Isoflurane does not vasodilate rat thoracic aortic rings by endothelium- derived relaxing factor or other cyclic GMP-mediated mechanisms

Endothelium-derived relaxing factor (EDRF) is a potent endogenous vasodilator that has been indirectly suggested to play a role in isoflurane- mediated vasodilation. To examine directly the possible role of EDRF in isoflurane-mediated vasodilation, isolated rat thoracic aortic rings were suspended for isometric tension measurements, equilibrated to a resting tension of 2 g, and constricted with a 50% maximal concentration (EC₅₀) dose of phenylephrine or KCl. Three groups of rings were studied: endothelium-intact, endothelium-denuded, and endothelium-intact rings treated with nitro-L-arginine methyl ester (L-NAME), a specific inhibitor of EDRF synthase. Isoflurane was then added at 1, 2, and 3% in a cumulative manner, allowing 10 min for each concentration to equilibrate. Indomethacin was present in all experiments to prevent the formation of vasoactive prostanoid metabolites. Since EDRF causes vascular relaxation by stimulating soluble guanylyl cyclase and increasing cyclic GMP, the effect of isoflurane on vascular ring cyclic GMP content was determined as an additional indicator of EDRF-mediated dilation. Rings with intact and denuded endothelium were isolated as described above, constricted with phenylephrine, and challenged with methacholine (positive control) or 1, 2, or 3% isoflurane. After 8 min exposure, the rings were flash-frozen in dry-ice-cooled acetone and homogenized in 1 N HCl for subsequent analysis of cyclic GMP content by radioimmunoassay. Isoflurane caused dose-dependent vasodilation of both KCl- and phenylephrine-constricted rings. In the phenylephrine group, at 2% and 3% isoflurane, endothelium-denuded and L-NAME-treated rings relaxed to a greater extent than endothelium-intact rings (P < 0.01). There were no differences in isoflurane-induced relaxation of any of the KCl-constricted groups. Methacholine, an endothelium-dependent vasodilator, increased cyclic GMP concentration of endothelium-intact vascular rings significantly above control (P < 0.001). Isoflurane 1, 2, and 3% had no effect on cyclic GMP content of either endothelium-intact or endothelium-denuded vessels. Vasodilation of the rat aorta by isoflurane is due to a direct effect on vascular smooth muscle and is independent of the stimulation of EDRF or other cyclic GMP-mediated mechanisms.