Drug-specific effects of volatile anesthetics on Ca²⁺ sensitization in airway smooth muscle

Halothane directly relaxes airway smooth muscle, partly by decreasing the Ca²⁺ sensitivity during membrane receptor stimulation. The effects of other volatile anesthetics on Ca²⁺ sensitivity are unclear. In the current study, we compared the ability of halothane, sevoflurane, and isoflurane to inhibit increases in Ca²⁺ sensitivity during muscarinic receptor stimulation. β-Escin-permeabilized canine tracheal smooth muscle strips were used. Anesthetics were applied during contractions induced by 3 μM acetylcholine and 10 μM guanosine 5'-triphosphate at a constant cytosolic Ca²⁺ concentration of 0.3 μM. Effects were evaluated as a percent relaxation from initial force corrected for time. Halothane significantly decreased force at both low (0.76 minimum alveolar anesthetic concentration [MAC]) and high (1.8 MAC) concentrations in a concentration-dependent manner. Sevoflurane also decreased force, significantly so at a high concentration (1.7 MAC). Isoflurane did not significantly affect force even at a high concentration (1.7 MAC). Halothane's relaxing effect was significantly greater than that of the other two anesthetics at each corresponding MAC concentration. Among these three volatile anesthetics compared at equipotent anesthetic concentrations, halothane was the most potent in reducing Ca²⁺ sensitivity during muscarinic receptor stimulation in canine tracheal smooth muscle. This may contribute to halothane's greater relaxing effect compared with isoflurane at the same MAC concentrations in intact airway smooth muscle. Implications: In this study, we showed that three volatile anesthetics (halothane, sevoflurane, and isoflurane) compared at equipotent anesthetic concentrations differed in their ability to inhibit Ca²⁺ sensitivity during muscarinic receptor stimulation in airway smooth muscle. The potency order was halothane > sevoflurane ≤ isoflurane.