A calcium pump is a transmembrane ATPase that pumps calcium (Ca2+) ions out of the cytosol, the aqueous fluid that fills the cytoplasm in the cell, into either the extracellular space or a calcium-sequestering organelle. In the plasma membrane of animal cells, the pump maintains a very low cytosolic concentration of Ca2+ ions, while the extracellular concentrations are very high. This steep gradient makes Ca2+ ions useful for many types of cell signaling. In addition, muscle cells have a Ca2+ ATPase in the membrane of an organelle called the sarcoplasmic reticulum, whose function is to regulate Ca2+ concentrations and activate muscle contraction. This Ca2+ ATPase pumps Ca2+ ions from the cytosol into the sarcoplasmic reticulum, where they are sequestered. Its operation is similar to that of the Na+/K+ pump, involving conformational changes associated with phosphorylation and dephosphorylation. It has two binding sites for Ca2+ ions, which it transports across the sarcoplasmic reticulum membrane with each ATPase cycle.
Ca2+ ATPase makes up 90% of the membrane protein of the sarcoplasmic reticulum. This membrane also contains Ca2+ release channels, which are opened when the muscle cell receives an electrical nerve impulse, or action potential. The sudden flow of Ca2+ ions into the cytosol causes a dramatic rise in cytosolic Ca2+ concentration. This is the initial event that triggers muscle contraction. The concentration is rapidly restored to its normal levels by the Ca2+ ATPase, pumping the Ca2+ back into the sarcoplasmic reticulum and allowing the muscle fibers to relax.