Mechanisms that Alter Capacity for Adenosine Triphosphate Production and Oxidative Phosphorylation: Insights from Avian Migration

Avian migration is among the most energetically demanding feats observed in animals. Studies evaluating the physiological underpinnings of migration have repeatedly shown that migratory birds display numerous adaptations that ultimately supply the flight muscle mitochondria with abundant fuel and oxygen during long-distance flights. To make use of this high input, the organs and mitochondria of migrants are predicted to display several traits that maximize their capacity to produce adenosine triphosphate (ATP). This review aims to introduce readers to several mechanisms by which organs and mitochondria can alter their capacity for oxidative phosphorylation and ATP production. The role of organ size, mitochondrial volume, substrate, and oxygen delivery to the electron transport system are discussed. A central theme of this review is the role of changes in electron chain complex activity, mitochondrial morphology and dynamics, and supercomplexes in allowing avian migrants and other taxa to alter the performance of the electron transport system with predictable shifts in demand. It is my hope that this review will serve as a springboard for future studies exploring the mechanisms that alter bioenergetic capacity across animal species.

A Mitochondrial Perspective on the Demands of Reproduction

The cost of supporting traits that increase mating opportunities and maximize the production of quality offspring is paid in energy. This currency of reproduction is enabled by bioenergetic adaptations that underlie the flexible changes in energy utilization that occur with reproduction. This review considers the traits that contribute to variation in the capacity of an organ to produce ATP. Further, it synthesizes findings from studies that have evaluated bioenergetic adaptations to the production of sexually selected traits and performance during reproduction and the role of change in mitochondrial respiratory performance in the tradeoff between reproduction and longevity. Cumulatively, these works provide evidence that in selecting for redder males, female finches will likely mate with a male with high mitochondrial respiratory performance and, potentially, a higher probability of mitonuclear compatibility. Females from diverse taxa allocate more to reproduction when the respiratory performance of mitochondria or density of the inner mitochondrial membrane in the liver or skeletal muscle is higher. Finally, reproduction does not appear to have persistent negative effects on mitochondrial respiratory performance, countering a role for mitochondria in the trade-off between reproduction and longevity. I close by noting that adaptations that improve mitochondrial respiratory performance appear vital for optimizing reproductive fitness.

Acute metformin induces hyperglycemia in healthy adult mourning doves, Zenaida macroura

Birds have the highest blood glucose among vertebrates. Several mechanisms may explain this including the lack of a functional insulin-responsive glucose transport protein, high glucagon concentrations, and reliance on lipid oxidation resulting in the production of gluconeogenic precursors. The hypothesis was that interruption of gluconeogenesis using the diabetes medication metformin would lower glucose concentrations in wild-caught birds. We captured two cohorts of adult mourning doves, Zenaida macroura, and acclimated them to captivity for two weeks. In this crossover study, cohort 1 was administered a single dose of one of the following oral treatments each week: metformin (150 or 300 mg/kg), glycogenolysis inhibitor (2.5 mg/kg 1,4-dideoxy-1,4-imino-D-arabinitol (DAB)), or water (50 μL). Whole blood glucose was measured using a glucometer at baseline, 30, 60, and 120 min following the oral doses. In contrast to mammals and chickens, 300 mg/kg metformin did not alter blood glucose (p > 0.05) whereas 150 mg/kg metformin increased blood glucose compared to water (p = 0.043). To examine whether 150 mg/kg metformin stimulated glycogenolysis, we co-administered 150 mg/kg metformin and 2.5 mg/kg DAB, which prevented the hyperglycemic response. Cohort 2 was administered the same treatments and the early response was examined (0, 5, 10, 15 min). Low-dose metformin increased blood glucose within 5 min (p = 0.039) whereas the high dose had no effect. DAB did not prevent the early response to metformin nor did it alter blood glucose concentrations when administered alone (p = 0.887). In conclusion, metformin increases endogenous blood glucose via glycogenolysis in healthy adult male mourning doves.

Flexibility underlies differences in mitochondrial respiratory performance between migratory and non-migratory White-crowned Sparrows (Zonotrichia leucophrys)

Migration is one of the most energy-demanding behaviors observed in birds. Mitochondria are the primary source of energy used to support these long-distance movements, yet how mitochondria meet the energetic demands of migration is scarcely studied. We quantified changes in mitochondrial respiratory performance in the White-crowned Sparrow (Zonotrichia leucophrys), which has a migratory and non-migratory subspecies. We hypothesized that the long-distance migratory Gambel's subspecies (Z. l. gambelii) would show higher mitochondrial respiratory performance compared to the non-migratory Nuttall's subspecies (Z. l. nuttalli). We sampled Gambel's individuals during spring pre-migration, active fall migration, and a period with no migration or breeding (winter). We sampled Nuttall's individuals during periods coinciding with fall migration and the winter period of Gambel's annual cycle. Overall, Gambel's individuals had higher citrate synthase, a proxy for mitochondrial volume, than Nuttall's individuals. This was most pronounced prior to and during migration. We found that both OXPHOS capacity (state 3) and basal respiration (state 4) of mitochondria exhibit high seasonal flexibility within Gambel's individuals, with values highest during active migration. These values in Nuttall's individuals were most similar to Gambel's individuals in winter. Our observations indicate that seasonal changes in mitochondrial respiration play a vital role in migration energetics.

ZHX2 emerges as a negative regulator of mitochondrial oxidative phosphorylation during acute liver injury

Mitochondria dysfunction contributes to acute liver injuries, and mitochondrial regulators, such as PGC-1α and MCJ, affect liver regeneration. Therefore, identification of mitochondrial modulators may pave the way for developing therapeutic strategies. Here, ZHX2 is identified as a mitochondrial regulator during acute liver injury. ZHX2 both transcriptionally inhibits expression of several mitochondrial electron transport chain genes and decreases PGC-1α stability, leading to reduction of mitochondrial mass and OXPHOS. Loss of Zhx2 promotes liver recovery by increasing mitochondrial OXPHOS in mice with partial hepatectomy or CCl4-induced liver injury, and inhibition of PGC-1α or electron transport chain abolishes these effects. Notably, ZHX2 expression is higher in liver tissues from patients with drug-induced liver injury and is negatively correlated with mitochondrial mass marker TOM20. Delivery of shRNA targeting Zhx2 effectively protects mice from CCl4-induced liver injury. Together, our data clarify ZHX2 as a negative regulator of mitochondrial OXPHOS and a potential target for developing strategies for improving liver recovery after acute injuries.

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.

Cold temperatures induce priming of the glucose stress response in tree swallows

Capricious environments often present wild animals with challenges that coincide or occur in sequence. Conceptual models of the stress response predict that one threat may prime or dampen the response to another. Although evidence has supported this for glucocorticoid responses, much less is known about the effects of previous challenges on energy mobilization. Food limitation may have a particularly important effect, by altering the ability to mobilize energy when faced with a subsequent challenge. We tested the prediction that challenging weather conditions, which reduce food availability, alter the energetic response to a subsequent acute challenge (capture and restraint). Using a three-year dataset from female tree swallows measured during three substages of breeding, we used a model comparison approach to test if weather (temperature, wind speed, and precipitation) over 3- or 72-hour timescales predicted baseline and post-restraint glucose levels, and if so which environmental factors were the strongest predictors. Contrary to our predictions, weather conditions did not affect baseline glucose; however, birds that had experienced lower temperatures over the preceding 72 h tended to have higher stress-induced glucose when faced with an acute stressor. We also saw some support for an effect of rainfall on stress-induced glucose: around the time that eggs hatched, birds that had experienced more rainfall over the preceding 72 h mounted lower responses. Overall, we find support in a wild animal for the idea that the glucose stress response may be primed by exposure to prior challenges.

Strategy of micro-environmental adaptation to cold seep among different brittle stars’ colonization

Diffusing fluid from methane seepage in cold seep field creates zones with physicochemical gradients and divergent ecosystems like the mussel beds and clam beds. Three species of brittle stars (Ophiuroidea) were discovered in the Haima cold seep fields, of which Ophiophthalmus serratus and Histampica haimaensis were found on top of or within mussel beds and clam beds, whereas Amphiura sp. was only collected from muds in the clam bed assemblage. Here, we evaluated the genetic signatures of micro-environmental adaptation of brittle stars to cold seep through the comparison of mitogenomes. This study provided two complete mitogenome sequences of O. serratus and Amphiura sp. and compared with those of H. haimaensis and other non-seep species. We found that the split events of the seep and non-seep species were as ancient as the Cretaceous period (∼148–98 Mya). O. serratus and H. haimaensis display rapid residue mutation and mitogenome rearrangements compared to their shallow or deep-sea relatives, in contrast, Amphiura sp. only show medium, regardless of nucleotide mutation rate or mitogenome rearrangement, which may correlate with their adaptation to one or two micro-ecosystems. Furthermore, we identified 10 positively selected residues in ND4 in the Amphiura sp. lineage, suggesting important roles of the dehydrogenase complex in Amphiura sp. adaptive to the cold seep environment. Our results shed light on the different evolutionary strategies during colonization in different micro-environments.

Ischemic damage to every segment of the oxidative phosphorylation cascade elevates ETC driving force and ROS production in cardiac mitochondria

Myocardial ischemia has long-lasting negative impacts on cardiomyocyte mitochondrial ATP production. However, the location(s) of damage to the oxidative phosphorylation pathway responsible for altered mitochondrial function is unclear. Mitochondrial reactive oxygen species (ROS) production increases following ischemia, but the specific factors controlling this increase are unknown. To determine how ischemia affects the mitochondrial energy conversion cascade and ROS production, mitochondrial driving forces [redox potential and membrane potential (ΔΨ)] were measured at resting, intermediate, and maximal respiration rates in mitochondria isolated from rat hearts after 60 min of control flow (control) or no-flow ischemia (ischemia). The effective activities of the dehydrogenase enzymes, the electron transport chain (ETC), and ATP synthesis and transport were computed using the driving forces and flux. Ischemia lowered maximal mitochondrial respiration rates and diminished the responsiveness of respiration to both redox potential and ΔΨ. Ischemia decreased the activities of every component of the oxidative phosphorylation pathway: the dehydrogenase enzymes, the ETC, and ATP synthesis and transport. ROS production was linearly related to driving force down the ETC; however, ischemia mitochondria demonstrated a greater driving force down the ETC and higher ROS production. Overall, results indicate that ischemia ubiquitously damages the oxidative phosphorylation pathway, reduces mitochondrial sensitivity to driving forces, and augments the propensity for electrons to leak from the ETC. These findings underscore that strategies to improve mitochondrial function following ischemia must target the entire mitochondrial energy conversion cascade.

NEW & NOTEWORTHY This integrative analysis is the first to assess how myocardial ischemia alters the mitochondrial driving forces and the degree to which individual segments of the mitochondrial energy transduction pathway contribute to diminished function following ischemia. This investigation demonstrates that increased reactive oxygen species production following ischemia is related to a lower effective activity of the electron transport chain and a greater driving force down the electron transport chain.

Revisiting glucose regulation in birds - A negative model of diabetes complications

Birds naturally have blood glucose concentrations that are nearly double levels measured for mammals of similar body size and studies have shown that birds are resistant to insulin-mediated glucose uptake into tissues. While a combination of high blood glucose and insulin resistance is associated with diabetes-related pathologies in mammals, birds do not develop such complications. Moreover, studies have shown that birds are resistant to oxidative stress and protein glycation and in fact, live longer than similar-sized mammals. This review seeks to explore how birds regulate blood glucose as well as various theories that might explain their apparent resistance to insulin-mediated glucose uptake and adaptations that enable them to thrive in a state of relative hyperglycemia.

Differences in advanced glycation endproducts (AGEs) in plasma from birds and mammals of different body sizes and ages

Birds and mammals provide a physiological paradox: similar-sized mammals live shorter lives than birds; yet, birds have higher blood glucose concentrations than mammals, and higher basal metabolic rates. We have previously shown that oxidative stress patterns between mammals and birds differ, so that birds, generally, have lower blood antioxidant capacity, and lower lipid peroxidation concentration. There is a close association between oxidative stress and the production of carbohydrate-based damaged biomolecules, Advanced Glycation End-products (AGEs). In mammals, AGEs can bind to their receptor (RAGE), which can lead to increases in reactive oxygen species (ROS) production, and can decrease antioxidant capacity. Here, we used plasma from birds and mammals to address whether blood plasma AGE-BSA concentration is associated with body mass and age in these two groups. We found a statistically significantly higher average concentrations of AGE-BSA in birds compared with mammals, and we found a significantly positive correlation between AGE-BSA and age in mammals, though, this correlation disappeared after phylogenetic correction. We propose that the higher AGE concentration in birds is mainly attributable to greater AGE-production due to elevated basal glucose concentrations and decreased AGE-clearance given differences in glomerular filtration rates in birds compared with mammals. Additionally, due to the potential lack of an AGE receptor in birds, AGE accumulation may not be closely linked to oxidative stress and therefore pose a lesser physiological challenge in birds compared to mammals.

Effects of acute temperature increases on House sparrow (Passer domesticus) pectoralis muscle myonuclear domain

With rapid climate change, heat wave episodes have become more intense and more frequent. This poses a significant threat to animals, and forces them to manage these physiologically challenging conditions by adapting and/or moving. As an invasive species with a large niche breadth, House sparrows (Passer domesticus) exhibit high phenotypic flexibility that caters to seasonal changes in function and metabolism. For example, their pectoral muscle complex exhibits size and mass plasticity with winter and summer acclimation. Here, we investigated the effects of acute whole-organism heat stress to 43°C on cellular-level changes in House sparrow pectoralis muscle myonuclear domain (MND), the volumetric portion each nucleus is responsible for, that have gone overlooked in the current literature. House sparrows were separated into a control group, a heat-shocked group subjected to thermal stress at 43°C for 24 h, and a recovery group that was returned to room temperature for 24 h after experiencing the same temperature treatment. Here, we found that heat-shocked and recovery groups demonstrated a decrease in number of nuclei per millimeter of fiber and increase in MND, when compared with the control. We also found a significant positive correlation between fiber diameter and MND in the recovery group, suggesting the possibility that nuclei number constrains the extent of muscle fiber size. Together, these results show that acute heat shock alters House sparrow pectoralis muscle cellular physiology in a rigid way that could prove detrimental to long-term muscle integrity and performance.

Oxidative phosphorylation K0.5ADP in vitro depends on substrate oxidative capacity: Insights from a luciferase-based assay to evaluate ADP kinetic parameters

The K_0.5ADP of oxidative phosphorylation (OxPhos) identifies the cytosolic ADP concentration which elicits one-half the maximum OxPhos rate. This kinetic parameter is commonly measured to assess mitochondrial metabolic control sensitivity. Here we describe a luciferase-based assay to evaluate the ADP kinetic parameters of mitochondrial ATP production from OxPhos, adenylate kinase (AK), and creatine kinase (CK). The high sensitivity, reproducibility, and throughput of the microplate-based assay enabled a comprehensive kinetic assessment of all three pathways in mitochondria isolated from mouse liver, kidney, heart, and skeletal muscle. Carboxyatractyloside titrations were also performed with the assay to estimate the flux control strength of the adenine nucleotide translocase (ANT) over OxPhos in human skeletal muscle mitochondria. ANT flux control coefficients were 0.91 ± 0.07, 0.83 ± 0.06, and 0.51 ± 0.07 at ADP concentrations of 6.25, 12.5, and 25 μM, respectively, an [ADP] range which spanned the K_0.5ADP. The oxidative capacity of substrate combinations added to drive OxPhos was found to dramatically influence ADP kinetics in mitochondria from several tissues. In mouse skeletal muscle ten different substrate combinations elicited a 7-fold range of OxPhos V_max, which correlated positively (R² = 0.963) with K_0.5ADP values ranging from 2.3 ± 0.2 μM to 11.9 ± 0.6 μM. We propose that substrate-enhanced capacity to generate the protonmotive force increases the OxPhos K_0.5ADP because flux control at ANT increases, thus K_0.5ADP rises toward the dissociation constant, K_dADP, of ADP-ANT binding. The findings are discussed in the context of top-down metabolic control analysis.

Fasting ameliorates oxidative stress: A review of physiological strategies across life history events in wild vertebrates

Fasting is a component of many species' life history due to environmental factors or behavioral patterns that limit access to food. Despite metabolic and physiological challenges associated with these life history stages, fasting-adapted wild vertebrates exhibit few if any signs of oxidative stress, suggesting that fasting promotes redox homeostasis. Here we review mammalian, avian, reptilian, amphibian, and piscine examples of animals undergoing fasting during prolonged metabolic suppression (e.g. hibernation and estivation) or energetically demanding processes (e.g. migration and breeding) to better understand the mechanisms underlying fasting tolerance in wild vertebrates. These studies largely show beneficial effects of fasting on redox balance via limited oxidative damage. Though some species exhibit signs of oxidative stress due to energetically or metabolically extreme processes, fasting wild vertebrates largely buffer themselves from the negative consequences of oxidative damage through specific strategies such as elevating antioxidants, selectively maintaining redox balance in critical tissues, or modifying behavioral patterns. We conclude with suggestions for future research to better elucidate the protective effects of fasting on oxidative stress as well as disentangle the impacts from other life history stages. Further research in these areas will facilitate our understanding of the mechanisms wild vertebrates use to mitigate the negative impacts associated with metabolically-extreme life history stages as well as potential translation into therapeutic interventions in non-fasting-adapted species including humans.

Evolution of Flight Muscle Contractility and Energetic Efficiency

The powered flight of animals requires efficient and sustainable contractions of the wing muscles of various flying species. Despite their high degree of phylogenetic divergence, flight muscles in insects and vertebrates are striated muscles with similarly specialized sarcomeric structure and basic mechanisms of contraction and relaxation. Comparative studies examining flight muscles together with other striated muscles can provide valuable insights into the fundamental mechanisms of muscle contraction and energetic efficiency. Here, we conducted a literature review and data mining to investigate the independent emergence and evolution of flight muscles in insects, birds, and bats, and the likely molecular basis of their contractile features and energetic efficiency. Bird and bat flight muscles have different metabolic rates that reflect differences in energetic efficiencies while having similar contractile machinery that is under the selection of similar natural environments. The significantly lower efficiency of insect flight muscles along with minimized energy expenditure in Ca2+ handling is discussed as a potential mechanism to increase the efficiency of mammalian striated muscles. A better understanding of the molecular evolution of myofilament proteins in the context of physiological functions of invertebrate and vertebrate flight muscles can help explore novel approaches to enhance the performance and efficiency of skeletal and cardiac muscles for the improvement of human health.

Comparative Pharmacokinetics and bioavailability of marbofloxacin in geese (Anser Anser domesticus) after two sites of intramuscular administrations

The pharmacokinetics of marbofloxacin (MAR) was compared in geese (Anser Anser domesticus) after single intravenous (IV) and intramuscular (IM) (thigh and pectoral muscles) administrations of 5 mg/kg. Serum concentrations of MAR were determined with high-performance liquid chromatography (HPLC) method. Serum MAR concentrations versus time were analyzed by a noncompartmental method. After IV administration, MAR showed high volume of distribution at steady state (Vdss ) of 5.24 ± 1.08 L/kg. The serum body clearance (Cl) and elimination half-life (T1/2 λz) of MAR were 0.79 ± 0.07 L hr-1  kg-1 and 6.94 ± 1.12 hr, respectively. The peak of MAR serum concentrations Cmax achieved at one and 0.50 hr after thigh and pectoral IM sites of injections, respectively, were 1.20 and 0.91 μg/ml. Significant differences were found in the mean absorption time (MAT), the systemic bioavailability (F%), and elimination parameters of MAR between two sites of injections, indicating that the absorption was fairly slow and complete after thigh IM injection. The pharmacokinetics of MAR in geese diverged according to the site of IM injection following a parallel study design. We recommend the thigh muscle as IM site of injection to obtain maximum concentrations of the administered drug in geese.

Comparative studies of mitochondrial reactive oxygen species in animal longevity: Technical pitfalls and possibilities

The mitochondrial oxidative theory of aging has been repeatedly investigated over the past 30 years by comparing the efflux of hydrogen peroxide (H₂O₂) from isolated mitochondria of long‐ and short‐lived species using horseradish peroxidase‐based assays. However, a clear consensus regarding the relationship between H₂O₂ production rates and longevity has not emerged. Concomitantly, novel insights into the mechanisms of reactive oxygen species (ROS) handling by mitochondria themselves should have raised concerns about the validity of this experimental approach. Here, we review pitfalls of the horseradish peroxidase/amplex red detection system for the measurement of mitochondrial ROS formation rates, with an emphasis on longevity studies. Importantly, antioxidant systems in the mitochondrial matrix are often capable of scavenging H₂O₂ faster than mitochondria produce it. As a consequence, as much as 84% of the H₂O₂ produced by mitochondria may be consumed before it diffuses into the reaction medium, where it can be detected by the horseradish peroxidase/amplex red system, this proportion is likely not consistent across species. Furthermore, previous studies often used substrates that elicit H₂O₂ formation at a much higher rate than in physiological conditions and at sites of secondary importance in vivo. Recent evidence suggests that the activity of matrix antioxidants may correlate with longevity instead of the rate of H₂O₂ formation. We conclude that past studies have been methodologically insufficient to address the putative relationship between longevity and mitochondrial ROS. Thus, novel methodological approaches are required that more accurately encompass mitochondrial ROS metabolism.

Roles and Mechanistic Bases of Glucocorticoid Regulation of Avian Reproduction

To maximize fitness, organisms must invest energetic and nutritional resources into developing, activating, and maintaining reproductive physiology and behavior. Corticosterone (CORT), the primary avian glucocorticoid, regulates energetic reserves to meet metabolic demands. At low (baseline) plasma levels, CORT activates avian mineralocorticoid receptors and may stimulate lipid mobilization, foraging activity, and feeding behavior. During stress in birds, elevated plasma CORT also stimulates glucocorticoid receptors and may promote glycemia, lipolysis, and proteolysis. Furthermore, CORT orchestrates physiological and behavioral adjustments to perceived threats. While many avian studies demonstrate effects of CORT on reproduction, few studies have elucidated the mechanisms, including receptor activation and site(s) of action, which underlie these effects. Even fewer studies have investigated how low and elevated plasma CORT regulates energetic reserves to meet the metabolic demands of reproduction. Here, we propose several hypotheses to clarify the direct and indirect effects of CORT on avian reproductive physiology and behavior. In addition, we emphasize the need for new manipulative studies involving alterations of endogenous plasma CORT levels and/or food availability to elucidate how CORT regulates the energetic demands of reproduction.

A Simple Hydraulic Analog Model of Oxidative Phosphorylation

Mitochondrial oxidative phosphorylation is the primary source of cellular energy transduction in mammals. This energy conversion involves dozens of enzymatic reactions, energetic intermediates, and the dynamic interactions among them. With the goal of providing greater insight into the complex thermodynamics and kinetics ("thermokinetics") of mitochondrial energy transduction, a simple hydraulic analog model of oxidative phosphorylation is presented. In the hydraulic model, water tanks represent the forward and back "pressures" exerted by thermodynamic driving forces: the matrix redox potential (ΔGredox), the electrochemical potential for protons across the mitochondrial inner membrane (ΔGH), and the free energy of adenosine 5'-triphosphate (ATP) (ΔGATP). Net water flow proceeds from tanks with higher water pressure to tanks with lower pressure through "enzyme pipes" whose diameters represent the conductances (effective activities) of the proteins that catalyze the energy transfer. These enzyme pipes include the reactions of dehydrogenase enzymes, the electron transport chain (ETC), and the combined action of ATP synthase plus the ATP-adenosine 5'-diphosphate exchanger that spans the inner membrane. In addition, reactive oxygen species production is included in the model as a leak that is driven out of the ETC pipe by high pressure (high ΔGredox) and a proton leak dependent on the ΔGH for both its driving force and the conductance of the leak pathway. Model water pressures and flows are shown to simulate thermodynamic forces and metabolic fluxes that have been experimentally observed in mammalian skeletal muscle in response to acute exercise, chronic endurance training, and reduced substrate availability, as well as account for the thermokinetic behavior of mitochondria from fast- and slow-twitch skeletal muscle and the metabolic capacitance of the creatine kinase reaction.

The role of the antioxidant system during intense endurance exercise: lessons from migrating birds

During migration, birds substantially increase their metabolic rate and burn fats as fuel and yet somehow avoid succumbing to overwhelming oxidative damage. The physiological means by which vertebrates such as migrating birds can counteract an increased production of reactive species (RS) are rather limited: they can upregulate their endogenous antioxidant system and/or consume dietary antioxidants (prophylactically or therapeutically). Thus, birds can alter different components of their antioxidant system to respond to the demands of long-duration flights, but much remains to be discovered about the complexities of RS production and antioxidant protection throughout migration. Here, we use bird migration as an example to discuss how RS are produced during endurance exercise and how the complex antioxidant system can protect against cellular damage caused by RS. Understanding how a bird's antioxidant system responds during migration can lend insights into how antioxidants protect birds during other life-history stages when metabolic rate may be high, and how antioxidants protect other vertebrates from oxidative damage during endurance exercise.

Current versus future reproduction and longevity: a re-evaluation of predictions and mechanisms

Oxidative damage is predicted to be a mediator of trade-offs between current reproduction and future reproduction or survival, but most studies fail to support such predictions. We suggest that two factors underlie the equivocal nature of these findings: (1) investigators typically assume a negative linear relationship between current reproduction and future reproduction or survival, even though this is not consistently shown by empirical studies; and (2) studies often fail to target mechanisms that could link interactions between sequential life-history events. Here, we review common patterns of reproduction, focusing on the relationships between reproductive performance, survival and parity in females. Observations in a range of species show that performance between sequential reproductive events can decline, remain consistent or increase. We describe likely bioenergetic consequences of reproduction that could underlie these changes in fitness, including mechanisms that could be responsible for negative effects being ephemeral, persistent or delayed. Finally, we make recommendations for designing future studies. We encourage investigators to carefully consider additional or alternative measures of bioenergetic function in studies of life-history trade-offs. Such measures include reactive oxygen species production, oxidative repair, mitochondrial biogenesis, cell proliferation, mitochondrial DNA mutation and replication error and, importantly, a measure of the respiratory function to determine whether measured differences in bioenergetic state are associated with a change in the energetic capacity of tissues that could feasibly affect future reproduction or lifespan. More careful consideration of the life-history context and bioenergetic variables will improve our understanding of the mechanisms that underlie the life-history patterns of animals.

Seasonal variation in glucocorticoid and mineralocorticoid receptors in metabolic tissues of the house sparrow (Passer domesticus)

Glucocorticoid hormones like corticosterone (CORT) play essential metabolic roles at both baseline and stress-induced concentrations, and CORT titers vary seasonally in patterns occurring across many different vertebrate species. It has been hypothesized that CORT may vary seasonally due to changing energy requirements at different times of year. However, hormone effects are dependent on binding to receptors in target tissues, and receptors might also vary seasonally. CORT alters metabolism primarily through binding to two receptors, the high-affinity mineralocorticoid receptor (MR) and low-affinity glucocorticoid receptor (GR). We quantified GR and MR in metabolic tissues (liver, kidney, omental and subcutaneous fat, and gastrocnemius and pectoralis muscle) of wild-caught house sparrows (Passer domesticus) to assess these tissues' capacity to respond to CORT-mediated metabolic demands. We quantified receptors using radioligand binding assays in early and late winter, pre-egg-laying, breeding, late breeding and molt (n=12 at each stage). MR binding did not vary significantly in any tissue over the course of the year. Because MR is associated with baseline CORT effects, this suggests that changing hormone titers may primarily regulate baseline CORT effects on metabolism. Seasonal modulation of GR binding occurred in every tissue but omental fat, though peak receptor density did not coincide with peak stress-induced CORT concentrations measured previously. Because GR is associated with stress-induced CORT effects, these data demonstrate seasonal patterns in stress-induced CORT are not driven by metabolic needs alone, although at different times of year sparrows may vary which tissue types respond to increased energy demands resulting from exposure to stressors.

Mitochondrial physiology in the major arbovirus vector Aedes aegypti: substrate preferences and sexual differences define respiratory capacity and superoxide production

Adult females of Aedes aegypti are facultative blood sucking insects and vectors of Dengue and yellow fever viruses. Insect dispersal plays a central role in disease transmission and the extremely high energy demand posed by flight is accomplished by a very efficient oxidative phosphorylation process, which take place within flight muscle mitochondria. These organelles play a central role in energy metabolism, interconnecting nutrient oxidation to ATP synthesis, but also represent an important site of cellular superoxide production. Given the importance of mitochondria to cell physiology, and the potential contributions of this organelle for A. aegypti biology and vectorial capacity, here, we conducted a systematic assessment of mitochondrial physiology in flight muscle of young adult A. aegypti fed exclusively with sugar. This was carried out by determining the activities of mitochondrial enzymes, the substrate preferences to sustain respiration, the mitochondrial bioenergetic efficiency and capacity, in both mitochondria-enriched preparations and mechanically permeabilized flight muscle in both sexes. We also determined the substrates preferences to promote mitochondrial superoxide generation and the main sites where it is produced within this organelle. We observed that respiration in A. aegypti mitochondria was essentially driven by complex I and glycerol 3 phosphate dehydrogenase substrates, which promoted distinct mitochondrial bioenergetic capacities, but with preserved efficiencies. Respiration mediated by proline oxidation in female mitochondria was strikingly higher than in males. Mitochondrial superoxide production was essentially mediated through proline and glycerol 3 phosphate oxidation, which took place at sites other than complex I. Finally, differences in mitochondrial superoxide production among sexes were only observed in male oxidizing glycerol 3 phosphate, exhibiting higher rates than in female. Together, these data represent a significant step towards the understanding of fundamental mitochondrial processes in A. aegypti, with potential implications for its physiology and vectorial capacity.

Metabolic and transcriptional responses of gilthead sea bream (Sparus aurata L.) to environmental stress: new insights in fish mitochondrial phenotyping

The aim of the current study was to phenotype fish metabolism and the transcriptionally-mediated response of hepatic mitochondria of gilthead sea bream to intermittent and repetitive environmental stressors: (i) changes in water temperature (T-ST), (ii) changes in water level and chasing (C-ST) and (iii) multiple sensory perception stressors (M-ST). Gene expression profiling was done using a quantitative PCR array of 60 mitochondria-related genes, selected as markers of transcriptional regulation, oxidative metabolism, respiration uncoupling, antioxidant defense, protein import/folding/assembly, and mitochondrial dynamics and apoptosis. The mitochondrial phenotype mirrored changes in fish performance, haematology and lactate production. T-ST especially up-regulated transcriptional factors (PGC1α, NRF1, NRF2), rate limiting enzymes of fatty acid β-oxidation (CPT1A) and tricarboxylic acid cycle (CS), membrane translocases (Tim/TOM complex) and molecular chaperones (mtHsp10, mtHsp60, mtHsp70) to improve the oxidative capacity in a milieu of a reduced feed intake and impaired haematology. The lack of mitochondrial response, increased production of lactate and negligible effects on growth performance in C-ST fish were mostly considered as a switch from aerobic to anaerobic metabolism. A strong down-regulation of PGC1α, NRF1, NRF2, CPT1A, CS and markers of mitochondrial dynamics and apoptosis (BAX, BCLX, MFN2, MIRO2) occurred in M-ST fish in association with the greatest circulating cortisol concentration and a reduced lactate production and feed efficiency, which represents a metabolic condition with the highest allostatic load score. These findings evidence a high mitochondrial plasticity against stress stimuli, providing new insights to define the threshold level of stress condition in fish.

Mitochondrial uncoupling as a regulator of life-history trajectories in birds: an experimental study in the zebra finch

Mitochondria have a fundamental role in the transduction of energy from food into ATP. The coupling between food oxidation and ATP production is never perfect, but may nevertheless be of evolutionary significance. The 'uncoupling to survive' hypothesis suggests that 'mild' mitochondrial uncoupling evolved as a protective mechanism against the excessive production of damaging reactive oxygen species (ROS). Because resource allocation and ROS production are thought to shape animal life histories, alternative life-history trajectories might be driven by individual variation in the degree of mitochondrial uncoupling. We tested this hypothesis in a small bird species, the zebra finch (Taeniopygia guttata), by treating adults with the artificial mitochondrial uncoupler 2,4-dinitrophenol (DNP) over a 32-month period. In agreement with our expectations, the uncoupling treatment increased metabolic rate. However, we found no evidence that treated birds enjoyed lower oxidative stress levels or greater survival rates, in contrast to previous results in other taxa. In vitro experiments revealed lower sensitivity of ROS production to DNP in mitochondria isolated from skeletal muscles of zebra finch than mouse. In addition, we found significant reductions in the number of eggs laid and in the inflammatory immune response in treated birds. Altogether, our data suggest that the 'uncoupling to survive' hypothesis may not be applicable for zebra finches, presumably because of lower effects of mitochondrial uncoupling on mitochondrial ROS production in birds than in mammals. Nevertheless, mitochondrial uncoupling appeared to be a potential life-history regulator of traits such as fecundity and immunity at adulthood, even with food supplied ad libitum.

Mitochondrial reactive oxygen species (ROS) in animal cells: relevance to aging and normal physiology

In animal mitochondria, the four electron reduction of molecular oxygen to produce water at respiratory complex IV is the terminal step in substrate oxidation. However, respiratory complexes I, II, and III can participate in the single electron reduction of oxygen to produce the radical species superoxide. This progenitor reactive oxygen species (ROS) participates in a number of reactions that generate other ROS. These molecules may react with, and damage, intracellular macromolecules, leading to cellular dysfunction. Mitochondrial ROS production is often considered from this perspective of macromolecular damage and is central to the “oxidative damage theory of aging”, which suggests the accumulation of oxidative damage in animal cells underlies the aging phenotype and limits lifespan. In this review, we discuss some experimental results accumulated over the past decade that are inconsistent with this theory. A limitation of the theory is that it presupposes mitochondrial ROS are inherently harmful. However, it is increasingly apparent that some basic cellular functions are physiologically regulated by normal levels of mitochondrial ROS. For example, cell growth and division, the apoptotic pathway, and mitochondrial fusion–fission dynamics all appear to be redox-regulated by mitochondrial ROS and perhaps the matrix manganese superoxide dismutase (MnSOD). Therefore, it is less clear how the balance between ROS regulation of normal cellular activities and ROS-mediated macromolecular damage is maintained and how this relates to aging and longevity in animals.

Comparison of metabolic substrates in alligators and several birds of prey

On average, avian blood glucose concentrations are 1.5-2 times those of mammals of similar mass and high concentrations of insulin are required to lower blood glucose. Whereas considerable data exist for granivorous species, few data are available for plasma metabolic substrate and glucoregulatory hormone concentrations for carnivorous birds and alligators. Birds and mammals with carnivorous diets have higher metabolic rates than animals consuming diets with less protein whereas alligators have low metabolic rates. Therefore, the present study was designed to compare substrate and glucoregulatory hormone concentrations in several birds of prey and a phylogenetically close relative of birds, the alligator. The hypothesis was that the combination of carnivorous diets and high metabolic rates favored the evolution of greater protein and fatty acid utilization leading to insulin resistance and high plasma glucose concentrations in carnivorous birds. In contrast, it was hypothesized that alligators would have low substrate utilization attributable to a low metabolic rate. Fasting plasma substrate and glucoregulatory hormone concentrations were compared for bald eagles (Haliaeetus leucocephalus), great horned owls (Bubo virginianus), red-tailed hawks (Buteo jamaicensis), and American alligators (Alligator mississippiensis). Avian species had high circulating β-hydroxybutyrate (10-21 mg/dl) compared to alligators (2.81 ± 0.16 mg/dl). In mammals high concentrations of this byproduct of fatty acid utilization are correlated with insulin resistance. Fasting glucose and insulin concentrations were positively correlated in eagles whereas no relationship was found between these variables for owls, hawks or alligators. Additionally, β-hydroxybutyrate concentrations were low in alligators. Similar to carnivorous mammals, ingestion of a high protein diet may have favored the utilization of fatty acids and protein for energy thereby promoting the development of insulin resistance and gluconeogenesis-induced high plasma glucose concentrations during periods of fasting in birds of prey.

A midlife crisis for the mitochondrial free radical theory of aging

Since its inception more than four decades ago, the Mitochondrial Free Radical Theory of Aging (MFRTA) has served as a touchstone for research into the biology of aging. The MFRTA suggests that oxidative damage to cellular macromolecules caused by reactive oxygen species (ROS) originating from mitochondria accumulates in cells over an animal’s lifespan and eventually leads to the dysfunction and failure that characterizes aging. A central prediction of the theory is that the ability to ameliorate or slow this process should be associated with a slowed rate of aging and thus increased lifespan. A vast pool of data bearing on this idea has now been published. ROS production, ROS neutralization and macromolecule repair have all been extensively studied in the context of longevity. We review experimental evidence from comparisons between naturally long- or short-lived animal species, from calorie restricted animals, and from genetically modified animals and weigh the strength of results supporting the MFRTA. Viewed as a whole, the data accumulated from these studies have too often failed to support the theory. Excellent, well controlled studies from the past decade in particular have isolated ROS as an experimental variable and have shown no relationship between its production or neutralization and aging or longevity. Instead, a role for mitochondrial ROS as intracellular messengers involved in the regulation of some basic cellular processes, such as proliferation, differentiation and death, has emerged. If mitochondrial ROS are involved in the aging process, it seems very likely it will be via highly specific and regulated cellular processes and not through indiscriminate oxidative damage to macromolecules.

Skeletal Muscle Fuel Selection Occurs at the Mitochondrial Level

Mammals exponentially increase the rate of carbohydrate oxidation as exercise intensity rises, while birds combust lipid almost exclusively while flying at high percentages of aerobic capacity. The fuel oxidized by contracting muscle depends on many factors: whole body fuel storage masses, mobilization, blood transport, cellular uptake, and substrate selection at the level of the mitochondrion. We examined the fuel preferences of mitochondria isolated from mammalian and avian locomotory muscles using two approaches. First, the influence of substrates on the kinetics of respiration (KMADP and Vmax) was evaluated. For all substrates and combinations, KMADP was generally two-fold higher in avian mitochondria. Second, fuel competition between pyruvate, glutamate, and/or palmitoyl-l-carnitine at three levels of ATP free energy was determined using the principle of mass balance and the measured rates of O2 consumption and metabolite accumulation/utilization. Avian mitochondria strongly spared pyruvate from oxidation when another substrate was available and fatty acid was the dominant substrate, regardless of energy state. Mammalian mitochondria exhibited some preference for fatty acid over pyruvate at lower flux (higher energy state), but exhibited much greater tendency to select pyruvate and glutamate when available. Studies in sonicated mitochondria revealed two-fold higher electron transport chain electron conductance in avian mitochondria. We conclude that substantial fuel selection occurs at the level of the mitochondrial matrix and that avian flight muscle mitochondria are particularly biased toward the selection of fatty acid, possibly by facilitating high β-oxidation flux by maintaining a more oxidized matrix.

Reciprocal inhibition of in vitro substrate movement into avian skeletal muscle

Plasma glucose and ketone concentrations are much higher in birds than in humans and birds exhibit resistance to insulin-mediated glucose uptake into muscle. Therefore, birds may offer a model in which to examine the effects of high plasma glucose and free fatty acid (FFA) concentrations on substrate preference. The present study examined the uptake of radiolabeled oleic acid (OA; C18:1) and radiolabeled glucose by skeletal muscle isolated from the forewing of English sparrows (Passer domesticus). In dose-response studies, unlabeled glucose and OA (20 mM each) inhibited the uptake of their respective radiolabeled counterparts. To examine the effects of glucose on OA uptake, muscles were incubated for 60 min in a buffer containing 20 mM glucose with the addition of radiolabeled OA. This level of glucose significantly decreased radiolabeled OA uptake by 36%. Using the same methodology, 20 mM OA significantly decreased radiolabeled glucose transport by 49%. Comparing control values for glucose (0.952 ± 0.04 μM/mg muscle) and OA uptake (2.20 ± 0.29 μM/mg muscle), it is evident that OA is preferentially taken up by avian skeletal muscle. As FFAs provide a greater amount of energy per mole (146 ATP/OA) than carbohydrates (36 ATP/glucose), storing and utilizing fats may be more energy-efficient for birds. As studies in mammals have shown that FFAs may impair glucose uptake pathways, it is suspected that high FFA uptake by avian skeletal muscle may induce their notably lower glucose transport.