Capacitative Ca2+ Entry During Ca2+ Undershoot in Bovine Airway Smooth Muscle
In numerous cells, Ca<sup>2+</sup> undershoot is commonly observed after withdrawing stimulus that release Ca<sup>2+</sup> from intracellular stores. In airway smooth muscle (ASM), the fast intracellular Ca<sup>2+</sup> concentration ([Ca<sup>2+</sup>]<sub>i</sub>) drop during undershoot is produced by sarcoplasmic reticulum (SR) reloading, but the mechanisms involved in the long lasting basal [Ca<sup>2+</sup>]<sub>i</sub> recovery are unknown. We investigated the post-caffeine Ca<sup>2+</sup> undershoot recovery in ASM isolated cells from bovine trachea. [Ca<sup>2+</sup>]<sub>i</sub> determination was done by a ratiometric method by incubating cells with Fura-2/AM. After inducing a transient response, caffeine withdrawn generated a Ca<sup>2+</sup> undershoot. SR-Ca<sup>2+</sup> content during maximum undershoot drop was ∼40% of SR caffeine-releasable Ca<sup>2+</sup> (SR-Ca<sup>2+</sup> load). Undershoot recovery rate increased in presence of cyclopiazonic acid (CPA, a SR-Ca<sup>2+</sup> ATPase inhibitor), but SR-Ca<sup>2+</sup> load was reduced. Genistein (a tyrosine kinase inhibitor) slowed down the Ca<sup>2+</sup> undershoot drop and the SR-Ca<sup>2+</sup> load but did not affect the undershoot recovery rate. Ni<sup>2+</sup> (a capacitative Ca<sup>2+</sup> inhibitor), but neither SKF-96365 (a passive Ca<sup>2+</sup> entry inhibitor) nor econazole (a capacitative Ca<sup>2+</sup> inhibitor in non-excitable cells), inhibited Ca<sup>2+</sup> undershoot recovery and SR-Ca<sup>2+</sup> load. Our data suggest that capacitative Ca<sup>2+</sup> entry is involved in bovine ASM Ca<sup>2+</sup> undershoot recovery, and that changes in Ca<sup>2+</sup> undershoot have an impact on SR-Ca<sup>2+</sup> loading which might affect in turn ASM excitability.