Bronchodilation by halothane is not modulated by airway epithelium

Halothane relaxes airway smooth muscles. To test the hypothesis that this relaxation is modulated by airway epithelium, we studied the effects of halothane on isolated second- and third-order canine bronchial rings and second-order canine bronchial segments. Paired rings or segments were examined, with the epithelium removed from one ring or segment of each pair. The bronchial rings were suspended in organ chambers and contracted with 10^-8-10^-3 M acetylcholine (ACh), 10^-8-10^-5 M 5-hydroxytryptamine (5HT), or 0.5-16 Hz electrical field stimulation (EFS, 15 V, 0.5-ms pulse duration). The tissue was contracted in the absence of halothane and during exposure to 1 and 2 MAC halothane. The bronchial segments were perfused intraluminally with physiologic salt solution (PSS) and contracted with 10^-6 M carbachol added to the tissue-bath PSS. One or 2 MAC halothane was then added to the perfusate. In the absence of halothane, epithelium removal increased the sensitivity of the bronchial rings to ACh and 5HT but not to EFS. Addition of 1 or 2 MAC halothane to the bathing fluid of the rings with or without epithelium decreased the sensitivity of the rings to ACh and 5HT. One MAC halothane decreased the sensitivity of the rings with and without epithelium to EFS. The decrease in sensitivity caused by halothane was not significantly different in rings with or without epithelium for any method of stimulation. In the bronchial segments, relaxations evoked by 1 or 2 MAC were not different in segments with or without epithelium. The time constants of relaxation also were not significantly different for segments with or without epithelium. We conclude that the relaxing effects of halothane on canine bronchial smooth muscle in vitro are not modulated by the airway epithelium, and the epithelium does not provide a detectable barrier to the diffusion of halothane from the bronchial lumen to the smooth muscle.