DEPLETION OF RYANODINE-SENSITIVE CA2-GLAND ACINAR-CELLS( STORE ACTIVATES CA2+ ENTRY IN RAT SUBMANDIBULAR)

The existence of ryanodine-sensitive Ca2+ stores and their role in the Ca2+ entry mechanism were examined in the rat submandibular gland aci nar cells, using the microfluorimetry of intracellular Ca2+ concentrat ion ([Ca2+](i)). In the presence of thapsigargin, a Ca2+-ATPase inhibi tor of inositol (1,4,5) triphosphate (InsP(3))-sensitive Ca2+ stores, caffeine caused an increase in [Ca2+](i), which was inhibited by treat ment with ryanodine (a ligand to the Ca2+-induced Ca2+ release channel s). In the cells treated with ryanodine, 1 mM Ca2+ addition to a Ca2+- free solution caused a marked increase in [Ca2+](i), which was elimina ted by application of Ni2+ or SK & F 96365, suggesting a Ca2+ entry tr iggered 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, w hich intense was caused by 10 mu M ryanodine. Emptying the InsP,-sensi tive stores by treatment with thapsigargin also caused Ca2+ entry, whi ch 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 parot idic 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 ry anodine-sensitive Ca2+ stores, can activate the Ca2+ entry. The abilit y for Ca2+ entry coupled with the ryanodine-sensitive Ca2+ stores seem s to be about 30% of the ability for Ca2+ entry coupled with the thaps igargin-sensitive Ca2+ stores.