Summary

Human platelets express two distinct G protein-coupled ADP receptors,one coupled to phospholipase C through Gq, P2Y1, and the otherto inhibition of adenylyl cyclase through Gi, P2T AC . We have recentlyshown that concomitant intracellular signaling from both the P2T AC andP2Y1 receptors is essential for ADP-induced platelet aggregation. Previousstudies have tested whether ADP causes a decrease in the basalcAMP level and this reduction promotes platelet aggregation, but didnot study the effect of decreased cAMP levels when the Gq pathway isselectively activated. Since we are now aware that platelet aggregationrequires activation of two receptors, we investigated whether thefunction of P2T AC receptor activation, leading to inhibition of plateletadenylyl cyclase, could be replaced by direct inhibition of adenylylcyclase, when Gq pathway is also activated, a possibility that has notbeen addressed to date. In the present study, we supplemented the P2Y1mediated Gq signaling pathway with inhibition of the platelet adenylylcyclase by using SQ22536 or dideoxyadenosine, or by selectiveactivation of the 2A adrenoceptors with epinephrine. AlthoughSQ22536, dideoxyadenosine, and epinephrine reduced the cAMPlevels, only epinephrine could mimic the P2T AC receptor mediatedsignaling events, suggesting that reduction in basal cAMP levels doesnot directly contribute to ADP-induced platelet activation. Adenosine-5’-phosphate-3’-phosphosulfate, a P2Y1 receptor antagonist, completelyblocked ADP-induced inositol 1,4,5-trisphosphate and inositol1,3,4-trisphosphate formation suggesting that P2T AC -mediated activationof G i (or other G proteins) does not activate phospholipase C.These results suggest that a signaling event downstream from Gi,independent of the inhibition of platelet adenylyl cyclase, contributes toalphaIIb beta3 activation.