The existence of ryanodine-sensitive Ca
2+ stores and their role in the Ca
2+ entry mechanism were examined in the rat submandibular gland acinar cells, using the microfluorimetry of intracellular Ca
2+ concentration ([Ca
2+]
i). In the presence of thapsigargin, a Ca
2+-ATPase inhibitor of inositol (1, 4, 5) triphosphate (InsP
3)- sensitive Ca
2+ stores, caffeine caused an increase in [Ca
2+]
i, which was inhibited by treatment with ryanodine (a ligand to the Ca
2+-induced Ca
2+ release channels). In the cells treated with ryanodine, 1 mM Ca
2+ addition to a Ca
2+-free solution caused a marked increase in [Ca
2+]
i, which was eliminated by application of Ni
2+ or SK & F 96365, suggesting a Ca
2+ entry triggered by ryanodine. The maximal change in the net increase in [Ca
2+]
i caused by the ryanodine-coupled Ca
2+ entry, was 104.0± 16.0 nM, which intense was caused by 10 μM ryanodine. Emptying the InsP
3-sensitive stores by treatment with thapsigargin also caused Ca
2+ entry, which maximally changed [Ca
2+]
i by 349.6± 15.1 nM. Ten μmol/liter ryanodine was confirmed to cause a release of
45Ca
2+ from the parotidic microsomal fraction enriched in endopaismic reticulum. We propose that ryanodine-sensitive Ca
2+ stores are present in rat submandibular gland acinar cells. We further propose that release of Ca
2+ from the ryanodine-sensitive stores, which means eventually depletion of the ryanodine-sensitive Ca
2+ stores, can activate the Ca
2+ entry. The ability for Ca
2+ entry coupled with the ryanodine-sensitive Ca
2+ stores seems to be about 30% of the ability for Ca
2+ entry coupled with the thapsigarginsensitive Ca
2+ stores.
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