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Objectives: The aim of the study was to determine if isosorbide-5-mononitrate (IS-5-MN) 120 mg, taken once daily for 7 days, is associated with evidence of endothelial dysfunction and whether this effect is determined by increased free radical production.Background: Tolerance to nitroglycerin is associated with increased free radical production and abnormal endothelial function. To date, no data is available concerning the effect of IS-5-MN, administered in clinically employed dosages, on endothelial function in humans.Methods: A total of 19 healthy volunteers were randomized in a double-blind fashion to therapy with IS-5-MN (120 mg once daily) or placebo. After 7 days of treatment, forearm blood flow responses to acetylcholine (Ach; 7.5, 15, and 30 microg/min) and N-monomethyl-L-arginine (L-NMMA; 1, 2, and 4 mumol/min) were measured. In a separate study, after 7 days of therapy with IS-5-MN 120 mg once daily, the responses to Ach were assessed during intra-arterial coinfusion of vitamin C (24 mg/min) or saline.Results: As compared with placebo, IS-5-MN caused significant blunting of the responses to both Ach (peak responses: placebo 127 +/- 31%; IS-5-MN 52 +/- 24%) and L-NMMA (peak responses: placebo 41 +/- 5%; IS-5-MN 22 +/- 8%). Vitamin C completely restored the forearm blood flow responses to Ach (peak responses: vitamin C 180 +/- 33%; saline 107 +/- 17%).Conclusions: We document for the first time that IS-5-MN impairs endothelial function in humans in vivo. Suggesting a role of oxygen free radicals, nitrate-induced abnormalities in endothelium-dependent vasomotor responses were reversed by the antioxidant vitamin C. [ABSTRACT FROM AUTHOR]