Inhibitors of Fibrinolysis in Platelets in Polycythaemia Vera and Thrombocytosis

Summary. The role of platelets in fibrinolysis was studied in relation to their content of fibrinolytic inhibitors. An antiplasmin was present in platelets when studied by the caseinolytic method, but this antiplasmin was weak when the fibrin plate method was used. Thus the fibrin plate method could be used for the study of the inhibitory action of platelets against three different types of plasminogen activator: urokinase, human heart tissue activator and human plasma plasminogen activator. It was found that normal human platelets contained inhibitors against all three plasminogen activators with the highest inhibitory activity against urokinase. Platelets from patients with thrombocytosis due to polycythaemia vera or secondary to bronchogenic carcinoma also showed similar inhibitory activity. The platelets from one patient with polycythaemia vera had an unusually high degree of inhibitory activity. The significance of these findings is discussed with respect to the relative resistance of a platelet thrombus to lysis and to its role in haemostasis.