The energy-state of mitochondria during the transport of Ca2+

Intermittent hypoxia leads to functional reorganization of mitochondria and affects cellular bioenergetics in marine molluscs

Fluctuations in oxygen (O2) concentrations represent a major challenge to aerobic organisms and can be extremely damaging to their mitochondria. Marine intertidal molluscs are well-adapted to frequent O2 fluctuations, yet it remains unknown how their mitochondrial functions are regulated to sustain energy metabolism and prevent cellular damage during hypoxia and reoxygenation (H/R). We used metabolic control analysis to investigate the mechanisms of mitochondrial responses to H/R stress (18 h at <0.1% O2 followed by 1 h of reoxygenation) using hypoxia-tolerant intertidal clams Mercenaria mercenaria and hypoxia-sensitive subtidal scallops Argopecten irradians as models. We also assessed H/R-induced changes in cellular energy balance, oxidative damage and unfolded protein response to determine the potential links between mitochondrial dysfunction and cellular injury. Mitochondrial responses to H/R in scallops strongly resembled those in other hypoxia-sensitive organisms. Exposure to hypoxia followed by reoxygenation led to a strong decrease in the substrate oxidation (SOX) and phosphorylation (PHOS) capacities as well as partial depolarization of mitochondria of scallops. Elevated mRNA expression of a reactive oxygen species-sensitive enzyme aconitase and Lon protease (responsible for degradation of oxidized mitochondrial proteins) during H/R stress was consistent with elevated levels of oxidative stress in mitochondria of scallops. In hypoxia-tolerant clams, mitochondrial SOX capacity was enhanced during hypoxia and continued rising during the first hour of reoxygenation. In both species, the mitochondrial PHOS capacity was suppressed during hypoxia, likely to prevent ATP wastage by the reverse action of FO,F1-ATPase. The PHOS capacity recovered after 1 h of reoxygenation in clams but not in scallops. Compared with scallops, clams showed a greater suppression of energy-consuming processes (such as protein turnover and ion transport) during hypoxia, indicated by inactivation of the translation initiation factor EIF-2α, suppression of 26S proteasome activity and a dramatic decrease in the activity of Na(+)/K(+)-ATPase. The steady-state levels of adenylates were preserved during H/R exposure and AMP-dependent protein kinase was not activated in either species, indicating that the H/R exposure did not lead to severe energy deficiency. Taken together, our findings suggest that mitochondrial reorganizations sustaining high oxidative phosphorylation flux during recovery, combined with the ability to suppress ATP-demanding cellular functions during hypoxia, may contribute to high resilience of clams to H/R stress and help maintain energy homeostasis during frequent H/R cycles in the intertidal zone.

Uncoupling and Toxic Action of Alkyltriphenylphosphonium Cations on Mitochondria and the Bacterium Bacillus subtilis as a Function of Alkyl Chain Length

A series of permeating cations based on alkyl derivatives of triphenylphosphonium (C(n)-TPP(+)) containing linear hydrocarbon chains (butyl, octyl, decyl, and dodecyl) was investigated in systems of isolated mitochondria, bacteria, and liposomes. In contrast to some derivatives (esters) of rhodamine-19, wherein butyl rhodamine possessed the maximum activity, in the case of C(n)-TPP a stimulatory effect on mitochondrial respiration steadily increased with growing length of the alkyl radical. Tetraphenylphosphonium and butyl-TPP(+) at a dose of several hundred micromoles exhibited an uncoupling effect, which might be related to interaction between C(n)-TPP(+) and endogenous fatty acids and induction of their own cyclic transfer, resulting in transport of protons across the mitochondrial membrane. Such a mechanism was investigated by measuring efflux of carboxyfluorescein from liposomes influenced by C(n)-TPP(+). Experiments with bacteria demonstrated that dodecyl-TPP(+), decyl-TPP(+), and octyl-TPP(+) similarly to quinone-containing analog (SkQ1) inhibited growth of the Gram-positive bacterium Bacillus subtilis, wherein the inhibitory effect was upregulated with growing lipophilicity. These cations did not display toxic effect on growth of the Gram-negative bacterium Escherichia coli. It is assumed that the difference in toxic action on various bacterial species might be related to different permeability of bacterial coats for the examined triphenylphosphonium cations.

Milestones and recent discoveries on cell death mediated by mitochondria and their interactions with biologically active amines

Mitochondria represent cell "powerhouses," being involved in energy transduction from the electrochemical gradient to ATP synthesis. The morphology of their cell types may change, according to various metabolic processes or osmotic pressure. A new morphology of the inner membrane and mitochondrial cristae, significantly different from the previous one, has been proposed for the inner membrane and mitochondrial cristae, based on the technique of electron tomography. Mitochondrial Ca(2+) transport (the transporter has been isolated) generates reactive oxygen species and induces the mitochondrial permeability transition of both inner and outer mitochondrial membranes, leading to induction of necrosis and apoptosis. In the mitochondria of several cell types (liver, kidney, and heart), mitochondrial oxidative stress is an essential step in the induction of cell death, although not in brain, in which the phenomenon is caused by a different mechanism. Mitochondrial permeability transition drives both apoptosis and necrosis, whereas mitochondrial outer membrane permeability is characteristic of apoptosis. Adenine nucleotide translocase remains the most important component involved in membrane permeability, with the opening of the transition pore, although other proteins, such as ATP synthase or phosphate carriers, have been proposed. Intrinsic cell death is triggered by the release from mitochondria of proteic factors, such as cytochrome c, apoptosis inducing factor, and Smac/DIABLO, with the activation of caspases upon mitochondrial permeability transition or mitochondrial outer membrane permeability induction. Mitochondrial permeability transition induces the permeability of the inner membrane in sites in contact with the outer membrane; mitochondrial outer membrane permeability forms channels on the outer membrane by means of various stimuli involving Bcl-2 family proteins. The biologically active amines, spermine, and agmatine, have specific functions on mitochondria which distinguish them from other amines. Enzymatic oxidative deamination of spermine by amine oxidases in tumor cells may produce reactive oxygen species, leading to transition pore opening and apoptosis. This process could be exploited as a new therapeutic strategy to combat cancer.

Interactive effects of pH and metals on mitochondrial functions of intertidal bivalves Crassostrea virginica and Mercenaria mercenaria

Intertidal bivalves experience broad fluctuations of environmental temperature, pH and oxygen content which could change their intracellular pH. They are also exposed to trace metals such as cadmium (Cd) and copper (Cu) that accumulate in their tissues and may negatively affect mitochondrial functions and bioenergetics. We determined the interactive effects of pH and trace metals (25 μM Cd or Cu) on mitochondrial functions (including respiration and membrane potentials in both ADP-stimulated (state 3) and resting (state 4) states) of two common marine bivalves, the hard clams (Mercenaria mercenaria) and eastern oysters (Crassostrea virginica). In the absence of the trace metals, mitochondrial functions of C. virginica and M. mercenaria were insensitive to pH in a broad physiologically relevant range (6.6-7.8). Mitochondrial respiration was generally suppressed by 25 μM Cd or Cu (with the stronger effects observed for ADP-stimulated compared to the resting respiration) while the mitochondrial membrane potential was unaffected. pH modulated the effects of Cu and Cd on mitochondrial respiration of the bivalves. In oysters, Cu suppressed ADP-stimulated mitochondrial respiration at high and low pH values (6.6 and 7.8, respectively), but had no effect in the intermediate pH range (7.0-7.4). In clams, the negative effect of Cu on ADP-stimulated respiration was only observed at extremely high pH (7.8). A decrease in pH was also protective against Cd in mitochondria of clams and oysters. In clams, 25 μM Cd suppressed ADP-stimulated respiration at all pH; however, at low pH (6.6-7.0) this suppression was paralleled by a decrease in the rates of proton leak thereby effectively restoring mitochondrial coupling. In oysters, the inhibitory effects of Cd on ADP-stimulated respiration were fully abolished at low pH (6.6-7.0). This indicates that moderate acidosis (such as occurs during exposure to air, extreme salinities or elevated CO2 levels in the intertidal zone) may have a beneficial side-effect of protecting mitochondria of clams and oysters against the toxic effects of trace metals in polluted estuaries.

Effects of temperature and cadmium exposure on the mitochondria of oysters (Crassostrea virginica) exposed to hypoxia and subsequent reoxygenation

Intertidal bivalves are commonly exposed to multiple stressors including periodic hypoxia, temperature fluctuations and pollution, which can strongly affect energy metabolism. We used top-down control and elasticity analyses to determine the interactive effects of intermittent hypoxia, cadmium (Cd) exposure and acute temperature stress on mitochondria of the eastern oyster Crassostrea virginica. Oysters were acclimated at 20°C for 30 days in the absence or presence of 50 μg l(-1) Cd and then subjected to a long-term hypoxia (6 days at <0.5% O(2) in seawater) followed by normoxic recovery. Mitochondrial function was assessed at the acclimation temperature (20°C), or at elevated temperature (30°C) mimicking acute temperature stress in the intertidal zone. In the absence of Cd or temperature stress, mitochondria of oysters showed high resilience to transient hypoxia. In control oysters at 20°C, hypoxia/reoxygenation induced elevated flux capacity of all three studied mitochondrial subsystems (substrate oxidation, phosphorylation and proton leak) and resulted in a mild depolarization of resting mitochondria. Elevated proton conductance and enhanced capacity of phosphorylation and substrate oxidation subsystems may confer resistance to hypoxia/reoxygenation stress in oyster mitochondria by alleviating production of reactive oxygen species and maintaining high aerobic capacity and ATP synthesis rates during recovery. Exposure to environmental stressors such as Cd and elevated temperatures abolished the putative adaptive responses of the substrate oxidation and phosphorylation subsystems, and strongly enhanced proton leak in mitochondria of oysters subjected to hypoxia/reoxygenation stress. Our findings suggest that Cd exposure and acute temperature stress may lead to the loss of mitochondrial resistance to hypoxia and reoxygenation and thus potentially affect the ability of oysters to survive periodic oxygen deprivation in coastal and estuarine habitats.

Inter-relationship between mitochondrial function and susceptibility to oxidative stress in red- and white-blooded Antarctic notothenioid fishes

It is unknown whether Antarctic fishes can defend themselves against oxidative stress induced by elevations in temperature. We hypothesized that Antarctic icefishes, lacking the oxygen-binding protein hemoglobin, might be more vulnerable to temperature-induced oxidative stress compared with red-blooded notothenioids because of differences in their mitochondrial properties. Mitochondria from icefishes have higher densities of phospholipids per mg of mitochondrial protein compared with red-blooded species, and these phospholipids are rich in polyunsaturated fatty acids (PUFA), which can promote the formation of reactive oxygen species (ROS). Additionally, previous studies have shown that multiple tissues in icefishes have lower levels of antioxidants compared with red-blooded species. We quantified several properties of mitochondria, including proton leak, rates of ROS production, membrane composition and susceptibility to lipid peroxidation (LPO), the activity of superoxide dismutase (SOD) and total antioxidant power (TAOP) in mitochondria isolated from hearts of icefishes and red-blooded notothenioids. Mitochondria from icefishes were more tightly coupled than those of red-blooded fishes at both 2°C and 10°C, which increased the production of ROS when the electron transport chain was disrupted. The activity of SOD and TAOP per mg of mitochondrial protein was equivalent between icefishes and red-blooded species, but TAOP normalized to mitochondrial phospholipid content was significantly lower in icefishes compared with red-blooded fishes. Additionally, membrane susceptibility to peroxidation was only detectable in icefishes at 1°C and not in red-blooded species. Together, our results suggest that the high density of mitochondrial phospholipids in hearts of icefishes may make them particularly vulnerable to oxidative stress as temperatures rise.

Minocycline exerts uncoupling and inhibiting effects on mitochondrial respiration through adenine nucleotide translocase inhibition

The present study was aimed to provide a better understanding of the mitochondria-targeted actions of minocycline (MC), a second-generation tetracycline which has cytoprotective effects. Although the specific mechanisms underlying its activity remained elusive, considerable amounts of data indicated mitochondria as the primary pharmacological target of MC. Previous reports have shown that MC affects the oxygen-uptake rate by isolated mitochondria in different respiratory states. Here, we report on the effect of MC, in the range 50-200μM, on mitochondrial respiration. State 3 respiration titration with carboxyatractyloside revealed that MC inhibits the adenine nucleotide translocase. Furthermore, we analyze MC channel-forming capacity in the lipid membrane bilayer. Our results confirmed the crucial role of Δψ and showed a dependence on Ca(2+) for MC to have an effect on mitochondria. Our data also indicated that outer and inner mitochondrial membranes contribute differently to this effect, involving the presence of Δψ (the inner membrane) and VDAC (the outer membrane). Data from three isosmotic media indicate that MC does not increase the permeability of the inner membrane to protons or potassium. In addition, by using mitoplasts and ruthenium red, we showed that Ca(2+) uptake is not involved in the MC effect, suggesting involvement of VDAC in the MC interaction with the outer membrane. Our data contribute to unravel the mechanisms behind the mitochondria-targeted activity of the cytoprotective drug MC.

Interactions of melatonin with mammalian mitochondria. Reducer of energy capacity and amplifier of permeability transition

Melatonin, a metabolic product of the amino acid tryptophan, induces a dose-dependent energy drop correlated with a decrease in the oxidative phosphorylation process in isolated rat liver mitochondria. This effect involves a gradual decrease in the respiratory control index and significant alterations in the state 4/state 3 transition of membrane potential (ΔΨ). Melatonin, alone, does not affect the insulating properties of the inner membrane but, in the presence of supraphysiological Ca2+, induces a ΔΨ drop and colloid-osmotic mitochondrial swelling. These events are sensitive to cyclosporin A and the inhibitors of Ca2+ transport, indicative of the induction or amplification of the mitochondrial permeability transition. This phenomenon is triggered by oxidative stress induced by melatonin and Ca2+, with the generation of hydrogen peroxide and the consequent oxidation of sulfydryl groups, glutathione and pyridine nucleotides. In addition, melatonin, again in the presence of Ca2+, can also induce substantial release of cytochrome C and AIF (apoptosis-inducing factor), thus revealing its potential as a pro-apoptotic agent.

Top-down control analysis of the cadmium effects on molluscan mitochondria and the mechanisms of cadmium-induced mitochondrial dysfunction

Cadmium (Cd) is a toxic metal and an important environmental pollutant that can strongly affect mitochondrial function and bioenergetics in animals. We investigated the mechanisms of Cd action on mitochondrial function of a marine mollusk (the eastern oyster Crassostrea virginica) by performing a top-down control analysis of the three major mitochondrial subsystems (substrate oxidation, proton leak, and phosphorylation). Our results showed that the substrate oxidation and proton leak subsystems are the main targets for Cd toxicity in oyster mitochondria. Exposure to 12.5 μM Cd strongly inhibited the substrate oxidation subsystem and stimulated the proton conductance across the inner mitochondrial membrane. Proton conductance was also elevated and substrate oxidation inhibited by Cd in the presence of a mitochondrially targeted antioxidant, MitoVitE, indicating that Cd effects on these subsystems were to a large extent ROS independent. Cd did not affect the kinetics of the phosphorylation system, indicating that it has negligible effects on F₁, F(O) ATP synthase and/or the adenine nucleotide transporter in oyster mitochondria. Cd exposure altered the patterns of control over mitochondrial respiration, increasing the degree of control conferred by the substrate oxidation subsystem, especially in resting (state 4) mitochondria. Taken together, these data suggest that Cd-induced decrease of mitochondrial efficiency and ATP production are predominantly driven by the high sensitivity of substrate oxidation and proton leak subsystems to this metal.

Effect of magnesium on calcium-induced depolarisation of mitochondrial transmembrane potential

An effect of magnesium on calcium-induced depolarisation of mitochondrial transmembrane potential (DeltaPsi(m)) was investigated. Depending on the presence of Mg(2+), addition of Ca(2+) to suspension of isolated rat heart mitochondria induced either reversible depolarisation or irreversible collapse of succinate-driven DeltaPsi(m). Irreversible collapse of DeltaPsi(m), observed in the absence of Mg(2+), was insensitive to Ca(2+) chelation, inhibition of Ca(2+) uptake and increased efflux of Ca(2+) from mitochondrial matrix. Based on these data, opening of mPTP in a high-conductance mode is considered to be a major cause of the Ca(2+)-induced irreversible collapse of DeltaPsi(m) in the absence of Mg(2+). Involvement of mPTP in the process of Ca(2+)-induced collapse of DeltaPsi(m) was further supported by protective effect of both CsA and ADP. Reversible collapse of DeltaPsi(m), observed in the presence of Mg(2+), was sensitive to EGTA, ADP; and inhibition of Ca(2+) uptake and increased efflux of Ca(2+) from mitochondrial matrix. This may represent selective induction of a low-conductance permeability pathway. Presented results indicate important role of Mg(2+) in the process of Ca(2+)-induced depolarisation of DeltaPsi(m) mainly through discrimination between low- and high-conductance modes of mPTP. Minor effect of Mg(2+) on Ca(2+)-induced depolarisation of DeltaPsi(m) was observed at the level of stimulation of DeltaPsi(m) generation and inhibition of mitochondrial Ca(2+) uptake.

Stimulation-induced changes in NADH fluorescence and mitochondrial membrane potential in lizard motor nerve terminals

To investigate mitochondrial responses to repetitive stimulation, we measured changes in NADH fluorescence and mitochondrial membrane potential (Psi(m)) produced by trains of action potentials (50 Hz for 10-50 s) delivered to motor nerve terminals innervating external intercostal muscles. Stimulation produced a rapid decrease in NADH fluorescence and partial depolarization of Psi(m). These changes were blocked when Ca2+ was removed from the bath or when N-type Ca2+ channels were inhibited with omega-conotoxin GVIA, but were not blocked when bath Ca2+ was replaced by Sr2+, or when vesicular release was inhibited with botulinum toxin A. When stimulation stopped, NADH fluorescence and Psi(m) returned to baseline values much faster than mitochondrial [Ca2+]. In contrast to findings in other tissues, there was usually little or no poststimulation overshoot of NADH fluorescence. These findings suggest that the major change in motor terminal mitochondrial function brought about by repetitive stimulation is a rapid acceleration of electron transport chain (ETC) activity due to the Psi(m) depolarization produced by mitochondrial Ca2+ (or Sr2+) influx. After partial inhibition of complex I of the ETC with amytal, stimulation produced greater Psi(m) depolarization and a greater elevation of cytosolic [Ca2+]. These results suggest that the ability to accelerate ETC activity is important for normal mitochondrial sequestration of stimulation-induced Ca2+ loads.

Nuclear and mitochondrial compartmentation of oxidative stress and redox signaling

New methods to measure thiol oxidation show that redox compartmentation functions as a mechanism for specificity in redox signaling and oxidative stress. Redox Western analysis and redox-sensitive green fluorescent proteins provide means to quantify thiol/disulfide redox changes in specific subcellular compartments. Analyses using these techniques show that the relative redox states from most reducing to most oxidizing are mitochondria > nuclei > cytoplasm > endoplasmic reticulum > extracellular space. Mitochondrial thiols are an important target of oxidant-induced apoptosis and necrosis and are especially vulnerable to oxidation because of the relatively alkaline pH. Maintenance of a relatively reduced nuclear redox state is critical for transcription factor binding in transcriptional activation in response to oxidative stress. The new methods are applicable to a broad range of experimental systems and their use will provide improved understanding of the pharmacologic and toxicologic actions of drugs and toxicants.

Control of oxidative phosphorylation during insect metamorphosis

The midgut of the tobacco hornworm ( Manduca sexta) is a highly aerobic tissue that is destroyed and replaced by a pupal epithelium at metamorphosis. To determine how oxidative phosphorylation is altered during the programmed death of the larval cells, top-down control analysis was performed on mitochondria isolated from the midguts of larvae before and after the commitment to pupation. Oxygen consumption and protonmotive force (measured as membrane potential in the presence of nigericin) were monitored to determine the kinetic responses of the substrate oxidation system, proton leak, and phosphorylation system to changes in the membrane potential. Mitochondria from precommitment larvae have higher respiration rates than those from postcommitment larvae. State 4 respiration is controlled by the proton leak and the substrate oxidation system. In state 3, the substrate oxidation system exerted 90% of the control over respiration, and this high level of control did not change with development. Elasticity analysis, however, revealed that, after commitment, the activity of the substrate oxidation system falls. This decline may be due, in part, to a loss of cytochrome c from the mitochondria. There are no differences in the kinetics of the phosphorylation system, indicating that neither the F1F0ATP synthase nor the adenine nucleotide translocase is affected in the early stages of metamorphosis. An increase in proton conductance was observed in mitochondria isolated from postcommitment larvae, indicating that membrane area, lipid composition, or proton-conducting proteins may be altered during the early stages of the programmed cell death of the larval epithelium.

Mitochondrial calcium in heart cells: beat-to-beat oscillations or slow integration of cytosolic transients?

Mitochondria have been implicated in intracellular Ca2+ signaling in many cell types. The inner mitochondrial membrane contains Ca2+-transporting proteins, which catalyze Ca2+ uptake and extrusion. Intramitochondrial (matrix) Ca2+, in turn, regulates the activity of Krebs cycle dehydrogenases and, ultimately, the rate of ATP synthesis. In the myocardium, controversy remains whether the fast cytosolic Ca2+ transients underlying excitation-contraction coupling in beating cells are rapidly transmitted into the matrix compartment or slowly integrated by the mitochondrial Ca2+ transporters. This mini-review critically summarizes the recent experimental work in this field.

Disturbance of myocardial energy metabolism in experimental virus myocarditis by antibodies against the adenine nucleotide translocator

OBJECTIVE

The adenine nucleotide translocator (ANT) of the inner mitochondrial membrane is an autoantigen in myocarditis and in dilated cardiomyopathy. Clinical and experimental studies showed that specific autoantibodies inhibit the transmembrane nucleotide transport. In isolated hearts of guinea pigs immunized with the ANT, energy metabolism is disturbed. This metabolic disorder is related to functionally active specific antibodies and to a reduced heart function. This study tests whether similar immunological, metabolical and functional responses also occur in experimental virus myocarditis.

METHODS AND RESULTS

Experimental virus myocarditis was induced in A.SW/SnJ-mice by Coxsackie B3 virus infection. Specific antibodies against the ANT were detected by Western Blot in 14 out of 19 infected animals. In the isolated perfused hearts of five of these 14 mice cytosolic and mitochondrial ATP/ADP-ratios, determined by nonaqueous fractionation, were significantly altered, signalling a reduced ANT function [cytosolic ATP/ADP: 59 +/- 18 vs. 136 +/- 20 (controls), mitochondrial ATP/ADP: 4.2 +/- 1.0 vs. 1.1 +/- 0.3], all P < 0.05. Also, left ventricular pressure [43 +/- 9 vs. 78 +/- 6 mmHg (noninfected controls)], rate-pressure product (15.8 +/- 3.2 vs. 30.5 +/- 3.0 mmHg/min/1000), dp/dt (2410 +/- 222 vs. 3250 +/- 118 mmHg/s), and oxygen consumption (4.7 +/- 0.9 vs. 7.3 +/- 0.7 mumol/g/min), all P < 0.05, were lowered.

CONCLUSION

The data support the hypothesis that a virus infection alters cardiac energy metabolism and function by an antibody-mediated modulation of the function of the ANT.

Changes in mitochondrial function resulting from synaptic activity in the rat hippocampal slice

Digital imaging microfluorimetry was used to visualize changes in mitochondrial potential and intracellular Ca2+ concentration, [Ca2+]i, in thick slices of rat hippocampus. Electrical activity, especially stimulus train-induced bursting (STIB) activity, produced slow, prolonged changes in mitochondrial potential within hippocampal slices as revealed by fluorescence measurements with rhodamine dyes. Changes in mitochondrial potential showed both temporal and spatial correlations with the intensity of the electrical activity. Patterned changes in mitochondrial potential were observed to last from tens of seconds to minutes as the consequence of epileptiform discharges. STIB-associated elevations in [Ca2+]i were also prolonged and exhibited a spatial pattern similar to that of the mitochondrial depolarization. The mitochondrial depolarization was sensitive to TTX and glutamate receptor blockers ([Mg2+]o and CNQX or DNQX plus D-AP-5) and to the inhibition of glutamate release by activation of presynaptic NPY receptors. The monitoring of mitochondrial potential in slice preparations provides a new tool for mapping synaptic activity in the brain and for determining the roles of mitochondria in regulation of brain synaptic activity.

Imaging the permeability pore transition in single mitochondria

In mitochondria the opening of a large proteinaceous pore, the "mitochondrial permeability transition pore" (MTP), is known to occur under conditions of oxidative stress and matrix calcium overload. MTP opening and the resulting cellular energy deprivation have been implicated in processes such as hypoxic cell damage, apoptosis, and neuronal excitotoxicity. Membrane potential (delta psi(m)) in single isolated heart mitochondria was measured by confocal microscopy with a voltage-sensitive fluorescent dye. Measurements in mitochondrial populations revealed a gradual loss of delta psi(m) due to the light-induced generation of free radicals. In contrast, the depolarization in individual mitochondria was fast, sometimes causing marked oscillations of delta psi(m). Rapid depolarizations were accompanied by an increased permeability of the inner mitochondrial membrane to matrix-entrapped calcein (approximately 620 Da), indicating the opening of a large membrane pore. The MTP inhibitor cyclosporin A significantly stabilized delta psi(m) in single mitochondria, thereby slowing the voltage decay in averaged recordings. We conclude that the spontaneous depolarizations were caused by repeated stochastic openings and closings of the transition pore. The data demonstrate a much more dynamic regulation of membrane permeability at the level of a single organelle than predicted from ensemble behavior of mitochondrial populations.

Interaction between Na+/phosphate-cotransporter and the adrenoceptors in myocardial depression

The primary objective of this study was to test the hypothesis that an increase in the alpha1-adrenoceptor tone would potentiate the myocardial biphasic contractile response to inorganic phosphate [Pi, the substrate of Na/Pi-cotransporter (NP)]. A second aim was to determine whether activation of alpha1-adrenoceptor is necessary for the NP-mediated increase in myocardial contractility (+dP/dt). Earlier study from this laboratory showed that high concentration of Pi (10 mM) produces a biphasic contractile response: initial increase in +dP/dt was followed by decline. In another study, Pi (3.5 mM) potentiated phenylephrine (PHE)-induced increase in +dP/dt. The alpha1-adrenoceptor was not blocked in these studies, and it can still be activated by the electrical stimulation of the sympathetic nerve terminals to the heart. Additionally, alpha1-adrenoceptor-activated increases in the activity of NP have been reported in numerous studies in noncardiac tissues and cell lines; therefore it is not clear whether Pi-induced increase in +dP/dt occurs only in the presence of alpha1-adrenoceptor activation. This study was performed by using isolated perfused rat heart in the condition of controlled extracellular calcium activity (0.72 mM); fixed preload (15 mm Hg); and constant heart rate (280 beats/min) and coronary flow (8 ml/min). The electronically differentiated value of the left ventricular pressure (LVP) signal was used as an index of myocardial contractility. The data show that activation of the alpha1-adrenoceptor is not necessary for the Pi-induced increase in +dP/dt (i.e., NP-mediated increase in +dP/dt has both the alpha1-adrenoceptor-dependent and alpha1-adrenoceptor-independent components. The interaction between alpha1-adrenoceptor agonist (PHE) and Pi (10 mM) did not produce a biphasic myocardial contractile response in the presence of propranolol. Because our earlier data on myocardial biphasic contractile response to 10 mM Pi was obtained when neither the beta- nor the alpha-adrenoceptor was blocked, we carried out more studies to see whether beta-adrenoceptor plays a role in this Pi-induced biphasic response. When both the alpha- and beta-adrenoceptors were activated with norepinephrine (NE), myocardial depression by high Pi concentration was markedly potentiated. This myocardial depression did not occur in the presence of phosphonoformate, a selective inhibitor of NP. It also did not occur when alpha1-adrenoceptor was blocked. Our data suggest that alpha1- and beta-adrenoceptors do not interact with the cardiac NP to potentiate the Pi-induced biphasic contractile response, but they interact in a manner that potentiates Pi-induced myocardial depression.

The effects of the ergosteroid 7-oxo-dehydroepiandrosterone on mitochondrial membrane potential: possible relationship to thermogenesis

Administered 3 beta-hydroxyandrost-5-ene-7,17-dione (7-oxo-DHEA) is more effective than 3 beta-hydroxyandrost-5-en-7-one (DHEA) as an inducer of liver mitochondrial sn-glycerol-3-phosphate dehydrogenase and cytosolic malic enzyme in rats. Like DHEA, the 7-oxo metabolite enhances liver catalase, fatty acylCoA oxidase, cytosolic sn-glycerol-3-phosphate dehydrogenase, mitochondrial substrate oxidation rate, and the reconstructed sn-glycerol 3-phosphate shuttle. The mitochondrial adenine nucleotide carrier is diminished by thyroidectomy and is restored to normal activity by administering 7-oxo-DHEA. The relationship between respiratory rate and proton motive force across the mitochondrial membrane was measured in the nonphosphorylating state. When treated with increasing concentrations of respiratory inhibitors liver mitochondria from rats treated with 7-oxo-DHEA or thyroid hormones show a more rapid decline of membrane potential than do normal liver mitochondria. Thus 7-oxo-DHEA induces an increased proton leak or slip as has been reported for the thyroid hormone by M.D. Brand [(1990) Biochem. Biophys. Acta 1018, 128-133]. This process may contribute to the enhanced thermogenesis caused by ergosteroids as well as by thyroid hormones.

Stimulation of Ca2+ release from rat liver mitochondria by the dithiol reagent alpha-lipoic acid

Rat liver mitochondria contain a Ca(2+)-specific release pathway stimulated by Ca(2+)-dependent hydrolysis of oxidized intramitochondrial pyridine nucleotides to ADP ribose and nicotinamide. We have previously shown that NAD+ hydrolysis and subsequent Ca2+ release are inhibited by cyclosporine A and that they are only possible when some critical thiols are cross-linked or oxidized, e.g. by phenylarsine oxide, gliotoxin, or peroxynitrite. We now report that the antioxidant alpha-lipoic acid stimulates Ca2+ release from intact mitochondria, i.e. with preservation of the mitochondrial membrane potential and without large-amplitude swelling. The release stimulated by alpha-lipoic acid is inhibited by cyclosporine A and is more effective when the pyridine nucleotides are oxidized. The results strongly suggest that alpha-lipoic acid stimulates the Ca(2+)-specific release pathway from intact mitochondria by oxidizing some vicinal thiols, thereby stimulating hydrolysis of oxidized pyridine nucleotides. These observations further corroborate that intact rat liver mitochondria contain a specific Ca2+ release pathway stimulated by modification of vicinal thiols. Prolonged stimulation of Ca2+ release by lipoic acid followed by its re-uptake (Ca2+ "cycling") may contribute to the detrimental, prooxidant-like effects seen with higher concentrations of lipoic acid.

Peroxynitrite stimulates the pyridine nucleotide-linked Ca2+ release from intact rat liver mitochondria

Rat liver mitochondria contain a specific Ca2+ release pathway which operates when oxidized mitochondrial pyridine nucleotides are hydrolyzed in a Ca2+-dependent manner to ADP-ribose and nicotinamide. We have previously shown that NAD+ hydrolysis is inhibited by cyclosporin A and is possible only when some vicinal thiols are cross-linked. Here we report that the thiol oxidant peroxynitrite (ONOO-), which can form from nitric oxide (nitrogen monoxide, NO.) and superoxide anion (O2-), at low concentrations stimulates the specific Ca2+ release pathway. Both peroxynitrite-induced pyridine nucleotide hydrolysis and Ca2+ release are inhibited by cyclosporin A, and peroxynitrite is ineffective when pyridine nucleotides are kept reduced. Ca2+ release induced by peroxynitrite occurs with maintenance of the mitochondrial membrane potential and is not accompanied by entry of sucrose into mitochondria. The results suggest that peroxynitrite stimulates the specific Ca2+ release from intact mitochondria by modifying critical mitochondrial thiols other than glutathione in such a way that hydrolysis of oxidized pyridine nucleotides is achieved. These findings provide further insight into the regulation of Ca2+ release from mitochondria by nitric oxide and its congeners.

Magnesium-protoporphyrin chelatase of Rhodobacter sphaeroides: reconstitution of activity by combining the products of the bchH, -I, and -D genes expressed in Escherichia coli

Magnesium-protoporphyrin chelatase lies at the branch point of the heme and (bacterio)chlorophyll biosynthetic pathways. In this work, the photosynthetic bacterium Rhodobacter sphaeroides has been used as a model system for the study of this reaction. The bchH and the bchI and -D genes from R. sphaeroides were expressed in Escherichia coli. When cell-free extracts from strains expressing BchH, BchI, and BchD were combined, the mixture was able to catalyze the insertion of Mg into protoporphyrin IX in an ATP-dependent manner. This was possible only when all three genes were expressed. The bchH, -I, and -D gene products are therefore assigned to the Mg chelatase step in bacteriochlorophyll biosynthesis. The mechanism of the Mg chelation reaction and the implications for chlorophyll biosynthesis in plants are discussed.

Dietary iron deficiency in the rat. I. Abnormalities in energy metabolism of the hepatic tissue

Severe iron deficiency was induced in rats by rearing nursing dams and their offspring on a diet comprising all the requisite nutrients and trace metals except iron. The iron deficient 5-week-old rats exhibited a severe anemia and a drastic decrease in iron content of the hepatic tissue and of the mitochondrial fraction. Cytochromes c + c1 and b were moderately but significantly reduced. A large increase in liver concentration was observed in iron-deficient animals; whereas there was no modification in total lipid, cholesterol, phospholipid and fatty acid composition of the mitochondrial membrane. Mitochondria from iron-deficient rats presented a partial uncoupling of the oxidative phosphorylation process. This functional derangement was completely reversed by the presence of either bovine serum albumin or L-carnitine plus ATP. This behaviour suggested that endogenous long-chain fatty acids could be primarily involved in the onset of mitochondrial dysfunction. The hepatic energy state of the liver appeared dramatically decreased under the pathological condition of severe iron-deficiency anemia. The possibility of a direct link between the partial loss of coupled functions observed in isolated mitochondria and the heavy energy deficit detected in the liver is discussed.

Dietary iron deficiency in the rat. II. Recovery from energy metabolism derangement of the hepatic tissue by iron therapy

Severe iron deficiency in rats was found to be associated with abnormal lipid accumulation in the liver and impairment of the oxidative metabolism in the hepatic tissue. Iron therapy, consisting in oral administration to iron-deficient 4-week-old rats of iron succinyl-albumin complex, at a daily dose of 10 mg/kg body weight, over a period of 7 days, almost completely corrected these functional anomalies. This treatment fully reverted severe anemia associated with iron deficiency. The level of iron in the hepatic tissue and in the mitochondrial fraction also increased largely. By contrast, no significant improvement in the lowered level of cytochromes occurred. Iron supplements significantly decreased the abnormal level of liver total lipids and serum triglycerides. Concomitantly, iron repletion fully reverted the partial loss of coupled function in isolated mitochondria and the energy state perturbation of the liver. A close relationship among abnormal lipid accumulation, impairment of mitochondrial oxidative phosphorylation and energy derangement in the hepatic cell in this experimental model of severe dietary iron deficiency anemia appears to be likely.

Effects of Ca2+ deregulation on mitochondrial membrane potential and cell viability in nucleated cells following lytic complement attack

We have previously shown [Papadimitriou JC. Ramm LE. Drachenberg CB. Trump BF. Shin ML. (1991) J. Immunol., 147, 212-217] that formation of lytic C5b-9 channels on Ehrlich ascites tumor cells induced rapid depletion of adenine nucleotides associated with prelytic leakage preceding cell death. Extracellular Ca2+ concentration ([Ca2+]e) reduction by chelation markedly delayed the onset of cell death, although the adenine nucleotide leakage was enhanced. In the present study, we examined the temporal relationships between ionized cytosolic Ca2+ ([Ca2+]i), mitochondrial membrane potential (delta psi m) and cell death in individual cells by digital imaging fluorescence microscopy (DIFM), during the earliest phase of C5b-9 attack. The results showed an immediate, > 20-fold rise in [Ca2+]i, rapidly followed by dissipation of delta psi m and subsequent acute cell death. These events were markedly delayed by chelation of Ca2+e, but not by nominally Ca2+ free medium. Differing from previous reports indicating propidium iodide labeling of viable cells bearing C5b-9 channels, with DIFM we observed nuclear fluorescence with that marker only in association with cell death. These findings indicate that Ca2+ influx through lytic C5b-9 channels is responsible for the massive increase in [Ca2+]i, as well as for the rapid loss of delta psi m, followed by acute cell death. When this [Ca2+]i increase is prevented, the cell death is probably related to metabolic depletion.

Lipid hydroperoxide induced mitochondrial dysfunction following acute ethanol intoxication in rats. The critical role for mitochondrial reduced glutathione

It has been found that acute ethanol (EtOH) intoxication of rats caused depletion of mitochondrial reduced glutathione (GSH) of approximately 40%. A GSH reduction of similar extent was also observed after the administration to rats of buthionine sulphoximine (BSO), a specific inhibitor of GSH synthesis. Combined treatment with BSO plus EtOH further decreased mitochondrial GSH up to 70% in comparison to control. Normal functional efficiency was encountered in BSO-treated mitochondria, as evaluated by membrane potential measurements during a complete cycle of phosphorylation. In contrast a partial loss of coupled functions occurred in mitochondria from EtOH- and BSO plus EtOH-treated rats. The presence in the incubation system of either GSH methyl monoester (GSH-EE), which normalizes GSH levels, or of EGTA, which chelates the available Ca2+, partially restores the mitochondrial phosphorylative efficiency. Following EtOH and BSO plus EtOH intoxication, the presence of fatty-acid-conjugated diene hydroperoxides, such as octadecadienoic acid hydroperoxide (HPODE), was detected in the mitochondrial membrane. Exogenous HPODE, when added to BSO-treated mitochondria, induced, in a concentration-dependent system, membrane potential derangement. The presence of either GSH-EE or EGTA fully prevented a drop in membrane potential. The results obtained suggest that fatty acid hydroperoxides, endogenously formed during EtOH metabolism, brought about non-specific permeability changes in the mitochondrial inner membrane whose extent was strictly dependent on the level of mitochondrial GSH.

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.

Further studies on the mechanism of action of gossypol on mitochondrial membrane

1. In order to explore the mechanism of inhibition of hydroxylases involved in steroidogenesis, by gossypol, we studied the effect of this drug on adrenal cortex mitochondria, and compared it with those on kidney and heart. 2. The uncoupler effect of gossypol (collapse of delta psi and Ca2+ efflux) was found to be lower in adrenal cortex mitochondria than in kidney and heart mitochondria. 3. Gossypol produced more extensive changes on the membrane lipidic matrix (increase in the order parameter for 5-doxylstearic acid) in adrenal cortex mitochondria than in the other mitochondria studied. 4. The results described above indicate that the mechanism of inhibition of gossypol of steroidogenic adrenal enzymes could be attributed to an alteration of the lipidic matrix which, in turn, modifies protein function.

Prooxidant-induced Ca2+ release from liver mitochondria. Specific versus nonspecific pathways

Ca2+ release from mitochondria can be induced by a variety of chemically different prooxidants. Release induced by these compounds is possibly regulated by protein mono(ADP)ribosylation, and leaves mitochondria initially intact. Excessive "cycling" (continuous release and uptake) of Ca2+ by mitochondria leads to their damage, as shown by a decreased membrane potential, fast Ca2+ release, and impairment of ATP synthesis. When cycling is prevented by Ca2+ chelators or by inhibition of the uptake route with ruthenium red, prooxidants still induce Ca2+ release but mitochondria remain intact. It has recently been suggested that formation of a "pore" in the inner mitochondrial membrane participates in the Ca2+ release mechanism. We find that the prooxidant-induced Ca2+ release is not paralleled by sucrose entry into, or K+ release from, or swelling of mitochondria, provided Ca2+ cycling is prevented. Thus, the prooxidant-induced Ca2+ release does not require formation of a "pore." We conclude that the release occurs via a specific pathway.

Fluorescamine-induced membrane permeability in mitochondria

1. Addition of fluorescamine (75 microM) to mitochondria induced an increase in membrane permeability. 2. The leakiness of the inner mitochondrial membrane is characterized by extensive release of accumulated Ca2+, collapse of the transmembrane potential, mitochondrial swelling and efflux of matrix proteins, among them, malate dehydrogenase. 3. These effects were diminished by supplementing the media with 1 mM phosphate, and partially prevented by Mg2+. 4. These results indicate that the primary amino groups of membrane components contribute, partially, to the maintenance of the permeability barrier in mitochondria.

ATPase-inhibitor proteins of brown-adipose-tissue mitochondria from warm- and cold-acclimated rats

1. A group of male Sprague-Dawley rats (5-6 weeks old) was cold-acclimated at 4 degrees C for 4 weeks. Warm-acclimated controls remained at 24 degrees C. Total protein content of brown adipose tissue (BAT) increased more than 3-fold and total uncoupling protein (UCP) content increased more than 6-fold upon cold-acclimation. The concentration of UCP in isolated BAT mitochondria almost doubled. 2. Specific ATPase activity of the non-thermogenic BAT mitochondria (from warm-acclimated controls) was low and increased about 6-fold on addition of 1 microM-Ca2+, which raised free Ca2+ levels (measured by Fura-2) in the incubation media from 1.32 +/- 0.28 microM (mean +/- S.E.M.) to 2.29 +/- 0.39 microM [at which the Ca(2+)-binding ATPase-inhibitor protein (CaBI) is inactivated]. Correspondingly, the specific ATP synthetase activity of the non-thermogenic BAT mitochondria was high and was decreased by 74% by addition of 1 microM-Ca2+. 3. In contrast, specific ATPase activity of thermogenic BAT mitochondria (from cold-acclimated rats) was 5 times that of the control group, and addition of Ca2+ had only a small stimulatory response. Correspondingly, the specific ATP synthetase activity of the thermogenic BAT mitochondria was low, and the decrease by Ca2+ was small, albeit significant. 4. Extracts of BAT mitochondria from both groups of animals contained significant amounts of the ATPase-inhibitor protein of Pullman and Monroy (PMI) as well as of CaBI, as shown by gel electrophoresis. Kinetic studies of inhibition of mitochondrial ATPase activity showed that PMI activity was unaltered in extracts from the thermogenic BAT mitochondria, whereas CaBI activity was slightly but significantly increased. 5. The presence of active ATPase-inhibitor proteins in BAT mitochondria was shown for the first time. We conclude that uncoupling of oxidative phosphorylation occurs in thermogenic BAT mitochondria, even in the presence of the ATPase-inhibitor proteins.

Regulation of oxidative activity and delta psi of liver mitochondria of active and hibernating gophers. The role of phospholipase A2

1. In the present work the initially lowered oxidase activity of liver mitochondria of hibernating gophers is shown to increase upon Ca(2+)-loading, after freezing-thawing repeated three times and after swelling in a medium containing potassium acetate as well as in a hypotonic sucrose medium. 2. In all cases the inhibition of phospholipase A2 hindered the increase of the oxidase activity of mitochondria. 3. Mitochondria of hibernating gophers have a lowered delta psi in comparison with active animals, which is restored in the hypotonic medium.

'Pore' formation is not required for the hydroperoxide-induced Ca2+ release from rat liver mitochondria

It has recently been suggested by several investigators that the hydroperoxide- and phosphate-induced Ca2+ release from mitochondria occurs through a non-specific 'pore' formed in the mitochondrial inner membrane. The aim of the present study was to investigate whether 'pore' formation actually is required for Ca2+ release. We find that the t-butyl hydroperoxide (tbh)-induced release is not accompanied by stimulation of sucrose entry into, K+ release from, and swelling of mitochondria provided re-uptake of the released Ca2+ ('Ca2+ cycling') is prevented. We conclude that (i) the tbh-induced Ca2+ release from rat liver mitochondria does not require 'pore' formation in the mitochondrial inner membrane, (ii) this release occurs via a specific pathway from intact mitochondria, and (iii) a non-specific permeability transition ('pore' formation) is likely to be secondary to Ca2+ cycling by mitochondria.

In vitro assay of the chlorophyll biosynthetic enzyme Mg-chelatase: resolution of the activity into soluble and membrane-bound fractions

The first committed step in chlorophyll synthesis is the Mg-chelatase-catalyzed insertion of magnesium into protoporphyrin IX. Since iron insertion into protoporphyrin leads to heme formation, Mg-chelastase lies at the branch point of heme and chlorophyll synthesis in chloroplasts. Little is known about the enzymology or regulation of Mg-chelatase, as it has been assayed only in intact cucumber chloroplasts. In this report we describe an in vitro assay for Mg-chelatase. Mg-chelatase activity in intact pea chloroplasts was 3- to 4-fold higher than in cucumber chloroplasts. This activity survived chloroplast lysis and could be fractionated by centrifugation into supernatant and pellet components. Both of these fractions were required to reconstitute Mg-chelatase activity, and both were inactivated by boiling indicating that the enzyme is composed of soluble and membrane-bound protein(s). The product of the reaction was confirmed fluorometrically as the magnesium chelate of the porphyrin substrate. The specific activity of the reconstituted system was typically 1 nmol of Mg-deuteroporphyrin per h per mg of protein, and activity was linear for at least 60 min under our assay conditions. ATP and magnesium were required for Mg-chelatase activity and the enzyme was sensitive to the sulfhydryl reagent N-ethylmaleimide (I50, 20 microM). Broken and reconstituted cucumber chloroplasts were unable to maintain Mg-chelatase activity. However, the cucumber supernatant fraction was active when combined with the pellet fraction of peas; the converse was not true, which suggested that the cucumber pellet was the component that lost activity during lysis.

Oxidative stress in mitochondria: its relationship to cellular Ca2+ homeostasis, cell death, proliferation, and differentiation

A variety of chemically different prooxidants causes Ca2+ release from mitochondria. This prooxidant-induced Ca2+ release occurs from intact mitochondria via a route which is physiologically relevant and may be regulated by protein monoADP-ribosylation. When the released Ca2+ is excessively 'cycled' by mitochondria (continuously taken up and released) the inner membrane is damaged. This leads to a decreased ability of mitochondria to retain Ca2+, uncoupling of mitochondria, and an impairment of ATP synthesis, which in turn deprives the cell of the energy necessary for the proper functioning of the Ca2+ ATPases of the endoplasmic (sarcoplasmic) reticulum, the nucleus and the plasma membrane. The ensuing rise of the cytosolic Ca2+ level cannot be counterbalanced by the damaged mitochondria which, under normoxic conditions, act as a safety device against an increase of the cytosolic Ca2+ concentration. The impaired ability of mitochondria to retain Ca2+ may lead to cell death. However, there is also evidence emerging that release of Ca2+ from mitochondria may be physiologically important for cell proliferation and differentiation.

Early mitochondrial disfunction in bromobenzene treated mice: a possible factor of liver injury

The membrane potential of liver mitochondria isolated from bromobenzene treated mice was studied. Specifically, the efficiency of the energy-transducing mitochondrial membrane was measured during the phase between the occurrence of a massive loss of hepatic GSH, after 2-3 hr of bromobenzene intoxication, and the appearance of lipid peroxidation and cell death (12-15 hr after treatment). Partial uncoupling of oxidative phosphorylation was observed in mitochondria during the early period of intoxication (3-9 hr). These anomalies in oxidative metabolism did not result in irreversible damage to the mitochondrial inner membrane. The possibility that phenolic metabolites of bromobenzene are responsible for the uncoupling effects was examined. Orto- and especially para-bromphenol reproduced the alterations of mitochondrial function when added to normal mitochondria at concentrations comparable to those found in the livers of the intoxicated animals. Since the concentration of the bromophenols (especially p-bromophenol) largely increases after the intoxication times as tested here, mitochondrial uncoupling may represent a mechanism of liver damage acting synergistically with or even independently of other factors such as oxidative stress and lipid peroxidation.

Effect of peroxide, sodium, and calcium on brain mitochondrial respiration in vitro: potential role in cerebral ischemia and reperfusion

Mitochondrial pyruvate-supported respiration was studied in vitro under conditions known to exist following ischemia, i.e., elevated extramitochondrial Ca2+, Na+, and peroxide. Ca2+ alone (7-10 nmol/mg) decreased state 3 and increased state 4 respiration to 81 and 141% of control values, respectively. Sodium (15 mM) and/or tert-butyl hydroperoxide (tBOOH; up to 2,000 nmol/mg protein) alone had no effect on respiration; however, Na+ or tBOOH in combination with Ca2+ dramatically altered respiration. Respiratory inhibition induced by Ca2+ and tBOOH does not involve pyruvate dehydrogenase (PDH) inhibition since PDH flux increased linearly with tBOOH concentration (R = 0.96). Calcium potentiated tBOOH-induced mitochondrial NAD(P)H oxidation and shifted the redox state of cytochrome b from 67 to 47% reduced. Calcium (5.5 nmol/mg) plus Na+ (15 mM) decreased state 3 and increased state 4 respiratory rates to 55 and 202% of control values, respectively. Sodium- as well as tBOOH-induced state 3 inhibition required mitochondrial Ca2+ uptake because ruthenium red addition before Ca2+ addition negated the effect. The increase in state 4 respiration involved Ca2+ cycling since ruthenium red immediately returned state 4 rates back to control values. The mechanisms for the observed Ca2(+)-, Na(+)-, and tBOOH-induced alterations in pyruvate-supported respiration in vitro are discussed and a multifactorial etiology for mitochondrial respiratory dysfunction following cerebral ischemia in vivo is proposed.

Quinone imine-induced Ca2+ release from isolated rat liver mitochondria

Incubation of Ca2(+)-loaded rat liver mitochondria with N-acetyl-p-benzoquinone imine (NAPQI) or its two dimethylated analogues resulted in a concentration dependent Ca2+ release, with the following order of potency: 2,6-(Me)2-NAPQI greater than NAPQI greater than 3,5-(Me)2-NAPQI. The quinone imine-induced Ca2+ release was associated with NAD(P)H oxidation and was prevented when NAD(P)+ reduction was stimulated by the addition of 3-hydroxybutyrate. Mitochondrial glutathione was completely depleted within 0.5 min by all three quinone imines, even at low concentrations that did not result in Ca2+ release. Depletion of mitochondrial GSH by pretreatment with 1-chloro-2,4-dinitrobenzene enhanced quinone imine-induced NAD(P)H oxidation and Ca2+ release. However, 3-hydroxybutyrate protected from quinone imine-induced Ca2+ release in GSH-depleted mitochondria. Mitochondrial membrane potential was lost after the addition of quinone imines at concentrations that caused rapid Ca2+ release; however, subsequent addition of EGTA led to the complete recovery of the transmembrane potential. In the absence of Ca2+, the quinone imines caused only a small and transient loss of the transmembrane potential. Taken together, our results suggests that the quinone imine-induced Ca2+ release from mitochondria is a consequence of NAD(P)H oxidation rather than GSH depletion, GSSG formation, or mitochondrial inner membrane damage.

The prooxidant-induced and spontaneous mitochondrial calcium release: inhibition by meta-iodo-benzylguanidine (MIBG), a substrate for mono (ADP-ribosylation)

The norepinephrine analogue meta-iodo-benzylguanidine (MIBG), a substrate for mono(ADP-ribosylation) and inhibitor of eukaryotic ADP-ribosyltransferases, inhibits the prooxidant-induced and spontaneous calcium release from intact rat liver mitochondria without affecting pyridine nucleotide oxidation and hydrolysis. This finding strongly suggests regulation of calcium release by ADP-ribosylation in mitochondria, and may be relevant for the cellular and pharmacological effects of MIBG.

Perturbation in liver mitochondrial Ca2+ homeostasis in experimental iron overload: a possible factor in cell injury

The functional state of isolated mitochondria and specifically the integrity of the inner membrane, were investigated in the liver of rats made siderotic by dietary supplementation with carbonyl iron. The concentration of iron in the hepatic tissue increased progressively up to nearly 40 days and reached a steady-state level. When the iron content reached a threshold value (higher than 90 nmol/mg protein) the occurrence of in vivo lipid peroxidation in the mitochondrial membrane was detected. This process did not result in gross alterations in the mitochondrial membrane, as indicated by electron microscopy, phosphorylative capability and membrane potential measurements. On the contrary, the induction of lipoperoxidative reaction appeared to be associated with the activation of Ca2+ release from mitochondria. This was shown to occur as a consequence of rather subtle modifications in the inner membrane structure via a specific efflux route, which appeared to be linked to the oxidation level of mitochondrial pyridine nucleotides. The induction of this Ca2+ release from iron-treated mitochondria resulted in enhancement of Ca2+ cycling, a process which dissipates energy to reaccumulate into mitochondria the released Ca2+. The perturbation in mitochondrial Ca2+ homeostasis reported here may be a factor in the onset of cell damage in this experimental model of hepatic iron overload.

Fluorescent cationic probes of mitochondria. Metrics and mechanism of interaction

Mitochondria strongly accumulate amphiphilic cations. We report here a study of the association of respiring rat liver mitochondria with several fluorescent cationic dyes from differing structural classes. Using gravimetric and fluorometric analysis of dye partition, we find that dyes and mitochondria interact in three ways: (a) uptake with fluorescence quenching, (b) uptake without change in fluorescence intensity, and (c) lack of uptake. For dyes that quench upon uptake, the extent of quenching correlates with the degree of aggregation of the dye to dimers, as predicted by theory (Tomov, T.C. 1986. J. Biochem. Biophys. Methods. 13:29-38). Also predicted is the relationship observed between quenching and the mitochondria concentration when constant dye is titrated with mitochondria. Not predicted is the relationship observed between quenching and dye concentration when constant mitochondria are titrated with dye. Because a limit to dye uptake exists, in this case, the degree of quenching decreases as dye is added. A Langmuir isotherm analysis gives phenomenological parameters that predict quenching when it is observed as a function of dye concentration. By allowing for a decrease in membrane potential, caused by incorporation of cationic dye into the lipid bilayer, a modification of the Tomov theory predicts the dye titration data. We present a model of cationic dye-mitochondria interaction and discuss the use of these as probes of mitochondrial membrane potential.

Role of sulfhydryl groups in benzoquinone-induced Ca2+ release by rat liver mitochondria

Incubation of rat liver mitochondria with benzoquinone derivatives in the presence of succinate plus rotenone has been shown to cause NAD(P)H oxidation followed by Ca2+ release. Further investigation revealed: (1)p-Benzoquinone-induced Ca2+ release was not initiated by a collapse of the mitochondrial membrane potential. However, Ca2+ release and subsequent Ca2+ cycling caused limited increased membrane permeability. (2) p-Benzoquinone-induced NAD(P)H oxidation and Ca2+ release were prevented by isocitrate, 3-hydroxybutyrate, and glutamate but not by pyruvate or 2-oxoglutarate. (3) Inhibition of pyruvate and 2-oxoglutarate dehydrogenases by p-benzoquinone was attributed to arylation of the SH groups of the cofactors, CoA and lipoic acid. Isocitrate dehydrogenase was also inhibited by p-benzoquinone, but the cofactors NAD(P)H and Mn2+ protected the enzyme. Glutamate dehydrogenase was not inhibited by p-benzoquinone. (4) Arylation of mitochondrial protein thiols by p-benzoquinone was associated with an inhibition of state 3 respiration, which was attributed to the inactivation of the phosphate translocase. In contrast, state 4 respiration, and the F1.F0-ATPase and ATP/ADP translocase activities were not inhibited. It was concluded that inhibition of mitochondrial NAD(P)H dehydrogenases by arylation of critical thiol groups will decrease the NAD(P)+-reducing capacity, and possibly lower the NAD(P)H/NAD(P)+ redox status in favor of Ca2+ release.

Quantitative microchemical imaging of calcium in Na-K pump inhibited heart cells

Quantitative electron probe X-ray imaging techniques have been utilized to determine simultaneously the element content within a single cultured embryonic chick heart cell and its intracellular compartments as well as the average elemental content of several heart cells within a population. These features of microchemical imaging have permitted establishment of data regarding: (1) the heterogeneity of calcium accumulation in mitochondrial, cytoplasmic and nuclear compartments under conditions which elevate total cell calcium without producing irreversible cell injury; and (2) the variability of calcium accumulation from cell to cell within the population sampled. The results indicate that during Na-K pump inhibition (K-free HT-BSS, 10(-4) M ouabain, 60 min) elevation of mitochondrial calcium, measured in situ by electron probe X-ray microanalysis, to levels more than 100 times greater than in the basal state, may not cause irreversible mitochondrial uncoupling and cell death.

Permeability of inner mitochondrial membrane and oxidative stress

The mechanism of increase in the inner membrane permeability induced by Ca2+ plus Pi, diamide and hydroperoxides has been analyzed. (1) The permeability increase is antagonized by oligomycin and favoured by atractyloside. The promoting effect of atractyloside is strongly reduced if the mitochondria are simultaneously treated with oligomycin. (2) Addition of the free-radical scavenger, butylhydroxytoluene, results in a complete protection of the membrane with respect to the permeability increase. (3) Although membrane damage and depression of the GSH concentration are often associated, there is no direct correlation between extent of membrane damage and concentration of reduced glutathione. Abolition of the permeability increase by butylhydroxytoluene or by oligomycin is not accompanied by maintenance of a high GSH concentration in the presence of diamide or hydroperoxides. The membrane damage induced by Ca2+ plus Pi is not accompanied by a depression of the GSH concentration. (4) It is proposed that a variety of processes causing an increased permeability of the inner mitochondrial membrane merge into some ultimate common steps involving the action of oxygen radicals.

Alterations in cation homeostasis in cultured chick ventricular cells during and after recovery from adenosine triphosphate depletion

Alterations in cation homeostasis during and after recovery from myocardial ischemia may account for some of the reversible and irreversible components of myocardial cell injury. To investigate possible mechanisms involved, we exposed cultured layers of spontaneously contracting chick embryo ventricular cells to media containing 1 mM cyanide (CN) and 20 mM 2-deoxyglucose (2-DG), and zero glucose for up to 6 h, and then allowed cultured cells to recover in serum-free culture medium for 24 h. Changes in Na, K, and Ca contents, 42K uptake and efflux, ATP content, cell water content, and lactate dehydrogenase (LDH) release were measured, and compared with changes produced by exposure to 10(-3) M ouabain and severe hypoxia. Exposure to CN and 2-DG caused marked increase in cell Na (sevenfold) and Ca (fivefold) contents, and a decrease in K content (one-fifth normal), coincident with ATP depletion to one-tenth normal levels. This produced only slight cell injury, evidenced by increased LDH release. Recovery for 24 h resulted in return to near normal values (expressed in nanomoles per milligram of protein) of Na, Ca, and ATP contents. However, there was failure of cell K content to return to normal, associated with a persistent reduced net uptake of 42K, and an increase in the rate of 42K efflux. These abnormalities in K homeostasis were associated with a decrease in cell volume and water content per milligram of protein. More marked ATP depletion (to 1/100 normal values) was produced by hypoxia plus 2-DG and zero glucose, and was associated with much more severe cell injury manifested by LDH loss. Ouabain exposure resulted in a much greater Ca gain (20-30-fold), relative to increase in Na content, than did either CN and 2-DG or hypoxia; and ouabain effects were not reversible (after a 15-fold or greater increase in Ca content was produced) and were associated with significant LDH release. We conclude that these cells are resistant to cell injury caused by moderately severe Ca overload and ATP depletion produced by exposure to CN and 2-DG. However, metabolic inhibition of ATP production produces persistent abnormalities in K homeostasis, associated with functional abnormalities.