Effects of 2-methylthioadenosine 5?-?,?-methylenetriphosphonate and 2-ethylthioadenosine 5?-monophosphate on human platelet activation induced by adenosine 5?-diphosphate

Abstract

Adenosine 5′-diphosphate (ADP) induces human platelet aggregation, increases intracellular levels of free calcium, and inhibits stimulated adenylate cyclase. These effects of ADP are mediated by P2T-purinoceptors that are inhibited specifically and competitively by adenosine 5′-triphosphate (ATP). Inhibition of ADP-induced aggregation and increases in calcium by 2-alkylthio analogs of ATP and of adenosine 5′-monophosphate (AMP) are also specific, but the inhibition is non-surmountable. To examine further the nature of inhibition of ADP-induced platelet activation by 2-alkylthio analogs, the effects of 2-methylthioadenosine 5′-β,γ-methylenetriphosphonate (2-MeS-AMP-PCP) and 2-ethylthioadenosine 5′-monophosphate (2-EtS-AMP) were tested on ADP-induced platelet aggregation and inhibition of adenylate cyclase. 2-MeS-AMP-PCP inhibited platelet aggregation induced by ADP but not by epinephrine, arachidonic acid, 5-hydroxytryptamine (5-HT), platelet activating factor (PAF), or 11α,9α-epoxymethano-prostaglandin H2 (U46619). Inhibition of ADP-induced platelet aggregation by 2-MeS-AMP-PCP was non-surmountable, and it achieved only 50% inhibition of ADP (5 μM)-induced aggregation. 2-MeS-AMP-PCP achieved 100% inhibition of ADP (5 μM)-induced inhibition of prostaglandin E1-stimulated adenylate cyclase, and Schild analysis showed the inhibition to be potent (pA2 7.3) and competitive (slope 1.12). 2-MeS-AMP-PCP inhibited platelet aggregation induced by adenosine 5′-O-2-thiodiphosphate (ADP-β-S), which inhibited stimulated adenylate cyclase activity, but did not inhibit aggregation induced by adenosine 5′-O-1-thiodiphosphate (ADP-α-S), which does not inhibit stimulated adenylate cyclase. 2-EtS-AMP behaved similarly to 2-MeS-AMP-PCP. These results suggest that ADP may induce aggregation by interacting with two forms of the calcium-mobilizing P2T-purinoceptor, only one of which is coupled to inhibition of adenylate cyclase and at which 2-alkylthio analogs of ATP and AMP are specific competitive antagonists. © 1996 Wiley-Liss, Inc.