Combined inhibitory effects of ethanol and adenosine on the responses of rabbit platelets to thrombin

We have previously shown that acutely administered ethanol, resulting in blood alcohol concentrations of 40-90 mM, inhibits experimentally induced arterial thrombosis in rabbits. This inhibition by ethanol in vivo is more pronounced than that observed on stimulated platelets in vitro, when a similar concentration of ethanol is added before an aggregating agent. It may be, then, that ethanol has combined effects in vivo with other inhibitors of platelet function. Adenosine has been found to be an important mediator of some of the in vivo effects of ethanol, and we investigated whether ethanol has combined inhibitory effects with adenosine on thrombin-stimulated platelet responses in vitro. Aggregation and secretion of [14C]serotonin from washed, prelabeled rabbit platelets, pretreated with aspirin, were studied. Maximal aggregation induced by 0.15 units thrombin/ml was slightly inhibited by 87 mM ethanol; secretion of serotonin was reduced from 24% to 12%. However, when thrombin-induced aggregation was significantly reduced by 1 microM adenosine, ethanol, at 44 and 87 mM, further inhibited aggregation. Secretion of [14C]serotonin was reduced to < 3%, with the combination of adenosine and the higher concentration of ethanol. Ethanol did not increase platelet cyclic AMP (cAMP) above basal levels, nor did it affect the increase in cAMP caused by adenosine. The adenosine receptor antagonist, 8-phenyltheophylline, at 1 microM, blocked the inhibitory effects of adenosine on platelet responses and prevented the adenosine-induced increase in cAMP. Unexpectedly, however, 8-phenyltheophylline (1-2 microM) did not completely block the combined inhibitory effects of ethanol and adenosine; this incomplete reversal was not associated with increases in cAMP over basal levels.(ABSTRACT TRUNCATED AT 250 WORDS)