Measurement of Intracellular Calcium with Fluorescent Calcium Indicators

Changes in cytosolic free calcium ion concentration ([Ca²⁺ ]_i ) accompany many cellular transitions and stimulation of cell surface receptors. Both release of Ca²⁺ from internal stores and Ca²⁺ entry through plasma membrane ion channels can contribute to cellular Ca²⁺ transients. In the past, direct measurement of [Ca²⁺ ]_i was limited mainly to cells that could be penetrated with Ca²⁺ electrodes or injected with membrane impermeant indicators, such as aequorin. Alternatives, such as ⁴⁵ Ca²⁺ flux measurements, reduction of extracellular Ca²⁺ , or artificial elevation of intracellular Ca²⁺ using ionophores, provided indirect evidence for the role of Ca²⁺ in particular cell processes. However, the clues provided by indirect approaches are not always reliable. Inward and outward Ca²⁺ fluxes can be triggered by depolarizing sea urchin eggs without measurable changes in [Ca²⁺ ]_i (M. Poenie, unpublished) or the activity of Ca²⁺ -dependent enzymes (Schmidt et a1.,1982 ). Although cells can transduce a change in membrane potential or [Ca²⁺ ]_i into a meaningful cell signal, the same cannot can be said for a Ca²⁺ current (Jaffe, 1986 )