Halothane attenuation of calcium sensitivity in airway smooth muscle: Mechanisms of action during muscarinic receptor stimulation

Background: In airway smooth muscle, muscarinic receptor stimulation is thought to increase calcium (Ca²⁺) sensitivity via a guanosine 5'- triphosphate (GTP)-binding protein/protein kinase C (PKC)-mediated mechanism. This study tested the hypothesis that halothane reduces Ca²⁺ sensitivity during muscarinic receptor stimulation by inhibiting these second messenger pathways. Methods: A β-escin permeabilized canine tracheal smooth muscle preparation was used in which the cytosolic Ca²⁺ concentration ([Ca²⁺](i)) is controlled and the GTP-binding protein/PKC pathways remain intact and can be activated. The muscarinic receptor was activated with acetylcholine plus GTP; the GTP-binding proteins were directly activated with a nonhydrolyzable form of GTP, guanosine 5'-O-(3-thiotriphosphate; GTPγS); and PKC was directly activated with the PKC agonist phorbol 12,13-dibutyrate (PDBu). Results: Free Ca²⁺ caused a concentration-dependent increase in force. Acetylcholine plus GTP significantly decreased the median effective concentration for free Ca²⁺ from 0.52 ± 0.06 μM to 0.21 ± 0.02 μM, demonstrating an increase in Ca²⁺ sensitivity. Halothane (0.99 ± 0.44 mM, equivalent to approximately 4 minimum alveolar concentration in dogs) significantly attenuated this increase in Ca²⁺ sensitivity induced by acetylcholine plus GTP, increasing the median effective concentration for free Ca²⁺ from 0.21 ± 0.02 μM to 0.31 ± 0.03 μM. However, halothane did not affect the increases in Ca²⁺ sensitivity induced by GTPγS or PDBu. Conclusions: Halothane had no effect on increased Ca²⁺ sensitivity caused by direct activation of GTP-binding proteins with GTPγS or PKC with PDBu, suggesting that halothane attenuates acetylcholine-induced Ca²⁺ sensitization via a mechanism independent of these pathways in β-escin- permeabilized canine tracheal smooth muscle.