Protein kinase C inhibitors enhance G-protein induced phospholipase A(2) activation in intact human platelets

Washed intact human platelets were prelabelled with [H-3]arachidonic acid ([H-3]AA) and stimulated with thrombin or with AlF4-, a known unspecific activator of G-proteins. Both stimuli induced the liberation of [H-3]AA, the release of beta-thromboglobulin (beta-TG) and platelet aggregation. PMA did not induce liberation of [H-3]AA although it induced beta-TG release and aggregation; preincubation with PMA did not modify significantly the amounts of [H-3]AA and beta-TG released by thrombin or AlF4-. Different inhibitors of PKC (staurosporine, H-7 and 4 calphostin C) increased the release of [H-3]AA and inhibited beta-TG release and aggregation induced by AlF4- but they had no effect when platelets were stimulated with thrombin (0.5 U/ml). Calphostin C was able to release [H-3]AA by itself without inducing aggregation or beta-TG release. Okadaic acid (a serine/threonine phosphoprotein phosphatase inhibitor) greatly inhibited the release of [H-3]AA, beta-TG and aggregation in AlF4--stimulated platelets. These results indicate the presence of a G-protein mediated mechanism for the activation of a platelet phospholipase A(2) which is negatively affected by a protein kinase, sensible to putative inhibitors of protein kinase C, and it is activated by a protein phosphatase, sensible to okadaic acid.