Dual effects of hexanol and halothane on the regulation of calcium sensitivity in airway smooth muscle

Background: Contraction of airway smooth muscle is regulated by receptor-coupled mechanisms that control the force developed for a given cytosolic calcium concentration (i.e., calcium sensitivity). Halothane antagonizes acetylcholine-induced increases in calcium sensitivity by inhibiting GTP-binding (G)-protein pathways. The authors tested the hypothesis that hexanol, like halothane, inhibits agonist-induced increases in calcium sensitivity in airway smooth muscle by inhibiting G-protein pathways. Methods: Calcium sensitivity was assessed using α-toxin-permeabilized canine tracheal smooth muscle. In selected experiments, regulatory myosin light chain phosphorylation was also determined by Western blotting in the presence and absence of 10 mM hexanol and/or 100 μM acetylcholine. Results: Hexanol (10 mM) and halothane (0.76 mM) attenuated acetylcholine-induced calcium sensitization by decreasing regulatory myosin light chain phosphorylation during receptor stimulation. Hexanol also inhibited increases in calcium sensitivity due to direct stimulation of heterotrimeric G-proteins with tetrafluoroaluminate but not with 3 μM GTPγS, consistent with prior results obtained with halothane. In contrast, in the absence of receptor stimulation, both compounds produced a small increase in calcium sensitivity by a G-protein - mediated increase in regulatory myosin light chain phosphorylation that was not affected by pertussis toxin treatment. Conclusions: The authors noted dual effects of hexanol and halothane. In the presence of muscarinic receptor stimulation, hexanol, like halothane, decreases calcium sensitivity by interfering with heterotrimeric G-protein function. However, in the absence of muscarinic receptor stimulation, hexanol and halothane slightly increase calcium sensitivity by a G-protein-mediated process not sensitive to pertussis toxin. Hexanol may represent a useful experimental tool to study the effect of anesthetics on heterotrimeric G-protein function.