Electroneutral H+-K+ exchange in liver mitochondria. Regulation by membrane potential

Ion channel-mediated mitochondrial volume regulation and its relationship with mitochondrial dynamics

The mitochondrion, one of the important cellular organelles, has the major function of generating adenosine triphosphate and plays an important role in maintaining cellular homeostasis, governing signal transduction, regulating membrane potential, controlling programmed cell death and modulating cell proliferation. The dynamic balance of mitochondrial volume is an important factor required for maintaining the structural integrity of the organelle and exerting corresponding functions. Changes in the mitochondrial volume are closely reflected in a series of biological functions and pathological changes. The mitochondrial volume is controlled by the osmotic balance between the cytoplasm and the mitochondrial matrix. Thus, any disruption in the influx of the main ion, potassium, into the cells can disturb the osmotic balance between the cytoplasm and the matrix, leading to water movement between these compartments and subsequent alterations in mitochondrial volume. Recent studies have shown that mitochondrial volume homeostasis is closely implicated in a variety of diseases. In this review, we provide an overview of the main influencing factors and research progress in the field of mitochondrial volume homeostasis.

Mitochondrial Volume Regulation and Swelling Mechanisms in Cardiomyocytes

Mitochondrion, known as the "powerhouse" of the cell, regulates ion homeostasis, redox state, cell proliferation and differentiation, and lipid synthesis. The inner mitochondrial membrane (IMM) controls mitochondrial metabolism and function. It possesses high levels of proteins that account for ~70% of the membrane mass and are involved in the electron transport chain, oxidative phosphorylation, energy transfer, and ion transport, among others. The mitochondrial matrix volume plays a crucial role in IMM remodeling. Several ion transport mechanisms, particularly K+ and Ca2+, regulate matrix volume. Small increases in matrix volume through IMM alterations can activate mitochondrial respiration, whereas excessive swelling can impair the IMM topology and initiates mitochondria-mediated cell death. The opening of mitochondrial permeability transition pores, the well-characterized phenomenon with unknown molecular identity, in low- and high-conductance modes are involved in physiological and pathological increases of matrix volume. Despite extensive studies, the precise mechanisms underlying changes in matrix volume and IMM structural remodeling in response to energy and oxidative stressors remain unknown. This review summarizes and discusses previous studies on the mechanisms involved in regulating mitochondrial matrix volume, IMM remodeling, and the crosstalk between these processes.

LETM1-Mediated K+ and Na+ Homeostasis Regulates Mitochondrial Ca2+ Efflux

Ca²⁺ transport across the inner membrane of mitochondria (IMM) is of major importance for their functions in bioenergetics, cell death and signaling. It is therefore tightly regulated. It has been recently proposed that LETM1—an IMM protein with a crucial role in mitochondrial K⁺/H⁺ exchange and volume homeostasis—also acts as a Ca²⁺/H⁺ exchanger. Here we show for the first time that lowering LETM1 gene expression by shRNA hampers mitochondrial K⁺/H⁺ and Na⁺/H⁺ exchange. Decreased exchange activity resulted in matrix K⁺ accumulation in these mitochondria. Furthermore, LETM1 depletion selectively decreased Na⁺/Ca²⁺ exchange mediated by NCLX, as observed in the presence of ruthenium red, a blocker of the Mitochondrial Ca²⁺ Uniporter (MCU). These data confirm a key role of LETM1 in monovalent cation homeostasis, and suggest that the effects of its modulation on mitochondrial transmembrane Ca²⁺ fluxes may reflect those on Na⁺/H⁺ exchange activity.

The effect of atp-dependent potassium uptake on mitochondrial functions under acute hypoxia

The opening of mitochondrial K(+) АТР-channel (mtK(+) АТР-channel) is supposed to be important in the modulation of mitochondrial functions under hypoxia, but the underlying mechanisms have not been clarified yet. The aim of this work was to study the effect of acute hypoxia on mtK(+) АТР-channel activity and to estimate the contribution of the channel in the modulation of mitochondrial functions. MtK(+) АТР-channel activity was assessed polarographically from the rate of State 4 respiration and by potentiometric monitoring of potassium efflux from deenergized mitochondria. It was shown that hypoxia reliably increased mtK(+) АТР-channel activity, which resulted in the changes of respiration rates (increase of State 4 and suppression of State 3 respiration), uncoupling (the decrease of respiratory control ratio) and suppression of phosphorylation. These effects were well mimicked by mtK(+) АТР-channel opener diazoxide (DZ) in isolated rat liver mitochondria. MtK(+) АТР-channel opening in vitro suppressed phosphorylation too, but increased phosphorylation efficiency, while mtK(+) АТР-channel blockers reduced it dramatically. The correlation was established between mtK(+) АТР-channel activity and the endurance of the rats to physical training under hypoxia. Hypoxia improved physical endurance, but treatment by mtK(+) АТР-channel blockers glibenklamide and 5-hydroxydecanoate (5-HD) prior to hypoxia strongly reduced both the channel activity and the endurance limits. This was in accord with the observation that under glibenklamide and 5-HD administration hypoxia failed to restore mtK(+) АТР-channel activity. Based on the experiments, we came to the conclusion that mtK(+) АТР-channel opening played a decisive role in the regulation of energy metabolism under acute hypoxia via the modulation of phosphorylation system in mitochondria.

Effect of potential-dependent potassium uptake on production of reactive oxygen species in rat brain mitochondria

The effect of potential-dependent potassium uptake on reactive oxygen species (ROS) generation in mitochondria of rat brain was studied. It was found that the effect of K+ uptake on ROS production in the brain mitochondria under steady-state conditions (state 4) was determined by potassium-dependent changes in the membrane potential of the mitochondria (ΔΨm). At K+ concentrations within the range of 0-120 mM, an increase in the initial rate of K(+)-uptake into the matrix resulted in a decrease in the steady-state rate of ROS generation due to the K(+)-induced depolarization of the mitochondrial membrane. The selective blockage of the ATP-dependent potassium channel (K(ATP)(+)-channel) by glibenclamide and 5-hydroxydecanoate resulted in an increase in ROS production due to the membrane repolarization caused by partial inhibition of the potential-dependent K+ uptake. The ATP-dependent transport of K+ was shown to be ~40% of the potential-dependent K+ uptake in the brain mitochondria. Based on the findings of the experiments, the potential-dependent transport of K+ was concluded to be a physiologically important regulator of ROS generation in the brain mitochondria and that the functional activity of the native K(ATP)(+)-channel in these organelles under physiological conditions can be an effective tool for preventing ROS overproduction in brain neurons.

Mitochondrial matrix K+ flux independent of large-conductance Ca2+-activated K+ channel opening

Large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) in the inner mitochondrial membrane may play a role in protecting against cardiac ischemia-reperfusion injury. NS1619 (30 microM), an activator of BK(Ca) channels, was shown to increase respiration and to stimulate reactive oxygen species generation in isolated cardiac mitochondria energized with succinate. Here, we tested effects of NS1619 to alter matrix K(+), H(+), and swelling in mitochondria isolated from guinea pig hearts. We found that 30 microM NS1619 did not change matrix K(+), H(+), and swelling, but that 50 and 100 microM NS1619 caused a concentration-dependent increase in matrix K(+) influx (PBFI fluorescence) only when quinine was present to block K(+)/H(+) exchange (KHE); this was accompanied by increased mitochondrial matrix volume (light scattering). Matrix pH (BCECF fluorescence) was decreased slightly by 50 and 100 microM NS1619 but markedly more so when quinine was present. NS1619 (100 microM) caused a significant leak in lipid bilayers, and this was enhanced in the presence of quinine. The K(+) ionophore valinomycin (0.25 nM), which like NS1619 increased matrix volume and increased K(+) influx in the presence of quinine, caused matrix alkalinization followed by acidification when quinine was absent, and only alkalinization when quinine was present. If K(+) is exchanged instantly by H(+) through activated KHE, then matrix K(+) influx should stimulate H(+) influx through KHE and cause matrix acidification. Our results indicate that KHE is not activated immediately by NS1619-induced K(+) influx, that NS1619 induces matrix K(+) and H(+) influx through a nonspecific transport mechanism, and that enhancement with quinine is not due to the blocking of KHE, but to a nonspecific effect of quinine to enhance current leak by NS1619.

Calcium-dependent spontaneously reversible remodeling of brain mitochondria

An exposure of cultured hippocampal neurons expressing mitochondrially targeted enhanced yellow fluorescent protein to excitotoxic glutamate resulted in reversible mitochondrial remodeling that in many instances could be interpreted as swelling. Remodeling was not evident if glutamate receptors were blocked with MK801, if Ca(2+) was omitted or substituted for Sr(2+) in the bath solution, if neurons were treated with carbonylcyanide p-trifluoromethoxyphenylhydrazone to depolarize mitochondria, or if neurons were pretreated with cyclosporin A or N-methyl-4-isoleucine-cyclosporin (NIM811) to inhibit the mitochondrial permeability transition. In the experiments with isolated brain synaptic or nonsynaptic mitochondria, Ca(2+) triggered transient, spontaneously reversible cyclosporin A-sensitive swelling closely resembling remodeling of organelles in cultured neurons. The swelling was accompanied by the release of cytochrome c, Smac/DIABLO, Omi/HtrA2, and AIF but not endonuclease G. Depolarization with carbonylcyanide p-trifluoromethoxyphenylhydrazone or inhibition of the Ca(2+) uniporter with Ru360 prevented rapid onset of the swelling. Sr(2+) depolarized mitochondria but failed to induce swelling. Neither inhibitors of the large conductance Ca(2+)-activated K(+) channel (charybdotoxin, iberiotoxin, quinine, and Ba(2+)) nor inhibitors of the mitochondrial ATP-sensitive K(+) channel (5-hydroxydecanoate and glibenclamide) suppressed swelling. Quinine, dicyclohexylcarbodiimide, and Mg(2+), inhibitors of the mitochondrial K(+)/H(+) exchanger, as well as external alkalization inhibited a recovery phase of the reversible swelling. In contrast to brain mitochondria, liver and heart mitochondria challenged with Ca(2+) experienced sustained swelling without spontaneous recovery. The proposed model suggests an involvement of the Ca(2+)-dependent transient K(+) influx into the matrix causing mitochondrial swelling followed by activation of the K(+)/H(+) exchanger leading to spontaneous mitochondrial contraction both in situ and in vitro.

Lack of manifestations of diazoxide/5-hydroxydecanoate-sensitive KATP channel in rat brain nonsynaptosomal mitochondria

Pharmacological modulation of the mitochondrial ATP-sensitive K+ channel (mitoKATP) sensitive to diazoxide and 5-hydroxydecanoate (5-HD) represents an attractive strategy to protect cells against ischaemia/reperfusion- and stroke-related injury. To re-evaluate a functional role for the mitoKATP in brain, we used Percoll-gradient-purified brain nonsynaptosomal mitochondria in a light absorbance assay, in radioisotope measurements of matrix volume, and in measurements of respiration, membrane potential (DeltaPsi) and depolarization-induced K+ efflux. The changes in mitochondrial morphology were evaluated by transmission electron microscopy (TEM). Polyclonal antibodies raised against certain fragments of known sulphonylurea receptor subunits, SUR1 and SUR2, and against different epitopes of K+ inward rectifier subunits Kir 6.1 and Kir 6.2 of the ATP-sensitive K+ channel of the plasma membrane (cellKATP), were employed to detect similar subunits in brain mitochondria. A variety of plausible blockers (ATP, 5-hydroxydecanoate, glibenclamide, tetraphenylphosphonium cation) and openers (diazoxide, pinacidil, chromakalim, minoxidil, testosterone) of the putative mitoKATP were applied to show the role of the channel in regulating matrix volume, respiration, and DeltaPsi and K+ fluxes across the inner mitochondrial membrane. None of the pharmacological agents applied to brain mitochondria in the various assays pinpointed processes that could be unequivocally associated with mitoKATP activity. In addition, immunoblotting analysis did not provide explicit evidence for the presence of the mitoKATP, similar to the cellKATP, in brain mitochondria. On the other hand, the depolarization-evoked release of K+ suppressed by ATP could be re-activated by carboxyatractyloside, an inhibitor of the adenine nucleotide translocase (ANT). Moreover, bongkrekic acid, another inhibitor of the ANT, inhibited K+ efflux similarly to ATP. These observations implicate the ANT in ATP-sensitive K+ transport in brain mitochondria.

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Stimulation of potassium cycling in mitochondria by long-chain fatty acids

Nonesterified long-chain fatty acids (myristic, palmitic, oleic and arachidonic), added at low amounts (around 20 nmol/mg protein) to rat liver mitochondria, energized by respiratory substrates and suspended in isotonic solutions of KCl, NaCl, RbCl or CsCl, adjusted to pH 8.0, induce a large-scale swelling followed by a spontaneous contraction. Such swelling does not occur in alkaline solutions of choline chloride or potassium gluconate or sucrose. These changes in the matrix volume reflect a net uptake, followed by net extrusion, of KCl (or another alkali metal chloride) and are characterized by the following features: (1) Lowering of medium pH from 8.0 to 7.2 results in a disappearance of the swelling-contraction reaction. (2) The contraction phase disappears when the respiration is blocked by antimycin A. (3) Quinine, an inhibitor of the K(+)/H(+) antiporter, does not affect swelling but suppresses the contraction phase. (4) The swelling phase is accompanied by a decrease of the transmembrane potential and an increase of respiration, whereas the contraction is followed by an increase of the membrane potential and a decrease of oxygen uptake. (5) Nigericin, a catalyst of the K(+)/H(+) exchange, prevents or partly reverses the swelling and partly restores the depressed membrane potential. These results indicate that long-chain fatty acids activate in liver mitochondria suspended in alkaline saline media the uniporter of monovalent alkali metal cations, the K(+)/H(+) antiporter and the inner membrane anion channel. These effects are presumably related to depletion of mitochondrial Mg(2+), as reported previously [Arch. Biochem. Biophys. 403 (2002) 16], and are responsible for the energy-dissipating K(+) cycling. The uniporter and the K(+)/H(+) antiporter are in different ways activated by membrane stretching and/or unfolding, resulting in swelling followed by contraction.

Mitochondrial alterations induced by the p13II protein of human T-cell leukemia virus type 1. Critical role of arginine residues

Human T-cell leukemia virus type 1 encodes a number of "accessory" proteins of unclear function; one of these proteins, p13(II), is targeted to mitochondria and disrupts mitochondrial morphology. The present study was undertaken to unravel the function of p13(II) through (i) determination of its submitochondrial localization and sequences required to alter mitochondrial morphology and (ii) an assessment of the biophysical and biological properties of synthetic peptides spanning residues 9-41 (p13(9-41)), which include the amphipathic mitochondrial-targeting sequence of the protein. p13(9-41) folded into an alpha helix in micellar environments. Fractionation and immunogold labeling indicated that full-length p13(II) accumulates in the inner mitochondrial membrane. p13(9-41) induced energy-dependent swelling of isolated mitochondria by increasing inner membrane permeability to small cations (Na(+), K(+)) and released Ca(2+) from Ca(2+)-preloaded mitochondria. These effects as well as the ability of full-length p13(II) to alter mitochondrial morphology in cells required the presence of four arginines, forming the charged face of the targeting signal. The mitochondrial effects of p13(9-41) were insensitive to cyclosporin A, suggesting that full-length p13(II) might alter mitochondrial permeability through a permeability transition pore-independent mechanism, thus distinguishing it from the mitochondrial proteins Vpr and X of human immunodeficiency virus type 1 and hepatitis B virus, respectively.

Na+/K+-ATPase inhibition during cardiac myocyte swelling: involvement of intracellular pH and Ca2+

Previous studies in chick embryo cardiac myocytes have shown that the inhibition of Na+/K+-ATPase with ouabain induces cell shrinkage in an isosmotic environment (290 mOsm). The same inhibition produces an enhanced RVD (regulatory volume decrease) in hyposmotic conditions (100 mOsm). It is also known that submitting chick embryo cardiomyocytes to a hyperosmotic solution induces shrinkage and a concurrent intracellular alkalization. The objective of this study was to evaluate the involvement of intracellular pH (pHi), intracellular Ca2+ ([Ca2+]i) and Na+/K+-ATPase inhibition during hyposmotic swelling. Changes in intracellular pH and Ca2+ were monitored using BCECF and fura-2, respectively. The addition of ouabain (100 microM) under both isosmotic and hyposmotic stimuli resulted in a large increase in [Ca2+]i (200%). A decrease in pHi (from 7.3 +/- 0.09 to 6.4 +/- 0.08, n = 6; p < 0.05) was only observed when ouabain was applied during hyposmotic swelling. This acidification was prevented by the removal of extracellular Ca2+. Inhibition of Na+/H+ exchange with amiloride (1 mM) had no effect on the ouabain-induced acidification. Preventing the mitochondrial accumulation of Ca2+ using CCCP (10 microM) resulted in a blockade of the progressive acidification normally induced by ouabain. The inhibition of mitochondrial membrane K+/H+ exchange with DCCD (1 mM) also completely prevented the acidification. Our results suggest that intracellular acidification upon cell swelling is mediated by an initial Ca2+ influx via Na+/Ca2+ exchange, which under hyposmotic conditions activates the K+ and Ca2+ mitochondrial exchange systems (K+/H+ and Ca2+/H+).

Matrix free Mg(2+) and the regulation of mitochondrial volume

Mitochondria must maintain volume homeostasis in order to carry out oxidative phosphorylation. It has been postulated that the concentration of free Mg(2+) ([Mg(2+)]) serves as the sensor of matrix volume and regulates a K(+)-extruding K(+)/H(+) antiport (K. D. Garlid. J. Biol. Chem. 255: 11273-11279, 1980). To test this hypothesis, the fluorescent probe furaptra was used to monitor [Mg(2+)] and free Ca(2+) concentration ([Ca(2+)]) in the matrix of isolated beef heart mitochondria, and K(+)/H(+) antiport activity was measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 microM matrix [Mg(2+)] and 2.2 microM [Ca(2+)] were determined for the K(+)/H(+) antiport. Untreated mitochondria average 670 microM matrix [Mg(2+)], a value that would permit <1% of maximum K(+)/H(+) antiport activity. Hypotonic swelling results in large decreases in matrix [Mg(2+)], but swelling due to accumulation of acetate salts does not alter [Mg(2+)]. Swelling in phosphate salts decreases matrix [Mg(2+)], but not to levels that permit appreciable antiport activity. We conclude that 1) it is unlikely that matrix [Mg(2+)] serves as the mitochondrial volume sensor, 2) if K(+)/H(+) antiport functions as a volume control transporter, it is probably regulated by factors other than [Mg(2+)], and 3) alternative mechanisms for mitochondrial volume control should be considered.

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

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

Mechanism of early carbon tetrachloride toxicity in cultured rat hepatocytes

CCl4 causes liver necrosis in a dose-dependent manner in vivo. However, we found that primary rat hepatocytes in culture were killed after a 2 hr incubation with carbon tetrachloride gas at CCl4 partial pressures above a threshold between 45 and 54 mmHg. Below this threshold concentration no increase in hepatocyte death was observed. We sought to explain the very abrupt CCl4 concentration threshold for hepatocyte death. Two inhibitors of cytochrome P450 2E1, cimetidine and diallyl sulfide, inhibited lipid peroxidation as measured by production of isoprostanes, but did not reduce hepatocyte death from CCl4. At 37 degrees, CCl4 accelerated the mitochondrial permeability transition in vitro, at a threshold CCl4 concentration similar to that which caused hepatocyte death. Phospholipase A2 inhibitors, mepacrine and 4-bromophenacyl bromide, inhibited the increase in mitochondrial permeability, but did not inhibit hepatocyte death caused by CCl4. Rat liver microsomal lipids were used to make liposomes loaded with Ponceau Red (FW 760.6). No leakage of Ponceau red was found at CCl4 concentrations greater than the threshold for cell death. However, CCl4 caused acceleration of liposome fusion, over the CCl4 concentration range spanning the threshold for hepatocyte death. Early hepatocyte death in cell culture is independent of metabolism of CCl4, and may be related to direct effects of CCl4 on intracellular membranes.

Cation transport systems in mitochondria: Na+ and K+ uniports and exchangers

It is now well established that mitochondria contain three antiporters that transport monovalent cations. A latent, allosterically regulated K+/H+ antiport appears to serve as a cation-extruding device that helps maintain mitochondrial volume homeostasis. An apparently unregulated Na+/H+ antiport keeps matrix [Na+] low and the Na(+)-gradient equal to the H(+)-gradient. A Na+/Ca2+ antiport provides a Ca(2+)-extruding mechanism that permits the mitochondrion to regulate matrix [Ca2+] by balancing Ca2+ efflux against influx on the Ca(2+)-uniport. All three antiports have well-defined physiological roles and their molecular properties and regulatory features are now being determined. Mitochondria also contain monovalent cation uniports, such as the recently described ATP- and glibenclamide-sensitive K+ channel and ruthenium red-sensitive uniports for Na+ and K+. A physiological role of such uniports has not been established and their properties are just beginning to be defined.

Effect of buffers and osmolality on anion uniport across the mitochondrial inner membrane

The effects of buffers and osmolality of the suspending medium on the pH-dependent anion uniport across the inner membrane of isolated rat liver mitochondria have been studied using the light scattering technique to measure passive osmotic swelling. In contrast to some other transport processes the rates of entry of chloride and other anions via the anion-conducting channel decreased steeply with increasing solute concentration. This effect appears to be a result of increased osmolality or decreased matrix volume rather than inhibition by the anion since it was also produced by increasing the osmolality by addition of non-penetrant solutes. The effects of some pH buffers on the mitochondrial anion-conducting channel were also investigated. Some zwitterionic buffers had little effect other than that produced by increasing osmolality but Tricine, Popso and Caps produced marked additional inhibition of anion uniport and several other zwitterionic buffers were also inhibitory. The correlation between increased anion conductivity and increased matrix volume supports the proposal that this channel functions in regulation of the volume of the mitochondrial matrix.

K+/H+ exchange in yeast mitochondria: sensitivity to inhibitors, solubilization and reconstitution of the activity in proteoliposomes

The K+/H+ exchange activity of the inner mitochondrial membrane was investigated in the yeast Saccharomyces cerevisiae. Swelling experiments in potassium acetate indicated that the K+/H+ exchange was active without any additional treatment after the mitochondria isolation, such as a Mg2+ depletion. As in mammalian mitochondria, the activity of yeast mitochondria was stimulated by increasing pH and was inhibited by the amphiphilic amines quinine and propranolol and by the carboxyl reagent dicyclohexylcarbodiimide. However, the activity was poorly inhibited by Mg2+ and consequently was only slightly stimulated by the Mg2+/H+ exchanger A23187. On the other hand, Zn2+ was very efficient for inhibiting the exchange and consequently the activity was strongly stimulated by the permeant metal-chelator o-phenanthroline. The [86Rb]Rb+ accumulation in mitochondria and mitoplasts was only partially inhibited by quinine and propranolol suggesting that part of the accumulation monitored under these conditions was due to cation leak through the inner membrane together with adsorption on the membrane. The DCCD-sensitive activity could be reconstituted from mitochondria and from mitoplasts solubilized with Triton X-100; this activity, measured by [86Rb]Rb+ accumulation, was quinine- and propranolol-sensitive. A spectrophotometric method, based on the capacity of negatively charged proteoliposomes to swell, was then developed in order to continuously follow the reconstituted activity.

Tracking of proton flow during transition from anaerobiosis to steady state. 1. Response of matrix pH indicators

1. The kinetics of acidification and realkalinization of the matrix after addition of nigericin to respiring and non-respiring mitochondria, recorded by intramitochondrial pH indicators such as neutral red and 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), is complementary to that recorded by extramitochondrial pH indicators. The extent of acidification decreases with the logarithm of the KCl concentration and is inhibited by Pi and ammonium ions. 2. Proton translocation during respiration has been compared with proton extraction from matrix bulk water. During oxygen pulses to EGTA-untreated mitochondria, BCECF records an extraction of protons from matrix bulk water of about 2-3 nmol H+/mg, reduced to 1-2 nmol H+/mg in EGTA-treated mitochondria. Since the amount of proton translocation required to achieve steady state is of the order of 6-7 nmol H+/mg, it appears that 75-90% of the protons are not extracted from matrix bulk water. Only a slight response is recorded by neutral red. 3. The effect of permeant cations and of uncouplers on the distribution of proton extraction between membrane and matrix bulk water has been studied in presteady state. During Sr2+ uptake, proton extrusion into cytosolic bulk water, as well as proton extraction from matrix bulk water, corresponds almost to 100% of the protons translocated by the redox proton pumps. In the absence of Sr2+, parallel to the disappearance of the proton extrusion in cytosolic bulk water, the proton extraction from matrix bulk water diminishes to about 20% of the proton translocation. 4. The mechanism by which divalent cation uptake and protonophoric uncouplers affect the distribution of proton extraction between matrix bulk water and membrane domains and the nature of the membrane domains are discussed.

Phenylarsine oxide induces the cyclosporin A-sensitive membrane permeability transition in rat liver mitochondria

This paper reports an investigation on the effects of the hydrophobic, bifunctional SH group reagent phenylarsine oxide (PhAsO) on mitochondrial membrane permeability. We show that PhAsO is a potent inducer of the mitochondrial permeability transition in a process which is sensitive to both the oxygen radical scavanger BHT and to cyclosporin A. The PhAsO-induced permeability transition is stimulated by Ca2+ but takes place also in the presence of EGTA in a process that maintains its sensitivity to BHT and cyclosporin A. Our findings suggest that, at variance from other known inducers of the permeability transition, PhAsO reacts directly with functional SH groups that are inaccessible to hydrophilic reagents in the absence of Ca2+.

Regulation of mitochondrial K+/H+ antiport activity by hydrogen ions

The effect of matrix pH (pHi) on the activity of the mitochondrial K+/H+ antiport has been studied using the fluorescence of 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) to monitor pHi and passive swelling in K+ acetate to follow antiport activity. Heart mitochondria suspended in hypotonic K+ acetate in the absence of respiration show an initial delta pH of -0.4 (interior acid) that decays slowly. Addition of A23187 to deplete matrix Mg2+ results in a further acid shift in pHi followed by equilibration of delta pH. This equilibration appears to depend on K+/H+ antiport and is slow at acid pHi but very rapid when the matrix is alkaline. Swelling of Mg(2+)-depleted mitochondria in K+ acetate is multiphasic with a slow initial rate, a period of maximum swelling, and a final period in which the rate declines. At constant external pH (pH0), the initial rate of swelling is faster with increasing pHi and the time to the onset of the maximum swelling rate decreases. The maximum swelling rate is initiated at pHi 7.4 when pH0 is 7.8 and at pHi 7.1 when pH0 is 7.4. The maximum rate of swelling increases linearly with increasing pH0 in the range from 7.0 to 8.2. This rate also shows a linear relationship to the value of pHi at the time the maximum rate is attained. Dixon plots of the reciprocal of the maximum swelling rate vs [H+]0 suggest that external [H+] is a noncompetitive inhibitor of K+ entry on the antiport. It is concluded that K+/H+ antiport in Mg(2+)-depleted heart mitochondria can be regulated by matrix [H+] (see Beavis, A. D., and Garlid, K. D. (1990) J. Biol. Chem. 265, 2538-2545), but that this antiport is also sensitive to external [H+] or to delta pH when it acts in the direction of K+ uptake.

A gated pathway for electrophoretic Na+ fluxes in rat liver mitochondria. Regulation by surface Mg2+

Addition of EDTA to mitochondria incubated aerobically in a phosphate-supplemented medium containing Na+ ions results in activation of cation uptake which is accompanied by membrane depolarization and stimulation of respiration. The same results are obtained in media containing Li+ but not K+, indicating that this pathway for cation transport is selective. The activation of Na+ transport is not accompanied by changes of matrix Mg2+, indicating that cation transport is controlled by surface-bound rather than intramitochondrial Mg2+. Na+ transport in respiring mitochondria is competitively inhibited by Mg2+ with a Ki in the nanomolar range. A Na+ current can also be induced by a K+ diffusion potential in the absence of respiration. The K(+)-diffusion-driven Na+ current has the same magnitude in the absence or presence of inorganic phosphate, suggesting that Na+ transport is mediated by Na+ uniport rather than by electrogenic nNa+/H+ antiport with n greater than 1. Analysis of the flow/force relationship indicates that the putative Na+ uniporter has a conductance of about 0.2 nmol Na+ x mg protein-1 x min-1 x mV-1, and that it is active only when the membrane potential exceeds about 150 mV.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

Cyanide toxicity in hepatocytes under aerobic and anaerobic conditions

The effect of cyanide on cell viability and mitochondrial function was studied in hepatocytes exposed to air or argon. Cells were more susceptible to cyanide toxicity under air than under argon. Analysis of the disposition of cyanide showed that the difference in susceptibility to KCN was not due to O2-dependent differences in cyanide metabolism or elimination. Studies of mitochondrial function revealed that cyanide under aerobic conditions resulted in substantial swelling of the mitochondria, which corresponded to a matrix loading of phosphate. In addition, cyanide caused a loss of the mitochondrial protonmotive force. This was in contrast to the results for cells exposed to 30 min of anoxia alone in which there was no loss of mitochondrial delta pH, no detectable change in mitochondrial volume, and little matrix loading of phosphate. These results show that at least some of the protective mechanisms elicited by anoxia (B. S. Andersson, T. Y. Aw, and D. P. Jones. Am. J. Physiol. 252 (Cell Physiol. 21): C349-C355, 1987) are not elicited by cyanide alone. Thus cyanide under aerobic conditions does not provide a completely valid model for simple anoxia. Moreover, the results suggest that the molecular sensor necessary to signal suppression of metabolic and transport functions during neahypoxia is dependent on O2 and is neither stimulated nor antagonized by KCN.

Intravesicular pH changes in submitochondrial particles induced by monovalent cations: relationship to the Na+/H+ and K+/H+ antiporters

The fluorescence of internalized fluorescein isothiocyanate dextran has been used to monitor the intravesicular pH of submitochondrial particles (SMP). Respiring SMP maintain a steady-state delta pH (interior acid) that results from the inwardly directed H+ flux of respiration and an opposing passive H+ leak. Addition of K+, Na+, or Li+ to SMP results in a shift to a more alkaline interior pH (pHi) in both respiring and nonrespiring SMP. The K+-dependent change in pHi, like the K+/H+ antiport in intact mitochondria, is inhibited by quinine and by dicyclohexylcarbodiimide. The Na+-dependent reaction is only partially inhibited by these reagents. Both the Na+- and the K+-dependent pH changes are sensitive to amiloride derivatives. The Km for both Na+ and K+ is near 20 mM whereas that for Li+ is closer to 10 mM. The K+/H+ exchange reaction is only slightly inhibited by added Mg2+, but abolished when A23187 is added with Mg2+. The passive exchange is optimal at pHi 6.5 with either Na+ or K+, and cannot be detected above pHi of 7.2. Both the Na+/H+ and the K+/H+ exchange reactions are optimal at an external pH of 7.8 in respiring SMP (pHi 7.1). Valinomycin stimulates the K+-dependent pH change in nonrespiring SMP, as does nigericin. It is concluded that SMP show K+/H+ antiport activity with properties distinct from those of Na+/H+ antiport. However, the properties of the K+/H+ exchange do not correspond in all respects to those of the antiport in intact mitochondria. Donnan equilibria and parallel uniport pathways for H+ and cations appear to contribute to cation-dependent pH changes in SMP.

Unique relationships between the rates of oxidation and phosphorylation and the protonmotive force in rat-liver mitochondria

The rate of ATP synthesis (JP) in isolated rat-liver mitochondria was strongly dependent on the magnitude of the protonmotive force (delta mu H+) across the mitochondrial inner membrane. Addition of different concentrations of various uncouplers or malonate to mitochondrial incubations in State 3 led to a depression of delta mu H+ and a concomitant decrease in JP. A unique relationship between JP and delta mu H+ was obtained, which was independent of the way in which delta mu H+ was varied. This unique relationship was observed when K+ (in the presence of valinomycin) was used as a probe for delta psi. Different relationships between JP and delta mu H+ were observed when K+ was used as a probe for delta psi and when K+ was measured after separation of the mitochondria by centrifugation without silicone oil. This led to a serious underestimation of delta psi, specifically when uncouplers were present, and non-unique flow-force relationships were thus obtained. Anomalous relationships between JP and delta mu H+ were also found when TPMP+ was used as a probe for delta psi. However, in uncoupler incubations the presence of TBP- strongly affected the TPMP+ accumulation ratio without any effect on the K+ accumulation or on JP and in the presence of TBP- unique relationships between JP and delta mu H+ were again obtained. This indicates that the accumulation of TPMP+ inside the mitochondria is not a straightforward function of delta psi but also depends on conditions like the presence of TBP- or uncouplers. We conclude that there is a unique relationship between the rate of phosphorylation and the protonmotive force in mitochondria and that under some conditions the behaviour of TPMP+ is anomalous.

Reconstitution of transmembrane K+ transport with a 53 kilodalton mitochondrial protein

A 53 kDa protein has been purified from a Triton X-100 extract of liver mitochondrial membranes, by affinity chromatography on immobilized quinine, a K+ transport inhibitor. KCl-containing lipid vesicles reconstituted with this protein lose K+ to a medium low in K+ faster than vesicles lacking protein. With bacteriorhodopsin reconstituted in vesicles containing K+, light induces faster development of a pH gradient if the 53 kDa protein is included during vesicle preparation. This effect is like that of valinomycin, which catalyzes K+ efflux, dissipating the membrane potential arising from H+ entry. Evidence that vesicles containing the 53 kDa protein are permeable to K+, but exhibit low permeability to H+, indicates that this protein acts as a K+ uniporter.

Membrane-potential-dependent changes in the stoichiometry of charge translocation by the mitochondrial electron transport chain

The charge/oxygen (q+/O) stoichiometry of mitochondria respiring on succinate was measured under conditions of high membrane potential (delta psi). The technique used was a variation of the steady-state method of Al-Shawi and Brand [(1981) Biochem. J. 200, 539-546]. We show that q+/O was about 2.7 at high values of delta psi (170 mV). As delta psi was lowered from 170 mV to 85 mV with the respiratory inhibitor malonate the q+/O stoichiometry increased to 6.0. A number of artefacts which could have led to an underestimation of the q+/O stoichiometry were eliminated. These included effects of any rapid change in mitochondrial volume, internal pH, activity of the endogenous K+/H+ exchanger or in H+ conductance due to changes in delta psi after the addition of inhibitor. The experiments presented here are the first direct demonstration that the stoichiometry of proton pumping by the mitochondrial respiratory chain changes as delta psi is varied.

K+/H+ antiport in mitochondria

Mitochondria contain a latent K+/H+ antiporter that is activated by Mg2+-depletion and shows optimal activity in alkaline, hypotonic suspending media. This K+/H+ antiport activity appears responsible for a respiration-dependent extrusion of endogenous K+, for passive swelling in K+ acetate and other media, for a passive exchange of matrix 42K+ against external K+, Na+, or Li+, and for the respiration-dependent ion extrusion and osmotic contraction of mitochondria swollen passively in K+ nitrate. K+/H+ antiport is inhibited by quinine and by dicyclohexylcarbodiimide when this reagent is reacted with Mg2+-depleted mitochondria. There is good suggestive evidence that the K+/H+ antiport may serve as the endogenous K+-extruding device of the mitochondrion. There is also considerable experimental support for the concept that the K+/H+ antiport is regulated to prevent futile influx-efflux cycling of K+. However, it is not yet clear whether such regulation depends on matrix free Mg2+, on membrane conformational changes, or other as yet unknown factors.

Respiration-dependent contraction of swollen heart mitochondria: participation of the K+/H+ antiporter

Respiration-dependent contraction of heart mitochondria swollen passively in K+ nitrate is activated by the ionophore A23187 and inhibited by Mg2+. Ion extrusion and osmotic contraction under these conditions are strongly inhibited by quinine, a known inhibitor of the mitochondrial K+/H+ antiporter, as measured in other systems. The inhibition by quinine is relieved by the exogenous antiporter nigericin. Respiration-dependent contraction is also inhibited by dicyclohexylcarbodiimide (DCCD) when reacted under conditions known to inhibit K+/H+ antiport (Martin et al., J. Biol. Chem. 259, 2062-2065, 1984). These studies strongly support the concept that K+ is extruded from the matrix by the endogenous K+/H+ antiporter and that inhibition of this component by quinine or DCCD inhibits respiration-dependent contraction. The extrusion of K+ nitrate is accompanied by a respiration-dependent efflux of a considerable portion of the endogenous Mg2+. This Mg2+ efflux does not occur in the presence of nigericin or when the mitochondrial Na+/H+ antiporter is active. Mg2+ efflux may take place on the K+/H+ antiporter. DCCD, reacted under conditions that do not result in inhibition of the K+/H+ antiporter, blocks a monovalent cation uniport pathway. This uniport contributes to futile cation cycling at elevated pH, and its inhibition by DCCD stimulates respiration-dependent contraction.

Sensitivity of mitochondrial Mg++ flux to reagents which affect K+ flux

Effects on Mg++ transport in rat liver mitochondria of three reagents earlier shown to affect mitochondrial K+ transport have been examined. The sulfhydryl reactive reagent phenylarsine oxide, which activates K+ flux into respiring mitochondria, also stimulates Mg++ influx. The K+ analog Ba++, when taken up into the mitochondrial matrix, inhibits influx of both K+ and Mg++. The effect on Mg++ influx is seen only if Mg++, which blocks Ba++ accumulation, is added after a preincubation with Ba++. Thus the inhibition of Mg++ influx appears to require interaction of Ba++ at the matrix side of the inner mitochondrial membrane. Added Ba++ also diminishes observed rates of Mg++ efflux but not K+ efflux. This difference may relate to a higher concentration of Ba++ remaining in the medium in the presence of Mg++ under the conditions of our experiments. Pretreatment of mitochondria with dicyclohexyl-carbodiimide (DCCD), under conditions which result in an increase in the apparent Km for K+ of the K+ influx mechanism, results in inhibition of Mg++ influx from media containing approximately 0.2 mM Mg++. The inhibitory effect of DCCD on Mg++ influx is not seen at higher external Mg++ (0.8 mM). This dependence on cation concentration is similar to the dependence on K+ concentration of the inhibitory effect of DCCD on K+ influx. Although mitochondrial Mg++ and K+ transport mechanisms exhibit similar reagent sensitivities, whether Mg++ and K+ share common transport catalysis remains to be established.

Valinomycin-induced chloride permeability in isolated rat liver mitochondria

1. Ionophore-induced osmotic swelling was used to study Cl- transport in isolated rat liver mitochondria. 2. Energy-dependent, neutral ionophore-induced swelling in Cl- salts at pH 7.2 required K+ and was preceded by a brief lag phase that was absent in chlorotributyltin-induced swelling. 3. Treatments that stimulated or inhibited mitochondrial K+/H+ exchange had qualitatively similar effects on both valinomycin-induced swelling and the associated lag phase. 4. The results suggest that valinomycin-induced Cl- permeability results from an interaction between the K+/H+ antiporter and neutral ionophore K+ complexes.

Pathways for Ca2+ efflux in heart and liver mitochondria

1. Two processes of Ruthenium Red-insensitive Ca2+ efflux exist in liver and in heart mitochondria: one Na+-independent, and another Na+-dependent. The processes attain maximal rates of 1.4 and 3.0 nmol of Ca2+.min-1.mg-1 for the Na+-dependent and 1.2 and 2.0 nmol of Ca2+.min-1.mg-1 for the Na+-independent, in liver and heart mitochondria, respectively. 2. The Na+-dependent pathway is inhibited, both in heart and in liver mitochondria, by the Ca2+ antagonist diltiazem with a Ki of 4 microM. The Na+-independent pathway is inhibited by diltiazem with a Ki of 250 microM in liver mitochondria, while it behaves as almost insensitive to diltiazem in heart mitochondria. 3. Stretching of the mitochondrial inner membrane in hypo-osmotic media results in activation of the Na+-independent pathway both in liver and in heart mitochondria. 4. Both in heart and liver mitochondria the Na+-independent pathway is insensitive to variations of medium pH around physiological values, while the Na+-dependent pathway is markedly stimulated parallel with acidification of the medium. The pH-activated, Na+-dependent pathway maintains the diltiazem sensitivity. 5. In heart mitochondria, the Na+-dependent pathway is non-competitively inhibited by Mg2+ with a Ki of 0.27 mM, while the Na+-independent pathway is less affected; similarly, in liver mitochondria Mg2+ inhibits the Na+-dependent pathway more than it does the Na+-independent pathway. In the presence of physiological concentrations of Na+, Ca2+ and Mg2+, the Na+-independent and the Na+-dependent pathways operate at rates, respectively, of 0.5 and 1.0 nmol of Ca2+.min-1.mg-1 in heart mitochondria and 0.9 and 0.2 nmol of Ca2+.min-1.mg-1 in liver mitochondria. It is concluded that both heart and liver mitochondria possess two independent pathways for Ca2+ efflux operating at comparable rates.

Suppression of mitochondrial respiratory function after short-term anoxia

Exposure of rat hepatocytes to 30 min anoxia resulted in a substantial decrease in O2 consumption on reoxygenation. Measurement of the sequestered Ca2+ pool of mitochondria by selective release with the protonophore, carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP), and quantitation with the metallochromic indicator, arsenazo III, showed that anoxia caused a marked decrease in mitochondrial Ca2+. This loss could, in part, be due to decreased electrophoretic uptake resulting from a 20% decrease in the magnitude of the mitochondrial transmembranal potential. The decrease was associated with a decrease in ATP synthase activity as expected from the Ca2+ dependence of endogenous inhibitor binding to the ATP synthase. These results show that short-term anoxia suppresses mitochondrial function in hepatocytes and suggest that mitochondrial Ca2+ content may be important in this regulation. Regulation of the ATP synthase and other ion transport systems may provide a means to preserve ion distribution and protonmotive force and thereby prolong the period during which cells can tolerate anoxia.

Mitochondrial transmembrane potential and pH gradient during anoxia

The effect of anoxia on the mitochondrial transmembrane potential and pH gradient was studied in a preparation of isolated hepatocytes. Transmembrane potential (delta psi) was calculated from the distribution of triphenylmethylphosphonium between the mitochondrial, cytosolic, and extracellular compartments, which were separated by digitonin fractionation and centrifugation. Mitochondrial and cytosolic pH values were calculated from the distribution of the weak acid, dimethadione, which was determined similarly. After 30 min anoxia, the magnitude of mitochondrial delta psi was decreased from -163 to -133 mV and the delta pH (mitochondria vs. cytoplasm) was essentially unchanged (aerobic, 0.78 +/- 0.08; anaerobic, 0.76 +/- 0.11). Thus the protonmotive force (delta p = delta psi-Z delta pH), is largely retained even in the absence of electron flow and ATP synthesis. Inhibitors of the ATP synthase (oligomycin), mitochondrial adenine nucleotide carrier (atractyloside), and glycolytic pathway (2-deoxy-D-glucose) do not affect the ability of the cell to maintain delta psi during anoxia. Therefore, the results indicate that retention of the protonmotive force is not due to utilization of ATP produced by glycolysis and suggest that mechanisms exist to preserve ion distribution during anoxia.

Mitochondrial transmembrane ion distribution during anoxia

The distribution of pyruvate, phosphate, malate, citrate, K+, aspartate, glutamate, ADP, and ATP between the mitochondrial and cytosolic compartments was studied in isolated rat hepatocytes exposed to 30 min anoxia. The results show that pyruvate and citrate gradients are comparable to aerobic values, indicating that the pH gradient across the membrane under anaerobic conditions is comparable to that under normal aerobic conditions. In contrast, the distribution of phosphate, malate, ATP, ADP, aspartate, and glutamate suggests that transport of these species may be inhibited during anoxia. The results are discussed in terms of potential regulation of mitochondrial function to provide a quiescent anoxic state that is capable of recovering normal function on reoxygenation.