Fluo-4, Pentapotassium Salt Fluorescent Calcium Indicator

To study the roles of calcium, a key intracellular messenger in cells, it is essential to quantitatively monitor its concentration. The most widely used method of Ca2+ detection is the use of fluorescent Ca2+ indicators. Fluo-4 is an analog of Fluo-3 with the two chlorine substituents replaced by fluorines, which results in increased fluorescence excitation at 488 nm that gives higher fluorescence signal. Fluo-4 has its absorption maximum at 494 nm, thus making it excitable by the argon-ion laser. Fluo-4 is essentially nonfluorescent without Ca2+ present, but the fluorescence increases at least 100 times on Ca2+ binding. Also, because Fluo-4 binds Ca2+ more weakly (higher Kd) than do fura-2 and indo-1, it is more useful for measuring high transient Ca2+ concentration during Ca2+ spikes. Fluo-4, pentapotassium is membrane-impermeant calcium indicator. It can be loaded into cells via microinjection or scrape loading.

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