Intracellular Calcium Dependent Activation of \>12syk in Platelets1

We previously demonstrated that thrombin-induced activation of p72s* * was independent of intracellular Ca2+ elevation in platelets. However, our previous studies also demonstrat-ed that activation of platelets by ionophore A23187 results in a dramatic increase in tyrosine phosphorylation of several cellular proteins. In the present study, we investigated the effect of Ca2+ elevation on the activity of p72s>*. When washed porcine platelets were stimulated with ionophore A23187 and the activity of p72s-v * was assessed by means of an immunoprecipitation kinase assay, A23187 caused a time- and dose-dependent increase in the specific activity of p72s>1*. In addition, pretreatment of platelets with both aspirin and ADP scavengers or chelation of extracellular Ca2+ by EGTA had no effect on the A23187-induced activation of p72s;v*. These results indicate that A23187-induced activation of p72s^* is independent of the formation of endoperoxide/thromboxane A2, released ADP and extracellular Ca2+, suggesting the existence of a novel pathway for activation of p72sy*. Furthermore, evidence is presented which indicates a synergistic effect of A23187 and thrombin on the activation of p72s:y \ and an inhibitory effect of pretreatment with phorbol 12-myristate 13-acetate, a protein kinase C activator, on the activation of p72sy * induced b