The sodium-calcium exchanger is an antiporter in the plasma membrane of a cell that transports sodium ions into the cell and calcium ions out of the cell by secondary active transport. It uses the downward Na+ gradient into the cell created by sodium pumps using ATP to power the uphill transport of Ca2+ out of the cell. It is effective at high Ca2+ concentrations.
The sodium-calcium exchanger is important in the regulation of calcium ions in a cell. Cytosolic calcium concentration is kept low because calcium ions can act as a trigger for many processes, such as muscle contraction and cell signalling pathways.