Calcium imaging

Calcium imaging is a scientific technique usually carried out in research which is designed to show the calcium (Ca²⁺) status of a cell, tissue or medium. Calcium imaging techniques take advantage of so-called calcium indicators, fluorescent molecules that can respond to the binding of Ca²⁺ ions by changing their fluorescence properties. Two main classes of calcium indicators exist: chemical indicators and genetically encoded calcium indicators (GECI). Calcium imaging can be used to optically probe intracellular calcium in living animals. This technique has allowed studies of calcium signalling in a wide variety of cell types and neuronal activity in hundreds of neurons and glial cells within neuronal circuits .

Chemical indicators are small molecules that can chelate calcium ions. All these molecules are based on an EGTA homologue called BAPTA, with high selectivity for calcium (Ca²⁺) ions versus magnesium (Mg²⁺) ions.

This group of indicators includes fura-2, indo-1, fluo-3, fluo-4, Calcium Green-1.

These dyes are often used with the chelator carboxyl groups masked as acetoxymethyl esters, in order to render the molecule lipophilic and to allow easy entrance into the cell. Once this form of the indicator is in the cell, cellular esterases will free the carboxyl groups and the indicator will be able to bind calcium. The free acid form of the dyes (i.e. without the acetoxymethyl ester modification) can also be directly injected into cells via a microelectrode or micropipette which removes uncertainties as to the cellular compartment holding the dye (the acetoxymethyl ester can also enter the endoplasmic reticulum and ). Binding of a Ca²⁺ ion to a fluorescent indicator molecule leads to either an increase in quantum yield of fluorescence or emission/excitation wavelength shift. Individual chemical Ca²⁺ fluorescent indicators are utilized for cytosolic calcium measurements in a wide variety of cellular preparations. The first real time (video rate) Ca²⁺ imaging was carried out in 1986 in cardiac cells using intensified video cameras. Later development of the technique using laser scanning confocal microscopes revealed sub-cellular Ca²⁺ signals in the form of Ca²⁺ sparks and Ca²⁺ blips. Relative responses from a combination of chemical Ca²⁺ fluorescent indicators were also used to quantify calcium transients in intracellular organelles such as .

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