SummaryThe fibrinolytic activity respectively the PA activity of normal veins before and after venous occlusion is measured by the histochemical method of Todd. The results show marked decreases of PA of the adventitia and an increase of PA of the endothelial type, after venous occlusion. This suggests a transfer of PA through the venous wall into the intravascular lumen, and may be the cause of increased fibrinolytic activity in venous blood after occlusion. Contrary assumptions are discussed.

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SummaryThe influence of the inhibitors of plasminogen activation by streptokinase, urokinase and tissue activator and of the two antiplasmins i.e. α1 antitrypsin and α2-macroglobulin, upon the formation and clearance of streptokinase-activator in vivo was investigated in 22 subjects. No influence of the above mentioned inhibitors on the formation of the streptokinase-activator could be found, whereas the clearance of the activator seems to be a function of the α2-macroglobulin level before streptokinase administration. The clearance rate of the streptokinase-activator differed significantly from the clearance rate of 131I labelled streptokinase caused by antigen-antibody complex formation [Fletcher et al. (10)], and was similar to the clearance rate of the histamine or nicotinic acid induced plasminogen activator. The possibility of a common clearing mechanism of fibrinolytic activators is discussed.The existence of 3 different inhibitors activities on plasminogen activation as tested in vitro i. e. inhibitor of streptokinase, urokinase and tissue activator, was demonstrated. These inhibitory activities are probably not the properties of 3 different serum proteins but rather the result of the action of α1antitrypsin and α2-macroglobulin. The degree of inhibition, however, of each of the 2 proteins on each of the 3 plasminogen activators (streptokinase, urokinase and tissue activator) seems to be different. This difference may be caused by a different inhibitory effect of α1 antitrypsin or α2-macroglobulin on each of the plasminogen activators.The plasmin formed after the streptokinase infusion is rapidly bound by the α2macroglobulin and this plasmin-antiplasmin complex is immediately removed from the blood, causing a decrease of the α2-macroglobulin level. This decrease of α2-macroglobulin is correlated with the initial level of α2-macroglobulin. This finding demonstrates the concentration depending formation of the plasmin-antiplasmin complex also in vivo.