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 beta-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; GTP gamma S); and PKC was directly activated with the PKC agonist phorbol 12,13-dibutyrate (PDBu). Results: Free Ca2+ caused a concentration-dependent in crease in force. Acetylcholine plus GTP significantly decreased the median effective concentration for free Ca2+ from 0.52 +/- 0.06 mu M to 0.21 +/- 0.02 mu M, demonstrating an increase in Ca2+ sensitivity. Halothane (0.99 +/- 0.04 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 mu M to 0.31 +/- 0.03 PM. However, halothane did not affect the increases in Ca2+ sensitivity induced by GTP gamma S or PDBu. Conclusions: Halothane had no effect on increased Ca2+ sensitivity caused by direct activation of GTP-binding proteins with GTP gamma S or PKC with PDBu, suggesting that halothane attenuates acetylcholine-induced Ca2+ sensitization via a mechanism independent of these pathways in beta-escin-permeabilized canine tracheal smooth muscle.