Concentration-dependent inhibition of calcium response by blockade of P2Y₁₂ Fura 2-loaded platelets were incubated either in the presence of various concentrations of AR-C69931MX for 5 minutes (A) or with or without 1 μM AR-C69931MX for 5 minutes (B). Platelets were then stimulated with ADP (10 μM; A) or various concentrations of ADP (B), as shown. The peak rise in cytosolic calcium concentration was measured and plotted against log concentration of antagonist (A) or agonist (B). Data shown are mean ± SEM (n = 3).