Ca2+-dependent NAD(P)+-induced alterations of rat liver and hepatoma mitochondrial membrane permeability

Feeding effects on liver mitochondrial bioenergetics of Boa constrictor (Serpentes: Boidae)

Snakes are interesting examples of taxa that can overcome energy metabolism challenges, as many species can endure long periods without feeding, and their eventual meals are of reasonably large sizes, thus exhibiting dual extreme adaptations. Consequently, metabolic rate increases considerably to attend to the energetic demand of digestion, absorption and protein synthesis. These animals should be adapted to transition from these two opposite states of energy fairly quickly, and therefore we investigated mitochondrial function plasticity in these states. Herein, we compared liver mitochondrial bioenergetics of the boid snake Boa constrictor during fasting and after meal intake. We fasted the snakes for 60 days, and then we fed a subgroup with 30% of their body size and evaluated their maximum postprandial response. We measured liver respiration rates from permeabilized tissue and isolated mitochondria. From isolated mitochondria, we also measured Ca2+ retention capacity and redox status. Mitochondrial respiration rates were maximized after feeding, reaching an approximately 60% increase from fasting levels when energized with complex I-linked substrates. Interestingly, fasting and fed snakes exhibited similar respiratory control ratios and citrate synthase activity. Furthermore, we found no differences in Ca2+ retention capacity, indicating no increase in susceptibility to mitochondrial permeability transition, and no changes in mitochondrial redox state, although fed animals exhibited increases in the release of H2O2. Thus, we conclude that liver mitochondria from B. constrictor snakes increase respiration rates during the postprandial period and quickly improve the bioenergetic capacity without compromising redox balance.

Oxidative Stress, Mitochondrial Dysfunction, and Neuroprotection of Polyphenols with Respect to Resveratrol in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disease and is characterized by dopaminergic neuronal loss. The exact pathogenesis of PD is complex and not yet completely understood, but research has established the critical role mitochondrial dysfunction plays in the development of PD. As the main producer of cytosolic reactive oxygen species (ROS), mitochondria are particularly susceptible to oxidative stress once an imbalance between ROS generation and the organelle’s antioxidative system occurs. An overabundance of ROS in the mitochondria can lead to mitochondrial dysfunction and further vicious cycles. Once enough damage accumulates, the cell may undergo mitochondria-dependent apoptosis or necrosis, resulting in the neuronal loss of PD. Polyphenols are a group of natural compounds that have been shown to offer protection against various diseases, including PD. Among these, the plant-derived polyphenol, resveratrol, exhibits neuroprotective effects through its antioxidative capabilities and provides mitochondria protection. Resveratrol also modulates crucial genes involved in antioxidative enzymes regulation, mitochondrial dynamics, and cellular survival. Additionally, resveratrol offers neuroprotective effects by upregulating mitophagy through multiple pathways, including SIRT-1 and AMPK/ERK pathways. This compound may provide potential neuroprotective effects, and more clinical research is needed to establish the efficacy of resveratrol in clinical settings.

Increasing Ca2+ in photoreceptor mitochondria alters metabolites, accelerates photoresponse recovery, and reveals adaptations to mitochondrial stress

Photoreceptors are specialized neurons that rely on Ca²⁺ to regulate phototransduction and neurotransmission. Photoreceptor dysfunction and degeneration occur when intracellular Ca²⁺ homeostasis is disrupted. Ca²⁺ homeostasis is maintained partly by mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniporter (MCU), which can influence cytosolic Ca²⁺ signals, stimulate energy production, and trigger apoptosis. Here we discovered that zebrafish cone photoreceptors express unusually low levels of MCU. We expected that this would be important to prevent mitochondrial Ca²⁺ overload and consequent cone degeneration. To test this hypothesis, we generated a cone-specific model of MCU overexpression. Surprisingly, we found that cones tolerate MCU overexpression, surviving elevated mitochondrial Ca²⁺ and disruptions to mitochondrial ultrastructure until late adulthood. We exploited the survival of MCU overexpressing cones to additionally demonstrate that mitochondrial Ca²⁺ uptake alters the distributions of citric acid cycle intermediates and accelerates recovery kinetics of the cone response to light. Cones adapt to mitochondrial Ca²⁺ stress by decreasing MICU3, an enhancer of MCU-mediated Ca²⁺ uptake, and selectively transporting damaged mitochondria away from the ellipsoid toward the synapse. Our findings demonstrate how mitochondrial Ca²⁺ can influence physiological and metabolic processes in cones and highlight the remarkable ability of cone photoreceptors to adapt to mitochondrial stress.

Saturated Fatty Acid-Enriched Diet-Impaired Mitochondrial Bioenergetics in Liver From Undernourished Rats During Critical Periods of Development

The nutritional transition that the western population has undergone is increasingly associated with chronic metabolic diseases. In this work, we evaluated a diet rich in saturated fatty acids (hyperlipidic, HL) after weaning of the offspring rats submitted to maternal protein restriction on the hepatic mitochondrial bioenergetics. Wistar rats were mated and during gestation and lactation, mothers received control diets (NP, normal protein content 17%) or low protein (LP, 8% protein). After weaning, rats received either NL (normolipidic) or HL (+59% SFA) diets up to 90 days of life. It was verified that all respiratory states of hepatic mitochondria showed a reduction in the LP group submitted to the post-weaning HL diet. This group also presented greater mitochondrial swelling compared to controls, potentiated after Ca2+ addition and prevented in the presence of EGTA (calcium chelator) and cyclosporin A (mitochondrial permeability transition pore inhibitor). There was also an increase in liver protein oxidation and lipid peroxidation and reduction in catalase and glutathione peroxidase activities in the LP group fed HL diet after weaning. Our data suggest that adult rats subjected to maternal protein restriction were more susceptible to hepatic mitochondrial damage caused by a diet rich in saturated fatty acids post-weaning.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

A spontaneous mutation in the nicotinamide nucleotide transhydrogenase gene of C57BL/6J mice results in mitochondrial redox abnormalities

NADPH is the reducing agent for mitochondrial H2O2 detoxification systems. Nicotinamide nucleotide transhydrogenase (NNT), an integral protein located in the inner mitochondrial membrane, contributes to an elevated mitochondrial NADPH/NADP(+) ratio. This enzyme catalyzes the reduction of NADP(+) at the expense of NADH oxidation and H(+) reentry to the mitochondrial matrix. A spontaneous Nnt mutation in C57BL/6J (B6J-Nnt(MUT)) mice arose nearly 3 decades ago but was only discovered in 2005. Here, we characterize the consequences of the Nnt mutation on the mitochondrial redox functions of B6J-Nnt(MUT) mice. Liver mitochondria were isolated both from an Nnt wild-type C57BL/6 substrain (B6JUnib-Nnt(W)) and from B6J-Nnt(MUT) mice. The functional evaluation of respiring mitochondria revealed major redox alterations in B6J-Nnt(MUT) mice, including an absence of transhydrogenation between NAD and NADP, higher rates of H2O2 release, the spontaneous oxidation of NADPH, the poor ability to metabolize organic peroxide, and a higher susceptibility to undergo Ca(2+)-induced mitochondrial permeability transition. In addition, the mitochondria of B6J-Nnt(MUT) mice exhibited increased oxidized/reduced glutathione ratios as compared to B6JUnib-Nnt(W) mice. Nonetheless, the maximal activity of NADP-dependent isocitrate dehydrogenase, which is a coexisting source of mitochondrial NADPH, was similar between both groups. Altogether, our data suggest that NNT functions as a high-capacity source of mitochondrial NADPH and that its functional loss due to the Nnt mutation results in mitochondrial redox abnormalities, most notably a poor ability to sustain NADP and glutathione in their reduced states. In light of these alterations, the potential drawbacks of using B6J-Nnt(MUT) mice in biomedical research should not be overlooked.

Peroxynitrite mobilizes calcium ions from ryanodine-sensitive stores, a process associated with the mitochondrial accumulation of the cation and the enforced formation of species mediating cleavage of genomic DNA

Peroxynitrite does not directly cause strand scission of genomic DNA. Rather, as we previously reported, the DNA cleavage is largely mediated by H(2)O(2) resulting from the dismutation of superoxide generated in the mitochondria upon peroxynitrite-dependent inhibition of complex III. The present study demonstrates that this process is strictly controlled by the availability of Ca(2+) in the mitochondrial compartment. Experiments using intact as well as permeabilized U937 cells showed that the DNA-damaging response evoked by peroxynitrite is enhanced by treatments causing an increase in mitochondrial Ca(2+) uptake and remarkably reduced under conditions leading to inhibition of mitochondrial Ca(2+) accumulation. An additional, important observation was that the source of the Ca(2+) mobilized by peroxynitrite is the ryanodine receptor; preventing the mobilization of Ca(2+) with ryanodine suppressed the mitochondrial formation of reactive oxygen species and the ensuing DNA strand scission. Identical results were obtained using PC12, C6, and THP-1 cells. These results, along with our previous findings indicating that the DNA damage induced by peroxynitrite is also suppressed by inhibition of the electron flow through complex I, e.g., by rotenone, or by the respiration-deficient phenotype, demonstrate that the mitochondrial formation of DNA-damaging species is critically regulated by the inhibition of complex III and by the availability of Ca(2+).

Disruption of mitochondrial membrane potential during apoptosis induced by PSC 833 and CsA in multidrug-resistant lymphoid leukemia

Previous findings from our laboratory demonstrated that when used at low concentration (0.1 microg ml(-1)), CsA as well as its analog PSC 833 were able to revert the MDR phenotype, while at high concentration (1 microg ml(-1)) were able to induce apoptosis. CsA induced apoptosis in leukemia cell lines sensitive (LBR-) and resistant to vincristine (LBR-V160), and doxorubicin (LBR-D160), while PSC 833 only induced apoptosis in vincristine-resistant cell line (LBR-V160). In this work, we investigated mitochondrial-associated mechanisms during CsA- and PSC 833-induced apoptosis. Mitochondrial function was evaluated by recording changes in its transmembrane potential, cytochrome c release, and caspase activation cascade. Results showed that CsA- and PSC 833-induced apoptosis was associated with mitochondrial depolarization, through potentiometric measurements with JC-1 and DiOC(6) probes. Collapse of mitochondrial potential in these cell lines after CsA treatment was followed by cytochrome c release to the cytosol, reaching an increase of 2.61-fold in LBR-, 1.98-fold in LBR-V160, and 3.01-fold in the case of LBR-D160. However, in the case of PSC 833 treatment, induction of apoptosis in LBR-V160 was associated with mitochondrial depolarization followed by a lower cytochrome c release of 1.15-fold as compared with untreated cells. Caspase 3 activation was clearly observed in LBR-, LBR-V160, and LBR-D160 after CsA treatment, while in LBR-V160, PSC 833 was less effective inducing activation of this caspase. Neither caspase 6 nor 8 activity was observed in these three cell lines. Our results suggest that during CsA- and PSC 833-induced apoptosis, mitochondrial dysfunction occurs. This is mediated through mitochondrial events, associated with an evident decrease in DeltaPsi(m), cytochrome c release and caspase 3 activation.

Biological significance of phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) in mammalian cells

Reactive oxygen species (ROS) are known mediators of intracellular signal cascades. Excessive production of ROS may lead to oxidative stress, loss of cell function, and cell death by apoptosis or necrosis. Lipid hydroperoxides are one type of ROS whose biological function has not yet been clarified. Phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) is a unique antioxidant enzyme that can directly reduce phospholipid hydroperoxide in mammalian cells. This contrasts with most antioxidant enzymes, which cannot reduce intracellular phospholipid hydroperoxides directly. In this review, we focus on the structure and biological functions of PHGPx in mammalian cells. Recently, molecular techniques have allowed overexpression of PHGPx in mammalian cell lines, from which it has become clear that lipid hydroperoxides also have an important function as activators of lipoxygenase and cyclooxygenase, participate in inflammation, and act as signal molecules for apoptotic cell death and receptor-mediated signal transduction at the cellular level.

Release of NO by a nitrosyl complex upon activation by the mitochondrial reducing power

The reaction of trans-[Ru(NH(3))(4)P(OEt)(3)NO](3+) and mitochondria was investigated through differential pulse polarography and fluorimetry. The nitrosyl complex undergoes one-electron reduction centered on the NO ligand site. The reaction between the mitochondrial reductor and trans-[Ru(NH(3))(4)P(OEt)(3)NO](3+) exhibits a second order specific rate constant calculated as k=2 x 10(1) M(-1) s(-1). The reduced species, trans-[Ru(NH(3))(4)P(OEt)(3)NO](2+), quickly releases NO, yielding trans-[Ru(NH(3))(4)P(OEt)(3)H(2)O](2+). The low toxicities of both trans-[Ru(NH(3))(4)P(OEt)(3)(NO)](2+) and trans-[Ru(NH(3))(4)P(OEt)(3)H(2)O](2+) and its ability to release NO after reductive activation in a biological medium make the nitrosyl compound a useful model of a hypotensive drug.

Suramin inhibits respiration and induces membrane permeability transition in isolated rat liver mitochondria

Suramin, a polysulfonated naphthylamine, caused a dose dependent inhibition of carbonyl cyanide p-(tri-fluoromethoxy)phenylhydrazone-stimulated respiration supported either by succinate or a cocktail of alphaketoglutarate, malate and isocitrate in isolated rat liver mitochondria. The half-maximum effect was obtained at 40 and 140 microM suramin for NADH- or FADH(2)-linked substrates, respectively. The respiration supported by N,N,N'N'-tetramethyl-p-phenylenediamine oxidation was unaffected by suramin (<or=500 microM). The latter respiratory substrate was used in the studies at examining the effects of suramin on the permeability properties of the inner mitochondrial membrane. These experiments provided evidence that suramin causes Ca(2+)-dependent mitochondrial swelling sensitive to the mitochondrial permeability transition (MPT) inhibitors cyclosporin A, ADP and Mg(2+). In addition, suramin decreased the content of reduced mitochondrial membrane protein thiols suggesting that thiol oxidation is the mechanism underlying suramin-induced MPT.

Mitochondrial permeability transition and oxidative stress

Mitochondrial permeability transition (MPT) is a non-selective inner membrane permeabilization that may precede necrotic and apoptotic cell death. Although this process has a specific inhibitor, cyclosporin A, little is known about the nature of the proteinaceous pore that results in MPT. Here, we review data indicating that MPT is not a consequence of the opening of a pre-formed pore, but the consequence of oxidative damage to pre-existing membrane proteins.

Ca2+ induces a cyclosporin A-insensitive permeability transition pore in isolated potato tuber mitochondria mediated by reactive oxygen species

Oxidative damage of mammalian mitochondria induced by Ca2+ and prooxidants is mediated by the attack of mitochondria-generated reactive oxygen species on membrane protein thiols promoting oxidation and cross-linkage that leads to the opening of the mitochondrial permeability transition pore (Castilho et al., 1995). In this study, we present evidence that deenergized potato tuber (Solanum tuberosum) mitochondria, which do not possess a Ca2+ uniport, undergo inner membrane permeabilization when treated with Ca2+ (>0.2 mM), as indicated by mitochondrial swelling. Similar to rat liver mitochondria, this permeabilization is enhanced by diamide, a thiol oxidant that creates a condition of oxidative stress by oxidizing pyridine nucleotides. This is inhibited by the antioxidants catalase and dithiothreitol. Potato mitochondrial membrane permeabilization is not inhibited by ADP, cyclosporin A, and ruthenium red, and is partially inhibited by Mg2+ and acidic pH, well known inhibitors of the mammalian mitochondrial permeability transition. The lack of inhibition of potato mitochondrial permeabilization by cyclosporin A is in contrast to the inhibition of the peptidylprolyl cis-trans isomerase activity, that is related to the cyclosporin A-binding protein cyclophilin. Interestingly, the monofunctional thiol reagent mersalyl induces an extensive cyclosporin A-insensitive potato mitochondrial swelling, even in the presence of lower Ca2+ concentrations (>0.01 mM). In conclusion, we have identified a cyclosporin A-insensitive permeability transition pore in isolated potato mitochondria that is induced by reactive oxygen species.

Mitochondrial damage induced by conditions of oxidative stress

Up to 2% of the oxygen consumed by the mitochondrial respiratory chain undergoes one electron reduction, typically by the semiquinone form of coenzyme Q, to generate the superoxide radical, and subsequently other reactive oxygen species such as hydrogen peroxide and the hydroxyl radical. Under conditions in which mitochondrial generation of reactive oxygen species is increased (such as in the presence of Ca2+ ions or when the mitochondrial antioxidant defense mechanisms are compromised), these reactive oxygen species may lead to irreversible damage of mitochondrial DNA, membrane lipids and proteins, resulting in mitochondrial dysfunction and ultimately cell death. The nature of this damage and the cellular conditions in which it occurs are discussed in this review article.

Ca2+-stimulated mitochondrial reactive oxygen species generation and permeability transition are inhibited by dibucaine or Mg2+

Mitochondrial swelling and membrane protein thiol oxidation associated with mitochondrial permeability transition induced by Ca2+ and t-butyl hydroperoxide or inorganic phosphate, but not 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid or phenylarsine oxide, are inhibited by the local anesthetic dibucaine. Dibucaine promotes an inhibition of the Ca2+-induced increase in mitochondrial H2O2 generation measured by the oxidation of scopoletin in the presence of horseradish peroxidase. This decrease in mitochondrial H2O2 generation may be attributed to the reduction of Ca2+ binding to the membrane induced by dibucaine, as assessed by measuring 45Ca2+ binding to the mitochondrial membrane. Mg2+ also inhibited Ca2+ binding to the mitochondrial membrane, mitochondrial swelling, membrane protein thiol oxidation, and H2O2 generation induced by Ca2+. Together, these results demonstrate that the mechanism by which dibucaine and Mg2+ inhibit mitochondrial permeability transition is related to the decrease in reactive oxygen species generation induced by Ca2+-promoted alterations of inner mitochondrial membrane properties.

Role of mitochondria in calcium regulation of spontaneously contracting cardiac muscle cells

Mitochondrial involvement in the regulation of cytosolic calcium concentration ([Ca2+]i) in cardiac myocytes has been largely discounted by many authors. However, recent evidence, including the results of this study, has forced a reappraisal of this role. [Ca2+]i and Ca2+ in the mitochondria ([Ca2+]m) were measured in this study with specific fluorescent probes, fluo-3 and di-hydro-rhod-2, respectively; mitochondrial membrane potential (DeltaPsim) was monitored with JC-1. Addition of uncouplers or inhibitors of the mitochondrial respiratory chain was found to cause a twofold decrease in the rate of removal of Ca2+ from the cytosol after a spontaneously generated Ca2+ wave. These agents also caused a progressive elevation of [Ca2+]i, an increase in the number of hotspots of Ca2+ release (Ca2+ sparks), and depression of mitochondrial potential. The Ca2+-indicative fluorophore dihydro-rhod-2 has a net positive charge that contributes to selective accumulation by mitochondria, as supported by its co-localization with other mitochondrial-specific probes (MitoTracker Green). Treatment of dihydro-rhod-2-loaded cells with NaCN resulted in rapid formation of "black holes" in the otherwise uniformly banded pattern. These are likely to represent individual or small groups of mitochondria that have depressed mitochondrial potential, or have lost accumulated rhod-2 and/or Ca2+; all of these eventualities are possible upon onset of the mitochondrial permeability transition. Release of Ca2+ from the sarcoplasmic reticulum and the resultant spontaneous contractility of cardiac muscle are proposed to be triggered by the induction of the mitochondrial permeability transition and the subsequent loss of [Ca2+]m.

Dietary restriction augments protection against induction of the mitochondrial permeability transition

Exposure to oxidants or phosphate, especially in the presence of calcium, has been long known to lead to mitochondrial structural alteration and damage. In the past 15 years, it has become increasingly appreciated that this damage is often the result of a cyclosporin A-sensitive event, the "permeability transition" (PT). Using liver mitochondria isolated from male Fischer 344 rats of 6-24 months of age, we now present evidence that long-term, life-prolonging, dietary restriction regimens greatly delay induction of a PT following challenge. Dietary restriction slowed induction by 25 microM calcium, or by calcium in conjunction with the strong oxidant t-butyl hydroperoxide, by approximately 50%. The increased resistance to PT induction was maintained through 24 months of age. Dietary restriction also protected against t-butyl hydroperoxide in the presence of high calcium challenges (250 microM), although the extent of this protection was age-dependent. Induction by 2.5 mM phosphate alone was blocked in most 6-month-old dietary restricted animals and was slowed by 50-100% in animals 12-24 months of age. Susceptibility to 25 microM calcium in conjunction with phosphate varied in an age-dependent manner, ranging from 4-12 times slower in the dietary restricted animals than in their ad lib fed counterparts. Together, these data provide evidence that the factors regulating PT induction are affected by long-term physiological and environmental conditions such as age and diet. The observed effects represent one of the largest recognized dietary restriction-mediated increases in a parameter related to antioxidant defenses. These data also suggest that the endogenous defense systems that protect mitochondria from calcium in conjunction with inorganic phosphate differ from those that protect against calcium in conjunction with an oxidant.

Ruthenium red-catalyzed degradation of peroxides can prevent mitochondrial oxidative damage induced by either tert-butyl hydroperoxide or inorganic phosphate

We have recently shown that ruthenium red, a non-competitive inhibitor of the mitochondrial Ca2+ uniporter, can reduce tert-butyl hydroperoxide via a Fenton-type reaction. In respiring mitochondrial preparations containing tert-butyl hydroperoxide, redox cycling of ruthenium red occurs and causes the amplification of methyl radical generation (Meinicke, A. R., Zavan, S. S., Ferreira, A. M. C., Vercesi, A. E., and Bechara, E. J. H. (1996) Arch. Biochem. Biophys. 328, 239-244). In this study we show that ruthenium red can act as an antioxidant preventing mitochondrial damage when the respiratory chain is reduced or when ascorbate is present. Ruthenium red can catalyze the degradation of hydrogen peroxide into H2O and O2. We show here that ruthenium red prevents both accumulation of mitochondrial generated H2O2 and swelling in the presence of the Ca2+ ionophore A23187. Under these conditions the damage induced by Ca2+ ions and either tert-butyl hydroperoxide or inorganic phosphate is promoted by mitochondrial-generated reactive oxygen species. Swelling induced by phenylarsine oxide, a thiol cross-linker, by a mechanism independent of free radicals is not inhibited by ruthenium red. These data provide evidence that the antioxidant behavior of ruthenium red under our conditions is due to its ability to destroy peroxides, which is related to its redox cycling and is prevalent over the Fenton-type reaction.

Ca2+ acting at the external side of the inner mitochondrial membrane can stimulate mitochondrial permeability transition induced by phenylarsine oxide

Mitochondrial permeability transition (MPT) induced by the thiol cross-linker phenylarsine oxide (PhAsO) in Ca(2+)-depleted mitochondria incubated in the presence of ruthenium red, an inhibitor of the Ca2+ uniporter, is stimulated by the addition of extramitochondrial Ca2+. The presence of extramitochondrial Ca2+ stimulates the reaction of mitochondrial membrane protein thiol groups with PhAsO. Both Ca(2+)-induced increase in mitochondrial membrane permeabilization and protein thiol group reaction with PhAsO are dependent on time (5-10 min to be complete) and the concentration of Ca2+ (1-25 microM). Mitochondrial permeabilization induced by PhAsO (15 microM) and extramitochondrial Ca2+ is inhibited by ADP, cyclosporin A, dibucaine and Mg2+, while mitochondrial permeabilization induced by high concentrations of PhAsO (60 microM) in the absence of Ca2+ is inhibited only by ADP and cyclosporin A. These results suggest that dibucaine and Mg2+ can inhibit mitochondrial permeabilization by antagonizing the effect of Ca2+ on the mitochondrial membrane. Once mitochondrial permeabilization induced by 15 microM PhAsO and extramitochondrial Ca2+ has already occurred, the addition of the Ca2+ chelator EGTA restores mitochondrial membrane potential (MPT pore closure), suggesting that the presence of Ca2+ is essential for the maintenance of the permeability of the mitochondrial membrane to protons (MPT pore opening). In conclusion, the results presented indicate that low Ca2+ concentrations acting at the external side of the inner mitochondrial membrane can stimulate mitochondrial permeability transition induced by PhAsO, due to increased accessibility of protein thiol groups to the reaction with PhAsO and increased probability of MPT pore opening.

Comparison of the effects of cyclosporine A and trimetazidine on Ca(2+)-dependent mitochondrial swelling

Cyclosporine A (CsA) is a known potent inhibitor of pro-oxidant-induced mitochondrial swelling. In the present study we show that CsA's effect is only transient when the liver mitochondrial swelling in induced by Ca2+ plus tert-butylhydroperoxide (t-BH). After an initial inhibition, swelling is worsened by CsA as evidenced by an extent of mitochondrial swelling that exceeds that of the control. Unlike CsA, trimetazidine (TMZ), an anti-ischemic drug decreases both the extent and the rate of the swelling with an IC50 value of 214 +/- 24 microM. Its inhibition effect on the initial swelling rate mimicks that of CsA but the mechanism may be independent. During long-term swelling. TMZ counteracts the worsening effect of CsA. The inhibition of swelling induced by TMZ is assessed by the fact that TMZ significantly increases the EC50 of Ca(2+)-induced mitochondrial swelling (46.6 +/- 6.0 to 85 +/- 10 microM, P < 0.01), without affecting its cooperativity. Apparently, TMZ seems to behave like trifluoperazine (TFP), a phospholipase A2 inhibitor that, under our experimental conditions, inhibits the mitochondrial swelling induced by Ca2+ and t-BH with an IC50 value of 25 +/- 10 microM. Both drugs are able to protect mitochondria from both phases (early and late) of the swelling, especially the late, which is enhanced in the presence of CsA. TFP and other phospholipase A2 inhibitors were able to displace [3H]TMZ from its mitochondrial binding sites whereas CsA was ineffective. We suggest that TMZ, like TFP, inhibits the CsA insensitive mechanism involved in the swelling process which is responsible for the worsening effect observed in the presence of CsA when the swelling is generated by Ca2+ and t-BH.

Two mechanisms by which ATP depletion potentiates induction of the mitochondrial permeability transition

The present and a previous study [J. W. Snyder, J. G. Pastorino, A. M. Attie, and J. L. Farber, Am. J. Physiol. 264 (Cell Physiol. 33): C709-C714, 1993] define two mechanisms whereby ATP depletion promotes liver cell death. ATP depletion and cell death are linked by the mitochondrial permeability transition (MPT). Mitochondrial deenergization promotes the MPT, and ATP maintains a membrane potential by reversal of ATP synthase. With an increased influx of Ca2+ induced by the ionophore A-23187, oligomycin depleted the cells of ATP without loss of the mitochondrial membrane potential and further elevated the intracellular Ca2+ concentration. Cyclosporin A (CyA) prevented the accompanying cell killing. Fructose also preserved the viability of the cells. With the increased cytosolic Ca2+ imposed by A-23187, viability is maintained by ATP-dependent processes. Upon depletion of ATP, Ca2+ homeostasis cannot be maintained, and the MPT is induced. Rotenone also depleted the cells of ATP, and A-23187 accelerated the loss of the mitochondrial membrane potential occurring with rotenone alone. CyA and fructose prevented the cell killing with rotenone and A-23187. Oligomycin did not prevent this action of fructose. We conclude that ATP is needed to maintain Ca2+ homeostasis to prevent the MPT and the resultant liver cell death. ATP is also needed to maintain mitochondrial energization when electron transport is inhibited.

The mitochondrial permeability transition pore is modulated by oxidative agents through both pyridine nucleotides and glutathione at two separate sites

We studied the modulation of the permeability transition pore (MTP), a cyclosporin-A-sensitive channel, in deenergized mitochondria. Rat liver mitochondria were incubated in a potassium gluconate medium and treated with uncoupler, valinomycin, oligomycin and A23187. Under these conditions the membrane and Donnan potentials are collapsed, and no ion gradients are maintained, allowing the study of the dependence of MTP opening on the Ca2+ concentration under a variety of oxidative conditions without the complexities arising from changes of the membrane potential and matrix pH, and from secondary-ion redistribution. Cross-linking of mitochondrial dithiols with arsenite or phenylarsine oxide, or treatment with tert-butylhydroperoxide leading to complete oxidation of glutathione, increased the sensitivity of MTP opening to Ca2+. This effect could be fully prevented by prior treatment of mitochondria with monobromobimane and restored by reduction with dithiothreitol. The effect of tert-butylhydroperoxide was not additive with that of AsO, and it was completely blocked by modification of reduced glutathione with 1-chloro 2,4-dinitrobenzene through glutathione-S-transferase, indicating that oxidized glutathione affects the pore through the AsO-reactive and PhAsO-reactive dithiol. Oxidation of mitochondrial pyridine nucleotides by a variety of treatments also increased the sensitivity of MTP opening to Ca2+ under conditions where glutathione was maintained in the reduced state. This effect could be fully prevented when reduced pyridine nucleotides levels were reestablished with 2-hydroxybutyrate but not by treatment with monobromobimane or dithiothreitol. The effects of dithiol cross-linking or oxidation, and of pyridine nucleotide oxidation on the MTP were additive. The contribution of each of these two oxidative events, when they were induced at the same time, could still be selectively blocked by monobromobimane and dithiothreitol. The effects of dithiol cross-linking or oxidation, and of pyridine nucleotide oxidation on the MTP were additive. The contribution of each of these two oxidative events,when they were induced at the same time, could still be selectively blocked by monobromobimane and dithiothreitol, or by 2-hydroxybutyrate, respectively. Complete oxidation of pyridine nucleotides did not affect the reactivity of the dithiol towards monobromobimane, indicating that it remained in the reduced state. After transient opening of the MTP, sensitivity to pyridine nucleotide oxidation was lost while sensitivity to dithiol cross-linking was retained. These data indicate that the dithiol belongs to an MTP regulatory component which is larger than the MTP cutoff of about 1500 Da, or is non-diffusible. Taken together, these findings indicate that the MTP is influenced by oxidation-reduction events at two separate sites that can be distinguished experimentally, and that these sites are not connected by common oxidation-reduction intermediates other than glutathione.

Opening of the mitochondrial permeability transition pore by uncoupling or inorganic phosphate in the presence of Ca2+ is dependent on mitochondrial-generated reactive oxygen species

In this study, we show that mitochondrial membrane permeability transition in Ca(2+)-loaded mitochondria treated with carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) or inorganic phosphate (P(i)) is preceded by enhanced production of H2O2. This production is inhibited either by ethylene glycobis(b-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA) or Mg2+, but not by cyclosporin A. Permeability transition is prevented either by EGTA, catalase or dithiothreitol, suggesting the involvement of Ca2+, H2O2 and oxidation of membrane protein thiols in this mechanism. When mitochondria are incubated under anaerobiosis, no permeabilization or H2O2 production occurs. Based on these results we conclude that mitochondrial permeability transition induced by P(i) or FCCP-uncoupling is dependent on mitochondrial-generated reactive oxygen species.

Ca(2+)-induced mitochondrial membrane permeabilization: role of coenzyme Q redox state

Rotenone-poisoned rat liver mitochondria energized by succinate addition, after a 5-min period of preincubation in presence of 10 microM Ca2+, produce H2O2 at much faster rates, undergo extensive swelling, and are not able to retain the membrane potential and accumulated Ca2+. Similar results were obtained when a suspension of rat liver mitochondria preincubated in anaerobic medium for 5 min was reoxygenated. The addition of either ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, ruthenium red, catalase, or dithiothreitol, just before succinate or O2 addition, prevented mitochondrial swelling, indicating the involvement of Ca2+, reactive oxygen species, and oxidation of membrane protein thiols in this process of membrane permeabilization. Inhibition of mitochondrial swelling by cyclosporin A suggests that the membrane alterations observed under these experimental conditions are related to opening of the permeability transition pore. The presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone, which prevents Ca2+ cycling across the membrane, did not inhibit mitochondrial swelling when Ca2+ influx into the mitochondrial matrix was driven by a high Ca2+ gradient. When rotenone plus antimycin A-poisoned mitochondria were energized by N,N,N',N'-tetramethyl-p-phenylenediamine, which reduces respiratory chain complex IV, mitochondrial swelling did not occur, unless succinate, which reduces coenzyme Q, was also added. It is concluded that reduced coenzyme Q is the electron source for oxygen radical production during Ca(2+)-stimulated oxidative damage of mitochondria.

Oxidative damage of mitochondria induced by 5-aminolevulinic acid: role of Ca2+ and membrane protein thiols

Reactive oxygen species (ROS) generated by metal-catalyzed 5-aminolevulinic acid (ALA) aerobic oxidation have been shown to damage the inner membrane of isolated rat liver mitochondria by a Ca(2+)-dependent mechanism. The present work describes experiments indicating that this damage can be prevented, but not completely reversed by the additions of catalase, ADP, cyclosporin A and dithiothreitol, as judged by the extent of delta psi regeneration by the injured mitochondria. In contrast, the addition of EGTA, which removes free Ca2+ and, possibly, Fe2+ present both in the intra- and extramitochondrial compartments, causes a prompt and complete regeneration of delta psi, even after long periods of mitochondrial incubations in the presence of ALA. This reversibility suggests that protein alterations such as protein thiol cross-linkings, evidenced by SDS-polyacrylamide gel electrophoresis, are the main cause of increased membrane permeability promoted by ALA oxidation. The inhibition of protein aggregation and fast regeneration of delta psi promoted by EGTA suggest that the binding of Ca2+ to some membrane proteins plays a crucial role in the mechanism of both protein polymerization (pore assembly) and pore opening. The implication of these results with the molecular pathology of acute intermittent porphyria is also discussed.

The different routes of calcium efflux from liver mitochondria

Calcium efflux from liver mitochondria, induced by an uncoupler during incubation at 20 degrees C, is largely inhibited by the prior addition of ruthenium red or EGTA. The inhibition by EGTA (i.e. by chelation of Ca2+) is impaired by the presence of EDTA (i.e. by chelation of Mg2+), and it is completely abolished by addition of spermine. In contrast, the inhibition of calcium efflux at 20 degrees C by ruthenium red is unaffected by EGTA or spermine. This latter pathway of calcium efflux therefore represents the reversal of the calcium uniporter. During incubation at 30 degrees C, uncoupler-induced calcium efflux is incompletely inhibited by ruthenium red, and the residual calcium efflux occurs via membrane transition. The kinetics of this process as well as its exceptionally strong dependence on temperature constitute the main evidence for considering that membrane transition modifies the uniporter, and that the modified uniporter is responsible for the residual calcium efflux. It was shown that the route of ruthenium red-insensitive calcium efflux from energized mitochondria under standard conditions is the same, irrespective of whether the uniporter is running or is blocked by ruthenium red. The development of methods for the clear experimental separation of these different routes of calcium efflux under different conditions is still critically important.

Calcium inhibition of the ATP in equilibrium with [32P]Pi exchange and of net ATP synthesis catalyzed by bovine submitochondrial particles

A previous communication (Fagian, M. M., Pereira da Silva, L. and Vercesi, A. E. (1986) Biochim. Biophys. Acta 852, 262-268) indicated that intramitochondrial calcium inhibits oxidative phosphorylation by decreasing the availability of adenine nucleotides to both the ADP/ATP translocase and the F0F1-ATP synthase complex. In this work we analyzed the interactions of calcium-nucleotide and magnesium-nucleotide complexes with the ATP synthase during catalysis of ATP in equilibrium with [32P]Pi exchange and net synthesis of ATP by submitochondrial particles. Concerning the ATP in equilibrium with [32P]Pi exchange reaction, calcium was ineffective as divalent cation when assayed alone. Furthermore, the addition of calcium increased the magnesium concentration required for half-maximal activation of the exchange, without changing Vmax. With respect to net ATP synthesis, the inhibition by calcium was shown to be due to formation of the CaADP- complex, which competes with MgADP- for the active site of the F0F1-ATP synthase. Moreover, ATP hydrolysis was competitively inhibited by CaATP2-, showing that calcium is able to interact with the enzyme in both forward and backward reactions in the same manner. That high calcium concentrations are required for significant inhibition of ATP synthesis indicates that this inhibition is relevant under conditions in which cytosolic calcium concentrations rise to pathological levels. Therefore, this mechanism may be responsible, in part, for the decrease in cellular ATP content that has been observed to occur when calcium accumulates in the cytosol.