Induction of mitochondrial Ca2+ uptake by mersalyl

In Saccharomyces cerevisiae, the phosphate carrier is a component of the mitochondrial unselective channel

The mitochondrial permeability transition (PT) involves the opening of a mitochondrial unselective channel (MUC) resulting in membrane depolarization and increased permeability to ions. PT has been observed in many, but not all eukaryotic species. In some species, PT has been linked to cell death, although other functions, such as matrix ion detoxification or regulation of the rate of oxygen consumption have been considered. The identification of the proteins constituting MUC would help understand the biochemistry and physiology of this channel. It has been suggested that the mitochondrial phosphate carrier is a structural component of MUC and we decided to test this in yeast mitochondria. Mersalyl inhibits the phosphate carrier and it has been reported that it also triggers PT. Mersalyl induced opening of the decavanadate-sensitive Yeast Mitochondrial Unselective Channel (YMUC). In isolated yeast mitochondria from a phosphate carrier-null strain the sensitivity to both phosphate and mersalyl was lost, although the permeability transition was still evoked by ATP in a decavanadate-sensitive fashion. Polyethylene glycol (PEG)-induced mitochondrial contraction results indicated that in mitochondria lacking the phosphate carrier the YMUC is smaller: complete contraction for mitochondria from the wild type and the mutant strains was achieved with 1.45 and 1.1 kDa PEGs, respectively. Also, as expected for a smaller channel titration with 1.1 kDa PEG evidenced a higher sensitivity in mitochondria from the mutant strain. The above data suggest that the phosphate carrier is the phosphate sensor in YMUC and contributes to the structure of this channel.

Cyclosporin A is unable to inhibit carboxyatractyloside-induced permeability transition in aged mitochondria

We studied the effect of mitochondrial ageing on membrane permeability transition. The results obtained indicate that aged mitochondria are neither able to retain Ca2+ nor to maintain a high transmembrane electric gradient. In addition, aged mitochondria undergo a large amplitude swelling. These dysfunctions were circumvented by the addition of cyclosporin A. Furthermore, it is shown that ageing-induced permeability transition causes oxidative damage on the matrix enzyme aconitase. The observed damage in aged mitochondria requires Ca2+ addition; therefore, it was not seen when Sr2+ replaced Ca2+. Two important findings in this work were the fact that despite of the presence of cyclosporin A, carboxyatractyloside was still able to induce permeability transition, and that ageing induced mitochondrial DNA disruption and release of cytochrome c. It is likely that the membrane's increased permeability is due to the effect of fatty acids, since bovine serum albumin makes mitochondria able to retain Ca2+. However, the possibility that the damage might be the result of oxidative stress cannot be discarded.

Hypothyroidism renders liver mitochondria resistant to the opening of membrane permeability transition pore

Membrane permeability was examined in liver mitochondria isolated from hypothyroid rats. It was found that such a thyroid status provides substantial protection from membrane leakiness as induced by Ca2+ loading. Thus, these mitochondria are less prone to undergoing permeability transition than mitochondria from euthyroid rats. The above conclusion was reached on the basis of the following two facts: (1) hypothyroid mitochondria are not strictly dependent on the addition of ADP to retain high matrix Ca2+ concentrations, and (2) carboxyatractyloside, antimycin A or carbonyl cyanide-m-chlorophenyl hydrazone failed to promote Ca2+ efflux. We discuss the possible relevance of the low content of membrane cardiolipin as well as the low expression of the adenine nucleotide translocase as responsible for the resistance to membrane damage.

On the mechanism by which 6-ketocholestanol protects mitochondria against uncoupling-induced Ca2+ efflux

This work shows that 6-ketocholestanol (kCh) inhibits the effect of carbonyl cyanide-m-chlorophenyl hydrazone (CCP) on mitochondrial Ca2+ efflux. Such an effect proved to be caused by diminution of membrane fluidity, therefore, it is affected by the incubation temperature. Furthermore, kCh reversed CCP-induced Ca2+ efflux depending on the accumulation of phosphate. It is also shown that kCh enhances the effect of carboxyatractyloside on membrane permeability transition.