Inhibitory effect of activated protein C on platelet aggregation induced by the prothrombin-converting reaction

The present study was undertaken to elucidate the effect on platelet aggregation of the prothrombin-converting reaction on platelets with or without activated protein C (APC). A reaction mixture of washed platelets from human individuals, Factor Xa and prothrombin markedly induced platelet aggregation; maximum aggregation rates, 31.3–92.5%, and times to reach to maximum aggregation, 11.6 to 20.1 min. This aggregation was inhibited by the addition of APC with 50% inhibition concentration (IC₅₀) value of 14.4 U/ml. APC also inhibited thrombin generation in the reaction mixture in a dose-dependent manner with IC50 value of 0.96 U/ml. However, APC did not inhibit the thrombin (0.1 CU/ml)-induced platelet aggregation at concentrations of up to 30 U/ml. These findings suggest that APC has no direct inhibitory effect on platelet aggregation and that APC inhibits platelet aggregation through inhibition of thrombin generation.