The signal transduction systems and intracellular Ca2+ dynamics in arachidonate-induced platelet activation.

Types II and III protein kinase C were expressed in human platelets and showed slightly different modes of activation and kinetic properties. Type III isozyme was more sensitive than type II for the activation of each isozyme with arachidonate (AA) although both isozymes were activated by diacylglycerol and phosphatidylserine in a similar manner. When human platelets were stimulated by AA, two types of platelet activation, a low level of AA (0.1-2.5 micrograms/ml)- and a high level of AA (10-50 micrograms/ml)-induced activations, were observed. These activations were associated with the phosphorylation of 40K and 20K proteins. Although a low level of AA (0.45-10.0 micrograms/ml) induced the formation of [32P] phosphatidate in intact platelets prelabeled with [32P] Pi, AA at high concentrations (20-50 micrograms/ml) did not stimulate phospholipase C. The incubation of fura 2 loaded platelets with a low level of AA evoked a rapid and transient elevation in [Ca2+] i. In contrast, a high level of AA induced an irreversible increase in [Ca2+] i but this [Ca2+] i elevation alone was not associated with platelet activation. These results suggest that the signal transduction system by a high level of AA on human platelets is different from that seen with a low level of AA. A high level of AA induces platelet activation, without phospholipase C stimulation, and therefore, the ability of AA to directly activate protein kinase C (pre-dominantly type III isozyme) may contribute toward the activation of platelets by a high level of AA.