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.
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