Membrane potential estimation by flow cytometry

Membrane potential (delta psi) is generated and maintained by concentration gradients of ions such as sodium, potassium, chloride, and hydrogen. Changes in cytoplasmic delta psi in the course of surface-receptor-mediated processes related to the development, function, and pathology of many cell types often play a role in transmembrane signaling. Cytoplasmic delta psi is also reduced to zero when the membrane is ruptured by chemical or physical agents. Mitochondrial delta psi is reduced when energy metabolism is disrupted, notably in apoptosis. In bacteria, which lack mitochondria, delta psi reflects both the state of energy metabolism and the physical integrity of the cytoplasmic membrane. Flow cytometry can be used to estimate membrane potential in eukaryotic cells, mitochondria in situ, isolated mitochondria, and bacteria. Older methods, using lipophilic cationic dyes such as the cyanines and rhodamine 123 or lipophilic anionic dyes such as the oxonols can detect relatively large changes in delta psi and identify heterogeneity of response in subpopulations comprising substantial fractions of a cell population. Newer ratiometric techniques allow precise measurement of delta psi to within 10 mV or less. Among other factors, action of efflux pumps, changes in membrane structure, and changes in protein or lipid concentration in the medium in which cells are suspended can produce changes in cellular fluorescence which may be misinterpreted as changes in delta psi. Techniques for estimation and measurement of Delta Psi therefore typically require careful control of cell and reagent concentrations and incubation times and selection of appropriate controls if they are to provide accurate information.