TRAP-induced reversion of cAMP formation by PGE₁ involves released ADP. The effects of apyrase and various ADP receptor antagonists on cAMP accumulation were measured in intact platelets stimulated by 5 μmol/L ADP (A). The effects of various concentrations of AR-C 66096 and ATPS on cAMP accumulation were measured in intact platelets stimulated by a high concentration of TRAP (100 μmol/L) (B) as described in Materials and Methods. Results are the means of triplicates ± SEM from 1 representative experiment of 3 independent experiments that gave very similar results.