[Protein-tyrosine phosphorylation of human platelets and its function]

Addition of ionophore A23187 to washed human platelets caused a time- and dose-dependent increase in the phosphotyrosyl content of 135, 124 and 76 kDa proteins. Platelets loaded with intracellular Ca2+ chelator 5,5'-demethyl-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid before addition of A23187 exhibited no protein-tyrosine phosphorylation. Replenishment of such platelets with extracellular CaCl2 restored A23187-induced protein-tyrosine phosphorylation. Upon stimulation with A23187, both aspirin and ADP scavengers-treated platelets exhibited protein-tyrosine phosphorylation without phosphoinositide hydrolysis or protein kinase C activation. Its protein-tyrosine phosphorylation was not inhibited by ML-9, a selective inhibitor of myosin light chain kinase. Genistein, a selective inhibitor of protein-tyrosine kinase, inhibited A23187-induced platelet aggregation but not secretion. These data show (a) that A23187 stimulates protein-tyrosine phosphorylation by elevation of intracellular Ca2+, (b) that A23187-induced protein-tyrosine phosphorylation is independent of formation of endoperoxides/thromboxane A2, released ADP, phosphoinositide hydrolysis, protein kinase C activation, fibrinogen binding and myosin light chain kinase, and (c) that A23187-induced protein-tyrosine phosphorylation may be involved in platelet aggregation but not in secretion. Furthermore, a synergistic effect of A23187 and protein kinase C activators in stimulating protein-tyrosine phosphorylation is suggested.