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 alpha subunit of the Gq/11 heterotrimeric G protein (Galphaq/11; i.e., they inhibit muscarinic receptor-Galphaq/11 coupling).
Methods
The effect of halothane (0.38 +/- 0.02 mm) and hexanol (10 mm) on basal and acetylcholine-stimulated Galphaq/11 guanosine nucleotide exchange was determined in membranes prepared from porcine tracheal smooth muscle. The nonhydrolyzable, radioactive form of guanosine-5'-triphosphate, [S]GTPgammaS, was used as the reporter for Galphaq/11 subunit dissociation from the membrane to soluble fraction, which was immunoprecipitated with rabbit polyclonal anti-Galphaq/11 antiserum.
Results
Acetylcholine caused a significant time- and concentration-dependent increase in the magnitude of Galphaq/11 nucleotide exchange compared with basal values (i.e., without acetylcholine), reaching a maximal difference at 100 microm (35.9 +/-2.9 vs. 9.8 +/-1.2 fmol/mg protein, respectively). Whereas neither anesthetic had an effect on basal Galphaq/11 nucleotide exchange, both halothane and hexanol significantly inhibited the increase in Galphaq/11 nucleotide exchange produced by 30 microm 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 Galphaq/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.
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