Inhibition of NO-mediated responses by 7-ethoxyresorufin, a substrate and competitive inhibitor of cytochrome P₄₅₀

1 The effects of 7-ethoxyresorufin (7-ER), which is a substrate for and competitive inhibitor of cytochrome P₄₅₀, were studied on responses to nitric oxide (NO), the NO donors sodium nitroprusside (SNP) and glyceryl trinitrate (GTN), acetylcholine-induced endothelium-dependent relaxations of rat and rabbit aortic rings and nitrergic nerve stimulation-induced relaxations of rat anococcygeus muscles. 2 In rat and rabbit aortic rings, 7-ER (2 μM) inhibited the relaxations to acetylcholine in endothelium-intact preparations and the relaxant action of NO in endothelium-denuded preparations. Relaxant responses to SNP and GTN were inhibited by 7-ER in the rat but not rabbit aortic rings. However, the relaxant actions of papaverine and 8-bromo-cyclic GMP were not affected by 7-ER. 3 In rat anococcygeus muscles, 7-ER (2 μM) inhibited the relaxant action of NO, but relaxations elicited by nitrergic nerve stimulation were only partly inhibited by a higher concentration of 7-ER (10 μM). 4 After inhibition by 7-ER, superoxide dismutase (100 u ml^-1) restored NO-induced relaxations of the rat aortic rings, but not acetylcholine-, SNP or GTN-induced relaxations, and restored NO- and nitrergic nerve stimulation-induced relaxations of anococcygeus muscles. 5 Another cytochrome P₄₅₀ inhibitor, troleandomycin (10-30 μM), had no effect on NO- or acetylcholine-induced relaxations of rat aortic rings and NO- or nitrergic nerve stimulation-induced relaxations of anococcygeus muscles. However, resorufin, an analogue of 7-ER, inhibited responses to acetylcholine, NO and GTN in rat aortic rings. 6 The results suggest that 7-ER inhibited responses to NO and nitrergic nerve stimulation through generation of superoxide radicals. However, an additional mechanism may be involved in the reduction in acetylcholine-induced responses in aortic rings. 7 A 7-ER sensitive P₄₅₀ system may be involved in the bioactivation of GTN and SNP in rat aortic rings, but not in rabbit aorta or rat anococcygeus muscles.