Halothane does not inhibit the functional coupling between the beta2-adrenergic receptor and the Galphas heterotrimeric G protein.
BACKGROUND
This study investigated whether halothane affects the functional coupling between the beta2 adrenergic receptor and the alpha subunit of its cognate stimulatory heterotrimeric guanosine-5'-triphosphate (GTP)-binding protein (Galphas). The authors hypothesized that halothane does not affect isoproterenol-promoted guanosine nucleotide exchange at Galphas and hence would not affect isoproterenol-induced relaxation of airway smooth muscle.


METHODS
Halothane effects on isoproterenol-induced inhibition of calcium sensitivity were measured in permeabilized porcine airway smooth muscle. Galphas nucleotide exchange was measured in crude membranes prepared from COS-7 cells transfected to transiently coexpress the human beta1 or beta2 receptor each with human short Galphas. A radioactive, nonhydrolyzable analog of GTP, [S]GTPgammaS, was used as the reporter for nucleotide exchange at Galphas.


RESULTS
Halothane (0.75 mm, approximately 2.8 minimum alveolar concentration [MAC] in pigs) did not affect isoproterenol-induced inhibition of calcium sensitivity. Isoproterenol caused a time- and concentration-dependent increase in Galphas nucleotide exchange. Halothane, even at concentrations of 1.5 mm (approximately 5.6 MAC), had no effect on basal Galphas nucleotide exchange in the absence of isoproterenol, whereas halothane inhibited isoproterenol-promoted Galphas nucleotide exchange in both the beta1-Galphas and beta2-Galphas expressing membranes. However, the effect was significantly greater on beta1-Galphas coupling compared with beta2-Galphas coupling, with no effect on beta2-Galphas coupling at 2.8 MAC halothane.


CONCLUSIONS
Halothane does not inhibit the biochemical coupling between the beta2 receptor and Galphas and hence does not affect the inhibition of calcium sensitivity induced by isoproterenol. Therefore, halothane should not affect the efficacy of beta2 agonists, as suggested by studies of in vivo animal models of asthma.
Hayashi
Masao
M
Department of Anesthesiology, Mayo Foundation, Rochester, Minnesota 55905, USA.
Penheiter
Sumedha G
SG
Nakayama
Tetsuzo
T
Penheiter
Alan R
AR
Warner
David O
DO
Jones
Keith A
KA
D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D019556:COS Cells; D002118:Calcium / Q000378:metabolism; D002522:Chlorocebus aethiops; D004305:Dose-Response Relationship, Drug; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000187:drug effects* / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006221:Halothane / Q000494:pharmacology*; D007545:Isoproterenol / Q000494:pharmacology; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects* / Q000502:physiology; D013552:Swine
D018685:Anesthetics, Inhalation; D018343:Receptors, Adrenergic, beta-2; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019205:GTP-Binding Protein alpha Subunits, Gs; D007545:Isoproterenol; D002118:Calcium; D006221:Halothane