EVIDENCE FOR A CA2-CELLS( POOL ASSOCIATED WITH SECRETORY GRANULES IN RAT SUBMANDIBULAR ACINAR)

Intracellular Ca2+ stores in rat submandibular acinar cells were chara cterized using the Ca2+-sensitive fluorescent indicator fura 2 and the radiotracer Ca-45(2+). Acetylcholine induced a rapid Ca2+ release fro m a store sensitive to inositol 1,4,5-trisphosphate (IP3) and to thaps igargin (TG). After this store was presumably depleted, ionomycin caus ed a further increase in cytosolic free Ca2+ concentration ([Ca2+](i)) , suggesting the presence of an IP3- insensitive Ca2+ release from a s tore that is more extensive and heterogeneous than the IP3-sensitive o ne and includes a small mitochondrial component. After both of these s tores had been discharged, exposure to monensin caused an additional r elease of Ca2+ from a third store. This store appears to be associated with secretory granules, since Ca2+ release was significantly reduced when degranulation was induced by isoprenaline. This third store appe ars to be insensitive to IP3, discharges Ca2+ when the pH gradient acr oss the limiting membrane is collapsed with monensin and only in the p resence of both ionomycin and monensin. Ca2+ release from this store i s not by Na+/Ca2+ exchange, since simply altering [Na+], did not cause significant Ca2+ release. In permeabilized cells, IP3 and TG released approx. 35% of Ca-45(2+), and ionomycin released an additional 57%, w hereas monensin only caused a small additional release, suggesting tha t only IP3- and ionomycin-sensitive stores are loaded with Ca-45(2+) u nder these conditions. The absence of significant isotope uptake into the ionomycin + monensin-sensitive store may result from a low rate of tracer accumulation or from the lack of Ca2+ pumps in the store. The pattern of response was similar in the presence and absence of mitocho ndrial inhibitors, indicating that the store is not located in mitocho ndria. In summary, these results suggest that a substantial IP3-insens itive Ca2+ store is present in secretory granules in rat submandibular acinar cells.