Swelling and shrinkage phenomena in liver mitochondria. VI. Metabolism-independent swelling coupled to ion movement

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

Ovarian tissue cryopreservation and transplantation: a review on reactive oxygen species generation and antioxidant therapy

Cancer is the leading cause of death worldwide. Fortunately, the survival rate of cancer continues to rise, owing to advances in cancer treatments. However, these treatments are gonadotoxic and cause infertility. Ovarian tissue cryopreservation and transplantation (OTCT) is the most flexible option to preserve fertility in women and children with cancer. However, OTCT is associated with significant follicle loss and an accompanying short lifespan of the grafts. There has been a decade of research in cryopreservation-induced oxidative stress in single cells with significant successes in mitigating this major source of loss of viability. However, despite its success elsewhere and beyond a few promising experiments, little attention has been paid to this key aspect of OTCT-induced damage. As more and more clinical practices adopt OTCT for fertility preservation, it is a critical time to review oxidative stress as a cause of damage and to outline potential ameliorative interventions. Here we give an overview of the application of OTCT for female fertility preservation and existing challenges; clarify the potential contribution of oxidative stress in ovarian follicle loss; and highlight potential ability of antioxidant treatments to mitigate the OTCT-induced injuries that might be of interest to cryobiologists and reproductive clinicians.

Mitochondria as a target of cardioprotection in models of preconditioning

Over the recent years the view on mitochondria in the heart as a cellular powerhouse providing ATP supply needed to sustain contractile function, basal metabolic processes, and ionic homeostasis has changed radically. At present it is known that dysfunctions of these organelles are essential in the development of a large number of diseases, including cardiovascular diseases. Moreover, mitochondria are considered to be a very promising target of endogenous strategies that are essential in the protection of the myocardium from acute ischemia/reperfusion injury. These strategies including ischemic preconditioning, remote ischemic preconditioning as well as the acute phase of streptozotocin-induced diabetes mellitus, provide a similar effect of protection. Alterations observed in the functional and structural properties of heart mitochondria caused by short-term pathological impulses are associated with endogenous cardioprotective processes. It seems that the extent of mitochondrial membrane fluidization could be an active response mechanism to injury with a subtle effect on membrane-associated processes which further affect the environment of the whole organelle, thus inducing metabolic changes in the heart. In this review article, we provide an overview of endogenous protective mechanisms induced by hypoxic, pseudohypoxic and ischemic conditions with special consideration of the role of heart mitochondria in these processes.

Anion Channels of Mitochondria

Mitochondria are the "power house" of a cell continuously generating ATP to ensure its proper functioning. The constant production of ATP via oxidative phosphorylation demands a large electrochemical force that drives protons across the highly selective and low-permeable mitochondrial inner membrane. Besides the conventional role of generating ATP, mitochondria also play an active role in calcium signaling, generation of reactive oxygen species (ROS), stress responses, and regulation of cell-death pathways. Deficiencies in these functions result in several pathological disorders like aging, cancer, diabetes, neurodegenerative and cardiovascular diseases. A plethora of ion channels and transporters are present in the mitochondrial inner and outer membranes which work in concert to preserve the ionic equilibrium of a cell for the maintenance of cell integrity, in physiological as well as pathophysiological conditions. For, e.g., mitochondrial cation channels KATP and BKCa play a significant role in cardioprotection from ischemia-reperfusion injury. In addition to the cation channels, mitochondrial anion channels are equally essential, as they aid in maintaining electro-neutrality by regulating the cell volume and pH. This chapter focusses on the information on molecular identity, structure, function, and physiological relevance of mitochondrial chloride channels such as voltage dependent anion channels (VDACs), uncharacterized mitochondrial inner membrane anion channels (IMACs), chloride intracellular channels (CLIC) and the aspects of forthcoming chloride channels.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Mitochondria are sources of metabolic sink and arrhythmias

Mitochondria have long been recognized for their central role in energy transduction and apoptosis. More recently, extensive work in multiple laboratories around the world has significantly extended the role of cardiac mitochondria from relatively static arbitrators of cell death and survival pathways to highly dynamic organelles that form interactive functional networks across cardiomyocytes. These coupled networks were shown to strongly affect cardiomyocyte responses to oxidative stress by modulating cell signaling pathways that strongly impact physiological properties. Of particular importance is the role of mitochondria in modulating key electrophysiological and calcium cycling properties in cardiomyocytes, either directly through activation of a myriad of mitochondrial ion channels or indirectly by affecting cell signaling cascades, ATP levels, and the over-all redox state of the cardiomyocyte. This important recognition has ushered a renewed interest in understanding, at a more fundamental level, the exact role that cardiac metabolism, in general and mitochondria, in particular, play in both health and disease. In this article, we provide an overview of recent advances in our growing understanding of the fundamental role that cardiac mitochondria play in the genesis of lethal arrhythmias.

Cardiac mitochondria and arrhythmias

Despite a high prevalence of sudden cardiac death throughout the world, the mechanisms that lead to ventricular arrhythmias are not fully understood. Over the last 20 years, a growing body of evidence indicates that cardiac mitochondria are involved in the genesis of arrhythmia. In this review, we have attempted to describe the role that mitochondria play in altering the heart's electrical function by introducing heterogeneity into the cardiac action potential. Specifically, we have focused on how the energetic status of the mitochondrial network can alter sarcolemmal potassium fluxes through ATP-sensitive potassium channels, creating a 'metabolic sink' for depolarizing wave-fronts and introducing conditions that favour catastrophic arrhythmia. Mechanisms by which mitochondria depolarize under conditions of oxidative stress are characterized, and the contributions of several mitochondrial ion channels to mitochondrial depolarization are presented. The inner membrane anion channel in particular opens upstream of other inner membrane channels during metabolic stress, and may be an effective target to prevent the metabolic oscillations that create action potential lability. Finally, we discuss therapeutic strategies that prevent arrhythmias by preserving mitochondrial membrane potential in the face of oxidative stress, supporting the notion that treatments aimed at cardiac mitochondria have significant potential in attenuating electrical dysfunction in the heart.

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.

Calcium activation of heart mitochondrial oxidative phosphorylation: rapid kinetics of mVO2, NADH, AND light scattering

Parallel activation of heart mitochondria NADH and ATP production by Ca(2+) has been shown to involve the Ca(2+)-sensitive dehydrogenases and the F(0)F(1)-ATPase. In the current study we hypothesize that the response time of Ca(2+)-activated ATP production is rapid enough to support step changes in myocardial workload ( approximately 100 ms). To test this hypothesis, the rapid kinetics of Ca(2+) activation of mV(O(2)), [NADH], and light scattering were evaluated in isolated porcine heart mitochondria at 37 degrees C using a variety of optical techniques. The addition of Ca(2+) was associated with an initial response time (IRT) of mV(O(2)) that was dose-dependent with a minimum IRT of 0.27 +/- 0.02 s (n = 41) at 535 nm Ca(2+). The IRTs for NADH fluorescence and light scattering in response to Ca(2+) additions were similar to mV(O(2)). The Ca(2+) IRT for mV(O(2)) was significantly shorter than 1.6 mm ADP (2.36 +/- 0.47 s; p < or = 0.001, n = 13), 2.2 mm P(i) (2.32 +/- 0.29, p < or = 0.001, n = 13), or 10 mm creatine (15.6.+/-1.18 s, p < or = 0.001, n = 18) under similar experimental conditions. Calcium effects were inhibited with 8 microm ruthenium red (2.4 +/- 0.31 s; p < or = 0.001, n = 16) and reversed with EGTA (1.6 +/- 0.44; p < or = 0.01, n = 6). Estimates of Ca(2+) uptake into mitochondria using optical Ca(2+) indicators trapped in the matrix revealed a sufficiently rapid uptake to cause the metabolic effects observed. These data are consistent with the notion that extramitochondrial Ca(2+) can modify ATP production, via an increase in matrix Ca(2+) content, rapidly enough to support cardiac work transitions in vivo.

Long-chain fatty acid-promoted swelling of mitochondria: further evidence for the protonophoric effect of fatty acids in the inner mitochondrial membrane

Swelling of non-respiring rat liver mitochondria suspended in isotonic potassium acetate at pH 6.5-7.4 in the presence of valinomycin was promoted by long-chain fatty acids, such as myristate, indicating a protonophoric mechanism. This swelling was partly inhibited by inhibitors or substrates of mitochondrial anion carriers. The results show that the fatty acid cycling mechanism responsible for uncoupling of oxidative phosphorylation can also operate in the direction opposite to that originally proposed [Skulachev, V.P. (1991) FEBS Lett. 294, 158-162], i.e. the inwardly directed transfer of the fatty acid anion accompanied by outwardly directed free passage of undissociated fatty acid. They also extend the list of mitochondrial anion carriers, that are involved in this process, over the mono- and tricarboxylate transporters. At pH 8, myristate, but not the synthetic protonophore, p-trifluoromethoxycarbonyl-cyanide phenylhydrazone, induced mitochondrial swelling in both potassium acetate and KCl media, that did not require the presence of valinomycin. This indicates that, at alkaline pH, myristate facilitates permeation of the inner mitochondrial membrane to monovalent cations and, possibly, activates the inner membrane anion channel.

Properties of the inner membrane anion channel in intact mitochondria

The mitochondrial inner membrane possesses an anion channel (IMAC) which mediates the electrophoretic transport of a wide variety of anions and is believed to be an important component of the volume homeostatic mechanism. IMAC is regulated by matrix Mg2+ (IC50 = 38 microM at pH 7.4) and by matrix H+ (pIC50 = 7.7). Moreover, inhibition by Mg2+ is pH-dependent. IMAC is also reversibly inhibited by many cationic amphiphilic drugs, including propranolol, and irreversibly inhibited by N,N'-dicyclohexylcarbodiimide. Mercurials have two effects on its activity: (1) they increase the IC50 values for Mg2+, H+, and propranolol, and (2) they inhibit transport. The most potent inhibitor of IMAC is tributyltin, which blocks anion uniport in liver mitochondria at about 1 nmol/mg. The inhibitory dose is increased by mercurials; however, this effect appears to be unrelated to the other mercurial effects. IMAC also appears to be present in plant mitochondria; however, it is insensitive to inhibition by Mg2+, mercurials, and N,N'-dicyclohexylcarbodiimide. Some inhibitors of the adenine nucleotide translocase also inhibit IMAC, including Cibacron Blue, agaric acid, and palmitoyl CoA; however, atractyloside has no effect.

N-ethylmaleimide and mercurials modulate inhibition of the mitochondrial inner membrane anion channel by H+, Mg2+ and cationic amphiphiles

Previously it has been shown that the mitochondrial inner membrane anion channel is reversibly inhibited by matrix Mg2+, matrix H+ and cationic amphiphiles such as propranolol. Furthermore, the IC50 values for both Mg2+ and cationic amphiphiles are dependent on matrix pH. It is now shown that pretreatment of mitochondria with N-ethylmaleimide, mersalyl and p-chloromercuribenzenesulfonate increases the IC50 values of these inhibitors. The effect of the mercurials is most evident when cysteine or thioglycolate is added to the assay medium to reverse their previously reported inhibitory effect (Beavis, A.D. (1989) Eur. J. Biochem. 185, 511-519). Although the IC50 values for Mg2+ and propranolol are shifted they remain pH dependent. Mersalyl is shown to inhibit transport even in N-ethylmaleimide-treated mitochondria indicating that N-ethylmaleimide does not react at the inhibitory mercurial site. However, the effects of N-ethylmaleimide and mersalyl on the IC50 for H+ are not additive which suggests that mercurials and N-ethylmaleimide react at the same 'regulatory' site. It is suggested that modification of this latter site exerts an effect on the binding of Mg2+, H+ and propranolol by inducing a conformational change. It is also suggested that a physiological regulator may exist which has a similar effect in vivo.

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.

The inner mitochondrial membrane contains ion-conducting channels similar to those found in bacteria

Patch-clamp experiments were performed on rat liver mitochondria inner membranes. Application of voltage gradients of either polarity revealed the presence of several different conductances, ranging up to 1.3 nS in symmetrical 150 mM KCl. Evidence is presented that at least those higher than 0.3 nS are substates of the highest conductance channel. Increasing matrix-side-positive (unphysiological) transmembrane voltage gradients favored the switch of the 1.3 nS channel to operation in lower conductance states. The size of these conductances, the presence of substates and the channel behavior are strongly reminiscent on one hand of the observations on the membrane of protoplasts from the gram-positive bacterium Streptococcus faecalis, [Zoratti, M. and Petronilli, V. (1988) FEBS Lett. 240, 105-109], and on the other of some properties of previously described channels of mitochondrial origin.

The mitochondrial inner-membrane anion channel possesses two mercurial-reactive regulatory sites

The mitochondrial inner membrane anion channel catalyzes the electrophoretic transport of a wide variety of anions and is inhibited by matrix divalent cations and protons. In this paper, evidence is provided that mersalyl and p-chloromercuribenzene-sulfonate each interact with this uniporter at two distinct sites. Binding to site 1 causes a shift in the pH dependence of transport, characterized by a decrease in the pIC50 for protons from about 7.8 to about 7.3, and leads to substantial stimulation of transport in the physiological pH range. This effect is not reversed by addition of thiols such as thioglycolate. Binding of mersalyl and p-chloromercuribenzenesulfonate to site 2 inhibits the transport of most anions including Pi, citrate, malonate, sulfate and ferrocyanide. The transport of Cl- is inhibited about 60% by mersalyl, but is not inhibited by p-chloromercuribenzenesulfonate. These data suggest that inhibition is a steric effect dependent on the size of the anion and the size of the R group of the mercurial. This inhibition is reversed by thioglycolate. Dose/response curves indicate that mersalyl binds to site 1 as the dose increased from 7 to 13 nmol/mg, whereas it binds to site 2 as the dose is increased from 10 to 18 nmol/mg. Thus, at certain pH values both stimulatory and inhibitory phases can be seen in the same dose/response curve. It is suggested that these sites may contain thiol groups and that physiological regulators may exist which can effect changes in activity of the inner membrane anion uniporter similar to those exerted by mercurials.

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.

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.

Evidence for the existence of an inner membrane anion channel in mitochondria

Mitochondria normally exhibit very low electrophoretic permeabilities to physiologically important anions such as chloride, bicarbonate, phosphate, succinate, citrate, etc. Nevertheless, considerable evidence has accumulated which suggests that heart and liver mitochondria contain a specific anion-conducting channel. In this review, a postulated inner membrane anion channel is discussed in the context of other known pathways for anion transport in mitochondria. This anion channel exhibits the following properties. It is anion-selective and inhibited physiologically by protons and magnesium ions. It is inhibited reversibly by quinine and irreversibly by dicyclohexylcarbodiimide. We propose that the inner membrane anion channel is formed by inner membrane proteins and that this pathway is normally latent due to regulation by matrix Mg2+. The physiological role of the anion channel is unknown; however, this pathway is well designed to enable mitochondria to restore their normal volume following pathological swelling. In addition, the inner membrane anion channel provides a potential futile cycle for regulated non-shivering thermogenesis and may be important in controlled energy dissipation.

Effects of quinine on K+ transport in heart mitochondria

Quinine inhibits the respiration-dependent extrusion of K+ from Mg2+-depleted heart mitochondria and the passive osmotic swelling of these mitochondria in K+ and Na+ acetate at alkaline pH. These observations concur with those of Nakashima and Garlid (J. Biol. Chem. 257, 9252, 1982) using rat liver mitochondria. Quinine also inhibits the respiration-dependent contraction of heart mitochondria swollen passively in Na+ or K+ nitrate and the increment of elevated respiration associated with the extrusion of ions from these mitochondria. Quinine, at concentrations up to 0.5 mM, inhibits the respiration-dependent 42K+/K+ exchange seen in the presence of mersalyl, but higher levels of the drug produce increased membrane permeability and net K+ loss from the matrix. These results are all consistent with an inhibition of the putative mitochondrial K+/H+ antiport by quinine. However, quinine has other effects on the mitochondrial membrane, and possible alternatives to this interpretation are discussed.

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

The paper analyzes the factors affecting the H+-K+ exchange catalyzed by rat liver mitochondria depleted of endogenous Mg2+ by treatment with the ionophore A23187. The exchange has been monitored as the rate of K+ efflux following addition of A23187 in low-K+ media. (1) The H+-K+ exchange is abolished by uncouplers and respiratory inhibitors. The inhibition is not related to the depression of delta pH, whereas a dependence is found on the magnitude of the transmembrane electrical potential, delta psi. Maximal rate of K+ efflux is observed at 180-190 mV, whereas K+ efflux is inhibited below 140-150 mV. (2) Activation of H+-K+ exchange leads to depression of delta pH but not of delta psi. Respiration is only slightly stimulated by the onset of H+-K+ exchange in the absence of valinomycin. These findings indicate that the exchange is electroneutral, and that the delta psi control presumably involves conformational changes of the carrier. (3) Incubation in hypotonic media at pH 7.4 or in isotonic media at alkaline pH results in a marked activation of the rate of H+-K+ exchange, while leaving unaffected the level of Mg2+ depletion. This type of activation results in partial 'uncoupling' from the delta psi control, suggesting that membrane stretching and alkaline pH induce conformational changes on the exchange carrier equivalent to those induced by high delta psi. (4) The available evidence suggests that the activity of the H+-K+ exchanger is modulated by the electrical field across the inner mitochondrial membrane.

Adenosine triphosphate synthesis coupled to K+ influx in mitochondria

The influx of K+ into swollen mitochondria in the presence of valinomycin results in the synthesis of adenosine triphosphate in which approximately one H+ disappears per adenosine triphosphate synthesized. The synthesis is blocked by atractyloside but is insensitive to oligomycin and relatively insensitive to uncouplers.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Ion transport in liver mitochondria from normal and thyroxine-treated rats

Liver mitochondria isolated from rats 24 h after a single subcutaneous injection of 8 mg thyroxine per kilogram body weight were compared with those isolated from control rats that received injections of isotonic saline at the same time. The mitochondria isolated from the thyroxine-treated rats show higher rates of energy-dependent K+ and phosphate accumulation than those from control animals. It was also found that mitochondria from the hormone-treated animals required a larger addition of Ca2+/mg mitochondrial protein in order to uncouple oxidative phosphorylation, and showed smaller tendency to swell in vitro under energizing conditions. The data obtained on ion movements support previous observations that the stimulation of the basal rate of mitochondrial respiration by thyroxine is associated with an increase in the transmembrane protonic electrochemical potential difference, and indicate the in vivo the hormone raises the intramitochondrial concentration of K+ and phosphate.

Swelling and contraction of heart mitochondria suspended in ammonium phosphate

Bovine heart mitochondria which have been allowed to swell in isotonic NH4+ phosphate contract in response to initiation of oxidative phosphorylation. The contraction occurs optimally at pH 6.0 and appears from inhibition studies to result from Pi uptake being slower than removal of internal Pi via phosphorylation of external ADP. Similar results are obtained when K+ + nigericin is substituted for NH4+. Mersalyl inhibition of Pi transport in respiring, nonphosphorylating mitochondria which have been allowed to swell in NH4+ phosphate reveals a contractile process having an alkaline pH optimum. This contraction resembles closely the contraction observed in salts of strong acids and presumably occurs by electrophoretic ejection of Pi anions driven by electrogenic H+ ejection.

Ion movements during energy-linked mitochondrial structural changes

The structure of isolated rat liver mitochondria has been observed in the electron microscope following incubation of the mitochondria in vitro under a variety of conditions. The results show that ultrastructural changes are only associated with the energization and deenergization of isolated mitochondria if the composition of the incubation medium permits ion movements in or out of the matrix. The mechanism of energy coupling does not appear to depend on these major mitochondrial structural changes. The addition of low levels of valinomycin greatly increases the rate at which the matrix compartment swells and shrinks on energization and deenergization even at low K+ concentrations.

[Permeability of yeast mitochondrial internal membrane: structure-activity relationship]

In order to investigate the possible relations between the anionic permeability and the functions (or the structure ) of the inner mitochondrial membrane, three types of organelles isolated from S. cerevisiae were tested: mitochondria (aerobic culture), promitochondria (anaerobic culture) and CAP-mitochondria (aerobic culture with chloramphenicol added). By using the technique of swelling in isoosmotic potassium salts, after a derermination of the isotonic conditions, it was possible to discriminate between an electrogenic (valinomycin induced) or an electroneutral (both valinomycin and uncoupler induced) translocation. 1) Mitochondria: The permeability properties of mitochondria are energy dependent: a) Respiring mitochondria are permeable to Cl-; Mg2+, however, inhibits this translocation. Phosphate transport seems to be exclusively electrogenic and mersalyl sensitive, but swelling inhibition by that thiol reagent is restored by Mg2+. b) Non respiring mitochondria are impermeable to Cl-, but ATP addition restores the permeability. Thiocyanate permeates as the anionic form and acetate as the undissociated form. The phosphate transport, sensitive to mersalyl, seems to be partially electrogenic. 2) Promitochondria: Deficient of respiratory enzymes but containing an oligomycin sensitive ATPase, they are impermeable to Cl- only when Mg2+ is added. In these conditions, an electrogenic phosphate transport, sensitive to mersalyl, is observed. 3) CAP-mitochondria: Although CAP-mitochondria are cytochrome deficient and contain an oligomycin insensitive ATPase, they are also impermeable to Cl- in presence of Mg2+. As in fully differenciated mitochondria, an electroneutral phosphate entry is observed; Mg2+ is required for mersalyl sensitivity.

Mechanism of active shrinkage in mitochondria. II. Coupling between strong electrolyte fluxes

1. Addition of succinate to valinomycin-treated mitochondria incubated in KCL causes a large electrolyte penetration. The process depends on a steady supply of energy and involves a continuous net extrusion of protons. Rates of respiration and of electrolyte penetration proceed in a parallel manner. 2. A passive penetration of K+ salt of permeant anions occurs in respiratory-inhibited mitochondria after addition of valinomycin. Addition of succinate at the end of the passive swelling starts an active extrusion of anions and cations with restoration of the initial volume. The shrinkage is accompanied by a slow reuptake of protons. The initiation of the active shrinkage correlates with the degree of stretching of the inner membrane. The extrusion of electrolytes is inhibited by nigericin, while it is only slightly sensitive to variations of the valinomycin concentration larger than two orders of magnitude. 3. Passive swelling and active shrinkage occurs also when K+ is replaced by a large variety of organic cations. The rate of organic cation penetration is enhanced by tetraphenylboron, while the rate of electrolyte extrusion is insensitive to variation of the tetraphenylboron concentration. 4. Active shrinkage, either with K+ or organic cation salts, is inhibited by weak acids. The phosphate inhibition is removed by SH inhibitors. The active shrinkage is also inhibited by mersalyl to an extent of about 60%. 5. Three models of active shrinkage are discussed: (a) mechanoprotein, (b) electrogenic proton pump, and (c) proton-driven cation anion pump.