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 s hort bursts of exocytosis by flash photolysis of caged Ca2+, and have tracked subsequent retrieval by measuring the plasma membrane capacita nce. We find that a limited amount of membrane can be retrieved with a time constant of 4 s at 21-26 degrees C, and that this occurs partial ly via structures larger than coated vesicles. This novel mechanism ma y be arrested at a late step. Incomplete retrieval structures then rem ain on the cell surface for minutes until the consequences of a renewe d increase in cytosolic [Ca2+] disconnect them from the cell surface i n <1 s. Our results provide evidence for a rapid, triggered membrane r etrieval pathway in excitable cells.