Dual-wavelength ratiometric fluorescence measurements of membrane potential.

This work shows that the voltage across membranes in two very different preparations, lipid vesicles in suspension and individual HeLa cells under a microscope, is linearly related to the ratio of fluorescence excited from the two wings of the absorption spectrum of a voltage-sensitive dye. The dye di-4-ANEPPS [1-(3-sulfonatopropyl)-4-[beta-[2-(di-n-butylamino)-6-naphthyl] vin yl]pyridinium betaine] is well characterized from earlier investigations and responds via a rapid (less than millisecond) spectral shift to membrane potential changes. The resultant small change in fluorescence intensity monitored at a single wavelength is useful for measurements of temporally well-defined voltage transients such as action potentials. The dual-wavelength approach described in this work extends the usefulness of this fast potentiometric dye by filtering out complex or artifactual changes in fluorescence intensity and providing a voltage-dependent signal that is internally standardized. Thus, rapid measurements of membrane potential are made possible in nonexcitable cells.