| Calcium ion (Ca2+) is a ubiquitous messenger and has an important role in the transduction of signals leading to cellular responses such as cell motility changes, egg fertilization, neurotransmission and protein secretion, as well as cell fusion, differentiation and proliferation.
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| Cells expend a considerable amount of energy maintaining a steep extracellular/intracellular Ca2+ concentration gradient: for example, the intracellular Ca2+ concentration in resting, unstimulated cells is of the order of 10-7 mol/L (100 nM), whereas the extracellular Ca2+ concentration is typically 10-3mol/L (1 mM). This steep gradient allows for rapid, abrupt, transient changes in Ca2+ concentration: ligation of a wide range of hormone receptors, for example, leads to a rapid (within seconds) and transient increase in intracellular Ca2+ concentration to the micromolar range. The rapid raising and lowering of Ca2+ concentrations is very tightly regulated and utilizes a variety of mechanisms involving cell compartmentalization. For example, intracellular Ca2+ concentrations can be lowered by sequestration of Ca2+ into the endoplasmic reticulum by Ca2+-ATPases, or into the mitochondria using the energy-driven electrochemical gradient. Alternatively, free Ca2+ can be chelated by Ca2+-binding proteins such as calsequestrin.
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