Liver mitochondrial coupling efficiency and its relationship to oxidative capacity and adenine nucleotide translocase content: A comparative study among crocodiles, birds and mammals.

The primary objective of this study was to assess whether adenine nucleotide translocase (ANT) content could be associated with phylogenetic disparities in mitochondrial coupling efficiency, within liver mitochondria obtained from rats, crocodiles, and ducklings. Our measurements included mitochondrial membrane conductance, ANT content, and oxidative phosphorylation fluxes at various steady-state rates. We observed significant variations in liver mitochondrial coupling efficiency across the three species. These variations correlated with interspecific differences in mitochondrial oxidative capacity and, to a lesser extent, the ANT content of liver mitochondria. These findings expand upon previous research by highlighting the pivotal role of oxidative capacity and ANT in modulating mitochondrial efficiency on an interspecific scale.

Antiobesity effects of intestinal gluconeogenesis are mediated by the brown adipose tissue sympathetic nervous system.

OBJECTIVE: Intestinal gluconeogenesis (IGN), via the initiation of a gut-brain nervous circuit, accounts for the metabolic benefits linked to dietary proteins or fermentable fiber in rodents and has been positively correlated with the rapid amelioration of body weight after gastric bypass surgery in humans with obesity. In particular, the activation of IGN moderates the development of hepatic steatosis accompanying obesity. In this study, we investigated the specific effects of IGN on adipose tissue metabolism, independent of its induction by nutritional manipulation. METHODS: We used two transgenic mouse models of suppression or overexpression of G6pc1, the catalytic subunit of glucose-6 phosphatase, which is the key enzyme of endogenous glucose production specifically in the intestine. RESULTS: Under a hypercaloric diet, mice overexpressing IGN showed lower adiposity and higher thermogenic capacities than wild-type mice, featuring marked browning of white adipose tissue (WAT) and prevention of the whitening of brown adipose tissue (BAT). Sympathetic denervation restricted to BAT caused the loss of the antiobesity effects associated with IGN. Conversely, IGN-deficient mice exhibited an increase in adiposity under a standard diet, which was associated with decreased expression of markers of thermogenesis in both BAT and WAT. CONCLUSIONS: IGN is sufficient to activate the sympathetic nervous system and prevent the expansion and the metabolic alterations of BAT and WAT metabolism under a high-calorie diet, thereby preventing the development of obesity. These data increase knowledge of the mechanisms of weight reduction in gastric bypass surgery and pave the way for new approaches to prevent or cure obesity.

The allometry of mitochondrial efficiency is tissue dependent: a comparison between skeletal and cardiac muscles of birds.

Body mass is known to be a fundamental driver of many biological traits, including metabolism. However, the effect of body mass on mitochondrial energy transduction is still poorly understood and has mainly been described in mammals. Using 13 species of birds ranging from 15 g (finches) to 160 kg (ostrich), we report here that the mitochondrial production of ATP, and the corresponding oxygen consumption, are negatively dependent on body mass in skeletal muscles but not in the heart. Results also showed that mitochondrial efficiency was positively correlated with body mass at sub-maximal phosphorylating states in the skeletal muscle, but not in the heart. This difference between muscle tissues is potentially linked to the difference in energetic demand expandability and the heavy involvement of skeletal muscle in thermoregulation.

Effect of physiological hyperthermia on mitochondrial fuel selection in skeletal muscle of birds and mammals.

Both birds and mammals have important thermogenic capacities allowing them to maintain high body temperatures, i.e., 37 °C and 40 °C on average in mammals and birds, respectively. However, during periods of high locomotor activity, the energy released during muscular contraction can lead to muscle temperature reaching up to 43-44 °C. Mitochondria are responsible for producing the majority of ATP through cellular respiration and metabolizing different substrates, including carbohydrates and lipids, to generate ATP. A limited number of studies comparing avian and mammalian species showed preferential utilization of specific substrates for mitochondrial energy at different metabolic intensities, but authors always measured at body temperature. The present study evaluated mitochondrial respiration rates and OXPHOS coupling efficiencies at 37 °C, 40 °C and 43 °C associated with pyruvate/malate (carbohydrate metabolism) or palmitoyl-carnitine/malate (lipid metabolism) as substrates in pigeons (Columba livia) and rats (Rattus norvegicus), a well-known pair in scientific literature and for their similar body mass. The data show different hyperthermia-induced responses between the two species with (i) skeletal muscle mitochondria from rats being more sensitive to rising temperatures than in pigeons, and (ii) the two species having different substrate preferences during hyperthermia, with rats oxidizing preferentially carbohydrates and pigeons lipids. By analyzing the interplay between temperature and substrate utilization, we describe a means by which endotherms deal with extreme muscular temperatures to provide enough ATP to support energy demands.

The aerotaxis of Dictyostelium discoideum is independent of mitochondria, nitric oxide and oxidative stress.

Spatial and temporal variations of oxygen environments affect the behaviors of various cells and are involved in physiological and pathological events. Our previous studies with Dictyostelium discoideum as a model of cell motility have demonstrated that aerotaxis toward an oxygen-rich region occurs below 2% O2. However, while the aerotaxis of Dictyostelium seems to be an effective strategy to search for what is essential for survival, the mechanism underlying this phenomenon is still largely unclear. One hypothesis is that an oxygen concentration gradient generates a secondary oxidative stress gradient that would direct cell migration towards higher oxygen concentration. Such mechanism was inferred but not fully demonstrated to explain the aerotaxis of human tumor cells. Here, we investigated the role on aerotaxis of flavohemoglobins, proteins that can both act as potential oxygen sensors and modulators of nitric oxide and oxidative stress. The migratory behaviors of Dictyostelium cells were observed under both self-generated and imposed oxygen gradients. Furthermore, their changes by chemicals generating or preventing oxidative stress were tested. The trajectories of the cells were then analyzed through time-lapse phase-contrast microscopic images. The results indicate that both oxidative and nitrosative stresses are not involved in the aerotaxis of Dictyostelium but cause cytotoxic effects that are enhanced upon hypoxia.

Body mass dependence of oxidative phosphorylation efficiency in liver mitochondria from mammals.

In eukaryotes, the performances of an organism are dependent on body mass and chemically supported by the mitochondrial production of ATP. Although the relationship between body mass and mitochondrial oxygen consumption is well described, the allometry of the transduction efficiency from oxygen to ATP production (ATP/O) is still poorly understood. Using a comparative approach, we investigated the oxygen consumption and ATP production of liver mitochondria from twelve species of mammals ranging from 5 g to 600 kg. We found that both oxygen consumption and ATP production are mass dependent but not the ATP/O at the maximal phosphorylating state. The results also showed that for sub-maximal phosphorylating states the ATP/O value positively correlated with body mass, irrespective of the metabolic intensity. This result contrasts with previous data obtained in mammalian muscles, suggesting a tissue-dependence of the body mass effect on mitochondrial efficiency.

Succinate oxidation rescues mitochondrial ATP synthesis at high temperature in Drosophila melanogaster.

Decreased NADH-induced and increased reduced FADH2 -induced respiration rates at high temperatures are associated with thermal tolerance in Drosophila. Here, we determined whether this change was associated with adjustments of adenosine triphosphate (ATP) production rate and coupling efficiency (ATP/O) in Drosophila melanogaster. We show that decreased pyruvate + malate oxidation at 35°C is associated with a collapse of ATP synthesis and a drop in ATP/O ratio. However, adding succinate triggered a full compensation of both oxygen consumption and ATP synthesis rates at this high temperature. Addition of glycerol-3-phosphate (G3P) led to a huge increase in respiration with no further advantage in terms of ATP production. We conclude that succinate is the only alternative substrate able to compensate both oxygen consumption and ATP production rates during oxidative phosphorylation at high temperature, which has important implications for thermal adaptation.

Increased retention of functional mitochondria in mature sickle red blood cells is associated with increased sickling tendency, hemolysis and oxidative stress.

Abnormal retention of mitochondria in mature red blood cells (RBC) has been recently reported in sickle cell anemia (SCA) but their functionality and their role in the pathophysiology of SCA remain unknown. The presence of mitochondria within RBC was determined by flow cytometry in 61 SCA patients and ten healthy donors. Patients were classified according to the percentage of mature RBC with mitochondria contained in the whole RBC population: low (0-4%), moderate (>4% and <8%), or high level (>8%). RBC rheological, hematological, senescence and oxidative stress markers were compared between the three groups. RBC senescence and oxidative stress markers were also compared between mature RBC containing mitochondria and those without. The functionality of residual mitochondria in sickle RBC was measured by high-resolution respirometry assay and showed detectable mitochondrial oxygen consumption in sickle mature RBC but not in healthy RBC. Increased levels of mitochondrial reactive oxygen species were observed in mature sickle RBC when incubated with Antimycin A versus without. In addition, mature RBC retaining mitochondria exhibited greater levels of reactive oxygen species compared to RBC without mitochondria, as well as greater Ca2+, lower CD47 and greater phosphatidylserine exposure. Hematocrit and RBC deformability were lower, and the propensity of RBC to sickle under deoxygenation was higher, in the SCA group with a high percentage of mitochondria retention in mature RBC. This study showed the presence of functional mitochondria in mature sickle RBC, which could favor RBC sickling and accelerate RBC senescence, leading to increased cellular fragility and hemolysis.

Temperature-dependent metabolic consequences of food deprivation in the European sardine.

Aquatic ecosystems can exhibit seasonal variation in resource availability and animals have evolved to cope with the associated caloric restriction. During winter in the NW Mediterranean Sea, the European sardine Sardina pilchardus naturally experiences caloric restriction owing to a decrease in the diversity and quantity of plankton. However, ongoing global warming has had deleterious effects on plankton communities such that food shortages may occur throughout the year, especially under warm conditions in the summer. We investigated the interactive effects of temperature and food availability on sardine metabolism by continuously monitoring whole-animal respiration of groups of control (fed) and food-deprived sardines over a 60-day experiment in winter (12°C) or summer (20°C) conditions under natural photoperiod. In addition, we measured mitochondrial respiration of red muscle fibres, biometric variables and energy reserves of individuals sampled at 30 and 60 days. This revealed that winter food deprivation elicits energy saving mechanisms at whole animal and cellular levels by maintaining a low metabolism to preserve energy reserves, allowing high levels of survival. By contrast, despite energy saving mechanisms at the mitochondrial level, whole animal metabolic rate was high during food deprivation in summer, causing increased consumption of energy reserves at the muscular level and high mortality after 60 days. Furthermore, a 5-day re-feeding did not improve survival, and mortalities continued, suggesting that long-term food deprivation at high temperatures causes profound stress in sardines that potentially impairs nutrient absorption.

Absence of mitochondrial responses in muscles of zebrafish exposed to several heat waves.

Heat waves are extreme thermal events whose frequency and intensity will increase with global warming. As metabolic responses to temperature are time-dependent, we explored the effects of an exposure to several heat waves on the mitochondrial metabolism of zebrafish Danio rerio. For this purpose, zebrafish were acclimated at 26 °C or 31 °C for 4 weeks and some fish acclimated at 26 °C underwent 2 types of heat waves: 2 periods of 5 days at 31 °C or 10 days at 31 °C. After this acclimation period, mitochondrial respiration of red muscle fibres was measured at 26 °C and 31 °C for each fish, with the phosphorylation (OXPHOS) and basal (LEAK) respirations obtained with activation of complex I, complex II or complexes I and II. The respiratory control ratio (RCR) and the mitochondrial aerobic scope (CAS) were also calculated at both temperatures after the activation of complexes I and II. Under our conditions, heat waves did not result in variations in any mitochondrial parameters, suggesting a high tolerance of zebrafish to environmental temperature fluctuations. However, an acute in vitro warming led to an increase in the LEAK respiration together with a higher temperature effect on complex II than complex I, inducing a decrease of mitochondrial efficiency to produce energy at high temperatures. Increased interindividual variability for some parameters at 26 °C or 31 °C also suggests that each individual has its own ability to cope with temperature fluctuations.

Mitochondrial oxidative phosphorylation response overrides glucocorticoid-induced stress in a reptile.

Stress hormones and their impacts on whole organism metabolic rates are usually considered as appropriate proxies for animal energy budget that is the foundation of numerous concepts and models aiming at predicting individual and population responses to environmental stress. However, the dynamics of energy re-allocation under stress make the link between metabolism and corticosterone complex and still unclear. Using ectopic application of corticosterone for 3, 11 and 21 days, we estimated a time effect of stress in a lizard (Zootoca vivipara). We then investigated whole organism metabolism, muscle cellular O2 consumption and liver mitochondrial oxidative phosphorylation processes (O2 consumption and ATP production) and ROS production. The data showed that while skeletal muscle is not impacted, stress regulates the liver mitochondrial functionality in a time-dependent manner with opposing pictures between the different time expositions to corticosterone. While 3 days exposition is characterized by lower ATP synthesis rate and high H2O2 release with no change in the rate of oxygen consumption, the 11 days exposition reduced all three fluxes of about 50%. Oxidative phosphorylation capacities in liver mitochondria of lizard treated with corticosterone for 21 days was similar to the hepatic mitochondrial capacities in lizards that received no corticosterone treatment but with 40% decrease in H2O2 production. This new mitochondrial functioning allows a better capacity to respond to the energetic demands imposed by the environment but do not influence whole organism metabolism. In conclusion, global mitochondrial functioning has to be considered to better understand the proximal causes of the energy budget under stressful periods.

Mice selected for a high basal metabolic rate evolved larger guts but not more efficient mitochondria.

Intra-specific variation in both the basal metabolic rate (BMR) and mitochondrial efficiency (the amount of ATP produced per unit of oxygen consumed) has profound evolutionary and ecological consequences. However, the functional mechanisms responsible for this variation are not fully understood. Mitochondrial efficiency is negatively correlated with BMR at the interspecific level but it is positively correlated with performance capacity at the intra-specific level. This discrepancy is surprising, as theories explaining the evolution of endothermy assume a positive correlation between BMR and performance capacity. Here, we quantified mitochondrial oxidative phosphorylation activity and efficiency in two lines of laboratory mice divergently selected for either high (H-BMR) or low (L-BMR) levels of BMR. H-BMR mice had larger livers and kidneys (organs that are important predictors of BMR). H-BMR mice also showed higher oxidative phosphorylation activity in liver mitochondria but this difference can be hypothesized to be a direct effect of selection only if the heritability of this trait is low. However, mitochondrial efficiency in all studied organs did not differ between the two lines. We conclude that the rapid evolution of BMR can reflect changes in organ size rather than mitochondrial properties, and does not need to be accompanied obligatorily by changes in mitochondrial efficiency.

Does high mitochondrial efficiency carry an oxidative cost? The case of the African pygmy mouse (Mus mattheyi).

Skeletal muscle mitochondria of the African pygmy mouse Mus mattheyi exhibit markedly reduced oxygen consumption and ATP synthesis rates but a higher mitochondrial efficiency than what would be expected from allometric trends. In the present study, we assessed whether such reduction of mitochondrial activity in M. mattheyi can limit the oxidative stress associated with an increased generation of mitochondrial reactive oxygen species. We conducted a comparative study of mitochondrial oxygen consumption, H2O2 release, and electron leak (%H2O2/O) in skeletal muscle mitochondria isolated from the extremely small African pygmy mouse (M. mattheyi, ~5 g) and Mus musculus, which is a larger Mus species (~25 g). Mitochondria were energized with pyruvate, malate, and succinate, after which fluxes were measured at different steady-state rates of oxidative phosphorylation. Overall, M. mattheyi exhibited lower oxidative activity and higher electron leak than M. musculus, while the H2O2 release did not differ significantly between these two Mus species. We further found that the high coupling efficiency of skeletal muscle mitochondria from M. mattheyi was associated with high electron leak. Nevertheless, data also show that, despite the higher electron leak, the lower mitochondrial respiratory capacity of M. mattheyi limits the cost of a net increase in H2O2 release, which is lower than that expected for a mammals of this size.

Different patterns of chronic hypoxia lead to hierarchical adaptive mechanisms in goldfish metabolism.

Some hypoxia-tolerant species, such as goldfish, experience intermittent and severe hypoxia in their natural habitat, causing them to develop multiple physiological adaptations. However, in fish, the metabolic impact of regular hypoxic exposure on swimming performance in normoxia is less well understood. Therefore, we experimentally tested whether chronic exposure to constant (30 days at 10% air saturation) or intermittent hypoxia (3 h in normoxia and 21 h in hypoxia, 5 days a week) would result in similar metabolic and swimming performance benefits after reoxygenation. Moreover, half of the normoxic and intermittent hypoxic fish were put on a 20-day normoxic training regime. After these treatments, metabolic rate (standard and maximum metabolic rates: SMR and MMR) and swimming performance [critical swimming speed (Ucrit) and cost of transport (COT)] were assessed. In addition, enzyme activities [citrate synthase (CS), cytochrome c oxidase (COX) and lactate dehydrogenase (LDH)] and mitochondrial respiration were examined in red muscle fibres. We found that acclimation to constant hypoxia resulted in (1) metabolic suppression (-45% SMR and -27% MMR), (2) increased anaerobic capacity (+117% LDH), (3) improved swimming performance (+80% Ucrit, -71% COT) and (4) no changes at the mitochondrial level. Conversely, the enhancement of swimming performance was reduced following acclimation to intermittent hypoxia (+45% Ucrit, -41% COT), with a 55% decrease in aerobic scope, despite a significant increase in oxidative metabolism (+201% COX, +49% CS). This study demonstrates that constant hypoxia leads to the greatest benefit in swimming performance and that mitochondrial metabolic adjustments only provide minor help in coping with hypoxia.

Changes in foraging mode caused by a decline in prey size have major bioenergetic consequences for a small pelagic fish.

Global warming is causing profound modifications of aquatic ecosystems and one major outcome appears to be a decline in adult size of many fish species. Over the last decade, sardine populations in the Gulf of Lions (NW Mediterranean Sea) have shown severe declines in body size and condition as well as disappearance of the oldest individuals, which could not be related to overfishing, predation pressure or epizootic diseases. In this study, we investigated whether this situation reflects a bottom-up phenomenon caused by reduced size and availability of prey that could lead to energetic constraints. We fed captive sardines with food items of two different sizes eliciting a change in feeding mode (filter-feeding on small items and directly capturing larger ones) at two different rations for several months, and then assessed their muscle bioenergetics to test for changes in cellular function. Feeding on smaller items was associated with a decline in body condition, even at high ration, and almost completely inhibited growth by comparison to sardines fed large items at high ration. Sardines fed on small items presented specific mitochondrial adjustments for energy sparing, indicating a major bioenergetic challenge. Moreover, mitochondria from sardines in poor condition had low basal oxidative activity but high efficiency of ATP production. Notably, when body condition was below a threshold value of 1.07, close to the mean observed in the wild, it was directly correlated with basal mitochondrial activity in muscle. The results show a link between whole-animal condition and cellular bioenergetics in the sardine, and reveal physiological consequences of a shift in feeding mode. They demonstrate that filter-feeding on small prey leads to poor growth, even under abundant food and an increase in the efficiency of ATP production. These findings may partially explain the declines in sardine size and condition observed in the wild.

Le changement global entraîne de profondes modifications des écosystèmes aquatiques, l'une des principales étant le déclin de la taille des adultes chez de nombreuses espèces de poissons. Au cours de la dernière décennie, les populations de sardines du Golfe du Lion (Nord-Ouest de la Méditerranée) ont montré une importante diminution de leur taille et de leur condition corporelle ainsi qu'une disparition des individus les plus âgés, qui n'ont pas pu être liées à la surpêche, à la pression de prédation ou aux épizooties. Dans cette étude, nous avons cherché à savoir si cette situation reflète un phénomène ascendant causé par la réduction de la taille et de la disponibilité des proies qui pourrait entraîner des contraintes énergétiques chez la sardine. Nous avons ainsi nourri des sardines captives avec des granulés de deux tailles différentes provoquant un changement de mode d'alimentation (filtration des petits granulés et capture directe des plus gros) et à deux rations différentes pendant plusieurs mois, puis nous avons évalué leur bioénergétique musculaire pour tester les changements au niveau de leur fonction cellulaire. L'alimentation à base de petits granulés a été associée à un déclin de la condition corporelle, même à une ration élevée, et à une croissance quasiment inhibée par rapport aux sardines nourries avec des plus gros granulés à une ration élevée. Les sardines nourries avec des petits granulés ont également présenté des ajustements mitochondriaux spécifiques pour économiser de l'énergie, indiquant un défi bioénergétique majeur. De plus, les mitochondries des sardines en mauvaise condition présentaient une faible activité oxydative basale, mais une efficacité élevée de production d'ATP. Notamment, lorsque la condition corporelle était inférieure à une valeur seuil de 1,07, proche de la moyenne observée dans la nature, elle était directement corrélée à l'activité mitochondriale basale dans le muscle. Ces résultats montrent un lien entre la condition de l'animal entier et la bioénergétique cellulaire chez la sardine, et révèlent les conséquences physiologiques d'un changement de mode d'alimentation. Ils démontrent que le nourrissage via la filtration de petites proies entraîne une faible croissance, même en cas de nourriture abondante, et une augmentation de l'efficacité de la production d'ATP. Ces résultats peuvent expliquer en partie le déclin de la taille et de la condition des sardines observé dans la nature.

Low oxygen levels can help to prevent the detrimental effect of acute warming on mitochondrial efficiency in fish.

Aerobic metabolism of aquatic ectotherms is highly sensitive to fluctuating climates. Many mitochondrial traits exhibit phenotypic plasticity in response to acute variations in temperature and oxygen availability. These responses are critical for understanding the effects of environmental variations on aquatic ectotherms' performance. Using the European seabass, Dicentrarchus labrax, we determined the effects of acute warming and deoxygenation in vitro on mitochondrial respiratory capacities and mitochondrial efficiency to produce ATP (ATP/O ratio). We show that acute warming reduced ATP/O ratio but deoxygenation marginally raised ATP/O ratio, leading to a compensatory effect of low oxygen availability on mitochondrial ATP/O ratio at high temperature. The acute effect of warming and deoxygenation on mitochondrial efficiency might be related to the leak of protons across the mitochondrial inner membrane, as the mitochondrial respiration required to counteract the proton leak increased with warming and decreased with deoxygenation. Our study underlines the importance of integrating the combined effects of temperature and oxygen availability on mitochondrial metabolism. Predictions on decline in performance of aquatic ectotherms owing to climate change may not be accurate, since these predictions typically look at respiratory capacity and ignore efficiency of ATP production.

Skeletal muscle metabolism in sea-acclimatized king penguins. II. Improved efficiency of mitochondrial bioenergetics.

At fledging, juvenile king penguins (Aptenodytes patagonicus) must overcome the tremendous energetic constraints imposed by their marine habitat, including during sustained extensive swimming activity and deep dives in cold seawater. Both endurance swimming and skeletal muscle thermogenesis require high mitochondrial respiratory capacity while the submerged part of dive cycles repeatedly and greatly reduces oxygen availability, imposing a need for solutions to conserve oxygen. The aim of the present study was to determine in vitro whether skeletal muscle mitochondria become more 'thermogenic' to sustain heat production or more 'economical' to conserve oxygen in sea-acclimatized immature penguins (hereafter 'immatures') compared with terrestrial juveniles. Rates of mitochondrial oxidative phosphorylation were measured in permeabilized fibers and mitochondria from the pectoralis muscle. Mitochondrial ATP synthesis and coupling efficiency were measured in isolated muscle mitochondria. The mitochondrial activities of respiratory chain complexes and citrate synthase were also assessed. The results showed that respiration, ATP synthesis and respiratory chain complex activities in pectoralis muscles were increased by sea acclimatization. Furthermore, muscle mitochondria were on average 30-45% more energy efficient in sea-acclimatized immatures than in pre-fledging juveniles, depending on the respiratory substrate used (pyruvate, palmitoylcarnitine). Hence sea acclimatization favors the development of economical management of oxygen, decreasing the oxygen needed to produce a given amount of ATP. This mitochondrial phenotype may improve dive performance during the early marine life of king penguins, by extending their aerobic dive limit.

Skeletal muscle metabolism in sea-acclimatized king penguins. I. Thermogenic mechanisms.

At fledging, king penguin juveniles undergo a major energetic challenge to overcome the intense and prolonged energy demands for thermoregulation and locomotion imposed by life in cold seas. Among other responses, sea acclimatization triggers fuel selection in skeletal muscle metabolism towards lipid oxidation in vitro, which is reflected by a drastic increase in lipid-induced thermogenesis in vivo However, the exact nature of skeletal muscle thermogenic mechanisms (shivering and/or non-shivering thermogenesis) remains undefined. The aim of the present study was to determine in vivo whether the capacity for non-shivering thermogenesis was enhanced by sea acclimatization. We measured body temperature, metabolic rate, heart rate and shivering activity in fully immersed king penguins (Aptenodytes patagonicus) exposed to water temperatures ranging from 12 to 29°C. Results from terrestrial pre-fledging juveniles were compared with those from sea-acclimatized immature penguins (hereafter 'immatures'). The capacity for thermogenesis in water was as effective in juveniles as in immatures, while the capacity for non-shivering thermogenesis was not reinforced by sea acclimatization. This result suggests that king penguins mainly rely on skeletal muscle contraction (shivering or locomotor activity) to maintain endothermy at sea. Sea-acclimatized immature penguins also exhibited higher shivering efficiency and oxygen pulse (amount of oxygen consumed or energy expended per heartbeat) than pre-fledging juvenile birds. Such increase in shivering and cardiovascular efficiency may favor a more efficient activity-thermoregulatory heat substitution providing penguins with the aptitude to survive the tremendous energetic challenge imposed by marine life in cold circumpolar oceans.

Mitochondrial threshold for H2O2 release in skeletal muscle of mammals.

The aim of the study was to evaluate the interplay between mitochondrial respiration and H2O2 release during the transition from basal non-phosphorylating to maximal phosphorylating states. We conducted a large scale comparative study of mitochondrial oxygen consumption, H2O2 release and electron leak (% H2O2/O) in skeletal muscle mitochondria isolated from mammal species ranging from 7 g to 500 kg. Mitochondrial fluxes were measured at different steady state rates in presence of pyruvate, malate, and succinate as respiratory substrates. Every species exhibited a burst of H2O2 release from skeletal muscle mitochondria at a low rate of oxidative phosphorylation, essentially once the activity of mitochondrial oxidative phosphorylation reached 26% of the maximal respiration. This threshold for ROS generation thus appears as a general characteristic of skeletal muscle mitochondria in mammals. These findings may have implications in situations promoting succinate accumulation within mitochondria, such as ischemia or hypoxia.

Accumulation of natriuretic peptides is associated with protein energy wasting and activation of browning in white adipose tissue in chronic kidney disease.

Protein energy wasting is a common feature of patients with chronic kidney disease (CKD) and is associated with poor outcomes. Protein energy wasting and cachexia, a severe form of protein energy wasting, are characterized by increased resting energy expenditure but the underlying mechanisms are unclear. Browning corresponds to the activation of inducible brown adipocytes in white adipose tissue and occurs in states of cachexia associated with hypermetabolic disease such as cancer. Here we tested the hypothesis that CKD-associated protein energy wasting could result from browning activation as a direct effect of the uremic environment on adipocytes. In a murine model of CKD (5/6 nephrectomy), there was increased resting energy expenditure, expression of uncoupling protein 1 (a thermogenic protein uncoupling oxidative phosphorylation in mitochondria) and citrate synthase activity (a proxy of mitochondrial density in white adipose tissue). Mice with CKD also exhibited increased levels of atrial natriuretic peptide, a well known activator of browning. The incubation of primary adipose cells with plasma from patients receiving dialysis treatment and having signs of protein energy wasting led to an increased synthesis of uncoupling protein 1. Similarly, primary adipose cells exposed to atrial natriuretic peptide at concentrations relevant of CKD led to a significant increase of uncoupling protein 1 content. Thus, accumulation of cardiac natriuretic peptides during CKD could contribute to the browning of white adipose tissue and protein energy wasting.