The effect of tetradecanoylphorbol acetate on calcium-ion mobilization, protein phosphorylation and cytoskeletal assembly induced by thrombin or arachidonate.

Tetradecanoylphorbol acetate (TPA) activates primarily only the protein kinase C pathway not the calcium ion-dependent pathway in platelets. The net effect of this split activation is that only the pseudopodal cytoskeleton assembles, not the contractile cytoskeleton needed for rapid secretion. In this study, platelets were first activated with TPA, then activated secondarily with either thrombin or arachidonate and the subsequent dense body secretion, calcium-ion mobilization, protein phosphorylation and cytoskeletal assembly compared to these same processes in control platelets activated solely with either thrombin or arachidonate. Secretion was reduced as the length of time between the primary and secondary activation was increased; but at a 2-3 min interval, where the activation by TPA was essentially complete, the reduction in the total radiolabeled serotonin secreted was small. Furthermore, nearly normal cytosolic calcium-ion increases, phosphorylation of myosin light chain and contractile cytoskeletal development were induced by thrombin or arachidonate after this interval. Prior treatment of the platelets with 100 microM acetylsalicylate to block the cyclooxygenase-dependent pathway caused minor reduction in dense-body secretion induced by TPA or thrombin or the combination of both, but otherwise the relative results were comparable to the untreated platelets. Therefore, short-term prior activation of gel-filtered platelets with TPA, even at concentrations in excess of 100-times that required to saturate protein kinase C, does not prevent normal activation of the calcium ion dependent processes through either the cyclooxygenase-dependent or -independent pathway. Longer-term preincubations with TPA differentially inhibit the secretion response induced by thrombin and arachidonate.