Failure of L-nitroarginine to inhibit the activity of aortic inducible nitric oxide synthase

Nitric oxide (NO) is produced by a family of three isoenzymes: the endothel ial, inducible and neuronal NO synthases. L-Nitroarginine methyl ester (L-N AME) is the most commonly used inhibitor of NO synthase activity. The goal of the present study was to evaluate to what extent L-nitroarginine (L-NA), the in vivo circulating metabolite of L-NAME, blocks NO production in the rat aorta depending on the NO synthase isoform expressed (and evidenced by Western blotting) and on the presence or absence of the extracellular NO sy nthase substrate L-arginine (100 muM, i.e. the plasma concentration). Intac t [endothelium present (E+)] control aortic rings express mainly endothelia l NO synthase. L-NA (30-100 muM) induced a dose-dependent contraction (due to blockade of the relaxant properties of NO) irrespective of the presence or absence of L-arginine. In deendothelialized (E-) control aortic rings, t he three isoforms of NO synthase are virtually absent (as demonstrated by W estern blotting) and L-NA does not elicit any contractile effect. E-aortic rings from lipopolysaccharide (LPS)-treated rats express mainly inducible N O synthase. In these rings, LNA induced a dose-dependent (0-100 muM) contra ction in the absence of extracellular L-arginine, whereas L-arginine (100 m uM) completely abrogated the contractile effect of the NO synthase inhibito r. Chronic L-NAME administration (50 mg/kg/day for 4 weeks) elicited the ao rtic expression of inducible NO synthase, but to a lesser extent (about 5-f old) than in LPS-treated rat aorta. The average plasma concentration of L-N A was 50 +/- 10 muM in these rats. In E- rings from these L-NAME-treated ra ts, L-NA induced a similar contractile response (but smaller in magnitude) to that observed in LPS-treated rat aorta. Altogether, these data suggest t hat (1) in the presence of a physiological concentration of extracellular L -arginine, L-NA fails to inhibit inducible NO synthase, and (2) chronic L-N AME administration, at a dose commonly given to block NO production in vivo , leaves the activity of inducible NO synthase unaffected. Copyright (C) 20 01 S. Karger AG, Basel.