Methodological aspects of measuring absolute values of membrane potential in human cells by flow cytometry

Abstract

The bis-barbituric acid oxonol, DiBAC 4 (3) is used as a standard potentiometric probe in human cells. However, its fluorescence depends not only on membrane potential but also varies with nonpotential related changes in the amount of intracellular free and bound dye. This study demonstrates the influence of different experimental conditions on this nonspecific fluorescence proportion. IGR1 melanoma cells as a model were specifically altered in cell volume and protein content by depolarizing treatments or cell cycle synchronization. Flow cytometry was performed over a wide range of extracellular DiBAC 4 (3) concentrations. Fixation and increase in protein content led to a nonspecifically enhanced fluorescence, while changes in the amount of free intracellular dye as a result of altered cell volume proved to be negligible. To establish a calibration curve using totally depolarized cells, the pore-forming action of gramicidin should be preferred to fixation. Below 100 nM DiBAC 4 (3), the logarithmic relation between cell fluorescence and dye concentration turned into a virtually linear function intersecting with zero. Consequently, calibration can then be confined to determination of the fluorescence of depolarized cells stained with the same concentration as used for the actual measurement of membrane potential. Unexpectedly, quenching of fluorescence occurred in totally depolarized cells at concentrations higher than 6,250 nM. Linearity and quenching could be confirmed by additional experiments on Chinese hamster ovary CHO-K1 and B lymphoblastoid LCL-HO cells. ' International Society for Advancement of CytometryKey terms bis-barbituric acid oxonol; flow cytometry; membrane potential; mV-scale calibration PATCH-clamp recording is considered to be the reference method to quantitatively measure the plasma membrane potential V p in human cells. A disadvantage in respect of routine use is that this method is time-consuming and laborious thus usually resulting in measuring values for only a small number of cells (1,2). Therefore, voltage-sensitive anionic and cationic dyes, which partition between the extracellular staining buffer and the cytoplasm under the influence of V p according to a Nernstian distribution, have been applied as an alternative (3-12).Due to their strong accumulation in mitochondria (13,14), cationic dyes report V p alone only when the mitochondrial membrane potential V m remains constant during the measurement. To ensure this condition, V m was specifically abolished by ionophores in some studies (15)(16)(17). With increasing concentration, the monomer form of cationic dyes, among them carbocyanines as the most popular dye class, is usually more and more displaced by dimers or higher aggregates. This process takes place both with dye dissolved in aqueous solution or bound to cell compartments, and is accompanied by distinct spectral shifts in absorbance and fluorescence maxima leading to quenching of the fluorescence of the monomers (18-21). For example, DiSC 3 (5) monomers show red flu...

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