Skeletal Muscle Oxidative Metabolism Research Articles

Prolonged Exposure to Hypoxia and Microgravity

PURPOSE: Skeletal muscle oxidative metabolism was evaluated in 12 young males (age: 27±5 years [mean±SD]) prior to (CTRL) and following a 21-day horizontal bed rest (simulation of microgravity), carried out in normobaric normoxic (PIO2=133mmHg; NBR) and hypoxic (PIO2=90mmHg; HBR) environments. HBR was aimed to simulate planetary habitats. The effects of a 21-day hypoxic ambulatory confinement (PIO2=90mmHg; HAMB) were also assessed. The study was conducted at the Olympic Sports Centre Planica (Rateče, Slovenia), situated at an altitude of 940m. METHODS: Pulmonary O2 uptake (V’O2) and vastus lateralis fractional O2 extraction (changes in deoxygenated hemoglobin+myoglobin concentration, Δ[deoxy(Hb+Mb)]; near-infrared spectroscopy) were evaluated during an incremental one-leg knee extension exercise to the limit of tolerance, carried out in normoxic environment. Mitochondrial respiration was evaluated ex vivo by high-resolution respirometry in permeabilized vastus lateralis fibers. RESULTS: V’O2peak was lower (P<0.05) after NBR (0.98±0.13 L/min) and after HBR (0.96±0.17) vs. CTRL (1.05±0.15). Δ[deoxy(Hb+Mb)]peak decreased (P<0.05) after NBR (63±13% of limb ischemia) and after HBR (62±13) vs. CTRL (72±11). No changes were observed after HAMB, in both variables. Maximal ADP-stimulated mitochondrial respiration (OXPHOS) was lower (P<0.05) in all conditions (43±10 pmol/s/mg ww in NBR, 38±10 in HBR; 41±12 in HAMB) vs. CTRL (51±10). Non-phosphorylating resting respiration (LEAK) and respiratory control ratio (the ratio between OXPHOS and LEAK) were not significantly affected by any condition. CONCLUSIONS: The impairment of the mitochondrial oxidative potential, described after all conditions, was not evident in muscle oxidative function in vivo after HAMB, presumably for adaptive changes in O2 delivery, peripheral O2 diffusion, intramuscular matching of O2 delivery/O2 utilization. Both ex vivo and in vivo, hypoxia does not exacerbate, nor attenuate, the impairment of skeletal muscle oxidative metabolism described following bed rest alone. EU VII Framework Programme (PlanHab project, grant No. 284438).

The coactivator PGC-1α regulates skeletal muscle oxidative metabolism independently of the nuclear receptor PPARβ/δ in sedentary mice fed a regular chow diet.

Physical activity improves oxidative capacity and exerts therapeutic beneficial effects, particularly in the context of metabolic diseases. The peroxisome proliferator-activated receptor (PPAR) γ coactivator-1α (PGC-1α) and the nuclear receptor PPARβ/δ have both been independently discovered to play a pivotal role in the regulation of oxidative metabolism in skeletal muscle, though their interdependence remains unclear. Hence, our aim was to determine the functional interaction between these two factors in mouse skeletal muscle in vivo. Adult male control mice, PGC-1α muscle-specific transgenic (mTg) mice, PPARβ/δ muscle-specific knockout (mKO) mice and the combination PPARβ/δ mKO + PGC-1α mTg mice were studied under basal conditions and following PPARβ/δ agonist administration and acute exercise. Whole-body metabolism was assessed by indirect calorimetry and blood analysis, while magnetic resonance was used to measure body composition. Quantitative PCR and western blot were used to determine gene expression and intracellular signalling. The proportion of oxidative muscle fibre was determined by NADH staining. Agonist-induced PPARβ/δ activation was only disrupted by PPARβ/δ knockout. We also found that the disruption of the PGC-1α-PPARβ/δ axis did not affect whole-body metabolism under basal conditions. As expected, PGC-1α mTg mice exhibited higher exercise performance, peak oxygen consumption and lower blood lactate levels following exercise, though PPARβ/δ mKO + PGC-1α mTg mice showed a similar phenotype. Similarly, we found that PPARβ/δ was dispensable for PGC-1α-mediated enhancement of an oxidative phenotype in skeletal muscle. Collectively, these results indicate that PPARβ/δ is not an essential partner of PGC-1α in the control of skeletal muscle energy metabolism.

Activating HSP72 in rodent skeletal muscle increases mitochondrial number and oxidative capacity and decreases insulin resistance.

Induction of heat shock protein (HSP)72 protects against obesity-induced insulin resistance, but the underlying mechanisms are unknown. Here, we show that HSP72 plays a pivotal role in increasing skeletal muscle mitochondrial number and oxidative metabolism. Mice overexpressing HSP72 in skeletal muscle (HSP72Tg) and control wild-type (WT) mice were fed either a chow or high-fat diet (HFD). Despite a similar energy intake when HSP72Tg mice were compared with WT mice, the HFD increased body weight, intramuscular lipid accumulation (triacylglycerol and diacylglycerol but not ceramide), and severe glucose intolerance in WT mice alone. Whole-body VO2, fatty acid oxidation, and endurance running capacity were markedly increased in HSP72Tg mice. Moreover, HSP72Tg mice exhibited an increase in mitochondrial number. In addition, the HSP72 coinducer BGP-15, currently in human clinical trials for type 2 diabetes, also increased mitochondrial number and insulin sensitivity in a rat model of type 2 diabetes. Together, these data identify a novel role for activation of HSP72 in skeletal muscle. Thus, the increased oxidative metabolism associated with activation of HSP72 has potential clinical implications not only for type 2 diabetes but also for other disorders where mitochondrial function is compromised.

Metabolic adaptations in skeletal muscle, adipose tissue, and whole-body oxidative capacity in response to resistance training

The effects of resistance training on mitochondrial biogenesis and oxidative capacity in skeletal muscle are not fully characterized, and even less is known about alterations in adipose tissue. We aimed to investigate adaptations in oxidative metabolism in skeletal muscle and adipose tissue after 8weeks of heavy resistance training in apparently healthy young men. Expression of genes linked to oxidative metabolism in the skeletal muscle and adipose tissue was assessed before and after the training program. Body composition, peak oxygen uptake (VO2 peak), fat oxidation, activity of mitochondrial enzyme in muscle, and serum adiponectin levels were also determined before and after resistance training. In muscle, the expression of the genes AdipoR1 and COX4 increased after resistance training (9 and 13%, respectively), whereas the expression levels of the genes PGC-1α, SIRT1, TFAM, CPT1b, and FNDC5 did not change. In adipose tissue, the expression of the genes SIRT1 and CPT1b decreased after training (20 and 23%, respectively). There was an increase in lean mass (from 59.7±6.1 to 61.9±6.2kg), VO2 peak (from 49.7±5.5 to 56.3±5.0ml/kg/min), and fat oxidation (from 6.8±2.1 to 9.1±2.7mg/kg fat-free mass/min) after training, whereas serum adiponectin levels decreased significantly and enzyme activity of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase did not change. Despite significant increases in VO2 peak, fat oxidation, and lean mass following resistance training, the total effect on gene expression and enzyme activity linked to oxidative metabolism was moderate.

Sarcolipin orchestrates calcium signaling to regulate muscle metabolism (1162.1)

Sarcolipin(SLN) is 31 amino acid long protein that regulates the Sarcoplasmic reticulum calcium ATPase (SERCA) activity in skeletal and cardiac muscle. Recently, our lab identified that SLN plays an important role in muscle‐based thermogenesis. Mice lacking SLN are cold sensitive and susceptible to diet induced obesity. Interestingly SLN is induced in skeletal muscle during neonatal development, high fat diet, muscle dystrophies, and inflammation. Since these are the states of increased oxidative metabolism in skeletal muscle we hypothesize that SLN is recruited to maintain this oxidative phenotype. Here we show that SLN overexpressing muscle fibers have high resting calcium and increased recruitment of calcium mediated signaling when there is increased demand. To determine the mechanism regulating SLN induction and downstream signaling we made use of C2C12 cells. We show that inhibiting the calcium signaling pathways regulated by calcineurin downregulates the expression of SLN. Based on our data we propose that SLN forms a feedback loop with calcium mediated signaling pathways and plays an important role to support the oxidative metabolism in skeletal muscle.Grant Funding Source: Supported by NIH, MDA

PPARβ/δ is not required by PGC‐1α to enhance skeletal muscle oxidative metabolism (1164.3)

Impaired skeletal muscle metabolism is a hallmark of numerous, chronic metabolic diseases. Exercise training efficiently improves skeletal muscle metabolic fitness, increases mitochondrial biogenesis and exerts beneficial therapeutic effects. Aberrant expression of metabolic‐related genes in skeletal muscle is thought to contribute to the development and progression of such pathologies. The peroxisome proliferator‐activated receptor γ (PPARγ) coactivator‐1α (PGC‐1α) and the nuclear receptor PPARβ/δ have both been independently shown to increase skeletal muscle oxidative capacity and exercise performance. Therefore, current understanding is that the PGC‐1α‐PPARβ/δ axis plays a pivotal role in the regulation of skeletal muscle metabolism. However, the interplay and hierarchical regulation of these two proteins remains unclear, with limited research conducted in vivo. In the present study, we demonstrate that the disruption of the PGC‐1α‐PPARβ/δ axis in mouse skeletal muscle does not affect body composition, whole body metabolism and glucose homeostasis under basal conditions. Importantly, PPARβ/δ seems dispensable for most of the effects of PGC‐1α on VO2 and lactate metabolism during exercise as well as the enhancement of skeletal muscle oxidative metabolism. Therefore, these results indicate that PPARβ/δ is not an essential partner of PGC‐1α in the control of skeletal muscle remodeling.Grant Funding Source: Supported by the Swiss National Science Foundation

The “second wind” in McArdle's disease patients during a second bout of constant work rate submaximal exercise

Patients with McArdle's disease (McA) typically show the "second-wind" phenomenon, a sudden decrease in heart rate (HR) and an improved exercise tolerance occurring after a few minutes of exercise. In the present study, we investigated whether in McA a first bout of exercise determines a second wind during a second bout, separated by the first by a few minutes of recovery. Eight McA (44 ± 4 yr) and a control group of six mitochondrial myopathy patients (51 ± 6 yr) performed two repetitions (CWR1 and CWR2) of 6-min constant work rate exercise (∼50% of peak work rate) separated by 6-min (SHORT) or 18-min (LONG) recovery. Pulmonary O2 uptake (Vo2), HR, cardiac output, rates of perceived exertion, vastus lateralis oxygenation {changes in deoxygenated Hb and myoglobin Mb concentrations, Δ[deoxy(Hb+Mb)], by near-infrared spectroscopy} were determined. In McA, Vo2 (0.86 ± 0.2 vs. 0.95 ± 0.1 l/min), HR (113 ± 10 vs. 150 ± 13 beats/min), cardiac output (11.6 ± 0.6 vs. 15.0 ± 0.8 l/min), and rates of perceived exertion (11 ± 2 vs. 14 ± 3) were lower, whereas Δ[deoxy(Hb+Mb)] was higher (14.7 ± 2.3 vs. -0.1 ± 4.6%) in CWR2-SHORT vs. CWR1; the "overshoot" of Δ[deoxy(Hb+Mb)] and the "slow component" of Vo2 kinetics disappeared in CWR2-SHORT. No differences (vs. CWR1) were observed in McA during CWR2-LONG, or in mitochondrial myopathy patients during both CWR2-SHORT and -LONG. A second-wind phenomenon was observed in McA during the second of two consecutive 6-min constant-work rate submaximal exercises. The second wind was associated with changes of physiological variables, suggesting an enhanced skeletal muscle oxidative metabolism. The second wind was not described after a longer (18-min) recovery period.

A PGC-1α- and muscle fibre type-related decrease in markers of mitochondrial oxidative metabolism in skeletal muscle of humans with inherited insulin resistance

Insulin resistance in obesity and type 2 diabetes is related to abnormalities in mitochondrial oxidative phosphorylation (OxPhos) in skeletal muscle. We tested the hypothesis that mitochondrial oxidative metabolism is impaired in muscle of patients with inherited insulin resistance and defective insulin signalling. Skeletal muscle biopsies obtained from carriers (n = 6) of a mutation in the tyrosine kinase domain of the insulin receptor gene (INSR) and matched healthy controls (n = 15) were used for discovery-mode microarray-based transcriptional profiling combined with biological pathway analysis. Findings were validated by quantitative real-time PCR, immunoblotting and activity assays. In INSR mutation carriers, insulin resistance was associated with a coordinated downregulation of OxPhos genes in skeletal muscle. This was related to a 46% decrease in mRNA levels (p = 0.036) of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), and 25-50% lower protein content of OxPhos subunits encoded by mitochondrial (ND6, p = 0.042) and nuclear DNA (UQCRC1, p = 0.001; SDHA, p = 0.067; COX5A, p = 0.017 and ATP5B, p = 0.005), as well as reduced citrate synthase activity (p = 0.025). Moreover, mutation carriers showed a marked reduction in type 1 muscle fibres (35% vs 62%, p = 0.0005) and increased type 2a fibres (53% vs 32%; p = 0.002) compared with controls. There were no differences in protein content and phosphorylation of 5' AMP-activated protein kinase, p38 mitogen-activated protein kinase, Erk1 and Erk2. These data indicate that inherited insulin resistance coincides with reduced mitochondrial oxidative capacity in a PGC-1α- and muscle fibre type-related manner. Whether this co-existence is directly or indirectly related to insulin resistance remains to be elucidated.

Road to exercise mimetics: targeting nuclear receptors in skeletal muscle.

Skeletal muscle is the largest organ in the human body and is the major site for energy expenditure. It exhibits remarkable plasticity in response to physiological stimuli such as exercise. Physical exercise remodels skeletal muscle and enhances its capability to burn calories, which has been shown to be beneficial for many clinical conditions including the metabolic syndrome and cancer. Nuclear receptors (NRs) comprise a class of transcription factors found only in metazoans that regulate major biological processes such as reproduction, development, and metabolism. Recent studies have demonstrated crucial roles for NRs and their co-regulators in the regulation of skeletal muscle energy metabolism and exercise-induced muscle remodeling. While nothing can fully replace exercise, development of exercise mimetics that enhance or even substitute for the beneficial effects of physical exercise would be of great benefit. The unique property of NRs that allows modulation by endogenous or synthetic ligands makes them bona fide therapeutic targets. In this review, we present an overview of the current understanding of the role of NRs and their co-regulators in skeletal muscle oxidative metabolism and summarize recent progress in the development of exercise mimetics that target NRs and their co-regulators.

Functional crosstalk of PGC-1 coactivators and inflammation in skeletal muscle pathophysiology

Skeletal muscle is an organ involved in whole body movement and energy metabolism with the ability to dynamically adapt to different states of (dis-)use. At a molecular level, the peroxisome proliferator-activated receptor γ coactivators 1 (PGC-1s) are important mediators of oxidative metabolism in skeletal muscle and in other organs. Musculoskeletal disorders as well as obesity and its sequelae are associated with PGC-1 dysregulation in muscle with a concomitant local or systemic inflammatory reaction. In this review, we outline the function of PGC-1 coactivators in physiological and pathological conditions as well as the complex interplay of metabolic dysregulation and inflammation in obesity with special focus on skeletal muscle. We further put forward the hypothesis that, in this tissue, oxidative metabolism and inflammatory processes mutually antagonize each other. The nuclear factor κB (NF-κB) pathway thereby plays a key role in linking metabolic and inflammatory programs in muscle cells. We conclude this review with a perspective about the consequences of such a negative crosstalk on the immune system and the possibilities this opens for clinical applications.

TWEAK promotes exercise intolerance by decreasing skeletal muscle oxidative phosphorylation capacity

BackgroundProinflammatory cytokine tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and its receptor Fn14 are the major regulators of skeletal muscle mass in many catabolic conditions. However, their role in muscle metabolism remains largely unknown. In the present study, we investigated the role of TWEAK on exercise capacity and skeletal muscle mitochondrial content and oxidative metabolism.MethodsWe employed wild-type and TWEAK-knockout (KO) mice and primary myotube cultures and performed biochemical, bioenergetics, and morphometric assays to evaluate the effects of TWEAK on exercise tolerance and muscle mitochondrial function and angiogenesis.ResultsTWEAK-KO mice showed improved exercise tolerance compared to wild-type mice. Electron microscopy analysis showed that the abundance of subsarcolemmal and intermyofibrillar mitochondria is significantly increased in skeletal muscle of TWEAK-KO mice compared to wild-type mice. Furthermore, age-related loss in skeletal muscle oxidative capacity was rescued in TWEAK-KO mice. Expression of a key transcriptional regulator peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and several other molecules involved in oxidative metabolism were significantly higher in skeletal muscle of TWEAK-KO mice. Moreover, treatment of primary myotubes with soluble TWEAK inhibited the expression of PGC-1α and mitochondrial genes and decreased mitochondrial respiratory capacity. Deletion of TWEAK also improved angiogenesis and transcript levels of vascular endothelial growth factor in skeletal muscle of mice.ConclusionsThese results demonstrate that TWEAK decreases mitochondrial content and oxidative phosphorylation and inhibits angiogenesis in skeletal muscle. Neutralization of TWEAK is a potential approach for improving exercise capacity and oxidative metabolism in skeletal muscle.

Leucine treatment enhances oxidative capacity through complete carbohydrate oxidation and increased mitochondrial density in skeletal muscle cells

Leucine has been largely implicated for increasing muscle protein synthesis in addition to stimulating mitochondrial biosynthesis. Limited evidence is currently available on the effects and potential benefits of leucine treatment on skeletal muscle cell glycolytic and oxidative metabolism. This work identified the effects of leucine treatment on oxidative and glycolytic metabolism as well as metabolic rate of human and murine skeletal muscle cells. Human rhabdomyosarcoma cells (RD) and mouse myoblast cells (C2C12) were treated with leucine at either 100 or 500μM for 24 or 48h. Glycolytic metabolism was quantified by measuring extracellular acidification rate (ECAR) and oxidative metabolism was quantified by measuring oxygen consumption rate. Peroxisome proliferator-activated receptor coactivator 1 alpha (PGC-1α), an important stimulator of mitochondrial biosynthesis, was quantified using flow cytometry and verified by immunofluorescent confocal microscopy. Mitochondrial content was quantified using mitochondrial and cytochrome C staining measured by flow cytometry and confirmed with confocal microscopy. Treatment with leucine significantly increased both basal and peak oxidative metabolism in both cell models. Leucine treated cells also exhibited significantly greater mitochondrial proton leak, which is associated with heightened energy expenditure. Basal ECAR was significantly reduced in both cell models following leucine treatment, evidence of reduced lactate export and more complete carbohydrate oxidation. In addition, both PGC-1α and cytochrome C expression were significantly elevated in addition to mitochondrial content following 48h of leucine treatment. Our observations demonstrated few dose-dependent responses induced by leucine; however, leucine treatment did induce a significant dose-dependent expression of PGC-1α in both cell models. Interestingly, C2C12 cells treated with leucine exhibited dose-dependently reduced ATP content, while RD ATP content remain unchanged. Leucine presents a potent dietary constituent with low lethality with numerous beneficial effects for increasing oxidative preference and capacity in skeletal muscle. Our observations demonstrate that leucine can enhance oxidative capacity and carbohydrate oxidation efficiency, as well as verify previous observations of increased mitochondrial content.

Impaired quality of life in growth hormone–deficient adults is independent of the altered skeletal muscle oxidative metabolism found in conditions with peripheral fatigue

Growth hormone-deficient (GHD) adults often report impaired quality of life (QoL) - with fatigue, a key element. This deficit can improve following GH replacement. The basis of this response is unclear. Perturbations in skeletal muscle metabolism have been demonstrated in several conditions in which fatigue is a prominent symptom. We wished to define the role of skeletal muscle metabolism in the impaired QoL observed in patients with GHD. To compare in vivo skeletal muscle mitochondrial oxidative phosphorylation using phosphorus-31 magnetic resonance spectroscopy in matched untreated GHD adults, treated GHD adults and healthy volunteers. Twenty-two untreated GHD adults, 23 treated GHD adults and 20 healthy volunteers were recruited at a regional centre. All patients underwent assessment of muscle mitochondrial function (τ₁/₂ PCr) and proton handling using spectroscopy. Fasting biochemical analyses and anthropometric measurement were obtained. All patients completed the QoL-AGHDA and physical activity assessment (IPAQ) questionnaires. Untreated and treated GHD adults complained of significantly increased fatigue and an impaired QoL (P = 0·002) when compared to healthy controls. There was no difference in maximal mitochondrial function (P = 0·53) nor pH recovery (P = 0·38) of skeletal muscle between the three groups. Untreated GHD patients had significantly lower IGF-1 than both treated GHD and healthy volunteers (P < 0·001), but there was no association between τ₁/₂ PCr and serum IGF-1 (r = -0·13, P = 0·32). The impaired QoL seen in GHD adults is not associated with the skeletal muscle spectroscopic 'footprint' of altered mitochondrial oxidative function, anaerobic glycolysis or proton clearance that are a feature of several conditions in which fatigue is a prominent feature. These data suggest that the pathophysiology of fatigue and impaired QoL in GHD may have a significant central rather than peripheral (skeletal muscle) component.

Inhibition of Class I Histone Deacetylases Unveils a Mitochondrial Signature and Enhances Oxidative Metabolism in Skeletal Muscle and Adipose Tissue

Chromatin modifications are sensitive to environmental and nutritional stimuli. Abnormalities in epigenetic regulation are associated with metabolic disorders such as obesity and diabetes that are often linked with defects in oxidative metabolism. Here, we evaluated the potential of class-specific synthetic inhibitors of histone deacetylases (HDACs), central chromatin-remodeling enzymes, to ameliorate metabolic dysfunction. Cultured myotubes and primary brown adipocytes treated with a class I–specific HDAC inhibitor showed higher expression of Pgc-1α, increased mitochondrial biogenesis, and augmented oxygen consumption. Treatment of obese diabetic mice with a class I– but not a class II–selective HDAC inhibitor enhanced oxidative metabolism in skeletal muscle and adipose tissue and promoted energy expenditure, thus reducing body weight and glucose and insulin levels. These effects can be ascribed to increased Pgc-1α action in skeletal muscle and enhanced PPARγ/PGC-1α signaling in adipose tissue. In vivo ChIP experiments indicated that inhibition of HDAC3 may account for the beneficial effect of the class I–selective HDAC inhibitor. These results suggest that class I HDAC inhibitors may provide a pharmacologic approach to treating type 2 diabetes.

The Corepressor NCoR1 Antagonizes PGC-1α and Estrogen-Related Receptor α in the Regulation of Skeletal Muscle Function and Oxidative Metabolism

Skeletal muscle exhibits a high plasticity and accordingly can quickly adapt to different physiological and pathological stimuli by changing its phenotype largely through diverse epigenetic mechanisms. The nuclear receptor corepressor 1 (NCoR1) has the ability to mediate gene repression; however, its role in regulating biological programs in skeletal muscle is still poorly understood. We therefore studied the mechanistic and functional aspects of NCoR1 function in this tissue. NCoR1 muscle-specific knockout mice exhibited a 7.2% higher peak oxygen consumption (VO(2peak)), a 11% reduction in maximal isometric force, and increased ex vivo fatigue resistance during maximal stimulation. Interestingly, global gene expression analysis revealed a high overlap between the effects of NCoR1 deletion and peroxisome proliferator-activated receptor gamma (PPARγ) coactivator 1α (PGC-1α) overexpression on oxidative metabolism in muscle. Importantly, PPARβ/δ and estrogen-related receptor α (ERRα) were identified as common targets of NCoR1 and PGC-1α with opposing effects on the transcriptional activity of these nuclear receptors. In fact, the repressive effect of NCoR1 on oxidative phosphorylation gene expression specifically antagonizes PGC-1α-mediated coactivation of ERRα. We therefore delineated the molecular mechanism by which a transcriptional network controlled by corepressor and coactivator proteins determines the metabolic properties of skeletal muscle, thus representing a potential therapeutic target for metabolic diseases.

Diet Enriched with Korean Pine Nut Oil Improves Mitochondrial Oxidative Metabolism in Skeletal Muscle and Brown Adipose Tissue in Diet-Induced Obesity

In this study, we investigated effects of pine nut oil (PNO) on high-fat-diet (HFD)-induced obesity and metabolic dysfunction in skeletal muscle and brown adipose tissue (BAT). Male C57BL/6 mice were fed a HFD with 15% energy from lard and 30% energy from either soybean oil (SBO-HFD) or PNO (PNO-HFD) for 12 weeks. The PNO-HFD resulted in less weight gain and intramuscular lipid accumulation than the SBO-HFD and was accompanied by upregulation of transcripts and proteins related to oxidative metabolism and phosphorylation of AMP-activated protein kinase (AMPK), as well as molecules selectively expressed in type I and type IIa muscle fibers. In addition, uncoupling protein-1 was upregulated in BAT. These beneficial metabolic effects were partly associated with the dual ligand activity of pinolenic acid, which is abundant in PNO, for peroxisome proliferator-activated receptors α and δ. Our findings suggest that PNO may have potential as a dietary supplement for counteracting obesity and metabolic dysregulation.

Abstract 18701: Activation of PPAR Delta Signaling Corrects Impaired Skeletal Muscle Oxidative Metabolism in Heart Failure

Background Exercise intolerance in heart failure (HF) has been linked to impaired skeletal muscle oxidative capacity. PPARδ signaling regulates oxidative metabolism and exercise capacity. We hypothesized that decreased skeletal muscle oxidative capacity in HF can be reverted by PPARδ agonists. Methods Left ventricular (LV) dysfunction was induced in C57BL/6 mice by left coronary artery ligature and confirmed by echocardiography. At 11 weeks post-MI, mice received the PPARδ agonist GW501516 (5 mg/kg body weight per day for 14 days, n=6) or vehicle (con, n=6). Exercise function was tested by swimming tests. Following sacrifice, skeletal muscle was harvested for RT-PCR, fatty acid (FA) and glucose oxidation assays, and ATP tissue content analysis. Cell culture analysis of signaling pathways was performed in C2C12 myotubes. Results Cardiac function decreased following MI (fractional shortening: 15.9±6.% vs 29.9±6.6% in sham; p&lt;0.001). FA oxidation was reduced in quadriceps muscle in MI mice (2.5±1.0 vs 4.0±0.8 pmol/mg tissue in sham; p=0.04) with a trend in glucose oxidation (-27%; p=0.07). Muscle ATP content decreased following MI (3.3±1.7 vs 6.1±1.1 uM ATP/ug protein in sham; p&lt;0.01). Expression of metabolic marker genes CPT1 (-86%), PDK4 (72%) and PGC1a (-94%) decreased in muscle post-MI (all p&lt;0.05). 2 weeks GW501516 administration increased muscle FA oxidation in animals with LV dysfunction (3.9±1.6 vs 1.8±0.4 pmol oleic acid/mg tissue in con; p=0.04) and also increased CPT1 expression by 309±175% (p&lt;0.001). There was a non-significant increase (+11%) in swimming duration in the GW501516 group. Treatment of C2C12 myotubes with 5 µM GW501516 protected from TNFα (25 ng/ml)-dependent decrease in FA oxidation (129.2±2.5 vs 237.8±6.8 pmol oleic acid/mg protein in TNFα+ GW501516 ; p&lt;0.001) Further, GW501516 prevented TNFα -dependent decrease in CPT1 expression (255±39 vs 58±5 % in TNFα alone, p&lt;0.01). Conclusion Treatment with the PPARδ agonist GW501516 corrects the impaired skeletal muscle oxidative capacity and metabolism in mice with ischemic LV dysfunction. Pharmacologic activation of PPARδ signaling could be an attractive intervention to counteract the progressive skeletal muscle dysfunction in HF.

Skeletal Muscle Oxidative Metabolism in an Animal Model of Pulmonary Emphysema

Skeletal muscle dysfunction is a significant contributor to exercise limitation in pulmonary emphysema. This study investigated skeletal muscle oxidative metabolism before and after aerosol exposure to a long-acting β-agonist (LABA), such as formoterol, in the pallid mouse (B6.Cg-Pldnpa/J), which has a deficiency in serum α(1)-antitrypsin (α(1)-PI) and develops spontaneous pulmonary emphysema. C57 BL/6J and its congener pallid mice of 8-12 and 16 months of age were treated with vehicle or formoterol aerosol challenge for 120 seconds. Morphological and morphometric studies and evaluations of mitochondrial adenosine diphosphate-stimulated respiration and of cytochrome oxidase activity on skeletal muscle were performed. Moreover, the mtDNA content in skeletal muscle and the mediators linked to muscle mitochondrial function and biogenesis, as well as TNF-α and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), were also evaluated. The lungs of pallid mice at 12 and 16 months of age showed patchy areas of airspace enlargements, with the destruction of alveolar septa. No significant differences were observed in basal values of mitochondrial skeletal muscle oxidative processes between C57 BL/6J and pallid mice. Exposure to LABA significantly improved mitochondrial skeletal muscle oxidative processes in emphysematous mice, where the mtDNA content was significantly higher with respect to 8-month-old pallid mice. This effect was compared with a significant increase of PGC-1α in skeletal muscles of 16-month-old pallid mice, with no significant changes in TNF-α concentrations. In conclusion, in emphysematous mice that showed an increased mtDNA content, exposure to inhaled LABA can improve mitochondrial skeletal muscle oxidative processes. PGC-1α may serve as a possible mediator of this effect.

Lack of β2‐AR improves exercise capacity and skeletal muscle oxidative phenotype in mice

β(2)-adrenergic receptor (β(2)-AR) agonists have been used as ergogenics by athletes involved in training for strength and power in order to increase the muscle mass. Even though anabolic effects of β(2)-AR activation are highly recognized, less is known about the impact of β(2)-AR in endurance capacity. We presently used mice lacking β(2)-AR [β(2)-knockout (β(2) KO)] to investigate the role of β(2)-AR on exercise capacity and skeletal muscle metabolism and phenotype. β(2) KO mice and their wild-type controls (WT) were studied. Exercise tolerance, skeletal muscle fiber typing, capillary-to-fiber ratio, citrate synthase activity and glycogen content were evaluated. When compared with WT, β(2) KO mice displayed increased exercise capacity (61%) associated with higher percentage of oxidative fibers (21% and 129% of increase in soleus and plantaris muscles, respectively) and capillarity (31% and 20% of increase in soleus and plantaris muscles, respectively). In addition, β(2) KO mice presented increased skeletal muscle citrate synthase activity (10%) and succinate dehydrogenase staining. Likewise, glycogen content (53%) and periodic acid-Schiff staining (glycogen staining) were also increased in β(2) KO skeletal muscle. Altogether, these data provide evidence that disruption of β(2)-AR improves oxidative metabolism in skeletal muscle of β(2) KO mice and this is associated with increased exercise capacity.

Lack of functional effects of neuromuscular electrical stimulation on skeletal muscle oxidative metabolism in healthy humans

A recent study has demonstrated that neuromuscular electrical stimulation (NMES) determines, in vitro, a fast-to-slow shift in the metabolic profile of muscle fibers. The aim of the present study was to evaluate if, in the same subjects, these changes would translate, in vivo, into an enhanced skeletal muscle oxidative metabolism. Seven young men were tested (cycle ergometer) during incremental exercises up to voluntary exhaustion and moderate and heavy constant-load exercises (CLE). Measurements were carried out before and after an 8-wk training program by isometric bilateral NMES (quadriceps muscles), which induced an ∼25% increase in maximal isometric force. Breath-by-breath pulmonary O(2) uptake (Vo(2)) and vastus lateralis oxygenation indexes (by near-infrared spectroscopy) were determined. Skeletal muscle fractional O(2) extraction was estimated by near-infrared spectroscopy on the basis of changes in concentration of deoxygenated hemoglobin + myoglobin. Values obtained at exhaustion were considered "peak" values. The following functional evaluation variables were unaffected by NMES: peak Vo(2); gas exchange threshold; the Vo(2) vs. work rate relationship (O(2) cost of cycling); changes in concentration of deoxygenated hemoglobin + myoglobin vs. work rate relationship (related to the matching between O(2) delivery and Vo(2)); peak fractional O(2) extraction; Vo(2) kinetics (during moderate and heavy CLE) and the amplitude of its slow component (during heavy CLE). Thus NMES did not affect several variables of functional evaluation of skeletal muscle oxidative metabolism. Muscle hypertrophy induced by NMES could impair peripheral O(2) diffusion, possibly counterbalancing, in vivo, the fast-to-slow phenotypic changes that were observed in vitro, in a previous work, in the same subjects of the present study.