Effects of Halothane and Isoflurane on Carbon Monoxide-induced Relaxations in the Rat Aorta

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Abstract
Background: Halothane and isoflurane previously were reported to attenuate endothelium-derived relaxing factor/nitric oxide-mediated vasodilation and cyclic guanosine monophosphate (cGMP) formation in isolated rat aortic rings. Carbon monoxide has many chemical and physiologic similarities to nitric oxide. This study was designed to investigate the effects of halothane and isoflurane on carbon monoxide-induced relaxations and cGMP formation in the isolated rat aorta. Methods: Isometric tension was recorded continuously from endothelium denuded rat aortic rings suspended in Krebs-filled organ baths. Rings precontracted with submaximal concentrations of norepinephrine were exposed to cumulative concentrations of carbon monoxide (26-176 microM). This procedure was repeated three times, with anesthetics delivered 10 min before the second procedure. Carbon monoxide responses of rings contracted with the same concentration of norepinephrine (10(-6) M and 2 x 10(-6) M) used in the anesthetic-exposed preparations also were examined. The concentrations of cGMP were determined in denuded rings using radioimmunoassay. The rings were treated with carbon monoxide (176 microM, 30 s) alone, or carbon monoxide after a 10-min incubation with halothane (0.34 mM or 0.72 mM). To determine whether the sequence of anesthetic delivery influenced results, vascular rings pretreated with halothane were compared with nonpretreated rings. Results: Carbon monoxide (26-176 microM) caused a dose-dependent reduction of norepinephrine-induced tension, with a maximal relaxation of 1.51 +/- 0.07 g (85 +/- 7% of norepinephrine-induced contraction). Halothane (0.34 mM and 0.72 mM) significantly attenuated the carbon monoxide-induced relaxations, but only the highest concentration of isoflurane (0.53 mM) significantly attenuated the carbon monoxide-induced relaxations. Carbon monoxide (176 microM) significantly increased cGMP content (+88.1 +/- 7.1%) and preincubation of the aortic rings with halothane (0.34 mM and 0.72 mM) inhibited this increase (-70.7 +/- 6.8% and -108.1 +/- 10.6%, respectively). When aortic rings and carbon monoxide were added simultaneously to Krebs solution equilibrated with halothane (0.72 mM), no inhibition of cGMP formation occurred. Conclusion: Carbon monoxide-induced endothelium-independent relaxations of rat aortic rings were decreased by clinically relevant concentrations of halothane and isoflurane. The carbon monoxide-induced elevations of cGMP were attenuated by halothane only when the anesthetic was incubated with aortic rings before carbon monoxide treatment. The possible clinical significance of the actions of the anesthetics on this endogenous vasodilator is yet to be determined.