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Monensin and hypo-osmolar medium cause calcium-independent beta-endorphin secretion from melanotropes
N Bäck1, S Soinila, K Törnquist
1Institute of Biomedicine, Department of Anatomy, University of Helsinki, Finland. nils.back@helsinki.fi
Abstract:
Monensin has been shown to cause nonexocytotic release of catecholamines from adrenal medullary and PC12 cells. We examined the effect of monensin on peptide secretion with cultured melanotropes from the rat pituitary as a model. 1 microM monensin caused an immediate, transient increase in beta-endorphin secretion. The effect was still seen in a calcium-free medium, but was totally abolished in a sodium-free medium. Intracellular calcium concentration was measured with Fura 2: no increase was observed during monensin stimulation. Hypo-osmolar medium mimicked the effect of monensin, causing a 12-fold transient increase in beta-endorphin secretion. This effect was not abolished in either calcium-free or sodium-free medium. No increase in the number of exocytotic figures captured by tannic acid incubation was observed during 5 min of incubation with 1 microM monensin or hypo-somolar medium. We thus show that monensin causes beta-endorphin secretion from the melanotrope and that this effect is due to sodium influx and resultant cell swelling. The calcium independency and lack of increase of exocytotic figures suggest that swelling-induced secretion is nonexocytotic, possibly via transient exocytotic pore opening.
Insights
Monensin triggers beta-endorphin release from pituitary cells via sodium influx and cell swelling, not calcium or exocytosis. This nonexocytotic pathway offers new insights into peptide secretion mechanisms.
Area of Science:
- Neuroendocrinology
- Cell Biology
- Pharmacology
Background:
- Monensin is known to induce nonexocytotic catecholamine release from adrenal cells.
- The mechanisms of peptide secretion from pituitary melanotropes are not fully understood.
Purpose of the Study:
- To investigate the effect of monensin on beta-endorphin secretion from cultured rat pituitary melanotropes.
- To elucidate the ionic requirements and cellular mechanisms underlying monensin-induced peptide release.
Main Methods:
- Primary rat melanotrope cell cultures were used.
- Beta-endorphin secretion was measured following monensin or hypo-osmolar stimulation.
- Intracellular calcium levels were monitored using Fura 2.
- Cellular responses were assessed in various ionic and osmotic conditions.
- Electron microscopy was employed to evaluate exocytotic figures.
Main Results:
- Monensin (1 microM) induced a rapid, transient increase in beta-endorphin secretion.
- This effect was dependent on sodium influx but independent of extracellular calcium.
- Hypo-osmolar medium mimicked monensin's effect, causing significant beta-endorphin release.
- Neither monensin nor hypo-osmolar conditions increased the number of exocytotic figures.
- Monensin stimulation did not elevate intracellular calcium concentrations.
Conclusions:
- Monensin stimulates beta-endorphin secretion from melanotropes through a nonexocytotic mechanism.
- The process involves sodium influx and subsequent cell swelling.
- Swelling-induced secretion may occur via transient pore opening, independent of classical exocytosis and calcium signaling.