Intracellular Cl- fluxes play a novel role in Ca2+ 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(i)(-)) 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(i)(-), 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(i)(-) 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(i)(-) fluxes may prove useful in the control of ASM hyperresponsiveness associated with asthma.
Hirota
Simon
S
Asthma Research Group, Firestone Institute for Respiratory Health, St. Joseph's Healthcare, 50 Charlton Avenue East, Hamilton, Ontario, Canada L8N 4A6.
Trimble
Nancy
N
Pertens
Evi
E
Janssen
Luke J
LJ
D000109:Acetylcholine / Q000494:pharmacology; D020533:Angiogenesis Inhibitors / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002417:Cattle; D002712:Chlorides / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D009544:Niflumic Acid / Q000494:pharmacology; D009579:Nitrobenzoates / Q000494:pharmacology; D014132:Trachea / Q000502:physiology*
D020533:Angiogenesis Inhibitors; D002712:Chlorides; D009579:Nitrobenzoates; C058176:5-nitro-2-(3-phenylpropylamino)benzoic acid; D009544:Niflumic Acid; D009543:Nifedipine; D000109:Acetylcholine; D002118:Calcium