In the present paper we compare prothrombin-converting activities of platelets non-activated, or activated by collagen, thrombin or collagen plus thrombin in the absence and presence of added factor Va. In all experiments described, the rate of thrombin formation for platelets activated by the combined action of collagen and thrombin is greater than that of platelets stimulated by collagen or thrombin alone. The presence of added factor Va enhanced the rate of thrombin formation in all cases, but the higher activity observed with platelets stimulated by collagen plus thrombin remains. When platelets are activated by collagen plus thrombin in the presence factor Xa and prothrombin, a lag period of approximately 10 min is observed before the rate of thrombin formation reaches a steady state. Addition of an excess of factor Va in this experiment reduces the lag time to 3 min. This lag period is interpreted as the time required to generate extra binding sites for the prothrombinase complex at the platelet surface. These extra sites explain the difference in thrombin formation rate between these platelets and platelets activated by either collagen or thrombin only.
The exposure of phospholipids at the platelet outer surface was studied with phospholipases after various activations of the platelets. It is demonstrated that activation by collagen plus thrombin is accompanied by increased susceptibility of platelet phospholipids towards phospholipase A2. Among these degradable phospho- lipids are 25% of the phosphatidylserine and 30% of the phosphatidylethanolamine. On the other hand, little no phosphatidylserine is exposed at the membrane exterior of thrombin-treated or control platelets. We propose that the exposure of phosphatidylserine at the outer surface of platelets activated with thrombin plus collagen essential for the rate enhancement of thrombin formation observed under these conditions. The possibility of transbilayer movement of phospholipids in the platelet membrane as a result of the activation process will be discussed.