after the cytosol reaches equilibrium allows the determination of R
max
, because
the 340-nm excitation signal is at its highest and the 380-nm excitation signal is
at its lowest. Subsequently, a calcium chelator (e.g., ethylene glycol tetraacetic
acid) is added to drop the intracellular [Ca
þ2
], and, at equilibrium, images are
acquired to measure R
min
. The value of K
D
is fluorophore specific and can be
obtained from the manufacturer. Ratio imaging can be performed using wide-
field, confocal, or two-photon microscopy, with the most critical requirement
being the simultaneous or near-simultaneous acquisition of two images at
different excitation or emission wavelengths [68].
12.7.5 Fluorescence Resonance Energy
Transfer (FRET)
Fluorescence resonance energy ...