Capacitative Ca2+ entry during Ca2+ undershoot in bovine airway smooth muscle

In numerous cells, Ca2+ undershoot is commonly observed after withdrawing stimulus that release Ca2+ from intracellular stores. In airway smooth muscle (ASM), the fast intracellular Ca2+ concentration ([Ca2+]i) drop during undershoot is produced by sarcoplasmic reticulum (SR) reloading, but the mechanisms involved in the long lasting basal [Ca2+]i recovery are unknown. We investigated the post-caffeine Ca2+ undershoot recovery in ASM isolated cells from bovine trachea. [Ca2+]i determination was done by a ratiometric method by incubating cells with Fura-2/AM. After inducing a transient response, caffeine withdrawn generated a Ca2+ undershoot. SR-Ca2+ content during maximum undershoot drop was approximately 40% of SR caffeine-releasable Ca2+ (SR-Ca2+ load). Undershoot recovery rate increased in presence of cyclopiazonic acid (CPA, a SR-Ca2+ ATPase inhibitor), but SR-Ca2+ load was reduced. Genistein (a tyrosine kinase inhibitor) slowed down the Ca2+ undershoot drop and the SR-Ca2+ load but did not affect the undershoot recovery rate. Ni2+ (a capacitative Ca2+ inhibitor), but neither SKF-96365 (a passive Ca2+ entry inhibitor) nor econazole (a capacitative Ca2+ inhibitor in non-excitable cells), inhibited Ca2+ undershoot recovery and SR-Ca2+ load. Our data suggest that capacitative Ca2+ entry is involved in bovine ASM Ca2+ undershoot recovery, and that changes in Ca2+ undershoot have an impact on SR-Ca2+ loading which might affect in turn ASM excitability.