Involvement of H2O2 in superoxide-dismutase-induced enhancement of endothelium-dependent relaxation in rabbit mesenteric resistance artery

1 The mechanism underlying the enhancement by superoxide dismutase ( SOD) of endothelium-dependent relaxation was investigated in rabbit mesenteric resistance arteries. 2 SOD (200 U ml(-1)) increased the production of H2O2 in smooth muscle cells ( as indicated by the use of an H2O2-sensitive fluorescent dye). 3 Neither SOD nor catalase (400 U ml(-1)) modified either the resting membrane potential or the hyperpolarization induced by acetylcholine (ACh, 1 mM) in smooth muscle cells. 4 In arteries constricted with noradrenaline, the endothelium-dependent relaxation induced by ACh (0.01 - 1 muM) was enhanced by SOD (200 U ml(-1)) (P<0.01). This action of SOD was inhibited by L-N-G-nitroarginine (nitric oxide (NO)-synthase inhibitor) but not by either charybdotoxin+ apamin (Ca2+-activated-K+-channel blockers) or diclofenac ( cyclooxygenase inhibitor). 5 Neither ascorbate (50 μM) nor tiron (0.3 mM), superoxide scavengers, had any effect on the ACh-induced relaxation, but each attenuated the enhancing effect of SOD on the ACh-induced relaxation. Similarly, catalase (400 U ml(-1)) inhibited the effect of SOD without changing the ACh-induced relaxation. 6 In endothelium-denuded strips constricted with noradrenaline, SOD enhanced the relaxation induced by the NO donor 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7) (P<0.05). Ascorbate and catalase each attenuated this effect of SOD. 7 H2O2 ( 1 mM) enhanced the relaxation on the noradrenaline contraction induced by NOC-7 and that induced by 8-bromo-cGMP, a membrane-permeable analogue of guanosine 30,50 cyclic monophosphate ( cGMP). 8 SOD had no effect on cGMP production, whether measured in endothelium-intact strips following an application of ACh (0.1 muM) or in endothelium-denuded strips following an application of NOC-7 ( 0.1 mM). 9 It is suggested that in rabbit mesenteric resistance arteries, SOD increases the ACh-induced, endothelium-dependent relaxation by enhancing the action of NO in the smooth muscle via its H2O2-producing action ( rather than via a superoxide-scavenging action).