Formation and disappearance of an endogenous uncoupling factor during swelling and contraction of mitochondria

Analysis of arsenic-modulated expression of hypothalamic estrogen receptor, thyroid receptor, and peroxisome proliferator-activated receptor gamma mRNA and simultaneous mitochondrial morphology and respiration rates in the mouse

Arsenic has been identified as an environmental toxicant acting through various mechanisms, including the disruption of endocrine pathways. The present study assessed the ability of a single intraperitoneal injection of arsenic, to modify the mRNA expression levels of estrogen- and thyroid hormone receptors (ERα,β; TRα,β) and peroxisome proliferator-activated receptor gamma (PPARγ) in hypothalamic tissue homogenates of prepubertal mice in vivo. Mitochondrial respiration (MRR) was also measured, and the corresponding mitochondrial ultrastructure was analyzed. Results show that ERα,β, and TRα expression was significantly increased by arsenic, in all concentrations examined. In contrast, TRβ and PPARγ remained unaffected after arsenic injection. Arsenic-induced dose-dependent changes in state 4 mitochondrial respiration (St4). Mitochondrial morphology was affected by arsenic in that the 5 mg dose increased the size but decreased the number of mitochondria in agouti-related protein- (AgRP), while increasing the size without affecting the number of mitochondria in pro-opiomelanocortin (POMC) neurons. Arsenic also increased the size of the mitochondrial matrix per host mitochondrion. Complex analysis of dose-dependent response patterns between receptor mRNA, mitochondrial morphology, and mitochondrial respiration in the neuroendocrine hypothalamus suggests that instant arsenic effects on receptor mRNAs may not be directly reflected in St3-4 values, however, mitochondrial dynamics is affected, which predicts more pronounced effects in hypothalamus-regulated homeostatic processes after long-term arsenic exposure.

The Haves and Have-Nots: The Mitochondrial Permeability Transition Pore across Species

The demonstration that F1FO (F)-ATP synthase and adenine nucleotide translocase (ANT) can form Ca2+-activated, high-conductance channels in the inner membrane of mitochondria from a variety of eukaryotes led to renewed interest in the permeability transition (PT), a permeability increase mediated by the PT pore (PTP). The PT is a Ca2+-dependent permeability increase in the inner mitochondrial membrane whose function and underlying molecular mechanisms have challenged scientists for the last 70 years. Although most of our knowledge about the PTP comes from studies in mammals, recent data obtained in other species highlighted substantial differences that could be perhaps attributed to specific features of F-ATP synthase and/or ANT. Strikingly, the anoxia and salt-tolerant brine shrimp Artemia franciscana does not undergo a PT in spite of its ability to take up and store Ca2+ in mitochondria, and the anoxia-resistant Drosophila melanogaster displays a low-conductance, selective Ca2+-induced Ca2+ release channel rather than a PTP. In mammals, the PT provides a mechanism for the release of cytochrome c and other proapoptotic proteins and mediates various forms of cell death. In this review, we cover the features of the PT (or lack thereof) in mammals, yeast, Drosophila melanogaster, Artemia franciscana and Caenorhabditis elegans, and we discuss the presence of the intrinsic pathway of apoptosis and of other forms of cell death. We hope that this exercise may help elucidate the function(s) of the PT and its possible role in evolution and inspire further tests to define its molecular nature.

Calcium Overload and Mitochondrial Metabolism

Mitochondria calcium is a double-edged sword. While low levels of calcium are essential to maintain optimal rates of ATP production, extreme levels of calcium overcoming the mitochondrial calcium retention capacity leads to loss of mitochondrial function. In moderate amounts, however, ATP synthesis rates are inhibited in a calcium-titratable manner. While the consequences of extreme calcium overload are well-known, the effects on mitochondrial function in the moderately loaded range remain enigmatic. These observations are associated with changes in the mitochondria ultrastructure and cristae network. The present mini review/perspective follows up on previous studies using well-established cryo-electron microscopy and poses an explanation for the observable depressed ATP synthesis rates in mitochondria during calcium-overloaded states. The results presented herein suggest that the inhibition of oxidative phosphorylation is not caused by a direct decoupling of energy metabolism via the opening of a calcium-sensitive, proteinaceous pore but rather a separate but related calcium-dependent phenomenon. Such inhibition during calcium-overloaded states points towards mitochondrial ultrastructural modifications, enzyme activity changes, or an interplay between both events.

Modulation and Pharmacology of the Mitochondrial Permeability Transition: A Journey from F-ATP Synthase to ANT

The permeability transition (PT) is an increased permeation of the inner mitochondrial membrane due to the opening of the PT pore (PTP), a Ca²⁺-activated high conductance channel involved in Ca²⁺ homeostasis and cell death. Alterations of the PTP have been associated with many pathological conditions and its targeting represents an incessant challenge in the field. Although the modulation of the PTP has been extensively explored, the lack of a clear picture of its molecular nature increases the degree of complexity for any target-based approach. Recent advances suggest the existence of at least two mitochondrial permeability pathways mediated by the F-ATP synthase and the ANT, although the exact molecular mechanism leading to channel formation remains elusive for both. A full comprehension of this to-pore conversion will help to assist in drug design and to develop pharmacological treatments for a fine-tuned PT regulation. Here, we will focus on regulatory mechanisms that impinge on the PTP and discuss the relevant literature of PTP targeting compounds with particular attention to F-ATP synthase and ANT.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

The mitochondrial permeability transition pore: molecular nature and role as a target in cardioprotection

The mitochondrial permeability transition (PT) - an abrupt increase permeability of the inner membrane to solutes - is a causative event in ischemia-reperfusion injury of the heart, and the focus of intense research in cardioprotection. The PT is due to opening of the PT pore (PTP), a high conductance channel that is critically regulated by a variety of pathophysiological effectors. Very recent work indicates that the PTP forms from the F-ATP synthase, which would switch from an energy-conserving to an energy-dissipating device. This review provides an update on the current debate on how this transition is achieved, and on the PTP as a target for therapeutic intervention. This article is part of a Special Issue entitled "Mitochondria: from basic mitochondrial biology to cardiovascular disease".

Taking control over intracellular fatty acid levels is essential for the analysis of thermogenic function in cultured primary brown and brite/beige adipocytes

Thermogenesis in brown adipocytes, conferred by mitochondrial uncoupling protein 1 (UCP1), is receiving great attention because metabolically active brown adipose tissue may protect humans from metabolic diseases. In particular, the thermogenic function of brown-like adipocytes in white adipose tissue, known as brite (or beige) adipocytes, is currently of prime interest. A valid procedure to quantify the specific contribution of UCP1 to thermogenesis is thus of vital importance. Adrenergic stimulation of lipolysis is a common way to activate UCP1. We here report, however, that in this frequently applied setup, taking control over intracellular fatty acid levels is essential for the analysis of thermogenic function in cultured brown and brite adipocytes. By the application of these findings, we demonstrate that UCP1 is functionally thermogenic in intact brite adipocytes and adrenergic UCP1 activation is largely dependent on adipose triglyceride lipase (ATGL) rather than hormone sensitive lipase (HSL).

Regulation of the Ca(2+)-independent phospholipase A2 in liver mitochondria by changes in the energetic state

The effect of electron transport chain redox status on activity of the mitochondrial Ca(2+)-independent phospholipase A2 (iPLA2) has been examined. When oxidizing NAD-linked substrates, the enzyme is not active unless deenergization occurs. Uncoupler, rotenone, antimycin A, and cyanide are equally effective at upregulating the enzyme, while oligomycin is ineffective. Thenoyltrifluoroacetone causes deenergization and activates the enzyme, but only if succinate is the respiratory substrate. These findings show that the mitochondrial iPLA2 responds to the energetic state overall, rather than to the redox status of individual electron transport chain complexes. With NAD-linked substrates, and using rotenone to deenergize, iPLA2 activation can be reversed by adding succinate to reestablish a membrane potential. For this purpose, ascorbate plus N,N,N'N'-tetramethyl-phenylenediamine can be used instead of succinate and is equally effective. With succinate as substrate, the membrane potential can be reduced in a graded and stable fashion by adding increasing concentrations of malonate, which is a competitive inhibitor of succinate utilization. A partial and stable activation of the iPLA2 accompanies partial deenergization. These findings suggest that in addition to the several functions that have been proposed, the mitochondrial iPLA2 may help to coordinate local capillary blood flow with changing energy demands.

The mitochondrial permeability transition pore: a mystery solved?

The permeability transition (PT) denotes an increase of the mitochondrial inner membrane permeability to solutes with molecular masses up to about 1500 Da. It is presumed to be mediated by opening of a channel, the permeability transition pore (PTP), whose molecular nature remains a mystery. Here I briefly review the history of the PTP, discuss existing models, and present our new results indicating that reconstituted dimers of the F_OF₁ ATP synthase form a channel with properties identical to those of the mitochondrial megachannel (MMC), the electrophysiological equivalent of the PTP. Open questions remain, but there is now promise that the PTP can be studied by genetic methods to solve the large number of outstanding problems.

The interplay of mitochondria with calcium: an historical appraisal

Indirect findings in the 1950s had indicated that mitochondria could accumulate Ca(2+), but only in 1961 isolated mitochondria were directly shown to take it up in a process driven by the activity of the respiratory chain or by the hydrolysis of added ATP. The uptake of Ca(2+) could be accompanied by the simultaneous uptake of inorganic phosphate, leading to the precipitation of hydroxyapatite in the matrix and to the effective buffering of the free Ca(2+) concentration in it. The uptake of Ca(2+) occurred via an electrophoretic uniporter that has been molecularly identified only recently. Ca(2+) was then released through a Na(+)/Ca(2+) exchanger that has also been identified very recently (a H(+)/Ca(2+) antiporter has also been described in some mitochondrial types). In the matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated by Ca(2+), providing a rationale for the Ca(2+) cycling process. The affinity of the uptake uniporter was found to be too low to efficiently regulate Ca(2+) in the low to mid nM concentration in the cytosol. However, a number of findings showed that energy linked transport of Ca(2+) did nevertheless occur in mitochondria in situ. The enigma was solved in the 1990s, when it was found that perimitochondrial Ca(2+) pools are created by the discharge of Ca(2+) from vicinal endoplasmic reticulum stores in which the concentration of Ca(2+) is high enough to satisfy the poor affinity of the uniporter. Thus, mitochondria have now regained a key role in the regulation of cytosolic Ca(2+) (not only of their own internal Ca(2+)).

The mitochondrial permeability transition from yeast to mammals

Regulated permeability changes have been detected in mitochondria across species. We review here their key features, with the goal of assessing whether a "permeability transition" similar to that observed in higher eukaryotes is present in other species. The recent discoveries (i) that treatment with cyclosporin A (CsA) unmasks an inhibitory site for inorganic phosphate (Pi) [Basso, E., Petronilli, V., Forte, M.A. and Bernardi, P. (2008) Phosphate is essential for inhibition of the mitochondrial permeability transition pore by cyclosporin A and by cyclophilin D ablation. J. Biol. Chem. 283, 26307-26311], the classical inhibitor of the permeability transition of yeast and (ii) that under proper experimental conditions a matrix Ca(2+)-dependence can be demonstrated in yeast as well [Yamada, A., Yamamoto, T., Yoshimura, Y., Gouda, S., Kawashima, S., Yamazaki, N., Yamashita, K., Kataoka, M., Nagata, T., Terada, H., Pfeiffer, D.R. and Shinohara Y. (2009) Ca(2+)-induced permeability transition can be observed even in yeast mitochondria under optimized experimental conditions. Biochim. Biophys. Acta 1787, 1486-1491] suggest that the mitochondrial permeability transition has been conserved during evolution.

The fateful encounter of mitochondria with calcium: how did it happen?

A number of findings in the 1950s had offered indirect indications that mitochondria could accumulate Ca2+. In 1961, the phenomenon was directly demonstrated using isolated mitochondria: the uptake process was driven by respiratory chain activity or by the hydrolysis of added ATP. It could be accompanied by the simultaneous uptake of inorganic phosphate, in which case precipitates of hydroxyapatite were formed in the matrix, buffering its free Ca2+ concentration. The properties of the uptake process were established in the 1960s and 1970s: the uptake of Ca2+ occurred electrophoretically on a carrier that has not yet been molecularly identified, and was released from mitochondria via a Na+/Ca2+ antiporter. A H+/Ca2+ release exchanger was also found to operate in some mitochondrial types. The permeability transition pore was later also found to mediate the efflux of Ca2+ from mitochondria. In the mitochondrial matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated in the matrix by the cycling of Ca2+ across the inner membrane. In conditions of cytoplasmic Ca2+ overload mitochondria could store for a time large amounts of precipitated Ca2+-phosphate, thus permitting cells to survive situations of Ca2+ emergency. The uptake process was found to have very low affinity for Ca2+: since the bulk concentration of Ca2+ in the cytoplasm is in the low to mid-nM range, it became increasingly difficult to postulate a role of mitochondria in the regulation of cytoplsmic Ca2+. A number of findings had nevertheless shown that energy linked Ca2+ transport occurred efficiently in mitochondria of various tissues in situ. The paradox was only solved in the 1990s, when it was found that the concentration of Ca2+ in the cytoplasm is not uniform: perimitochondrial micropools are created by the agonist-promoted discharge of Ca2+ from vicinal stores in which the concentration of Ca2+ is high enough to activate the low affinity mitochondrial uniporter. Mitochondria thus regained center stage as important regulators of cytoplasmic Ca2+ (not only of their own internal Ca2+). Their Ca2+ uptake systems was found to react very rapidly to cytoplasmic Ca2+ demands, even in the 150-200 msec time scale of processes like the contraction and relaxation of heart. An important recent development in the area of mitochondrial Ca2+ transport is its involvement in the disease process. Ca2+ signaling defects are now gaining increasing importance in the pathogenesis of diseases, e.g., neurodegenerative diseases. Since mitochondria have now regained a central role in the regulation of cytoplasmic Ca2+, dysfunctions of their Ca2+ controlling systems have expectedly been found to be involved in the pathogenesis of numerous disease processes.

L-Carnitine suppresses oleic acid-induced membrane permeability transition of mitochondria

Membrane permeability transition (MPT) of mitochondria has an important role in apoptosis of various cells. The classic type of MPT is characterized by increased Ca(2+) transport, membrane depolarization, swelling, and sensitivity to cyclosporin A. In this study, we investigated whether L-carnitine suppresses oleic acid-induced MPT using isolated mitochondria from rat liver. Oleic acid-induced MPT in isolated mitochondria, inhibited endogenous respiration, caused membrane depolarization, and increased large amplitude swelling, and cytochrome c (Cyt. c) release from mitochondria. L-Carnitine was indispensable to beta-oxidation of oleic acid in the mitochondria, and this reaction required ATP and coenzyme A (CoA). In the presence of ATP and CoA, L-carnitine stimulated oleic acid oxidation and suppressed the oleic acid-induced depolarization, swelling, and Cyt. c release. L-Carnitine also contributed to maintaining mitochondrial function, which was decreased by the generation of free fatty acids with the passage of time after isolation. These results suggest that L-carnitine acts to maintain mitochondrial function and suppresses oleic acid-mediated MPT through acceleration of beta-oxidation.

Evidence for involvement of nonesterified fatty acid-induced protonophoric uncoupling during mitochondrial dysfunction caused by hypoxia and reoxygenation

BACKGROUND

Proximal tubules subjected to hypoxia in vitro under conditions relevant to ischaemia in vivo develop an energetic deficit that is not corrected even after full reoxygenation. We have provided evidence that accumulation of nonesterified fatty acids (NEFA) is the primary reason for this energetic deficit. In this study, we have further investigated the mechanism for the NEFA-induced energetic deficit.

METHODS

Mitochondrial membrane potential (Deltapsi) was measured in digitonin-permeabilized, freshly isolated proximal tubules by safranin O uptake. Addition of the potassium/proton exchanger nigericin enables the determination of the mitochondrial proton motive force (Deltap) and the proton gradient (DeltapH). ATP was measured luminometrically and NEFA colorimetrically.

RESULTS

Tubule ATP content was depleted after hypoxia and recovered incompletely, even after full reoxygenation. Mitochondrial safranin O uptake was decreased in proximal tubules after hypoxia and reoxygenation (H/R). This decrease was attenuated by delipidated bovine serum albumin (dBSA) or citrate. Addition of nigericin increased safranin O uptake of mitochondria in normoxic proximal tubules, but not in proximal tubules after H/R. Addition of dBSA restored the effect of nigericin to increase mitochondrial safranin O uptake. Addition of the NEFA oleate had the same impact on mitochondrial safranin O uptake as subjecting proximal tubules to H/R.

CONCLUSION

The mechanism of the NEFA-induced energetic deficit in freshly isolated rat proximal tubules induced by H/R is characterized by impaired ATP production after full reoxygenation, impaired recovery of Deltapsi and Deltap, abrogation of DeltapH and sensitivity to citrate, consistent with involvement of the tricarboxylate carrier. The data support the concept that protonophoric uncoupling by NEFA movement on anion carriers plays a critical role in proximal tubule mitochochondrial dysfunction after H/R.

The mitochondrial permeability transition from in vitro artifact to disease target

The mitochondrial permeability transition pore is a high conductance channel whose opening leads to an increase of mitochondrial inner membrane permeability to solutes with molecular masses up to approximately 1500 Da. In this review we trace the rise of the permeability transition pore from the status of in vitro artifact to that of effector mechanism of cell death. We then cover recent results based on genetic inactivation of putative permeability transition pore components, and discuss their meaning for our understanding of pore structure. Finally, we discuss evidence indicating that the permeability transition pore plays a role in pathophysiology, with specific emphasis on in vivo models of disease.

Accumulation of nonesterified fatty acids causes the sustained energetic deficit in kidney proximal tubules after hypoxia-reoxygenation

Kidney proximal tubules exhibit decreased ATP and reduced, but not absent, mitochondrial membrane potential (Deltapsi(m)) during reoxygenation after severe hypoxia. This energetic deficit, which plays a pivotal role in overall cellular recovery, cannot be explained by loss of mitochondrial membrane integrity, decreased electron transport, or compromised F1F0-ATPase and adenine nucleotide translocase activities. Addition of oleate to permeabilized tubules produced concentration-dependent decreases of Deltapsi(m) measured by safranin O uptake (threshold for oleate = 0.25 microM, 1.6 nmol/mg protein; maximal effect = 4 microM, 26 nmol/mg) that were reversed by delipidated BSA (dBSA). Cell nonesterified fatty acid (NEFA) levels increased from <1 to 17.4 nmol/mg protein during 60- min hypoxia and remained elevated at 7.6 nmol/mg after 60 min reoxygenation, at which time ATP had recovered to only 10% of control values. Safranin O uptake in reoxygenated tubules, which was decreased 85% after 60-min hypoxia, was normalized by dBSA, which improved ATP synthesis as well. dBSA also almost completely normalized Deltapsi(m) when the duration of hypoxia was increased to 120 min. In intact tubules, the protective substrate combination of alpha-ketoglutarate + malate (alpha-KG/MAL) increased ATP three- to fourfold, limited NEFA accumulation during hypoxia by 50%, and lowered NEFA during reoxygenation. Notably, dBSA also improved ATP recovery when added to intact tubules during reoxygenation and was additive to the effect of alpha-KG/MAL. We conclude that NEFA overload is the primary cause of energetic failure of reoxygenated proximal tubules and lowering NEFA substantially contributes to the benefit from supplementation with alpha-KG/MAL.

Mitochondrial energy dissipation by fatty acids. Mechanisms and implications for cell death

For most cell types, fatty acids are excellent respiratory substrates. After being transported across the outer and inner mitochondrial membranes they undergo beta-oxidation in the matrix and feed electrons into the mitochondrial energy-conserving respiratory chain. On the other hand, fatty acids also physically interact with mitochondrial membranes, and possess the potential to alter their permeability. This occurs according to two mechanisms: an increase in proton conductance of the inner mitochondrial membrane and the opening of the permeability transition pore, an inner membrane high-conductance channel that may be involved in the release of apoptogenic proteins into the cytosol. This article addresses in some detail the mechanisms through which fatty acids exert their protonophoric action and how they modulate the permeability transition pore and discusses the cellular effects of fatty acids, with specific emphasis on their role as potential mitochondrial mediators of apoptotic signaling.

Roles of long chain fatty acids and carnitine in mitochondrial membrane permeability transition

Palmitoyl-CoA (Pal-CoA) lowered the respiratory control ratio (RCR), and induced mitochondrial membrane permeability transition (MPT) and cytochrome c (Cyt. c) release from isolated rat liver mitochondria. L-Carnitine suppressed the Pal-CoA-induced dysfunction, MPT, and Cyt. c release of isolated mitochondria. This suppression was inhibited by cephaloridine, an inhibitor of carnitine uptake into mitochondria. Cyclosporin A (CsA), an inhibitor of MPT, and BSA also suppressed the Pal-CoA-induced MPT. In the presence of inorganic phosphate (P(i)), Ca2+-induced MPT was suppressed by BSA, L-carnitine, and chlorpromazine, an inhibitor of phospholipase A2. In the presence of a low concentration of Ca2+, 3,3',5-triiodothyronine, long chain fatty acids, salicylic acid, and diclofenac induced MPT by a mechanism that was suppressed by BSA, L-carnitine, or chlorpromazine. During the incubation of mitochondria on ice, their respiratory competence decreased; L-carnitine and BSA also prevented this decrease. Mitochondrial depolarization in pheochromocytoma PC12 cells was induced by either serum deprivation or arachidonic acid by a mechanism that was suppressed by acetyl-L-carnitine. These results indicate that some MPTs may be regulated by fatty acid metabolism and that the Pal-CoA-induced MPT plays an important role in the induction of apoptosis.

Suppression of T(3)- and fatty acid-induced membrane permeability transition by L-carnitine

Cytochrome c (Cyt. c) is known to be released from the mitochondria into the cytosol by means of the membrane permeability transition (MPT) mechanism, thereby activating caspase cascade activity, and inducing cell apoptosis. Recently we reported that L-carnitine suppressed palmitoyl-CoA-induced MPT as well as apoptosis in some cell types (Biochem. Pharmacol, in press). In the present study T(3) was found to induce MPT and Cyt. c release, while cyclosporin A (CsA), bovine serum albumin (BSA) and L-carnitine were found to inhibit this action in a concentration-dependent manner. Similarly, long chain fatty acid (LCFA) also induced MPT and Cyt. c release, which was then inhibited by CsA, BSA and L-carnitine. From these results the authors postulate that T(3)-induced MPT is in part regulated by fatty acid metabolism through a dynamic balance between LCFAs and L-carnitine.

Effects of cresols (o-, m-, and p-isomers) on the bioenergetic system in isolated rat liver mitochondria

It is known that o-, m- and p-cresols exert a toxic effect on rat liver cells. However, there is little information on the mechanism for the hepatotoxicity of cresols. We, therefore, investigated the effects of o-, m-, and p-cresols on the bioenergetic system using isolated rat liver mitochondria. When o-, m- or p-cresol was added to liver mitochondria with glutamate or succinate at concentrations of 0.3 to 6.0 mumol/mg protein, each cresol isomer reduced the rate of state 3 respiration dose-dependently. Three cresol isomers at 6.0 mumol/mg protein each inhibited state 3 respiration in liver mitochondria with glutamate or succinate by about 60 or 20%, respectively. The three isomers affected NAD- and succinate-linked respirations in liver mitochondria, by which the respiratory control ratio was dose-dependently attenuated. The inhibitory effects of o-, m- and p-cresols on the NAD-linked respiration were stronger than those on the succinate-linked respiration. However, three cresol isomers had little effect on the P/O ratio in liver mitochondria with glutamate or succinate. Three cresol isomers at 15 mumol/mg protein each induced the swelling in the absence of Ca2+ in medium and accelerated the swelling of liver mitochondria in the presence of Ca2+ in medium. These results indicate that o-, m- and p-cresols inhibit liver mitochondrial respiration and induce or accelerate the swelling of liver mitochondria, and suggest that liver mitochondria may be one of the targets for the hepatotoxic actions of cresols.

Swelling of fish mitochondria

The physical properties of fish liver and rat liver mitochondria were compared as a function of temperature and osmotic pressure. The data indicate that fish mitochondria are more flexible and swell at a more rapid rate over a 0 to 30 degrees C temperature range, whereas the rates of swelling at 30 to 40 degrees C are comparable. The swelling rates of both fish and rat mitochondria vary with temperature and approximate the Arrhenius relationship. Apparent energies of activation for swelling averaged 26.5 kcal and 12.9 kcal for rat and fish, respectively. Fish mitochondria were less stable than rat mitochondria to osmotic variation, and the disparity in initial swelling rates became increasingly greater with lower osmotic pressure. The hypotonic swelling of both fish and rat mitochondria was readily reversed osmotically; however, there was a very rapid decay of reversal in fish mitochondria and only a very slow decay in the case of rat. All the data indicate that under comparable conditions the fish mitochondrial membranes are more flexible and presumably more permeable and labile than rat mitochondrial membranes. The findings are discussed in relation to the general metabolic implications and the possible contributions of the membrane constituents to membrane behavior.

Fatty acid-induced uncoupling of oxidative phosphorylation is partly due to opening of the mitochondrial permeability transition pore

Addition of myristate at low concentration (30-60 nmol/mg protein) to energized rat liver mitochondria resulted in dissipation of the electric membrane potential which, in Ca2+-free media, could be partly reversed by carboxyatractyloside but not by cyclosporin A. In contrast, in mitochondria preloaded with Ca2+ this energy-dissipating effect of fatty acid was partly prevented or reversed by cyclosporin A or ADP. In sucrose media, myristate, but not the protonophore carbonyl cyanide m-chlorophenylhydrazone, induced swelling of Ca2+-loaded mitochondria which was inhibited by cyclosporin A and ADP. We conclude that long-chain fatty acids may induce opening of the mitochondrial permeability transition pore not only because of their protonophoric effect mediated by mitochondrial anion carriers [Skulachev, V.P., FEBS Lett. 294 (1991) 158-162; Wieckowski, M.R. and Wojtczak, L., Biochem. Biophys. Res. Commun. (1997) 232, 414-417] but also by a direct interaction with the pore assembly.

Thyroxine induces cyclosporin A-insensitive, Ca2+-dependent reversible permeability transition pore in rat liver mitochondria

The effect of thyroxine on Ca2+-dependent mitochondrial permeability transition has been examined. It is shown that 40 microM thyroxine induces high amplitude swelling and decrease in membrane potential in Ca2+-loaded rat liver mitochondria, both in the presence and absence of cyclosporin A. Thyroxine-induced decrease in membrane potential is partially or completely reversed by addition of EGTA into the incubation medium. Nigericin and ADP are shown to prevent, or significantly delay, the effects of thyroxine on both mitochondrial swelling and membrane potential, whereas nicotinamide potentiates the permeabilisation of mitochondria. It is suggested that thyroxine induced reversible, cyclosporin A-insensitive permeability transition pore (PTP) opening in the inner mitochondrial membrane.

CALCIUM ION ACCUMULATION AND VOLUME CHANGES OF ISOLATED LIVER MITOCHONDRIA. REVERSAL OF CALCIUM ION-INDUCED SWELLING

1. The excessive accumulation of Ca(2+) by mitochondria suspended in an iso-osmotic buffered potassium chloride medium containing oxidizable substrate and phosphate led to extensive swelling and release of accumulated Ca(2+) from the mitochondria. When the Ca(2+) was removed from the medium by chelation with ethylene glycol bis(aminoethyl)tetra-acetate, the swelling was reversed in a respiration-dependent contraction. The contracted mitochondria were shown to have regained some degree of respiratory control. 2. The respiration-dependent contraction could be supported by electron transport through a restricted portion of the respiratory chain, and by substrates donating electrons at different levels in the respiratory chain. 3. Respiratory inhibitors appropriate to the substrate present completely inhibited the contraction. Uncoupling agents, and the inhibitors oligomycin and atractyloside, were without effect. 4. When the reversal of swelling had been prevented by respiratory inhibitors, the addition of ATP induced a contraction of the mitochondria. In the absence of added chelating agent the contraction was very slow. The ATP-induced contraction was completely inhibited by oligomycin and atractyloside, was incomplete in the presence of uncoupling agents and was unaffected by respiratory inhibitors. 5. The relationship between the energy requirements of respiration-dependent contraction and the requirements of ion transport and other contractile systems are discussed.

CALCIUM ION ACCUMULATION AND VOLUME CHANGES OF ISOLATED LIVER MITOCHONDRIA. CALCIUM ION-INDUCED SWELLING

1. Liver mitochondria suspended in an iso-osmotic buffered potassium chloride medium containing an oxidizable substrate and phosphate accumulated added Ca(2+). During this process H(+) appeared in the medium and the mitochondrial suspension showed increased light-scattering. Respiration was markedly stimulated. 2. The addition of excess of Ca(2+), respiratory inhibitors or uncoupling agents caused extensive mitochondrial swelling associated with release of Ca(2+) into the suspending medium. When the suspension became anaerobic extensive swelling also occurred. Only under conditions when the addition of uncoupling agents would have produced high rates of electron transport, e.g. in the presence of succinate, was the structural integrity of the mitochondrion maintained after Ca(2+) accumulation. 3. Conditions that prevented respiration-dependent Ca(2+) accumulation also prevented Ca(2+)-induced swelling. Bovine plasma albumin was without effect, indicating that U-factor was not involved. Oligomycin together with ADP or ATP partially stabilized the mitochondria against Ca(2+)-induced swelling. 4. It is suggested that a ;high-energy' intermediate generated by coupled electron transport is required to prevent the mitochondrial swelling that results as a consequence of Ca(2+) accumulation.

Effect of fatty acids on energy coupling processes in mitochondria

Long-chain fatty acids are natural uncouplers of oxidative phosphorylation in mitochondria. The protonophoric mechanism of this action is due to transbilayer movement of undissociated fatty acid in one direction and the passage of its anion in the opposite direction. The transfer of the dissociated form of fatty acid can be, at least in some kinds of mitochondrion, facilitated by adenine nucleotide translocase. Apart from dissipating the electrochemical proton gradient, long-chain fatty acids decrease the activity of the respiratory chain by mechanism(s) not fully understood. In intact cells and tissues fatty acids operate mostly as excellent respiratory substrates, providing electrons to the respiratory chain. This function masks their potential uncoupling effect which becomes apparent only under special physiological or pathological conditions characterized by unusual fatty acid accumulation. Short- and medium-chain fatty acids do not have protonophoric properties. Nevertheless, they contribute to energy dissipation because of slow intramitochondrial hydrolysis of their activation products, acyl-AMP and acyl-CoA. Long-chain fatty acids increase permeability of mitochondrial membranes to alkali metal cations. This is due to their ionophoric mechanism of action. Regulatory function of fatty acids with respect to specific cation channels has been postulated for the plasma membrane of muscle cells, but not demonstrated in mitochondria. Under cold stress, cold acclimation and arousal from hibernation the uncoupling effect of fatty acids may contribute to increased thermogenesis, especially in the muscle tissue. In brown adipose tissue, the special thermogenic organ of mammals, long-chain fatty acids promote operation of the unique natural uncoupling protein, thermogenin. As anionic amphiphiles, long-chain fatty acids increase the negative surface charge of biomembranes, thus interfering in their enzymic and transporting functions.

Loss of mitochondrial respiratory function and its suppression during cold ischemic preservation of rat livers with University of Wisconsin solution

Preservation of the liver involves a period of cold (0 degrees to 4 degrees C) ischemia; the longer the ischemic period, the greater the injury to the liver. The mechanisms for cold-induced ischemic injury are not known, but it is clear that after preservation the liver has a reduced capacity to regenerate high-energy phosphate compounds (ATP). One cause for the delayed rate of ATP synthesis could be injury to the mitochondria. The effects of long-term (more than 24 hr) preservation on liver mitochondrial function have not been previously studied. In this study, rat livers were preserved in University of Wisconsin solution at 4 degrees C for up to 96 hr. After preservation, mitochondrial respiratory function was assayed in a homogenate and in isolated mitochondria. We saw a progressive increase in oligomycin-sensitive respiration with time of preservation (from 1.2 +/- 0.09 mumol.min-1.gm tissue-1 at 0 hr to 3.8 +/- 0.2 mumol.min-1.gm tissue-1 after 96 hr). The increase after 24-hr preservation (2.1 +/- 0.2 mumol.min-1.gm tissue-1) was also significantly greater than 0 time values (p less than 0.05). No decrease was found in uncoupler-stimulated respiration for up to 48 hr of preservation; only a small decrease was seen after 72 hr of preservation (about 30%). The cause of the increase in oligomycin-sensitive respiration appeared to be related to free fatty acids (or another uncoupling factor) generated during preservation. This was suggested from the fact that bovine serum albumin prevented the increase in oligomycin-sensitive respiration after all periods of preservation.(ABSTRACT TRUNCATED AT 250 WORDS)

The protecting effect of L-carnitine on Ca(2+)-loaded rat liver mitochondria

It is shown that L-carnitine strongly increases the ability of rat liver mitochondria to respond to the train of Ca2+ additions by a transient stimulation of the State-4 respiration rate. Such an effect requires ATP and the L-carnitine efficiency strongly decreases when ATP is omitted. Oleate influences the mitochondria in a fashion opposite to that of L-carnitine. The oleate effect is strongly diminished by L-carnitine. Again, the L-carnitine effect requires ATP, and D-carnitine fails to substitute for L-carnitine. It is suggested that L-carnitine removes, in an ATP-dependent manner, endogenous or added fatty acids, which are involved in oxidative damage of Ca(2+)-loaded mitochondria.

A possible mechanism of mitochondrial dysfunction during cerebral ischemia: inhibition of mitochondrial respiration activity by arachidonic acid

The dramatic increase in the arachidonic acid (AA) level in the brain is a well-known molecular event during cerebral ischemia. As mitochondria are known to be one possible site of the cell damage, the effects of AA on the respiratory activity of rat brain mitochondria were investigated in vitro using an oxygen electrode. In NAD-linked respiration, respiratory control ratio was decreased significantly by AA, with an IC50 of 6.0 microM. AA had the dual effect on mitochondrial respiration, a decrease in state 3 and uncoupled state and an increase in state 4 (i.e., uncoupling) as reported by Hillered and Chan (J. Neurosci. Res. 19, 94-100, 1988). Furthermore, we found that other unsaturated long-chain free fatty acids (C18:1-C18:3, C20:1-C20:5) also showed such a dual effect. Cyclooxygenase metabolites of AA such as prostaglandins (D2, E2, F2 alpha, E1) and thromboxane B2, and lipoxygenase metabolites such as leukotrienes (D4, B4) and 5- or 12-hydroperoxyeicosatetraenoic acid had no significant effect. The inhibition of the uncoupled state by AA was more marked in NAD-linked than that in FAD-linked respiration, while the degree of uncoupling by AA were the same in both respirations. In spectrophotometrical measurement, the reduction of cytochromes and flavo-protein was markedly inhibited by AA in NAD-linked respiration, but not in the FAD-linked one. In addition, the activity of cytochrome c oxidase was scarcely inhibited by AA. These data suggest that AA itself, not its metabolites, may inhibit mitochondrial ATP production during brain ischemia and that AA may act on the site(s) closely related to NAD-linked respiration, but not the FAD-linked one, in addition to its uncoupling effect.

Brain mitochondrial swelling induced by arachidonic acid and other long chain free fatty acids

Polyunsaturated fatty acids (PUFAs), arachidonic acid in particular, are well known, potent inducers of edema in the brain, while monounsaturated and saturated long chain fatty acids do not possess this quality. This investigation has compared the ability of some free fatty acids (FFAs), known to be released during cerebral ischemia, to induce brain mitochondrial swelling in vitro. The PUFAs tested, especially arachidonic acid (20:4), were more potent in causing swelling than saturated or monounsaturated ones, as measured by the decrease in light absorbance of the mitochondrial suspension. This finding is in line with the unique potency of 20:4 to induce brain edema. Incubation of brain mitochondria with 20:4 for 20 min caused a dose-dependent swelling. ATP-MgCl2 both prevented and reversed this swelling, while binding of the 20:4 by the addition of bovine serum albumin could only prevent but not reverse the swelling. The contraction of the swollen mitochondria appeared to be mediated by a mechanism dependent upon high-energy phosphates, potentiated by MgCl2. The concentration of 20:4 required to induce swelling was about 20 times higher than the concentration required to induce inhibition of mitochondrial respiratory function (L Hillered and P H Chan: J Neurosci Res 19:94-100, 1988a). Moreover, reversal of the swelling occurred without recovery of respiratory function. These results suggest that swelling is a phenomenon of minor importance as an indicator of brain mitochondrial dysfunction, at least when induced by 20:4 in vitro.

The ATP/ADP-antiporter is involved in the uncoupling effect of fatty acids on mitochondria

The ATP/ADP-antiporter inhibitors and the substrate ADP suppress the uncoupling effect induced by low (10-20 microM) concentrations of palmitate in mitochondria from skeletal muscle and liver. The inhibitors and ADP are found to (a) inhibit the palmitate-stimulated respiration in the controlled state and (b) increase the membrane potential lowered by palmitate. The degree of efficiency decreases in the order: carboxyatractylate (CAtr) greater than ADP greater than bongkrekic acid, atractylate. GDP is ineffective, Mg.ADP is of much smaller effect, whereas ATP is effective at much higher concentration than is ADP. Inhibitor concentrations, which maximally suppress the palmitate-stimulated respiration, correspond to those needed for arresting the state 3 respiration. The extent of the CAtr-sensitive stimulation of respiration by palmitate has been found to decrease with an increase in palmitate concentration. Stimulation of the controlled respiration by p-trifluoromethoxycarbonylcyanide phenylhydrozone (FCCP) and gramicidin D at any concentrations of these uncouplers is CAtr-insensitive, whereas that caused by a low concentrations of 2,4-dinitrophenol and dodecyl sulfate is inhibited by CAtr. The above effect of palmitate develops immediately after addition of the fatty acid. It is resistant to EGTA as well as to inhibitors of phospholipase (nupercain) and of lipid peroxidation (ionol). Moreover, palmitate accelerates spontaneous release of the respiratory control, developing in rat liver mitochondria under certain conditions. This effect takes several minutes, being sensitive to EGTA, nupercain and ionol. Like the fast uncoupling, this slow effect is inhibited by ADP but CAtr and atractylate are stimulatory rather than inhibitory. In artificial planar phospholipid membrane, palmitate does not increase the membrane conductance, FCCP increases it strongly and dinitrophenol only slightly. In cytochrome oxidase proteoliposomes, FCCP, gramicidin and dinitrophenol (less effectively) lower, whereas palmitate enhances the cytochrome-oxidase-generated membrane potential. In this system, monensin substitutes for palmitate. It is concluded that the ATP/ADP antiporter is somehow involved in the uncoupling effect caused by low concentrations of palmitate and, partially, of dinitrophenol, whereas uncoupling produced by FCCP and gramicidin is due to their action on the phospholipid part of the mitochondrial membrane. A possible mechanism of this effect is discussed.