Cytosolic Free Calcium Measurements in Single Cells Using Calcium-Sensitive Fluorochromes

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.