Halothane does not inhibit the functional coupling between the β₂-adrenergic receptor and the Gα_s heterotrimeric G protein

Background: This study investigated whether halothane affects the functional coupling between the β₂ adrenergic receptor and the α subunit of its cognate stimulatory heterotrimeric guanosine-5′- triphosphate (GTP)-binding protein (Gα_s). The authors hypothesized that halothane does not affect isoproterenol-promoted guanosine nucleotide exchange at Gα_s 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. Gα_s nucleotide exchange was measured in crude membranes prepared from COS-7 cells transfected to transiently coexpress the human β₁ or β₂ receptor each with human short Gα_s. A radioactive, nonhydrolyzable analog of GTP, [³⁵S]GTPγS, was used as the reporter for nucleotide exchange at Gα_s. 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 Gα_s nucleotide exchange. Halothane, even at concentrations of 1.5 mM (approximately 5.6 MAC), had no effect on basal Gα_s nucleotide exchange in the absence of isoproterenol, whereas halothane inhibited isoproterenol-promoted Gα_s nucleotide exchange in both the β₁-Gα_s and β₂- Gα_s expressing membranes. However, the effect was significantly greater on β₁-Gα_s coupling compared with β₂-Gα_s coupling, with no effect on β₂-Gα_s coupling at 2.8 MAC halothane. Conclusion: Halothane does not inhibit the biochemical coupling between the β₂ receptor and Gα_s and hence does not affect the inhibition of calcium sensitivity induced by isoproterenol. Therefore, halothane should not affect the efficacy of β₂ agonists, as suggested by studies of in vivo animal models of asthma.