Depletion of Ryanodine-Sensitive Ca2⁺ Store Activates Ca2⁺ Entry in Rat Submandibular Gland Acinar Cells

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The existence of ryanodine-sensitive Ca²⁺ stores and their role in the Ca²⁺ entry mechanism were examined in the rat submandibular gland acinar cells, using the microfluorimetry of intracellular Ca²⁺ concentration ([Ca²⁺]_i). In the presence of thapsigargin, a Ca²⁺-ATPase inhibitor of inositol (1, 4, 5) triphosphate (InsP₃)- sensitive Ca²⁺ stores, caffeine caused an increase in [Ca²⁺]_i, which was inhibited by treatment with ryanodine (a ligand to the Ca²⁺-induced Ca²⁺ release channels). In the cells treated with ryanodine, 1 mM Ca²⁺ addition to a Ca²⁺-free solution caused a marked increase in [Ca²⁺]_i, which was eliminated by application of Ni²⁺ or SK & F 96365, suggesting a Ca²⁺ entry triggered by ryanodine. The maximal change in the net increase in [Ca²⁺]_i caused by the ryanodine-coupled Ca²⁺ entry, was 104.0± 16.0 nM, which intense was caused by 10 μM ryanodine. Emptying the InsP₃-sensitive stores by treatment with thapsigargin also caused Ca²⁺ entry, which maximally changed [Ca²⁺]_i by 349.6± 15.1 nM. Ten μmol/liter ryanodine was confirmed to cause a release of ⁴⁵Ca²⁺ from the parotidic microsomal fraction enriched in endopaismic reticulum. We propose that ryanodine-sensitive Ca²⁺ stores are present in rat submandibular gland acinar cells. We further propose that release of Ca²⁺ from the ryanodine-sensitive stores, which means eventually depletion of the ryanodine-sensitive Ca²⁺ stores, can activate the Ca²⁺ entry. The ability for Ca²⁺ entry coupled with the ryanodine-sensitive Ca²⁺ stores seems to be about 30% of the ability for Ca²⁺ entry coupled with the thapsigarginsensitive Ca²⁺ stores.