Inhibition of PAR4 Signaling Mediates Ethanol‐Induced Attenuation of Platelet Function In Vitro

BACKGROUND: Reduction in coronary heart disease morbidity in response to moderate consumption of alcoholic beverages may be partly mediated by ethanol-induced inhibition of platelet function. However, the precise mechanisms by which ethanol modulates platelet activation induced by thrombin, which plays a central role in hemostasis, remain unclear. The goal of this study was to investigate ethanol-induced changes in platelet function and clarify the underlying mechanisms including PAR1 and PAR4 activity and [Ca2+]i dynamics in vitro. METHODS: Platelet aggregation, increase in intracellular calcium ([Ca2+]i), and release of platelet factor 4 and beta-thromboglobulin induced by alpha-thrombin, PAR1-agonist peptide (AP), or PAR4-AP were assessed in the presence or absence of ethanol. RESULTS: Ethanol exposure inhibited low-dose thrombin (0.5 nM)-induced aggregation but not an increase in [Ca2+]i. In contrast, ethanol had no effect on high-dose thrombin (10 nM)-induced aggregation or the [Ca2+]i increase. Ethanol did not significantly inhibit thrombin-induced release of platelet factor 4 and beta-thromboglobulin. Ethanol reduced PAR1-AP-induced aggregation, but did not affect the spike form of [Ca2+]i increase. In contrast, ethanol inhibited the increase in [Ca2+]i as well as the aggregation in response to PAR4-AP and resulted in delayed [Ca2+]i peak time. Furthermore, ethanol inhibited both PAR1-AP- and PAR4-AP-induced platelet factor 4 and beta-thromboglobulin release. CONCLUSIONS: These data suggest that ethanol inhibits platelet aggregation via inhibition of PAR4 signaling and subsequent inhibition of Ca2+ influx and granule release. This phenomenon may contribute to the reduction in coronary heart disease morbidity in response to consumption of alcoholic beverages.