Where is intracellular calcium stored?

Intracellular calcium is stored in organelles which repetitively release and then reaccumulate Ca2+ ions in response to specific cellular events. The storage sites include mitochondria and the endoplasmic reticulum. The endoplasmic reticulum (ER) is the largest store of releasable calcium in the cell. Many cellular functions including contraction, protein synthesis, secretion, and reproduction, depend on the release of the right amount of calcium at a specific time and place. Intracellular calcium in the reticulum is repeatedly released and re-accumulated in response to specific cellular events. The amount of intracellular calcium that is released to perform its required function relies on changes in the concentration of sodium between the cell’s outer membrane and the intracellular calcium stores. The smallest change in sodium concentration can produce significant changes in calcium stores in the endoplasmic reticulum.