title: Monensin and hypo-osmolar medium cause calcium-independent beta-endorphin secretion from melanotropes.
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.
authors.affiliation: Institute of Biomedicine, Department of Anatomy, University of Helsinki, Finland. nils.back@helsinki.fi
authors.forename: N
authors.initials: N
authors.lastname: Bäck
authors.forename: S
authors.initials: S
authors.lastname: Soinila
authors.forename: K
authors.initials: K
authors.lastname: Törnquist
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D003470:Culture Media / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D005260:Female; D007038:Hypotonic Solutions / Q000494:pharmacology*; D007476:Ionophores / Q000494:pharmacology*; D008297:Male; D008854:Microscopy, Electron; D008985:Monensin / Q000494:pharmacology*; D009994:Osmolar Concentration; D010903:Pituitary Gland, Anterior / Q000166:cytology* / Q000378:metabolism*; D011863:Radioimmunoassay; D051381:Rats; D017208:Rats, Wistar; D014882:Water-Electrolyte Balance / Q000187:drug effects / Q000502:physiology; D001615:beta-Endorphin / Q000378:metabolism*
chemical_list: D003470:Culture Media; D007038:Hypotonic Solutions; D007476:Ionophores; D001615:beta-Endorphin; D008985:Monensin; D002118:Calcium