Stimulation of catecholamine secretion from cultured chromaffin cells by an ionophore-mediated rise in intracellular sodium.
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.
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002395:Catecholamines / Q000378:metabolism*; D002417:Cattle; D002478:Cells, Cultured; D002838:Chromaffin System / Q000378:metabolism* / Q000648:ultrastructure; D007476:Ionophores / Q000494:pharmacology*; D008854:Microscopy, Electron; D008985:Monensin / Q000494:pharmacology; D012964:Sodium / Q000502:physiology*; D013997:Time Factors
D002395:Catecholamines; D007476:Ionophores; D008985:Monensin; D012964:Sodium; D002118:Calcium