title: Halothane attenuation of calcium sensitivity in airway smooth muscle. Mechanisms of action during muscarinic receptor stimulation.
abstract: 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 treated 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 increase in force. Acetylcholine plus GTP significantly decreased the median effective concentration for free Ca2+ from 0.52 +/- 0.06 microM to 0.21 +/- 0.02 microM, 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 microM to 0.31 +/- 0.03 microM. 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.
authors.affiliation: Department of Anesthesiology, Physiology, Mayo Clinic, Rochester, Minnesota 55905, USA.
authors.forename: D H
authors.initials: DH
authors.lastname: Bremerich
authors.forename: A
authors.initials: A
authors.lastname: Hirasaki
authors.forename: K A
authors.initials: KA
authors.lastname: Jones
authors.forename: D O
authors.initials: DO
authors.lastname: Warner
mesh_terms: D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000502:physiology*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D006221:Halothane / Q000494:pharmacology*; D008297:Male; D018721:Muscarinic Agonists / Q000494:pharmacology; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology; D011976:Receptors, Muscarinic / Q000502:physiology*; D012143:Respiratory Physiological Phenomena; D012137:Respiratory System / Q000187:drug effects*
chemical_list: D018685:Anesthetics, Inhalation; D018721:Muscarinic Agonists; D011976:Receptors, Muscarinic; D002118:Calcium; D006221:Halothane