Role of extracellular Ca2+ in acetylcholine-induced repetitive Ca2+ release in submandibular gland acinar cells of the rat.

Acetylcholine (ACh) caused repetitive transient Cl- currents activated by intracellular Ca2+ in single rat submandibular grand acinar cells. As the concentration of ACh increased the amplitude and the frequency of the transient Cl- currents increased. These responses occurred also in the absence of extracellular Ca2+ but disappeared after several minutes. Repetitive transient Cl- currents were restored by readmission of Ca2+ to the extracellular solution. The higher the concentration of extracellular Ca2+ readmitted, the larger the amplitude of the transient Cl- currents. Ca2+ entry through a store-coupled pathway was detected by application of Ca2+ to the extracellular solution during a brief cessation of stimulation with ACh. In these experiments too, the higher the concentration of Ca2+, the larger the transient Cl- currents activated by Ca2+ released from the stores. The time course of decrease in total charge movements of repetitive transient responses to ACh with removal of extracellular Ca2+ depended on a decrease in charge movements of each transient event rather than a decrease in frequency of the repetitive events. The decrease of charge movements of each transient event was due to a decrease in its amplitude rather than its duration. The results suggest that in this cell type and amplitude-modulated mechanism is involved in repetitive Ca2+ release and that Ca2+ entry is essential to maintain the repetitive release of Ca2+. The results further suggest that the magnitude of Ca2+ entry determines the number of unitary stores filled with Ca2+ which can synchronously respond to ACh.