On the mechanism of heparin-induced potentiation of platelet aggregation.

The role of antithrombin III (AT III) in heparin-induced potentiation of platelet aggregation was investigated using purified AT III and AT III depleted plasma. When ADP or epinephrine was added to citrated platelet rich plasma (PRP) one minute after addition of heparin, marked enhancement of platelet aggregation was observed, compared with the degree of platelet aggregation in the absence of heparin. However, heparin exhibited no potentiating effect on ADP- or epinephrine-induced platelet aggregation when platelets were resuspended in AT III depleted plasma prepared by immunosorption using matrix-bound antibodies to AT III. When purified AT III was added to AT III depleted plasma at a concentration of 20 microgram/ml, potentiation of platelet aggregation by heparin was clearly demonstrated. These results suggest that the effect of heparin on platelet aggregation is also mediated by AT III.