The calcium loading of secretory granules. A possible key event in stimulus‐secretion coupling

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Abstract
Summary—: 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 index. 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 (calciumloaded) 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.