Halothane inhibits agonist-induced potentiation of rmlc phosphorylation in permeabilized airway smooth muscle

Agonist-induced increases in Ca²⁺ sensitivity are mediated in part by mechanisms that increase phosphorylation of the regulatory myosin light chain (rMLC) at constant cytosolic Ca²⁺ concentration ([Ca²⁺](i)). The current study tested the hypothesis that halothane inhibits acetylcholine (ACh)- induced potentiation of rMLC phosphorylation in β-escin-permeabilized canine tracheal smooth muscle. ACh plus GTP significantly potentiated the increase in isometric force and rMLC phosphorylation induced by 0.8 μM free Ca²⁺. However, whereas the potentiation of isometric force was sustained, the potentiation of rMLC phosphorylation was biphasic, peaking at 0.5 min and then declining by ~10 min to a steady-state level significantly above that induced by 0.8 μM free Ca²⁺ alone. This finding suggests that mechanisms in addition to changes in rMLC phosphorylation may mediate ACh-induced Ca²⁺ sensitization, as has been reported for vascular smooth muscle. Halothane (0.91 ± 0.10 mM) significantly inhibited ACh plus GTP-induced potentiation of rMLC phosphorylation and isometric force after 2 (peak rMLC phosphorylation) and 15 (steady-state rMLC phosphorylation) min of stimulation. However, the effect of halothane on the potentiation of isometric force was significantly less than that expected from its effect on rMLC phosphorylation (i.e., halothane changed the relationship between rMLC phosphorylation and isometric force). These results demonstrate that halothane inhibits the ACh-induced increase in Ca²⁺ sensitivity by inhibiting the membrane receptor-coupled mechanisms that increase rMLC phosphorylation at constant submaximal [Ca²⁺](i). Possible additional effects of halothane on rMLC phosphorylation-independent mechanisms cannot be ruled out.