| Abstract: | An increase in cytoplasmic free calcium, Ca2+]i, is thought to be the trigger for secretory exocytosis in many cells. In blood platelets, large rises in Ca2+]i can cause secretion and calcium has been regarded as the final common activator not only for secretion but also for shape-change and aggregation. We have shown that while thrombin and platelet-activating factor (PAF) normally elevate Ca2+]i, they can also stimulate shape-change and secretion even when the Ca2+]i rise is suppressed. The present results strongly implicate diacylglycerol, produced by stimulus-dependent breakdown of phosphoinositide, in this calcium-independent activation. Exogenous diacylglycerol activates a protein kinase (C-kinase) in platelets as do PAF, thrombin and collagen. 12-O-tetradecanoyl phorbol-13-acetate (TPA) also activates C-kinase and is a potent stimulus for secretion and aggregation. It is shown here that the exogenous diacylglycerol 1-oleoyl-2-acetyl-glycerol (OAG) and TPA evoke similar secretion and aggregation without elevating Ca2+]i above the basal level of 0.1 microM. The pattern of secretion resembles that produced by collagen and thrombin when Ca2+]i remains at basal levels. Modest increases in Ca2+]i, insufficient to stimulate secretion, markedly accelerate the responses to TPA and OAG. |
