Measurements of membrane potentials using the dye safranine

A metachromatic shift in the spectrum of the cationic dye safranine occurs upon induction of electrical potentials across the mitochondrial membrane by adding respiratory substrate, ATP or a cation conductor valinomycin (when a potassium gradient exists across the membrane) to a mitochondrial suspension. The extent of spectral change correlates linearily to the membrane potential. During the spectral change safranine is taken up by the mitochondria and most of the dye can be recovered in the pellet after centrifugation. By measuring the spectral changes a fairly good estimate of membrane potentials in mitochondria and bacteria is obtained. This method is compared to other optical methods of studying membrane potentials and is possible applications in different systems in the future is discussed.