Despite a 10,000-fold gradient of Ca2+ across the cell membrane, the concentration of cytosolic free calcium, [Ca2+ ]i , is regulated with remarkable constancy. A combination of mechanisms precisely regulate [Ca2+ ]i at nanomolar levels. These include influx of Ca2+ via plasma membrane calcium channels, release and redistribution of Ca2+ from internal stores, and efflux of Ca2+ by the action of ATP-driven calcium pumps.
