Effects of halothane on sarcoplasmic reticulum calcium release channels inporcine airway smooth muscle cells

Background Volatile anesthetics relax airway smooth muscle (ASM) by alterin g intracellular Ca2+ concentration ([Ca2+],). The authors hypothesized that relaxation is produced by decreasing sarcoplasmic reticulum Ca2+ content v ia increased Ca2+ "leak" through both inositol trisphosphate (IP3) and ryan odine receptor channels. Methods: Enzymatically dissociated porcine ASM cells were exposed to acetyl choline in the presence or absence of 2 minimum alveolar concentration (MAC ) halothane, and IF, levels were measured using radioimmunoreceptor assay, Other cells were loaded with the Ca2+ indicator fluo-3 and imaged using rea l-time confocal microscopy, Results: Halothane increased IF, concentrations in the presence and absence of acetylcholine. Inhibition of phospholipase C blunted the IF3 response t o halothane, Exposure to 2 MAC halothane induced a transient [Ca2+], respon se, suggesting depletion of sarcoplasmic reticulum Ca2+. Exposure to 20 muM Xestospongin D, a cell-permeant IF, receptor antagonist, resulted in a 45 +/- 13% decrease in the [Ca2+], response to halothane compared with halotha ne exposure alone. In permeabilized cells, Xestospongin D or 0.5 mg/ml hepa rin decreased the [Ca2+], response to halothane by 65 +/- 13% and 68 +/- 22 %, respectively, compared with halothane alone. In both intact and permeabi lized cells, 20 muM ryanodine blunted the [Ca2+], response to halothane by 32 +/- 13% and 39 +/- 21%, respectively, compared with halothane alone. Sim ultaneous exposure to Xestospongin D and ryanodine completely inhibited the [Ca2+], response to halothane, Conclusions: The authors conclude that halothane reduces sarcoplasmic retic ulum Ca2+ content in ASM cells via increased Ca2+ leak through both IF, rec eptor and ryanodine receptor channels. Effects on IF, receptor channels are both direct and indirect via elevation of IF, levels.