The plasma membrane Ca2+ ATPase and the sodium-calcium exchanger are together the main regulators of cytoplasmic Ca2+ concentrations.
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.
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.
[3] The structure of calcium pumps found in the sarcoplasmic reticulum of skeletal muscle was elucidated in 2000 by Toyoshima, et al. using microscopy of tubular crystals and 3D microcrystals.
[4] Classical theory of active transport for P-type ATPases [5] Data from crystallography studies by Chikashi Toyoshima applied to the above cycle [6][7]