Membrane transport in the cellular homeostasis of calcium

The messenger function of Ca2+ requires its maintenance at a very low concentration level within cells. In this way, significant changes in concentration can be achieved by mobilizing relatively minor amounts of the cation. The reversible complexation by specific membranous and nonmembranous binding proteins is the basic mechanism for the control of cellular Ca2+. Specific membrane proteins transport Ca between cells and the environment, and between cytoplasm and the space within organelles. Numerous Ca2+ transport systems have been described, but they correspond to only four transport modes: ATPases, (Na+/Ca2+) exchangers, channels, and electrophoretic uniporters. The transport systems interact with Ca2+ with either low or high affinity, thus playing different roles in the functional cycle of cells. From the standpoint of the division of labor among cell organelles, endo(sarco)plasmic reticulum can be considered as the structure presiding over the fine (high-affinity) and rapid tuning of cell Ca2+, and the mitochondria as long-term, low-affinity Ca2+ buffering structures. Their most important task may be the regulation of Ca2+ in their matrix, rather than in the cytosol.