADP and 2-MeS-ADP (Ki values in the nanomolar range) might be involved in the ADP-induced shape change, a clopidogrel-resistant ADP-induced event. Using clopidogrel which acts via a direct and irreversible inhibition of ADP binding to its adenylyl cyclase-coupled receptor sites on platelets, we were able to discriminate between two types of ADP receptor sites. The former which was clopidogrel-sensitive represented about 70% of the total [3H]-2-MeS-ADP receptors and was responsible for ADP-induced platelet aggregation and adenylyl cyclase inhibition. The latter which was not affected by clopidogrel might be involved in ADP-induced shape-change.

Adenosine Diphosphate↗

PubMed1994

Malformin-A1 inhibits the binding of interleukin-1 beta (IL1 beta) and suppresses the expression of tissue factor in human endothelial cells and monocytes.

Malformin-A1, a cyclic pentapeptide of microbial origin, antagonized in a competitive manner the binding of 125I-IL1 beta (interleukin-1 beta) to human monocytes and cultured human umbilical vein endothelial cells (HUVEC) with IC50 values (doses which reduce specific binding by 50%) of 250 +/- 80 and 230 +/- 25 nM, respectively (N = 3). IL1 increased in a dose-dependent manner the expression of tissue factor, a ubiquitous membrane-anchored glycoprotein that initiates blood coagulation at the surface of HUVEC and human monocytes. Malformin-A1 strongly inhibited IL1-induced tissue factor expression in HUVEC and monocytes with IC50 values of 420 +/- 35 and 105 +/- 25 nM, respectively (N = 3), and reduced IL1-induced expression of intercellular adhesion molecule-1 (ICAM-1, CD54) on HUVEC (IC50 = 125 +/- 18 nM) (N = 4). These observations demonstrate that malformin-A1 recognizes and blocks IL1 beta binding to its receptor sites on monocytes and endothelial cells and protects these cells from IL1-induced procoagulant changes.

Cells, Cultured↗

PubMed1994

Characterization of specific binding sites for [3H]2-MeS-ADP on megakaryocytoblastic cell lines in culture.

Binding of [3H]2-methyl thio-adenosine 5' diphosphate ([3H]2-MeS-ADP), a stable analogue of adenosine 5' diphosphate (ADP) to DAMI and Meg-01, two megakaryocytoblastic cell lines, was time-dependent, reversible and saturable. Scatchard analysis of the saturation binding data indicated that [3H]2-MeS-ADP bound to one class of specific binding sites with high affinity (dissociation constants = 45.3 +/- 13.4 and 48.2 +/- 17.7 nM, and maximum binding capacities = 341.2 +/- 31.1 and 903 +/- 98 fmole/10(6) cells for DAMI and Meg-01, respectively) (N = 3). Unlabelled 2-MeS-ADP competitively and selectively inhibited the specific binding of [3H]2-MeS-ADP on DAMI and Meg-01 with inhibitory constant values of 118 +/- 11 and 38 +/- 11 nM, respectively (N = 3). ADP was 3 to 10 times less potent than 2-Mes-ADP in displacing [3H]2-MeS-ADP from its binding sites on DAMI and Meg-01, whereas other ADP analogues, such as AMP, GDP, UDP, adenosine or FSBA, did not interfere with the binding of [3H]2-MeS-ADP, suggesting that DAMI and Meg-01 contain ADP-specific receptors.

Adenosine Diphosphate↗

PubMed1994

Effect of aspirin and clopidogrel on platelet-dependent tissue factor expression in endothelial cells.

Tissue factor is an ubiquitous membrane-anchored glycoprotein that initiates blood coagulation by forming a complex with circulating factors VII and VIIa. Under normal circumstances, endothelial cells do not express tissue factor but, in some pathological situations, when the endothelium or the monocytes are exposed to inflammatory mediators, they can acquire procoagulant properties. When rat platelets were incubated with cultured bovine aortic endothelial cells, a significant increase in tissue factor expression was obtained. When added simultaneously, thrombin or 2-methylthio-ADP, a stable analogue of ADP, potentiated in a time and dose-dependent manner the effect obtained with platelets alone. In order to determine if antiplatelet agents can modulate these effects, the activity of two compounds was evaluated. When administered orally at the dose of 25 mg/kg, clopidogrel, a potent and selective analogue of ticlopidine, was able to inhibit platelet-induced tissue factor expression whereas aspirin (100 mg/kg, p.o.) was ineffective. These effects were closely correlated to their respective anti-aggregatory and antithrombotic activities showing that platelet activation which has already been shown to be mainly involved in arterial-type thrombosis could also play an important role in the initiation of venous thrombosis where tissue factor expression is thought to play a major role.

Adenosine Diphosphate↗

PubMed1994

The antiaggregating activity of clopidogrel is due to a metabolic activation by the hepatic cytochrome P450-1A.

Clopidogrel and ticlopidine are two well known selective anti-ADP agents which are inactive in vitro and must be administered in vivo to fully exhibit their antiaggregating and antithrombotic effects. Since previous studies have clearly demonstrated that the activation steps take place in the liver, we examined the effect of specific induction or inhibition of cytochrome P450 subfamilies on the antiaggregating activity of clopidogrel. SKF 525-A, a global cytochrome P450 inhibitor, dramatically decreased the antiaggregating effect of clopidogrel, therefore indicating that cytochrome P450 enzymes are involved in the hepatic activation of clopidogrel. The efficacy of clopidogrel was increased in animals pretreated with 3-methylcholanthrene and beta-naphthoflavone, indicating that the cytochrome P450-1A subfamily pathway was mainly involved in the activating metabolism of clopidogrel. The use of specific antibodies directed against the various cytochrome P450 subfamilies ascertained this observation.

Adenosine Diphosphate↗

PubMed1994

Binding of [3H]-2-methylthio ADP to rat platelets--effect of clopidogrel and ticlopidine.

Thienopyridine compounds, including ticlopidine and clopidogrel, have been found to selectively inhibit adenosine 5' diphosphate (ADP)-induced platelet aggregation and adenylyl cyclase ex vivo, but the mechanism of their antiplatelet action remains to be determined. This study was aimed at investigating the effect of clopidogrel and ticlopidine on the binding of [3H]-2-methylthio- adenosine-5'-diphosphate (2-MeS-ADP) to rat platelets. Binding of [3H]-2-MeS-ADP to rat platelets was time-dependent and saturable. Scatchard analysis of the saturation binding data indicated that [3H]-2-MeS-ADP bound to one population of specific binding sites with high affinity (KD = 0.78 +/- 0.05 nM; Bmax = 156.3 +/- 4.8 fmole/10(8) cells) (n = 3). Unlabeled 2-MeS-ADP and ADP competitively and selectively inhibited the specific binding of [3H]-2-MeS-ADP with IC50 values of 11.3 +/- 1.2 nM and 11.3 +/- 0.7 microM, respectively (n = 3). Other nucleotide analogs such as ADP-beta S, ATP and ATP-alpha S also antagonized [3H]-2-MeS-ADP binding. When administered orally at doses ranging from 1 to 25 mg/kg, clopidogrel inhibited ADP- or 2-MeS-ADP-induced platelet aggregation as well as ADP or 2-MeS-ADP-induced inhibition of intraplatelet adenylyl cyclase. When measured in parallel, clopidogrel reduced in a dose-dependent manner the binding of [3H]-2-MeS-ADP to rat platelets ex vivo. Clopidogrel administration resulted in the decrease of [3H]-2-MeS-ADP binding sites on platelets without any significant change in the affinity; this indicates noncompetitive binding. Ticlopidine (200 mg/kg a day for 3 days) behaved in the same way.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Diphosphate↗

PubMed1993

Chelerythrine, a selective protein kinase C inhibitor, counteracts pyrogen-induced expression of tissue factor without effect on thrombomodulin down-regulation in endothelial cells.

Endotoxin, interleukin 1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF-alpha) dose-dependently increased the expression of tissue factor and at the same time induced thrombomodulin down-regulation on the surface of cultured bovine aortic endothelial cells. Chelerythrine, a selective protein kinase C inhibitor, strongly reduced endotoxin-, IL1 beta- and TNF alpha-induced tissue factor expression but remained without effect with regard to thrombomodulin down-regulation measured in parallel. On the contrary, staurosporine, a highly potent, non-selective PKC inhibitor, simultaneously abolished tissue factor expression and thrombomodulin down-regulation induced by endotoxin, IL1 beta and TNF alpha. These results show that protein kinase C is deeply involved in the process leading to pyrogen-induced tissue factor expression and suggest that thrombomodulin down-regulation is regulated by a different pathway.

Alkaloids↗

PubMed1993

IL-4 and IL-13 exhibit comparable abilities to reduce pyrogen-induced expression of procoagulant activity in endothelial cells and monocytes.

Endotoxin (LPS), interleukin-1 beta (IL-1) and tumor necrosis factor-alpha (TNF) increased the expression of tissue factor, a membrane-anchored glycoprotein that initiates blood coagulation on the surface of cultured bovine aortic endothelial cells (ABAE) and human monocytes. These compounds simultaneously reduced the amount of thrombomodulin on the endothelial cell surface, further contributing to the procoagulant activity of the endothelium or monocytes. On endothelial cells and monocytes, interleukin-4 (IL-4) and interleukin-13 (IL-13), a newly described lymphokine, both strongly inhibited LPS-induced tissue factor expression, a similar activity also being obtained with regard to the pyrogenic effects of IL-1 or TNF. When measured in parallel, IL-4 and IL-13 counteracted thrombomodulin down-regulation induced by LPS, IL-1 or TNF in endothelial cells. These results therefore show that both IL-4 and IL-13 protect the endothelial and the monocyte surface against inflammatory mediator-induced procoagulant changes.

Animals↗

PubMed1993

Biochemical and pharmacological properties of SR 27388, a dual antioxidant and PAF receptor antagonist.

SR 27388 (N-(2-dimethylaminoethyl)-N-(3-pyridinylmethyl[4-(3,5-di(tert- butyl)-4-hydroxylphenyl)thiazol-2-yl]amine) is a potent and competitive antagonist of the binding of [3H]PAF to its receptor on rabbit platelets exhibiting an equilibrium inhibition constant for PAF binding of 10.5 +/- 1.2 nM (n = 3). SR 27388 potently inhibited PAF-induced aggregation of rabbit platelets in vitro (IC50 = 65 +/- 12 nM) (n = 4). In this respect, SR 27388 was as potent as the triazolothienodiazepine WEB-2086 against PAF-induced aggregation of rabbit platelets and had no effect on the action of other platelet aggregating agents. SR 27388 prevented in a dose-dependent manner the formation of thiobarbituric acid reactive substances during membrane peroxidation (IC50 = 0.7 microM) and inhibited reduction of the stable 1,1-diphenyl-2-picrylhydrazyl radical, indicating that the antioxidant potency of SR 27388 was due to an efficient radical scavenging activity. SR 27388 displayed marked in vitro inhibition of zymosan-induced oxidative burst in human monuclear cells (IC50 = 3 microM). In vivo, SR 27388 protected mice from 100 micrograms/kg PAF-induced death with an ED50 value of 500 micrograms/kg, when given i.v., 5 min before PAF challenge or p.o. (ED50 = 800 micrograms/kg) when given 1 h before PAF administration. Similarly, i.v. or oral doses of SR 27388 afforded in mice complete protection against endotoxin-induced lethality (ED50 values were 250 micrograms/kg and 1.3 mg/kg, respectively). Neither BHT, vitamin E nor catechin exhibited significant protection against PAF- or endotoxin-induced death. In ovalbumin-presensitized rabbits, SR 27388 premixed with the allergen inhibited in a dose-dependent manner allergen-induced oedema formation in the skin (ED50 = 0.1 mumol/site). After an i.v. administration of 10 mg/kg, SR 27388 significantly protected mice against alloxan-induced diabetes. These results show that SR 27388 is a potent and orally active dual PAF receptor antagonist and antioxidant.

Animals↗

PubMed1993

ADP-dependence of platelet activation induced by a thrombin receptor agonist.

The synthetic peptide SFLLRNPNDKYEPF (Thrombin Receptor Agonist: TRA) has recently been shown to mimic the new amino-terminus created by cleavage of the thrombin receptor on platelets and therefore to act as a receptor agonist. In the present work, this peptide proved a potent aggregating agent for human platelets with an ED50 of 3 microM. The ADP removing enzyme system creatine phosphate/creatine phosphokinase (CP/CPK) and the ADP receptor antagonist ATP alpha S strongly inhibited platelet aggregation in response to low doses of TRA, indicating that TRA-induced platelet aggregation, like thrombin-induced aggregation is an ADP mediated event. CP/CPK had however no effect on aggregation in response to the agonist peptide at higher concentrations. Similar results were obtained with regard to the influence of TRA and thrombin on platelet adenylyl cyclase activity, while both agents induced nucleotide release from platelets in a dose-dependent manner. These results confirm that the aggregating effect of alpha-thrombin on human platelets is closely linked to its esterolytic activity at the receptor level and show that this aggregation is an ADP mediated event.

Adenosine Diphosphate↗

PubMed1992

The NK1 receptor is involved in the neurokinin-induced shape change of rabbit platelets.

Substance P and selective neurokinin receptor agonists have been tested for their ability to induce shape change in rabbit platelets. Substance P and the NK1 receptor agonist Ac [Arg6,Sar9,Met(O2)11]-substance P (6-11) induced shape change (EC50 = 3 and 6 nM, respectively), whereas the selective NK2 agonist [Nle10]-Neurokinin A (4-10) and the selective NK3 agonist [MePhe7]-Neurokinin B did not show any effect. Moreover, the specific NK1 receptor antagonist CP-96,345 selectively and dose-dependently counteracted the effect of substance P or of the NK1 receptor agonist (IC50 = 2 and 0.8 nM, respectively), whereas the selective NK2 receptor antagonist, SR 48968, had no effect. Unlike for serotonin or low doses of ADP, epinephrine did not allow substance P or the NK1 receptor agonist to become a proaggregating substance. These data therefore show that the NK1 receptor is solely involved in the neurokinin-induced shape change of rabbit platelets.

Animals↗

PubMed1992

IL-4 inhibits LPS-, IL-1 beta- and TNF alpha-induced expression of tissue factor in endothelial cells and monocytes.

Inflammatory mediators such as endotoxin, interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) dose-dependently increased the expression of tissue factor on the surface of cultured bovine aortic endothelial cells (ABAE), human umbilical vein endothelial cells (HUVEC) and human monocytes. In ABAE, endotoxin-, IL-1 beta- and TNF alpha-induced tissue factor expression was suppressed by interleukin-4 (IL-4) which also neutralized the pyrogenic effect of endotoxin in HUVEC and monocytes. IL-4 did not alter TNF-alpha-induced procoagulant changes in HUVEC and monocytes but strongly protected the monocyte surface against IL-1 beta-induced procoagulant changes.

Animals↗

PubMed1992

Importance of hepatic metabolism in the antiaggregating activity of the thienopyridine clopidogrel.

The thienopyridine clopidogrel is not active in vitro and must be administered i.v. or orally, suggesting that metabolism is necessary for activity. To verify whether the effect after i.v. administration was consecutive to recycling by hepatic bile secretion of clopidogrel or its metabolite(s) in the digestive tract, a catheter was implanted in the choledocus of rats, preventing bile and pancreatic secretions from being excreted into the digestive tractus. Two hours after clopidogrel administration (10 mg/kg, i.v.), blood was withdrawn and platelet-rich plasma aggregation was measured after the addition of 5 microM ADP. Clopidogrel treatment was equally efficient for sham-operated and catheterized animals (% inhibition of platelet aggregation: 76% and 59%, respectively) suggesting that the i.v. effect of clopidogrel was independent of re-absorption of biliary-excreted products and consequently that enteric metabolism is not necessary for activity. The antiaggregating activity of clopidogrel in rats before and after functional hepatectomy by a porto-jugular shunt was then studied. A great difference between treated animals was observed 30 min after i.v. administration of 25 mg/kg of clopidogrel. Per cent inhibition of platelet aggregation was 76% and 6% (P less than 0.001) for sham-operated and hepatectomized animals, respectively. Similar results were obtained after intraduodenal administration of clopidogrel, showing that the treatment was completely ineffective in hepatectomized animals. In isolated, blood-perfused rat livers, clopidogrel inhibited ADP-induced platelet aggregation, thereby supporting the theory that the activity of clopidogrel is highly dependent on hepatic metabolism.

Adenosine Diphosphate↗

PubMed1992

ADP receptor induced activation of guanine nucleotide binding proteins in rat platelet membranes--an effect selectively blocked by the thienopyridine clopidogrel.

The thienopyridine clopidogrel, a potent analog of ticlopidine, is a powerful inhibitor of ADP induced platelet aggregation and ADP induced inhibition of cyclic AMP accumulation in intact platelets but not of ADP induced shape change. We have recently demonstrated that ADP stimulates the binding of GTP gamma S to GTP binding proteins (G proteins) in human platelet membranes. We now studied the effects of clopidogrel, a specific inhibitor of ADP induced platelet aggregation on the stimulation of GTP gamma S binding to rat platelet membranes by ADP. Using the non hydrolyzable stable analog of ADP, 2MeSADP, we demonstrate that 2MeSADP stimulates the binding of [35S]GTP gamma S to rat platelet membranes in a concentration dependent manner, that this effect is inhibited by the specific ADP receptor antagonist Sp-ATP alpha S and that clopidogrel completely and selectively blocks the stimulation by 2MeSADP of [35S]GTP gamma S binding to platelet membranes of treated rats. We conclude that: i) rat platelet membranes possess an ADP receptor coupled to unidentified G protein(s) and ii) the thienopyridine clopidogrel impairs the interaction of the ADP receptor with its G protein by an irreversible modification the ADP receptor itself or its putative G protein.

Adenylyl Cyclases↗