Assessment of mitochondrial membrane potential in proximal tubules after hypoxia-reoxygenation

Proximal tubules develop a severe energetic deficit during hypoxia-reoxygenation (H/R) that previous studies using fluorescent potentiometric probes have suggested is characterized by sustained, partial mitochondrial deenergization. To validate the primary occurrence of mitochondrial deenergization in the process, optimize approaches for estimating changes in mitochondrial membrane potential (Delta psi(m)) in the system, and clarify the mechanisms for the defect, we further investigated the behavior of 5,5', 6,6'-tetrachloro-1,1', 3,3'-tetraethylbenzimidazocarbocyanine iodide (JC-1) in these cells and introduce a more dynamic and quantitative approach employing safranin O for use with the tubule system. Although use of JC-1 can be complicated by decreases in the plasma membrane potential that limit cellular uptake of JC-1 and such behavior was demonstrated in ouabain-treated tubules, changes in Delta psi(m) entirely accounted for the decreases in the formation of red fluorescent JC-1 aggregates and in the ratio of red/green fluorescence observed after H/R. The red JC-1 aggregates did not readily dissociate when tubules were deenergized after JC-1 uptake, making it unsuitable for dynamic studies of energization. Safranin O uptake by digitonin-permeabilized tubules required very small numbers of tubules, permitted measurements of Delta psi(m) for relatively prolonged periods after the end of the experimental maneuvers, was rapidly reversible during deenergization, and allowed for direct assessment of both substrate-dependent, electron transport-mediated Delta psi(m), and ATP hydrolysis-supported Delta psi(m). Both types of energization measured using safranin O in tubules permeabilized after H/R were impaired, but combining substrates and ATP substantially restored Delta psi(m).