We have studied the role of changes in mitochondrial membrane potential (v v8 8) in two widely-used models of apoptosis, such as dexamethasone-treated rat thymocytes and U937 human cells treated with tumor necrosis factor-K K and cycloheximide. To dissipate v v8 8, we used low concentrations of valinomycin, unable per se to induce apoptosis, and demonstrated that the decline in v v8 8 exerts opposite effects in the two models. Indeed, in U937 cells, depolarization of mitochondria increased apoptosis, which decreased in rat thymocytes. This leads to the suggestion that disruption of v v8 8 plays opposite roles depending on the experimental model. In U937 cells, the drop of v v8 8 is a possible contributory cause for the apoptotic process; in rat thymocytes, it could be a limiting factor. We propose that these opposite effects could be due to the different ATP requirement of each apoptotic pathway.z 2000 Federation of European Biochemical Societies.

The effect of the in vivo thyroid status on mitochondrial membrane potential (v v8 8 m ) in isolated rat hepatocytes was studies by means of a cytofluorimetric technique and the v v8 8 mspecific probe JC-1. It is shown that the v v8 8 m level decreases in the order hypothyroid s euthyroid s hyperthyroid. Polarographic measurement of the hepatocyte respiratory rates revealed an opposite trend of values: the highest respiratory rate in hepatocytes from hyperthyroid animals, the lowest in those from hypothyroid ones. This means that mitochondrial energy coupling is highest in hypothyroid hepatocytes and lowest in hyperthyroid hepatocytes. 6-Ketocholestanol added to hepatocytes failed to counterbalance the uncoupling effect of thyroid hormones on v v8 8 m and respiration rate. Under the same conditions, 6-ketocholestanol appeared to be effective in recoupling of respiration uncoupled by low concentrations of the artificial protonophore FCCP. The mechanism and possible physiological functions of the thyroid hormone-induced decrease in mitochondrial energy coupling are discussed.z 1998 Federation of European Biochemical Societies.

The specific involvement of cardiolipin in modulating and/or controlling the activity of a number of mitochondrial carriers, enzymes and receptors is well documented; however, comparatively less understood is its role for the integrated functions of intact mitochondria. The aim of the present research was to get a better insight into this problem by investigating the effect of in vitro addition of cardiolipin on the properties of isolated liver mitochondria.The results obtained show that cardiolipin induces extensive structural and functional perturbations at the level of the inner mitochondrial membrane. In fact, addition of cardiolipin to intact mitochondria causes a significant increase of proton leak associated with a parallel increase of respiratory rate in State 4. Concomitantly, a slight uncoupling of phosphorylation associated with a moderate increase in ATPase activity is observed. Furthermore, the pore-mediated membrane permeability to calcium is drastically modified, an effect that can be reversed by addition of cycIosporin.