Microfilament-associated degranulation of sensitized guinea-pig lung mast cells.

When sensitized guinea-pig lung mast cells were exposed to antigen, granules were pushed out on to the cell surface. Subsequently, thin filaments, some extending as long as 15 microns, were projected radially with the extruded granules. The latter became swollen in the extracellular medium and the elongated filaments became shorter, until, within 7-8 min, the granules were reincorporated into the cytoplasm. The time course of morphological changes corresponded approximately to that of changes in the intracellular Ca2+ concentrations. The filaments connecting the extruded granules to the cell surface were stained with rhodamine-phalloidin, indicating that they consisted mainly of actin.