Effect of halothane on the guanosine 5′ triphosphate binding activity of G-protein αi subunits

Abstract

Background: Receptor-mediated increases in the force produced by airway smooth muscle are attenuated by anesthetics such as halothane. Guanosine 5′-triphosphate (GTP) binding protein α subunits (Gαi) are known to participate in the regulation of force in airway smooth muscle. The authors hypothesized that halothane would inhibit the ability of Gαi subunits to bind a nonhydrolyzable analog of GTP (GTPγS). Methods: The effect of halothane on both GTPase-specific activity and [35S]GTPγS binding were assayed using purified, recombinant Gαi1. In separate experiments, [35S]GTPγS binding to Gαi in crude airway smooth muscle membrane preparations was assayed using an immunoprecipitation technique in the presence and absence of halothane. Results: The steady state GTPase-specific activity of the recombinant Gαi1 was 0.033 ± 0.018 (mean ± SD) mole Pi mole Gαi1-1 min-1 under control conditions and 0.035 ± 0.015 mole Pi mole Gαi1-1 min-1 in the presence of 1.1 ± 0.2 mM halothane, a difference that is not significant. The mole fractions of recombinant Gαi1 bound to [35S]GTPγS were 0.49 ± 0.02 and 0.60 ± 0.02 at 10 and 20 min, respectively. The addition of halothane (1.26 ± 0.07 mM) did not significantly change these values. Halothane did not affect the binding of [35S]GTPγS to Gαi subunits in membrane fractions of airway smooth muscle as measured using immunoprecipitation. Validity of the assays was confirmed using suramin, an inhibitor of GTP binding. Conclusion: These results suggest that halothane, which inhibits receptor-activated Gαi-coupled pathways in intact airway smooth muscle, must functionally target a component of the G protein-coupled receptor complex other than Gαi.