Effect of monensin on chromaffin cells and the mechanism of organelle swelling

Chromaffin cells dissociated from bovine adrenal medulla display morphological changes consistent with lysis of the chromaffin granules after brief exposure to the cation carrier monensin. Monensin lysed isolated granules rapidly in Na + or K + -containing isotonic media. Lysis could be prevented either by hyperosmotic media or by lowering the external pH to 5.5. When granules were suspended in hyperosmotic sucrose to prevent lysis, Na + -monensin equalised external and internal pH as measured by intragranular ATP 31 P-NMR. A mechanism for the in vivo lysis of chromaffin granules and vacuolation of Golgi membranes by monensin in intact cells is proposed.