ADP receptors in platelet activation and aggregation.

ADP plays a central role in platelet aggregation. Activation of platelets via ADP proceeds via three membrane receptor proteins, two of which are coupled to a G protein and the third one constituting an ion channel mediating rapid Ca2+-influx. Via Ca2+-mobilization, the Gq-coupled P2Y(1) receptor acts in concert with the Gi-coupled P2T(AC) receptor, which functions by lowering intracellular cAMP levels. The importance of Ca2+-influx via the P2X(1) ion channel remains to be elucidated.