Depletion of ryanodine-sensitive Ca2+ store activates Ca2+ entry in rat submandibular gland acinar cells

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 (InsP(3))-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 mu M ryanodine. Emptying the InsP,-sensitive stores by treatment with thapsigargin also caused Ca2+ entry, which maximally changed [Ca2+](i) by 349.6 +/- 15.1 nM. Ten mu mol/liter ryanodine was confirmed to cause a release of Ca-45(2+) from the parotidic microsomal fraction enriched in endopalsmic reticulum. me 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 thapsigargin-sensitive Ca2+ stores.