Characterization of P2x1 purinoreceptors on rat platelets: effect of clopidogrel

This study aimed to determine the binding characteristics of 3Halpha,beta-Me-ATP, a specific ligand of the P2x1 receptors to rat platelets, and to investigate the effect of clopidogrel, a thienopyridine compound which has been found to selectively inhibit ADP-induced platelet aggregation and adenylyl cyclase ex vivo. Binding of 3Halpha,beta-Me-ATP to rat platelets was time-dependent and saturable. Scatchard analysis of the saturation binding data indicated that 3Halpha,beta-Me-ATP bound to one population of specific binding sites with high affinity (KD = 23.6 +/- 1.6 nM; Bmax = 690 +/- 24 fmole/108cells) (n=3). Unlabelled alpha,beta-Me-ATP as well as 2-MeS-ADP and ADP competitively inhibited the specific binding of 3Halpha,beta-Me-ATP with IC50 values of 19.0 +/- 6.6, 103 +/- 20 and 1120 +/- 80 nM respectively (n=3). Other nucleotide analogues such as ATP, ATP-gammaS, UTP and GTP also antagonized 3Halpha,beta-Me-ATP binding. When administered orally (10mg/kg, p.o.), clopidogrel inhibited ADP- or 2-MeS-ADP-induced platelet aggregation but did not affect the binding of 3Halpha,beta-Me-ATP to rat platelets ex vivo. In vitro, alpha,beta-Me-ATP did not induce the aggregation or shape change of rat platelets and did not interfere with ADP-induced platelet aggregation.