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 (Δψm and Δψp) 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 ([Re]) were measured over time. R123 [Re] fell from 10 nM to 6.6 ± 0.1 (SE) nM over 120 min. TMRE [Re] 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 [Re], and P-glycoprotein (Pgp) inhibition had the opposite effect, demonstrating the qualitative impact of these processes on [Re]. A kinetic model incorporating a modified Goldman-Hodgkin-Katz model was fit to [Re] vs. time data for R123 and TMRE, respectively, under various conditions to obtain (means ± 95% confidence intervals) Δψm (−130 ± 7 and −133 ± 4 mV), Δψp (−36 ± 4 and −49 ± 4 mV), and a Pgp activity parameter (KPgp, 25 ± 5 and 51 ± 11 μl/min). The higher membrane permeability of TMRE also allowed application of steady-state analysis to obtain Δψm (−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 Δψm and Δψp. Finally, steady-state analysis revealed that although room air- and hyperoxia-exposed (95% O2 for 48 h) cells have equivalent resting Δψm, hyperoxic cell Δψm was more sensitive to depolarization with protonophore, consistent with previous observations of pulmonary endothelial hyperoxia-induced mitochondrial dysfunction