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Channels selective for potassium or chloride ions are present in all intracellular membranes such as mitochondrial membranes, sarcoplasmic/endoplasmic reticulum, nuclear membrane and chromaffin granule membranes. They probably play an important role in events such as acidification of intracellular compartments and regulation of organelle volume. Additionally, intracellular ion channels are targets for pharmacologically active compounds, e.g. mitochondrial potassium channels interact with potassium channel openers such as diazoxide. This review describes current observations concerning the properties and functional roles of intracellular potassium and chloride channels.

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We investigated the effect of the potassium channel openers diazoxide and RP66471 on mitochondrial membrane potential and mitochondrial respiration in digitonin-treated rat hippocampal homogenates. Both diazoxide and RP66471 induced a dose-dependent decrease of mitochondrial membrane potential. Concomitant with the depolarization was an increase of mitochondrial respiration. Furthermore, the mitochondrial membrane depolarization induced by diazoxide and RP66471 was significantly larger in the presence of potassium ions than in the presence of sodium ions. The diazoxide-induced (but not RP66471-induced) mitochondrial membrane depolarization was partially inhibited by blockers of the ATP-regulated potassium channel, 5-hydroxydecanoic acid or the antidiabetic sulfonylurea glibenclamide. In addition, the potassium channel openers diazoxide and RP66471 increased mitochondrial matrix volume and induced a release of cytochrome c from hippocampal mitochondria. These results indicate the presence of a mitochondrial ATP-regulated potassium channel in rat hippocampus being a target for potassium channel openers.

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Ion channels selective for potassium or chloride ions are present in membranes of intracellular organelles such as mitochondria, sarcoplasmic (endoplasmic) reticulum, nucleus, synaptic vesicles, and chromaffin, and zymogen granules. They are probably important in cellular events such as compensation of electrical charges during intracellular transport of Ca2+ and H+ and regulation of organelle volume changes. This review describes the basic properties, and current hypotheses concerning the functional role, and some aspects of experimental methodology of intracellular ion channels studies.

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It is shown that 2-10 microM Zn2+ induces swelling of rat liver mitochondria incubated in a buffered sucrose medium either with valinomycin or with FCCP, Ca2+, ionophore A23187, oligomycin, and nigericin. This swelling was associated with the release of GSH from mitochondria. Both processes were sensitive to known inhibitors of the mitochondrial permeability transition (MPT), cyclosporin A, and Mg2+. Mitochondrial swelling induced by Zn2+ was also inhibited by rotenone, antymycin A, N-ethylmaleimide, butylhydroxytoluene, and spermine, whereas it was stimulated by tert-butyl hydroperoxide, diamide, and monobromobimane. It did not require the addition of phosphate. The same sensitivity to pH of the mitochondrial swelling induced by Zn2+ and by phenylarsine oxide suggests the same site of the interaction, namely, thiol groups. The ability of Zn2+ to induce mitochondrial swelling gradually decreased along with its increasing concentration above 10 microM. It is concluded that micromolar Zn2+ induces the MPT presumably by the interaction with cysteinyl residues. This process is independent of the mitochondrial membrane potential.

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The oxidation of some mitochondrial sulfhydryl groups acts as an inducer of the mitochondrial membrane permeability transition. This membrane damage can be reversed by regeneration of the sulfhydryl groups. Hence the mitochondrial activities reducing disulfides are especially important for the defense against oxidative injury. The ability of isolated rat liver mitochondria to reduce disulfides was examined by the reduction of 5,5'-dithiobis-(2 nitro-benzoic acid) (DTNB), the reaction catalyzed by thioredoxin reductase and glutathione reductase. The incubation of mitochondria with DTNB induced their swelling which was prevented by EGTA, cyclosporin A, Mg2+, and pyrophosphate. The rate of DTNB reduction by mitochondria was compared with the changes in their volume and in the total content of their sulfhydryl groups. The reduction of DTNB was stimulated by cation chelators, both unphysiological and physiological, and was suppressed by up to 65% during mitochondrial swelling. In mitochondria treated with tert-butylhydroperoxide the total content of mitochondrial sulfhydryl groups decreased in the correlation with the suppression of DTNB reduction and with mitochondrial swelling. The content of nonprotein sulfhydryl groups was significantly lowered under all conditions applied. However, the protein sulfhydryl groups were preserved in the presence of EGTA or Mg2+ when the rate of DTNB reduction was relatively high and this was associated with the strong inhibition of mitochondrial swelling. It is suggested that during oxidative stress mitochondrial activities reducing disulfides are involved in the maintenance of sulfhydryl groups in mitochondria thus protecting their membranes against permeabilization.

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