a2-Antiplasmin Supplementation Inhibits Tissue Plasminogen Activator-induced Fibrinogenolysis and Bleeding with Little Effect on Thrombolysis

Tissue plasminogen activator (t-PA) causes fibrinogen proteo-lysis when a2-antiplasmin levels fall, and this may contribute to t-PA-induced hemorrhage. Because clot-bound plasmin is pro-tected from a2-antiplasmin inhibition, we tested the possibility that a2-antiplasmin supplementation would block t-PA-in-duced fibrinogenolysis and bleeding without affecting thrombol-ysis. When added to human or rabbit plasma, a2-antiplasmin inhibits t-PA-induced fibrinogenolysis, but has little effect on the lysis of 1251-fibrin clots. To examine its effect in vivo, rab-bits with preformed 125I-labeled-jugular vein thrombi were ran-domized to receive t-PA, t-PA and a2-antiplasmin, or saline. a2-Antiplasmin infusion produced a modest decrease in t-PA-induced thrombolysis (from 40.2 % to 30.1%, P = 0.12), but reduced fibrinogen consumption from 87 % to 27 % (P = 0.0001), and decreased blood loss from standardized ear in-cisions from 5,594 to 656,ul (P < 0.0001). We hypothesize that a2-antiplasmin limits t-PA-induced hemorrhage by inhibit-ing fibrinogenolysis and subsequent fragment X formation be-cause (a) SDS-PAGE and immunoblot analysis indicate less fragment X formation in a2-antiplasmin treated animals, and (b) when added to a solution of fibrinogen and plasminogen clotted with thrombin in the presence of t-PA, fragment X shortens the lysis time in a concentration-dependent fashion. These findings suggest that fragment X incorporation into he-mostatic plugs contributes to t-PA-induced bleeding. By block-ing t-PA-mediated fibrinogenolysis, a2-antiplasmin supple-mentation may improve the safety of fibrin-specific plasmino