Inhibition of cAMP was measured after incubation of cells with different concentrations of ADP (0.1-30 μM) and stimulated for 5 min with or without 30 nM AR-C69931MX in presence of 20 μM forskolin. Data were normalized to the maximal response obtained in the absence of agonist and antagonist, which served as control (mean ± S.E.M., n=3). ## P<0.01, ### P<0.001, significant differences vs. respective control without agonist and antagonist. ** P<0.01, *** P<0.001, significant differences vs. respective values in the absence of the antagonist. (ANOVA followed by the Bonferroni post-test).