Modulation of platelet-activating-factor-induced calcium influx and intracellular calcium release in platelets by phorbol esters.

The mechanisms by which platelet-activating factor (PAF) and thrombin increase intracellular calcium were examined. Platelets were loaded with the calcium-sensitive fluorescent probe Quin 2 and then were suspended in buffer containing 0.5 mM-Mn2+ in order to quantify simultaneously calcium release from intracellular stores and divalent cation influx. Pretreating platelets with agents which activate protein kinase C [the phorbol ester phorbol myristate acetate (PMA) or the diacylglycerol 1-oleoyl-2-acetylglycerol (OAG)] inhibited increased intracellular calcium by PAF and thrombin in a dose-related manner. That protein kinase C regulates intracellular calcium by phosphorylating proteins in two distinct pathways was suggested by two observations. PAF-induced calcium release was more sensitive to inhibition by PMA and OAG than was manganese influx and the kinetics of recovery from inhibition were different for the two pathways. Both PMA and OAG aggregated Quin 2-loaded platelets without eliciting measurable increases in intracellular calcium. In contrast, prostacyclin, which increases intracellular cyclic AMP, inhibited calcium release and influx in parallel, suggesting that this agent acts at a step common to both pathways.