Assessment of membrane potential using confocal microspectrofluorimetry.

Some slow potentiometric dyes, e.g., 3,3'-dipropylthiacarbocyanine and tetramethylrhodamine methyl ester, exhibit fluorescence spectral changes on redistributing from aqueous medium to cells. This effect has been used for spectroscopic discrimination of the emissions from free and bound dyes in cells. Such a discrimination can, in principle, allow for the assessment of cell membrane potential in individual cells, using the Nernst equation applied to the ratio of free dye fluorescence intensities inside and outside of the cell.