P2Y₁₂ potentiates the P2Y₁-dependent calcium response. Fura 2-loaded platelets (A-B) or 1321N1 cells (C) were preincubated for 5 minutes with either AR-C69931MX (ARC; 1 μM), 2MeSAMP (10 μM), A3P5P (1 mM), or vehicle as control, as indicated. Cells were then stimulated with ADP (10 μM; arrow) and 340:380 fluorescence ratio was followed as shown (A-B). Traces shown are representative of at least 3 separate experiments. (C) Mean peak responses are represented relative to the control response to 10 μM ADP in the absence of P2Y₁₂ receptor antagonists. Data shown are mean ± SEM (n = 3).