Regulation of intracellular calcium oscillations in porcine tracheal smooth muscle cells

Using real-time confocal microscopy, we examined the dynamic intracellular Ca²⁺ concentration ([Ca²⁺](i)) response of porcine tracheal smooth muscle (TSM) cells to acetylcholine (ACh). Exposure to ACh caused regenerative, propagating [Ca²⁺](i) oscillations. The amplitude and fall time of the [Ca²⁺](i) oscillations were inversely correlated to basal [Ca²⁺](i), whereas the frequency and rise time were directly correlated to basal [Ca²⁺](i). ACh-induced [Ca²⁺](i) oscillations were initiated in the absence of extracellular Ca²⁺ and after membrane depolarization with KCl, suggesting that 1) [Ca²⁺](i) oscillations primarily arise by release from internal stores such as the sarcoplasmic reticulum (SR) and 2) Ca²⁺ influx is necessary for maintenance of oscillations. Exposure to both caffeine and ryanodine inhibited ongoing ACh-induced [Ca²⁺](i) oscillations, suggesting a role for caffeine-sensitive ryanodine receptor (RyR) SR Ca²⁺ channels. Inhibition of SR Ca²⁺ reuptake by thapsigargin increased basal [Ca²⁺](i) and decreased [Ca²⁺](i) oscillation amplitude, suggesting that Ca²⁺ reuptake is also essential. The present results suggest that [Ca²⁺](i) oscillations in porcine TSM cells involve repetitive Ca²⁺ release and reuptake from RyR channels, perhaps through a Ca²⁺-induced Ca²⁺ release mechanism.