ADENOSINE 5′‐DIPHOSPHATE ANTAGONISTS AND HUMAN PLATELETS: NO EVIDENCE THAT AGGREGATION AND INHIBITION OF STIMULATED ADENYLATE CYCLASE ARE MEDIATED BY DIFFERENT RECEPTORS

Abstract

1 Adenosine 5′-diphosphate (ADP) induces human platelet aggregation and noncompetitively inhibits stimulated human platelet adenylate cyclase; it has been suggested that these two effects are mediated by separate ADP receptors on the platelet surface. 2 Adenosine 5′-triphosphate and seven adenine nucleotide analogues were tested as inhibitors of both effects of ADP on human platelets, and were found to be competitive. 3 pA₂ values were calculated for each antagonist for inhibition of both effects of ADP, and a good correlation (correlation coefficient 0.87; P < 0.01) was found between the pA₂ values for inhibition of ADP-induced aggregation and the pA₂ values for inhibition of the effect of ADP on stimulated adenylate cyclase. 4 Such a correlation does not support the suggestion that ADP-induced aggregation and the inhibition by ADP of stimulated adenylate cyclase are mediated by two separate receptors.