Mast cell secretion. Rapid sealing of exocytotic cavities demonstrated by cytofluorometry

Mast cell secretion is closely associated with extrusion of granules from the cell, and results in rapid release of both amines and heparin. Amines may also be released from intracellular granules communicating with the extracellular environment through channels formed by membrane fusions. We have recently reported the use of the fluorescent, cationic dye berberine as a vital stain marking amine-releasing granules located within the domains of the cell. The present report concerns early membrane events and the fate of such granules after termination of the secretory process. Purified mast cells were stimulated to a moderate secretory response by a low concentration of polymyxin B. The cells were then kept in a medium without the secretagogue. The fluorescence intensity of individual cells after vital berberine staining (reflecting the number of intracellular granules in contact with the extracellular environment) decreased rapidly after terminated mast cell secretion. The heparin content (reflecting the total number of intracellular granules) and the histamine content of the cells, both reduced by the secretory process, remained constant over a period of 1 h after terminated mast cell secretion. These findings suggest that mast cell membranes are rapidly restored after secretion, permitting selective amine release without accompanying release of heparin or other matrix components.