Mitochondrial pyruvate transport and its hormonal regulation

AMPK inhibits liver gluconeogenesis: fact or fiction?

Is there a role for AMPK in the control of hepatic gluconeogenesis and could targeting AMPK in liver be a viable strategy for treating type 2 diabetes? These are frequently asked questions this review tries to answer. After describing properties of AMPK and different small-molecule AMPK activators, we briefly review the various mechanisms for controlling hepatic glucose production, mainly via gluconeogenesis. The different experimental and genetic models that have been used to draw conclusions about the role of AMPK in the control of liver gluconeogenesis are critically discussed. The effects of several anti-diabetic drugs, particularly metformin, on hepatic gluconeogenesis are also considered. We conclude that the main effect of AMPK activation pertinent to the control of hepatic gluconeogenesis is to antagonize glucagon signalling in the short-term and, in the long-term, to improve insulin sensitivity by reducing hepatic lipid content.

Isoorientin Decreases Cell Migration via Decreasing Functional Activity and Molecular Expression of Proton-Linked Monocarboxylate Transporters in Human Lung Cancer Cells

Aggressive tumor cells mainly rely on glycolysis, and further release vast amounts of lactate and protons by monocarboxylate transporter (MCT), which causes a higher intracellular pH (pH_i) and acidic extracellular pH. Isoorientin, a principle flavonoid compound extracted from several plant species, shows various pharmacological activities. However, effects of isoorientin on anticancer and MCT await to explore in human lung cancer cells. Human lung cancer tissues were obtained from cancer patients undergoing surgery, while the human lung adenocarcinoma cells (A549) were bought commercially. Change of pH_i was detected by microspectrofluorometry method with a pH-sensitive fluorescent dye, BCECF. MTT and wound-healing assay were used to detect the cell viability and migration, respectively. Western blot techniques and immunocytochemistry staining were used to detect the protein expression. Our results indicated that the expression of MCTs1/4 and CD147 were upregulated significantly in human lung tissues. In experiments of A549 cells, under HEPES-buffer, the resting pH_i was 7.47, and isoorientin (1-300μM) inhibited functional activity of MCT concentration-dependently (up to -42%). Pretreatment with isoorientin (3-100μM) for 24h, MCT activity and cell migration were significantly inhibited (-25% and -40%, respectively), while the cell viability was not affected. Moreover, the expression of MCTs1/4, CD147, and matrix metalloproteinase (MMP) 2/9 were significantly down regulated. In summary, MCTs1/4 and CD147 are significantly upregulated in human lung adenocarcinoma tissues, and isoorientin inhibits cells-migration by inhibiting activity/expression of MCTs1/4 and MMPs2/9 in human lung cancer cells. These novel findings suggest that isoorientin could be a promising pharmacological agent for lung cancer.

Association of NKAPL, TSPAN18, and MPC2 gene variants with schizophrenia based on new data and a meta-analysis in Han Chinese

BACKGROUND

Schizophrenia (SZ) is suggested to be a complex polygenetic disorder with high heritability. Genome-wide association studies have found that the rs1635, rs11038167, and rs10489202 polymorphisms are associated with SZ in Han Chinese. However, results of validation studies are inconsistent. This study aimed to test the association between the NKAPL rs1635, TSPAN18 rs11038167, and MPC2 rs10489202 polymorphisms and SZ in a Chinese population.

METHODS

This study contained 700 unrelated SZ patients (300 Zhuang and 400 Han) and 700 gender- and age-matched controls (300 Zhuang and 400 Han). The polymorphisms in TSPAN18 (rs11038167), NKAPL (rs1635), and MPC2 (rs10489202) were genotyped using the Sequenom MassARRAY method. Statistical analyses were performed with PLINK program and SPSS l6.0 for Windows. STATA11.1 was used for meta-analysis.

RESULTS

No statistically significant difference was found in different allele and genotype frequencies of rs1635, rs11038167, and rs10489202 between SZ cases and controls of Zhuang and Han ethnicities and the total samples (all p>0.05). Further meta-analysis suggested that single-nucleotide polymorphism rs10489202 was significantly associated with SZ in a Han Chinese population (p OR=0.002).

CONCLUSIONS

Our case-control study failed to validate the significant association of NKAPL rs1635, TSPAN18 rs11038167, and MPC2 rs10489202 polymorphisms with SZ susceptibility in the southern Zhuang or Han Chinese population. However, meta-analysis showed a significant association between MPC2 variant rs10489202 and SZ susceptibility in Han Chinese.

Pioglitazone inhibits mitochondrial pyruvate metabolism and glucose production in hepatocytes

Pioglitazone is used globally for the treatment of type 2 diabetes mellitus (T2DM) and is one of the most effective therapies for improving glucose homeostasis and insulin resistance in T2DM patients. However, its mechanism of action in the tissues and pathways that regulate glucose metabolism are incompletely defined. Here we investigated the direct effects of pioglitazone on hepatocellular pyruvate metabolism and the dependency of these observations on the purported regulators of mitochondrial pyruvate transport, MPC1 and MPC2. In cultured H4IIE hepatocytes, pioglitazone inhibited [2-¹⁴ C]-pyruvate oxidation and pyruvate-driven oxygen consumption and, in mitochondria isolated from both hepatocytes and human skeletal muscle, pioglitazone selectively and dose-dependently inhibited pyruvate-driven ATP synthesis. Pioglitazone also suppressed hepatocellular glucose production (HGP), without influencing the mRNA expression of key HGP regulatory genes. Targeted siRNA silencing of MPC1 and 2 caused a modest inhibition of pyruvate oxidation and pyruvate-driven ATP synthesis, but did not alter pyruvate-driven HGP and, importantly, it did not influence the actions of pioglitazone on either pathway. In summary, these findings outline a novel mode of action of pioglitazone relevant to the pathogenesis of T2DM and suggest that targeting pyruvate metabolism may lead to the development of effective new T2DM therapies.

Transport of haloacids across biological membranes

Haloacids are considered to be environmental pollutants, but some of them have also been tested in clinical research. The way that haloacids are transported across biological membranes is important for both biodegradation and drug delivery purposes. In this review, we will first summarize putative haloacids transporters and the information about haloacids transport when studying carboxylates transporters. We will then introduce MCT1 and SLC5A8, which are respective transporter for antitumor agent 3-bromopyruvic acid and dichloroacetic acid, and monochloroacetic acid transporters Deh4p and Dehp2 from a haloacids-degrading bacterium. Phylogenetic analysis of these haloacids transporters and other monocarboxylate transporters reveals their evolutionary relationships. Haloacids transporters are not studied to the extent that they deserve compared with their great application potentials, thus future inter-discipline research are desired to better characterize their transport mechanisms for potential applications in both environmental and clinical fields.

The mitochondrial pyruvate carrier: has it been unearthed at last?

The mitochondrial pyruvate carrier (MPC) is essential for several major pathways of carbohydrate, fat, and amino acid metabolism, yet its molecular identity has remained elusive. Two recent papers in Science (Herzig et al., 2012; Bricker et al., 2012) implicate three newly identified inner mitochondrial membrane proteins as MPC components.

Glucose is a pH-dependent motor for sperm beat frequency during early activation

To reach the egg in the ampulla, sperm have to travel along the female genital tract, thereby being dependent on external energy sources and substances to maintain and raise the flagellar beat. The vaginal fluid is rich in lactate, whereas in the uterine fluid glucose is the predominant substrate. This evokes changes in the lactate content of sperm as well as in the intracellular pH (pH(i)) since sperm possess lactate/proton co-transporters. It is well documented that glycolysis yields ATP and that HCO(3)- is a potent factor in the increase of beat frequency. We here show for the first time a pathway that connects both parts. We demonstrate a doubling of beat frequency in the mere presence of glucose. This effect can reversibly be blocked by 2-deoxy-D-glucose, dichloroacetate and aminooxyacetate, strongly suggesting that it requires both glycolysis and mitochondrial oxidation of glycolytic end products. We show that the glucose-mediated acceleration of flagellar beat and ATP production are hastened by a pH(i) ≥7.1, whereas a pH(i) ≤7.1 leaves both parameters unchanged. Since we observed a diminished rise in beat frequency in the presence of specific inhibitors against carbonic anhydrases, soluble adenylyl cyclase and protein kinase, we suggest that the glucose-mediated effect is linked to CO(2) hydration and thus the production of HCO(3)- by intracellular CA isoforms. In summary, we propose that, in sperm, glycolysis is an additional pH(i)-dependent way to produce HCO(3)-, thus enhancing sperm beat frequency and contributing to fertility.

N-acetyl cysteine improves glycogenesis after segmental liver ischemia and reperfusion injury in pigs

OBJECTIVE

N-acetylcysteine (NAC) is an antioxidative molecule known to protect liver tissue from oxygen radical species generated during ischemia and reperfusion (IR). Nutritional and toxicology studies have shown that NAC also improves glucose metabolism and glycogen stores. We hypothesized that NAC improves glycogenesis and that impaired glycogenesis is a key element in IR injury.

MATERIAL AND METHODS

In an experimental model, 80 min of segmental liver ischemia was induced in 16 pigs and the reperfusion was followed for 360 min. Eight animals received NAC 150 mg/kg as a bolus injection followed by an infusion of NAC 50 mg/kg/h intravenously.

RESULTS

AST and leukocyte density were lower in the NAC-treated animals, unrelated to the glutathione levels or apoptosis. Glycogen stores returned to a higher degree in the NAC-treated animals and microdialysis revealed lower levels of lactate during the reperfusion phase. Nitrite/Nitrate levels in the NAC group were lower in both serum and microdialysates, indicating that NAC scavenges radical nitrosative species.

CONCLUSIONS

NAC treatment improves glycogenesis after liver IR injury and reduces the level of intraparenchymal lactate during reperfusion, possibly due to the scavenging of radical nitrosative species.

The monocarboxylate transporter family--role and regulation

Monocarboxylate transporter (MCT) isoforms 1-4 catalyze the proton-linked transport of monocarboxylates such as L-lactate across the plasma membrane, whereas MCT8 and MCT10 are thyroid hormone and aromatic amino acid transporters, respectively. The importance of MCTs is becoming increasingly evident as their extensive physiological and pathological roles are revealed. MCTs 1-4 play essential metabolic roles in most tissues with their distinct properties, expression profile, and subcellular localization matching the particular metabolic needs of a tissue. Important metabolic roles include energy metabolism in the brain, skeletal muscle, heart, tumor cells, and T-lymphocyte activation, gluconeogenesis in the liver and kidney, spermatogenesis, bowel metabolism of short-chain fatty acids, and drug transport. MCT8 is essential for thyroid hormone transport across the blood-brain barrier. Genetic perturbation of MCT function may be involved in disease states such as pancreatic β-cell malfunction (inappropriate MCT1 expression), chronic fatigue syndromes (impairment of muscle MCT function), and psychomotor retardation (MCT8 mutation). MCT expression can be regulated at both the transcriptional and post-transcriptional levels. Of particular importance is the upregulation of muscle MCT1 expression in response to training and MCT4 expression in response to hypoxia. The latter is mediated by hypoxia inducible factor 1α and often observed in tumor cells that rely almost entirely on glycolysis for their energy provision. The recent discovery of potent and specific MCT1 inhibitors that prevent proliferation of T-lymphocytes confirms that MCTs may be promising pharmacological targets including for cancer chemotherapy.

Cardiovascular applications of hyperpolarized contrast media and metabolic tracers

Modern hyperpolarization technology enhances the recordable magnetic resonance signal four to five orders of magnitude, making in vivo assessments of tracer pathways and metabolic compartments feasible. Existing hyperpolarization instrumentation and previous tracer studies using hydroxyethylpropionate (HEP) as an extracellular marker and 14-carbon label pyruvate as examples are described and reviewed as applicable to the working heart. Future metabolic imaging based on the use of hyperpolarized pyruvate needs to consider extra- and intra-cellular label dilution due to glycolysis, lactate oxidation and protein degradation. This dilution can substantially decrease the recordable signals from PDH flux (oxidative decarboxylation of pyruvate) and other pyruvate pathways. The review of previous literature and data suggests that the (13)C-alanine signal is a better index of mitochondrially oxidized pyruvate than L-lactate. These facts and considerations will help in the interpretation of the in vivo recorded hyperpolarization signals of metabolic tracers and contrast media.

Phosphoenolpyruvate metabolism in Jerusalem artichoke mitochondria

We report here initial studies on phosphoenolpyruvate metabolism in coupled mitochondria isolated from Jerusalem artichoke tubers. It was found that: (1) phosphoenolpyruvate can be metabolized by Jerusalem artichoke mitochondria by virtue of the presence of the mitochondrial pyruvate kinase, shown both immunologically and functionally, located in the inner mitochondrial compartments and distinct from the cytosolic pyruvate kinase as shown by the different pH and inhibition profiles. (2) Jerusalem artichoke mitochondria can take up externally added phosphoenolpyruvate in a proton compensated manner, in a carrier-mediated process which was investigated by measuring fluorimetrically the oxidation of intramitochondrial pyridine nucleotide which occurs as a result of phosphoenolpyruvate uptake and alternative oxidase activation. (3) The addition of phosphoenolpyruvate causes pyruvate and ATP production, as monitored via HPLC, with their efflux into the extramitochondrial phase investigated fluorimetrically. Such an efflux occurs via the putative phosphoenolpyruvate/pyruvate and phosphoenolpyruvate/ATP antiporters, which differ from each other and from the pyruvate and the adenine nucleotide carriers, in the light of the different sensitivity to non-penetrant compounds. These carriers were shown to regulate the rate of efflux of both pyruvate and ATP. The appearance of citrate and oxaloacetate outside mitochondria was also found as a result of phosphoenolpyruvate addition.

Mitochondrial transport in proline catabolism in plants: the existence of two separate translocators in mitochondria isolated from durum wheat seedlings

Abiotic stresses, such as high salinity or drought, can cause proline accumulation in plants. Such an accumulation involves proline transport into mitochondria where proline catabolism occurs. By using durum wheat seedlings as a plant model system, we investigated how proline enters isolated coupled mitochondria. The occurrence of two separate translocators for proline, namely a carrier solely for proline and a proline/glutamate antiporter, is shown in a functional study in which we found the following: (1) Mitochondria undergo passive swelling in isotonic proline solutions in a stereospecific manner. (2) Externally added L: -proline (Pro) generates a mitochondrial membrane potential (Delta Psi) with a rate depending on the transport of Pro across the mitochondrial inner membrane. (3) The dependence of the rate of generation of Delta Psi on increasing Pro concentrations exhibits hyperbolic kinetics. Proline transport is inhibited in a competitive manner by the non-penetrant thiol reagent mersalyl, but it is insensitive to the penetrant thiol reagent N-ethylmaleimide (NEM). (4) No accumulation of proline occurs inside the mitochondria as a result of the addition of proline externally, whereas the content of glutamate increases both in mitochondria and in the extramitochondrial phase. (5) Glutamate efflux from mitochondria occurs at a rate which depends on the mitochondrial transport, and it is inhibited in a non-competitive manner by NEM. The dependence of the rate of glutamate efflux on increasing proline concentration shows saturation kinetics. The physiological role of carrier-mediated transport in the regulation of proline catabolism, as well as the possible occurrence of a proline/glutamate shuttle in durum wheat seedlings mitochondria, are discussed.

Identification and characterisation of a new class of highly specific and potent inhibitors of the mitochondrial pyruvate carrier

Two novel thiazolidine compounds, GW604714X and GW450863X, were found to be potent inhibitors of mitochondrial respiration supported by pyruvate but not other substrates. Direct measurement of pyruvate transport into rat liver and yeast mitochondria confirmed that these agents inhibited the mitochondrial pyruvate carrier (MPC) with K(i) values <0.1 muM. Inhibitor titrations of pyruvate-dependent respiration by heart mitochondria gave values (+/-S.E.) for the concentration of inhibitor binding sites (pmol per mg protein) and their K(i) (nM) of 56.0+/-0.9 and 0.057+/-0.010 nM for the more hydrophobic GW604714X; for GW450863X the values were 59.9+/-4.6 and 0.60+/-0.12 nM. [(3)H]-methoxy-GW450863X binding was also used to determine the MPC content of the heart, kidney, liver and brain mitochondria giving values of 56, 40, 26 and 20 pmol per mg protein respectively. Binding to yeast mitochondria was <10% of that in rat liver mitochondria, consistent with the slow rate of pyruvate transport into yeast mitochondria. [(3)H]-methoxy-GW450863X binding was inhibited by GW604714X and by the established MPC inhibitor, UK5099. The absorbance spectra of GW450863X and GW604714X were markedly changed by the addition of beta-mercaptoethanol suggesting that the novel inhibitors, like alpha-cyanocinnamate, possess an activated double bond that attacks a critical cysteine residue on the MPC. However, no labelled protein was detected following SDS-PAGE suggesting that the covalent modification is reversible. GW604714X and GW450863X inhibited l-lactate transport by the plasma membrane monocarboxylate transporter MCT1, but at concentrations more than four orders of magnitude greater than the MPC.

Partial reconstruction of in vitro gluconeogenesis arising from mitochondrial l-lactate uptake/metabolism and oxaloacetate export via novel L-lactate translocators

In the light of the occurrence of L-lactate dehydrogenase inside the mitochondrial matrix, we looked at whether isolated rat liver mitochondria can take up and metabolize L-lactate, and provide oxaloacetate outside mitochondria, thus contributing to a partial reconstruction of gluconeogenesis in vitro. We found that: (1) L-lactate (10 mM), added to mitochondria in the presence of a cocktail of glycolysis/gluconeogenesis enzymes and cofactors, can lead to synthesis of glyceraldehyde-3-phosphate at a rate of about 7 nmol/min per mg mitochondrial protein. (2) Three novel translocators exist to mediate L-lactate traffic across the inner mitochondrial membrane. An L-lactate/H+ symporter was identified by measuring fluorimetrically the rate of endogenous pyridine nucleotide reduction. Consistently, L-lactate oxidation was found to occur with P/O ratio=3 (where P/O ratio is the ratio of mol of ATP synthesized to mol of oxygen atoms reduced to water during oxidative phosphorylation) and with generation of membrane potential. Proton uptake, which occurred as a result of addition of L-lactate to RLM together with electron flow inhibitors, and mitochondrial swelling in ammonium L-lactate solutions were also monitored. L-Lactate/oxaloacetate and L-lactate/pyruvate anti-porters were identified by monitoring photometrically the appearance of L-lactate counter-anions outside mitochondria. These L-lactate translocators, which are distinct from the monocarboxylate carrier, were found to differ from each other in V(max) values and in inhibition and pH profiles, and proved to regulate mitochondrial L-lactate metabolism in vitro. The role of lactate/mitochondria interactions in gluconeogenesis is discussed.

Identification of the mitochondrial pyruvate carrier in Saccharomyces cerevisiae

Mitochondrial pyruvate transport is fundamental for metabolism and mediated by a specific inhibitable carrier. We have identified the yeast mitochondrial pyruvate carrier by measuring inhibitor-sensitive pyruvate uptake into mitochondria from 18 different Saccharomyces cerevisiae mutants, each lacking an unattributed member of the mitochondrial carrier family (MCF). Only mitochondria from the YIL006w deletion mutant exhibited no inhibitor-sensitive pyruvate transport, but otherwise behaved normally. YIL006w encodes a 41.9 kDa MCF member with homologous proteins present in both the human and mouse genomes.

Apoptotic pathways of oxidative damage to renal tubular epithelial cells

Toxic renal failure induced by gentamicin, glycerol, or cisplatin, as well as ischemic renal failure in vivo and hypoxia/reoxygenation of tubular epithelial cells in vitro, induces the production of reactive oxygen metabolites (ROM). Generation of ROM is responsible for the induction of tubular epithelial cell death, which is mediated by caspases and/or endonucleases. Scavenging of ROM protects tubular epithelium from caspase and endonuclease activation and from cell death. Thus, the inhibition of ROM production combined with the pharmacological control of caspase and endonuclease pathways may provide future modalities in the prevention or treatment of acute renal failure in humans.

D-Lactate transport and metabolism in rat liver mitochondria

In the present study we investigated whether isolated rat liver mitochondria can take up and metabolize D-lactate. We found the following: (1) externally added D-lactate causes oxygen uptake by mitochondria [P/O ratio (the ratio of mol of ATP synthesized to mol of oxygen atoms reduced to water during oxidative phosphorylation)=2] and membrane potential (Delta(psi)) generation in processes that are rotenone-insensitive, but inhibited by antimycin A and cyanide, and proton release from coupled mitochondria inhibited by alpha-cyanocinnamate, but not by phenylsuccinate; (2) the activity of the putative flavoprotein (D-lactate dehydrogenase) was detected in inside-out submitochondrial particles, but not in mitochondria and mitoplasts, as it is localized in the matrix phase of the mitochondrial inner membrane; (3) three novel separate translocators exist to mediate D-lactate traffic across the mitochondrial inner membrane: the D-lactate/H(+) symporter, which was investigated by measuring fluorimetrically the rate of endogenous flavin reduction, the D-lactate/oxoacid antiporter (which mediates both the D-lactate/pyruvate and D-lactate/oxaloacetate exchanges) and D-lactate/malate antiporter studied by monitoring photometrically the appearance of the D-lactate counteranions outside mitochondria. The D-lactate translocators, in the light of their different inhibition profiles separate from the monocarboxylate carrier, were found to differ from each other in the V(max) values and in the inhibition and pH profiles and were shown to regulate mitochondrial D-lactate metabolism in vitro. The D-lactate translocators and the D-lactate dehydrogenase could account for the removal of the toxic methylglyoxal from cytosol, as well as for D-lactate-dependent gluconeogenesis.

Pyruvate-induced long-term maintenance of glutathione s-transferase in rat hepatocyte cultures

The addition of pyruvate to the culture medium has been reported to improve the maintenance of P450-dependent enzyme expression in primary rat hepatocyte cultures. In this study, the effects of 30mM pyruvate on cell morphology, albumin secretion and glutathione S-transferase (GST) expression were investigated as a function of the time in culture. The effect of triiodothyronine (T3) exposure on GST expression was also measured in pyruvate-treated cultures. Transmission electron microscopy showed that untreated hepatocytes deteriorated after culture for 7 days, whereas the morphology of the pyruvate-treated cells was similar to that observed in intact liver tissue. The albumin secretion rate was significantly higher in rat hepatocytes exposed to pyruvate than in control cells. In the presence of pyruvate, mu and alpha class GST activities were well maintained, whereas GST pi activity was increased over the entire culture period. HPLC analysis revealed that the complement of GST subunits present in hepatocytes is altered during culture with pyruvate: mu,class proteins remained relatively constant, whereas a decrease in the alpha class content was accompanied by a strong increase in GST subunit P1 (GSTP1). The induction of GSTP1 was confirmed at the mRNA level. In control cultures, pi class GST activity was increased, but total, mu, and alpha class GST activities continuously declined as a function of culture time and became undetectable beyond 7 days in culture. At the protein and mRNA levels, a much smaller increase in GSTP1 was observed than in the pyruvate cultures. When the pyruvate-treated cell cultures were exposed to T3, an inhibitory effect on GST activities and proteins was found. These results indicate that this simple culture model could be useful for studying the expression and regulation of GST.

Metabolism of citric acid production by Aspergillus niger: model definition, steady-state analysis and constrained optimization of citric acid production rate

In an attempt to provide a rational basis for the optimization of citric acid production by A. niger, we developed a mathematical model of the metabolism of this filamentous fungus when in conditions of citric acid accumulation. The present model is based in a previous one, but extended with the inclusion of new metabolic processes and updated with currently available kinetic data. Among the different alternatives to represent the system behavior we have chosen the S-system representation within power-law formalism. This type of representation allows us to verify not only the ability of the model to exhibit a stable steady state of the integrated system but also the robustness and quality of the representation. The model analysis is shown to be self-consistent, with a stable steady state, and in good agreement with experimental evidence. Moreover, the model representation is sufficiently robust, as indicated by sensitivity and steady-state and dynamic analyses. From the steady-state results we concluded that the range of accuracy of the S-system representation is wide enough to model realistic deviations from the nominal steady state. The dynamic analysis indicated a reasonable response time, which provided further indication that the model is adequate. The extensive assessment of the reliability and quality of the model put us in a position to address questions of optimization of the system with respect to increased citrate production. We carried out the constrained optimization of A. niger metabolism with the goal of predicting an enzyme activity profile yielding the maximum rate of citrate production, while, at the same time, keeping all enzyme activities within predetermined, physiologically acceptable ranges. The optimization is based on a method described and tested elsewhere that utilizes the fact that the S-system representation of a metabolic system becomes linear at steady state, which allows application of linear programming techniques. Our results show that: (i) while the present profile of enzyme activities in A. niger at idiophase steady state yields high rates of citric acid production, it still leaves room for changes and suggests possible optimization of the activity profile to over five times the basal rate synthesis; (ii) when the total enzyme concentration is allowed to double its basal value, the citric acid production rate can be increased by more than 12-fold, and even larger values can be attained if the total enzyme concentration is allowed to increase even more (up to 50-fold when the total enzyme concentration may rise up to 10-fold the basal value); and (iii) the systematic search of the best combination of subsets of enzymes shows that, under all conditions assayed, a minimum of 13 enzymes need be modified if significant increases in citric acid are to be obtained. This implies that improvements by single enzyme modulation are unlikely, which is in agreement with the findings of some investigators in this and other fields.

Protective effect of pyruvate upon cultured mesothelial cells exposed to 2 mM hydrogen peroxide

Rat peritoneal mesothelial cells in culture have the capability of generating hydrogen peroxide. Exposure of these cells to glucose-enriched, lactated-buffered fluids for peritoneal dialysis significantly increases the production of H(2)O(2). Increased liberation of oxygen radicals also involves the risk of damaging the peritoneal membrane. Pyruvate being a natural oxidant scavenger abundantly present in mammalian cells, we hypothesized that its protective effects facing H(2)O(2) can eventually be of relevance for the mesothelial monolayer of patients on long-term peritoneal dialysis. So far, we designed an experimental study in which rat peritoneal mesothelial cells in culture were exposed to 2 mM H(2)O(2). Cell damage was estimated in terms of decreased capability of the mitochondrial dehydrogenases to reduce MTT. Addition of 2 mM sodium pyruvate to the medium prevented the negative effect of hydrogen peroxide. The MTT/protein values for the control group were 0.00357 +/- 0.00075. The ratio after exposure to 2 mM H(2)O(2) was 0. 00217 +/- 0.00028, whereas that detected in cells incubated in H(2)O(2) plus pyruvate was 0.00325 +/- 0.0082 (p < 0.05). These results indicate that pyruvate protected rat peritoneal mesothelial cells in culture against oxidant injury. These data are one more piece of evidence pointing at pyruvate as a potentially useful buffer for peritoneal dialysis solutions.

Pyruvate: metabolic protector of cardiac performance

Pyruvate, a metabolic product of glycolysis and an oxidizable fuel in myocardium, increases cardiac mechanical performance and energy reserves, especially when supplied at supraphysiological concentrations. The inotropic effects of pyruvate are most impressive in hearts that have been reversibly injured (stunned) by ischemia/reperfusion stress. Glucose appears to be an essential co-substrate for pyruvate's salutary effects in stunned hearts, but other fuels including lactate, acetate, fatty acids, and ketone bodies produce little or no improvement in postischemic function over glucose alone. In contrast to pharmacological inotropism by catecholamines, metabolic inotropism by pyruvate increases cardiac energy reserves and bolsters the endogenous glutathione antioxidant system. Pyruvate enhancement of cardiac function may result from one or more of the following mechanisms: increased cytosolic ATP phosphorylation potential and Gibbs free energy of ATP hydrolysis, enhanced sarcoplasmic reticular calcium ion uptake and release, decreased cytosolic inorganic phosphate concentration, oxyradical scavenging via direct neutralization of peroxides and/or enhancement of the intracellular glutathione/NADPH antioxidant system, and/or closure of mitochondrial permeability transition pores. This review aims to summarize evidence for each of these mechanisms and to consider the potential utility of pyruvate as a therapeutic intervention for clinical management of cardiac insufficiency.

Oxidant mechanisms in toxic acute renal failure

Over the last decade, there is accumulating evidence for a role of reactive oxygen metabolites in the pathogenesis of a variety of renal diseases, including gentamicin, glycerol, cisplatin, and cyclosporine A models of toxic acute renal failure. Gentamicin has been shown both in in vitro and in vivo studies to enhance the generation of reactive oxygen metabolites. Iron is important in models of tissue injury, presumably because it is capable of catalyzing free-radical formation. Gentamicin has been shown to cause release of iron from renal cortical mitochondria. Scavengers of reactive oxygen metabolites as well as iron chelators provide protection in gentamicin-induced nephrotoxicity. In glycerol-induced acute renal failure, an animal model of rhabdomyolysis, there is enhanced generation of hydrogen peroxide, and scavengers of reactive oxygen metabolites and iron chelators provide protection. Although the dogma is that the myoglobin is the source of iron, recent studies suggest that cytochrome P450 may be an important source of iron in this model. In addition, there are marked alterations in antioxidant defenses, such as glutathione, as well as changes in heme oxygenase. Several recent in vitro and in vivo studies indicate an important role of reactive oxygen metabolites in cisplatin-induced nephrotoxicity. Thus, catalytic iron is increased both in vitro and in vivo by cisplatin, and iron chelators as well as hydroxyl radical scavengers have been shown to be protective. Recent studies indicate that cytochrome P450 may also be an important source of the catalytic iron in cisplatin nephrotoxicity. Cyclosporine A has been shown to enhance generation of hydrogen peroxide in vitro and enhance lipid peroxidation in vitro and in vivo. Antioxidants have been shown to be protective in cyclosporine A nephrotoxicity. This collective body of evidence suggests an important role for reactive oxygen metabolites in toxic acute renal failure and may provide therapeutic opportunities of preventing or treating acute renal failure in humans.

Oxidant mechanisms in toxic acute renal failure

Over the last decade, there has been accumulating evidence for a role of reactive oxygen metabolites in the pathogenesis of a variety of renal diseases, including gentamicin, glycerol, and cyclosporine A models of toxic acute renal failure. Gentamicin has been shown in both in vitro and in vivo studies to enhance the generation of reactive oxygen metabolites. Iron is important in models of tissue injury, presumably because it is capable of catalyzing free radical formation. Gentamicin has been shown to cause release of iron from renal cortical mitochondria. Scavengers of reactive oxygen metabolites as well as iron chelators provide protection in gentamicin-induced nephrotoxicity. In glycerol-induced acute renal failure, an animal model of rhabdomyolysis, there is enhanced generation of hydrogen peroxide, and scavengers of reactive oxygen metabolites and iron chelators provide protection. Although the dogma is that the myoglobin is the source of iron, the results of recent studies suggest that cytochrome P-450 may be an important source of iron in this model. In addition, there are marked alterations in antioxidant defenses, such as glutathione, as well as changes in heme oxygenase. Cyclosporine A has been shown to enhance the generation of hydrogen peroxide in vitro and lipid peroxidation in vitro and in vivo. Antioxidants have been shown to be protective in cyclosporine A nephrotoxicity. This collective body of evidence suggests an important role for reactive oxygen metabolites in toxic acute renal failure and may provide therapeutic opportunities of preventing or treating acute renal failure in humans.

High resolution 1H NMR investigations of the reactivities of alpha-keto acid anions with hydrogen peroxide

The chemical reactivity of various alpha-keto acid anions (beta-hydroxypyruvate, beta-phenylpyruvate, 2-ketobutyrate and 2-ketoglutarate) with hydrogen peroxide (H2O2) was investigated at physiological pH (7.4) and a temperature of 25 degrees C. The initial concentration of the alpha-keto acid anions was kept constant at 1.00 mM whilst that of added H2O2 was varied from 0.25 to 1.00 mM, and the rate and extent of these reactions was evaluated using 1H NMR spectroscopy. At all H2O2 concentrations utilised, the order of reactivity of the alpha-keto acid anions was beta-hydroxypyruvate > beta-phenylpyruvate > 2-ketobutyrate > 2-ketoglutarate. The results obtained are in agreement with a proposed mechanism for these reactions, involving nucleophilic attack of the mono-deprotonated peroxide species (HO2-) at the C-2 carbonyl group carbon centre. The antioxidant capacity of such alpha-keto acids is discussed in terms of their potential use as therapeutic agents in clinical conditions where H2O2 has been shown to play a critical role in the disease process, i.e., those involving 'oxidative stress'.

Multicomponent investigations of the hydrogen peroxide- and hydroxyl radical-scavenging antioxidant capacities of biofluids: the roles of endogenous pyruvate and lactate. Relevance to inflammatory joint diseases

High field proton (1H) nuclear magnetic resonance (NMR) analysis of biofluids (health human blood sera and inflammatory knee-joint synovial fluids) has been employed to evaluate the hydrogen peroxide (H2O2)- and hydroxyl radical (0OH)- scavenging antioxidant capacities of a range of polar, low-molecular-mass endogenous metabolites therein. Data obtained indicate that consumption of H2O2 by pyruvate (generating acetate and CO2 via an oxidative decarboxylation reaction) and 0OH radical by lactate (generating pyruvate, and subsequently acetate and CO2) may serve to protect alternative biofluid components (e.g., macromolecules) against reactive oxygen species-mediated oxidative damage in vivo. The mechanistic, physiological and potential therapeutic implications of these results are discussed with special reference to inflammatory joint diseases.

Mitochondrial pyruvate metabolism in liver and kidney during acidosis

Pyruvate transport and carboxylation have been determined in mitochondria from liver and kidney cortex isolated from Wistar rats with acidosis produced by three different treatments: fasting, exercise and ingestion of ammonium chloride. Fasting for 48 h or swimming for 2 h resulted in an increased rate of CO2 fixation by mitochondria from both organs incubated with pyruvate. This increase was accompanied by a rise in the rate of pyruvate transport in all cases except in mitochondria derived from the kidney of the fasted animals. Acute acidosis produced by the ingestion of ammonium chloride resulted in increases in pyruvate transport and carboxylation in kidney mitochondria, but a drop in pyruvate carboxylation was observed in mitochondria from the liver. The results are discussed in terms of the differential regulation of the mitochondria steps for gluconeogenesis from three carbon precursors in liver and kidney, taking into consideration the hormonal status of the animals and the prevailing available substrates in each condition.

Effect of pyruvate on oxidant injury to isolated and cellular DNA

Drawing upon the capacity of pyruvate to detoxify H2O2, we demonstrate that pyruvate (i) protects against H2O2-dependent, hydroxyl radical-mediated degradation of isolated DNA; (ii) reduces the amount of 8-hydroxy-2-deoxyguanosine detected following oxidative injury to isolated DNA and (iii) diminishes the amounts of detectable hydroxyl radical generated by a H2O2-dependent system. Compared to mannitol, pyruvate protects weakly against oxidative degradation of DNA induced by a H2O2-independent, hydroxyl radical-generating system. The protective effects of pyruvate against H2O2-instigated DNA damage were also evinced in cells in culture exposed to H2O2. In contrast to its protective effects against H2O2-dependent injury to DNA, pyruvate failed to offer convincing protection to another intracellular, H2O2-vulnerable target, glyceraldehyde-3-phosphate dehydrogenase. The protection conferred by pyruvate to intracellular H2O2-vulnerable targets is thus influenced by the nature of the target exposed to H2O2. Pyruvate was markedly protective in a model of cytotoxicity induced by the concomitant depletion of cellular glutathione and inhibition of catalase activity; pyruvate can thus function as an intracellular antioxidant and in this latter model, no evidence of DNA damage was observed. Pyruvate, in contrast to catalase, is a potent protector against cytotoxicity induced by organic peroxides, a finding that cannot be explained by the scavenging of organic peroxides, differences in glutathione content or attenuation in oxidative injury to DNA. We conclude that while DNA damage is a key pathogenetic event in oxidative stress induced by H2O2, such nuclear changes may not universally subserve a critical role in models of H2O2-dependent cell death. We also conclude that the antioxidant capabilities of pyruvate extend beyond scavenging of H2O2 to include potent protection against cytotoxicity induced by organic peroxides.

Structure, function and regulation of the tricarboxylate transport protein from rat liver mitochondria

Recent progress is summarized on the structure, function, and regulation of the tricarboxylate (i.e., citrate) transport protein (CTP) from the rat liver mitochondrial inner membrane. The transporter has been purified and its reconstituted function characterized. A cDNA clone encoding the CTP has been isolated and sequenced, thus enabling a deduction of the complete amino acid sequence of this 32.6 kDa transport protein. Dot matrix analysis and sequence alignment indicate that based on structural considerations the CTP can be assigned to the mitochondrial carrier family. Hydropathy analysis of the transporter sequence indicates six putative membrane-spanning alpha-helices and has permitted the development of an initial model for the topography of the CTP within the inner membrane. The questions as to whether more than one gene encodes the CTP and whether more than one isoform is expressed remain unanswered at this time. Studies documenting a diabetes-induced alteration in the function of several mitochondrial anion transporters, which can be reversed by treatment with insulin, provide a physiologically/pathologically relevant experimental system for studying the molecular mechanism(s) by which mitochondrial transporters are regulated. Potential future research directions are discussed.

Mitochondrial pyruvate transport in working guinea-pig heart. Work-related vs. carrier-mediated control of pyruvate oxidation

Myocardial pyruvate oxidation is work- or calcium-load-related, but control of pyruvate dehydrogenase (PDH) by the specific mitochondrial pyruvate transporter has also been proposed. To test the transport hypothesis distribution of pyruvate across the cell membrane as well as rates of mitochondrial pyruvate net transport plus oxidation were examined in isolated perfused but stable and physiologically working guinea-pig hearts. 150 microM-1.2 mM alpha-cyanohydroxycinnamate proved to specifically block mitochondrial pyruvate uptake in these hearts. When perfusate glucose as cytosolic pyruvate precursor was supplied in combination with octanoate (0.2 or 0.5 mM) as diffusible alternative fatty acid substrate, alpha-cyanohydroxycinnamate produced up to 20- and 3-fold increases in pyruvate and lactate efflux, respectively. Cinnamates did not alter myocardial hemodynamics nor sarcolemmal pyruvate and lactate export. In contrast the tested concentrations of cinnamate produced reversible, dose-dependent decreases in 14CO2 production from [1-14C]pyruvate or [U-14C]glucose by inhibiting mitochondrial pyruvate uptake. Linear least-squares estimates of available cinnamate-sensitive total pyruvate transport potential yielded rates close to 110 mumol/min per g dry mass at S0.5 approximately 120 microM, which compared reasonably well with literature values from isolated cardiac mitochondria. This transport potential was severalfold larger than total extractable myocardial PDH activity of approximately 32 mumol/min per g dry mass at 37 degrees C. Even when cytosolic pyruvate levels were in the lower physiologic range of about 90 microM, pyruvate oxidation readily kept pace with mitochondrial respiration over a wide range of workload and inotropism. Furthermore, dichloroacetate, a selective activator of PDH, stimulated pyruvate oxidation without affecting myocardial O2 consumption, regardless of the metabolic or inotropic state of the hearts. Consequently, little or no regulatory function with regard to pyruvate oxidation could be assigned to the native mitochondrial pyruvate carrier of the working heart. Therefore, mitochondrial pyruvate-H+ symport was the normal, highly efficient (rather than controlling) mechanism for pyruvate entry into the mitochondria where PDH regulation controlled pyruvate oxidation.

The activity of pyruvate carrier in a reconstituted system: substrate specificity and inhibitor sensitivity

The pyruvate carrier, of molecular mass 34 kDa, was purified from mitochondria isolated from rat liver, rat brain, and bovine heart, by affinity chromatography on immobilized 2-cyano-4-hydroxycinnamate. Its activity after reconstitution in phosphatidylcholine vesicles was measured either as uptake of [1-14C]pyruvate or as exchange with different 2-oxoacids. All preparations exhibited similar apparent Km values for pyruvate, but somewhat different V(max) values. The ability to exchange different anions of physiological significance, including branched-chain 2-oxoacids, confirmed the known substrate specificity described for the pyruvate carrier in mitochondria. The sensitivity of pyruvate transport toward phenylglyoxal suggested an important role of arginyl residues in the transport activity, while a role of lysyl and histidyl residues was not confirmed.

Pyruvate/malate antiporter in rat liver mitochondria

To gain some insight into the process by which both acetylCoA and NADPH, needed for fatty acid synthesis, are obtained, in the cytosol, from the effluxed intramitochondrial citrate, via citrate lyase and malate dehydrogenase plus malic enzyme respectively, the capability of externally added pyruvate to cause efflux of malate from rat liver mitochondria was tested. The occurrence of a pyruvate/malate translocator is here shown: pyruvate/malate exchange shows saturation features (Km and Vmax values, measured at 20 degrees C and at pH 7.20, were found to be about 0.25 mM and 2.7 nmoles/min x mg mitochondrial protein, respectively) and is inhibited by certain impermeable compounds. This carrier, together with the previously reported tricarboxylate and oxodicarboxylate translocators proved to allow for citrate and oxaloacetate efflux due to externally added pyruvate.

Hydrogen peroxide-induced renal injury. A protective role for pyruvate in vitro and in vivo

Hydrogen peroxide (H2O2) contributes to renal cellular injury. alpha-Keto acids nonenzymatically reduce H2O2 to water while undergoing decarboxylation at the 1-carbon (1-C) position. We examined, in vitro and in vivo, the protective role of sodium pyruvate in H2O2-induced renal injury. Pyruvate effectively scavenged H2O2 in vitro, and suppressed H2O2-induced renal lipid peroxidation. Injury to LLC-PK1 cells induced by hydrogen peroxide was attenuated by pyruvate to an extent comparable to that seen with catalase. Studies utilizing [1-14C]pyruvate further demonstrated 1-C decarboxylation concurrent with cytoprotection by pyruvate from H2O2-induced injury. Pyruvate was also protective in vivo. Infusion of pyruvate before and during the intrarenal infusion of H2O2 attenuated H2O2-induced proteinuria. Systemic administration of pyruvate was also protective in the glycerol model of acute renal failure, a model also characterized by increased generation of H2O2. These findings indicate that pyruvate, a ubiquitous alpha-keto acid, scavenges H2O2 and protects renal tissue in vitro and in vivo from H2O2-mediated injury. These data suggest a potential therapeutic role for pyruvate in diseases in which increased generation of H2O2 is incriminated in renal damage.

The effect of insulin supplementation on diabetes-induced alterations in the extractable levels of functional mitochondrial anion transport proteins

The effect of insulin supplementation on diabetes-induced alterations in the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach consisted of the extraction of the pyruvate, dicarboxylate, and citrate transport proteins from the mitochondrial inner membrane with Triton X-114 using rat liver mitoplasts (prepared from control, diabetic, or insulin-supplemented diabetic animals) as the starting material, followed by the reconstitution of the function of each transporter in a proteoliposomal system. This experimental strategy permitted the quantification of the functional levels of these three transporters in the absence of the complications that arise when such measurements are carried out with intact mitochondria (or mitoplasts). We found that treatment of diabetic rats (i.e., animals that were injected with streptozotocin 3 weeks earlier) on a daily basis with insulin for 3 weeks resulted in a reversal of the diabetes-induced (a) increase in the extractable and reconstitutable total (and specific) transport activities of the pyruvate and dicarboxylate transporters and (b) decrease in the activity of the citrate transporter. These findings indicate that diabetes-induced alterations in the functional levels of mitochondrial anion transport proteins are a direct consequence of the insulin insufficiency that characterizes this disease. Furthermore, this study provides the first demonstration that insulin participates in the regulation of the functional levels of liver mitochondrial anion transport proteins.

Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

The effect of streptozotocin-induced diabetes on the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach utilized for these studies consisted of the extraction of each of four mitochondrial anion transport proteins from rat liver mitoplasts (isolated from diabetic and control animals) with the nonionic detergent Triton X-114, followed by the functional reconstitution of each transporter in a liposomal system via the freeze-thaw-sonication technique. This approach permitted the quantification of transporter function without the complications that occur when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental diabetes caused an increase in the extractable and reconstitutable specific (and total) transport activities of the pyruvate and dicarboxylate transporters, a decrease in the activity of the citrate transporter, and no significant change in the activity of the phosphate transporter relative to control values. An examination of the time course of the appearance of changes in the reconstitutable activities of the pyruvate and citrate transporters following the injection of streptozotocin revealed differences. Thus, whereas the activity of the pyruvate transporter displayed the most pronounced increase (193%) 1 week following streptozotocin injection and then subsequently declined from this peak and plateaued at later times (99% and 96% increases at 3 and 8 weeks, respectively), the activity of the citrate transporter progressively decreased with time (31-51% decreases at 1-8 weeks). We suggest that the observed diabetes-induced changes in mitochondrial anion transporter function are predictable on the basis of diabetes-induced alterations in the activities of enzymes that constitute metabolic pathways to which these transporters either supply substrate or remove product. Furthermore, we speculate that mitochondrial anion transport proteins may be regulated in coordination with the enzymes of such associated metabolic pathways.

Age-related changes in the activity of the pyruvate carrier and in the lipid composition in rat-heart mitochondria

The effect of aging on the activity of the pyruvate translocator and on the lipid composition in rat-heart mitochondria has been investigated. It has been found that the rate of pyruvate transport in mitochondria from aged rats (28 months old) is markedly reduced (38%) as compared with that obtained with mitochondria from young adults rats (4 months old). Kinetic analysis of the pyruvate transport shows that only the Vmax of this process is decreased, while there is no change in the Km values. The age-related decrement in the activity of the pyruvate carrier is not due to a decrease in the transmembrane delta pH value, neither does it depend on a decrease in the total number of the pyruvate carrier molecules, titrated with radioactive alpha-cyanocinnamate. The lower activity of the pyruvate translocator in mitochondria from aged rats is associated to a parallel decrement of the rate of pyruvate-dependent oxygen uptake. There is, however no appreciable difference in either the respiratory control ratios or in the ADP/O ratios between these two types of mitochondrion. The Arrhenius plot characteristics differ for pyruvate transport activity in mitochondria from aged rats as compared with young rats in that the break point of the biphasic plot is shifted to a higher temperature. The heart mitochondrial lipid composition is significantly altered in aged rats. The total cholesterol increases (43%), the phospholipids decrease (15%) and the cholesterol/phospholipid molar ratio increases (68%). Among phospholipids, cardiolipin shows the greatest alteration (28% decrease in aged rats). The lower activity of the pyruvate carrier in mitochondria from aged rats may be ascribed to changes in the lipid domain surrounding the carrier molecule in the membrane.

Stimulation of mitochondrial pyruvate transport in rat renal cortex by phenylephrine

Phenylephrine effect on liver and kidney cortex mitochondrial pyruvate concentration was investigated. While in liver the alpha 1-adrenergic agent produced a decrease in pyruvate content, a significant increase was observed in kidney, even in the presence of 0.5 mM alpha-cyano-4-hydroxy-cinnamate. These changes were not observed when pyruvate was formed by intramitochondrial transamination of alanine, suggesting a role for the pyruvate transport across mitochondrial membranes in the regulation of mitochondrial pyruvate metabolism in kidney cortex. This was corroborated measuring the phenylephrine effect on pyruvate carboxylation.

The kinetics of transport of lactate and pyruvate into isolated cardiac myocytes from guinea pig. Kinetic evidence for the presence of a carrier distinct from that in erythrocytes and hepatocytes

1. Time courses for the uptake of L-lactate, D-lactate and pyruvate into isolated cardiac ventricular myocytes from guinea pig were determined at 11 degrees C or 0 degrees C (for pyruvate) in a citrate-based buffer by using a silicone-oil-filtration technique. These conditions enabled initial rates of transport to be measured without interference from metabolism of the substrates. 2. At a concentration of 0.5 mM, transport of all these substrates was inhibited by approx. 90% by 5 mM-alpha-cyano-4-hydroxycinnamate; at 10 mM-L-lactate a considerable portion of transport could not be inhibited. 3. Initial rates of L-lactate and pyruvate uptake in the presence of 5 mM-alpha-cyano-4-hydroxycinnamate were linearly related to the concentration of the monocarboxylate and probably represented diffusion of the free acid. The inhibitor-sensitive component of uptake obeyed Michaelis-Menten kinetics, with Km values for L-lactate and pyruvate of 2.3 and 0.066 mM respectively. 4. Pyruvate and D-lactate inhibited the transport of L-lactate, with Ki values (competitive) of 0.077 and 6.6 mM respectively; the Ki for pyruvate was very similar to its Km for transport. The Ki for alpha-cyano-4-hydroxycinnamate as a non-competitive inhibitor was 0.042 mM. 5. These results indicate that L-lactate, D-lactate and pyruvate share a common carrier in guinea-pig cardiac myocytes; the low stereoselectivity for L-lactate over D-lactate and the high affinity for pyruvate distinguish it from the carrier in erythrocytes and hepatocytes. The metabolic roles for this novel carrier in heart are discussed.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.