THROMBIN BINDING TO PLATELETS DEFINES FUNCTIONAL RECEPTORS - INHIBITION OF THROMBIN-INDUCED PLATELET ACTIVATION BY CATALYTICALLY-INACTIVATED THROMBIN

It has been widely questioned as to whether the observed binding of al pha-thrombin to intact platelets defines receptors coupled to signal t ransduction or merely thrombin binding sites, We have now shown that a t alpha-thrombin concentrations sufficient to induce a full shape chan ge response without aggregation (0.1 nM), PPACK-thrombin (that is, alp ha-thrombin treated with the irreversible active site inhibitor D-phen ylalanyl-L-prolyl-L-arginine cholormethylketone) dose-dependently inhi bits platelet shape change (IC50 similar to 70 nM), the concomitant in creases in [Ca2+](i) (IC50 similar to 75 nM) and ATP secretion (IC50 s imilar to 50 nM). Since PPACK-thrombin competes fully in the binding o f alpha-thrombin to high, moderate and low affinity sites on intact pl atelets, these results show that this binding defines functional recep tors coupled to platelet activation.