Effects of phosphate-modified analogs of adenosine 5'-diphosphate and adenosine 5'-triphosphate at P-2T-purinoceptors mediating human platelet activation by ADP

Adenosine 5'-diphosphate (ADP) induces human platelet aggregation and inhibits stimulated adenylate cyclase by an action at P-2T-purinoceptors. Both of these effects of ADP are inhibited competitively by ATP. Structure-activity relationships for phosphate-modified analogs of ADP and adenosine 5'-triphosphate (ATP) were studied by testing their effects on human platelet activation, Of the ADP analogs, only beta,gamma-imido-ADP (AMPNHP) induced platelet aggregation, but was a weak partial agonist (pA(50) 4.53). ADP-induced platelet aggregation was antagonized noncompetitively by beta,gamma-methylene-ADP (AMPCP) (pA(2) 3.2), beta,gamma-ethylene-ADP (AMPCCP)(pA(2) 4.42), and beta,gamma-difluoromethylene-ADP (AMPCF(2)P) (pA(2) 4.77), and competitively by beta,gamma-dichloromethylene-ADP (AMPCCI(2)P) (pA(2) 4.68). None of the ADP analogs inhibited prostaglandin E(1) (PGE(1))-stimulated adenylate cyclase, and ADP-induced inhibition of PGE(1)-stimulated adenylate cyclase was unaffected by AMPNHP, AMPCP, or AMPCCP (100 mu M), but was antagonized by AMPCF(2)P (pA(2) 4.36) and AMPCCI(2)P (4.24). ADP-induced platelet aggregation was antagonized competitively by the ATP analogs beta,gamma-difluoromethylene-ATP (AMP-PCF2P) (pA(2) 4.55), beta,gamma-dichloromethylene-ATP (AMP-PCCI2P) (pA(2) 4.42), and beta,gamma-imido-ATP (AMP-PNHP) (pA(2) 4.32) and non-competitively by 2-methylthio-beta,gamma-methylene-ATP (2-MeS-AMP-PCP), 2-methylthio-beta,gamma-difluoromethylene-ATP (2-MeS-AMP-PC2P), and 2-methylthio-beta,gamma-dichloromethylene-ATP (2-MeS-AMP-PCCI2P). In summary, agonist activity at the human platelet P-2T-purinoceptor was extremely sensitive to alterations to the diphosphate chain of ADP, and only AMPNHP induced platelet aggregation. increasing the electronegativity of the methylene group by halogen substitution increased the antagonist potency of the ADP analog AMPCP but resulted in little or no change in the antagonist potencies of the ATP analogs AMP-PCP and 2-MeS-AMP-PCP. (C) 1996 Wiley-Liss, Inc.