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, but can be loaded into cells via microinjection or scrape loading. We also cell a membrane-permeant version, Fluo-4 AM Ester.    Kd: ~335 nM  &lamba;Ex/&lamba;Em (low or high [Ca2 ]) = 494nm/516nm  ε(494 nm) = 82,000 M-1cm-1  Orange solid soluble in DMSO and water (pH >6)  Store at 4°C. Protect from light, especially when in solution  C36H25F2K5N2O13  MW: 927.09