Calcium dynamics in the secretory granules of neuroendocrine cells

Cellular Ca(2+)signaling results from a complex interplay among a variety of Ca(2+) fluxes going across the plasma membrane and across the membranes of several organelles, together with the buffering effect of large numbers of Ca(2+)-binding sites distributed along the cell architecture. Endoplasmic and sarcoplasmic reticulum, mitochondria and even nucleus have all been involved in cellular Ca(2+) signaling, and the mechanisms for Ca(2+) uptake and release from these organelles are well known. In neuroendocrine cells, the secretory granules also constitute a very important Ca(2+)-storing organelle, and the possible role of the stored Ca(2+) as a trigger for secretion has attracted considerable attention. However, this possibility is frequently overlooked, and the main reason for that is that there is still considerable uncertainty on the main questions related with granular Ca(2+) dynamics, e.g., the free granular [Ca(2+)], the physical state of the stored Ca(2+) or the mechanisms for Ca(2+) accumulation and release from the granules. This review will give a critical overview of the present state of knowledge and the main conflicting points on secretory granule Ca(2+) homeostasis in neuroendocrine cells.