Incubation of human platelets with protein kinase C activator 4 beta-phorbol-12 beta-myristate-13 alpha-acetate (PMA) abolished stimulation of membrane high-affinity GTPase by platelet-activating factor (PAF). GTPase stimulation by epinephrine decreased by 30%, while the prostaglandin E1 (PGE1) effect was unchanged. Basal GTPase activity (22.4 +/- 1.1 pmol Pi/min per mg protein) was not affected by PMA. Therefore, a study was performed of the effect of endogenous protein kinase C activation on adenylate cyclase regulation by agonists. PMA pretreatment completely suppressed PAF inhibition of basal adenylate cyclase activity but hardly influenced the inhibition by PAF of forskolin-stimulated activity. Adenylate cyclase inhibition by epinephrine in the presence of propranolol was not suppressed completely after platelet incubation with PMA. Epinephrine effects on basal and forskolin-stimulated activities decreased equally. Platelet pretreatment with PMA increased PGE1-stimulated activity by abolishing the inhibitory effect of high GTP concentrations. These studies indicate that protein kinase C selectively inhibits PAF effects, presumably by inactivating a GTP-binding protein coupled with PAF receptors.
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