Thrombin and adenosine diphosphate (ADP) supported the binding of 125I- fibrinogen to washed human platelets with similar kinetics and affinity. Platelet secretion, as measured by 14C-serotonin release, and fibrinogen binding exhibited an identical dependence on thrombin concentration. Enzymatic removal of ADP with apyrase or creatine phosphate/creatine phosphokinase (CP/CPK) from thrombin-stimulated platelets markedly inhibited 125I-fibrinogen binding, but pretreatment of platelets with CP/CPK prior to thrombin stimulation was without effect. Thus, ADP, released from the platelet, participates in the binding of fibrinogen to thrombin-stimulated platelets.

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