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The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger, NCX, or exchange protein) is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium (Na+) by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions (Ca2+). The NCX removes a single calcium ion in exchange for the import of three sodium ions. The exchanger exists in many different cell types and animal species. The NCX is considered one of the most important cellular mechanisms for removing Ca2+.〔 The exchanger is usually found in the plasma membranes and the mitochondria and endoplasmic reticulum of excitable cells. == Function == The Na+/Ca2+ exchanger does not bind very tightly to Ca2+ (has a low affinity), but it can transport the ions rapidly (has a high capacity), transporting up to five thousand Ca2+ ions per second. Therefore, it requires large concentrations of Ca2+ to be effective, but is useful for ridding the cell of large amounts of Ca2+ in a short time, as is needed in a neuron after an action potential. Thus, the exchanger also likely plays an important role in regaining the cell's normal calcium concentrations after an excitotoxic insult.〔 Another, more ubiquitous transmembrane pump that exports calcium from the cell is the plasma membrane Ca2+ ATPase (PMCA), which has a much higher affinity but a much lower capacity. Since the PMCA is capable of effectively binding to Ca2+ even when its concentrations are quite low, it is better suited to the task of maintaining the very low concentrations of calcium that are normally within a cell. Therefore the activities of the NCX and the PMCA complement each other. The exchanger is involved in a variety of cell functions including the following:〔 *control of neurosecretion *activity of photoreceptor cells *cardiac muscle relaxation *maintenance of Ca2+ concentration in the sarcoplasmic reticulum in cardiac cells *maintenance of Ca2+ concentration in the endoplasmic reticulum of both excitable and nonexcitable cells *excitation-contraction coupling *maintenance of low Ca2+ concentration in the mitochondria The exchanger is also implicated in the cardiac electrical conduction abnormality known as delayed afterdepolarization.〔Lilly, L: "Pathophysiology of Heart Disease", chapter 11: "Mechanisms of Cardiac Arrhythmias", Lippencott, Williams and Wilkens, 2007〕 It is thought that intracellular accumulation of Ca2+ causes the activation of the Na+/Ca2+ exchanger. The result is a brief influx of a net positive charge (remember 3 Na+ in, 1 Ca2+ out), thereby causing cellular depolarization.〔 This abnormal cellular depolarization can lead to a cardiac arrhythmia. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sodium-calcium exchanger」の詳細全文を読む スポンサード リンク
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