Role of G protein-gated inwardly rectifying potassium channels in P2Y ₁₂ receptor-mediated platelet functional responses

The role of the G_i-coupled platelet P2Y₁₂ receptor in platelet function has been well established. However, the functional effector or effectors contributing directly to αIIbβ3 activation in human platelets has not been delineated. As the P2Y₁₂ receptor has been shown to activate G protein-gated, inwardly rectifying potassium (GIRK) channels, we investigated whether GIRK channels mediate any of the functional responses of the platelet P2Y₁₂ receptor. Western blot analysis revealed that platelets express GIRK1, GIRK2, and GIRK4. In aspirin-treated and washed human platelets, 2 structurally distinct GIRK inhibitors, SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride) and U50488H (trans-(±)-3,4-dichloro-N-methyl-N-[2- (pyrrolidinyl)cyclohexyl] benzeneacetamide methanesulfonate), inhibited adenosine diphosphate (ADP)-, 2-methylthioADP (2-MeSADP)-, U46619-, and low-dose thrombin-mediated platelet aggregation. However, the GIRK channel inhibitors did not affect platelet aggregation induced by high concentrations of thrombin, AYPGKF, or convulxin. Furthermore, the GIRK channel inhibitors reversed SFLLRN-induced platelet aggregation, inhibited the P2Y₁₂-mediated potentiation of dense granule secretion and Akt phosphorylation, and did not affect the agonist-induced G_q-mediated platelet shape change and intracellular calcium mobilization. Unlike AR-C 69931MX, a P2Y₁₂ receptor-selective antagonist, the GIRK channel blockers did not affect the ADP-induced adenlylyl cyclase inhibition, indicating that they do not directly antagonize the P2Y₁₂ receptor. We conclude that GIRK channels are important functional effectors of the P2Y₁₂ receptor in human platelets.