Sequential-replenishment mechanism of exocytosis in pancreatic acini

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Abstract
Here we report exocytosis of zymogen granules, as examined by multiphoton excitation imaging in intact pancreatic acini. Cholecystokinin induces Ca2+ oscillations that trigger exocytosis when the cytosolic Ca2+ concentration exceeds 1 μM. Zymogen granules fused with the plasma membrane maintain their Ω-shaped profile for an average of 220 s and serve as targets for sequential fusion of granules that are located within deeper layers of the cell. This secondary exocytosis occurrs as rapidly as the primary exocytosis and accounts for most exocytotic events. Granule–granule fusion does not seem to precede primary exocytosis, indicating that secondary fusion events may require a plasma-membrane factor. This sequential-replenishment mechanism of exocytosis allows the cell to take advantage of a large supply of fusion-ready granules without needing to transport them to the plasma membrane.