Differential effects of volatile anesthetics on M₃ muscarinic receptor coupling to the Gα_q heterotrimeric G protein

BACKGROUND: Halothane inhibits airway smooth muscle contraction in part by inhibiting the functional coupling between muscarinic receptors and one of its cognate heterotrimeric G proteins, Gαq. Based on previous studies indicating a more potent effect of halothane and sevoflurane on airway smooth muscle contraction compared with isoflurane, the current study hypothesized that at anesthetic concentrations of 2 minimum alveolar concentration (MAC) or less, halothane and sevoflurane but not isoflurane inhibit acetylcholine-promoted Gαq guanosine nucleotide exchange. METHODS: Gαq guanosine nucleotide exchange was measured in crude membranes prepared from COS-7 cells transiently coexpressing the human M3 muscarinic receptor and human Gαq. A radioactive, nonhydrolyzable analog of guanosine-5′-triphosphate, [S]GTPγS, was used as a reporter for nucleotide exchange at Gαq. RESULTS: Acetylcholine caused a concentration-dependent increase in Gαq [S]GTPγS-GDP exchange. Neither anesthetic affected constitutive Gαq [S]GTPγS-GDP exchange in the absence of acetylcholine. Conversely, each anesthetic caused a concentration-dependent and reversible inhibition of Gαq [S]GTPγS-GDP exchange when promoted by acetylcholine. At concentrations of 3 MAC or less, the effect of halothane and sevoflurane were significantly greater than that of isoflurane, with only a minimal inhibition by isoflurane observed at 2 MAC. CONCLUSION: The differential effects of volatile anesthetics on acetylcholine-promoted guanosine nucleotide exchange at Gαq are consistent with the apparent more potent direct effect of halothane and sevoflurane compared with isoflurane on muscarinic receptor-mediated contraction of isolated airway smooth muscle. These differential effects also suggest a mode of anesthetic action that could be due to anesthetic-protein interactions and not simply anesthetic accumulation in the lipid membrane.