title: Effects of halothane on sarcoplasmic reticulum calcium release channels in porcine airway smooth muscle cells.
abstract: BACKGROUND
  Volatile anesthetics relax airway smooth muscle (ASM) by altering intracellular Ca2+ concentration ([Ca2+]i). The authors hypothesized that relaxation is produced by decreasing sarcoplasmic reticulum Ca2+ content via increased Ca2+ "leak" through both inositol trisphosphate (IP3) and ryanodine receptor channels.
  
  
  METHODS
  Enzymatically dissociated porcine ASM cells were exposed to acetylcholine in the presence or absence of 2 minimum alveolar concentration (MAC) halothane, and IP3 levels were measured using radioimmunoreceptor assay. Other cells were loaded with the Ca2+ indicator fluo-3 and imaged using real-time confocal microscopy.
  
  
  RESULTS
  Halothane increased IP3 concentrations in the presence and absence of acetylcholine. Inhibition of phospholipase C blunted the IP3 response to halothane. Exposure to 2 MAC halothane induced a transient [Ca2+]i response, suggesting depletion of sarcoplasmic reticulum Ca2+. Exposure to 20 microM Xestospongin D, a cell-permeant IP3 receptor antagonist, resulted in a 45+/-13% decrease in the [Ca2+]i response to halothane compared with halothane exposure alone. In permeabilized cells, Xestospongin D or 0.5 mg/ml heparin decreased the [Ca2+]i response to halothane by 65+/-13% and 68+/-22%, respectively, compared with halothane alone. In both intact and permeabilized cells, 20 microM ryanodine blunted the [Ca2+]i response to halothane by 32+/-13% and 39+/-21%, respectively, compared with halothane alone. Simultaneous exposure to Xestospongin D and ryanodine completely inhibited the [Ca2+]i response to halothane.
  
  
  CONCLUSIONS
  The authors conclude that halothane reduces sarcoplasmic reticulum Ca2+ content in ASM cells via increased Ca2+ leak through both IP3 receptor and ryanodine receptor channels. Effects on IP3 receptor channels are both direct and indirect via elevation of IP3 levels.
authors.affiliation: Department of Anesthesiology, Mayo Clinic and Foundation, Rochester, Minnesota 55905, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Pabelick
authors.forename: Y S
authors.initials: YS
authors.lastname: Prakash
authors.forename: M S
authors.initials: MS
authors.lastname: Kannan
authors.forename: D O
authors.initials: DO
authors.lastname: Warner
authors.forename: G C
authors.initials: GC
authors.lastname: Sieck
mesh_terms: D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002138:Calibration; D004928:Escin / Q000494:pharmacology; D006221:Halothane / Q000494:pharmacology*; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018613:Microscopy, Confocal; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000378:metabolism; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013552:Swine
chemical_list: D018685:Anesthetics, Inhalation; D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D019837:Ryanodine Receptor Calcium Release Channel; D004928:Escin; D015544:Inositol 1,4,5-Trisphosphate; D006221:Halothane