Characteristics of collagen-induced fibrinogen binding to human platelets

Polymerized type I calf skin collagen induced a time-dependent specific binding of 125I-fibrinogen to washed human platelets. Binding occurred more rapidly in a shaken rather than in an unstirred system. It was linear in the range 0.05-0.3 microM added fibrinogen and was saturated at higher fibrinogen concentrations (more than 0.8 microM). Scatchard analysis showed a single population of binding sites (16530 +/- 5410 per platelet) with a Kd = 0.53 +/- 0.23 microM. Collagen-induced 125I-fibrinogen binding to platelets was completely inhibited by ADP antagonists such as creatine phosphate/creatine phosphokinase and AMP, and partially inhibited by pretreatment of the platelets with aspirin. With both normal and aspirin-treated platelets a close correlation was observed between the amount of 125I-fibrinogen bound and the extent of dense granule secretion. Our results confirm that fibrinogen becomes bound to platelet surface receptors during collagen-induced platelet aggregation and suggest that secreted ADP is an essential cofactor in this process.