Failure of L-Nitroarginine to Inhibit the Activity of Aortic Inducible Nitric Oxide Synthase

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Abstract
Nitric oxide (NO) is produced by a family of three isoenzymes: the endothelial, inducible and neuronal NO synthases. L-Nitroarginine methyl ester (L-NAME) 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 synthase substrate L-arginine (100 µM, i.e. the plasma concentration). Intact [endothelium present (E+)] control aortic rings express mainly endothelial NO synthase. L-NA (30–100 µM) 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, the three isoforms of NO synthase are virtually absent (as demonstrated by Western blotting) and L-NA does not elicit any contractile effect. E– aortic rings from lipopolysaccharide (LPS)-treated rats express mainly inducible NO synthase. In these rings, L-NA induced a dose-dependent (0–100 µM) contraction in the absence of extracellular L-arginine, whereas L-arginine (100 µM) completely abrogated the contractile effect of the NO synthase inhibitor. Chronic L-NAME administration (50 mg/kg/day for 4 weeks) elicited the aortic expression of inducible NO synthase, but to a lesser extent (about 5-fold) than in LPS-treated rat aorta. The average plasma concentration of L-NA was 50 ± 10 µM in these rats. In E– rings from these L-NAME-treated rats, L-NA induced a similar contractile response (but smaller in magnitude) to that observed in LPS-treated rat aorta. Altogether, these data suggest that (1) in the presence of a physiological concentration of extracellular L-arginine, L-NA fails to inhibit inducible NO synthase, and (2) chronic L-NAME administration, at a dose commonly given to block NO production in vivo, leaves the activity of inducible NO synthase unaffected.