Fibrinopeptide A Cleavage And Platelet Release In Whole Blood In Vitro

The relationship between platelet release and fibrinopeptide A (FPA) cleavage from fibrinogen to form fibrin I was examined in blood allowed to clot in vitro. FPA and the platelet release products (platelet factor 4 and (β-thrombo- globulin) were quantitated by radioimmunoassay. In undisturbed blood platelet release and FPA cleavage occurred simultaneously. Addition of hirudin prevented platelet release as well as FPA cleavage, indicating that thrombin was essential for platelet release. Addition of collagen increased platelet release and then FPA cleavage, and addition of ADP had a similar but less marked effect, suggesting that increased platelet activation accelerated thrombin formation. PGE₁ and theophylline inhibited platelet release but did not affect FPA cleavage in the initial stages of incubation. When blood was agitated, the effects of collagen and PGE₁ and theophylline were qualitatively similar to those in undisturbed blood, but addition of hirudin no longer affected platelet release, suggesting that in this case platelet release occurred independently of thrombin action. Addition of dilute thrombin or tissue thromboplastin to undisturbed blood accelerated FPA cleavage with little effect on platelet release, indicating that exogenous thrombin or thrombin generated by the extrinsic system behaves differently from that generated by the intrinsic system. Finally, thrombin concentrations were compared in platelet rich and platelet free plasma and were found to be increased in platelet rich plasma in spontaneously clotting blood but not in blood to which tissue thromboplastin was added. These relationships suggest that thrombin formation via the intrinsic system occurs on the platelet surface leading to simultaneous platelet release and FPA cleavage, while thrombin generated via tissue thromboplastin leads to thrombin formation in the plasma and FPA cleavage precedes platelet release. Collagen primarily accelerates platelet release and secondarily increases thrombin formation.