DIFFERENCES IN THE MODE OF ACTION OF 1-OLEOYL-2-ACETYL-GLYCEROL AND PHORBOL ESTER IN PLATELET ACTIVATION

Abstract

The ADP-hydrolyzing enzyme apyrase inhibited platelet aggregation and phosphorylation of a 40,000 dalton platelet protein (P40) induced by 1-oleoyl-2-acetylglycerol (OAG), indicating a dependence on secreted ADP. Apyrase also enhanced OAG-induced potentiation of forskolin or prostaglandin I2 activation of cyclic AMP formation in platelets. Cyclic AMP formation induced by OAG alone could be demonstrated in the presence of a phosphodiesterase inhibitor. Elevation of cyclic AMP level inhibits platelet aggregation so that secreted ADP may be required to inhibit OAG-activated adenylate cyclase for aggregation to proceed. In contrast, apyrase only partially affected phosphorylation of P40 and aggregation induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). TPA caused marked inhibition of forskolin-stimulated cyclic AMP formation. TPA inhibition of cyclic AMP formation was largely reversed by apyrase, indicating that it was mainly due to release of ADP.