Involvement of a calcium-independent pathway in plasmin-induced platelet shape change

Plasmin-induced platelet activation is considered to be a cause of reocclus ion after thrombolytic therapy with plasminogen activators. However, little is known regarding its mechanism and regulation, particularly with respect to the initial step shape change. We here demonstrate that a Ca2+-independ ent pathway is involved in plasmin-induced human platelet shape change, and that Rho-kinase plays an important role in this pathway. When the increase in cytosolic Ca2+ was prevented by an intracellular Ca2+ chelator. 5,5'-di methyl-BAPTA, plasmin-induced platelet shape change was partially inhibited but still occurred. In the presence of 5,5'-dimethyl-BAPTA, a specific Rho -kinase inhibitor, Y-27632, completely inhibited the shape change. Phosphor ylation of myosin light chain, a key regulator of platelet shape change, wa s completely inhibited by Y-27632 in 5,5'-dimethyl-BAPTA-treated platelets, Although plasmin caused tyrosine phosphorylation of the 80 kDa protein dur ing the shape change. it did not seem to have a critical role. cAMP-elevati ng agents inhibited plasmin-induced shape change in 5,5'-dimethyl-BAPTA- or Y-27632-treated platelets with similar efficiency. These results indicated that plasmin causes platelet shape change by activating Ca2+-dependent and Ca2+-independent-Rho-kinase-dependent pathways, both of which are sensitiv e to cAMP. (C) 2001 Elsevier Science Inc. All rights reserved.