Abstract

The mechanisms of vasorelaxation elicited byN^ω-hydroxy-l-arginine (l-NOHA) and other compounds bearing a C=NOH function and the structural determinants governing this effect were investigated in rat aorta. l-NOHA, formamidoxime, five aromatic monosubstituted amidoximes, and one aromatic monosubstituted ketoxime elicited relaxation in endothelium-denuded rings.N-Hydroxyguanidine and substitutedN-hydroxyguanidines were markedly less active. Relaxations induced by l-NOHA and by the most active studied compound, 4-chlorobenzamidoxime (ClBZA), were unmodified by the presence of endothelium. In endothelium-denuded rings, they were blunted by the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (300 μM) and by the inhibitor of guanylyl-cyclase activation 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (1 μM). In addition, l-NOHA- and ClBZA both caused cGMP accumulation. l-Arginine, but notd-arginine (1 mM), antagonized the effect ofl-NOHA but not ClBZA. Both l-NOHA- and ClBZA-induced relaxations were inhibited by the NAD(P)H-dependent enzymes inhibitor diphenyliodonium (30 μM) and the NAD(P)H-dependent reductases inhibitor 7-ethoxyresorufin (10 μM), but they were unmodified by the cytochrome P450 (P450) inhibitor proadifen (10 μM) and by the NO synthase inhibitorN^ω-nitro-l-arginine methyl ester (l-NAME, 300 μM). These results show thatl-NOHA and other compounds with a C=NOH function can cause endothelium-independent relaxation in the rat aorta. They suggest that activation of guanylyl cyclase and NO formation is implicated in relaxation and that a 7-ethoxyresorufin-sensitive NAD(P)H-dependent pathway is involved. On one hand, l-NOHA and amidoximes may be useful tools for characterizing this pathway in blood vessels and, on the other, may offer a novel approach for treating vascular diseases with impaired endothelial NO activity.