TOXI-SIM-A simulation tool for the analysis of mitochondrial and plasma membrane potentials

Measurement of the Absolute Magnitude and Time Courses of Mitochondrial Membrane Potential in Primary and Clonal Pancreatic Beta-Cells

The aim of this study was to simplify, improve and validate quantitative measurement of the mitochondrial membrane potential (ΔψM) in pancreatic β-cells. This built on our previously introduced calculation of the absolute magnitude of ΔψM in intact cells, using time-lapse imaging of the non-quench mode fluorescence of tetramethylrhodamine methyl ester and a bis-oxonol plasma membrane potential (ΔψP) indicator. ΔψM is a central mediator of glucose-stimulated insulin secretion in pancreatic β-cells. ΔψM is at the crossroads of cellular energy production and demand, therefore precise assay of its magnitude is a valuable tool to study how these processes interplay in insulin secretion. Dispersed islet cell cultures allowed cell type-specific, single-cell observations of cell-to-cell heterogeneity of ΔψM and ΔψP. Glucose addition caused hyperpolarization of ΔψM and depolarization of ΔψP. The hyperpolarization was a monophasic step increase, even in cells where the ΔψP depolarization was biphasic. The biphasic response of ΔψP was associated with a larger hyperpolarization of ΔψM than the monophasic response. Analysis of the relationships between ΔψP and ΔψM revealed that primary dispersed β-cells responded to glucose heterogeneously, driven by variable activation of energy metabolism. Sensitivity analysis of the calibration was consistent with β-cells having substantial cell-to-cell variations in amounts of mitochondria, and this was predicted not to impair the accuracy of determinations of relative changes in ΔψM and ΔψP. Finally, we demonstrate a significant problem with using an alternative ΔψM probe, rhodamine 123. In glucose-stimulated and oligomycin-inhibited β-cells the principles of the rhodamine 123 assay were breached, resulting in misleading conclusions.

Mitochondrial membrane potential probes and the proton gradient: a practical usage guide

Fluorescent probes for monitoring mitochondrial membrane potential are frequently used for assessing mitochondrial function, particularly in the context of cell fate determination in biological and biomedical research. However, valid interpretation of results obtained with such probes requires careful consideration of numerous controls, as well as possible effects of non-protonic charges on dye behavior. In this context, we provide an overview of some of the important technical considerations, controls, and parallel complementary assays that can be employed to help ensure appropriate interpretation of results, thus providing a practical usage guide for monitoring mitochondrial membrane potentials with cationic probes. In total, this review will help illustrate both the strengths and potential pitfalls of common mitochondrial membrane potential dyes, and highlight best-usage approaches for their efficacious application in life sciences research.

Quantifying mitochondrial and plasma membrane potentials in intact pulmonary arterial endothelial cells based on extracellular disposition of rhodamine dyes

Our goal was to quantify mitochondrial and plasma potential (Δψ(m) and Δψ(p)) based on the disposition of rhodamine 123 (R123) or tetramethylrhodamine ethyl ester (TMRE) in the medium surrounding pulmonary endothelial cells. Dyes were added to the medium, and their concentrations in extracellular medium ([R(e)]) were measured over time. R123 [R(e)] fell from 10 nM to 6.6 ± 0.1 (SE) nM over 120 min. TMRE [R(e)] fell from 20 nM to a steady state of 4.9 ± 0.4 nM after ∼30 min. Protonophore or high K(+) concentration ([K(+)]), used to manipulate contributions of membrane potentials, attenuated decreases in [R(e)], and P-glycoprotein (Pgp) inhibition had the opposite effect, demonstrating the qualitative impact of these processes on [R(e)]. A kinetic model incorporating a modified Goldman-Hodgkin-Katz model was fit to [R(e)] vs. time data for R123 and TMRE, respectively, under various conditions to obtain (means ± 95% confidence intervals) Δψ(m) (-130 ± 7 and -133 ± 4 mV), Δψ(p) (-36 ± 4 and -49 ± 4 mV), and a Pgp activity parameter (K(Pgp), 25 ± 5 and 51 ± 11 μl/min). The higher membrane permeability of TMRE also allowed application of steady-state analysis to obtain Δψ(m) (-124 ± 6 mV). The consistency of kinetic parameter values obtained from R123 and TMRE data demonstrates the utility of this experimental and theoretical approach for quantifying intact cell Δψ(m) and Δψ(p.) Finally, steady-state analysis revealed that although room air- and hyperoxia-exposed (95% O(2) for 48 h) cells have equivalent resting Δψ(m), hyperoxic cell Δψ(m) was more sensitive to depolarization with protonophore, consistent with previous observations of pulmonary endothelial hyperoxia-induced mitochondrial dysfunction.

Quantitative analysis of membrane potentials

The changes that occur in electrochemical gradients across biological membranes provide us with invaluable information on physiological responses, pathophysiological processes and drug actions/toxicity. This chapter aims to provide researchers with sufficient information to carry out a quantitative assessment of mitochondrial energetics at a single-cell level thereby providing output on changes in the mitochondrial membrane potential (Deltapsi(m)) through the utilization of potentiometric fluorescent probes (TMRM, TMRE, Rhodamine 123). As these cationic probes behave in a Nernstian fashion, changes at the plasma membrane potential (Deltapsi(p)) need also to be accounted for in order to validate the responses obtained with Deltapsi(m)-sensitive fluorescent probes. To this end techniques that utilize Deltapsi(p)-sensitive anionic fluorescent probes to monitor changes in the plasma membrane potential will also be discussed. In many biological systems multiple changes occur at both a Deltapsi(m) and Deltapsi(p) level that often makes the interpretation of the cationic fluorescent responses much more difficult. This problem has driven the development of computational modelling techniques that utilize the redistribution properties of the cationic and anionic fluorescent probes within the cell to provide output on changes in Deltapsi(m) and Deltapsi(p).