The role of ADP secretion and thromboxane synthesis in factor VIII binding to platelets

Following stimulation with arachidonic acid, collagen, U-46619 (a stable analogue of prostaglandin endoperoxide/thromboxane-A2), thrombin, or adenosine diphosphate (ADP), unstirred human platelet suspensions bound labeled factor VIII in a reaction that reached equilibrium within 10 min. Apyrase inhibited binding induced by arachidonic acid, collagen, U-46619, and thrombin by less than 40%, but inhibited ADP-induced binding by 95%. Binding to aspirin-treated platelets was normal in response to U-46619, reduced by 60%-70% in response to ADP, collagen, and thrombin, and absent in response to arachidonic acid. Binding in response to U-46619 was not altered by the combination of apyrase and aspirin. Binding of factor VIII was decreased by 90% when 10 mM EDTA was added before each agonist, but it was inhibited less than 30% when EDTA was added following platelet stimulation. We conclude that arachidonic acid, collagen, and thrombin can expose binding sites for factor VIII independently of released ADP; that Ca++ is required for activation but probably not for binding of factor VIII to platelets; and that platelet thromboxane synthesis plays a major role in the binding of factor VIII to platelets induced by thrombin, ADP, or collagen.