Activation of the blood fibrinolytic enzyme system by platelets

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Abstract
Fibrinolytic activity in human blood develops after the occurrence of viscous metamorphosis (VM) of platelets. Freshly drawn venous blood was held at room temperature until spontaneous VM of platelets occurred and then introduced into oxalate solution to prevent coagulation. As compared to the control (blood added to anticoagulant before VM) plasma of such samples exhibited increased fibrinolytic activity, detected by increased serial thrombin times. Clots from platelet-rich plasma exhibited an increase in the convergence of branches of the thrombelastograph and a decrease in weight after incubation for 24 hr or more. The increase in branch convergence was roughly proportional to the platelet count and was reduced or absent in thrombocytopenic blood or platelet-poor plasma. Dimethylsulfoxide blocked VM and prevented branch convergence (without preventing clot retraction). Epsilon-aminocaproic acid, an inhibitor of plasminogen activator, blocked or inhibited fibrinolytic activity without arresting VM. This evidence suggests that platelets contain significant proactivator or activator of plasminogen (or a factor which initiates their activity) which is released when VM occurs.