EVIDENCE FOR THE EXISTENCE OF 2 DIFFERENT ADP-BINDING SITES ON RAT PLATELETS

[H-3]-2-Methylthio-ADP ([H-3]-2-MeS-ADP), a stable analogue of ADP bou nd to one type of specific binding sites on rat platelets (K-D = 0.77 +/- 0.07 nM, Bmax = 160 +/- 11 fmol/10(8) cells). 2-MeS-ADP and ADP an tagonized [H-3]-2-MeS-ADP binding, showing respective Ki values of 1.4 +/- 0.1 nM and 486 +/- 78 nM. Clopidogrel, a potent and specific inhi bitor of ADP-induced platelet aggregation partially inhibited (similar to 70% inhibition) the binding of [H-3]-2-MeS-ADP at the same time it abrogated 2-MeS-ADP-and ADP-induced adenylyl cyclase inhibition and a ggregation. A population of clopidogrel-resistant [H-3]-2-MeS-ADP bind ing sites was detected on platelets from treated animals. These recept or sites (K-D = 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 th e nanomolar range) might be involved in the ADP-induced shape change, a clopidogrel-resistant ADP-induced event. Using clopidogrel which act s via a direct and irreversible inhibition of ADP binding to its adeny lyl cyclase-coupled receptor sites on platelets, we were able to discr iminate between two types of ADP receptor sites. The former which was clopidogrel-sensitive represented about 70% of the total [H-3]-2-MeS-A DP 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.