title: Calcium loading of secretory granules in stimulated neurohypophysial nerve endings.
abstract: The total calcium content of secretory granules, Cag, was evaluated in isolated neurohypophysial nerve endings. The Cag in the resting state, as measured by X-ray microanalysis, is relatively high with an average of 7.4 +/- 0.6 mmol/kg wet weight. Following a depolarizing potassium challenge, a subpopulation of granules with even higher Cag could be detected, dispersed over a wider range of concentrations (up to 70 mmol/kg wet weight). After subsequent rinsing in physiological saline, Cag decreased to control values. This could have resulted from Ca2+ extrusion, or from preferential secretion of calcium-enriched granules. Our data can be interpreted in favor of the second explanation since no decrease in Cag was observed when secretion was blocked by a hyperosmotic saline. The effect of hyperosmotic conditions on isolated nerve endings was further studied by monitoring free cytoplasmic Ca2+ with the calcium-sensitive dye Fura-2 and by conventional electron microscopy. It was demonstrated that hyperosmotic treatment alone did not increase basal cytosolic Ca2+ concentrations but did significantly reduce the potassium-induced cytosolic rise in Ca2+. Electron microscopy of nerve endings in hyperosmotic conditions showed numerous exocytotic figures at various stages. The observed changes in Cag are in accord with a published hypothesis which proposes that intragranular calcium is a significant variable in regulated secretion.
authors.affiliation: Laboratoire de Physiologie Cellulaire et Comparée, Université de Nice-Sophia Antipolis, France.
authors.forename: S
authors.initials: S
authors.lastname: Thirion
authors.forename: E L
authors.initials: EL
authors.lastname: Stuenkel
authors.forename: G
authors.initials: G
authors.lastname: Nicaise
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003594:Cytoplasmic Granules / Q000378:metabolism*; D006982:Hypertonic Solutions; D007552:Isotonic Solutions; D009411:Nerve Endings / Q000502:physiology*; D010904:Pituitary Gland, Posterior / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013194:Staining and Labeling
chemical_list: D006982:Hypertonic Solutions; D007552:Isotonic Solutions; D011188:Potassium; D002118:Calcium