Mammalian fuel utilization during sustained exercise

Commentary: Tracing the fate of metabolic substrates during changes in whole-body energy expenditure in mice

For small mammals, such as mice, cannulation procedures can be quite challenging, limiting research associated with tracing isotopically labelled substrates at the whole-animal level. When cannulation in mice is possible, assessment of substrate use is further limited to when mice are either under anesthesia or are at rest, as there are no studies directly quantifying substrate use during exercise in mice. The use of isotopic tracer techniques has greatly advanced our knowledge in understanding how metabolic substrates (carbohydrates, amino acids, and fatty acids) contribute to whole-body metabolism. However, research regarding tissue-specific fuel use contributions to whole-body energy expenditure in mice at varying metabolic intensities (i.e., exercise) is lacking, despite the popularity of using mice in a variety of metabolic models. In this commentary, we briefly discuss the methodologies, advantages, and disadvantages of using radiolabelled, positron emission, and stable isotopes with a specific focus on fatty acids. We highlight recent mouse studies that have used creative experimental designs employing the use of isotopic tracer techniques and we briefly discuss how these methodologies can be further pursued to deepen our understanding of substrate use during exercise. Lastly, we show findings of a recent study we performed using a radiolabelled fatty acid tracer (14C-bromopalmitic acid) to determine fatty acid uptake in 16 muscles, two brown and two white adipose tissue depots during submaximal exercise in deer mice.

How the 'Aerobic/Anaerobic Glycolysis' Meme Formed a 'Habit of Mind' Which Impedes Progress in the Field of Brain Energy Metabolism

The division of glycolysis into two separate pathways, aerobic and anaerobic, depending on the presence or absence of oxygen, respectively, was formulated over eight decades ago. The former ends with pyruvate, while the latter ends with lactate. Today, this division is confusing and misleading as research over the past 35 years clearly has demonstrated that glycolysis ends with lactate not only in cancerous cells but also in healthy tissues and cells. The present essay offers a review of the history of said division and the more recent knowledge that has been gained about glycolysis and its end-product, lactate. Then, it presents arguments in an attempt to explain why separating glycolysis into aerobic and anaerobic pathways persists among scientists, clinicians and teachers alike, despite convincing evidence that such division is not only wrong scientifically but also hinders progress in the field of energy metabolism.

Invasive dynamic electrical impedance myography during 100 s of moderate contraction in rats' gastrocnemius muscle

GOAL

The present study aimed to investigate the behavior of dynamic electrical impedance myography (dEIM) signals during a 100-s period of the dynamic contraction of Wistar rats' gastrocnemius evoked by electrical stimulation and to link the variations in bioimpedance with muscular energy systems.

METHODS

Muscle contraction used 30% of the maximum muscular force and persisted for 100 s, along which dynamic bioimpedance signals were acquired. Based on the bioimpedance signals, two parameters, ΔZc and ΔZpc, were calculated to allow the analysis of their changes with the energy systems that supplied adenosine triphosphate (ATP) to the muscle. ΔZc indicated the variation of impedance of a twitch cycle compared to the values observed during the rest period preceding the cycle, and ΔZpc indicated slow bioimpedance variations compared to the values obtained during the rest period.

RESULTS

The results indicated that ΔZc followed the force behavior, achieving a change rate of ∼14%. This parameter was associated with instantaneous impedance changes owing to the occurrence of each twitch.

CONCLUSION

Although the findings of this study were linked to energy system processes, future studies are required for improving the understanding of the underlying mechanisms involved in dEIM.

SIGNIFICANCE

The results contributed to understanding the relation of energy systems that supply ATP to the muscles with dEIM variations that occurred during muscle activity.

History and Function of the Lactate Receptor GPR81/HCAR1 in the Brain: A Putative Therapeutic Target for the Treatment of Cerebral Ischemia

GPR81 is a G-protein coupled receptor (GPCR) discovered in 2001, but deorphanized only 7 years later, when its affinity for lactate as an endogenous ligand was demonstrated. More recently, GPR81 expression and distribution in the brain were also confirmed and the function of lactate as a volume transmitter has been suggested since then. These findings shed light on a new function of lactate acting as a signaling molecule in the central nervous system, in addition to its well-known role as a metabolic fuel for neurons. GPR81 seems to act as a metabolic sensor, coupling energy metabolism, synaptic activity, and blood flow. Activation of this receptor leads to Gi-mediated downregulation of adenylyl cyclase and subsequent reduction in cAMP levels, regulating several downstream pathways. Recent studies have also suggested the potential role of lactate as a neuroprotective agent, mainly under brain ischemic conditions. This effect is usually attributed to the metabolic role of lactate, but the underlying mechanisms need further investigation and could be related to lactate signaling via GPR81. The activation of GPR81 showed promising results for neuroprotection: it modulates many processes involved in the pathophysiology of ischemia. In this review, we summarize the history of GPR81, starting with its deorphanization; then, we discuss GPR81 expression and distribution, signaling transduction cascades, and neuroprotective roles. Lastly, we propose GPR81 as a potential target for the treatment of cerebral ischemia.

Low carbohydrate high fat ketogenic diets on the exercise crossover point and glucose homeostasis

In exercise science, the crossover effect denotes that fat oxidation is the primary fuel at rest and during low-intensity exercise with a shift towards an increased reliance on carbohydrate oxidation at moderate to high exercise intensities. This model makes four predictions: First, >50% of energy comes from carbohydrate oxidation at ≥60% of maximum oxygen consumption (VO2max), termed the crossover point. Second, each individual has a maximum fat oxidation capacity (FATMAX) at an exercise intensity lower than the crossover point. FATMAX values are typically 0.3–0.6 g/min. Third, fat oxidation is minimized during exercise ≥85%VO2max, making carbohydrates the predominant energetic substrate during high-intensity exercise, especially at >85%VO2max. Fourth, high-carbohydrate low-fat (HCLF) diets will produce superior exercise performances via maximizing pre-exercise storage of this predominant exercise substrate. In a series of recent publications evaluating the metabolic and performance effects of low-carbohydrate high-fat (LCHF/ketogenic) diet adaptations during exercise of different intensities, we provide findings that challenge this model and these four predictions. First, we show that adaptation to the LCHF diet shifts the crossover point to a higher %VO2max (>80%VO2max) than previously reported. Second, substantially higher FATMAX values (>1.5 g/min) can be measured in athletes adapted to the LCHF diet. Third, endurance athletes exercising at >85%VO2max, whilst performing 6 × 800 m running intervals, measured the highest rates of fat oxidation yet reported in humans. Peak fat oxidation rates measured at 86.4 ± 6.2%VO2max were 1.58 ± 0.33 g/min with 30% of subjects achieving >1.85 g/min. These studies challenge the prevailing doctrine that carbohydrates are the predominant oxidized fuel during high-intensity exercise. We recently found that 30% of middle-aged competitive athletes presented with pre-diabetic glycemic values while on an HCLF diet, which was reversed on LCHF. We speculate that these rapid changes between diet, insulin, glucose homeostasis, and fat oxidation might be linked by diet-induced changes in mitochondrial function and insulin action. Together, we demonstrate evidence that challenges the current crossover concept and demonstrate evidence that a LCHF diet may also reverse features of pre-diabetes and future metabolic disease risk, demonstrating the impact of dietary choice has extended beyond physical performance even in athletic populations.

Bioactive nutraceuticals oligo-lactic acid and fermented soy extract alleviate cognitive decline in mice in part via anti-neuroinflammation and modulation of gut microbiota

Introduction

Cognition decline is associated with aging and certain diseases, such as neurodegenerative or neuropsychiatric disorders, diabetes and chronic kidney disease. Inflammation/neuroinflammation is considered an important causal factor, and experimental evidence suggests that anti-inflammatory natural compounds may effectively prevent cognitive decline. The goal of this study was to evaluate the effects of two natural bioactive agents, oligo-lactic acid (LAP) and fermented soy extract (ImmunBalance, IMB), on cognition in an adenine-induced cognitive impairment mouse model and to investigate the modulation of related biomarkers.

Methods

Male C57 black mice were randomly assigned into the following experimental groups and received the corresponding treatments for 2 weeks before the use of adenine for model development: (1) negative control; (2) model control: injection of adenine at 50 mg/kg daily for 4 weeks; (3, 4) IMB groups: adenine injection and IMB oral gavage at 250 and 1,000 mg/kg BW, respectively; and (5) LAP group: adenine injection and LAP oral gavage at 1,000 mg/kg BW. One week after the model was developed, mice were evaluated for cognitive performances by using Y maze test, novel object recognition test, open field test, and Barnes maze tests. At the end of the experiment, brain tissues and cecum fecal samples were collected for analysis of gene expression and gut microbiota.

Results

Mice treated with LAP or IMB had significantly improved spatial working memory, spatial recognition memory (LAP only), novel object recognition, and spatial learning and memory, compared with those in the model group. Gene expression analysis showed that, among a panel of cognition related genes, six of them (ELOVL2, GLUT4, Nestein, SNCA, TGFB1, and TGFB2) were significantly altered in the model group. LAP treatment significantly reversed expression levels of inflammatory/neuroinflammatory genes (SNCA, TGFB1), and IMB significantly reversed expression levels of genes related to inflammation/neuroinflammation, neurogenesis, and energy metabolism (ELOVL2, GLUT4, Nestin, TGFB1, and TGFB2). The altered microbiome was attenuated only by IMB.

Discussion

In conclusion, our data showed that LAP improved cognition associated with regulating biomarkers related to neuroinflammation and energy metabolism, whereas IMB improved cognition associated with regulating biomarkers related to neuroinflammation, energy metabolism, and neurogenesis, and modulating gut microbiota. Our results suggest that LAP and IMB may improve cognitive performance in mice via distinct mechanisms of action.

Effects of Wild Blueberries on Fat Oxidation Rates in Aerobically Trained Males

Wild blueberries (WBs) have been documented to decrease oxidative stress in active and sedentary populations as well as influence lipolytic enzymes and increase the rate of fat oxidation (FAT-ox) during rest. To examine the effect of WBs on the rate of FAT-ox and lipid peroxidation during submaximal exercise, 11 healthy, aerobically trained males (26 ± 7.5 years, 74.9 ± 7.54 kg, 10.5 ± 3.2% BF) completed a 2-week washout avoiding foods high in anthocyanins, then completed a control exercise protocol cycling at 65% of VO2peak for 40 min. Participants then consumed 375 g/d of anthocyanins for two weeks before repeating the exercise protocol. WBs increased FAT-ox when cycling at 65% of VO2peak by 19.7% at 20, 43.2% at 30, and 31.1% at 40 min, and carbohydrate oxidation (CHO-ox) decreased by 10.1% at 20, 19.2% at 30, and 14.8% at 40 min of cycling at 65% of VO2peak. Lactate was lower with WBs at 20 (WB: 2.6 ± 1.0, C: 3.0 ± 1.1), 30 (WB: 2.2 ± 0.9, C: 2.9 ± 1.0), and 40 min (WB: 1.9 ± 0.8, C: 2.5 ± 0.9). Results indicate that WBs may increase the rate of FAT-ox during moderate-intensity activity in healthy, active males.

Lactate as a myokine and exerkine: drivers and signals of physiology and metabolism

No longer viewed as a metabolic waste product and cause of muscle fatigue, a contemporary view incorporates the roles of lactate in metabolism, sensing and signaling in normal as well as pathophysiological conditions. Lactate exists in millimolar concentrations in muscle, blood, and other tissues and can rise more than an order of magnitude as the result of increased production and clearance limitations. Lactate exerts its powerful driver-like influence by mass action, redox change, allosteric binding, and other mechanisms described in this article. Depending on the condition, such as during rest and exercise, following carbohydrate nutrition, injury, or pathology, lactate can serve as a myokine or exerkine with autocrine-, paracrine-, and endocrine-like functions that have important basic and translational implications. For instance, lactate signaling is: involved in reproductive biology, fueling the heart, muscle adaptation, and brain executive function, growth and development, and a treatment for inflammatory conditions. Lactate also works with many other mechanisms and factors in controlling cardiac output and pulmonary ventilation during exercise. Ironically, lactate can be disruptive of normal processes such as insulin secretion when insertion of lactate transporters into pancreatic β-cell membranes is not suppressed, and in carcinogenesis when factors that suppress carcinogenesis are inhibited, whereas factors that promote carcinogenesis are upregulated. Lactate signaling is important in areas of intermediary metabolism, redox biology, mitochondrial biogenesis, neurobiology, gut physiology, appetite regulation, nutrition, and overall health and vigor. The various roles of lactate as a myokine and exerkine are reviewed.NEW & NOTEWORTHY Lactate sensing and signaling is a relatively new and rapidly changing field. As a physiological signal lactate works both independently and in concert with other signals. Lactate operates via covalent binding and canonical signaling, redox change, and lactylation of DNA. Lactate can also serve as an element of feedback loops in cardiopulmonary regulation. From conception through aging lactate is not the only a myokine or exerkine, but it certainly deserves consideration as a physiological signal.

Low and high carbohydrate isocaloric diets on performance, fat oxidation, glucose and cardiometabolic health in middle age males

High carbohydrate, low fat (HCLF) diets have been the predominant nutrition strategy for athletic performance, but recent evidence following multi-week habituation has challenged the superiority of HCLF over low carbohydrate, high fat (LCHF) diets, along with growing interest in the potential health and disease implications of dietary choice. Highly trained competitive middle-aged athletes underwent two 31-day isocaloric diets (HCLF or LCHF) in a randomized, counterbalanced, and crossover design while controlling calories and training load. Performance, body composition, substrate oxidation, cardiometabolic, and 31-day minute-by-minute glucose (CGM) biomarkers were assessed. We demonstrated: (i) equivalent high-intensity performance (@∼85%VO_2max), fasting insulin, hsCRP, and HbA_1c without significant body composition changes across groups; (ii) record high peak fat oxidation rates (LCHF:1.58 ± 0.33g/min @ 86.40 ± 6.24%VO_2max; 30% subjects > 1.85 g/min); (iii) higher total, LDL, and HDL cholesterol on LCHF; (iv) reduced glucose mean/median and variability on LCHF. We also found that the 31-day mean glucose on HCLF predicted 31-day glucose reductions on LCHF, and the 31-day glucose reduction on LCHF predicted LCHF peak fat oxidation rates. Interestingly, 30% of athletes had 31-day mean, median and fasting glucose > 100 mg/dL on HCLF (range: 111.68-115.19 mg/dL; consistent with pre-diabetes), also had the largest glycemic and fat oxidation response to carbohydrate restriction. These results: (i) challenge whether higher carbohydrate intake is superior for athletic performance, even during shorter-duration, higher-intensity exercise; (ii) demonstrate that lower carbohydrate intake may be a therapeutic strategy to independently improve glycemic control, particularly in those at risk for diabetes; (iii) demonstrate a unique relationship between continuous glycemic parameters and systemic metabolism.

Tumor lactic acid: a potential target for cancer therapy

Tumor development is influenced by circulating metabolites and most tumors are exposed to substantially elevated levels of lactic acid and low levels of nutrients, such as glucose and glutamine. Tumor-derived lactic acid, the major circulating carbon metabolite, regulates energy metabolism and cancer cell signaling pathways, while also acting as an energy source and signaling molecule. Recent studies have yielded new insights into the pro-tumorigenic action of lactic acid and its metabolism. These insights suggest an anti-tumor therapeutic strategy targeting the oncometabolite lactic acid, with the aim of improving the efficacy and clinical safety of tumor metabolism inhibitors. This review describes the current understanding of the multifunctional roles of tumor lactic acid, as well as therapeutic approaches targeting lactic acid metabolism, including lactate dehydrogenase and monocarboxylate transporters, for anti-cancer therapy.

Scaling with body mass and age in glycolytic enzymes of domestic dogs

Animals produce ATP through oxidative phosphorylation using oxygen, but cellular energy can also be obtained through glycolysis when oxygen is not present at sufficient levels. Although most mammals of larger body mass have longer life spans, small dog breeds tend to outlive large breeds. Primary fibroblast cells from larger breeds of dogs have previously been shown to have increased dependency on glycolytic phenotypes across their lifespan. Different levels of activity of the glycolytic enzymes pyruvate kinase (PK), lactate dehydrogenase (LDH), and phosphoenolpyruvate carboxykinase (PEPCK) may provide insight to a mechanism that leads to the different metabolic phenotype observed in different sized breeds as they age. In this study, 1) we measured the activities of PK, LDH, and PEPCK in primary fibroblasts from dogs of different breed sizes and age classes and 2) measured the activities of PK and LDH in plasma from dogs of different breed sizes and age classes. We found that there was no significant relationship between body mass and PK, LDH and PEPCK activity in primary fibroblasts. Further, there were not significant differences with activity in these enzymes for old dogs compared to young dogs. In plasma, we found a negative correlation between PK activity and body mass and no relationship between LDH activity and body mass. There was a negative relationship between LDH activity and age in dogs. Further, while a negative correlational relationship between PK activity and age was only marginal, a best subsets regression model demonstrated a significant marginal effect of age on PK activity. PK and LDH may provide intermediates for other metabolic pathways in small breeds. However, large breed dogs may demonstrate a deficiency in metabolism at the PK level, a cellular metabolic pathway that may potentially aid in tumor progression.

A remarkable adaptive paradigm of heart performance and protection emerges in response to marked cardiac-specific overexpression of ADCY8

Adult (3 month) mice with cardiac-specific overexpression of adenylyl cyclase (AC) type VIII (TGAC8) adapt to an increased cAMP-induced cardiac workload (~30% increases in heart rate, ejection fraction and cardiac output) for up to a year without signs of heart failure or excessive mortality. Here, we show classical cardiac hypertrophy markers were absent in TGAC8, and that total left ventricular (LV) mass was not increased: a reduced LV cavity volume in TGAC8 was encased by thicker LV walls harboring an increased number of small cardiac myocytes, and a network of small interstitial proliferative non-cardiac myocytes compared to wild type (WT) littermates; Protein synthesis, proteosome activity, and autophagy were enhanced in TGAC8 vs WT, and Nrf-2, Hsp90α, and ACC2 protein levels were increased. Despite increased energy demands in vivo LV ATP and phosphocreatine levels in TGAC8 did not differ from WT. Unbiased omics analyses identified more than 2,000 transcripts and proteins, comprising a broad array of biological processes across multiple cellular compartments, which differed by genotype; compared to WT, in TGAC8 there was a shift from fatty acid oxidation to aerobic glycolysis in the context of increased utilization of the pentose phosphate shunt and nucleotide synthesis. Thus, marked overexpression of AC8 engages complex, coordinate adaptation "circuity" that has evolved in mammalian cells to defend against stress that threatens health or life (elements of which have already been shown to be central to cardiac ischemic pre-conditioning and exercise endurance cardiac conditioning) that may be of biological significance to allow for proper healing in disease states such as infarction or failure of the heart.

Beyond Mechanical Tension: A Review of Resistance Exercise-Induced Lactate Responses & Muscle Hypertrophy

The present review aims to explore and discuss recent research relating to the lactate response to resistance training and the potential mechanisms by which lactate may contribute to skeletal muscle hypertrophy or help to prevent muscle atrophy. First, we will discuss foundational information pertaining to lactate including metabolism, measurement, shuttling, and potential (although seemingly elusive) mechanisms for hypertrophy. We will then provide a brief analysis of resistance training protocols and the associated lactate response. Lastly, we will discuss potential shortcomings, resistance training considerations, and future research directions regarding lactate's role as a potential anabolic agent for skeletal muscle hypertrophy.

Tracing the lactate shuttle to the mitochondrial reticulum

Isotope tracer infusion studies employing lactate, glucose, glycerol, and fatty acid isotope tracers were central to the deduction and demonstration of the Lactate Shuttle at the whole-body level. In concert with the ability to perform tissue metabolite concentration measurements, as well as determinations of unidirectional and net metabolite exchanges by means of arterial–venous difference (a-v) and blood flow measurements across tissue beds including skeletal muscle, the heart and the brain, lactate shuttling within organs and tissues was made evident. From an extensive body of work on men and women, resting or exercising, before or after endurance training, at sea level or high altitude, we now know that Organ–Organ, Cell–Cell, and Intracellular Lactate Shuttles operate continuously. By means of lactate shuttling, fuel-energy substrates can be exchanged between producer (driver) cells, such as those in skeletal muscle, and consumer (recipient) cells, such as those in the brain, heart, muscle, liver and kidneys. Within tissues, lactate can be exchanged between white and red fibers within a muscle bed and between astrocytes and neurons in the brain. Within cells, lactate can be exchanged between the cytosol and mitochondria and between the cytosol and peroxisomes. Lactate shuttling between driver and recipient cells depends on concentration gradients created by the mitochondrial respiratory apparatus in recipient cells for oxidative disposal of lactate.

Studies using isotope tracer technologies have significantly improved understanding of how lactate, a metabolite produced as fuel during normal metabolism and in response to exercise, moves or ‘shuttles’ throughout the body. George Brooks and colleagues at the University of California, Berkeley, USA, reviewed the history of the understanding of lactate shuttling, which has largely been informed by human studies using isotope tracer infusions during rest and exercise. Such research highlights continuous organ–organ, cell–cell, and intracellular lactate shuttling. Lactate moves between producer cells such as skeletal muscle cells and consumer cells in tissues including the heart and brain, where it is preferred over glucose as an energy source. Shuttling depends on lactate concentration gradients created by mitochondrial networks in recipient cells. Lactate is disposed of via oxidation.

Lactate metabolism in human health and disease

The current understanding of lactate extends from its origins as a byproduct of glycolysis to its role in tumor metabolism, as identified by studies on the Warburg effect. The lactate shuttle hypothesis suggests that lactate plays an important role as a bridging signaling molecule that coordinates signaling among different cells, organs and tissues. Lactylation is a posttranslational modification initially reported by Professor Yingming Zhao’s research group in 2019. Subsequent studies confirmed that lactylation is a vital component of lactate function and is involved in tumor proliferation, neural excitation, inflammation and other biological processes. An indispensable substance for various physiological cellular functions, lactate plays a regulatory role in different aspects of energy metabolism and signal transduction. Therefore, a comprehensive review and summary of lactate is presented to clarify the role of lactate in disease and to provide a reference and direction for future research. This review offers a systematic overview of lactate homeostasis and its roles in physiological and pathological processes, as well as a comprehensive overview of the effects of lactylation in various diseases, particularly inflammation and cancer.

The potential mechanisms of lactate in mediating exercise-enhanced cognitive function: a dual role as an energy supply substrate and a signaling molecule

Lactate has previously been considered a metabolic waste and is mainly involved in exercise-induced fatigue. However, recent studies have found that lactate may be a mediator of the beneficial effects of exercise on brain health. Lactate plays a dual role as an energy supply substrate and a signaling molecule in this process. On the one hand, astrocytes can uptake circulating glucose or degrade glycogen for glycolysis to produce lactate, which is released into the extracellular space. Neurons can uptake extracellular lactate as an important supplement to their energy metabolism substrates, to meet the demand for large amounts of energy when synaptic activity is enhanced. Thus, synaptic activity and energy transfer show tight metabolic coupling. On the other hand, lactate acts as a signaling molecule to activate downstream signaling transduction pathways by specific receptors, inducing the expression of immediate early genes and cerebral angiogenesis. Moderate to high-intensity exercise not only increases lactate production and accumulation in muscle and blood but also promotes the uptake of skeletal muscle-derived lactate by the brain and enhances aerobic glycolysis to increase brain-derived lactate production. Furthermore, exercise regulates the expression or activity of transporters and enzymes involved in the astrocyte-neuron lactate shuttle to maintain the efficiency of this process; exercise also activates lactate receptor HCAR1, thus affecting brain plasticity. Rethinking the role of lactate in cognitive function and the regulatory effect of exercise is the main focus and highlights of the review. This may enrich the theoretical basis of lactate-related to promote brain health during exercise, and provide new perspectives for promoting a healthy aging strategy.

Lactate inhibits naked mole-rat cardiac mitochondrial respiration

In aerobic conditions, the proton-motive force drives oxidative phosphorylation (OXPHOS) and the conversion of ADP to ATP. In hypoxic environments, OXPHOS is impaired, resulting in energy shortfalls and the accumulation of protons and lactate. This results in cellular acidification, which may impact the activity and/or integrity of mitochondrial enzymes and in turn negatively impact mitochondrial respiration and thus aerobic ATP production. Naked mole-rats (NMRs) are among the most hypoxia-tolerant mammals and putatively experience intermittent hypoxia in their underground burrows. However, if and how NMR cardiac mitochondria are impacted by lactate accumulation in hypoxia is unknown. We predicted that lactate alters mitochondrial respiration in NMR cardiac muscle. To test this, we used high-resolution respirometry to measure mitochondrial respiration in permeabilized cardiac muscle fibres from NMRs exposed to 4 h of in vivo normoxia (21% O₂) or hypoxia (7% O₂). We found that: (1) cardiac mitochondria cannot directly oxidize lactate, but surprisingly, (2) lactate inhibits mitochondrial respiration, and (3) decreases complex IV maximum respiratory capacity. Finally, (4) in vivo hypoxic exposure decreases the magnitude of lactate-mediated inhibition of mitochondrial respiration. Taken together, our results suggest that lactate may retard electron transport system function in NMR cardiac mitochondria, particularly in normoxia, and that NMR hearts may be primed for anaerobic metabolism.

Kaempferol increases intracellular ATP content in C2C12 myotubes under hypoxic conditions by suppressing the HIF-1α stabilization and/or by enhancing the mitochondrial complex IV activity

Kaempferol (KMP) has numerous important biological functions, and we recently showed that it remarkably increased intracellular adenosine triphosphate (ATP) content in C₂C₁₂ myotubes under hypoxic conditions. Since intracellular ATP is generated by aerobic energy metabolism or anaerobic glycolysis, hypoxia inducible factor-1α (HIF-1α) has been shown to be associated with metabolic remodeling and causes metabolic shift from aerobic energy metabolism to anaerobic glycolysis in response to hypoxic conditions. Here, we investigate the effects of KMP under hypoxic conditions on the stabilization of HIF-1α in C₂C₁₂ myotubes and its underlying molecular mechanisms. Constitutive HIF-1α protein expression was observed in C₂C₁₂ myotubes, and its expression under hypoxic conditions was remarkably suppressed by KMP by reducing its stability; thus, resulting in an increase in ATP content. Furthermore, KMP strikingly increased the ubiquitination of HIF-1α and promoted its degradation via the ubiquitin proteasome system. Inhibition of HIF-1α by KMP resulted in the abrogation of the expression of glycolytic enzymes such as lactate dehydrogenase A and pyruvate dehydrogenase kinase isozyme 1. In addition, the metabolome profiling showed that KMP promoted oxidative energy production, while the mitochondrial complex activity assay indicated that KMP increased the activity of mitochondrial complex IV. Finally, we showed that KMP inhibited HIF-1α expression and increased intracellular ATP content in the soleus muscle of rats. Taken together, these results suggest that KMP increases intracellular ATP content under hypoxic conditions by suppressing the HIF-1α stabilization and/or by enhancing the mitochondrial complex IV activity in muscle.

Voluntary Behavior and Training Conditions Modulate in vivo Extracellular Glucose and Lactate in the Mouse Primary Motor Cortex

Learning or performing new behaviors requires significant neuronal signaling and is metabolically demanding. The metabolic cost of performing a behavior is mitigated by exposure and practice which result in diminished signaling and metabolic requirements. We examined the impact of novel and habituated wheel running, as well as effortful behaviors on the modulation of extracellular glucose and lactate using biosensors inserted in the primary motor cortex of mice. We found that motor behaviors produce increases in extracellular lactate and decreases in extracellular glucose in the primary motor cortex. These effects were modulated by experience, novelty and intensity of the behavior. The increase in extracellular lactate appears to be strongly associated with novelty of a behavior as well as the difficulty of performing a behavior. Our observations are consistent with the view that a main function of aerobic glycolysis is not to fuel the current neuronal activity but to sustain new bio-infrastructure as learning changes neural networks, chiefly through the shuttling of glucose derived carbons into the pentose phosphate pathway for the biosynthesis of nucleotides.

AMPK-Regulated Astrocytic Lactate Shuttle Plays a Non-Cell-Autonomous Role in Neuronal Survival

Lactate is used as an energy source by producer cells or shuttled to neighboring cells and tissues. Both glucose and lactate fulfill the bioenergetic demand of neurons, the latter imported from astrocytes. The contribution of astrocytic lactate to neuronal bioenergetics and the mechanisms of astrocytic lactate production are incompletely understood. Through in vivo¹H magnetic resonance spectroscopy, ¹³C glucose mass spectroscopy, and electroencephalographic and molecular studies, here we show that the energy sensor AMP activated protein kinase (AMPK) regulates neuronal survival in a non-cell-autonomous manner. Ampk-null mice are deficient in brain lactate and are seizure prone. Ampk deletion in astroglia, but not neurons, causes neuronal loss in both mammalian and fly brains. Mechanistically, astrocytic AMPK phosphorylated and destabilized thioredoxin-interacting protein (TXNIP), enabling expression and surface translocation of the glucose transporter GLUT1, glucose uptake, and lactate production. Ampk loss in astrocytes causes TXNIP hyperstability, GLUT1 misregulation, inadequate glucose metabolism, and neuronal loss.

Muraleedharan et al. demonstrate that AMPK is required for astrocytic glycolysis, lactate production, and lactate shuttle as an energy source to neurons such that AMPK loss in glia causes non-cell-autonomous neuronal loss in the mammalian and fly brain.

Lipid metabolism in adaptation to extreme nutritional challenges

Food shortages represent a common challenge for most animal species. As a consequence, many have evolved metabolic strategies encompassing extreme starvation-resistance capabilities, going without food for months or even years. One such strategy is to store substantial levels of fat when food is available and release these energy-rich lipids during periods of dearth. In this review, we provide an overview of the strategies and pathways underlying the extreme capacity for animals to store and mobilize lipids during nutritionally stressful environmental conditions and highlight accompanying resilience phenotypes that allow these animals to develop and tolerate such profound metabolic phenotypes.

The effect of 10 days of energy-deficit diet and high-intensity exercise training on the plasma high-density-lipoprotein (HDL) level among healthy collegiate males

ABSTRACTAn energy-deficit (ED) diet increases lipid mobilisation, while endurance exercise improves lipid profile by promoting formation of high-density lipoproteins (HDLs) among moderately active population. However, it is not clear whether ED with high-intensity exercise training can improve lipid profiles. Therefore, 20 recreationally active males (20.3 ± 2.7 years old with peak oxygen consumption (V˙O₂peak) of 2.8 ± 0.2 L min^-1) with HDL concentration of 1.44 ± 0.32 mmol L^-1, were evenly allocated to either an ED group (1590 ± 79 kcal day^-1: 40% of total caloric omitted at lunchtime) or a control (CON) group (2570 ± 139 kcal day^-1). Participants in both groups performed high-intensity cycling at 90% V˙O₂peak at a constant workload for 8 min, 2 h after breakfast on day D₀, D₂, D₄, D₆, D₈ and D₁₀ (D_n is day number), after which blood samples were collected. In ED, compared to D₀, triglycerides (TG) decreased on D_6-10 (p < .01) while total low-density lipoprotein (LDL), total cholesterol (TC) increased on D_2-10, while HDL progressively increased on D_6-10 (p = .01) with a final value of 1.67 ± 0.24 mmol L^-1. In CON, there were no changes in TG, TC and HDL while LDL was reduced on D_8-10 (p = .01). Moreover, the proportions of TC/HDL and LDL/HDL increased in ED on D_2-10 and D_2-8 (p = .05), respectively, while LDL/HDL in CON was reduced on D_8-10 (p = .01). In brief, 10 days of ED and a series of high-intensity exercise sessions show progressive elevation of HDL which suggest longer period is required to observe changes in lipid ratios.

Cytotoxic T-cells mediate exercise-induced reductions in tumor growth

Exercise has a wide range of systemic effects. In animal models, repeated exertion reduces malignant tumor progression, and clinically, exercise can improve outcome for cancer patients. The etiology of the effects of exercise on tumor progression are unclear, as are the cellular actors involved. We show here that in mice, exercise-induced reduction in tumor growth is dependent on CD8+ T cells, and that metabolites produced in skeletal muscle and excreted into plasma at high levels during exertion in both mice and humans enhance the effector profile of CD8+ T-cells. We found that activated murine CD8+ T cells alter their central carbon metabolism in response to exertion in vivo, and that immune cells from trained mice are more potent antitumor effector cells when transferred into tumor-bearing untrained animals. These data demonstrate that CD8+ T cells are metabolically altered by exercise in a manner that acts to improve their antitumoral efficacy.

The role of alternative sugars on endurance capacity in balb/c mice

Alternative sugars containing isomaltulose were investigated to confirm the hypothesis that isomaltulose ingestion affects endurance capacity due to slow rates of hydrolysis and absorption rate at the intestine. A swimming time of the control group tends to decrease, but the group administrated with low glycemic index (GI) sweeteners tend to increase gradually. Fructo-oligosaccharide (FOS) and inverted sugar (IS), contained isomaltulose, groups showed a significant difference of change in blood glucose and lactic acid level than control group (p < .05). Serum creatine phosphokinase (CPK) and serum lactate dehydrogenases (LDH) of PAL100 were significantly lower than that of the control group (p < .05). IS showed a significant difference in glutathione peroxidase (GSH-Px) compared to Con and PAL20 (20% isomaltulose) group (p < .05). Consuming FOS seems to increase an endurance capacity since fructose and FOS based in isomaltulose contained syrup showed low absorption rate and GI level. PRACTICAL APPLICATIONS: Sugar is a major energy source for exercise, but it causes excessive intake because of the short duration of sweetness, which causes diseases such as obesity, diabetes, and skin aging. An alternative sugar complex containing isomatulose was found to be a sugar substitute for athletic performance. Athletic performance is not just for athletes or active people. In general, elderly people with low muscle mass have low mobility. Alternative sugars can be a good source of supplements to help them perform smoothly with less intake. Therefore, an endurance test with alternative sugars is and important study for energy supplements industry.

Complex I is bypassed during high intensity exercise

Human muscles are tailored towards ATP synthesis. When exercising at high work rates muscles convert glucose to lactate, which is less nutrient efficient than respiration. There is hence a trade-off between endurance and power. Metabolic models have been developed to study how limited catalytic capacity of enzymes affects ATP synthesis. Here we integrate an enzyme-constrained metabolic model with proteomics data from muscle fibers. We find that ATP synthesis is constrained by several enzymes. A metabolic bypass of mitochondrial complex I is found to increase the ATP synthesis rate per gram of protein compared to full respiration. To test if this metabolic mode occurs in vivo, we conduct a high resolved incremental exercise tests for five subjects. Their gas exchange at different work rates is accurately reproduced by a whole-body metabolic model incorporating complex I bypass. The study therefore shows how proteome allocation influences metabolism during high intensity exercise.

Targeting L-Lactate Metabolism to Overcome Resistance to Immune Therapy of Melanoma and Other Tumor Entities

Although immunotherapy plays a significant role in tumor therapy, its efficacy is impaired by an immunosuppressive tumor microenvironment. A molecule that contributes to the protumor microenvironment is the metabolic product lactate. Lactate is produced in large amounts by cancer cells in response to either hypoxia or pseudohypoxia, and its presence in excess alters the normal functioning of immune cells. A key enzyme involved in lactate metabolism is lactate dehydrogenase (LDH). Elevated baseline LDH serum levels are associated with poor outcomes of current anticancer (immune) therapies, especially in patients with melanoma. Therefore, targeting LDH and other molecules involved in lactate metabolism might improve the efficacy of immune therapies. This review summarizes current knowledge about lactate metabolism and its role in the tumor microenvironment. Based on that information, we develop a rationale for deploying drugs that target lactate metabolism in combination with immune checkpoint inhibitors to overcome lactate-mediated immune escape of tumor cells.

Glycolysis Paradigm Shift Dictates a Reevaluation of Glucose and Oxygen Metabolic Rates of Activated Neural Tissue

In 1988 two seminal studies were published, both instigating controversy. One concluded that “the energy needs of activated neural tissue are minimal, being fulfilled via the glycolytic pathway alone,” a conclusion based on the observation that neural activation increased glucose consumption, which was not accompanied by a corresponding increase in oxygen consumption (Fox et al., 1988). The second demonstrated that neural tissue function can be supported exclusively by lactate as the energy substrate (Schurr et al., 1988). While both studies continue to have their supporters and detractors, the present review attempts to clarify the issues responsible for the persistence of the controversies they have provoked and offer a possible rationalization. The concept that lactate rather than pyruvate, is the glycolytic end-product, both aerobically and anaerobically, and thus the real mitochondrial oxidative substrate, has gained a greater acceptance over the years. The idea of glycolysis as the sole ATP supplier for neural activation (glucose → lactate + 2ATP) continues to be controversial. Lactate oxidative utilization by activated neural tissue could explain the mismatch between glucose and oxygen consumption and resolve the existing disagreements among users of imaging methods to measure the metabolic rates of the two energy metabolic substrates. The postulate that the energy necessary for active neural tissue is supplied by glycolysis alone stems from the original aerobic glycolysis paradigm. Accordingly, glucose consumption is accompanied by oxygen consumption at 1–6 ratio. Since Fox et al. (1988) observed only a minimal if non-existent oxygen consumption compared to glucose consumption, their conclusion make sense. Nevertheless, considering (a) the shift in the paradigm of glycolysis (glucose → lactate; lactate + O₂ + mitochondria → pyruvate → TCA cycle → CO₂ + H₂O + 17ATP); (b) that one mole of lactate oxidation requires only 50% of the amount of oxygen necessary for the oxidation of one mole of glucose; and (c) that lactate, as a mitochondrial substrate, is over eight times more efficient at ATP production than glucose as a glycolytic substrate, suggest that future studies of cerebral metabolic rates of activated neural tissue should include along with the measurements of CMR_O2 and CMR_glucose the measurement of CMR_lactate.

Cell-Specific "Competition for Calories" Drives Asymmetric Nutrient-Energy Partitioning, Obesity, and Metabolic Diseases in Human and Non-human Animals

The mammalian body is a complex physiologic “ecosystem” in which cells compete for calories (i.e., nutrient-energy). Axiomatically, cell-types with competitive advantages acquire a greater number of consumed calories, and when possible, increase in size and/or number. Thus, it is logical and parsimonious to posit that obesity is the competitive advantages of fat-cells (adipocytes) driving a disproportionate acquisition and storage of nutrient-energy. Accordingly, we introduce two conceptual frameworks. Asymmetric Nutrient-Energy Partitioning describes the context-dependent, cell-specific competition for calories that determines the partitioning of nutrient-energy to oxidation, anabolism, and/or storage; and Effective Caloric Intake which describes the number of calories available to constrain energy-intake via the inhibition of the sensorimotor appetitive cells in the liver and brain that govern ingestive behaviors. Inherent in these frameworks is the independence and dissociation of the energetic demands of metabolism and the neuro-muscular pathways that initiate ingestive behaviors and energy intake. As we demonstrate, if the sensorimotor cells suffer relative caloric deprivation via asymmetric competition from other cell-types (e.g., skeletal muscle- or fat-cells), energy-intake is increased to compensate for both real and merely apparent deficits in energy-homeostasis (i.e., true and false signals, respectively). Thus, we posit that the chronic positive energy balance (i.e., over-nutrition) that leads to obesity and metabolic diseases is engendered by apparent deficits (i.e., false signals) driven by the asymmetric inter-cellular competition for calories and concomitant differential partitioning of nutrient-energy to storage. These frameworks, in concert with our previous theoretic work, the Maternal Resources Hypothesis, provide a parsimonious and rigorous explanation for the rapid rise in the global prevalence of increased body and fat mass, and associated metabolic dysfunctions in humans and other mammals inclusive of companion, domesticated, laboratory, and feral animals.

Reduced non-bicarbonate skeletal muscle buffering capacity in mice with the mini-muscle phenotype

Muscle pH decreases during exercise, which may impair function. Endurance training typically reduces muscle buffering capacity as a result of changes in fiber-type composition, but existing comparisons of species that vary in activity level are ambiguous. We hypothesized that high-runner (HR) lines of mice from an experiment that breeds mice for voluntary wheel running would have altered muscle buffering capacity as compared with their non-selected control counterparts. We also expected that 6 days of wheel access, as used in the selection protocol, would reduce buffering capacity, especially for HR mice. Finally, we expected a subset of HR mice with the 'mini-muscle' phenotype to have relatively low buffering capacity as a result of fewer type IIb fibers. We tested non-bicarbonate buffering capacity of thigh muscles. Only HR mice expressing the mini-muscle phenotype had significantly reduced buffering capacity, females had lower buffering capacity than males, and wheel access had no significant effect.

The Science and Translation of Lactate Shuttle Theory

Once thought to be a waste product of anaerobic metabolism, lactate is now known to form continuously under aerobic conditions. Shuttling between producer and consumer cells fulfills at least three purposes for lactate: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. "Lactate shuttle" (LS) concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signaling. In medicine, it has long been recognized that the elevation of blood lactate correlates with illness or injury severity. However, with lactate shuttle theory in mind, some clinicians are now appreciating lactatemia as a "strain" and not a "stress" biomarker. In fact, clinical studies are utilizing lactate to treat pro-inflammatory conditions and to deliver optimal fuel for working muscles in sports medicine. The above, as well as historic and recent studies of lactate metabolism and shuttling, are discussed in the following review.

Obese super athletes: fat-fueled migration in birds and bats

Migratory birds are physiologically specialized to accumulate massive fat stores (up to 50-60% of body mass), and to transport and oxidize fatty acids at very high rates to sustain flight for many hours or days. Target gene, protein and enzyme analyses and recent -omic studies of bird flight muscles confirm that high capacities for fatty acid uptake, cytosolic transport, and oxidation are consistent features that make fat-fueled migration possible. Augmented circulatory transport by lipoproteins is suggested by field data but has not been experimentally verified. Migratory bats have high aerobic capacity and fatty acid oxidation potential; however, endurance flight fueled by adipose-stored fat has not been demonstrated. Patterns of fattening and expression of muscle fatty acid transporters are inconsistent, and bats may partially fuel migratory flight with ingested nutrients. Changes in energy intake, digestive capacity, liver lipid metabolism and body temperature regulation may contribute to migratory fattening. Although control of appetite is similar in birds and mammals, neuroendocrine mechanisms regulating seasonal changes in fuel store set-points in migrants remain poorly understood. Triacylglycerol of birds and bats contains mostly 16 and 18 carbon fatty acids with variable amounts of 18:2n-6 and 18:3n-3 depending on diet. Unsaturation of fat converges near 70% during migration, and unsaturated fatty acids are preferentially mobilized and oxidized, making them good fuel. Twenty and 22 carbon n-3 and n-6 polyunsaturated fatty acids (PUFA) may affect membrane function and peroxisome proliferator-activated receptor signaling. However, evidence for dietary PUFA as doping agents in migratory birds is equivocal and requires further study.

Lean, mean, lipolytic machines: lipid mobilization in rainbow trout during graded swimming

The mobilization of mammalian lipid reserves is strongly stimulated during exercise to reach a maximum at moderate intensities, but the effects of swimming speed on fish lipolysis have never been quantified. Continuous infusion of 2-[³H]glycerol was used to measure the rate of appearance of glycerol or lipolytic rate (R_a glycerol) in rainbow trout kept at rest, or during graded exercise in a swim tunnel up to critical swimming speed (U_crit). Results show that R_a glycerol is 1.67±0.18 µmol kg^-1 min^-1 in control animals, and remains at a steady level of 1.24±0.10 µmol kg^-1 min^-1 in exercising fish at all swimming intensities. Baseline lipolytic rate provides more than enough fatty acids from lipid reserves to accommodate all the oxidative fuel requirements for swimming at up to 2 body lengths per second (BL s^-1), and more than 50% of the energy needed at U_crit (3.4±0.1 BL s^-1). Such 'excess lipolysis' also means that trout sustain high rates of fatty acid reesterification. Maintaining steady lipolysis at rest and throughout graded swimming is strikingly different from mammals that stimulate R_a glycerol by twofold to fivefold to support exercise. Instead, trout act like 'lipolytic machines' that do not modulate R_a glycerol even when their metabolic rate triples - a strategy that eliminates the need to increase lipolytic rate during exercise. This study also supports the notion that maintaining a high rate of reesterification (or triacylglycerol/fatty acid cycling) may be a mechanism widely used by ectotherms to achieve rapid membrane remodelling in variable environments.

Lactate metabolism: historical context, prior misinterpretations, and current understanding

Lactate (La^-) has long been at the center of controversy in research, clinical, and athletic settings. Since its discovery in 1780, La^- has often been erroneously viewed as simply a hypoxic waste product with multiple deleterious effects. Not until the 1980s, with the introduction of the cell-to-cell lactate shuttle did a paradigm shift in our understanding of the role of La^- in metabolism begin. The evidence for La^- as a major player in the coordination of whole-body metabolism has since grown rapidly. La^- is a readily combusted fuel that is shuttled throughout the body, and it is a potent signal for angiogenesis irrespective of oxygen tension. Despite this, many fundamental discoveries about La^- are still working their way into mainstream research, clinical care, and practice. The purpose of this review is to synthesize current understanding of La^- metabolism via an appraisal of its robust experimental history, particularly in exercise physiology. That La^- production increases during dysoxia is beyond debate, but this condition is the exception rather than the rule. Fluctuations in blood [La^-] in health and disease are not typically due to low oxygen tension, a principle first demonstrated with exercise and now understood to varying degrees across disciplines. From its role in coordinating whole-body metabolism as a fuel to its role as a signaling molecule in tumors, the study of La^- metabolism continues to expand and holds potential for multiple clinical applications. This review highlights La^-'s central role in metabolism and amplifies our understanding of past research.

Cardiopulmonary Exercise Testing

Cardiopulmonary exercise testing (CPET) is an objective assessment of exercise capacity. It has become increasingly popular in clinical, research, and athletic performance settings. CPET allows for investigation of the cardiovascular, pulmonary, and skeletal muscle systems during exercise-induced stress. The main variable of maximal oxygen uptake (VO₂max) reflects the gold standard measure of exercise capacity. This chapter will describe the method of performing a graded maximal CPET with the Vmax 229 Cardiopulmonary Exercise Testing Instrument and CardioSoft program.

Sugar Metabolism in Hummingbirds and Nectar Bats

Hummingbirds and nectar bats coevolved with the plants they visit to feed on floral nectars rich in sugars. The extremely high metabolic costs imposed by small size and hovering flight in combination with reliance upon sugars as their main source of dietary calories resulted in convergent evolution of a suite of structural and functional traits. These allow high rates of aerobic energy metabolism in the flight muscles, fueled almost entirely by the oxidation of dietary sugars, during flight. High intestinal sucrase activities enable high rates of sucrose hydrolysis. Intestinal absorption of glucose and fructose occurs mainly through a paracellular pathway. In the fasted state, energy metabolism during flight relies on the oxidation of fat synthesized from previously-ingested sugar. During repeated bouts of hover-feeding, the enhanced digestive capacities, in combination with high capacities for sugar transport and oxidation in the flight muscles, allow the operation of the “sugar oxidation cascade”, the pathway by which dietary sugars are directly oxidized by flight muscles during exercise. It is suggested that the potentially harmful effects of nectar diets are prevented by locomotory exercise, just as in human hunter-gatherers who consume large quantities of honey.

Oxidative Phosphorylation System in Gastric Carcinomas and Gastritis

Switching of cellular energy production from oxidative phosphorylation (OXPHOS) by mitochondria to aerobic glycolysis occurs in many types of tumors. However, the significance of this switching for the development of gastric carcinoma and what connection it may have to Helicobacter pylori infection of the gut, a primary cause of gastric cancer, are poorly understood. Therefore, we investigated the expression of OXPHOS complexes in two types of human gastric carcinomas ("intestinal" and "diffuse"), bacterial gastritis with and without metaplasia, and chemically induced gastritis by using immunohistochemistry. Furthermore, we analyzed the effect of HP infection on several key mitochondrial proteins. Complex I expression was significantly reduced in intestinal type (but not diffuse) gastric carcinomas compared to adjacent control tissue, and the reduction was independent of HP infection. Significantly, higher complex I and complex II expression was present in large tumors. Furthermore, higher complex II and complex III protein levels were also obvious in grade 3 versus grade 2. No differences of OXPHOS complexes and markers of mitochondrial biogenesis were found between bacterially caused and chemically induced gastritis. Thus, intestinal gastric carcinomas, but not precancerous stages, are frequently characterized by loss of complex I, and this pathophysiology occurs independently of HP infection.

Mitochondrial Metabolism in a Large-Animal Model of Huntington Disease: The Hunt for Biomarkers in the Spermatozoa of Presymptomatic Minipigs

BACKGROUND

Huntington disease (HD) is a fatal neurodegenerative disorder involving reduced muscle coordination, mental and behavioral changes, and testicular degeneration. In order to further clarify the decreased fertility and penetration ability of the spermatozoa of transgenic HD minipig boars (TgHD), we applied a set of mitochondrial metabolism (MM) parameter measurements to this promising biological material, which can be collected noninvasively in longitudinal studies.

OBJECTIVE

We aimed to optimize methods for MM measurements in spermatozoa and to establish possible biomarkers of HD in TgHD spermatozoa expressing the N-terminal part of mutated human huntingtin.

METHODS

Semen samples from 12 TgHD and wild-type animals, aged 12-65 months, were obtained repeatedly during the study. Respiration was measured by polarography, MM was assessed by the detection of oxidation of radiolabeled substrates (mitochondrial energy-generating system; MEGS), and the content of the oxidative phosphorylation system subunits was detected by Western blot. Three possibly interfering factors were statistically analyzed: the effect of HD, generation and aging.

RESULTS

We found 5 MM parameters which were significantly diminished in TgHD spermatozoa and propose 3 specific MEGS incubations and complex I-dependent respiration as potential biomarkers of HD in TgHD spermatozoa.

CONCLUSIONS

Our results suggest a link between the gain of toxic function of mutated huntingtin in TgHD spermatozoa and the observed MM and/or glycolytic impairment. We determined 4 biomarkers useful for HD phenotyping and experimental therapy monitoring studies in TgHD minipigs.

Effect of resistant and digestible rice starches on human cytokine and lactate metabolic networks in serum

Resistant starch generated after treating ordinary starch is of great significance to human health in the countries with overnutrition. However, its functional evaluation in the human body has been rarely reported. By determining the lactate metabolic flux, 12 serum enzymes expression level and 38 serum cytokines in healthy volunteers, the variation in cytokine network and lactate metabolic network in serum were investigated to compare the mechanism of the physiological effects between the two starches. The results indicated that compared with digestible starch, resistant starch had anti-inflammatory effects, increased anabolism, and decreased catabolism. Further, the intercellular communication networks including cytokine and lactate metabolic networks were mapped out. The relationship suggested that resistant starch might affect and control the secretion of cytokines to regulate lactate metabolic network in the body, promoting the development of immunometabolism.

Lactic acidosis: an update

Lactate is one of the most crucial intermediates in carbohydrate and nonessential amino acid metabolism. The complexity of cellular interactions and metabolism means that lactate can be considered a waste product for one cell but a useful substrate for another. The presence of elevated lactate levels in critically ill patients has important implications for morbidity and mortality. In this review, we provide a brief outline of the metabolism of lactate, the pathophysiology of lactic acidosis, the clinical significance of D-lactate, the role of lactate measurement in acutely ill patients, the methods used to measure lactate in blood or plasma and some of the methodological issues related to interferences in these assays, especially in the case of ethylene glycol poisoning.

ADAPTAÇÃO DO MÚSCULO ESQUELÉTICO AO EXERCÍCIO FÍSICO: CONSIDERAÇÕES MOLECULARES E ENERGÉTICAS

RESUMO Os benefícios para a saúde e as adaptações fisiológicas ao exercício regular são amplamente conhecidos e, com o advento das ciências ômicas e moleculares, revelou-se uma complexa rede de vias de sinalização e moléculas reguladoras que coordenam a resposta adaptativa do músculo esquelético ao exercício. As mudanças orgânicas transientes, porém, são cumulativas no pós-exercício. Elas incluem, de forma principal, a transcrição de genes relacionados aos fatores regulatórios da miogênese, ao metabolismo de carboidratos, à mobilização de gorduras, ao transporte e oxidação de substratos, ao metabolismo mitocondrial através da fosforilação oxidativa e, por fim, à regulação transcricional de genes envolvidos na biogênese mitocondrial. Tendo em vista o grande impacto científico, resumiram-se neste trabalho, além de algumas das principais respostas moleculares sofridas pelo músculo esquelético com o exercício físico, fatores que coordenam a plasticidade muscular para o ganho de desempenho. Foram citadas dezenas de biomarcadores ligados a alguns aspectos moleculares das adaptações do músculo esquelético ao exercício físico, algumas principais vias sinalizadoras e o papel mitocondrial, revelando alguns novos paradigmas para o entendimento desta área científica.

Mitochondrial physiology in the skeletal and cardiac muscles is altered in torrent ducks, Merganetta armata, from high altitudes in the Andes

Torrent ducks inhabit fast-flowing rivers in the Andes from sea level to altitudes up to 4500 m. We examined the mitochondrial physiology that facilitates performance over this altitudinal cline by comparing the respiratory capacities of permeabilized fibers, the activities of 16 key metabolic enzymes and the myoglobin content in muscles between high- and low-altitude populations of this species. Mitochondrial respiratory capacities (assessed using substrates of mitochondrial complexes I, II and/or IV) were higher in highland ducks in the gastrocnemius muscle - the primary muscle used to support swimming and diving - but were similar between populations in the pectoralis muscle and the left ventricle. The heightened respiratory capacity in the gastrocnemius of highland ducks was associated with elevated activities of cytochrome oxidase, phosphofructokinase, pyruvate kinase and malate dehydrogenase (MDH). Although respiratory capacities were similar between populations in the other muscles, highland ducks had elevated activities of ATP synthase, lactate dehydrogenase, MDH, hydroxyacyl CoA dehydrogenase and creatine kinase in the left ventricle, and elevated MDH activity and myoglobin content in the pectoralis. Thus, although there was a significant increase in the oxidative capacity of the gastrocnemius in highland ducks, which correlates with improved performance at high altitudes, the variation in metabolic enzyme activities in other muscles not correlated to respiratory capacity, such as the consistent upregulation of MDH activity, may serve other functions that contribute to success at high altitudes.

Impaired glucose tolerance after brief heat exposure: a randomized crossover study in healthy young men

A high demand on thermoregulatory processes may challenge homoeostasis, particularly regarding glucose regulation. This has been understudied, although it might concern millions of humans. The objective of this project was to examine the isolated and combined effects of experimental short-term mild heat exposure and metabolic level on glucoregulation. Two experimental randomized crossover studies were conducted. Ten healthy young men participated in study A, which comprises four sessions in a fasting state at two metabolic levels [rest and exercise at 60% of maximal oxygen uptake (O2) for 40 min] in two environmental temperatures (warm: 31°C and control: 22°C). Each session ended with an ad libitum meal, resulting in similar energy intake across sessions. In study B, 12 healthy young men underwent two 3 h oral glucose tolerance tests (OGTTs) in warm and control environmental temperatures. Venous blood was sampled at several time points. In study A, repeated measure ANOVAs revealed higher postprandial serum glucose and insulin levels with heat exposure. Glycaemia following the OGTT was higher in the warm temperature compared with control. The kinetics of the serum glucose response to the glucose load was also affected by the environmental temperature (temperature-by-time interaction, P=0.030), with differences between the warm and control conditions observed up to 90 min after the glucose load (all P<0.033). These studies provide evidence that heat exposure alters short-term glucoregulation. The implication of this environmental factor in the physiopathology of Type 2 diabetes has yet to be investigated.

Graded Maximal Exercise Testing to Assess Mouse Cardio-Metabolic Phenotypes

Functional assessments of cardiovascular fitness (CVF) are needed to establish animal models of dysfunction, test the effects of novel therapeutics, and establish the cardio-metabolic phenotype of mice. In humans, the graded maximal exercise test (GXT) is a standardized diagnostic for assessing CVF and mortality risk. These tests, which consist of concurrent staged increases in running speed and inclination, provide diagnostic cardio-metabolic parameters, such as, VO_2max, anaerobic threshold, and metabolic crossover. Unlike the human-GXT, published mouse treadmill tests have set, not staged, increases in inclination as speed progress until exhaustion (PXT). Additionally, they often lack multiple cardio-metabolic parameters. Here, we developed a mouse-GXT with the intent of improving mouse-exercise testing sensitivity and developing translatable parameters to assess CVF in healthy and dysfunctional mice. The mouse-GXT, like the human-GXT, incorporated staged increases in inclination, speed, and intensity; and, was designed by considering imitations of the PXT and differences between human and mouse physiology. The mouse-GXT and PXTs were both tested in healthy mice (C57BL/6J, FVBN/J) to determine their ability to identify cardio-metabolic parameters (anaerobic threshold, VO_2max, metabolic crossover) observed in human-GXTs. Next, theses assays were tested on established diet-induced (obese-C57BL/6J) and genetic (cardiac isoform Casq2^-/-) models of cardiovascular dysfunction. Results showed that both tests reported VO_2max and provided reproducible data about performance. Only the mouse-GXT reproducibly identified anaerobic threshold, metabolic crossover, and detected impaired CVF in dysfunctional models. Our findings demonstrated that the mouse-GXT is a sensitive, non-invasive, and cost-effective method for assessing CVF in mice. This new test can be used as a functional assessment to determine the cardio-metabolic phenotype of various animal models or the effects of novel therapeutics.

Diacylglycerol kinase-δ regulates AMPK signaling, lipid metabolism, and skeletal muscle energetics

Decrease of AMPK-related signal transduction and insufficient lipid oxidation contributes to the pathogenesis of obesity and type 2 diabetes. Previously, we identified that diacylglycerol kinase-δ (DGKδ), an enzyme involved in triglyceride biosynthesis, is reduced in skeletal muscle from type 2 diabetic patients. Here, we tested the hypothesis that DGKδ plays a role in maintaining appropriate AMPK action in skeletal muscle and energetic aspects of contraction. Voluntary running activity was reduced in DGKδ(+/-) mice, but glycogen content and mitochondrial markers were unaltered, suggesting that DGKδ deficiency affects skeletal muscle energetics but not mitochondrial protein abundance. We next determined the role of DGKδ in AMPK-related signal transduction and lipid metabolism in isolated skeletal muscle. AMPK activation and signaling were reduced in DGKδ(+/-) mice, concomitant with impaired lipid oxidation and elevated incorporation of free fatty acids into triglycerides. Strikingly, DGKδ deficiency impaired work performance, as evident by altered force production and relaxation dynamics in response to repeated contractions. In conclusion, DGKδ deficiency impairs AMPK signaling and lipid metabolism, thereby highlighting the deleterious role of excessive lipid metabolites in the development of peripheral insulin resistance and type 2 diabetes pathogenesis. DGKδ deficiency also influences skeletal muscle energetics, which may lead to low physical activity levels in type 2 diabetes.

Coping with an exogenous glucose overload: glucose kinetics of rainbow trout during graded swimming

This study examines how chronically hyperglycemic rainbow trout modulate glucose kinetics in response to graded exercise up to critical swimming speed (Ucrit), with or without exogenous glucose supply. Our goals were 1) to quantify the rates of hepatic glucose production (Ra glucose) and disposal (Rd glucose) during graded swimming, 2) to determine how exogenous glucose affects the changes in glucose fluxes caused by exercise, and 3) to establish whether exogenous glucose modifies Ucrit or the cost of transport. Results show that graded swimming causes no change in Ra and Rd glucose at speeds below 2.5 body lengths per second (BL/s), but that glucose fluxes may be stimulated at the highest speeds. Excellent glucoregulation is also achieved at all exercise intensities. When exogenous glucose is supplied during exercise, trout suppress hepatic production from 16.4 ± 1.6 to 4.1 ± 1.7 μmol·kg(-1)·min(-1) and boost glucose disposal to 40.1 ± 13 μmol·kg(-1)·min(-1). These responses limit the effects of exogenous glucose to a 2.5-fold increase in glycemia, whereas fish showing no modulation of fluxes would reach dangerous levels of 114 mM of blood glucose. Exogenous glucose reduces metabolic rate by 16% and, therefore, causes total cost of transport to decrease accordingly. High glucose availability does not improve Ucrit because the fish are unable to take advantage of this extra fuel during maximal exercise and rely on tissue glycogen instead. In conclusion, trout have a remarkable ability to adjust glucose fluxes that allows them to cope with the cumulative stresses of a glucose overload and graded exercise.