THE CALCIUM LOADING OF SECRETORY GRANULES - A POSSIBLE KEY EVENT IN STIMULUS-SECRETION COUPLING

The review focuses on calcium accumulation by secretory organelles. The observation that secretory granules contain variable and often important quantities of calcium (1-200 mM of total calcium) can be interpreted as a maturation in ex. A progressive loading with calcium would be permitted by a Ca2+-transport mechanism on the granular membrane and calcium-binding molecules in the granular core. The saturation of this store by the stimulus-induced calcium transient would permit in mature (calcium-loaded) granules the ionic crisis leading to exocytosis. The inside of secretory organelles being acidic, calcium influx into the granule can be driven by calcium-proton exchange. The calcium-proton exchanger could be a Ca2+-ATPase.