Duality of plasmin effect on cytosolic free calcium in human platelets

Plasmin caused a modest and gradual increase in platelet cytosolic Ca²⁺, mediated through both Ca²⁺ mobilization and external Ca²⁺ entry. This response was associated with accelerated Ca²⁺ extrusion and protein tyrosine phosphorylation. Plasmin-enhanced external Ca²⁺ entry and Ca²⁺ extrusion (but not Ca²⁺ mobilization) were attenuated by the tyrosine kinase inhibitor, genistein. Plasmin inhibited the thrombin-evoked increase in cytosolic Ca²⁺ and also inhibited the Ca²⁺ response to the tethered peptide TRAP-6 of the thrombin receptor. Furthermore, plasmin inhibited the binding of ¹²⁵I-labeled α-thrombin to platelets. The inhibitory effect of plasmin on the thrombin response shared some characteristics with the effect of protein kinase C stimulators but was not reversed by protein kinase C inhibitors. Plasmin did not change platelet cyclic nucleotides. These results suggest a dual effect of plasmin. Plasmin produces a small rise in platelet cytosolic Ca²⁺ and a tyrosine kinase-dependent enhancement of Ca²⁺ turnover (external Ca²⁺ influx and Ca²⁺ efflux). However, it also attenuates the thrombin-evoked cytosolic Ca²⁺ response by blocking Ca²⁺ mobilization and slowing the rate of external Ca²⁺ influx. The latter feature would result in a plasmin-induced inhibition of thrombogenesis.