Microfilament-associated, local degranulation of rat peritoneal mast cells.

When compound 48/80 was applied by means of microelectrophoresis to the surface of a rat peritoneal mast cell, localized degranulation was observed in the area close to the microelectrode tip. The extruded granules were connected to the cell surface by filaments. The filaments were elongated radially and, in some occasions, projected to a length of 5 microns. A few minutes later, the length of the protruded filaments became shorter and shorter and, finally, the extruded granules were reincorporated into the cell. When rhodamine-phalloidin, an F-actin-specific dye, was perfused the extruded granules and filaments were stained by this dye. This indicates that actin filaments or fragments of them exist on the granule surface and on the cell surface at the site of degranulation. These actin filaments bound to the mast cell granules may play an important role, not only for the extrusion of the granules, but also for the reuptake of extruded granules into the cytoplasm.