Regulation of [Ca2+]i in canine airway smooth muscle by Ca(2+)-ATPase and Na+/Ca2+ exchange mechanisms
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We investigated Ca2+ handling in airway smooth muscle (SM) using fura 2 fluorescence, ion currents, and contractions as indexes of intracellular Ca2+ concentration ([Ca2+]i). Carbachol evoked a transient elevation of [Ca2+]i, the magnitude of which was smaller and the rate of decay faster at 37 degrees C, indicating that some temperature-sensitive mechanism contributed to recovery. Removal of external Na+ had no effect on agonist-evoked Ca2+ transients or contractions or on spontaneous Ca(2+)-dependent K+ currents. Cyclopiazonic acid, a selective inhibitor of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, evoked a transient elevation of [Ca2+]i and contraction, markedly slowed recovery of the cholinergic Ca2+ transient, and depleted the SR. Sodium vanadate evoked a sustained elevation of [Ca2+]i and markedly slowed the decay of the cholinergic Ca2+ transient. We conclude that, in canine airway SM, 1) Na+/Ca2+ exchange makes at best only a minor contribution to Ca2+ homeostasis, 2) the SR Ca(2+)-ATPase compensates for spontaneous and agonist-triggered release of Ca2+, and 3) [Ca2+]i homeostasis involves some other extrusion pathway, likely the plasmalemmal Ca(2+)-ATPase.
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