Stimulation of catecholamine secretion from cultured chromaffin cells by an ionophore-mediated rise in intracellular sodium.

Abstract

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 Ba 2÷ stimulation, overall granule distribution remains similar to that in untreated cells. Monensin-stimulated catecholamine release requires extracellular Na ÷ but not Ca 2+ whereas carbachol-stimulated catecholamine release requires extracellular Ca 2÷ 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 Ca ~+ into the cell. We propose that the monensin-stimulated increase in intracellular Na + levels causes an increase in the availability of intracellular Ca 2+ 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 Ca 2+ that enters the cell as a result of acetylcholine stimulation.The key role of Ca ~+ in stimulus-secretion coupling in secretory cells was first recognized by Douglas and Rubin (1). The subsequent availability of divalent cation ionophores, e.g. A23187, lasalocid (X-537A), ionomycin, prompted an extensive series of investigations of the ability of these agents to promote cellular secretion by admitting extracellular Ca 2+ into cells (2-11).A number of recent studies indicate that a rise in the cytoplasmic Na + concentration also stimulates the secretory activity of cells (12)(13)(14)(15). The Na ÷ may dislodge a small fraction of the relatively large store of intracellularly sequestered Ca ~÷ so that it becomes available to the presumptive intracellular Ca 2+ receptors, calmodulin or troponin C (16). Alternately, increased intracellular Na + may interfere with the ability of the intracellular Ca2+-sequestering mechanisms to intercept Ca 2÷ that is brought into the cytosol by secretagogue-initiated processes. Ouabain, which alters the egress of intracellular Na + by a specific inhibition of the Na+-K+-ATPase (Na ÷ pump), elevates intracellular Na ÷ levels and thereby stimulates secretion of catecholamines, acetylcholine, and insulin (17)(18)(...

Calcium/sodium exchange in purified secretory vesicles from bovine neurohypophyses

“…Whether Na+ dependent Ca 2 + efflux from secretory vesicles may also occur in stimulated or resting cells is not easy to evaluate. It should be noted, however, that increased intracellular Na+, elicited by specific inhibition of the Na+/ K+ ATPase or the Na+ ionophore monensin, initiates secretion even in the absence of extracellular Ca 2 + (27). This means that the amount of Ca 2 + present in secretory vesicles, which can be released by Na+, would be easily sufficient to trigger exocytosis.…”

Ca2+-dependent and -independent release of neurotransmitters from PC12 cells: a role for protein kinase C activation?

“…Previous data on monensin (which has a high affinity for Na') were conflicting . Results oftransmitter release in isolated chromafftn cells were interpreted as stimulation of exocytosis (37). But in PC12 cells, redistribution and leakage of catecholamines was reported to be responsible for the monensininduced DA release (29).…”

“…In rat brain synaptosomes, the release of catecholamines induced by X537A has been attributed to the ionophore effect of the drug for amines (i .e ., redistribution of the transmitter from granules to the cytoplasm and the extracellular space), rather than to stimulation of exocytosis (12). The mechanism of the monensin-induced stimulation ofcatecholamine release from neurosecretory cells (whether transmitter redistribution [29] or stimulation of exocytosis [37]) is still debated.…”

“…X537A exchanges H+ for various monovalent and divalent cations, whereas monensin exchanges H+ only with monovalent cations, with marked preference for Na+ (12,28,29,37) . In rat brain synaptosomes, the release of catecholamines induced by X537A has been attributed to the ionophore effect of the drug for amines (i .e ., redistribution of the transmitter from granules to the cytoplasm and the extracellular space), rather than to stimulation of exocytosis (12).…”

Effect of sodium and calcium on basal secretory activity of rat neurohypophysial peptidergic nerve terminals.

“…[Na+]i (Suchard, Lattanzio, Rubin & Pressman, 1982), the release of y-aminobutyric acid (GABA) from brain slices (Cunningham & Neal, 1981) or the stimulation of insulin release from pancreatic ,-cells (Donatsch, Lowe, Richardson & Taylor, 1977). In all these reports the observed effects have been explained by the capacity of increased intracellular Na+ to release Ca2+ from internal stores.…”