Measurement of changes in membrane surface morphology associated with exocytosis using scanning ion conductance microscopy

The extent that vesicles maintain a distinct identity and morphology after fusing with the plasma membrane is controversial. We used scanning ion conductance microscopy to image changes in the surface membrane of adrenal chromaffin cells after stimulation of exocytosis with a high K(+) solution. Within several minutes after stimulation, punctate depressions, 100-600 nm wide, were noted from 16% of the cells. The depressions were not randomly distributed, but appeared in clusters of two or more within a approximately 1 microm(2) area and disappeared after several minutes. Increases in membrane surface area, consistent with the fusion and collapse of one or more vesicles into the surface membrane, were observed in 64% of the cells after high K(+) stimulation. Surface area increases did not occur if the high K(+) solution did not contain Ca(2+). We conclude that scanning ion conductance microscopy can be used to follow the time course of surface membrane changes resulting from exocytosis and endocytosis.