Intermittent hypobaric hypoxia combined with aerobic exercise improves muscle morphofunctional recovery after eccentric exercise to exhaustion in trained rats

Simulated altitude is medicine: intermittent exposure to hypobaric hypoxia and cold accelerates injured skeletal muscle recovery

Muscle injuries are the leading cause of sports casualties. Because of its high plasticity, skeletal muscle can respond to different stimuli to maintain and improve functionality. Intermittent hypobaric hypoxia (IHH) improves muscle oxygen delivery and utilization. Hypobaria coexists with cold in the biosphere, opening the possibility to consider the combined use of both environmental factors to achieve beneficial physiological adjustments. We studied the effects of IHH and cold exposure, separately and simultaneously, on muscle regeneration. Adult male rats were surgically injured in one gastrocnemius and randomly assigned to the following groups: (1) CTRL: passive recovery; (2) COLD: intermittently exposed to cold (4°C); (3) HYPO: submitted to IHH (4500 m); (4) COHY: exposed to intermittent simultaneous cold and hypoxia. Animals were subjected to these interventions for 4 h/day for 9 or 21 days. COLD and COHY rats showed faster muscle regeneration than CTRL, evidenced after 9 days at histological (dMHC-positive and centrally nucleated fibre reduction) and functional levels after 21 days. HYPO rats showed a full recovery from injury (at histological and functional levels) after 9 days, while COLD and COHY needed more time to induce a total functional recovery. IHH can be postulated as an anti-fibrotic treatment since it reduces collagen I deposition. The increase in the pSer473Akt/total Akt ratio observed after 9 days in COLD, HYPO and COHY, together with the increase in the pThr172AMPKα/total AMPKα ratio observed in the gastrocnemius of HYPO, provides clues to the molecular mechanisms involved in the improved muscle regeneration. KEY POINTS: Only intermittent hypobaric exposure accelerated muscle recovery as early as 9 days following injury at histological and functional levels. Injured muscles from animals treated with intermittent (4 h/day) cold, hypobaric hypoxia or a simultaneous combination of both stimuli regenerated histological structure and recovered muscle function 21 days after injury. The combination of cold and hypoxia showed a blunting effect as compared to hypoxia alone in the time course of the muscle recovery. The increased expression of the phosphorylated forms of Akt observed in all experimental groups could participate in the molecular cascade of events leading to a faster regeneration. The elevated levels of phosphorylated AMPKα in the HYPO group could play a key role in the modulation of the inflammatory response during the first steps of the muscle regeneration process.

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions

BACKGROUND

Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions.

METHODS

We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18-30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively.

RESULTS

Males were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males.

CONCLUSION

This increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.

Histomorphological and functional contralateral symmetry in the gastrocnemius muscles of the laboratory rat

It is usual in anatomical and physiological research to assess the effects of some intervention on extremities (e.g., training programmes or injury recovery protocols) using one muscle for the intervention and its contralateral as control. However, the existence of laterality (left‐handedness or right‐handedness) in athletes of different specialities is widely recognized. In rats, gastrocnemius is one of the muscles most widely used because of its importance in locomotion and high relative limb mass. Since we have not found studies reporting laterality assessment on the morphology and function in rat gastrocnemius, our study aimed to evaluate the fibre histochemical, morphometrical and muscle force contractile properties between right and left gastrocnemius of the laboratory rat. Fibre‐type proportion, fibre morphometrical measurements, muscle capillarization and muscle force properties were analysed in the right and left gastrocnemius of six male rats. No statistically significant differences (p = 0.265) were found in gastrocnemius to body weight ratio (‰) between right (6.55 ± 0.40) and left (6.49 ± 0.40) muscles. The muscles analysed showed a great degree of heterogeneity in fibre type distribution, having three clearly distinguished regions named red, mixed and white. In the three regions, there were no statistical differences in fibre type proportions between right and left gastrocnemius, as is indicated by the p‐values (from 0.203 to 0.941) obtained after running t‐Student paired tests for each fibre type. When analysing fibre cross‐sectional area, individual fibre capillarization and fibre circularity, no significant differences between right and left gastrocnemius in any of these morphometrical parameters were found in any muscle region or fibre type. Most of the p‐values (70%) resulting from running t‐Student paired tests were higher than 0.400, and the lowest p‐value was 0.115. Seemingly, global capillary and fibre densities were not statistically different between right and left sides in all muscle regions with p‐values ranging from 0.337 to 0.812. Force parameters normalized to gastrocnemius mass (mN g⁻¹) did not show any significant difference between right (PF = 74.0 ± 13.4, TF = 219.4 ± 13.0) and left (PF = 70.9 ± 10.7, TF = 213.0 ± 18.0) muscles with p = 0.623 (PF) and p = 0.514 (TF). Twitch time parameters (ms) also lacked significant differences between the two sides (CT: 43.4 ± 8.6 vs. 45.0 ± 14.3, p = 0.639; HRT: 77.6 ± 15.0 vs. 82.3 ± 25.3, p = 0.475). Finally, both muscles also showed similar (p = 0.718) fatigue properties. We did find an absence of laterality at the morphological and functional levels, which raises the possibility of using right and left gastrocnemius muscles interchangeably for experimental designs where one muscle is used to analyse data after a physiological intervention and its contralateral muscle plays the control role, thus allowing unbiased paired comparisons to derive accurate conclusions.

Elucidating the combined effect of intermittent hypoxia training and acetazolamide on hypoxia induced hematological and physiological changes

As the number of people travelling to altitude increases, the risk of life threatening medical emergencies also increases. It is important that we have effective strategies to minimize the risk of altitude illness. In this study, an attempt was made to investigate the combined effect of non-pharmacological (Intermittent hypoxia training; IHT) and pharmacological (acetazolamide; ACZ) intervention as a prophylactic strategy in order to minimize the risk of high altitude hypoxic related problems using rats as an animal model. Male Sprague Dawley rats were subjected to IHT for 4 h consecutively for 5 days at 12% FiO₂ under normobaric conditions with and without oral ACZ administration at 25 mg/kg body weight. Validation of the intervention was performed by exposing the rats to extreme hypoxia (EH) at 8% FiO₂ to further assess the effect of IHT and ACZ on hypoxic acclimatization. The principal findings of this study is that the combined effect of IHT and ACZ improves the arterial oxygenation by alterations in hemodynamics and in blood gasometry, thereby resulting into an increase in the oxygen carrying capacity of the blood with increase in SpO₂ (peripheral oxygen saturation). The present study showed that the combined effect of IHT with ACZ could be refined as a prophylactic measure for better outcomes during altitude ascent and rapid altitude acclimatization rather than IHT or ACZ alone.

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Combination of IHT with ACZ attenuates the inhibitory effect of respiratory alkalosis.

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It also minimizes pathological changes in hematology and blood gas.

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The strategy improved the oxygen carrying capacity of the blood by increasing SpO₂.

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Thus, it leads to ventilatory Acclimatization.

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The combined treatment could be refined as a prophylactic measure for altitude maladies.

Chronic intermittent hypobaric hypoxia attenuates ischemic limb injury by promoting angiogenesis in mice

This study aimed to evaluate the protective effect of chronic intermittent hypobaric hypoxia (CIHH) against limb ischemic injury. C57BL/6 mice were randomly divided into three groups: limb ischemic injury group (Ischemia, induced by ligation and excision of the left femoral artery), limb ischemia following CIHH pretreatment group (CIHH+Ischemia, simulated a 5000 m altitude hypoxia, 6 h per day for 28 days, before induction of hind-limb ischemia), and sham group (Sham). The blood flow in the mouse models of hind-limb ischemia was examined using laser doppler imaging. The functional and morphological performance of ischemic muscle was evaluated using contraction force and hematoxylin-eosin and Masson's trichrome staining. Angiogenesis was determined by immunohistochemistry staining of the endothelial markers CD31 and CD34. The protein expressions of angiogenesis-related genes were detected using Western blot assay. Chronic ischemia resulted in reduced blood perfusion, decreased contraction tension, and morphological destruction in gastrocnemius muscle. CIHH pretreatment increased the contractile force and muscle fiber diameter and decreased necrosis and fibrosis of the ischemic muscle. Also, CIHH significantly increased the density of CD31+ and CD34+ cells and promoted the expression of angiogenesis-related molecules in ischemic muscle. These data demonstrate that CIHH has a protective effect against chronic limb ischemia by promoting angiogenesis.

Physiological Effects of Intermittent Passive Exposure to Hypobaric Hypoxia and Cold in Rats

The benefits of intermittent hypobaric hypoxia (IHH) exposure for health and its potential use as a training tool are well-documented. However, since hypobaric hypoxia and cold are environmental factors always strongly associated in the biosphere, additive or synergistic adaptations could have evolved in animals’ genomes. For that reason, the aim of the present study was to investigate body composition and hematological and muscle morphofunctional responses to simultaneous intermittent exposure to hypoxia and cold. Adult male rats were randomly divided into four groups: (1) control, maintained in normoxia at 25°C (CTRL); (2) IHH exposed 4 h/day at 4,500 m (HYPO); (3) intermittent cold exposed 4 h/day at 4°C (COLD); and (4) simultaneously cold and hypoxia exposed (COHY). At the end of 9 and 21 days of exposure, blood was withdrawn and gastrocnemius (GAS) and tibialis anterior muscles, perigonadal and brown adipose tissue, diaphragm, and heart were excised. GAS transversal sections were stained for myofibrillar ATPase and succinate dehydrogenase for fiber typing and for endothelial ATPase to assess capillarization. Hypoxia-inducible factor 1α (HIF-1α), vascular endothelial growth factor (VEGF), and glucose transporter 1 (GLUT1) from GAS samples were semi-quantified by Western blotting. COLD and HYPO underwent physiological adjustments such as higher brown adipose tissue weight and increase in blood-related oxygen transport parameters, while avoiding some negative effects of chronic exposure to cold and hypoxia, such as body weight and muscle mass loss. COHY presented an additive erythropoietic response and was prevented from right ventricle hypertrophy. Intermittent cold exposure induced muscle angiogenesis, and IHH seems to indicate better muscle oxygenation through fiber area reduction.

Implication of gut microbiota in the physiology of rats intermittently exposed to cold and hypobaric hypoxia

This study examines the influence of intermittent exposure to cold, hypobaric hypoxia, and their combination, in gut microbiota and their metabolites in vivo, and explores their effects on the physiology of the host. Sprague-Dawley rats were exposed to cold (4°C), hypobaric hypoxia (462 torr), or both simultaneously, 4 h/day for 21 days. Biometrical and hematological parameters were monitored. Gut bacterial subgroups were evaluated by qPCR and short-chain fatty acids were determined by gas chromatography in caecum and feces. Cold increased brown adipose tissue, Clostridiales subpopulation and the concentration of butyric and isovaleric acids in caecum. Hypobaric hypoxia increased hemoglobin, red and white cell counts and Enterobacteriales, and reduced body and adipose tissues weights and Lactobacilliales. Cold plus hypobaric hypoxia counteracted the hypoxia-induced weight loss as well as the increase in white blood cells, while reducing the Bacteroidetes:Firmicutes ratio and normalizing the populations of Enterobacteriales and Lactobacilliales. In conclusion, intermittent cold and hypobaric hypoxia exposures by themselves modified some of the main physiological variables in vivo, while their combination kept the rats nearer to their basal status. The reduction of the Bacteroidetes:Firmicutes ratio and balanced populations of Enterobacteriales and Lactobacilliales in the gut may contribute to this effect.

Local Heat Therapy to Accelerate Recovery After Exercise-Induced Muscle Damage

The prolonged impairment in muscle strength, power, and fatigue resistance after eccentric exercise has been ascribed to a plethora of mechanisms, including delayed muscle refueling and microvascular and mitochondrial dysfunction. This review explores the hypothesis that local heat therapy hastens functional recovery after strenuous eccentric exercise by facilitating glycogen resynthesis, reversing vascular derangements, augmenting mitochondrial function, and stimulating muscle protein synthesis.

A three-criteria performance score for rats exercising on a running treadmill

In this study, we propose a novel three-criteria performance score to semiquantitatively classify the running style, the degree of involvement and compliance and the validity of electric shock count for rats exercising on a treadmill. Each score criterion has several style-marks that are based on the observational registry of male Sprague-Dawley rats running for 4-7 weeks. Each mark was given a score value that was averaged throughout a session-registry and resulting in a session score for each criterion, ranging from "0" score for a hypothetical "worst runner", to score "1" for a hypothetical "perfect runner" rat. We found significant differences throughout a training program, thus providing evidence of sufficient sensitivity of this score to reflect the individual evolution of performance improvement in exercise capacity due to training. We hypothesize that this score could be correlated with other physiological or metabolic parameters, thus refining research results and further helping researchers to reduce the number of experimental subjects.

Recent Data on Cellular Component Turnover: Focus on Adaptations to Physical Exercise

Significant progress has expanded our knowledge of the signaling pathways coordinating muscle protein turnover during various conditions including exercise. In this manuscript, the multiple mechanisms that govern the turnover of cellular components are reviewed, and their overall roles in adaptations to exercise training are discussed. Recent studies have highlighted the central role of the energy sensor (AMP)-activated protein kinase (AMPK), forkhead box class O subfamily protein (FOXO) transcription factors and the kinase mechanistic (or mammalian) target of rapamycin complex (MTOR) in the regulation of autophagy for organelle maintenance during exercise. A new cellular trafficking involving the lysosome was also revealed for full activation of MTOR and protein synthesis during recovery. Other emerging candidates have been found to be relevant in organelle turnover, especially Parkin and the mitochondrial E3 ubiquitin protein ligase (Mul1) pathways for mitochondrial turnover, and the glycerolipids diacylglycerol (DAG) for protein translation and FOXO regulation. Recent experiments with autophagy and mitophagy flux assessment have also provided important insights concerning mitochondrial turnover during ageing and chronic exercise. However, data in humans are often controversial and further investigations are needed to clarify the involvement of autophagy in exercise performed with additional stresses, such as hypoxia, and to understand the influence of exercise modality. Improving our knowledge of these pathways should help develop therapeutic ways to counteract muscle disorders in pathological conditions.

Recombinant human glucagon-like peptide-1 protects against chronic intermittent hypoxia by improving myocardial energy metabolism and mitochondrial biogenesis

BACKGROUND AND AIMS

Obstructive sleep apnea syndrome is a chronic disease associated with intermittent hypoxia (IH) and is an important risk factor for cardiovascular disease. Glucagon-like peptide (GLP-1) is a naturally occurring incretin used as a promising therapeutic agent in the treatment of acute myocardial infarction, dilated cardiomyopathy, and advanced heart failure. However, whether GLP-1 can protect against IH-induced cardiac injury is still unclear. Accordingly, in this study, we evaluated the effects of recombinant human GLP-1 (rhGLP-1) on cardiac health in mice.

METHODS

Mice were subjected to repetitive 5% O₂ for 30 s and 21% O₂ for 30 s, for a total of 8 h/day for 4 weeks. Subsequently, mice received subcutaneous injection of saline or rhGLP-1 (100 μg/kg, three times per day). Cardiac function, myocardial apoptosis and fibrosis, energy metabolism, and mitochondrial biogenesis were examined for evaluation of cardiac injury.

RESULTS

A reduction in diastolic function (E/A ratio) in mice exposed to IH was significantly reversed by rhGLP-1. IH induced marked cardiomyocyte apoptosis and myocardial fibrosis. Additionally, IH resulted in a shift from fatty acid to glucose metabolism in the myocardium with downregulation of peroxisome proliferator-activated receptor (PPAR) α and PPARγ. Moreover, IH caused a reduction in mitochondrial DNA (mtDNA) replication and transcription, together with reduced mtDNA content and impaired mitochondrial ultrastructure. These changes were abolished by rhGLP-1 via activation of PGC-1α and Akt signaling.

CONCLUSIONS

rhGLP-1 protects against IH-induced cardiac injury by improving myocardial energy metabolism and enhancing the early adaptive changes of mitochondrial biogenesis.

Impact of heat therapy on recovery after eccentric exercise in humans

The purpose of this study was to investigate the effects of heat therapy (HT) on functional recovery, the skeletal muscle expression of angiogenic factors, macrophage content, and capillarization after eccentric exercise in humans. Eleven untrained individuals (23.8 ± 0.6 yr) performed 300 bilateral maximal eccentric contractions of the knee extensors. One randomly selected thigh was treated with five daily 90-min sessions of HT, whereas the opposite thigh received a thermoneutral intervention. Peak isokinetic torque of the knee extensors was assessed at baseline and daily for 4 days and fatigue resistance was assessed at baseline and 1 and 4 days after the eccentric exercise session. Muscle biopsies were obtained 2 wk before and 1 and 5 days after the eccentric exercise bout. There were no differences between thighs in the overall recovery profile of peak torque. However, the thigh exposed to HT had greater fatigue resistance than the thigh exposed to the thermoneutral intervention. The change from baseline in mRNA expression of vascular endothelial growth factor (VEGF) was higher at day 1 in the thigh exposed to HT. Protein levels of VEGF and angiopoietin 1 were also significantly higher in the thigh treated with HT. The number of capillaries around type II fibers decreased similarly in both thighs at day 5. Exposure to HT had no impact on macrophage content. These results suggest that HT accelerates the recovery of fatigue resistance after eccentric exercise and promotes the expression of angiogenic factors in human skeletal muscle. NEW & NOTEWORTHY We investigated whether exposure to local heat therapy (HT) accelerates recovery after a bout of eccentric exercise in humans. Compared with a thermoneutral control intervention, HT improved fatigue resistance of the knee extensors and enhanced the expression of the angiogenic mediators vascular endothelial growth factor and angiopoietin 1. These results suggest that HT hastens functional recovery and enhances the expression of regulatory factors involved in muscle repair after eccentric exercise in humans.

Physiological and Biological Responses to Short-Term Intermittent Hypobaric Hypoxia Exposure: From Sports and Mountain Medicine to New Biomedical Applications

In recent years, the altitude acclimatization responses elicited by short-term intermittent exposure to hypoxia have been subject to renewed attention. The main goal of short-term intermittent hypobaric hypoxia exposure programs was originally to improve the aerobic capacity of athletes or to accelerate the altitude acclimatization response in alpinists, since such programs induce an increase in erythrocyte mass. Several model programs of intermittent exposure to hypoxia have presented efficiency with respect to this goal, without any of the inconveniences or negative consequences associated with permanent stays at moderate or high altitudes. Artificial intermittent exposure to normobaric hypoxia systems have seen a rapid rise in popularity among recreational and professional athletes, not only due to their unbeatable cost/efficiency ratio, but also because they help prevent common inconveniences associated with high-altitude stays such as social isolation, nutritional limitations, and other minor health and comfort-related annoyances. Today, intermittent exposure to hypobaric hypoxia is known to elicit other physiological response types in several organs and body systems. These responses range from alterations in the ventilatory pattern to modulation of the mitochondrial function. The central role played by hypoxia-inducible factor (HIF) in activating a signaling molecular cascade after hypoxia exposure is well known. Among these targets, several growth factors that upregulate the capillary bed by inducing angiogenesis and promoting oxidative metabolism merit special attention. Applying intermittent hypobaric hypoxia to promote the action of some molecules, such as angiogenic factors, could improve repair and recovery in many tissue types. This article uses a comprehensive approach to examine data obtained in recent years. We consider evidence collected from different tissues, including myocardial capillarization, skeletal muscle fiber types and fiber size changes induced by intermittent hypoxia exposure, and discuss the evidence that points to beneficial interventions in applied fields such as sport science. Short-term intermittent hypoxia may not only be useful for healthy people, but could also be considered a promising tool to be applied, with due caution, to some pathophysiological states.

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.