Prolonged Hormone Secretion from Neuroendocrine Cells of Aplysia is Indepentdent of Extracellular Calcium

The role of Ca 2+ from extracellular and intracellular sources in stimulating neurosecretion was investigated in four experiments using neuroendocrine bag cells of the marine mollusk Aplysia . (i) Bag cells were treated with either an extracellular calcium chelator (BAPTA) or Co 2+ ‐substitution within 30 s after onset of an electrical afterdischarge to prevent influx of Ca 2+ from extracellular fluid. These treatments shortened the duration of the afterdischarge, but did not significantly affect the overall pattern or total amount of egg laying hormone (ELH) secretion, suggesting that extracellular Ca 2+ is not required for maintenance of ELH release. (ii) Substitution of Ba 2+ for Ca 2+ has previously been shown to support bag cell afterdischarges that trigger transient elevations in intracellular Ca 2+ . We showed that this treatment also stimulates ELH secretion, suggesting that Ca 2+ release from intracellular stores can stimulate ELH secretion. (iii) To raise intracellular Ca 2+ levels in the absence of an afterdischarge, the calcium ionophore X537A was used to transport Ca 2+ across plasma and organelle membranes. When this treatment was combined with extracellular calcium chelators so that the only source of Ca 2+ was from intracellular compartments, ELH secretion was stimulated. Taken together, these findings are consistent with the hypothesis that release of Ca 2+ from intracellular stores is sufficient to stimulate ELH secretion.