Calcium and Cell Function

The crucial role of Ca²⁺ in the regulation of cell functions has been appreciated for a long time. In eukaryotic cells, the role of Ca²⁺ as a second messenger is as important as the more widely known messenger role of cAMP. Depending on the cell and on the stimulus, cytosolic Ca2+ concentration may increase, either as consequence of the inflow of extracellular Ca²⁺ or because Ca²⁺ is released from intracellular stores. Calmodulin has been found in so many tissues of so many different organisms as to make it very likely that it is a component of all eukaryotic cells. The electroplax of electric eel, mammalian brain, and mammalian testis have a particulary high content of calmodulin. Present experimental evidence seems to favor the idea that in most cells the main intracellular store of messenger Ca²⁺ is the endoplasmic reticulum rather than the mitochondria.