Calcium loading of secretory granules in stimulated neurohypophysical nerve endings

• Published in: Neuroscience Vol. 64; no. 1; pp. 125 - 137
• Main Authors: Thirion, S., Stuenkel, E.L., Nicaise, G.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1995
• Subjects: Ca g; EGTA; ethyleneglycoltetra-acetate; HEPES; intragranular total calcium concentration; N-2-hydrpxyethylpiperazine- N′-2-ethanesulfonic acid; HEPES; Ca g; EGTA
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/0306-4522(94)00414-Z

The total calcium content of secretory granules, Ca g, was evaluated in isolated neurohypophysical nerve endings. The Ca g 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 ptassium challenge, a subpopulation of granules with even higher Ca g could be detected, dispersed over a wider range of concentrations (up to 70 mmol/kg wet weight). After subsequent rinsing in physiological saline, Ca g decreased to control values. This could have resulted from Ca 2+ extrusion, or from preferential secretion of calcium-enriched granules. Our data can be interpreted in favor of the second explanation since no decrease in Ca g was observed when secretion was blocked by a hyperosmostic saline. The effect of hyperosmotic conditions on isolated nerve endings was further studied by monitoring free cytoplasmic Ca 2+ with the calcium-sensitive dye Fura-2 and by conventional electron microscopy. It was demonstrated that hyperosmotic treatment alone did not increase basal cytosolic Ca 2= concentrations but did significantly reduce the potassium-induced cytosolic rise in Ca 2+. Electron microscopy of nerve endings in hyperosmotic conditions showed numerous exocytotic figures at various stages. The observed changes in Ca g are in accord with a published hypothesis which proposes that intragranular calcium is a significant variable in regulated secretion.