Aerobic capacity and skeletal muscle properties of normoxic and hypoxic rats in response to training

Effect of hypobaric hypoxia on the fiber type transition of skeletal muscle: a synergistic therapy of exercise preconditioning with a nanocurcumin formulation

Hypobaric hypoxia (HH) leads to various adverse effects on skeletal muscles, including atrophy and reduced oxidative work capacity. However, the effects of HH on muscle fatigue resistance and myofiber remodeling are largely unexplored. Therefore, the present study aimed to explore the impact of HH on slow-oxidative fibers and to evaluate the ameliorative potential of exercise preconditioning and nanocurcumin formulation on muscle anti-fatigue ability. C2C12 cells (murine myoblasts) were used to assess the effect of hypoxia (0.5%, 24 h) with and without the nanocurcumin formulation (NCF) on myofiber phenotypic conversion. To further validate this hypothesis, male Sprague Dawley rats were exposed to a simulated HH (7620 m) for 7 days, along with NCF administration and/or exercise training. Both in vitro and in vivo studies revealed a significant reduction in slow-oxidative fibers (p < 0.01, 61% vs. normoxia control) under hypoxia. There was also a marked decrease in exhaustion time (p < 0.01, 65% vs. normoxia) in hypoxia control rats, indicating a reduced work capacity. Exercise preconditioning along with NCF supplementation significantly increased the slow-oxidative fiber proportion and exhaustion time while maintaining mitochondrial homeostasis. These findings suggest that HH leads to an increased transition of slow-oxidative fibers to fast glycolytic fibers and increased muscular fatigue. Administration of NCF in combination with exercise preconditioning restored this myofiber remodeling and improved muscle anti-fatigue ability.

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

Contractile Activity Is Necessary to Trigger Intermittent Hypobaric Hypoxia-Induced Fiber Size and Vascular Adaptations in Skeletal Muscle

Altitude training has become increasingly popular in recent decades. Its central and peripheral effects are well-described; however, few studies have analyzed the effects of intermittent hypobaric hypoxia (IHH) alone on skeletal muscle morphofunctionality. Here, we studied the effects of IHH on different myofiber morphofunctional parameters, investigating whether contractile activity is required to elicit hypoxia-induced adaptations in trained rats. Eighteen male Sprague-Dawley rats were trained 1 month and then divided into three groups: (1) rats in normobaria (trained normobaric inactive, TNI); (2) rats subjected daily to a 4-h exposure to hypobaric hypoxia equivalent to 4,000 m (trained hypobaric inactive, THI); and (3) rats subjected daily to a 4-h exposure to hypobaric hypoxia just before performing light exercise (trained hypobaric active, THA). After 2 weeks, the tibialis anterior muscle (TA) was excised. Muscle cross-sections were stained for: (1) succinate dehydrogenase to identify oxidative metabolism; (2) myosin-ATPase to identify slow- and fast-twitch fibers; and (3) endothelial-ATPase to stain capillaries. Fibers were classified as slow oxidative (SO), fast oxidative glycolytic (FOG), fast intermediate glycolytic (FIG) or fast glycolytic (FG) and the following parameters were measured: fiber cross-sectional area (FCSA), number of capillaries per fiber (NCF), NCF per 1,000 μm² of FCSA (CCA), fiber and capillary density (FD and CD), and the ratio between CD and FD (C/F). THI rats did not exhibit significant changes in most of the parameters, while THA animals showed reduced fiber size. Compared to TNI rats, FOG fibers from the lateral/medial fields, as well as FIG and FG fibers from the lateral region, had smaller FCSA in THA rats. Moreover, THA rats had increased NCF in FG fibers from all fields, in medial and posterior FIG fibers and in posterior FOG fibers. All fiber types from the three analyzed regions (except the posterior FG fibers) displayed a significantly increased CCA ratio compared to TNI rats. Global capillarisation was also increased in lateral and medial fields. Our results show that IHH alone does not induce alterations in the TA muscle. The inclusion of exercise immediately after the tested hypoxic conditions is enough to trigger a morphofunctional response that improves muscle capillarisation.

IGF-1 Attenuates Hypoxia-Induced Atrophy but Inhibits Myoglobin Expression in C2C12 Skeletal Muscle Myotubes

Chronic hypoxia is associated with muscle wasting and decreased oxidative capacity. By contrast, training under hypoxia may enhance hypertrophy and increase oxidative capacity as well as oxygen transport to the mitochondria, by increasing myoglobin (Mb) expression. The latter may be a feasible strategy to prevent atrophy under hypoxia and enhance an eventual hypertrophic response to anabolic stimulation. Mb expression may be further enhanced by lipid supplementation. We investigated individual and combined effects of hypoxia, insulin-like growth factor (IGF)-1 and lipids, in mouse skeletal muscle C2C12 myotubes. Differentiated C2C12 myotubes were cultured for 24 h under 20%, 5% and 2% oxygen with or without IGF-1 and/or lipid treatment. In culture under 20% oxygen, IGF-1 induced 51% hypertrophy. Hypertrophy was only 32% under 5% and abrogated under 2% oxygen. This was not explained by changes in expression of genes involved in contractile protein synthesis or degradation, suggesting a reduced rate of translation rather than of transcription. Myoglobin mRNA expression increased by 75% under 5% O₂ but decreased by 50% upon IGF-1 treatment under 20% O₂, compared to control. Inhibition of mammalian target of rapamycin (mTOR) activation using rapamycin restored Mb mRNA expression to control levels. Lipid supplementation had no effect on Mb gene expression. Thus, IGF-1-induced anabolic signaling can be a strategy to improve muscle size under mild hypoxia, but lowers Mb gene expression.

Interleukin-6 Signaling Pathway and Its Role in Kidney Disease: An Update

Interleukin-6 (IL-6) is a pleiotropic cytokine that not only regulates the immune and inflammatory response but also affects hematopoiesis, metabolism, and organ development. IL-6 can simultaneously elicit distinct or even contradictory physiopathological processes, which is likely discriminated by the cascades of signaling pathway, termed classic and trans-signaling. Besides playing several important physiological roles, dysregulated IL-6 has been demonstrated to underlie a number of autoimmune and inflammatory diseases, metabolic abnormalities, and malignancies. This review provides an overview of basic concept of IL-6 signaling pathway as well as the interplay between IL-6 and renal-resident cells, including podocytes, mesangial cells, endothelial cells, and tubular epithelial cells. Additionally, we summarize the roles of IL-6 in several renal diseases, such as IgA nephropathy, lupus nephritis, diabetic nephropathy, acute kidney injury, and chronic kidney disease.

Alterations to mitochondrial fatty-acid use in skeletal muscle after chronic exposure to hypoxia depend on metabolic phenotype

We investigated the effects of chronic hypoxia on the maximal use of and sensitivity of mitochondria to different substrates in rat slow-oxidative (soleus, SOL) and fast-glycolytic (extensor digitorum longus, EDL) muscles. We studied mitochondrial respiration in situ in permeabilized myofibers, using pyruvate, octanoate, palmitoyl-carnitine (PC), or palmitoyl-coenzyme A (PCoA). The hypophagia induced by hypoxia may also alter metabolism. Therefore, we used a group of pair-fed rats (reproducing the same caloric restriction, as observed in hypoxic animals), in addition to the normoxic control fed ad libitum. The resting respiratory exchange ratio decreased after 21 days of exposure to hypobaric hypoxia (simulated elevation of 5,500 m). The respiration supported by pyruvate and octanoate were unaffected. In contrast, the maximal oxidative respiratory rate for PCoA, the transport of which depends on carnitine palmitoyltransferase 1 (CPT-1), decreased in the rapid-glycolytic EDL and increased in the slow-oxidative SOL, although hypoxia improved affinity for this substrate in both muscle types. PC and PCoA were oxidized similarly in normoxic EDL, whereas chronic hypoxia limited transport at the CPT-1 step in this muscle. The effects of hypoxia were mediated by caloric restriction in the SOL and by hypoxia itself in the EDL. We conclude that improvements in mitochondrial affinity for PCoA, a physiological long-chain fatty acid, would facilitate fatty-acid use at rest after chronic hypoxia independently of quantitative alterations of mitochondria. Conversely, decreasing the maximal oxidation of PCoA in fast-glycolytic muscles would limit fatty-acid use during exercise.

NEW & NOTEWORTHY Affinity for low concentrations of long-chain fatty acids (LCFA) in mitochondria skeletal muscles increases after chronic hypoxia. Combined with a lower respiratory exchange ratio, this suggests facility for fatty acid utilization at rest. This fuel preference is related to caloric restriction in oxidative muscle and to hypoxia in glycolytic one. In contrast, maximal oxidation for LCFA is decreased by chronic hypoxia in glycolytic muscle and can explain glucose dependence at exercise.

Acclimation to hypoxia increases carbohydrate use during exercise in high-altitude deer mice

The low O₂ experienced at high altitude is a significant challenge to effective aerobic locomotion, as it requires sustained tissue O₂ delivery in addition to the appropriate allocation of metabolic substrates. Here, we tested whether high- and low-altitude deer mice (Peromyscus maniculatus) have evolved different acclimation responses to hypoxia with respect to muscle metabolism and fuel use during submaximal exercise. Using F₁ generation high- and low-altitude deer mice that were born and raised in common conditions, we assessed 1) fuel use during exercise, 2) metabolic enzyme activities, and 3) gene expression for key transporters and enzymes in the gastrocnemius. After hypoxia acclimation, highland mice showed a significant increase in carbohydrate oxidation and higher relative reliance on this fuel during exercise at 75% maximal O₂ consumption. Compared with lowland mice, highland mice had consistently higher activities of oxidative and fatty acid oxidation enzymes in the gastrocnemius. In contrast, only after hypoxia acclimation did activities of hexokinase increase significantly in the muscle of highland mice to levels greater than lowland mice. Highland mice also responded to acclimation with increases in muscle gene expression for hexokinase 1 and 2 genes, whereas both populations increased mRNA expression for glucose transporters. Changes in skeletal muscle with acclimation suggest that highland mice had an increased capacity for the uptake and oxidation of circulatory glucose. Our results demonstrate that highland mice have evolved a distinct mode of hypoxia acclimation that involves an increase in carbohydrate use during exercise.

Diaphragm Muscle Adaptation to Sustained Hypoxia: Lessons from Animal Models with Relevance to High Altitude and Chronic Respiratory Diseases

The diaphragm is the primary inspiratory pump muscle of breathing. Notwithstanding its critical role in pulmonary ventilation, the diaphragm like other striated muscles is malleable in response to physiological and pathophysiological stressors, with potential implications for the maintenance of respiratory homeostasis. This review considers hypoxic adaptation of the diaphragm muscle, with a focus on functional, structural, and metabolic remodeling relevant to conditions such as high altitude and chronic respiratory disease. On the basis of emerging data in animal models, we posit that hypoxia is a significant driver of respiratory muscle plasticity, with evidence suggestive of both compensatory and deleterious adaptations in conditions of sustained exposure to low oxygen. Cellular strategies driving diaphragm remodeling during exposure to sustained hypoxia appear to confer hypoxic tolerance at the expense of peak force-generating capacity, a key functional parameter that correlates with patient morbidity and mortality. Changes include, but are not limited to: redox-dependent activation of hypoxia-inducible factor (HIF) and MAP kinases; time-dependent carbonylation of key metabolic and functional proteins; decreased mitochondrial respiration; activation of atrophic signaling and increased proteolysis; and altered functional performance. Diaphragm muscle weakness may be a signature effect of sustained hypoxic exposure. We discuss the putative role of reactive oxygen species as mediators of both advantageous and disadvantageous adaptations of diaphragm muscle to sustained hypoxia, and the role of antioxidants in mitigating adverse effects of chronic hypoxic stress on respiratory muscle function.

Alterations in Skeletal Muscle Oxidative Phenotype in Mice Exposed to 3 Weeks of Normobaric Hypoxia

Skeletal muscle of patients with chronic respiratory failure is prone to loss of muscle mass and oxidative phenotype. Tissue hypoxia has been associated with cachexia and emphysema in humans. Experimental research on the role of hypoxia in loss of muscle oxidative phenotype, however, has yielded inconsistent results. Animal studies are frequently performed in young animals, which may hinder translation to generally older aged patients. Therefore, in this study, we tested the hypothesis that hypoxia induces loss of skeletal muscle oxidative phenotype in a model of aged (52 weeks) mice exposed to 3 weeks of hypoxia. Additional groups of young (4 weeks) and adult (12 weeks) mice were included to examine age effects. To verify hypoxia-induced cachexia, fat pad and muscle weights as well as muscle fiber cross-sectional areas were determined. Muscle oxidative phenotype was assessed by expression and activity of markers of mitochondrial metabolism and fiber-type distribution. A profound loss of muscle and fat was indeed accompanied by a slightly lower expression of markers of muscle oxidative capacity in the aged hypoxic mice. In contrast, hypoxia-associated changes of fiber-type composition were more prominent in the young mice. The differential response of the muscle of young, adult, and aged mice to hypoxia suggests that age matters and that the aged mouse is a better model for translation of findings to elderly patients with chronic respiratory disease. Furthermore, the findings warrant further mechanistic research into putative accelerating effects of hypoxia-induced loss of oxidative phenotype on the cachexia process in chronic respiratory disease.

HIF-1-driven skeletal muscle adaptations to chronic hypoxia: molecular insights into muscle physiology

Skeletal muscle is a metabolically active tissue and the major body protein reservoir. Drop in ambient oxygen pressure likely results in a decrease in muscle cells oxygenation, reactive oxygen species (ROS) overproduction and stabilization of the oxygen-sensitive hypoxia-inducible factor (HIF)-1α. However, skeletal muscle seems to be quite resistant to hypoxia compared to other organs, probably because it is accustomed to hypoxic episodes during physical exercise. Few studies have observed HIF-1α accumulation in skeletal muscle during ambient hypoxia probably because of its transient stabilization. Nevertheless, skeletal muscle presents adaptations to hypoxia that fit with HIF-1 activation, although the exact contribution of HIF-2, I kappa B kinase and activating transcription factors, all potentially activated by hypoxia, needs to be determined. Metabolic alterations result in the inhibition of fatty acid oxidation, while activation of anaerobic glycolysis is less evident. Hypoxia causes mitochondrial remodeling and enhanced mitophagy that ultimately lead to a decrease in ROS production, and this acclimatization in turn contributes to HIF-1α destabilization. Likewise, hypoxia has structural consequences with muscle fiber atrophy due to mTOR-dependent inhibition of protein synthesis and transient activation of proteolysis. The decrease in muscle fiber area improves oxygen diffusion into muscle cells, while inhibition of protein synthesis, an ATP-consuming process, and reduction in muscle mass decreases energy demand. Amino acids released from muscle cells may also have protective and metabolic effects. Collectively, these results demonstrate that skeletal muscle copes with the energetic challenge imposed by O2 rarefaction via metabolic optimization.

Altered Oxygen Utilisation in Rat Left Ventricle and Soleus after 14 Days, but Not 2 Days, of Environmental Hypoxia

The effects of environmental hypoxia on cardiac and skeletal muscle metabolism are dependent on the duration and severity of hypoxic exposure, though factors which dictate the nature of the metabolic response to hypoxia are poorly understood. We therefore set out to investigate the time-dependence of metabolic acclimatisation to hypoxia in rat cardiac and skeletal muscle. Rats were housed under normoxic conditions, or exposed to short-term (2 d) or sustained (14 d) hypoxia (10% O₂), after which samples were obtained from the left ventricle of the heart and the soleus for assessment of metabolic regulation and mitochondrial function. Mass-corrected maximal oxidative phosphorylation was 20% lower in the left ventricle following sustained but not short-term hypoxia, though no change was observed in the soleus. After sustained hypoxia, the ratio of octanoyl carnitine- to pyruvate- supported respiration was 11% and 12% lower in the left ventricle and soleus, respectively, whilst hexokinase activity increased by 33% and 2.1-fold in these tissues. mRNA levels of PPARα targets fell after sustained hypoxia in both tissues, but those of PPARα remained unchanged. Despite decreased Ucp3 expression after short-term hypoxia, UCP3 protein levels and mitochondrial coupling remained unchanged. Protein carbonylation was 40% higher after short-term but not sustained hypoxic exposure in the left ventricle, but was unchanged in the soleus at both timepoints. Our findings therefore demonstrate that 14 days, but not 2 days, of hypoxia induces a loss of oxidative capacity in the left ventricle but not the soleus, and a substrate switch away from fatty acid oxidation in both tissues.

Skeletal muscle energy metabolism in environmental hypoxia: climbing towards consensus

Skeletal muscle undergoes metabolic remodelling in response to environmental hypoxia, yet aspects of this process remain controversial. Broadly, environmental hypoxia has been suggested to induce: (i) a loss of mitochondrial density; (ii) a substrate switch away from fatty acids and towards other substrates such as glucose, amino acids and ketone bodies; and (iii) a shift from aerobic to anaerobic metabolism. There remains a lack of a consensus in these areas, most likely as a consequence of the variations in degree and duration of hypoxic exposure, as well as the broad range of experimental parameters used as markers of metabolic processes. To attempt to resolve some of the controversies, we performed a comprehensive review of the literature pertaining to hypoxia-induced changes in skeletal muscle energy metabolism. We found evidence that mass-specific mitochondrial function is decreased prior to mass-specific mitochondrial density, implicating intra-mitochondrial changes in the response to environmental hypoxia. This loss of oxidative capacity does not appear to be matched by a loss of glycolytic capacity, which on the whole is not altered by environmental hypoxia. Environmental hypoxia does however induce a selective attenuation of fatty acid oxidation, whilst glucose uptake is maintained or increased, perhaps to support glycolysis in the face of a downregulation of oxidative metabolism, optimising the pathways of ATP synthesis for the hypoxic environment.

Increased oxidative metabolism and myoglobin expression in zebrafish muscle during chronic hypoxia

Fish may be extremely hypoxia resistant. We investigated how muscle fibre size and oxidative capacity in zebrafish (Danio rerio) adapt during severe chronic hypoxia. Zebrafish were kept for either 3 or 6 weeks under chronic constant hypoxia (CCH) (10% air/90%N2 saturated water). We analyzed cross-sectional area (CSA), succinate dehydrogenase (SDH) activity, capillarization, myonuclear density, myoglobin (Mb) concentration and Mb mRNA expression of high and low oxidative muscle fibres. After 3 weeks of CCH, CSA, SDH activity, Mb concentration, capillary and myonuclear density of both muscle fibre types were similar as under normoxia. In contrast, staining intensity for Mb mRNA of hypoxic high oxidative muscle fibres was 94% higher than that of normoxic controls (P<0.001). Between 3 and 6 weeks of CCH, CSA of high and low oxidative muscle fibres increased by 25 and 30%, respectively. This was similar to normoxic controls. Capillary and myonuclear density were not changed by CCH. However, in high oxidative muscle fibres of fish maintained under CCH, SDH activity, Mb concentration as well as Mb mRNA content were higher by 86%, 138% and 90%, respectively, than in muscle fibres of fish kept under normoxia (P<0.001). In low oxidative muscle fibres, SDH activity, Mb and Mb mRNA content were not significantly changed. Under normoxia, the calculated interstitial oxygen tension required to prevent anoxic cores in muscle fibres (PO2crit) of high oxidative muscle fibres was between 1.0 and 1.7 mmHg. These values were similar at 3 and 6 weeks CCH. We conclude that high oxidative skeletal muscle fibres of zebrafish continue to grow and increase oxidative capacity during CCH. Oxygen supply to mitochondria in these fibres may be facilitated by an increased Mb concentration, which is regulated by an increase in Mb mRNA content per myonucleus.

Hypoxia differentially regulates muscle oxidative fiber type and metabolism in a HIF-1α-dependent manner

Loss of skeletal muscle oxidative fiber types and mitochondrial capacity is a hallmark of chronic obstructive pulmonary disease and chronic heart failure. Based on in vivo human and animal studies, tissue hypoxia has been hypothesized as determinant, but the direct effect of hypoxia on muscle oxidative phenotype remains to be established. Hence, we determined the effect of hypoxia on in vitro cultured muscle cells, including gene and protein expression levels of mitochondrial components, myosin isoforms (reflecting slow-oxidative versus fast-glycolytic fibers), and the involvement of the regulatory PPAR/PGC-1α pathway. We found that hypoxia inhibits the PPAR/PGC-1α pathway and the expression of mitochondrial components through HIF-1α. However, in contrast to our hypothesis, hypoxia stimulated the expression of slow-oxidative type I myosin via HIF-1α. Collectively, this study shows that hypoxia differentially regulates contractile and metabolic components of muscle oxidative phenotype in a HIF-1α-dependent manner.

The different mechanisms of hypoxic acclimatization and adaptation in Lizard Phrynocephalus vlangalii living on Qinghai-Tibet Plateau

Phrynocephalus vlangalii is a species of lizard endemic in China, which lives on Qinghai-Tibet Plateau ranging from 2000 to 4600 m above sea level. In this study, P. vlangalii were collected from low altitude (2750 m) and high altitude (4564 m). The lizards from low altitude were acclimatized in simulated hypoxic chamber (equivalent to 4600 m) for 7, 15, and 30 days. The hematological parameters, heart weight, myocardial capillary density, and myocardial enzyme activities were examined. The results showed that acclimatization to hypoxia significantly increased hemoglobin concentration ([Hb]), hematocrit (Hct), heart weight (HW), heart weight to body mass (HW/BM), lactate dehydrogenase (LDH) activity, but markedly decreased mean corpuscular hemoglobin concentration (MCHC) and succinate dehydrogenase (SDH) activity. Red blood cell (RBC) count, body mass (BM), myocardial capillary density did not change markedly during hypoxic acclimatization. On the other hand, [Hb], Hct, MCHC, HW/BM, myocardium capillary density, and SDH activity of P. vlangalii from high altitude were remarkably higher than those from low-altitude; however, LDH activity of high-altitude P. vlangalii was lower than that of low-altitude lizards. There was no significant difference in HW or BM between populations of high-altitude and low-altitude. Based on the present data, we suggest that P. vlangalii has special anatomical, physiological, and biochemical accommodate mechanisms to live in hypoxic environment, and the regulative mechanisms are different between hypoxic acclimatization and adaptation.

Living at high altitude in combination with sea-level sprint training increases hematological parameters but does not improve performance in rats

The regimen of aerobic training at sea level with recovery at high altitude has been used by athletes to improve performance. However, little is known about the effects of hypoxia when combined with sprint interval training on performance. The aim of the present study was to determine the effect of a "living high-sprint training low" strategy on hemoglobin, hematocrit and erythropoietin levels in rats. We also wanted to test whether the addition of a hypoxic stress to the program of daily treadmill running at high speeds induces expressional adaptations in skeletal muscle and affects performance. The protein content of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), cytochrome C, pyruvate dehydrogenase kinase (PDK1), heat shock protein 70 (HSP70), manganese superoxide dismutase (MnSOD) and citrate synthase activity were determined in different muscle fiber types in our animals (red and white gastrocnemius muscle). We also determined the maximal aerobic velocity (MAV) before and after the training period. A total of 24 male Wistar rats (3 month old) were randomly divided into four experimental groups: the normoxic control group (n = 6), the normoxic trained group (n = 6), the hypoxic control group (12 h pO(2) 12%/12 h pO(2) 21%) (n = 6) and the hypoxic trained group (12 h pO(2) 12%/12 h pO(2) 21%). Living in normobaric hypoxia condition for 21 days significantly increased hemoglobin, hematocrit and erythropoietin levels in both the rest and the trained groups. The trained animals (normoxia and hypoxia) significantly increased their maximal aerobic velocity. No changes were found in the skeletal muscle in PGC-1α, cytochrome C, PDK1, HSP70, MnSOD protein content and in the citrate synthase activity in any experimental group. Regardless of whether it is combined with sprint interval training or not, after 21 days of living at high altitude we found a significant increase in the hematological values determined in our study. However, contrary to our starting hypothesis, the combination of normobaric hypoxia and sprint training did not improve MAV in our animals.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

The effect of chronically increased intra-abdominal pressure on rectus abdominis muscle histology an experimental study on rabbits

BACKGROUND

The aim of this study was to specify the histologic response of the rectus abdominis muscle of the rabbit, to the chronically increased intra-abdominal pressure.

MATERIALS AND METHODS

Forty-five New Zealand white rabbits were divided into three groups. In all groups, a rubber bag was implanted into the peritoneal cavity. In group A (n=15) the bags were kept empty. In group B (n=15) the bags were filled with normal saline in order to achieve an intra-abdominal pressure of over 12 mmHg. This pressure was kept at this level for 8 wk. In group C (n=15) the intra-abdominal rubber bags were filled with lead covered by silicone, equiponderant to the mean weight of the normal saline insufflated in group B. After 8 wk we took biopsies of the rectus abdominis muscle and counted the proportion of the different types of muscular fibers (type I, IIA, and IIB/X).

RESULTS

Significant difference was found in the proportion of the three types of muscle fibers. Intra-abdominal hypertension led to an increase in type I fibers (P=0.008). No difference was noticed between groups A and C.

CONCLUSIONS

The histologic response to the increased intra-abdominal pressure was an increase in type I muscle fibers. Charging with lead did not cause any significant change in the proportion of muscular fibers.

Different adaptations of alpha-actinin isoforms to exercise training in rat skeletal muscles

AIM

Alpha (alpha)-actinins are located in the skeletal muscle Z-line and form actin-actin cross-links. Mammalian skeletal muscle has two isoforms: alpha-actinin-2 and alpha-actinin-3. However, the response of alpha-actinin to exercise training is little understood. Therefore, the current study examined the effects of exercise training on the expression level of two alpha-actinin isoforms in skeletal muscles.

METHODS

Twelve male Wistar rats were assigned randomly to a control (C; n = 6) or exercise training (T; n = 6) group. After T animals were trained on an animal treadmill for 9 weeks, alpha-actinin-2 and alpha-actinin-3 levels in the plantaris, white and red gastrocnemius muscles were analysed. In addition, changes in the myosin heavy chain (MyHC) composition were assessed, and muscle bioenergetic enzyme activities were measured.

RESULTS

Results show that exercise training increased alpha-actinin-2 expression levels in all muscles (P < 0.05). However, no significant difference was found in alpha-actinin-3 expression levels between C and T animals. Subsequent MyHC analyses of all muscle showed an MyHC shift with direction from IIb to IIa. Furthermore, enzymatic analysis revealed that exercise training improved enzyme activities related to aerobic metabolism.

CONCLUSION

The results of this study demonstrate that exercise training alters the expression level of alpha-actinin at the isoform level. Moreover, the increase in expression levels of alpha-actinin-2 is apparently related to alteration of skeletal muscle: its aerobic capacity is improved.

Region-specific adaptations in determinants of rat skeletal muscle oxygenation to chronic hypoxia

Chronic exposure to hypoxia is associated with muscle atrophy (i.e., a reduction in muscle fiber cross-sectional area), reduced oxidative capacity, and capillary growth. It is controversial whether these changes are muscle and fiber type specific. We hypothesized that different regions of the same muscle would also respond differently to chronic hypoxia. To investigate this, we compared the deep (oxidative) and superficial (glycolytic) region of the plantaris muscle of eight male rats exposed to 4 wk of hypobaric hypoxia (410 mmHg, Po(2): 11.5 kPa) with those of nine normoxic rats. Hematocrit was higher in chronic hypoxic than control rats (59% vs. 50%, P < 0.001). Using histochemistry, we observed 10% fiber atrophy (P < 0.05) in both regions of the muscle but no shift in the fiber type composition and myoglobin concentration of the fibers. In hypoxic rats, succinate dehydrogenase (SDH) activity was elevated in fibers of each type in the superficial region (25%, P < 0.05) but not in the deep region, whereas in the deep region but not the superficial region the number of capillaries supplying a fiber was elevated (14%, P < 0.05). Model calculations showed that the region-specific alterations in fiber size, SDH activity, and capillary supply to a fiber prevented the occurrence of anoxic areas in the deep region but not in the superficial region. Inclusion of reported acclimatization-induced increases in mean capillary oxygen pressure attenuated the development of anoxic tissue areas in the superficial region of the muscle. We conclude that the determinants of tissue oxygenation show region-specific adaptations, resulting in a marked differential effect on tissue Po(2).

Cellular distribution of Hsp70 expression in rat skeletal muscles. Effects of moderate exercise training and chronic hypoxia

Rat hindlimb muscles constitutively express the inducible heat shock protein 72 (Hsp70), apparently in proportion to the slow myosin content. Since it remains controversial whether chronic Hsp70 expression reflects the overimposed stress, we investigated Hsp70 cellular distribution in fast muscles of the posterior rat hindlimb after (1) mild exercise training (up to 30 m/min treadmill run for 1 h/day), which induces a remodeling in fast fiber composition, or (2) prolonged exposure to normobaric hypoxia (10%O(2)), which does not affect fiber-type composition. Both conditions increased significantly protein Hsp70 levels in the skeletal muscle. Immunohistochemistry showed the labeling for Hsp70 in subsets of both slow/type 1 and fast/type 2A myofibers of control, sedentary, and normoxic rats. Endurance training increased about threefold the percentage of Hsp70-positive myofibers (P < 0.001), and changed the distribution of Hsp70 immunoreactivity, which involved a larger subset of both type 2A and intermediate type 2A/2X myofibers (P < 0.001) and vascular smooth muscle cells. Hypoxia induced Hsp70 immunoreactivity in smooth muscle cells of veins and did not increase the percentage of Hsp70-positive myofibers; however, sustained exposure to hypoxia affected the distribution of Hsp70 immunoreactivity, which appeared detectable in a very small subset of type 2A fibers, whereas it concentrated in type 1 myofibers (P < 0.05) together with the labeling for heme-oxygenase isoform 1, a marker of oxidative stress. Therefore, the chronic induction of Hsp70 expression in rat skeletal muscles is not obligatory related to the slow fiber phenotype but reveals the occurrence of a stress response.

Capillary supply, fibre types and fibre morphometry in rat tibialis anterior and diaphragm muscles after intermittent exposure to hypobaric hypoxia

Three groups of sedentary male rats were exposed to intermittent hypobaric hypoxia (IHH) for 22 days (4 h/day, 5 days/week) in a hypobaric chamber at a simulated altitude of 5,000 m. Tibialis anterior (TA) and diaphragm (DG) were removed at the end of the programme (H group), and 20 or 40 days later (P20 and P40 groups). A control group (C) was maintained at sea-level pressure and their TA and DG were compared to those of the experimental rats at the end of the IHH programme, and also 20 and 40 days later. We measured the fibre morphometry and capillaries of each muscle. Our results demonstrate that IHH does not change the fibre type composition (with reference to either their contractile or oxidative properties) for most muscle regions of the muscles analysed analysed. We found few significant differences in muscle capillarity and fibre morphometry for TA after IHH. However, IHH did induce some statistically significant changes in DG: capillary density of the H rats (736 capillaries/mm2) increased compared to C animals (610 capillaries/mm2). Although IHH did not change the fibre capillarization or morphometric parameters of fast fibre types, we observed reductions ranging from 7 to 13% in fibre area, perimeter and diffusion distances between C and H for slow fibres. Moreover, these morphometric changes accounted for increases of 10-20% in capillarization, fibre unit area and fibre unit perimeter. This indicates that SO fibres are more sensitive to IHH than both fast fibre types.

Effect of l-Carnitine Supplementation on Endurance Exercise in Normobaric/Normoxic and Hypobaric/Hypoxic Conditions

OBJECTIVE

To evaluate the effect of L-carnitine supplementation on improving endurance exercise in normobaric/normoxic and hypobaric/hypoxic environments.

METHODS

Six-week-endurance-trained male Sprague-Dawley rats (n = 24) were randomly divided into 2 groups: control and experimental; the latter group was supplemented with L-carnitine, administered orally in a dose of 100 mg x kg(-1) body weight. The animals were supplemented for 25 days under ambient normobaric/normoxic conditions and thereafter were exposed to 72 hours of hypobaric hypoxia equivalent to 6100 m. The supplementation was continued during the exposure. "Run to exhaustion" was recorded on day 1 (R1) (presupplementation) and on days 7 (R2), 14 (R3), 21 (R4), and 28 (R5, which followed the last 72 hours of hypoxic exposure) of supplementation. Food intake, body weight, and the biochemical measures of plasma glucose, total cholesterol, and high-density lipoprotein (HDL) cholesterol were recorded.

RESULTS

There was a significant improvement in endurance exercise, as indicated by an increase in run to exhaustion following L-carnitine supplementation under normobaric normoxia (36%-39%) and hypobaric hypoxia (50%). L-carnitine supplementation had no effect on plasma glucose levels either at sea level or after hypoxic exposure. Total cholesterol was decreased in normoxic and HDL cholesterol was increased in normoxic and hypoxic conditions, indicating a beneficial effect of exercise.

CONCLUSION

L-carnitine supplementation improved exercise endurance in rats exposed to normobaric normoxic and hypobaric hypoxic conditions. Such supplementation would be beneficial in delaying the onset of fatigue during prolonged exercise in both conditions, indicating its potentially beneficial use at high altitude.

Exercise and hypoxia: The role of the autonomic nervous system

The reduction in maximal oxygen consumption in hypoxia can be due to physiological factors, the relative importance of which depends on the degree of hypoxia: the reduction in inspired PO2, the impairment of lung gas exchange contributing to an exercise-induced decrease in arterial O(2) saturation, the reduction in maximal cardiac output and the limitation in tissue diffusion. This paper focuses on two aspects of this oxygen cascade. First, the decrease in heart rate at maximal exercise in prolonged exposure to hypoxia is discussed and the role of changes in the autonomous nervous system is emphasised. The desensitization of the beta-adrenergic pathway and the upregulation of the muscarinic pathway, both using G-protein systems, contribute to limit the myocardial O(2) consumption in face of reduced O(2) availability during maximal exercise in hypoxia. The changes in O(2) diffusion to the tissues are discussed in relation to the expression of hypoxia inducible factor (HIF-1alpha) and vascular endothelial growth factor (VEGF) and their possible changes induced by training and/or hypoxic exposure.

Fat to the fire: the regulation of lipid oxidation with exercise and environmental stress

Lipids are an important fuel for submaximal aerobic exercise. The ways in which lipid oxidation is regulated during locomotion is an area of active investigation. Indeed, the integration between cellular regulation of lipid metabolism and whole-body exercise performance is a fascinating but often overlooked research area. Additionally, the interaction between environmental stress, exercise, and lipid oxidation has not been sufficiently examined. There are many functional and structural steps as fatty acids are mobilized, transported, and oxidized in working muscle, which may serve either as regulatory points for responding to acute or chronic stimuli or as raw material for natural selection. At the whole-animal level, the partitioning of lipids and carbohydrates across exercise intensities is remarkably similar among mammals, which suggests that there is conservation in regulatory mechanisms. Conversely, the proportions of circulatory and intramuscular fuels differ between species and across exercise intensities. Responses to acute and chronic environmental stress likely involve the interaction of genetic and nongenetic changes in the fatty acid pathway. Determining which of these factors help regulate the fatty acid pathway and what impact they have on whole-animal lipid oxidation and performance is an important area of future research. Using an integrative approach to complete the information loop from gene to physiological function provides the most powerful mode of analysis.

Acclimatization to 4100 m does not change capillary density or mRNA expression of potential angiogenesis regulatory factors in human skeletal muscle

Increased skeletal muscle capillary density would be a logical adaptive mechanism to chronic hypoxic exposure. However, animal studies have yielded conflicting results, and human studies are sparse. Neoformation of capillaries is dependent on endothelial growth factors such as vascular endothelial growth factor (VEGF), a known target gene for hypoxia inducible factor 1 (HIF-1). We hypothesised that prolonged exposure to high altitude increases muscle capillary density and that this can be explained by an enhanced HIF-1alpha expression inducing an increase in VEGF expression. We measured mRNA levels and capillary density in muscle biopsies from vastus lateralis obtained in sea level residents (SLR; N=8) before and after 2 and 8 weeks of exposure to 4100 m altitude and in Bolivian Aymara high-altitude natives exposed to approximately 4100 m altitude (HAN; N=7). The expression of HIF-1alpha or VEGF mRNA was not changed with prolonged hypoxic exposure in SLR, and both genes were similarly expressed in SLR and HAN. In SLR, whole body mass, mean muscle fibre area and capillary to muscle fibre ratio remained unchanged during acclimatization. The capillary to fibre ratio was lower in HAN than in SLR (2.4+/-0.1 vs 3.6+/-0.2; P<0.05). In conclusion, human muscle VEGF mRNA expression and capillary density are not significantly increased by 8 weeks of exposure to high altitude and are not increased in Aymara high-altitude natives compared with sea level residents.

Matched adaptations of electrophysiological, physiological, and histological properties of skeletal muscles in response to chronic hypoxia

This study tried to differentiate the consequences of chronic hypoxia on the electrophysiological and physiological properties and the histological characteristics of slow and fast muscles in rats. Animals inhaled a 10% O(2) concentration for a 1-month period. Then, slow [soleus (SOL)] and fast [extensor digitorum longus (EDL)] muscles were analyzed in vitro by physiological and electrophysiological measurements and histological analyses. The results were compared to those obtained in corresponding muscles of an age-matched normoxic group. After exposure to hypoxia: (1) in SOL, there was a tendency to elevated F(max), a significant increase in twitch force and tetanic frequency and a shortening of M-wave duration, and a reduced percentage of type I fibres, whereas the proportion of type IIa fibres doubled; (2) in EDL, F(max) and tetanic frequency were lowered, the muscle became less resistant to fatigue, and the proportion of type IId/x fibres was halved. Then, after 1 month of hypoxia, in the SOL muscle, both the contractile and histological properties resemble those of a fast muscle. By contrast, the EDL became slower, despite its histology was modestly affected. Reduced muscle use in hypoxia could explain the tendency for deteriorating adaptations in EDL, and the faster properties of SOL could result from hypoxia-induced inhibition of the growth-related fast-to-slow shift in muscle fibre types.

Stimulation of HSP72 expression following ATP depletion and short-term exercise training in fast-twitch muscle

AIM

Previous data have reported increases in HSP72 expression in skeletal muscles after endurance training but the physiological and biochemical signals that induce HSP72 accumulation remain largely unknown. In this study, we tested the hypothesis that energy status is a key regulatory event for HSP72 accumulation in skeletal muscles.

METHODS

Reduction of high-energy phosphate levels was induced by supplementation with a creatine analogue, beta-guanidinopropionic acid (GPA) for 3 weeks while control rats received distilled water in the same conditions. Half of the animals were kept sedentary while the others were submitted to a short-term (2 weeks) training program on a treadmill (30 m min-1, 0% slope; 50-70 min day-1).

RESULTS

GPA supplementation resulted in a large drop ( approximately 50%) in adenosine triphosphate (ATP) level in both fast and slow muscles whether the animals were trained or remained sedentary. HSP72 level did not change with GPA alone, but the training-induced increase in HSP72 level was strongly enhanced by superimposition of GPA diet in fast but not in slow skeletal muscles. The changes in HSP72 level were not linked to changes in fibre typology and/or mitochondrial capacities.

CONCLUSIONS

The results of the present investigation indicate that levels of high-energy phosphate per se do not play a direct role in determining HSP72 level in skeletal muscles. However, during superimposition of training to GPA, then the adaptive strategy of fast-twitch muscle (e.g. plantaris) seems to be directed towards appearance of some properties of red, oxidative fibres (increase in oxidative capacities and HSP72 level).

Allometric scaling of maximal metabolic rate in mammals: muscle aerobic capacity as determinant factor

Maximal metabolic rate (MMR) of mammals scales differently from basal metabolic rate (BMR). This is first shown by scrutinizing data reported on exercise-induced Vo2 max in 34 eutherian mammalian species covering a body mass range of 7 g-500 kg. Vo2 max was found to scale with the 0.872 (+/-0.029, 95% confidence limits 0.813-0.932) power of body mass which is significantly different from the 3/4 power reported for basal metabolic rate. The aerobic scope is higher in athletic than non-athletic species, and it is also higher in large than in small species. Integrated structure-function studies on a subset of 11 species (body mass 20 g-450 kg) show that the variation of Vo2 max with body size is tightly associated with the aerobic capacity of the locomotor musculature: the scaling exponents for Vo2 max, the total volume of mitochondria, and the volume of capillaries are nearly identical. The higher Vo2 max of athletic species is tightly linked to proportionally larger mitochondrial and capillary volumes in animals of the same size class. As a result Vo2 max is linearly related to both total mitochondrial and capillary erythrocyte volumes. We conclude that the scaling of maximal metabolic rate is explained by features and mechanisms different from those determining basal metabolic rate.

Capillary-to-fiber ratio of hind limb muscles in the male Syrian golden hamster

The hamster has been the accepted model of emphysema since the 1970s, demonstrating disease-related effects on respiratory skeletal muscle. However, there is scant information available about the model's ability to replicate the peripheral skeletal muscle changes seen in human disease, such as alterations in capillarity. The present study described the capillary-to-fiber ratio (C/F) of normal hamster plantaris, gastrocnemius, and soleus muscles in eight animals. C/F was 1.72 +/- 0.38 for plantaris, 1.95 +/- 0.40 for gastrocnemius, and 2.22 +/- 0.43 for soleus. C/F of soleus was significantly greater (P < 0.01) than plantaris. The C/F of hamster hindlimb muscles varies from those seen in rat species, and having baseline data on hamsters makes it possible to determine the effects of emphysema on C/F in this model.

Myogenin induces higher oxidative capacity in pre-existing mouse muscle fibres after somatic DNA transfer

Muscle is a permanent tissue, and in the adult pronounced changes can occur in pre-existing fibres without the formation of new fibres. Thus, the mechanisms responsible for phenotype transformation in the adult might be distinct from mechanisms regulating muscle differentiation during muscle formation and growth. Myogenin is a muscle-specific, basic helix-loop-helix transcription factor that is important during early muscle differentiation. It is also expressed in the adult, where its role is unknown. In this study we have overexpressed myogenin in glycolytic fibres of normal adult mice by electroporation and single-cell intracellular injection of expression vectors. Myogenin had no effects on myosin heavy chain fibre type, but induced a considerable increase in succinate dehydrogenase and NADH dehydrogenase activity, with some type IIb fibres reaching the levels observed histochemically in normal type IIx and IIa fibres. mRNA levels for malate dehydrogenase were similarly altered. The size of the fibres overexpressing myogenin was reduced by 30-50 %. Thus, the transfected fibres acquired a phenotype reminiscent of the phenotype obtained by endurance training in man and other animals, with a higher oxidative capacity and smaller size. We conclude that myogenin can alter pre-existing glycolytic fibres in the intact adult animal.

Citrate synthase expression and enzyme activity after endurance training in cardiac and skeletal muscles

The present study was designed to examine the acute and chronic effects of endurance treadmill training on citrate synthase (CS) gene expression and enzymatic activity in rat skeletal and cardiac muscles. Adult rats were endurance trained for 8 wk on a treadmill. They were killed 1 h (T(1), n = 8) or 48 h (T(48), n = 8) after their last bout of exercise training. Eight rats were sedentary controls (C) during the training period. CS mRNA levels and enzymatic activities of the soleus and ventricle muscles were determined. Training resulted in higher CS mRNA levels in both the soleus muscles (21% increase in T(1); 18% increase in T(48), P < 0.05) and ventricle muscles (23% increase in T(1); 17% increase in T(48), P < 0.05) when compared with the C group. The CS enzyme activities were 42 (P < 0.01) and 25% (P < 0.01) greater in the soleus muscles of T(1) and T(48) groups, respectively, when compared with that of the C group. Soleus CS enzyme activity was significantly greater in the T(1) vs. T(48) groups (P < 0.05). However, no appreciable alterations in CS enzyme activities were observed in the ventricle muscles in both training groups. These findings suggest differential responses of skeletal and cardiac muscles in CS enzymatic activity but similar responses in CS gene expression at 1 and 48 h after the last session of endurance training. Moreover, our data support the existence of an acute effect of exercise on the training-induced elevation in CS activity in rat soleus but not ventricle muscles.

Developmental plasticity in aerobic performance in deer mice (Peromyscus maniculatus)

While several studies have examined the abiotic effects of altitude (low ambient temperatures and hypoxia) on the aerobic performance of small mammals, few have explored the effects of development and maturation at different altitudes on aerobic performance as adults. We examined the basal metabolism and aerobic performance of deer mice (Peromyscus maniculatus) under four different developmental and testing regimes: (1) reared (gestation through weaning) and tested at high altitude; (2) reared and tested at low altitude; (3) reared at low altitude and tested at high altitude after acclimation; and (4) reared at low altitude and tested in hypoxia without acclimation. We found that mice that developed and were tested at low altitudes had a higher aerobic capacity (both aerobic performance and basal metabolic rate) than those that developed, or were acclimated as adults, at high altitudes. In addition, we found that mice that developed at high altitude did not have a higher aerobic capacity than those that developed at low altitude and were acclimated to high altitude as adults. Both groups tested at high altitudes had higher hematocrits (% red blood cells) and hemoglobin than mice tested at low altitudes. Surprisingly, mice acclimated to low altitudes and given an instantaneous exposure to hypoxia did not suffer a depression in aerobic performance.

Effect of a carbohydrate supplement on feeding behaviour and exercise in rats exposed to hypobaric hypoxia

The effect of a carbohydrate supplement, offered as a diet option, on feeding behaviour, body weight gain, and endurance exercise was studied in rats exposed to hypobaric hypoxia. Male albino rats (n = 35) were randomly divided into 5 groups; hypoxic supplemented and control groups; normoxic supplemented and control groups, and an untreated control group. After treadmill training for 5 days, the hypoxic groups were exposed to simulated high altitude equivalent to 6960 m for 18 days continuously. Food and water intakes, body weight and endurance exercise were recorded before and during the exposure period. Blood glucose, insulin, muscle and liver glycogen were assayed at the end of the exposure period. Hypobaric hypoxia resulted in a significant decrease in food and water intake, and body weight, and reduced endurance exercise capacity compared to the basal and normoxic group values. The carbohydrate supplement did not ameliorate the hypoxia-induced loss in body weight, but however, significantly delayed the onset of fatigue during exercise in the supplemented rats compared to the hypoxic control group.

Changes in MCT 1, MCT 4, and LDH expression are tissue specific in rats after long-term hypobaric hypoxia

Little is known about the effect of chronic hypobaric hypoxia on the enzymes and transporters involved in lactate metabolism. We looked at the protein expression of monocarboxylate transporters MCT 1, MCT 2, and MCT 4, along with total lactate dehydrogenase (LDH) and LDH isozymes in skeletal muscle, cardiac muscle, and liver. Expression of these components of the lactate shuttle affects the ability to transport and oxidize lactate. We hypothesized that the expression of MCTs and LDH would increase after acclimation to high altitude (HA). The response to acclimation to HA was, however, tissue specific. In addition, the response was different in whole muscle (Mu) and mitochondria-enriched (Mi) fractions. Heart, soleus, and plantaris muscles showed the greatest response to HA. Acclimation resulted in a 34% increase in MCT 4 in heart and a decrease in MCT 1 (-47%) and MCT 4 (-47%) in plantaris Mu. In Mi fractions, the heart had an increase (+40%) and soleus a decrease (-40%) in LDH. HA also had a significant effect on the LDH isozyme composition of both the Mu and Mi fractions. Mitochondrial density was decreased in both the soleus (-17%) and plantaris (-44%) as a result of chronic hypoxia. We conclude that chronic hypoxia had a tissue-specific effect on MCTs and LDH (that form the lactate shuttle) but did not produce a consistent increase in these components in all tissues.

High-altitude acclimation increases the triacylglycerol/fatty acid cycle at rest and during exercise

High-altitude acclimation alters lipid metabolism during exercise, but it is unknown whether this involves changes in rates of lipolysis or reesterification, which form the triacylglycerol/fatty acid (TAG/FA) cycle. We combined indirect calorimetry with [2-(3)H]glycerol and [1-(14)C]palmitate infusions to simultaneously measure total lipid oxidation, lipolysis, and rate of appearance (R(a)) of nonesterified fatty acids (NEFA) in high-altitude-acclimated (HA) rats exercising at 60% maximal O(2) uptake (VO(2 max)). During exercise, relative total lipid oxidation (%VO(2)) equaled sea-level control (SL) values; however, acclimation greatly stimulated lipolysis (+75%) but had no effect on R(a) NEFA. As a result, TAG/FA cycling increased (+119%), due solely to an increase in recycling (+144%) within adipocytes. There was no change in either group in these variables with the transition from rest to exercise. We conclude that, in HA, 1) acclimation is a potent stimulator of lipolysis; 2) rats do not modify TAG/FA cycling with the transition to exercise; and 3) in normoxia, HA and SL derive the same fraction of their total energy from lipids and carbohydrates.

Skeletal muscle capillarity and angiogenic mRNA levels after exercise training in normoxia and chronic hypoxia

Gene expression of vascular endothelial growth factor (VEGF), and to a lesser extent of transforming growth factor-beta(1) (TGF-beta(1)) and basic fibroblast growth factor (bFGF), has been found to increase in rat skeletal muscle after a single exercise bout. In addition, acute hypoxia augments the VEGF mRNA response to exercise, which suggests that, if VEGF is important in muscle angiogenesis, hypoxic training might produce greater capillary growth than normoxic training. Therefore, we examined the effects of exercise training (treadmill running at the same absolute intensity) in normoxia and hypoxia (inspired O(2) fraction = 0.12) on rat skeletal muscle capillarity and on resting and postexercise gene expression of VEGF, its major receptors (flt-1 and flk-1), TGF-beta(1), and bFGF. Normoxic training did not alter basal or exercise-induced VEGF mRNA levels but produced a modest twofold increase in bFGF mRNA (P < 0.05). Rats trained in hypoxia exhibited an attenuated VEGF mRNA response to exercise (1.8-fold compared 3.4-fold with normoxic training; P < 0.05), absent TGF-beta(1) and flt-1 mRNA responses to exercise, and an approximately threefold (P < 0.05) decrease in bFGF mRNA levels. flk-1 mRNA levels were not significantly altered by either normoxic or hypoxic training. An increase in skeletal muscle capillarity was observed only in hypoxically trained rats. These data show that, whereas training in hypoxia potentiates the adaptive angiogenic response of skeletal muscle to a given absolute intensity of exercise, this was not evident in the gene expression of VEGF or its receptors when assessed at the end of training.

Relationship between capillary angiogenesis, fiber type, and fiber size in chronic systemic hypoxia

Whether chronic hypoxia causes angiogenesis in skeletal muscle is controversial. Male Wistar rats, 5--6 wk of age, were kept at constant 12% O(2) for 3 wk, and frozen sections of their postural soleus (SOL), phasic extensor digitorum longus (EDL), and tibialis anterior (TA) muscles were compared with those of normoxic controls. Capillary supply increased in SOL muscles [capillary-to-fiber ratio (C/F) = 2.55 +/- 0.09 hypoxia vs. 2.17 +/- 0.06 normoxia; capillary density (CD) = 942 +/- 14 hypoxia vs. 832 +/- 20 mm(-2) normoxia, P < 0.01] but not in EDL muscles (C/F = 1.44 +/- 0.04 hypoxia vs. 1.42 +/- 0.04 normoxia; CD = 876 +/- 52 hypoxia vs. 896 +/- 24 mm(-2) normoxia). The predominantly glycolytic cortex of TA muscles showed higher C/F after hypoxia (1.79 +/- 0.09 vs. 1.53 +/- 0.05 normoxia, P < 0.05), whereas the mainly oxidative TA core with smaller fibers showed no change in capillarity. The region of the SOL muscle with large-sized (mean fiber area 2,843 +/- 128 microm(2)) oxidative fibers (90% type I) had a higher C/F (by 30%) and CD (by 25%), whereas there was no angiogenesis in the region with sparse (76%) and smaller-sized (2,200 +/- 85 microm(2)) type I fibers. Thus systemic hypoxia differentially induces angiogenesis between and within hindlimb skeletal muscles, with fiber size contributing either directly (via a metabolic stimulus) or indirectly (via a mechanical stimulus) to the process.

Effects of Altitude and Temperature on Organ Phenotypic Plasticity Along an Altitudinal Gradient

Small mammals living in high-altitude environments must endure decreased ambient temperatures and hypoxic conditions relative to sea-level environments. Previously, it was noted that heart, lung and digestive tract masses and blood hematocrit increase along an altitudinal gradient in small mammals. Increases in digestive organ mass were attributed to lower ambient temperatures and greater food intake, and increases in lung mass and hematocrit were attributed to hypoxia, but these assumptions were not explicitly tested. In addition, it was not clear whether changes in heart and lung mass were a function of an increase in organ blood content or of an increase in organ tissue mass. We used captive deer mice (Peromyscus maniculatus sonoriensis) to determine the relative effects of ambient temperature and oxygen concentration (PO2) on organ mass and blood hematocrit along an altitudinal gradient. We also exsanguinated hearts and lungs to determine whether changes in mass were associated with the blood content or with increases in tissue mass. We found that small intestine mass was, as expected, correlated positively with energy intake and negatively with ambient temperature. Heart mass was also negatively correlated with temperature. Lung mass and hematocrit were, as expected, positively correlated with altitude (and PO2). Interestingly, the masses of both small intestine and kidney were negatively correlated with altitude. For kidney mass, this correlation was apparent in cold-exposed mice but not in warm-exposed mice. We also found that changes in both heart and lung mass were mainly a function of changes in tissue mass rather than blood content. These data show that different abiotic variables have different effects on organ masses at high altitude, but also that phenotypic plasticity in response to cold temperatures and low oxygen pressures at altitude is widespread across several different organ systems, suggesting a general elevated whole-body response.

Differential responses to chronic hypoxia and dietary restriction of aerobic capacity and enzyme levels in the rat myocardium

Chronic exposure of mammals to hypoxia induces a state of anorexia. We aimed to determine the role played by diet restriction in the alterations of myocardial energy metabolism occurring under chronic hypoxia in order to detect the specific effects of hypoxia per se. Adult female rats were exposed to normobaric hypoxia (FiO2 = 0.10) for three weeks; pair-fed rats, kept under normoxic conditions, received the same amount of food as hypoxic rats. The oxidative capacity of myocardial ventricles and some skeletal muscles was evaluated using permeabilized fibers. Several metabolic enzyme activities were measured on extracts from myocardium and soleus. Diet restriction increased the activity of lactate dehydrogenase in both ventricles while it augmented phosphofructokinase and pyruvate kinase activities only in the left ventricle and depressed the respiratory rate in the right ventricle only. Hypoxia per se induced a rise in hexokinase activity in all studied oxidative muscles and a fall of hydroxy-acyl CoA-dehydrogenase activity in both myocardial ventricles. The respiratory rate and the citrate synthase activities were unaffected by hypoxia. We conclude that chronic hypoxia per se leads to specific alterations in myocardial metabolism that could favor the use of exogenous glucose at the expense of free fatty acids without any change in the oxidative capacity.

Physiological adjustments and arteriolar remodelling within skeletal muscle during acclimation to chronic hypoxia in the rat

1. We have investigated the physiological and structural changes that occur in skeletal muscle vasculature during acclimation to chronic hypoxia in rats exposed to 12 % O2 in a hypoxic chamber for 7 or 18 days (7CH and 18CH rats, respectively) and in age-matched normoxic (7N and 18N) rats. 2. Under anaesthesia and breathing 12 % O2, 7CH and 18CH rats had lower arterial blood pressure (ABP) than 7N and 18N rats breathing air, but the haematocrit of the CH rats was increased so that their arterial O2 content equalled that of N rats. Blood flow recorded from the iliac or femoral artery and used to compute muscle vascular conductance (MVC: blood flow/ABP) showed that, in 18CH rats, MVC was comparable with that of 18N rats. 3. Maximal MVC induced by infusion of sodium nitroprusside (SNP) was used as an index of structural vascular conductance and compared with the MVC evoked by acute hypoxia (breathing 8 % O2). Hypoxia induced similar increases in MVC in 7N and 7CH rats and in 18N and 18CH rats, even though N rats were switched from air to 8 % O2 and CH rats were switched from 12 to 8 % O2. The MVCs attained with 8 % O2 and SNP were similar in 7N and 18N rats. However, the MVCs attained with 8 % O2 in 7CH and 18CH rats were only approximately 60 % of those evoked by SNP, while the MVC attained with SNP was greater in 18CH than in 18N rats. 4. Vascular casts of the spinotrapezius muscle analysed ex vivo showed that interbranch intervals along primary, secondary and terminal arterioles (22-50, 13-18 and 7-13 microm diameter, respectively) were 30-50 % shorter in 7CH and 18CH rats than in 7N and 18N rats. Further, the proportions of branches that were of the secondary and terminal arteriolar categories were increased such that the mean diameter of the branches was lower in 7CH than in 7N rats and lower in 18CH than in 18N rats. 5. These results indicate that arteriolar remodelling and angiogenesis occurs in skeletal muscle during acclimation to chronic hypoxia, beginning by the 7th day and progressing at least until the 18th day, so that the number of small arterioles and the functional size of the vascular bed is increased. We propose that these structural and functional changes enhance the ability of skeletal muscle to respond to acute hypoxia by facilitating the increase in vascular conductance, blood flow and thereby the O2 that can be delivered to muscle.

Myogenin induces a shift of enzyme activity from glycolytic to oxidative metabolism in muscles of transgenic mice

Physical training regulates muscle metabolic and contractile properties by altering gene expression. Electrical activity evoked in muscle fiber membrane during physical activity is crucial for such regulation, but the subsequent intracellular pathway is virtually unmapped. Here we investigate the ability of myogenin, a muscle-specific transcription factor strongly regulated by electrical activity, to alter muscle phenotype. Myogenin was overexpressed in transgenic mice using regulatory elements that confer strong expression confined to differentiated post-mitotic fast muscle fibers. In fast muscles from such mice, the activity levels of oxidative mitochondrial enzymes were elevated two- to threefold, whereas levels of glycolytic enzymes were reduced to levels 0.3-0.6 times those found in wild-type mice. Histochemical analysis shows widespread increases in mitochondrial components and glycogen accumulation. The changes in enzyme content were accompanied by a reduction in fiber size, such that many fibers acquired a size typical of oxidative fibers. No change in fiber type-specific myosin heavy chain isoform expression was observed. Changes in metabolic properties without changes in myosins are observed after moderate endurance training in mammals, including humans. Our data suggest that myogenin regulated by electrical activity may mediate effects of physical training on metabolic capacity in muscle.

Carbohydrate utilization during exercise after high-altitude acclimation: a new perspective

At high altitude (HA), carbohydrate (CHO) is thought to be the preferred fuel because of its higher yield of ATP per mole of O2. We used indirect calorimetry and D-[6-3H]glucose infusions to determine total CHO and circulatory glucose utilization during exercise in HA-acclimated and sea level (SL) rats. We hypothesized that the percent contribution of CHO to total metabolism (VO2) is determined by exercise intensity relative to an aerobic maximum (% VO2max). HA rats run under hypoxia (FIO2 = 0.12) showed a decrease in VO2max compared with SL (67.55 +/- 1.26 vs. 89.30 +/- 1.23 ml kg-1 min-1). When exercised at 60% of their respective VO2max, both groups showed the same relative use of CHO (38 +/- 3% and 38 +/- 5% of VO2, at the beginning of exercise, in HA and SL, respectively). In both HA and SL, circulatory glucose accounted for approximately 20% of VO2, the balance was provided by muscle glycogen (approximately 18% of VO2). After 20 min at a higher intensity of 80% VO2max, 54 +/- 5% (HA) and 59 +/- 4% (SL) of VO2 was accounted for by CHO. We conclude the following: (i) the relative contributions of total CHO, circulatory glucose, and muscle glycogen do not increase after HA acclimation because the O2-saving advantage of CHO is outweighed by limited CHO stores; and (ii) relative exercise intensity is the major determinant of metabolic fuel selection at HA, as well as at SL.

Effect of endurance training on mRNA expression of uncoupling proteins 1, 2, and 3 in the rat

Endurance exercise training has been shown to decrease diet-induced thermogenesis (DIT) in rats and humans. In rodents, most thermogenesis is thought to occur in brown adipose tissue via activation of the uncoupling protein-1 (UCP1) and in skeletal muscle. Since the level of UCP1 mRNA in rat BAT was reported to be unmodified by exercise training, the newly described uncoupling proteins UCP2 and UCP3 could be responsible for the decreased DIT in trained rats. UCP3 mRNA levels in endurance-trained rats were found to be reduced by 76% and 59% in tibialis anterior and soleus muscles, respectively. UCP2 mRNA levels were also decreased in tibialis anterior and in heart by 54% and 41%, respectively. Neither white adipose tissue UCP2 nor brown adipose tissue UCP1, UCP2, and UCP3 mRNA levels were modified. The results of this study show that a need for a higher metabolic efficiency is associated with decreased mRNA expression of the uncoupling proteins in skeletal and heart muscles, which would decrease energy dissipation in these tissues. The down-regulation of UCP3 and UCP2 expressions might also contribute to the rapid weight gain known to occur when exercise training ceased.