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 (Gαs). The authors hypothesized that halothane does not affect isoproterenol-promoted guanosine nucleotide exchange at Gαs 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. Gαs 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 Gαs. A radioactive, nonhydrolyzable analog of GTP, [35S]GTPγS, was used as the reporter for nucleotide exchange at Gαs. 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 Gαs nucleotide exchange. Halothane, even at concentrations of 1.5 mM (approximately 5.6 MAC), had no effect on basal Gαs nucleotide exchange in the absence of isoproterenol, whereas halothane inhibited isoproterenol-promoted Gαs nucleotide exchange in both the β1-Gαs and β2- Gαs expressing membranes. However, the effect was significantly greater on β1-Gαs coupling compared with β2-Gαs coupling, with no effect on β2-Gαs coupling at 2.8 MAC halothane. Conclusion: Halothane does not inhibit the biochemical coupling between the β2 receptor and Gαs 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.
ASJC Scopus subject areas
- Anesthesiology and Pain Medicine