Stimulation of Catecholamine Secretion from Cultured Chromaffin Cells by an Ionophore-Mediated Rise in Intracellular Sodium

• Published in: The Journal of cell biology 1982-09, Vol.94 (3), p.531-539
• Main Authors: Suchard, Suzanne J., Lattanzio, Frank A., Rubin, Robert W., Pressman, Berton C.
• Format: Article
• Language: English
• Subjects: Adrenal medulla; Animals; Calcium; Calcium - physiology; Catecholamines; Catecholamines - metabolism; Cattle; Cell membranes; Cells; Cells, Cultured; Chromaffin cells; Chromaffin System - metabolism; Chromaffin System - ultrastructure; Cultured cells; Exocytosis; Ionophores - pharmacology; Microscopy, Electron; Monensin - pharmacology; Secretion; Sodium; Sodium - physiology; Time Factors
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.94.3.531

The significance of intracellular Na+ concentration in catecholamine secretion of cultured bovine adrenal chromaffin cells was investigated using the monovalent carboxylic ionophore monensin. This ionophore, which is known to mediate a one-for-one exchange of intracellular K+ for extracellular Na+, induces a slow, prolonged release of catecholamines which, at 6 h, amounts of 75-90% of the total catecholamines; carbachol induces a rapid pulse of catecholamine secretion of 25-35%. Although secretory granule numbers appear to be qualitatively reduced after carbachol, multiple carbachol, or Ba2+ stimulation, overall granule distribution remains similar to that in untreated cells. Monensin-stimulated catecholamine release requires extracellular Na+ but not Ca2+ whereas carbachol-stimulated catecholamine release requires extracellular Ca2+ and is partially dependent on extracellular Na+. Despite its high selectivity for monovalent ions, monensin is considerably more effective in promoting catecholamine secretion than the divalent ionophores, A23187 and ionomycin, which mediate a more direct entry of extracellular Ca2+ into the cell. We propose that the monensin-stimulated increase in intracellular Na+ levels causes an increase in the availability of intracellular Ca2+ which, in turn, stimulates exocytosis. This hypothesis is supported by the comparable stimulation of catecholamine release by ouabain which inhibits the outwardly directed Na+ pump and thus permits intracellular Na+ to accumulate. The relative magnitudes of the secretion elicited by monensin, carbachol, and the calcium ionophores, are most consistent with the hypothesis that, under normal physiological conditions, Na+ acts by decreasing the propensity of Ca2+-sequestering sites to bind the Ca2+ that enters the cell as a result of acetylcholine stimulation.