In vivo actions of organic nitrates on platelet function in humans

Nitroglycerin and isosorbide dinitrate (ISDN), the most commonly used nitrate vasodilators, have been shown to possess antiplatelet properties. It has also been shown, interestingly, that their inhibition of aggregation (mostly upon ADP and adrenaline) occurs in vivo at concentrations 1-2 log orders lower than in vitro, and in the full therapeutic range. Many different hypotheses may explain such an in vivo/in vitro difference: 1) that nitrates induce prostacyclin synthesis; 2) that they synergize prostacyclin; 3) that they give rise to more active in vivo metabolites; 4) that some other requirement for their action, such as the availability of reducing thiols, may be a limiting factor in the in vitro setting. The discrimination among such hypotheses should contribute new insights into nitrate action at the platelet level. On the basis of experiments on cultured endothelial cells and vascular fragments, we had previously concluded against prostacyclin induction by nitrates. On the other hand, ISDN may decrease the IC50 for prostacyclin in aggregation by suprathreshold doses of various aggregating agents, and therefore, be synergistic with this endogenous antiplatelet substance. Compared to ISDN, the two longer-lived metabolites isosorbide-2- and isosorbide-5-mononitrates (IS-2-MN, IS-5-MN) appear remarkably different in terms of antiplatelet potency in vitro (minimum effective concentrations 10(-7)-10(-6)M for IS-2-MN, 10(-4)M for IS-5-MN, for ADP-and adrenaline-induced aggregation and thromboxane (TX) B2 production).(ABSTRACT TRUNCATED AT 250 WORDS)