Involvement of NO in the endothelium-independent relaxing effects of Nω-Hydroxy-L-arginine and other compounds bearing a C=NOH function in the rat aorta

The mechanisms of vasorelaxation elicited by N-omega-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 substituted N-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 muM) and by the inhibitor of guanylylcyclase activation 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (1 muM). In addition, L-NOHA- and ClBZA both caused cGMP accumulation. L-Arginine, but not D-arginine (1 mM), antagonized the effect of L-NOHA but not ClBZA. Both L-NOHA- and ClBZA-induced relaxations were inhibited by the NAD(P)H-dependent enzymes inhibitor diphenyliodonium (30 muM) and the NAD(P)H-dependent reductases inhibitor 7-ethoxyresorufin (10 muM), but they were unmodified by the cytochrome P450 (P450) inhibitor proadifen (10 muM) and by the NO synthase inhibitor N-omega-nitro-L-arginine methyl ester (L-NAME, 300 muM). These results show that L-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.