Absence of a Role for Superoxide Anion, Hydrogen Peroxide and Hydroxyl Radical in Endothelium-Mediated Relaxation of Rabbit Aorta

Abstract

We tested the hypothesis that the endothelium-dependent relaxation of rabbit thoracic aorta in vitro is mediated by reduced metabolites of oxygen. Helical vascular strips were contracted with either norepinephrine or phenylephrine. Oxygen metabolites, generated by the xanthine oxidase reaction, completely relaxed norepinephrine-induced contractile tone but not tone induced by phenylephrine. A mixture of oxygen metabolite scavengers (superoxide dismutase, catalase and mannitol) eliminated the relaxation induced by the xanthine oxidase products. Acetylcholine caused a dose-dependent and endothelium-dependent relaxation of the strips; this was not inhibited by the presence of the scavengers. We conclude that reduced oxygen metabolites have little direct effect on rabbit aortic smooth muscle in vitro, although they indirectly but specifically relax norepinephrine-induced tone, presumably by oxidation of norepinephrine. Oxygen metabolites do not appear to mediate the endothelium-dependent relaxation response of this tissue to acetylcholine.