The development of Ca2+ indicators: a breakthrough in pharmacological research

The development, beginning in 1979, of fluorescent Ca2+-specific indicators as research tools has revolutionized transmembrane signaling studies. In this article, the state of the art in the 'pre-Ca2+-indicator' era and the rationale for the development of indicators trapped in the cytosol to investigate the cytosolic concentration of Ca2+ in mammalian cells are summarized. Subsequent extension of these studies to the level of the single cell, together with the unique impact that Ca2+ indicators have had on signaling research and the introduction of specific, fluorescent gene constructs that provide direct, high-resolution information about the intracellular concentration of Ca2+, are also discussed.