The Calcium Entry Pas de Deux

摘要

Calcium ions (Ca~(2+)) are universal secondary messengers that are key players in many cellular signal transduc-tion pathways. There are two sources of these signaling cations in the cell: internal stores that release Ca~(2+), and channels in the plasma membrane that open to allow external Ca~(2+) to flow into the cell. Internal Ca~(2+) stores—located in the sarcoplasmic reticu-lum of muscle cells and the endoplasmic reticulum (ER) of other cells—have a limited capacity for Ca~(2+) storage, and so they have to be replenished through entry of Ca~(2+) from the external environment. Putney first recognized that the processes of emptying and replenishing internal Ca~(2+) stores must be linked. Somehow, empty Ca~(2+) stores activate store-operated channels (SOCs) in the plasma membrane that then allow Ca~(2+) ions to enter the cell. Putney termed this mechanism "capacitative calcium entry" (CCE) because the stores behave like a capacitor in an electrical circuit. When Ca~(2+) stores are replete the SOCs are closed, but once the stores discharge their contents, the SOCs open and Ca~(2+) ions enter the cell.