Membrane-potential-dependent changes of the lipid microviscosity of mitochondria and phospholipid vesicles.

The effects of a transmembrane potential difference upon the lipid microviscosity of cytochrome oxidase vesicles (COVs) and rat liver mitochondria (RLM) were investigated. COVs and RLM were labelled with the fluorescent probe 1,6-diphenylhexa-1,3,5-triene (DPH). The fluorescence polarization of the probe was then measured when potentials of different magnitudes were induced across the membranes of these particles. It was shown that the absolute value of the microviscosity changes to quite a significant extent, owing to the imposition of large membrane potentials. On relaxation of the membrane potential the lipid microviscosity was also shown to return to the value before the induction of the potential. The largest change in lipid microviscosity was observed when coupled respiration was initiated. This occurred in both the COV system and the RLM system. The absolute value of the lipid microviscosity was shown to change by as much as 22% with the induction of membrane potentials, owing to respiration. To confirm the viscosity measurements made with DPH, lipid microviscosity was also measured with the spin-labelled fatty acid 5-doxyl stearate. Measurements of the order parameters indicated that, in agreement with the results of fluorescence experiments, viscosity changes occurred that were due to the induction of a membrane potential. The significance of these findings to the regulation of metabolism is briefly discussed, the main conclusion being that, although there is certainly a significant variation of lipid microviscosity with electric field, mechanistic interpretations will require further studies.