Inhibition of fibrinogen receptor expression and serotonin release by leupeptin and antipain

Human platelet agonists such as thrombin, ADP, and collagen stimulate the rapid expression of fibrinogen receptors. In other cell types, calcium-activated proteases have been suggested to participate in the mechanism of expression of cell surface receptors (Lynch, G., and Baudry, M. (1984) Science 224, 1057-1063). In platelets the majority of the neutral protease activity is calcium-activated protease. We examined the effects of leupeptin and antipain, two calcium-activated protease inhibitors, on the expression of platelet fibrinogen receptors. These inhibitors abolished thrombin and ADP-induced fibrinogen binding. This inhibition required the addition of leupeptin or antipain prior to the agonist and was not due to displacement of fibrinogen from its receptor or inhibition of agonist binding to platelets. Leupeptin and antipain also inhibited fibrinogen-independent thrombin-stimulated release of serotonin. These results are discussed in relation to the involvement of calcium-activated protease in early events of platelet activation.