Comparison of relaxant effects of propofol on methacholine‐induced bronchoconstriction in dogs with and without vagotomy

Propofol has been suggested to have in vivo airway relaxant effects, although the mechanism is still unclear. In this study, we determined whether propofol could antagonize methacholine-induced bronchoconstriction and determined whether vagotomy modifies this relaxant effect. Fourteen mongrel dogs anaesthetized with pentobarbital and pancuronium were assigned to a control group ( n =7) and a vagotomy group ( n =7). The trachea was intubated with a special endotracheal tube that had a second lumen for insertion of the bronchoscope. Bronchial cross-sectional area, which was monitored continuously through the bronchoscope, was measured with image analysis software. Bronchoconstriction was elicited with methacholine (0.5 μg kg −1  + 5.0 μg kg −1  min −1 ) until the end of the experiment. Thirty minutes after the start of methacholine infusion, propofol 0, 0.2, 2.0 and 20 mg kg −1 was administered. Changes in bronchial cross-sectional area were expressed as percentages of the basal area. Plasma concentrations of propofol and catecholamine were measured by high-performance liquid chromatography. Maximal inhibition (bronchoconstriction = 0%, baseline=100%) and IC 50 (concentration producing 50% inhibition of maximal effect) produced by propofol was obtained from each concentration–response curve using a curve-fitting program. Methacholine decreased bronchial cross-sectional area to 49.3% (95% confidence interval 38.5–60.1%) and 45.3% (34.8–55.7%) of the baseline value. Propofol 20 mg kg −1 significantly reversed this effect: bronchial cross-sectional area was reduced to 77.8% (66.2–89.6%) and 75.9% (64.0–87.9) in the control and vagotomy groups respectively. The two groups did not differ significantly in the maximal inhibitory effect of propofol [control group, 61.1% (46.3–75.9%), vagotomy group, 64.2% (40.1–88.3%)] or pIC 50 [control group 5.03 (4.55–5.51), vagotomy group 4.86 (4.49–5.24)]. Therefore, the relaxant effects of propofol on methacholine-induced bronchoconstriction may not be mediated centrally. Propofol may relax airway smooth muscles directly or through the peripheral vagal pathway.