title: Differential effects of volatile anesthetics on M3 muscarinic receptor coupling to the Galphaq heterotrimeric G protein.
abstract: 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, Galphaq. 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 Galphaq guanosine nucleotide exchange.
  
  
  METHODS
  Galphaq guanosine nucleotide exchange was measured in crude membranes prepared from COS-7 cells transiently coexpressing the human M3 muscarinic receptor and human Galphaq. A radioactive, nonhydrolyzable analog of guanosine-5'-triphosphate, [35S]GTPgammaS, was used as a reporter for nucleotide exchange at Galphaq.
  
  
  RESULTS
  Acetylcholine caused a concentration-dependent increase in Galphaq [35S]GTPgammaS-GDP exchange. Neither anesthetic affected constitutive Galphaq [35S]GTPgammaS-GDP exchange in the absence of acetylcholine. Conversely, each anesthetic caused a concentration-dependent and reversible inhibition of Galphaq [35S]GTPgammaS-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.
  
  
  CONCLUSIONS
  The differential effects of volatile anesthetics on acetylcholine-promoted guanosine nucleotide exchange at Galphaq 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.
authors.affiliation: Department of Anesthesiology, Mayo Foundation, Rochester, Minnesota 55905, USA.
authors.forename: Tetsuzo
authors.initials: T
authors.lastname: Nakayama
authors.forename: Alan R
authors.initials: AR
authors.lastname: Penheiter
authors.forename: Sumedha G
authors.initials: SG
authors.lastname: Penheiter
authors.forename: Eduardo N
authors.initials: EN
authors.lastname: Chini
authors.forename: Michael
authors.initials: M
authors.lastname: Thompson
authors.forename: David O
authors.initials: DO
authors.lastname: Warner
authors.forename: Keith A
authors.initials: KA
authors.lastname: Jones
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000465:Algorithms; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D019556:COS Cells; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D003593:Cytoplasm / Q000378:metabolism; D003627:Data Interpretation, Statistical; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000187:drug effects*; D006150:Guanine Nucleotides / Q000378:metabolism; D006221:Halothane / Q000494:pharmacology; D000441:Hexanols / Q000494:pharmacology; D006801:Humans; D015151:Immunoblotting; D007530:Isoflurane / Q000494:pharmacology; D007537:Isometric Contraction; D008738:Methyl Ethers / Q000494:pharmacology; D043587:Receptor, Muscarinic M3 / Q000187:drug effects*; D000077149:Sevoflurane
chemical_list: D018685:Anesthetics, Inhalation; D006150:Guanine Nucleotides; D000441:Hexanols; D008738:Methyl Ethers; D043587:Receptor, Muscarinic M3; D000077149:Sevoflurane; D007530:Isoflurane; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D000109:Acetylcholine; D002118:Calcium; D006221:Halothane