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Platelet activation by sustained exposure to low-dose plasmin

Plasmin has been reported to activate and inhibit platelet function depending on dose and exposure temperature. The present study examines the induction of fibrinogen-dependent platelet aggregation following prolonged (60 min) platelet exposure to very low doses of plasmin (0.05 CU/ml) at either 22 or 37°C. Maximum aggregation [mean ± SD, 60 ± 19 light transmission units (LTU); n = 43] occurred following platelet exposure to plasmin at 22°C, but significant platelet aggregation (28 ± 4 LTU, n = 3) also occurred following plasmin treatment at 37°C. Plasmin-induced platelet aggregates appeared microscopically larger than aggregates of adenosine diphosphate (ADP)-activated platelets, and were less reversible. Aggregated plasmin-treated platelets also expressed more procoagulant activity than platelets aggregated with ADP, as reflected by shortening of the plasma kaolin recalcification time. Aggregation of platelets exposed to very low doses of plasmin was not accompanied by dense or alpha-granule secretion, and was unaffected by ADP antagonists or aspirin. Partial inhibition of platelet aggregation, however, was achieved with metabolic inhibitors, PGE1, and inhibitors of phosphoinositide 3-kinase or protein kinase C. Although fibrinogen was required for plasmin-treated platelet aggregation, [125I]-fibrinogen binding comprised only 58 ± 3% (n = 3) of fibrinogen binding associated with ADP aggregated platelets. This was consistent with observed decreases in reptilase-induced fibrin clot retraction. Taken together, these data suggest that sustained exposure of platelets to very low plasmin doses leads to platelet activation and thus may contribute to thrombotic complications in vivo.