Intracellular Cl fluxes play a novel role in Ca handling in airway smooth muscle

Hirota, Simon, Nancy Trimble, Evi Pertens, and Luke J. Janssen. Intracellular Cl fluxes play a novel role in Ca handling in airway smooth muscle. Am J Physiol Lung Cell Mol Physiol 290: L1146–L1153, 2006. First published January 20, 2006; doi:10.1152/ajplung.00393.2005.—Intracellular Ca is actively sequestered into the sarcoplasmic reticulum (SR), whereas the release of Ca from the SR can be triggered by activation of the inositol 1,4,5-trisphosphate and ryanodine receptors. Uptake and release of Ca across the SR membrane are electrogenic processes; accumulation of positive or negative charge across the SR membrane could electrostatically hinder the movement of Ca into or out of the SR, respectively. We hypothesized that the movement of intracellular Cl (Cli ) across the SR membrane neutralizes the accumulation of charge that accompanies uptake and release of Ca . Thus inhibition of SR Cl fluxes will reduce Ca sequestration and agonist-induced release. The Cl channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 10 4 M), previously shown to inhibit SR Cl channels, significantly reduced the magnitude of successive acetylcholine-induced contractions of airway smooth muscle (ASM), suggesting a “run down” of sequestered Ca within the SR. Niflumic acid (10 4 M), a structurally different Cl channel blocker, had no such effect. Furthermore, NPPB significantly reduced caffeine-induced contraction and increases in intracellular Ca concentration ([Ca ]i). Depletion of Cli , accomplished by bathing ASM strips in Cl -free buffer, significantly reduced the magnitude of successive acetylcholine-induced contractions. In addition, Cl depletion significantly reduced caffeine-induced increases in [Ca ]i. Together these data suggest a novel role for Cli fluxes in Ca handling in smooth muscle. Because the release of sequestered Ca is the predominate source of Ca for contraction of ASM, targeting Cli fluxes may prove useful in the control of ASM hyperresponsiveness associated with asthma.