The effects of the P2-purinoceptor antagonist, suramin, on ADP-induced increases in human platelet cytosolic calcium concentration ([Ca2+]i) and inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase activity were investigated. Suramin (50-200 microM) acted as an antagonist of ADP-induced increases in [Ca2+]i, causing parallel, rightward shifts of the log concentration-response curve to ADP with no apparent depression of the maximal response. However, the slope of the Schild plot was 2.3 +/- 0.3, similar to that obtained in previous studies on aggregation, indicating that the antagonism was not simply competitive. The apparent pA2 for suramin, taken from the Schild plot, was 4.63, similar to that for suramin's inhibition of aggregation, which suggests that these two effects are closely related. Suramin was not specific for the ADP receptor, however, as it was also able to inhibit, non-competitively, increases in [Ca2+]i induced by 5-hydroxytryptamine. Suramin (50-400 microM) also inhibited the effect of ADP on PGE1-stimulated accumulation of cyclic AMP, causing parallel shifts of the log concentration-response curve to ADP, with a Schild plot slope of 1.00 +/- 0.10, suggesting competitive antagonism, and a pA2 value of 5.09. Suramin (400 microM) did not reduce the inhibition of cyclic AMP accumulation by adrenaline, although it was able to inhibit the accumulation of cyclic AMP caused by PGE1, again showing that suramin has some non-specific effects. These data suggest that suramin is an antagonist at the platelet ADP receptor mediating increases in [Ca2+]i and inhibition of adenylate cyclase, but that it also shows non-specific effects and can depress platelet responses to other agonists. In addition, the similar pA2 value of suramin for the two effects of ADP does not support suggestion that they are mediated by two different receptors on human platelets.