t-Butylhydroperoxide-induced Ca2+ efflux from liver mitochondria in the presence of physiological concentrations of Mg2+ and ATP

The Many Roles Mitochondria Play in Mammalian Aging

Significance: Aging is a natural process that affects most living organisms, resulting in increased mortality. As the world population ages, the prevalence of age-associated diseases, and their associated health care costs, has increased sharply. A better understanding of the molecular mechanisms that lead to cellular dysfunction may provide important targets for interventions to prevent or treat these diseases. Recent Advances: Although the mitochondrial theory of aging had been proposed more than 40 years ago, recent new data have given stronger support for a central role for mitochondrial dysfunction in several pathways that are deregulated during normal aging and age-associated disease. Critical Issues: Several of the experimental evidence linking mitochondrial alterations to age-associated loss of function are correlative and mechanistic insights are still elusive. Here, we review how mitochondrial dysfunction may be involved in many of the known hallmarks of aging, and how these pathways interact in an intricate net of molecular relationships. Future Directions: As it has become clear that mitochondrial dysfunction plays causative roles in normal aging and age-associated diseases, it is necessary to better define the molecular interactions and the temporal and causal relationship between these changes and the relevant phenotypes seen during the aging process. Antioxid. Redox Signal. 36, 824-843.

Mitochondrial bioenergetics and redox dysfunctions in hypercholesterolemia and atherosclerosis

In the first part of this review, we summarize basic mitochondrial bioenergetics concepts showing that mitochondria are critical regulators of cell life and death. Until a few decades ago, mitochondria were considered to play essential roles only in respiration, ATP formation, non-shivering thermogenesis and a variety of metabolic pathways. However, the concept presented by Peter Mitchell regarding coupling between electron flow and ATP synthesis through the intermediary of a H⁺ electrochemical potential leads to the recognition that the proton-motive force also regulates a series of relevant cell signalling processes, such as superoxide generation, redox balance and Ca²⁺ handling. Alterations in these processes lead to cell death and disease states. In the second part of this review, we discuss the role of mitochondrial dysfunctions in the specific context of hypercholesterolemia-induced atherosclerosis. We provide a literature analysis that indicates a decisive role of mitochondrial redox dysfunction in the development of atherosclerosis and discuss the underlying molecular mechanisms. Finally, we highlight the potential mitochondrial-targeted therapeutic strategies that are relevant for atherosclerosis.

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.

High susceptibility of activated lymphocytes to oxidative stress-induced cell death

The present study provides evidence that activated spleen lymphocytes from Walker 256 tumor bearing rats are more susceptible than controls to tert-butyl hydroperoxide (t-BOOH)-induced necrotic cell death in vitro. The iron chelator and antioxidant deferoxamine, the intracellular Ca2+ chelator BAPTA, the L-type Ca2+ channel antagonist nifedipine or the mitochondrial permeability transition inhibitor cyclosporin A, but not the calcineurin inhibitor FK-506, render control and activated lymphocytes equally resistant to the toxic effects of t-BOOH. Incubation of activated lymphocytes in the presence of t-BOOH resulted in a cyclosporin A-sensitive decrease in mitochondrial membrane potential. These results indicate that the higher cytosolic Ca2+ level in activated lymphocytes increases their susceptibility to oxidative stress-induced cell death in a mechanism involving the participation of mitochondrial permeability transition.

Involvement of ryanodine-operated channels in tert-butylhydroperoxide-evoked Ca2+ mobilisation in pancreatic acinar cells

Reactive oxygen species and related oxidative damage have been implicated in the initiation of acute pancreatitis, a disease characterised in its earliest stages by disruption of intracellular Ca2+ homeostasis. The present study was carried out in order to establish the effect of the organic pro-oxidant, tert-butylhydroperoxide (tBHP), on the mobilisation of intracellular Ca2+ stores in isolated rat pancreatic acinar cells and the mechanisms underlying this effect. Cytosolic free Ca2+ concentrations ([Ca2+]c) were monitored using a digital microspectrofluorimetric system in fura-2 loaded cells. In the presence of normal extracellular Ca2+ concentrations ([Ca2+]o), perfusion of pancreatic acinar cells with 1 mmol l-1 tBHP caused a slow sustained increase in [Ca2+]c. This increase was also observed in a nominally Ca2+-free medium, indicating a release of Ca2+ from intracellular stores. Pretreatment of cells with tBHP abolished the typical Ca2+ response of both the physiological agonist CCK-8 (1 nmol l-1) and thapsigargin (TPS, 1 micromol l-1), an inhibitor of the SERCA pump, in the absence of extracellular Ca2+. Similar results were observed with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP, 0.5 micromol l-1), a mitochondrial uncoupler. In addition, depletion of either agonist-sensitive Ca2+ pools by CCK-8 or TPS or mitochondrial Ca2+ pools by FCCP were unable to prevent the tBHP-induced Ca2+ release. By contrast, simultaneous administration of TPS and FCCP clearly abolished the tBHP-induced Ca2+ release. These results show that tBHP releases Ca2+ from agonist-sensitive intracellular stores and from mitochondria. On the other hand, simultaneous application of FCCP and of 2-aminoethoxydiphenylborane (2-APB), a blocker of IP3-mediated Ca2+ release, was unable to suppress the increase in [Ca2+]c induced by tBHP, while the application of 50 micromol l-1 of ryanodine (which is able to block the ryanodine channels) inhibits tBHP-evoked Ca2+ mobilisation. These findings indicate that tBHP releases Ca2+ from non-mitochondrial Ca2+ pools through ryanodine channels.

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+).

The redox state of endogenous pyridine nucleotides can determine both the degree of mitochondrial oxidative stress and the solute selectivity of the permeability transition pore

Acetoacetate, an NADH oxidant, stimulated the ruthenium red-insensitive rat liver mitochondrial Ca(2+) efflux without significant release of state-4 respiration, disruption of membrane potential (Deltapsi) or mitochondrial swelling. This process is compatible with the opening of the currently designated low conductance state of the permeability transition pore (PTP) and, under our experimental conditions, was associated with a partial oxidation of the mitochondrial pyridine nucleotides. In contrast, diamide, a thiol oxidant, induced a fast mitochondrial Ca(2+) efflux associated with a release of state-4 respiration, a disruption of Deltapsi and a large amplitude mitochondrial swelling. This is compatible with the opening of the high conductance state of the PTP and was associated with extensive oxidation of pyridine nucleotides. Interestingly, the addition of carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone to the acetoacetate experiment promoted a fast shift from the low to the high conductance state of the PTP. Both acetoacetate and diamide-induced mitochondrial permeabilization were inhibited by exogenous catalase. We propose that the shift from a low to a high conductance state of the PTP can be promoted by the oxidation of NADPH. This impairs the antioxidant function of the glutathione reductase/peroxidase system, strongly strengthening the state of mitochondrial oxidative stress.

Mitochondrial transport of cations: channels, exchangers, and permeability transition

This review provides a selective history of how studies of mitochondrial cation transport (K+, Na+, Ca2+) developed in relation to the major themes of research in bioenergetics. It then covers in some detail specific transport pathways for these cations, and it introduces and discusses open problems about their nature and physiological function, particularly in relation to volume regulation and Ca2+ homeostasis. The review should provide the basic elements needed to understand both earlier mitochondrial literature and current problems associated with mitochondrial transport of cations and hopefully will foster new interest in the molecular definition of mitochondrial cation channels and exchangers as well as their roles in cell physiology.

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.

Noxious effects of oxygen reactive species on energy-coupling processes in Ehrlich ascites tumor mitochondria and the protection by pantothenic acid

Irradiation of Ehrlich ascites tumor cells with ultraviolet light or exposure to the Fenton reaction results in lesions in the mitochondrial energy-coupling system. Formation of the membrane potential and its utilization for ATP synthesis are more affected than the respiratory chain. Preincubation of the cells with pantothenic acid or its derivatives which can serve as precursors of CoA largely protects against the damage of mitochondrial energetics by oxygen reactive species formed by UV light or the Fenton reaction. Incubation of Ehrlich ascites tumor cells with pantothenic acid increases their content of glutathione (most of which is present in the reduced form) by 40%. It is concluded that the protective effect of precursors of CoA against lesions of the mitochondrial energy-coupling system by oxygen reactive species is mainly due to removal of free radicals and peroxides by glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase.

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.

Involvement of the ADP/ATP carrier in permeabilization processes of the inner mitochondrial membrane

The effect of different agents on inner-mitochondrial-membrane permeabilization and lipoperoxidation induced by Ca2+ and the pyridine-nucleotide oxidant t-butylhydroperoxide or inorganic phosphate was investigated. Comparing the protection conferred by ADP, a substrate of the ADP/ATP carrier, dithiothreitol, a disulfide reductant and butylhydroxytoluene, a radical scavenger, it was found that ADP was always the most effective against mitochondrial damage, when present in the incubation medium from the beginning. Moreover, carboxyatractyloside, a specific inhibitor of the ADP/ATP carrier, abolished completely the protective effect of ADP on both the lipoperoxidation and mitochondrial swelling processes. Experiments where deenergized mitochondria were previously incubated with Ca2+ showed a decrease in the content of active ADP/ATP carrier, indicating a direct involvement of this protein in the formation of a non-specific Ca(2+)-dependent pore. Our results also eliminate the possibility of an attack of oxygen radicals on lipids or proteins of the mitochondrial membrane as the primary event triggering the permeability transition of the inner mitochondrial membrane.

Protective effect of trifluoperazine on the mitochondrial damage induced by Ca2+ plus prooxidants

Isolated rat liver mitochondria undergo extensive swelling and disruption of membrane potential when they accumulate Ca2+ in the presence of a prooxidant such as diamide or t-butylhydroperoxide. The phenothiazinic drug trifluoperazine, at concentrations (15-35 microM) which do not inhibit respiration or the influx of Ca2+ into mitochondria, significantly protected mitochondria against the deleterious effects of Ca2+ plus a prooxidant. In contrast, at concentrations higher than 100 microM the drug potentiated these deleterious effects of Ca2+ and prooxidants and had a damaging effect per se on the inner mitochondrial membrane. It is proposed that the protection conferred by the drug is mediated by changes in membrane protein structure that decrease the production of protein thiol cross-linkings which occur when mitochondria accumulate calcium under oxidant stress conditions.

Alterations to the permeability of plant and animal mitochondria submitted to Ca2+ releasing agents

1. Mitochondria from different rat tissues and from plants were compared as regards their sensitivity towards Ca2+ in the presence of different Ca2+ releasing agents, and the phospholipase A2 activity was evaluated in the different mitochondrial preparations. 2. The mitochondria were exposed to Ca2+ and an oxidant such as t-butylhydroperoxide or diamide or to Ca2+ and inorganic phosphate, and plant mitochondria were seen to be much more resistant than liver, brain or kidney mitochondria of rats to the deleterious effects of these agents. 3. The phospholipase A2 activity is not directly involved in the alterations of the mitochondrial inner membrane permeability within the first 10 min of incubation under our experimental conditions. 4. The protection conferred by ATP and Mg2+ against Ca2+ efflux from mitochondria or the decrease in the mitochondrial transmembrane electrical potential was also observed under our experimental conditions, but cannot be attributed to an enhancement of the reacylation of lysophospholipids resulting from the phospholipase A2 activity.

A mechanism of latent cryoinjury and reparation of mitochondria

Changes in the systems of oxidative phosphorylation and ion transport of rat liver mitochondria have been studied during storage or incubation after freeze-thawing. It has been found that two different processes take place in frozen-thawed mitochondria, one of them tends toward resealing membrane defects and is accompanied by a partial reparation of function; and another one leads to a decrease of the membrane potential, release of K+ and Ca2+ from the matrix, accumulation of lipid peroxides, and uncoupling of oxidative phosphorylation (latent cryoinjury). The latent cryoinjury appears as a result of oxidation of endogenous pyridine nucleotides under conditions of high permeability of the inner membrane and low membrane potential, thus causing activation of the membrane lipid peroxidation and enzyme hydrolysis, leakage of cations, and deenergization of mitochondria. Inhibition of the latent cryoinjury favors the restoration of mitochondrial function after freeze-thawing.

Alterations in mitochondrial Ca2+ flux by the antibiotic X-537A (lasalocid-A)

A previous communication (Pereira da Silva, L., Bernardes, C.F. and Vercesi, A.E. (1984) Biochem. Biophys. Res. Commun. 124, 80-86) presented evidence that lasalocid-A, at concentrations far below those required to act as a Ca2+ ionophore, significantly inhibits Ca2+ efflux from liver mitochondria. In the present work we have studied the mechanism of this inhibition in liver and heart mitochondria. It was observed that lasalocid-A (25-250 nM), like nigericin, promotes the electroneutral exchange of K+ for H+ across the inner mitochondrial membrane and as a consequence can cause significant alterations in delta pH and delta psi. An indirect effect of these changes that might lead to inhibition of mitochondrial Ca2+ release was ruled out by experiments showing that the three observed patterns of lasalocid-A effect depend on the size of the mitochondrial Ca2+ load. At low Ca2+ loads (5-70 nmol Ca2+/mg protein), under experimental conditions in which Ca2+ release is supposed to be mediated by a Ca2+/2H+ antiporter, the kinetic data indicate that lasalocid-A inhibits the efflux of the cation by a competitive mechanism. The Ca2+/2Na+ antiporter, the dominant pathway for Ca2+ efflux from heart mitochondria, is not affected by lasalocid-A. At intermediate Ca2+ loads (70-110 nmol Ca2+/mg protein), lasalocid-A slightly stimulates Ca2+ release. This effect appears to be due to an increase in membrane permeability caused by the displacement of a pool of membrane bound Mg2+ possibly involved in the maintenance of membrane structure. Finally, at high Ca2+ loads (110-140 nmol Ca2+/mg protein) lasalocid-A enhances Ca2+ retention by liver mitochondria even in the presence of Ca2(+)-releasing agents such as phosphate and oxidants of the mitochondrial pyridine nucleotides. The maintenance of a high membrane potential under these conditions may indicate that lasalocid-A is a potent inhibitor of the Ca2(+)-induced membrane permeabilization. Nigericin, whose chemical structure resembles that of lasalocid-A, caused similar results.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

Control of pyruvate carboxylase activity by the pyridine-nucleotide redox state in mitochondria from rat liver

Pyruvate carboxylation by isolated mitochondria from rat liver is inhibited by t-butylhydroperoxide in a fully reversible manner. The rate of malate formation at 10 mM pyruvate was decreased by some 80% by 30 microM t-butylhydroperoxide. The effective peroxide concentration was dependent on the mitochondrial hydrogen supply, being increased to about 120 microM in the presence of 50 microM palmitoylcarnitine. Regarding the mechanism(s) of the t-butylhydroperoxide action, pyruvate transport and intramitochondrial energy or activator supply are unlikely involved, because the effect also took place with alanine as the substrate and was not accompanied by a change in the intramitochondrial levels of adenine nucleotides and acetyl-CoA respectively. However, t-butylhydroperoxide caused a rapid fall in the 3-hydroxybutyrate/acetoacetate ratio and a marked increase in the oxidized glutathione content. Therefore, experiments were designed to disclose the participation of the respective redox couples in the expression of pyruvate carboxylase activity. From measurements of NADPH, NADH, oxidized and reduced glutathione contents of mitochondria incubated under a variety of conditions, evidence has been obtained indicating that the mitochondrial NADH supply represents an important factor in the regulation of pyruvate carboxylase activity. The results presented seemingly provide a new basis for the understanding of the functional relationship between beta-oxidation and pyruvate carboxylation.

Quinone toxicity in hepatocytes: studies on mitochondrial Ca2+ release induced by benzoquinone derivatives

Hepatocyte cytotoxicity caused by substituted benzoquinones was associated with increased cytosolic Ca2+ concentration. p-Benzoquinone-induced hepatotoxicity was enhanced when the hepatocytes were loaded with Ca2+ by preincubation with ATP. A similar order of potency of the substituted benzoquinones in releasing Ca2+ from isolated mitochondria and inducing hepatocyte cytotoxicity was found; in decreasing order, this was 2-Br-, unsubstituted-, 2-CH3-, 2,6-(CH3O)2-, 2,6-(CH3)2-, 2,5-(CH3)2-, 2,3,5-(CH3)3-, and 2,3,5,6-(CH3)4-benzoquinones (duroquinone). The cellular products of quinone metabolism, hydroquinones and glutathione conjugates, did not cause mitochondrial Ca2+ release. Benzoquinone-induced mitochondrial Ca2+ release was preceded by GSH conjugate formation and NAD(P)H oxidation but followed by mitochondrial swelling. With duroquinone, a slow GSH and NADPH oxidation preceded Ca2+ release, but GSH oxidation did not occur with Se-deficient mitochondria lacking glutathione peroxidase activity. Cyanide-insensitive respiration was also observed with duroquinone but not with benzoquinone, suggesting that duroquinone undergoes redox cycling. GSH was depleted by both arylation and oxidation with 2,6-(CH3O)2-, 2,6-(CH3)2-, 2,5(CH3)2-, and 2,3,5-(CH3)3-benzoquinones. Benzoquinone concentrations that totally depleted GSH did not cause Ca2+ release until intramitochondrial NAD(P)H was oxidized. Ca2+ release was also prevented when NAD(P)H generation was stimulated by the presence of isocitrate or 3-hydroxybutyrate. This suggests that mitochondrial Ca2+ release is associated with NAD(P)H oxidation catalyzed by NADH dehydrogenase with benzoquinone or by the glutathione peroxidase-glutathione reductase system with duroquinone.

The effect of a ferric iron complex on isolated rat-liver mitochondria. III. Mechanistic aspects of iron-induced calcium efflux

Addition of iron(III)-gluconate complex to isolated rat liver mitochondria induced a net efflux of Ca2+ which was not inhibited by ruthenium red. This process resulted in the enhancement of Ca2+ cycling and a consequent membrane potential drop. Under these experimental conditions the content of mitochondrial glutathione did not appear to be critically modified, whereas an extensive oxidation of mitochondrial pyridine nucleotides was parallelly detected. Iron failed to induce appreciable changes in the oxidation level of pyridine nucleotides in mitochondria isolated from rats fed a selenium deficient diet, a condition in which mitochondrial glutathione peroxidase resulted inhibited by 80%. The iron-induced Ca2+ release in Se-deficient mitochondria appeared largely delayed and the membrane potential of these mitochondrial did not present gross alterations. Iron was also found to induce a transient increase in the mitochondrial cyanide-insensitive oxygen consumption. This effect was largely prevented by the addition of the hydrogen peroxide scavenger catalase. It was concluded that iron induced the activation of a specific Ca2+ efflux pathway via the oxidation of pyridine nucleotides due to the hydrogen peroxide metabolism by glutathione enzyme system.

The participation of NADP, the transmembrane potential and the energy-linked NAD(P) transhydrogenase in the process of Ca2+ efflux from rat liver mitochondria

The pyridine nucleotide specificity, the participation of delta psi, and the energy-linked transhydrogenase in the process of Ca2+ efflux stimulated by the oxidized state of NAD(P) were examined in rat liver mitochondria energized by ascorbate + TMPD. The following observations were made: The Ca2+ efflux rate is independent of the redox state of mitochondrial NAD, but is at a minimum when mitochondrial NADP is in the reduced state and accelerated several-fold when it is in the oxidized state. When the redox state of NADP is shifted to a more oxidized state, the steady-state level of Ca2+ in the medium increased and delta psi decreased in proportion to the mitochondrial NADP+ level. The activity of the energy-linked NAD(P) transhydrogenase seems to be a key element in determining the redox state of NADP and thus of Ca2+ retention and efflux from mitochondria.