?Fura-2 AM is a noninvasive, intracellular calcium indicator that is UV light-excitable. This AM ester form crosses cell membranes and once inside the cell is converted by cellular esterases to fura-2, the pentacarboxylate calcium indicator. The spectral properties of fura-2 in the presence of low calcium concentrations (excitation max = 362 nm, emission max = 512 nm) differ from those during high calcium conditions (excitation max = 335 nm, emission max = 505 nm). Measurement of Ca2+-induced fluorescence at excitation wavelengths of both 340 and 380 nm with a constant emission wavelength of 505 nm allows for calculation of calcium concentrations based on a ratio of the 340/380 values. Employing this ratio method cancels out certain variables that could be considered artifacts when attempting to image calcium concentrations in cells.
