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Calcium pump
There is a very large transmembrane electrochemical gradient of Ca2+ driving the entry of the ion into cells, yet it is very important for cells to maintain low concentrations of Ca2+ for proper cell signalling; thus it is necessary for the cell to employ ion pumps to remove the Ca2+.
The PMCA and the sodium calcium exchanger (NCX) are together the main regulators of intracellular Ca2+ concentrations.
== Why cells need calcium pumps ==
Ca2+ has many important roles as an intracellular messenger. The release of a large amount of free Ca2+ can trigger a fertilized egg to develop, skeletal muscle cells to contract, secretion by secretory cells and interactions with Ca2+ -responsive proteins like calmodulin. To maintain low concentrations of free Ca2+ in the cytosol, cells use membrane pumps like calcium ATPase found in the membranes of sarcoplasmic reticulum of skeletal muscle. These pumps are needed to provide the steep electrochemical gradient that allows Ca2+ to rush into the cytosol when a stimulus signal opens the Ca2+ channels in the membrane. The pumps are also necessary to actively pump the Ca2+ back out of the cytoplasm and return the cell to its pre-signal state.〔Bruce Alberts, ''Essential Cell Biology", third edition,New York, Garland Science, Taylor & Francis Group,LLC, 2010 ISBN 978-0-8153-4129-1, pages 552-553〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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