Intracellular Cl − fluxes play a novel role in Ca 2+ handling in airway smooth muscle

Intracellular Ca 2+ is actively sequestered into the sarcoplasmic reticulum (SR), whereas the release of Ca 2+ from the SR can be triggered by activation of the inositol 1,4,5-trisphosphate and ryanodine receptors. Uptake and release of Ca 2+ across the SR membrane are electrogenic processes; accumulation of positive or negative charge across the SR membrane could electrostatically hinder the movement of Ca 2+ into or out of the SR, respectively. We hypothesized that the movement of intracellular Cl − (Cl[Formula: see text]) across the SR membrane neutralizes the accumulation of charge that accompanies uptake and release of Ca 2+ . Thus inhibition of SR Cl − fluxes will reduce Ca 2+ 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 2+ 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 2+ concentration ([Ca 2+ ] i ). Depletion of Cl[Formula: see text], 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 2+ ] i . Together these data suggest a novel role for Cl[Formula: see text] fluxes in Ca 2+ handling in smooth muscle. Because the release of sequestered Ca 2+ is the predominate source of Ca 2+ for contraction of ASM, targeting Cl[Formula: see text] fluxes may prove useful in the control of ASM hyperresponsiveness associated with asthma.