Halothane Does Not Inhibit the Functional Coupling between the β2-Adrenergic Receptor and the Gαs Heterotrimeric G Protein

Background: This study investigated whether halothane affects the functional coupling between the [beta]2 adrenergic receptor and the [alpha] subunit of its cognate stimulatory heterotrimeric guanosine-5'-triphosphate (GTP)-binding protein (G[alpha]s). The authors hypothesized that halothane does not affect isoproterenol-promoted guanosine nucleotide exchange at G[alpha]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. G[alpha]s nucleotide exchange was measured in crude membranes prepared from COS-7 cells transfected to transiently coexpress the human [beta]1 or [beta]2 receptor each with human short G[alpha]s. A radioactive, nonhydrolyzable analog of GTP, [35S]GTP[gamma]S, was used as the reporter for nucleotide exchange at G[alpha]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 G[alpha]s nucleotide exchange. Halothane, even at concentrations of 1.5 mm (approximately 5.6 MAC), had no effect on basal G[alpha]s nucleotide exchange in the absence of isoproterenol, whereas halothane inhibited isoproterenol-promoted G[alpha]s nucleotide exchange in both the [beta]1-G[alpha]s and [beta]2-G[alpha]s expressing membranes. However, the effect was significantly greater on [beta]1-G[alpha]s coupling compared with [beta]2-G[alpha]s coupling, with no effect on [beta]2-G[alpha]s coupling at 2.8 MAC halothane.