Halothane attenuates calcium sensitization in airway smooth muscle by inhibiting G-proteins

Background: Halothane directly relaxes airway smooth muscle partly by decreasing the Ca²⁺ sensitivity. In smooth muscle, receptor stimulation is thought to increase Ca²⁺ sensitivity via a cascade of heterotrimeric and small monomeric guanine nucleotide-binding proteins (G-proteins). Whether this model is applicable in the airway and where halothane acts in this pathway were investigated. Methods: A β-escin-permeabilized canine tracheal smooth muscle preparation was used. Exoenzyme C3 of Clostridium botulinum, which inactivates Rho monomeric G-proteins, was used to evaluate the involvement of this protein in the Ca²⁺ sensitization pathway. The effects of halothane on different stimulants acting at different levels of signal transduction were compared: acetylcholine on the muscarinic receptor, aluminum fluoride (AlF₄^-) on heterotrimeric G-proteins, and guanosine 5'- O-(3-thiotriphosphate) (GTP(γ)S) on all G-proteins. Results: Exoenzyme C3 equally attenuated acetylcholine- and AlF₄^--induced Ca²⁺ sensitization, suggesting that these pathways are both mediated by Rho. Halothane applied before stimulation equally attenuated acetylcholine- and AlF₄^--induced Ca²⁺ sensitization. However, when added after Ca²⁺ sensitization was established, the effect of halothane was greater during Ca²⁺ sensitization induced by acetylcholine compared with AlF₄^-, which, along with the previous result, suggests that halothane may interfere with dissociation of heterotrimeric G-proteins. Halothane applied during GTP(γ)S-induced Ca²⁺ sensitization had no significant effect on force, suggesting that halothane has no effect downstream from monomeric G-proteins. Conclusion: Halothane inhibits increases in Ca²⁺ sensitivity of canine tracheal smooth muscle primarily by interfering with the activation of heterotrimeric G-proteins, probably by inhibiting their dissociation.