Heparin neutralizes serum antiplasmin inhibition of peripheral blood leukocyte fibrinolytic activity.

Serum inhibition of peripheral blood fibrinolytic activity has been evaluated with the 125I-fibrin coated well method. The inhibitory activity was found in a 140,000 d serum fraction that contained alpha 2-antiplasmin. Addition of heparin to cell cultures at concentrations in the range of values obtained during anticoagulant therapy has been demonstrated to counteract such inhibitory activity. The phenomenon has been shown to be linked with plasminogen activation in the presence of fibrin, to lead to a weakening of antiplasmin activity. By clarifying an important aspect of the mechanism of heparin action, our findings support the view that heparin can be usefully employed in treating thrombotic syndromes, not only as an anticoagulant, but also as a "profibrinolytic" agent.