ADP acts through the P2cyc receptor.

(A) The selective P2cyc receptor antagonist AR-C69931MX (10 μmol/L) and clopidogrel (100 mg/kg) inhibited the potentiation by ADP of collagen-induced aggregation of Gαq-deficient mouse platelets. (B) In contrast, the effect of adrenaline was not inhibited by the P2cyc-selective compounds. (C) By itself, ADP (10 μmol/L) still inhibited cAMP production after PGE₁ (10 μmol/L)–induced stimulation in Gαq-deficient mouse platelets, an effect that was blocked by AR-C69931MX (10 μmol/L) and by clopidogrel (100 mg/kg). (D) Blockade of the ADP-induced inhibition of adenylyl cyclase even in the presence of collagen by AR-C69931MX and by clopidogrel. (E) No inhibition of adrenaline-induced inhibition of adenylyl cyclase by AR-C69931MX or by clopidogrel in the presence of collagen.