Measurement of Intracellular Calcium with Fluorescent Calcium Indicators
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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outward Ca2+ fluxes can be triggered by depolarizing sea urchin eggs without measurable changes in [Ca2+ ]i (M. Poenie, unpublished) or the activity of Ca2+ -dependent enzymes (Schmidt et a1.,1982 ).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 )