A triggered mechanism retrieves membrane in seconds after Ca²⁺- stimulated exocytosis in single pituitary cells

In neuroendocrine cells, cytosolic Ca²⁺ triggers exocytosis in tens of milliseconds, yet known pathways of endocytic membrane retrieval take minutes. To test for faster retrieval mechanisms, we have triggered short bursts of exocytosis by flash photolysis of caged Ca²⁺, and have tracked subsequent retrieval by measuring the plasma membrane capacitance. We find that a limited amount of membrane can be retrieved with a time constant of 4 s at 21-26°C, and that this occurs partially via structures larger than coated vesicles. This novel mechanism may be arrested at a late step. Incomplete retrieval structures then remain on the cell surface for minutes until the consequences of a renewed increase in cytosolic [Ca²⁺] disconnect them from the cell surface in <1 s. Our results provide evidence for a rapid, triggered membrane retrieval pathway in excitable cells.