Halothane reduces myofilament Ca²⁺ sensitivity during muscarinic receptor stimulation of airway smooth muscle

This study used a β-escin-permeabilized canine tracheal smooth muscle preparation to test the hypothesis that the volatile anesthetic halothane decreases myofilament Ca²⁺ sensitivity by inhibiting the membrane receptor- linked second messenger systems that regulate myofilament Ca²⁺ sensitivity and not by inhibiting Ca²⁺-calmodulin activation of the contractile proteins. Acetylcholine (ACh) caused a GTP-dependent increase in force at constant submaximal cytosolic Ca²⁺ concentration. ACh, guanosine-5'-O-(3- thiotriphosphate), and the protein kinase C agonist 12, 13-phorbol dibutyrate each significantly decreased the concentration of free Ca²⁺ producing a half-maximal response from 0.77 ± 0.09 μM (Ca²⁺ alone) to 0.16 ± 0.01, 0.19 ± 0.02, and 0.37 ± 0.03 μM, respectively, demonstrating an increase in myofilament Ca²⁺ sensitivity. Halothane (0.92 ± 0.12 mM) had no effect on the free Ca²⁺ concentration-response curves generated by Ca²⁺ alone. However, in the presence of 3 μM ACh plus 10 μM GTP to maximally activate muscarinic receptors, halothane significantly increased the EC₅₀ for free Ca²⁺ from 0.17 ± 0.01 μM to 0.38 ± 0.03 μM. These findings suggest that halothane decreases myofilament Ca²⁺ sensitivity in β-escin-permeabilized canine tracheal smooth muscle by inhibiting the membrane receptor-linked second messenger systems that regulate myofilament Ca²⁺ sensitivity.