Correlated effluxes of adenine nucleotides, Mg2+ and Ca2+ induced in rat-liver mitochondria by external Ca2+ and phosphate

Adenylate kinase derived ATP shapes respiration and calcium storage of isolated mitochondria

The ratio of ADP and ATP is a natural indicator of cellular bioenergetic state and thus a prominent analyte in metabolism research. Beyond adenylate interconversion via oxidative phosphorylation and ATPase activities, ADP and ATP act as steric regulators of enzymes, e.g. cytochrome C oxidase, and are major factors in mitochondrial calcium storage potential. Consideration of all routes of adenylate conversion is critical to successfully predict their abundance in an experimental system and to correctly interpret many aspects of mitochondrial function. We showcase here how adenylate kinases elicit considerable impact on the outcome of a variety of mitochondrial assays through their drastic manipulation of the adenylate profile. Parameters affected include cytochrome c oxidase activity, P/O ratio, and mitochondrial calcium dynamics. Study of the latter revealed that the presence of ATP is required for mitochondrial calcium to be shaped into a particularly dense form of mitochondrial amorphous calcium phosphate.

To involvement the conformation of the adenine nucleotide translocase in opening the Tl(+)-induced permeability transition pore in Ca(2+)-loaded rat liver mitochondria

The conformation of adenine nucleotide translocase (ANT) has a profound impact in opening the mitochondrial permeability transition pore (MPTP) in the inner membrane. Fixing the ANT in 'c' conformation by phenylarsine oxide (PAO), tert-butylhydroperoxide (tBHP), and carboxyatractyloside as well as the interaction of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) with mitochondrial thiols markedly attenuated the ability of ADP to inhibit the MPTP opening. We earlier found (Korotkov and Saris, 2011) that calcium load of rat liver mitochondria in medium containing TlNO3 and KNO3 stimulated the Tl(+)-induced MPTP opening in the inner mitochondrial membrane. The MPTP opening as well as followed increase in swelling, a drop in membrane potential (ΔΨmito), and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration were visibly enhanced in the presence of PAO, tBHP, DIDS, and carboxyatractyloside. However, these effects were markedly inhibited by ADP and membrane-penetrant hydrophobic thiol reagent, N-ethylmaleimide (NEM) which fix the ANT in 'm' conformation. Cyclosporine A additionally potentiated these effects of ADP and NEM. Our data suggest that conformational changes of the ANT may be directly involved in the opening of the Tl(+)-induced MPTP in the inner membrane of Ca(2+)-loaded rat liver mitochondria. Using the Tl(+)-induced MPTP model is discussed in terms finding new transition pore inhibitors and inducers among different chemical and natural compounds.

Bongkrekic acid analogue, lacking one of the carboxylic groups of its parent compound, shows moderate but pH-insensitive inhibitory effects on the mitochondrial ADP/ATP carrier

Bongkrekic acid, isolated from Burkholderia cocovenenans, is known to specifically inhibit the mitochondrial ADP/ATP carrier. However, the manner of its interaction with the carrier remains elusive. In this study, we tested the inhibitory effects of 17 bongkrekic acid analogues, derived from the intermediates obtained during its total synthesis, on the mitochondrial ATP/ATP carrier. Rough screening of these chemicals, performed by measuring their inhibitory effects on the mitochondrial ATP synthesis, revealed that 4 of them, KH-1, KH-7, KH-16, and KH-17, had moderate inhibitory effects. Further characterization of the actions of these 4 analogues on mitochondrial function showed that KH-16 had moderate; KH-1 and KH-17, weak; and KH-7, negligible side effects of both permeabilization of the mitochondrial inner membrane and inhibition of the electron transport, indicating that only KH-7 had a specific inhibitory effect on the mitochondrial ADP/ATP carrier. Although the parental bongkrekic acid showed a strong pH dependency of its action, the inhibitory effect of KH-7 was almost insensitive to the pH of the reaction medium, indicating the importance of the 3 carboxyl groups of bongkrekic acid for its pH-dependent action. A direct inhibitory effect of KH-7 on the mitochondrial ADP/ATP carrier was also clearly demonstrated.

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.

Permeability transition-independent release of mitochondrial cytochrome c induced by valinomycin

To examine whether valinomycin induces a mitochondrial permeability transition (PT), we investigated its effects on mitochondrial functions under various conditions. The acceleration of mitochondrial respiration and swelling, induced by valinomycin, were found to be insensitive to inhibitors of the ordinary PT, indicating that valinomycin does not induce the ordinary PT. Results of experiments using mitochondria isolated from transgenic mice expressing human bcl-2 also supported this conclusion. Furthermore, evidence for induction of PT pores by valinomycin was not obtained by either electron microscopic analysis of mitochondrial configurations or by measurement of the permeability of the inner mitochondrial membrane by use of polyethylene glycol. However, valinomycin did induce a significant release of cytochrome c, and thus it may be a nice tool to study the processes of mitochondrial cytochrome c release.

Oxidant, mitochondria and calcium: an overview

Mitochondria are active in the continuous generation of reactive oxygen species (ROS), (e.g., superoxide), thereby favouring a situation of mitochondrial oxidative stress. Under oxidative stress--for example, ischaemia-reoxygenation injury to cells--mitochondria form superoxide, which in turn is converted to hydrogen peroxide and the potent reactive species, hydroxyl radical. Alternatively, mitochondrial superoxide may react with nitric oxide to form potent oxidant peroxynitrite and as a consequence, mitochondrial function is altered. An increase in the release of calcium from mitochondria by oxidants stimulates calcium-dependent enzymes such as calcium-dependent proteases, nucleases, and phospholipases, which subsequently trigger apoptosis of the cells. In principle, calcium can leave mitochondria by different ways: by non-specific leakage through the inner membrane by "pore formation," by changes in the membrane lipid phase, by reversal of the uniport influx carrier, by the specific calcium/hydrogen (or sodium) antiport system, by channel-mediated release pathways, or by a combination of two or more of these pathways. Additionally, the release of calcium from mitochondria can also occur either by oxidation of internal nicotinamide adenine nucleotides to ADP ribose and nicotinamide or by oxidation of thiols in membrane proteins. Once calcium efflux has been triggered, a series of common pathways of apoptosis are initiated, each of which may be sufficient to destroy the cell. Apoptosis requires the active participation of cellular components, and several genes have been suggested to control apoptosis. The proto-oncogene bcl-2 suppresses apoptosis through mitochondrial effects. Overexpression of bcl-2 in the mitochondrial membrane inhibits calcium efflux, but the underlying mechanisms are not clearly known. Further studies are needed to explore the nature of the apoptosis-inducing pathways, the precise mechanisms of calcium efflux, the molecular partners of bcl-2 oncoproteins at the level of the outer-inner membrane contact sites, the molecular biology of the apoptosis-inducing factor formation and release, and the essential molecular targets of apoptosis-inducing proteases. Clarification of these issues might facilitate the understanding of mitochondrial response on cellular calcium dynamics under oxidant stress.

Periodate-oxidized ATP stimulates the permeability transition of rat liver mitochondria

Periodate-oxidized ADP (oADP)2 and periodate-oxidized ATP (oATP) stimulate the permeability transition in energized rat liver mitochondria measured as the Ca2+-efflux induced by Ca2+ and Pi. In the presence of Mg2+ and Pi, mitochondria lose intramitochondrial adenine nucleotides at a slow rate. oATP induces a strong decrease of the matrix adenine nucleotides which is inhibited by carboxyatractyloside. Under these conditions, Mg2+ prevents the opening of the permeability transition pore. EGTA prevents the Pi-induced slow efflux of adenine nucleotides, but is without effect on the oATP-induced strong decrease of adenine nucleotides. This oATP-induced strong adenine nucleotide efflux is inhibited by ADP. oATP reduces the increase of matrix adenine nucleotides occurring when the mitochondria are incubated with Mg2+ and ATP. This effect of oATP is also prevented by carboxyatractyloside. oATP is not taken up by the mitochondria. It is suggested that oATP induces a strong efflux of matrix adenine nucleotides by the interaction with the ADP/ATP carrier from the cytosolic side. The induction of the mitochondrial permeability transition by oADP and oATP is attributed to two mechanisms-a strong decrease in the intramitochondrial adenine nucleotide content, especially that of ADP, and a stabilization of the c-conformation of the ADP/ATP carrier.

Triphenyltin as inductor of mitochondrial membrane permeability transition

The effect of triphenyltin on mitochondrial Ca2+ content was studied. It was found that this trialkyltin compound induces an increase in membrane permeability that leads to Ca2+ release, drop of the transmembrane potential, and efflux of matrix proteins. Interestingly, cyclosporin A was unable to inhibit triphenyltin-induced Ca2+ release. Based on these results it is proposed that the hyperpermeable state is produced by modification of 2.25 nmol of membrane thiol groups.

On the involvement of a mitochondrial pore in reperfusion injury

We review evidence implicating mitochondrial dysfunction in the pathogenesis of ischaemia/reperfusion injury. The lesion has been identified as a non selective pore that is triggered by Ca2+ and particular metabolic derangements associated with this form of injury, namely falling ATP, raised Pi and oxidative stress. Once activated, the pore flickers between open and closed states and disrupts mitochondrial energy transduction, allowing ATP hydrolysis by the F1F0 ATPase. Pore activation is prevented by cyclosporin A, which also retards the onset of necrosis in heart cells subjected to substrate-free anoxia and allows partial regeneration of ATP on reoxygenation.

Calcium-dependent mitochondrial oxidative damage promoted by 5-aminolevulinic acid

Swelling of isolated rat liver mitochondria is shown to be induced by metal-catalyzed 5-aminolevulinic acid (ALA) aerobic oxidation, a putative endogenous source of reactive oxygen species (ROS), at concentrations as low as 50-100 microM. In this concentration range, ALA is estimated to occur in the liver of acute intermittent porphyria patients. Removal of Ca2+ (10 microM) from the suspension of isolated rat liver mitochondria by added EGTA abolishes both the ALA-induced transmembrane-potential collapse and mitochondrial swelling. Prevention of the ALA-induced swelling by addition of ruthenium red prior to mitochondrial energization by succinate demonstrates the deleterious involvement of internal Ca2+. Addition of MgCl2 at concentrations higher than 2.5 mM, prevents the ALA-induced mitochondrial swelling, transmembrane potential collapse and Ca2+ efflux. This indicates that Mg2+ protects against the mitochondrial damage promoted by ALA-generated ROS. The ALA-induced mitochondrial damage might be a key event in the liver mitochondrial damage of acute intermittent porphyria patients reported elsewhere.

Fluorescamine-induced membrane permeability in mitochondria

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

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.

On the mechanism of oligomycin inhibition of Ca(2+)-induced mitochondrial respiration

The addition of oligomycin in the presence of Ca2+ increased the ADP pool in mitochondrial suspension. It is suggested that oligomycin inhibition of Ca(2+)-induced mitochondrial respiratory activation is the function of the increased endogenous ADP pool. Low ADP concentrations (5-20 microM) produce the same inhibitory effect as oligomycin. The increase of ADP levels in the presence of glucose plus hexokinase resulted in the inhibition of Ca(2+)-induced respiration, while the addition of phosphoenol pyruvate plus pyruvate kinase followed by a reduction in ADP levels, reversed the oligomycin inhibitory effect. One of the essential stages of ADP accumulation in mitochondrial suspensions in the presence of oligomycin and Ca2+ is proposed to be the formation of ADP from AMP and ATP, effected by adenylate kinase.

The presence of two classes of high-affinity cyclosporin A binding sites in mitochondria. Evidence that the minor component is involved in the opening of an inner-membrane Ca(2+)-dependent pore

The inner membrane of rat liver mitochondria contains a reversible Ca(2+)-dependent pore, opening of which is largely blocked by cyclosporin A. Analyses of [3H]cyclosporin binding to rat liver mitochondria demonstrate two classes of high-affinity binding site with capacities of less than 5 pmol and approximately 60 pmol cyclosporin.mg mitochondrial protein-1 in addition to partitioning into membrane phospholipids (0.03 pmol.mg mitochondrial protein.nM-1). Direct measurement [14C]sucrose entry into the matrix space indicates that cyclosporin A inhibits pore opening by interacting with the low-capacity sites. The same low-capacity sites (Kd cyclosporin, 8 nM) are possibly attributable to peptidylprolyl cis-trans-isomerase, although investigation of pore state interconversion from the rapid kinetics of [14C]sucrose entrapment in the matrix space does not indicate that cyclosporin-sensitive prolyl isomerization occurs at the actual step of pore opening/closure. It is suggested that the low-capacity cyclosporin-binding component may stabilize the open pore state; this is supported by the observations that Ca2+ decreases cyclosporin binding to this component and that cyclosporin brings about closure of the pre-opened pore. The implications for the possible number of functional pores in mitochondria are discussed.

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+.

Ca2+-mediated action of long-chain acyl-CoA on liver mitochondria energy-linked processes

The decrease of steady-state transmembrane potential (delta psi) and loss of accumulated Ca2+ are magnified if palmitoyl-CoA is added to rat liver mitochondria exposed to Ca2+ and phosphate. The extent of this damage increases with increasing concentration of long-chain acyl-CoA. Addition of L-carnitine with or without the addition of palmitoyl-CoA considerably delays the deenergization. In the latter case, there is a substantial decrease in the assayed endogenous long-chain acyl-CoA content. This protective action of L-carnitine is abolished by L-aminocarnitine, a powerful inhibitor of carnitine palmitoyl transferase (palmitoyl-CoA: L-carnitine O-palmitoyltransferase, EC 2.3.1.21.). The removal of Ca2+ by EGTA, or the inhibition of its uptake by Ruthenium red or Mg2+ further enhances the degree of protection.

Interaction of Sr2+ with Ca2+-induced Ca2+ release in mitochondria

Respiring rat liver mitochondria are known to spontaneously release the Ca2+ taken up when they have accumulated Ca2+ over a certain threshold, while Sr2+ and Mn2+ are well tolerated and retained. We have studied the interaction of Sr2+ with Ca2+ release. When Sr2+ was added to respiring mitochondria simultaneously with or soon after the addition of Ca2+, the release was potently inhibited or reversed. On the other hand, when Sr2+ was added before Ca2+, the release was stimulated. Ca2+-induced mitochondrial damage and release of accumulated Ca2+ is generally believed to be due to activation of mitochondrial phospholipase A (EC 3.1.1.4.) by Ca2+. However, isolated mitochondrial phospholipase A activity was little if at all inhibited by Sr2+. The Ca2+-release may thus be triggered by some Ca2+-dependent function other than phospholipase.

Protective action of methylglyoxal bis (guanylhydrazone) on the mitochondrial membrane

At low concentrations (0.5-1.0 mM) methylglyoxal bis (guanylhydrazone) (MGBG) exhibited a clearcut protection of rat liver mitochondria against the deenergizing action of either Ca2+, or oxidizing agents (butylhydroperoxide and oxaloacetate). Such a protection resulted from the prevention of transmembrane potential decay, discharge of accumulated Ca2+, release of mitochondrial Mg2+, adenine nucleotides and pyridine nucleotides and mitochondrial swelling. At high concentrations (5-10 mM) MGBG induced functional alterations of mitochondria (decrease of transmembrane potential, lower capability to accumulate and to retain Ca2+) which can be reversed by resuspension of mitochondria in a MGBG free medium. These reversible mitochondrial alterations by high MGBG concentrations are interpreted as a consequence of an aggregation and coprecipitation of suspended mitochondria.

Alleviation of cyclosporine nephrotoxicity with verapamil and ATP-MgCl2. Mitochondrial respiratory and calcium studies

Although recent studies have shown that combined treatment with verapamil and ATP-MgCl2 (ATP) prevents cyclosporine (CyA)-induced nephrotoxicity, the mechanism of these effects remains unknown. To study this, rat kidneys were perfused at 100 mmHg for 100 minutes with Krebs buffer containing 7.5 g/dL of albumin and substrates. After an equilibration period of 30 minutes, 500 ng/mL CyA was added. In some experiments 1 microgram/mL verapamil was added 10 minutes prior to CyA and in others 2 mM ATP was added to CyA. At the end of the perfusion, cortical mitochondria (mito) were isolated and mito Ca2+ and Mg2+ (mumoles/g protein) and respiratory control ratios (RCR) were measured. In addition, total tissue Ca2+ and Mg2+ levels were measured. The results indicate that CyA treatment leads to an accumulation of mito Ca2+ and a decrease in ADP/O ratio. Simultaneous administration of ATP with CyA led to an increased mito Ca2+ accumulation and depressed RCR, which were corrected by verapamil pretreatment. The combination of verapamil pretreatment and ATP cotreatment with CyA increased tissue ATP levels from 0.8 +/- 0.4 (control) to 1.4 +/- 0.1 mumol/g. This pharmacologic regimen may prevent CyA-induced nephrotoxicity by preventing mito Ca2+ accumulation and by preserving mitochondrial respiratory function. This allows a more efficient generation of ATP and consequently preservation of renal function.

Ion permeability induction by the SH cross-linking reagents in rat liver mitochondria is inhibited by the free radical scavenger, butylhydroxytoluene

The hydrophobic, potentially SH cross-linking reagent, phenylarsine oxide (PhAsO), was found to induce K+ and Ca2+ effluxes from mitochondria and to accelerate the respiration rate in state 4. The hydrophobic monofunctional electrophilic agent, N-ethylmaleimide, does not exhibit this effect but prevents the action of PhAsO. The polar potentially SH cross-linking regents (arsenite, diamide) induce ion fluxes only in the presence of Pi. Ion fluxes induced by the SH reagents are inhibited by butylhydroxytoluene (an inhibitor of free radical reactions), and N,N'-dicyclohexylcarbodiimide, not by oligomycin. It is inferred that the induction of ion fluxes in mitochondria caused by cross-linking of two juxtaposed SH groups is related to the development of free radical reactions.

Ca2+ transport in isolated mouse liver mitochondria; role of reductive carboxylation and citrate?

The uptake of Ca2+ in isolated mouse liver mitochondria respiring on succinate in the presence of rotenone and added Pi, was inhibited by dibucaine, fluorocitrate, p-hydroxymercuribenzoate (PMB), malonate, palmitoyl-CoA, succinyl-CoA and trifluoroperazine. The release of accumulated Ca2+ was stimulated by arsenite, malonate, PMB, palmitoyl-CoA and succinyl-CoA, whereas the release was inhibited by dibucaine, fluorocitrate, trifluoroperazine, and by oligomycin, especially in the presence of ADP. The pyridine nucleotides were oxidized in mitochondria incubated with PMB. The observations suggest a possible contributory role of reductive carboxylation for the uptake of Ca2+, and a possible role of citrate for the retention of Ca2+ in isolated mouse liver mitochondria.

t-Butylhydroperoxide-induced Ca2+ efflux from liver mitochondria in the presence of physiological concentrations of Mg2+ and ATP

Isolated rat liver mitochondria, energized either by succinate oxidation or by ATP hydrolysis, present a transient increase in the rate of Ca2+ efflux concomitant to NAD(P)H oxidation by hydroperoxides when suspended in a medium containing 3 mM ATP, 4 mM Mg2+ and acetate as permeant anion. This is paralleled by an increase in the steady-state concentration of extramitochondrial Ca2+, a small decrease in delta psi and an increase in the rate of respiration and mitochondrial swelling. With the exception of mitochondrial swelling all other events were found to be reversible. If Ca2+ cycling was prevented by ruthenium red, the changes in delta psi, the rate of respiration and the extent of mitochondrial swelling were significantly diminished. In addition, there was no significant decrease in the content of mitochondrial pyridine nucleotides. Mitochondrial coupling was preserved after a cycle of Ca2+ release and re-uptake under these experimental conditions. It is concluded that hydroperoxide-induced Ca2+ efflux from intact mitochondria is related to the redox state of pyridine nucleotides.

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.

Stabilising action of carnitine on energy linked processes in rat liver mitochondria

Rat liver mitochondria exposed to stressing conditions - ageing at room temperature, incubation in the presence of t-butyl hydroperoxide or damaging concentrations of Ca2+ and phosphate- undergo a rapid fall in their membrane potential (delta psi) with a concomitant release of endogenous Mg2+ and accumulated Ca2+. Addition of L-carnitine to the incubation medium considerably delays mitochondrial deenergization. A similar, though lower, protection has also been observed in L-carnitine pretreated and subsequently washed rat liver mitochondria. Furthermore mitochondria isolated from livers of starved rats, treated with L-carnitine 30 minutes before death and exposed to the same stressing conditions show similar delay in the decrease of delta psi and concurrent energy linked processes as compared with untreated animals. Both the in vitro and in vivo results strongly indicate that the stabilising action of L-carnitine on liver mitochondria is due to the removal of membrane bound long chain acyl CoA.

Uncoupler-stimulated release of Ca2+ from Ehrlich ascites tumor cell mitochondria

Ruthenium red-insensitive, uncoupler-stimulated release of Ca2+ from Ehrlich ascites tumor cell mitochondria is much slower than from rat liver mitochondria under comparable conditions. In the presence of Pi and at moderate or high Ca2+ loads, ruthenium red-insensitive Ca2+ efflux elicited with uncoupler is approximately 20 times more rapid for rat liver than Ehrlich cell mitochondria. This is attributed to resistance of tumor mitochondria to damage by Ca2+ due to a high level of endogenous Mg2+ that also attenuates Ca2+ efflux. Calcium release from rat liver and tumor mitochondria is inhibited by exogenous Mg2+. This applies to ruthenium red-insensitive spontaneous Ca2+ efflux associated with Ca2+ uptake and uncoupling, and (b) ruthenium red-insensitive Ca2+ release stimulated by uncoupling agent. The endogenous Mg2+ level of Ehrlich tumor mitochondria is approximately three times that of rat liver mitochondria. Endogenous Ca2+ is also much greater (six fold) in Ehrlich tumor mitochondria compared to rat liver. Despite the quantitative difference in endogenous Mg2+, the properties of internal Mg2+ are much the same for rat liver and Ehrlich cell mitochondria. Ehrlich ascites tumor mitochondria exhibit slow, metabolically dependent Mg2+ release and rapid limited release of Mg2+ during Ca2+ uptake. Both have been observed with rat liver and other types of mitochondria. The proportions of apparently "bound" and "free" Mg2+ (inferred from release by the ionophore, A23187) do not differ significantly between tumor and liver mitochondria. Thus, the endogenous Mg2+ of tumor mitochondria has no unusual features but is simply elevated substantially. Ruthenium red-insensitive Ca2+ efflux, when expressed as a function of the intramitochondrial Ca2+/Mg2+ ratio, is quite similar for tumor and rat liver. It is proposed, therefore, that endogenous Mg2+ is a major regulatory factor responsible for differences in the sensitivity to damage by Ca2+ and Ca2+ release by Ehrlich ascites tumor mitochondria compared to mitochondria from normal tissues.

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.

Alloxan effects on mitochondria: study of oxygen consumption, fluxes of Mg2+, Ca2+, K+ and adenine nucleotides, membrane potential and volume change in vitro

Isolated mouse liver mitochondria incubated with alloxan showed stimulated resting (state 4) respiration with succinate, and inhibited resting respiration with pyridine-linked substrates, whereas active (state 3) respiration was decreased with both kinds of substrates. The effects were dependent on the concentration of alloxan, on the energy state, and on transport of inorganic phosphate and uptake of Ca2+. Using succinate as substrate, the effects of alloxan on endogenous Mg2+, K+ and adenine nucleotides, uptake of K+, accumulated Ca2+, membrane potential and volume were studied in liver mitochondria, and in addition efflux of endogenous K+ and accumulated Ca2+ were investigated in mouse islet mitochondria. High concentrations of alloxan (greater than or equal to 3 mmol/l) induced efflux of endogenous Mg2+, K+ and adenine nucleotides, efflux of accumulated Ca2+, inhibition of uptake of K+, loss of membrane potential, and swelling. Low concentrations of alloxan (less than 3 mmol/l) had similar effects only in the presence of added Ca2+ and inorganic phosphate. The influence of potentially protective agents was studied mainly with regard to alloxan induced swelling. Complete or partial protection was offered by antimycin A, malonate, La3+, Ni2+, ruthenium red, mersalyl and N-ethylmaleimide, suggesting requirement for energized transport of Ca2+ and uptake of inorganic phosphate. The start of the respiratory changes, decrease of membrane potential and loss of Mg2+ preceded the release of accumulated Ca2+, which occurred in parallel with efflux of K+ and swelling. The loss of Ca2+ in association with swelling agrees with data previously obtained using qualitative and quantitative electron microscopy and X-ray microanalysis of islet beta cells from alloxan-treated mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation of NAD(P)+-stimulated mitochondrial Ca2+ efflux from swelling and membrane damage

NAD(P)+-stimulated Ca2+ efflux from mitochondria in a high-sucrose medium is irreversible and is accompanied by large-amplitude mitochondrial swelling and membrane damage. If sucrose is partially replaced by polyethylene glycol (Mr approximately equal to 1000) as osmolar supporting medium, Ca2+ efflux is still stimulated by NAD(P)+ but mitochondrial swelling is eliminated. Other experiments in a high-sucrose medium showed that the lag phase between NAD(P)H oxidation and the beginning of net Ca2+ efflux decreases with increasing temperature. At 37 degrees C Ca2+ efflux precedes mitochondrial swelling, even in a high-sucrose medium, showing that the mitochondrial damage, as reflected by large-amplitude swelling, is not obligatory for Ca2+ efflux induced by the oxidized state of mitochondrial NAD(P)+. If a high-sucrose medium is supplemented with 20 mM potassium acetate, longer periods of Ca2+ release can be observed before the appearance of swelling. Under these experimental conditions the release of Ca2+ can be completely reversed if the rereduction of NAD(P)+ is brought about by the addition of the reductants beta-hydroxybutyrate and isocitrate.

Possible participation of membrane thiol groups on the mechanism of NAD(P)+-stimulated Ca2+ efflux from mitochondria

NAD(P)+-stimulated Ca2+ efflux from mitochondria is inhibited by bongkrekate and slightly stimulated by carboxyatractylate. Addition of oxaloacetate, an NAD(P) oxidant, or diamide, a thiol oxidant, to de-energized mitochondria incubated in Ca2+ -free medium induced a small decrease in turbidity of the mitochondrial suspension compatible with small structural changes of mitochondria. Similar to NADP+-stimulated Ca2+ efflux these changes were also inhibited by bongkrekate and slightly stimulated by carboxyatractylate. The similarity between the effects of oxaloacetate and diamide, on both Ca2+ efflux and mitochondrial structure, indicates the existence of a common denominator, possibly the oxidation of specific thiol groups, regarding the mechanism by which these agents stimulate Ca2+ efflux from mitochondria.

Calcium efflux parallel to total phosphate retention in rat liver mitochondria

Phosphate efflux from uncoupled rat liver mitochondria was completely inhibited when mersalyl plus butylmalonate and ATP were added to a sucrose suspending medium. Despite the total retention of phosphate a calcium efflux was observed even in presence of ruthenium red. Under the above conditions no phosphate is transported in association with the ADP/ATP carrier. While mersalyl completely blocked the phosphate release induced by ruthenium red or EGTA from coupled mitochondria it only partially inhibited the CA2+-efflux. The inhibition of Ca2+ efflux was almost completely abolished in the presence of acetate. The existence of a co-transport of Ca2+ associated with phosphate is discussed.

On the mechanism of citrate and isocitrate protective action on rat liver mitochondria

Both citrate and isocitrate prevent the damage (efflux of endogenous Mg2+ and pyridine nucleotides, decay of delta psi and release of accumulated Ca2+) induced in rat liver mitochondria by Ca2+ and phosphate fluxes. Addition of fluorocitrate suppresses the action of isocitrate, but not that of citrate. The same results have been obtained with mitochondria isolated from animals treated with fluoroacetate. It is suggested that citrate directly and isocitrate by prior conversion into citrate exert the protective action by chelating and retaining Mg2+ within the mitochondria.

Involvement of mitochondria in ischemic cell injury and in regulation of intracellular calcium

Ischemia causes a pathological drop in the cellular energy state due to inhibition of mitochondrial oxidative phosphorylation. The reversibility of this condition depends on the damage to mitochondrial membrane-linked activities during the period of ischemia or during reoxygenation of the tissue. It is likely that the ischemia-induced damage is due to a combination of factors including an increase in the cytosolic free Ca2+ concentration, a triggering of phospholipase and protease activities, an increase in cellular free fatty acids, and a decrease in pH. Mitochondrial damage that occurs during reperfusion is probably a consequence of excessive mitochondrial Ca2+ accumulation under adverse intracellular conditions. Mitochondria normally have an extremely high capacity for sequestering and buffering cytosolic Ca2+. However, during postischemic reperfusion these processes are inhibited due to existing conditions that potentiate Ca2+ uptake-induced irreversible mitochondrial damage.

Inhibition of oxidative phosphorylation by a Ca2+-induced diminution of the adenine nucleotide translocator

The mechanism through which internal Ca2+ inhibits oxidative phosphorylation of rat heart mitochondria has been explored. In parallel to a Ca2+-induced diminution of the activity of the adenine nucleotide translocator, an efflux of internal adenine nucleotides is observed. The efflux of adenine nucleotides depends on the amount of Ca2+ accumulated by the mitochondria and on the time that Ca2+ remains in the mitochondria; this efflux is atractyloside insensitive. These results suggest that internal Ca2+, by inducing a lowering of the internal concentration of adenine nucleotides, diminishes the rate of exchange of adenine nucleotides via the translocase, and in consequence of oxidative phosphorylation. Under conditions in which the Ca2+-induced release of adenine nucleotides takes place, no gross changes of the permeability properties of the membrane are observed. As revealed by studies with arsenate, respiratory activity and the function of the ATPase in the direction of ATP synthesis are not affected by internal Ca2+.

Mn2+ prevents the Ca2+-induced inhibition of ATP synthesis in brain mitochondria

Uptake of Ca2+ by rat brain mitochondria causes an inhibition of respiratory stimulation by ADP, and the inhibition is relieved upon Na+-induced release of Ca2+ from the mitochondria, in accordance with earlier reports. We show that simultaneous uptake of Ca2+ and Mn2+ results in no inhibition of ADP-stimulated respiration, indicating that Mn2+ prevents the Ca2+-induced inhibition of ATP synthesis, without preventing Ca2+ accumulation in the mitochondria. The results are discussed in relation to a possible involvement of the mitochondrial ATPase-inhibitor protein in the observed effects of Ca2+ and Mn2+.

Net adenine nucleotide transport in rat liver mitochondria is affected by both the matrix and the external ATP/ADP ratios

The adenine nucleotide pool size of rat liver mitochondria may be regulated by a transport mechanism which allows net movement of ATP or ADP across the inner mitochondrial membrane (J. R. Aprille and J. Austin (1981) Arch. Biochem. Biophys. 212, 689-699). Since transport can occur in either direction, and ATP is the preferred substrate, variations in the matrix or external ATP/ADP ratio were examined for an effect on the direction and rate of net adenine nucleotide flux. Fastest rates of net uptake were seen when the external ATP/ADP ratio was high and the internal ratio was low. Conversely, if the external ratio was low and the internal ratio was high, net loss was observed. The rate and direction of net transport were also changed by simply varying the external ATP concentration alone between 0 and 4 mM. Concentrations of less than 1 mM produced net loss of matrix adenines, whereas higher concentrations caused a net increase. The results suggest that changes in relative concentrations of ATP in the external and internal compartments can result in changes in the adenine nucleotide pool size. Another important result was clarification of earlier ambiguous findings with carboxyatractyloside. If care was taken to avoid changes in the matrix ATP/ADP ratio, the addition of carboxyatractyloside had no effect on either net uptake or net efflux of adenine nucleotides.

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

The presence of ATP or ADP in the incubation medium prevents the collapse of membrane potential induced by external Ca2+ and phosphate. The same adenine nucleotides are unable to restore collapsed membrane potential unless Mg2+ are also added. Bongkrekate is also able to prevent the effects of external Ca2+ and phosphate and when added after membrane potential has collapsed strongly potentiates the restorative action of ATP or ADP. Atractyloside has an opposite effect.

On the relationship between calcium and phosphate transport, transmembrane potential and acetoacetate-induced oxidation of pyridine nucleotides in rat-liver mitochondria

Acetoacetate addition to rat liver mitochondria induces a complete oxidation of pyridine nucleotides, a collapse of membrane potential, a release of mitochondrial Ca2+ and a loss of respiratory control only in the presence of external phosphate. Acetoacetate also enhances the efflux of mitochondrial Mg2+ promoted by phosphate. All these effects are not only prevented but also reversed, except the oxidation of pyridine nucleotides, by the combined addition of Mg2+, ADP and dithioerythritol to damaged mitochondria. It is concluded that acetoacetate, through the oxidation of mitochondrial pyridine nucleotides, potentiates the action of phosphate in altering the mitochondrial permeability barrier, which is closely dependent on the maintenance of membrane thiol groups in a reduced form.