Fluo-4, pentapotassium is a membrane-impermeant green-fluorescent calcium indicator that can be loaded into cells via microinjection or scrape loading. It is an improved analog of Fluo-3 with the two chlorine substituents replaced by fluorines, resulting in increased fluorescence excitation at 488 nm and higher fluorescence signal. Fluo-4 is essentially nonfluorescent without Ca2+ but upon Ca2+ binding, its fluorescence increases at least 100 times. Fluo-4 has a higher Kd than fura-2 and indo-1, it is more useful for measuring high transient Ca2+ concentration during Ca2+ spikes. We also offer a membrane-permeant version, Fluo-4 AM Ester.Features: Kd: ~335 nM; Ex/Em nm (low or high [Ca2+]) = 494/516 nm, excitable by the argon-ion laser; Ext. coeff. (494 nm) = 82,000 M-1 cm-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