Zymogen granules of mouse parotid acinar cells are acidified in situ in an ATP-dependent manner.

The mechanism for acidification of zymogen granules in acinar cells of mouse parotid gland was explored using acridine orange, lysosomotropic agents, and an inhibitor of cellular ATP production. Methylamine and monensin reversibly collapsed the pH gradient of granules without affecting cellular ATP levels. Depletion of cellular ATP with antimycin A did not collapse the pH gradient. However, recovery of acidity in the granules, after collapse of the pH gradient by methylamine, was blocked by depletion of cellular ATP. These results demonstrate that zymogen granules of parotid gland are acidic in situ and that ATP is required for acidification of the granules.