Cation-dependent lysis of chromaffin granules ? An alternative hypothesis for osmotically-driven exocytosis

The sodium-selective ionophore monensin produced rapid lysis of chromaffin granules under iso-osmotic conditions. Lysis occurred at ionophore concentrations down to 15ng/ml and resulted from a dramatic increase in the osmotic activity of the granule contents. The significant features of this osmotical1y-driven fission are the low effective cation concentration (half maximal lysis at 11mM Na+) and the absence of a requirement for a permeant anion. It is suggested that an increase in Na+ (or K+) permeability of the granule membrane following stimulation of the chromaffin cell could produce a similar response in the intact cell.