Role of intracellular signaling events in ADP-induced platelet aggregation

Human platelets express two distinct G protein-coupled ADP receptors, one c oupled to phospholipase C through Gq, P2Y1, and the other to inhibition of adenylyl cyclase through Gi, P2T(AC). We have recently shown that concomita nt intracellular signaling from both the P2T(AC) and P2Y1 receptors is esse ntial for ADP-induced platelet aggregation. Previous studies have tested wh ether ADP causes a decrease in the basal cAMP level and this reduction prom otes platelet aggregation, but did not study the effect of decreased cAMP I evels when the Gq pathway is selectively activated. Since we are now aware that platelet aggregation requires activation of two receptors, we investig ated whether the function of P2T(AC) receptor activation, leading to inhibi tion of platelet adenylyl cyclase, could be replaced by direct inhibition o f adenylyl cyclase, when Gq pathway is also activated, a possibility that h as not been addressed to date,In the present study, we supplemented the P2Y 1 mediated Gq signaling pathway with inhibition of the platelet adenylyl cy clase by using SQ22536 or dideoxyadenosine, or by selective activation of t he alpha(2A) adrenoceptors with epinephrine. Although SQ22536, dideoxyadeno sine, and epinephrine reduced the cAMP levels, only epinephrine could mimic the P2T(AC) receptor mediated signaling events, suggesting that reduction in basal cAMP levels does not directly contribute to ADP-induced platelet a ctivation. Adenosine-5'-phosphate-3'-phosphosulfate, a P2Y1 receptor antago nist, completely blocked ADP-induced inositol 1,4,5-trisphosphate and inosi tol 1,3,4-trisphosphate formation suggesting that P2T(AC)-mediated activati on of G(i) (or other G proteins) does not activate phospholipase C. These r esults suggest that a signaling event downstream from Gi, independent of th e inhibition of platelet adenylyl cyclase, contributes to alpha(IIb)beta(3) activation.