On the mechanism by which Mg2+ and adenine nucleotides restore membrane potential in rat liver mitochondria deenergized by Ca2+ and phosphate

The Role of Adenine Nucleotide Translocase in the Mitochondrial Permeability Transition

The mitochondrial permeability transition, a Ca2+-induced significant increase in permeability of the inner mitochondrial membrane, plays an important role in various pathologies. The mitochondrial permeability transition is caused by induction of the permeability transition pore (PTP). Despite significant effort, the molecular composition of the PTP is not completely clear and remains an area of hot debate. The Ca2+-modified adenine nucleotide translocase (ANT) and F0F1 ATP synthase are the major contenders for the role of pore in the PTP. This paper briefly overviews experimental results focusing on the role of ANT in the mitochondrial permeability transition and proposes that multiple molecular entities might be responsible for the conductance pathway of the PTP. Consequently, the term PTP cannot be applied to a single specific protein such as ANT or a protein complex such as F0F1 ATP synthase, but rather should comprise a variety of potential contributors to increased permeability of the inner mitochondrial membrane.

A bioenergetic model of the mitochondrial population undergoing permeability transition

Mitochondrial permeability transition (MPT) is a highly regulated complex phenomenon that is a type of ischemia/reperfusion injury that can lead to cell death and ultimately organ dysfunction. A novel population transition and detailed permeability transition pore regulation model were integrated with an existing bioenergetics model to describe MPT induction under a variety of conditions. The framework of the MPT induction model includes the potential states of the mitochondria (aggregated, orthodox and post-transition), their transitions from one state to another as well as their interaction with the extra-mitochondrial environment. The model encodes the three basic necessary conditions for MPT: a high calcium load, alkaline matrix pH and circumstances which favor de-energization. The MPT induction model was able to reproduce the expected bioenergetic trends observed in a population of mitochondria subjected to conditions that favor MPT. The model was corroborated and used to predict that MPT in an acidic environment is mitigated by an increase in activity of the mitochondrial potassium/hydrogen exchanger. The model was also used to present the beneficial impact of reducing the duration mitochondria spend in the orthodox state on preserving the extra-mitochondrial ATP levels. The model serves as a tool for investigators to use to understand the MPT induction phenomenon, explore alternative hypotheses for PTP regulation, as well as identify endogenous pharmacological targets and evaluate potential therapeutics for MPT mitigation.

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.

Re-evaluation of the distinction between type I and type II cells: The necessary role of the mitochondria in both the extrinsic and intrinsic signaling pathways upon fas receptor activation

Cyclosporin A (CyA) and bongkrekic acid (BK) prevented Fas-induced apoptosis in two type I cell lines (H9 and SKW6.4) and two type II cell lines (Jurkat and CEM). CyA and BK inhibited the release of cytochrome c in all four cell lines. In type I cells and in CEM cells, CyA and BK did not prevent the translocation of Bax to the mitochondria. In these same cells, full-length Bid decreased in the mitochondria and cytosol. The cleavage product of Bid, tBid, appeared in the cytosol and to a lesser extent in the mitochondria. In Jurkat cells, Bid also decreased in the cytosol, but increased in the mitochondria. Similar to the other cells, tBid appeared in the mitochondria and cytosol. In the type I H9 and SKW6.4 cells and type II Jurkat cells, the caspase-8 inhibitor Z-Ile-Glu(OMe)-Thr-Asp(OMe)-CH2F (IETD) prevented the cell killing. In the type I cells, IETD prevented the translocation of Bax, the degradation of Bid and the accumulation of tBid. By contrast, IETD only marginally protected the type II CEM cells. In these cells in the presence of IETD, Bax translocated to the mitochondria, in the absence of any degradation of Bid or accumulation of tBid. In the type I H9 cells, IETD produced a depletion of ATP, an effect that did not occur in the type II CEM cells. It is concluded that in type I cells the extrinsic signaling pathway is mitochondrial dependent to the same extent as is the intrinsic pathway in type II cells.

A historical review of cellular calcium handling, with emphasis on mitochondria

Calcium ions are of central importance in cellular physiology, as they carry the signal activating cells to perform their programmed function. Ca(2+) is particularly suitable for this role because of its chemical properties and because its free concentration gradient between the extra-cellular and the cytosolic concentrations is very high, about four orders of magnitude. The cytosolic concentration of Ca(2+) is regulated by binding and chelation by various substances and by transport across plasma and intracellular membranes. Various channels, transport ATPases, uniporters, and antiporters in the plasma membrane, endoplasmic and sarcoplasmic reticulum, and mitochondria are responsible for the transport of Ca(2+). The regulation of these transport systems is the subject of an increasing number of studies. In this short review, we focus on the mitochondrial transporters, i.e. the calcium uniporter used for Ca(2+) uptake, and the antiporters used for the efflux, i.e. the Ca(2+)/Na(+) antiporter in mitochondria and the plasma membrane of excitable cells, and the Ca(2+)/nH(+) antiporter in liver and some other mitochondrial types. Mitochondria are of special interest in that Ca(2+) stimulates respiration and oxidative phosphorylation to meet the energy needs of activated cells. The studies on Ca(2+) and mitochondria began in the fifties, but interest in mitochondrial Ca(2+) handling faded in the late seventies since it had become apparent that mitochondria in resting cells contain very low Ca(2+). Interest increased again in the nineties also because it was discovered that mitochondria and Ca(2+) had a central role in apoptosis and necrosis. This is of special interest in calcium overload and oxidative stress conditions, when the opening of the mitochondrial permeability transition pore is stimulated.

Menadione induces a low conductance state of the mitochondrial inner membrane sensitive to bongkrekic acid

When rat liver mitochondria are allowed to cycle Ca(2+) and are incubated in the presence of the pro-oxidant menadione, they undergo swelling, membrane potential (DeltaPsi) collapse, and ion release. These effects, which are inhibited by cyclosporin A (CsA), are fully consistent with the opening of the so-called permeability transition pore. However, when Ca(2+) cycling is abolished by EGTA, the mitochondria remain energized (DeltaPsi collapse and swelling are avoided), but Ca(2+) efflux, promoted by the chelating agent, is stimulated by menadione. This stimulation goes together with the release of Mg(2+), K(+), and adenine nucleotides (AdN) and is inhibited by bongkrekic acid (BKA). The effect of menadione is also characterized by biphasic NAD(P)H oxidation which becomes monophasic in the presence of BKA, CsA, or EGTA and by the oxidation of thiol groups not restrained by the above-mentioned inhibitors. These results suggest that BKA acts indirectly by preserving in the matrix a critical amount of AdN without modifying the monophasic oxidation of pyridine nucleotides by menadione. A critical number of thiol groups also seems to be involved in the phenomenon. Their oxidation most probably causes a conformational change on adenine nucleotide translocase with the opening of the "low-conductance state" of the mitochondrial permeability transition, resulting in ion permeability without DeltaPsi disruption and mitochondrial swelling.

Hypothyroidism renders liver mitochondria resistant to the opening of membrane permeability transition pore

Membrane permeability was examined in liver mitochondria isolated from hypothyroid rats. It was found that such a thyroid status provides substantial protection from membrane leakiness as induced by Ca2+ loading. Thus, these mitochondria are less prone to undergoing permeability transition than mitochondria from euthyroid rats. The above conclusion was reached on the basis of the following two facts: (1) hypothyroid mitochondria are not strictly dependent on the addition of ADP to retain high matrix Ca2+ concentrations, and (2) carboxyatractyloside, antimycin A or carbonyl cyanide-m-chlorophenyl hydrazone failed to promote Ca2+ efflux. We discuss the possible relevance of the low content of membrane cardiolipin as well as the low expression of the adenine nucleotide translocase as responsible for the resistance to membrane damage.

Mitochondrial permeability transition as induced by cross-linking of the adenine nucleotide translocase

Mitochondrial permeability transition is caused by the opening of a transmembrane pore whose chemical nature has not been well established yet. The present work was aimed to further contribute to the knowledge of the membrane entity comprised in the formation of the non-specific channel. The increased permeability was established by analyzing the inability of rat kidney mitochondria to take up and accumulate Ca2+, as well as their failure to build up a transmembrane potential, after the cross-linking of membrane proteins by copper plus ortho-phenanthroline. To identify the cross-linked proteins, polyacrylamide gel electrophoresis was performed. The results are representative of at least three separate experiments. It is indicated that 30 microM Cu2+ induced the release of 4.3 nmol Ca2+ per mg protein. However, in the presence of 100 microM ortho-phenanthroline only 2 microM Cu2+ was required to attain the total release of the accumulated Ca2+; it should be noted that such a reaction is not inhibited by cyclosporin. The increased permeability corresponds to cross-linking of membrane proteins in which approximately 4 nmol thiol groups per mg protein appear to be involved. Such a linking process is inhibited by carboxyatractyloside. By using the fluorescent probe eosin-5-maleimide the label was found in a cross-linking 60 kDa dimer of two 30 kDa monomers. From the data presented it is concluded that copper-o-phenanthroline induces the intermolecular cross-linking of the adenine nucleotide translocase which in turn is converted to non-specific pore.

Characterization of a high capacity calcium transport system in mitochondria of the yeast Endomyces magnusii

The Ca2+ transport system of Endomyces magnusii mitochondria has been shown previously to be activated by spermine. Here we report it to be regulated also by low, physiological ADP concentrations, by the intramitochondrial NADH/NAD+ ratio, and by Ca2+ ions. The combination of all these physiological modulators induced high initial rates of Ca2+ uptake and high Ca2+-buffering capacity of yeast mitochondria, enabling them to lower the medium [Ca2+] to approximately 0.2 microM. The mechanisms of stimulation by these agents are discussed.

Human papillomavirus (HPV) 16 E6 sensitizes cells to atractyloside-induced apoptosis: role of p53, ICE-like proteases and the mitochondrial permeability transition

Infection of cervical epithelial cells with certain high risk HPV genotypes is thought to play an etiologic role in the development of cervical cancer. In particular, HPV type 16 and 18 early protein 6 (E6) is thought to contribute to epithelial transformation by binding to the tumor suppressor protein p53, targeting it for rapid proteolysis, resulting in loss of its cell cycle arrest and apoptosis-inducing activities. Recent data indicate that factors responsible for triggering apoptosis reside in the cytoplasm of cells, and not in the nucleus. In particular, the findings that mitochondria are required in certain cell-free models for induction of apoptosis and that bcl-2 is localized to mitochondria have focused attention on the role of the mitochondrial membrane permeability transition (MPT) in apoptosis. Here we present data to indicate that HPV 16 E6 expression sensitizes cells to MPT-induced apoptosis. We also report that HPV 16 E6 sensitization of cells to MPT-induced apoptosis occurs only in the presence of wildtype (wt) p53 expression. The extent of apoptosis induced by atractyloside (an inducer of the MPT) in normal, temperature-sensitive (ts) p53, and HPV-16 E6 transfected J2-3T3 cells, and the HPV expressing cervical carcinoma cell lines SiHa, Hela and CaSki was determined. C33A cells, which express mutant p53 but not HPV, were also exposed to atractyloside in the presence or absence of HPV 16 E6 expression. Dose-dependent apoptosis induced by atractyloside in normal J2-3T3 cells and cervical carcinoma cells was measured by loss of cell viability, nuclear fragmentation and DNA laddering. The sensitivity of cells to atractyloside-induced apoptosis was found to be: HPV 16 E6-J2-3T3 > CaSki > normal-J2-3T3 cells approximately ts p53-J2-3T3 approximately vector-J2-3T3 cells > Hela > SiHa > C33A approximately C33A 16 E6. Cyclosporin A (CsA), an inhibitor of the MPT, and ICE-I, a protease inhibitor, provided protection against atractyloside-induced apoptosis. These findings indicate that: 1) high risk HPV 16 E6 protein is capable of sensitizing cells to apoptosis; 2) HPV 16 E6 sensitization of cells to atractyloside-induced apoptosis occurs in a p53-dependent fashion; 3) the target of HPV 16 E6 sensitization of cells to atractyloside-induced apoptosis is the mitochondria; and 4) HPV 16 E6 sensitization of cells to atroctycoside-induced apoptosis involves an ICE-like protease-sensitive mechanism, regulating the onset of the MPT. These findings constitute the first evidence that mitochondria play a role in HPV 16 E6 modulation of apoptosis.

Differential effects of zidovudine and zidovudine triphosphate on mitochondrial permeability transition and oxidative phosphorylation

1. The effects of zidovudine (ZDV) and zidovudine triphosphate (ZDV-3P) on Ca2+-induced mitochondrial permeability transition (MPT), respiratory control ratio (RCR) and ATP synthesis have been investigated on isolated rat liver mitochondria. 2. ZDV slightly but significantly decreased RCR and ATP synthesis but was ineffective in inhibiting MPT. In contrast, ZDV-3P did not alter RCR and ATP synthesis but strongly inhibited MPT (IC50 = 3.0 +/- 0.9 microM). 3. The effect of ZDV-3P on mitochondrial swelling required a preincubation time. When incubated 10 min with mitochondria, ZDV-3P (8 microM) totally inhibited the rate of swelling. 4. ADP, ATP and atractyloside, which are agents known to interact with the mitochondrial adenine nucleotide carrier (ANC), antagonized the effect of ZDV-3P on mitochondrial swelling. Indeed, the IC50 value of ZDV-3P increased from 3.0 to 17.4, 93.6 and 66.5 microM, in the presence of 20 microM, ADP, ATP or atractyloside, respectively. 5. ZDV-3P did not displace [3H]-ATP from its mitochondrial binding site(s) whereas ADP and atractyloside did, suggesting that ZDV-3P and [3H]-ATP do not share the same binding sites. 6. ZDV-3P did not affect either mitochondrial respiration or ATP synthesis but inhibited Ca2+-dependent mitochondrial swelling. It was concluded that mitochondrial toxic effects observed during the chronic administration of ZDV cannot be related to its active metabolite (ZDV-3P).

Inhibitors of permeability transition interfere with the disruption of the mitochondrial transmembrane potential during apoptosis

In a number of experimental systems, the early stage of the apoptotic process, i.e. the stage which precedes nuclear disintegration, is characterized by the breakdown of the inner mitochondrial transmembrane potential (delta psi m). Here we address the question as to whether mitochondrial permeability transition (PT) pores may account for the delta psi m dissipation in lymphocyte apoptosis. Drugs known for their PT-inhibitory potential (bongkrekic acid, cyclosporin A, and the non-immunosuppressive cyclosporin A analogue N-methyl-Val-4-cyclosporin A) are capable of preventing the apoptotic delta psi m disruption. Moreover, pharmacological modulation of PT-mediated delta psi m dissipation can prevent apoptosis. Thus, while suppressing the delta psi m disruption, bongkrekic acid also inhibits the apoptotic chromatinolysis. In conclusion, these data are compatible with the hypothesis that apoptotic delta psi m disruption is mediated by the formation of PT pores and that PT-mediated delta psi m disruption is a critical event of the apoptotic cascade.

Mitochondrial control of nuclear apoptosis

Anucleate cells can be induced to undergo programmed cell death (PCD), indicating the existence of a cytoplasmic PCD pathway that functions independently from the nucleus. Cytoplasmic structures including mitochondria have been shown to participate in the control of apoptotic nuclear disintegration. Before cells exhibit common signs of nuclear apoptosis (chromatin condensation and endonuclease-mediated DNA fragmentation), they undergo a reduction of the mitochondrial transmembrane potential (delta psi m) that may be due to the opening of mitochondrial permeability transition (PT) pores. Here, we present direct evidence indicating that mitochondrial PT constitutes a critical early event of the apoptotic process. In a cell-free system combining purified mitochondria and nuclei, mitochondria undergoing PT suffice to induce chromatin condensation and DNA fragmentation. Induction of PT by pharmacological agents augments the apoptosis-inducing potential of mitochondria. In contrast, prevention of PT by pharmacological agents impedes nuclear apoptosis, both in vitro and in vivo. Mitochondria from hepatocytes or lymphoid cells undergoing apoptosis, but not those from normal cells, induce disintegration of isolated Hela nuclei. A specific ligand of the mitochondrial adenine nucleotide translocator (ANT), bongkreik acid, inhibits PT and reduces apoptosis induction by mitochondria in a cell-free system. Moreover, it inhibits the induction of apoptosis in intact cells. Several pieces of evidence suggest that the proto-oncogene product Bcl-2 inhibits apoptosis by preventing mitochondrial PT. First, to inhibit nuclear apoptosis, Bcl-2 must be localized in mitochondrial but not nuclear membranes. Second, transfection-enforced hyperexpression of Bcl-2 directly abolishes the induction of mitochondrial PT in response to a protonophore, a pro-oxidant, as well as to the ANT ligand atractyloside, correlating with its apoptosis-inhibitory effect. In conclusion, mitochondrial PT appears to be a critical step of the apoptotic cascade.

The alterations in the energy linked properties induced in rat liver mitochondria by acetylsalicylate are prevented by cyclosporin A or Mg2+

The alterations in rat liver mitochondria induced by acetylsalicylate in the presence of low concentrations of Ca2+ (large amplitude swelling, permeability to 14C]sucrose, collapse of transmembrane potential and effluxes of endogenous Mg2+ and accumulated Ca2+) were fully prevented by either cyclosporin A or Mg2+. Cyclosporin A and Mg2+ were also capable of restoring transmembrane potential upon its decrease induced by acetylsalicylate. The loss of endogenous Mg2+ was the primary effect promoted by acetylsalicylate; the other noxious effects followed. These results indicate that Mg2+ are fundamental components of the mitochondrial permeability barrier and that their loss might be responsible for the membrane transition induced by acetylsalicylate.

Recent progress on regulation of the mitochondrial permeability transition pore; a cyclosporin-sensitive pore in the inner mitochondrial membrane

The mitochondrial permeability transition pore allows solutes with a m.w. approximately less than 1500 to equilibrate across the inner membrane. A closed pore is favored by cyclosporin A acting at a high-affinity site, which may be the matrix space cylophilin isozyme. Early results obtained with cyclosporin A analogs and metabolites support this hypothesis. Inhibition by cyclosporin does not appear to require inhibition of calcineurin activity; however, it may relate to inhibition of cyclophilin peptide bond isomerase activity. The permeability transition pore is strongly regulated by both the membrane potential (delta psi) and delta pH components of the mitochondrial protonmotive force. A voltage sensor which is influenced by the disulfide/sulhydryl state of vicinal sulfhydryls is proposed to render pore opening sensitive to delta psi. Early results indicate that this sensor is also responsive to membrane surface potential and/or to surface potential gradients. Histidine residues located on the matrix side of the inner membrane render the pore responsive to delta pH. The pore is also regulated by several ions and metabolites which act at sites that are interactive. There are many analogies between the systems which regulate the permeability transition pore and the NMDA receptor channel. These suggest structural similarities and that the permeability transition pore belongs to the family of ligand gated ion channels.

Ion permeability induced in artificial membranes by the ATP/ADP antiporter

The hypothesis on the additional function of the ATP/ADP antiporter (ANT) as uncoupling protein has been tested in proteoliposomes and planar bilayer phospholipid membranes (BLM). It is found that dissipation of the light-induced delta pH in the dark is very much faster in ANT-bacteriorhodopsin proteoliposomes than in proteoliposomes containing bacteriorhodopsin as the only protein. Mersalyl treatment of ANT-bacteriorhodopsin proteoliposomes causes further increase in the delta pH dissipation rate due to formation of a high conductance pore. The properties of this pore are studied on ANT incorporated to BLM. They proved to be similar to those of so-called multiple conductance channel or permeability transition pore of inner mitochondrial membrane. The conductance of the single channel is as high as 2.2 nS. The channel fails to discriminate between K+, Na+, H+ and Cl-. Thus the obtained data are consistent with the assumption that native and modified ANT might function as an H(+)-specific conductor and as a permeability transition pore, respectively.

Effects of palmitoyl CoA and palmitoyl carnitine on the membrane potential and Mg2+ content of rat heart mitochondria

Palmitoyl CoA and palmitoyl carnitine added to rat heart mitochondria in amounts above 20 and 50 nmoles/mg protein, respectively, induced a fall in transmembrane potential and loss of endogenous Mg2+. The dissipation of membrane potential by low concentrations of palmitoyl CoA in the presence of Ca2+, but not that of high concentrations of palmitoyl CoA alone, was prevented by either ruthenium red, Cyclosporin A or Mg2+, but reversed only by Mg2+. The fall of membrane potential induced by palmitoyl carnitine was not prevented by any of these factors. It is suggested that the action of both palmitoyl CoA and palmitoyl carnitine at high concentrations is due to a non specific disruption of membrane architecture, while that of low concentrations of palmitoyl CoA in the presence of Ca2+ is associated specifically with energy dissipation due to Ca2+ cycling.

Effects of the membrane potential upon the Ca(2+)- and cumene hydroperoxide-induced permeabilization of the inner mitochondrial membrane

A protonophore-induced delta psi decrease in a 180-140 mV range causes an increase in the lag-period of Ca(2+)-induced mitochondrial permeabilization but has little effect on the cumene hydroperoxide-induced permeability transition of mitochondria. Suppression of the non-specific permeability induction seems to be mediated by an increase in [ADP] in the mitochondrial matrix. A further decrease in delta psi leads to additional suppression of the non-specific permeability as a result of a partial ruthenium red-sensitive efflux of the previously accumulated Ca2+. On the other hand, complete dissipation of delta psi causes immediate induction of the non-specific permeability. It is concluded that only complete dissipation of delta psi caused by H+ leakages may act as a trigger for non-specific permeability induction.

Protective role of chlorpromazine on lead-induced damage to heart mitochondria

1. The protective effect of chlorpromazine (CPZ) on the toxic effects of lead in mitochondrial functions was studied. 2. The findings indicate that CPZ at a concentration of 50 microM protects heart mitochondria against lead-induced Ca2+ uptake inhibition. 3. In addition, CPZ inhibits the drop of the transmembrane potential, as well as mitochondrial swelling as induced by 10 microM Pb2+. 4. It is proposed that the protective effect of chlorpromazine can be due to its stabilizing action on biological membranes.

Effect of ADP/ATP antiporter conformational state on the suppression of the nonspecific permeability of the inner mitochondrial membrane by cyclosporine A

The influence of the conformational state of ADP/ATP antiporter on the efficiency of the inhibitory effect of cyclosporine A on the Ca2(+)-induced nonspecific permeability of the inner mitochondrial membrane has been studied. Carboxyatractiloside, the inhibitor of ADP/ATP-antiporter, was shown to prevent the cyclosporine A-induced suppression of the nonspecific permeability. The carboxyatractiloside effect was displayed only in mitochondria depleted of adenine nucleotides. Bifunctional SH reagent, phenylarsine oxide, was also able to reverse the effect of cyclosporine A. The data are consistent with the suggestion that cyclosporine A causes suppression of the nonspecific permeability due to its effect on the ADP/ATP antiporter conformation.

Extensive Ca2+ release from energized mitochondria induced by disulfiram

The effect of the alcohol-deterrent drug, disulfiram, on mitochondrial Ca2+ content was studied. Addition of this drug (20 microM) to mitochondria induces a complete loss of accumulated Ca2+. The calcium release is accompanied by a collapse of the transmembrane potential, mitochondrial swelling, and a diminution of the NAD(P)H/NAD(P) radio. These effects of disulfiram depend on Ca2+ accumulation; thus, ruthenium red reestablished the membrane delta psi and prevents the oxidation of pyridine nucleotides. The binding of disulfiram to the membrane sulfhydryls appeared to depend on the metabolic state of mitochondria, as well as on the mitochondrial configuration. In addition, it is shown that modification of 9 nmol -SH groups per mg protein suffices to induce the release of accumulated Ca2+.

Induction of mitochondrial Ca2+ uptake by mersalyl

1. The addition of mersalyl to aged mitochondria from rat kidneys, is followed by induction of an ATP-driven Ca2+ uptake which is sensitive to Ruthenium Red. 2. This Ca2+ influx requires Mg2+, albumin, and is accomplished by membrane energization. 3. The activation of Ca2+ uptake by the mercurial in the presence of ATP can be explained if it is assumed that the inorganic phosphate generated by ATPase activity, and trapped in the matrix by the thiol reagent, provides the negative potential which results in an electrophoresis cation influx.

Temperature dependence of the atractyloside-induced mitochondrial Ca2+ release

1. Mitochondrial Ca2+, accumulated by succinate oxidation was released by addition of 50 microM atractyloside. Beside this Ca2+ efflux, a large oxidation of pyridine nucleotides and sustained membrane depolarization occurs. An absolute requirement for acetate to support Ca2+ release is demonstrated. 2. Membrane de-energization, NAD(P)H oxidation, and Ca2+ efflux as induced by atractyloside were temperature-dependent, since it occurs when mitochondria are incubated at 22 degrees C and was abolished at 4 degrees C. 3. Taking into account this latter, the effects of atractyloside on mitochondrial Ca2+ release appears not to be a simple result of the binding of the inhibitor to adenine nucleotide translocase. 4. It is proposed that the mechanism involved in atractyloside-driven membrane permeability to Ca2+ must be related with the transference of the conformational change of the carrier, to another membrane structure responsible for the maintenance permeability to ions.

Control of mitochondrial Ca2+ retention by ADP-stimulated glutamic dehydrogenase

The protective effect of ADP on unspecific Ca2+ release and collapse of the transmembrane potential was analyzed in mitochondria from kidneys of rats. The presence of ADP in the incubation mixture prevents Ca2+ leakage and collapse of delta psi in sucrose-containing medium, but fails to do so in KCl medium. The effect of the adenine nucleotide in sucrose media correlates with an increase in the level of reduced pyridine nucleotides; the increase was due to a stimulatory effect on the activity of glutamic dehydrogenase. It also was observed that in KCl media, in the presence and in the absence of ADP the rate of NADH oxidation through the respiratory chain was higher than in sucrose; in this latter medium a high level of reduced pyridine nucleotides was found, in comparison to KCl media. It is proposed that the role of ADP is to increase glutamic dehydrogenase activity and in consequence to provoke a higher rate of formation of NADH which in turn controls Ca2+ release.

The influence of age on the calcium-efflux pathway and matrix calcium buffering power in brain mitochondria

The variations with age of the ruthenium red-insensitive calcium efflux rate have been studied in rat brain mitochondria. Both H+- and Na+-dependent effluxes are decreased with age when expressed as a function of calcium taken up in mitochondria incubated in the presence of 0.8 mM inorganic phosphate (Pi) and 0.2 mM ADP. However, the age-dependent differences in calcium efflux rates disappear when mitochondria are incubated in the absence of ADP and Pi. It is suggested that the decrease in efflux rate observed with age corresponds to an increased calcium buffering power of the mitochondrial matrix due to an increase in mitochondrial Pi. The causes of the increased Pi accumulation in old-rat-brain mitochondria are yet unknown but possibly not due to differences in the Pi efflux. The results suggest that the age-dependent lowering of the free calcium concentration in the brain mitochondrial matrix together with the reduced activity of the calcium uniporter (Vitórica, J. and Satrústegui, J. (1986) Brain Research 378, 36-48) could lead to an impaired activation of mitochondrial dehydrogenases after a rise in cytosolic calcium.

Uncoupling of oxidative phosphorylation by divalent cationic cyanine dye. Participation of phosphate transporter

The trinuclear cationic cyanine dye tri-S-C4(5) was found to be an uncoupler of oxidative phosphorylation. Its uncoupling required inorganic phosphate (Pi) or arsenate, which is transported into mitochondria via the Pi transport system, and was abolished by the Pi-transport inhibitor N-ethylmaleimide or mersalyl. The dye stimulated Pi uptake into mitochondria, and its uncoupling action was accompanied by swelling of the mitochondria. The adenine nucleotides ADP and ATP protected mitochondria from uncoupling by the dye. The dye taken up by mitochondria was released into the incubation medium on induction of uncoupling. In the absence of Pi, the dye did not cause uncoupling, but its uptake was much greater than in the presence of Pi. The cyanine dye is suggested to induce uncoupling by acting on the membrane, rather than after its electrophoretic transfer into the mitochondria.

Adenosine 5'-triphosphate stimulates the release of polypeptides from mitochondria

There was release of polypeptides to the medium when mitochondria containing labeled proteins were incubated with a rat liver post-mitochondrial supernatant. The release of polypeptides increased with the amount of rat liver extract added. Addition of cycloheximide did not inhibit the effect. Heating the post-mitochondrial supernatant did not inhibit the release of mitochondrial proteins, indicating that it was due to a heat-stable factor. The factor responsible has been isolated and identified as ATP. The presence of EDTA inhibits the release of polypeptides caused by ATP and Mg2+ stimulates it. The possible role of ATP in the turnover of mitochondrial proteins is briefly discussed.

The role of ADP in the modulation of the calcium-efflux pathway in rat brain mitochondria

The role of ADP in the regulation of Ca2+ efflux in rat brain mitochondria was investigated. ADP was shown to inhibit Ruthenium-Red-insensitive H+- and Na+-dependent Ca2+-efflux rates if Pi was present, but had no effect in the absence of Pi. The primary effect of ADP is an inhibition of Pi efflux, and therefore it allows the formation of a matrix Ca2+-Pi complex at concentrations above 0.2 mM-Pi and 25 nmol of Ca2+/mg of protein, which maintains a constant free matrix Ca2+ concentration. ADP inhibition of Pi and Ca2+ efflux is nucleotide-specific, since in the presence of oligomycin and an inhibitor of adenylate kinase ATP does not substitute for ADP, is dependent on the amount of ADP present, and requires ADP concentrations in excess of the concentrations of translocase binding sites. Brain mitochondria incubated with 0.2 mM-Pi and ADP showed Ca2+-efflux rates dependent on Ca2+ loads at Ca2+ concentrations below those required for the formation of a Pi-Ca2+ complex, and behaved as perfect cytosolic buffers exclusively at high Ca2+ loads. The possible role of brain mitochondrial Ca2+ in the regulation of the tricarboxylic acid-cycle enzymes and in buffering cytosolic Ca2+ is discussed.