Novel role of calcium in exocytosis: mechanism of nematocyst discharge as shown by x-ray microanalysis.

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Abstract
Mature nematocysts of the sea anemones Rhodactis rhodostoma and Anthopleura elegantissima contain a fluid that has a high concentration of solutes and is extraordinarily rich in Ca (approximately 500-600 mmol/kg wet wt); this contrasts with the surrounding cytoplasm which is rich in K but poor in Ca. The undischarged capsule is surrounded by a membrane that probably acts as a selective permeability barrier between the cytoplasm and the nematocyst fluid. During discharge the nematocyst moves to the surface of the nematocyte and comes into contact with the external sea water medium. Ca, which may be bound to proteins in the undischarged state, is rapidly lost from the fluid; at the same time, sea water enters the capsule. In vitro experiments already have shown that Ca loss increases the osmotic pressure of the capsular fluid, causing an influx of water from the external medium; this influx appears to increase the hydrostatic pressure inside the capsule to the point that the thread everts explosively.