Anesthetics inhibit acetylcholine-promoted guanine nucleotide exchange of heterotrimeric G proteins of airway smooth muscle

Background: Anesthetics inhibit airway smooth muscle contraction in part by a direct effect on the smooth muscle cell. This study tested the hypothesis that the anesthetics halothane and hexanol, which both relax airway smooth muscle in vitro, inhibit acetylcholine-promoted nucleotide exchange at the α subunit of the G _q/11 heterotrimeric G protein (Gα _q/11; i.e., they inhibit muscarinic receptor-Gα _q/11 coupling). Methods: The effect of halothane (0.38 ± 0.02 mM) and hexanol (10 mM) on basal and acetylcholine-stimulated Gα _q/11 guanosine nucleotide exchange was determined in membranes prepared from porcine tracheal smooth muscle. The nonhydrolyzable, radioactive form of guanosine-5′-triphosphate, [ ³⁵S] GTPγS, was used as the reporter for Gα _q/11 subunit dissociation from the membrane to soluble fraction, which was immunoprecipitated with rabbit polyclonal anti-Gα _q/11, antiserum. Results: Acetylcholine caused a significant time- and concentration-dependent increase in the magnitude of Gα _q/11 nucleotide exchange compared with basal values (i.e., without acetylcholine), reaching a maximal difference at 100 μM (35.9 ± 2.9 vs. 9.8 ± 1.2 fmol/mg protein, respectively). Whereas neither anesthetic had an effect on basal Gα _q/11 nucleotide exchange, both halothane and hexanol significantly inhibited the increase in Gα _q/11 nucleotide exchange produced by 30 μM acetylcholine (by 59% and 68%, respectively). Conclusions: Halothane and hexanol interact with the receptor-heterotrimeric G-protein complex in a manner that prevents acetylcholine-promoted exchange of guanosine-5′-triphosphate for guanosine-5′-diphosphate at Gα _q/11. These data are consistent with the ability of anesthetics to interfere with cellular processes mediated by heterotrimeric G proteins in many cells, including effects on muscarinic receptor-G-protein regulation of airway smooth muscle contraction.