Morphological changes in rat hindlimb muscle fibres during recovery from disuse atrophy

Development of Connective Tissue Area Increases by Initial Impact With High-Intensity Exercise After Reloading in Rat Soleus Muscle

OBJECTIVE

The aim of the study is to clarify whether high-intensity exercise in atrophied skeletal muscle after reloading (1) increases the injury and promotes the development of the muscle connective tissue area in the soleus muscle and (2) delays muscle strength recovery.

DESIGN

Rats had their hindlimbs suspended for 14 days followed by hindlimb reloading and squat exercise of 30% or 70%-one-repetition maximum exercise until 14 days of reloading. The influences of reloading and exercise in muscles were examined by histological and immunofluorescence analyses based on the exercise load.

RESULTS

Seventy percent-one-repetition maximum exercise increased the extramyofiber area more than the control group and the central nuclear fiber number than the other groups after 7 days of reloading. Seventy percent-one-repetition maximum exercise resulted in a larger muscle connective tissue area than the control and reload groups after 14 days of reloading. Myogenin and M2-type macrophages after 7 days of reloading increased by reloading but were not increased by the exercise load.

CONCLUSIONS

Seventy percent-one-repetition maximum exercise to atrophied muscle increased injury early in postreloading and increased the muscle connective tissue area after 14 days of reloading, but evidence that it delays muscle strength recovery was lacking. Therefore, muscle connective tissue area induced by high-intensity exercise after reloading seems to be associated with initial exercise damage but not with repetition over 14 days.

Construction and Analysis of Disuse Atrophy Model of the Gastrocnemius Muscle in Chicken

Disuse muscle atrophy is identified as the physiological, biochemical, morphological, and functional changes during restricted movement, immobilization, or weightlessness. Although its internal mechanism has been extensively studied in mammals and was thought to be mainly related to oxidative stress, it was unclear whether it behaved consistently in non-mammals such as chickens. In this study, we tried to construct a disuse atrophy model of the gastrocnemius muscle in chickens by limb immobilization, and collected the gastrocnemius muscles of the fixed group and the control group for RNA sequencing. Through analysis of muscle loss, HE staining, immunohistochemistry, and oxidative stress level, we found that limb immobilization could lead to loss of muscle mass, decrease in muscle fiber diameter, decrease in the proportion of slow muscle fibers, and increase in the proportion of fast muscle fibers, and also cause elevated levels of oxidative stress. In addition, a total of 565 different expression genes (DEGs) were obtained by RNA sequencing, which was significantly enriched in the biological processes such as cell proliferation and apoptosis, reactive oxygen species metabolism, and fast and slow muscle fiber transformation, and it showed that the FOXO signaling pathway, closely related to muscle atrophy, was activated. In brief, we initially confirmed that limb immobilization could induce disuse atrophy of skeletal muscle, and oxidative stress was involved in the process of disuse muscle atrophy.

Twenty-one days of low-intensity eccentric training improve morphological characteristics and function of soleus muscles of mdx mice

Duchene muscular dystrophy (DMD) is caused by the absence of the protein dystrophin, which leads to muscle weakness, progressive degeneration, and eventually death due to respiratory failure. Low-intensity eccentric training (LIET) has been used as a rehabilitation method in skeletal muscles after disuse. Recently, LIET has also been used for rehabilitating dystrophic muscles, but its effects are still unclear. The purpose of this study was to investigate the effects of 21 days of LIET in dystrophic soleus muscle. Thirty-six male mdx mice were randomized into six groups (n = 6/each): mdx sedentary group; mdx training group-3 days; mdx training group-21 days; wild-type sedentary group; wild-type training group-3 days and wild-type training group-21 days. After the training sessions, animals were euthanized, and fragments of soleus muscles were removed for immunofluorescence and histological analyses, and measurements of active force and Ca²⁺ sensitivity of the contractile apparatus. Muscles of the mdx training group-21 days showed an improvement in morphological characteristics and an increase of active force when compared to the sedentary mdx group. The results show that LIET can improve the functionality of dystrophic soleus muscle in mice.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.

Bone muscle crosstalk targets muscle regeneration pathway regulated by core circadian transcriptional repressors DEC1 and DEC2

Deletion of proprotein convertase Mbtps1 in bone osteocytes leads to a significant postnatal increase in skeletal muscle size and contractile function, while causing only a 25% increase in stiffness in long bones. Concerns about leakiness in skeletal muscle were discounted since Cre recombinase expression does not account for our findings, and, Mbtps1 protein and mRNA is not deleted. Interestingly, the response of normal skeletal muscle to exercise and the regenerative response of skeletal muscle to the deletion of Mbtps1 in bone share some key regulatory features including a preference for slow twitch muscle fibers. In addition, transcriptional regulators PPAR, PGC-1α, LXR, and repressors DEC1 and DEC2 all occupy central positions within these two pathways. We hypothesize that the age-dependent muscle phenotype in Dmp1-Cre Mbtps1 cKO mice is due to bone→muscle crosstalk. Many of the myogenic genes altered in this larger and functionally improved muscle are regulated by circadian core transcriptional repressors DEC1 and DEC2, and furthermore, display a temporal coordination with Dec1 and Dec2 expression consistent with a regulatory co-dependency. These considerations lead us to propose that Dmp1-Cre Mbtps1 cKO osteocytes activate myogenesis by increased release of an activator of muscle PPAR-gamma, for example, PGE₂ or sphingosine-1-P, or, by diminished release of an inhibitor of LXR, for example, long-chain polyunsaturated fatty acids. We hope that further investigation of these interacting pathways in the Dmp1-Cre Mbtps1 cKO model will lead to clinically translatable findings applicable to age-related sarcopenia and other muscle wasting syndromes.

Time-Course of Muscle Mass Loss, Damage, and Proteolysis in Gastrocnemius following Unloading and Reloading: Implications in Chronic Diseases

BACKGROUND

Disuse muscle atrophy is a major comorbidity in patients with chronic diseases including cancer. We sought to explore the kinetics of molecular mechanisms shown to be involved in muscle mass loss throughout time in a mouse model of disuse muscle atrophy and recovery following immobilization.

METHODS

Body and muscle weights, grip strength, muscle phenotype (fiber type composition and morphometry and muscle structural alterations), proteolysis, contractile proteins, systemic troponin I, and mitochondrial content were assessed in gastrocnemius of mice exposed to periods (1, 2, 3, 7, 15 and 30 days) of non-invasive hindlimb immobilization (plastic splint, I cohorts) and in those exposed to reloading for different time-points (1, 3, 7, 15, and 30 days, R cohorts) following a seven-day period of immobilization. Groups of control animals were also used.

RESULTS

Compared to non-exposed controls, muscle weight, limb strength, slow- and fast-twitch cross-sectional areas, mtDNA/nDNA, and myosin content were decreased in mice of I cohorts, whereas tyrosine release, ubiquitin-proteasome activity, muscle injury and systemic troponin I levels were increased. Gastrocnemius reloading following splint removal improved muscle mass loss, strength, fiber atrophy, injury, myosin content, and mtDNA/nDNA, while reducing ubiquitin-proteasome activity and proteolysis.

CONCLUSIONS

A consistent program of molecular and cellular events leading to reduced gastrocnemius muscle mass and mitochondrial content and reduced strength, enhanced proteolysis, and injury, was seen in this non-invasive mouse model of disuse muscle atrophy. Unloading of the muscle following removal of the splint significantly improved the alterations seen during unloading, characterized by a specific kinetic profile of molecular events involved in muscle regeneration. These findings have implications in patients with chronic diseases including cancer in whom physical activity may be severely compromised.

Sensorial, structural and functional response of rats subjected to hind limb immobilization

AIMS

This study analyzed the sensorial, structural and functional response of rats subjected to paw immobilization.

MAIN METHODS

Animal pelvis, hip, knee and ankle were immobilized using waterproof tape during two weeks for assessment of sensorial response to thermal (hot plate test) and mechanical stimuli (Von Frey test), motor system structure (histology and radiography) and muscle function (soleus contractility).

KEY FINDINGS

Disuse animals became more responsive to thermal stimuli (49%), although less responsive to mechanical challenge (58%). Disuse animals showed local injuries such as reduction in muscle fiber diameter (16.7% in gastrocnemius, 5.7% in soleus), contractile activity (55% of the control maximal tonic contraction) and tibia cortical thickness (9.3%), besides increased nitrite:protein ratio, suggestive of protein degradation. Disuse also evoked systemic adaptations that include increase in serum lactate dehydrogenase (36.1%) and alkaline phosphatase (400%), but reduction in calcium (8.4%) and total serum protein (5.5%), especially albumin (34.2%).

SIGNIFICANCE

Two weeks of functional paw disuse leads to local and systemic harmful adaptive changes in sensorial and structural systems. This study brings new insights into nervous and motor system mechanism associated with therapeutic limb immobilization in muscle and skeletal pathological conditions.

Impact of high intensity exercise on muscle morphology in EAE rats

The impact of high-intensity exercise on disease progression and muscle contractile properties in experimental autoimmune encephalomyelitis (EAE) remains unclear. Control (CON) and EAE rats were divided into sedentary and exercise groups. Before onset (experiment 1, n=40) and after hindquarter paralysis (experiment 2, n=40), isokinetic foot extensor strength, cross sectional area (CSA) of tibialis anterior (TA), extensor digitorum longus (EDL) and soleus (SOL) and brain-derived neurotrophic factor (BDNF) levels were assessed. EAE reduced muscle fiber CSA of TA, EDL and SOL. In general, exercise was not able to affect CSA, whereas it delayed hindquarter paralysis peak. CON muscle work peaked and declined, while it remained stable in EAE. BDNF-responses were not affected by EAE or exercise. In conclusion, EAE affected CSA-properties of TA, EDL and SOL, which could, partly, explain the absence of peak work during isokinetic muscle performance in EAE-animals. However, exercise was not able to prevent muscle fiber atrophy.

Histomorphometric alterations of muscle soleus provoked by drawn out immobilization: experimental study with wistar lineage rats

Objective This study has as objective to analyze the effect of joint immobilization of the soleus muscle of posterior members of rats on morphometric profile view, at periods of 21 and 45 days. Materials and methods Ten male Rattus novergicus albinus, Wistar variety, were used, separated into two groups (group I and II), of 5 animals each, with the first group submitted to immobilization for 21 days and the second for 45 days. The experiment control was acquired from the right contralateral member of each animal. The immobilization of the left posterior member was done by adapted orthesis. The morphometric analysis of soleus was by non-serial cross sections, 8μm thick. Results From the obtained images, the muscle fibers areas were analyzed, including the form of the fibers, comparing with the control group. In both the periods of immobilization cross sections of staple fibres had reduction of the section area, front to the comparison to its respective member has controlled, with significant important (p < 0.0001), especially it enters the experimental groups of 21 and 45 days. Conclusion It can be concluded that immobilization to articulate of rats per 21 and 45 days modifies the section area more transversa of staple fibres of the sóleo muscle and deleterious effect of more significant the cellular components for drawn out periods.

Agent-based computational model investigates muscle-specific responses to disuse-induced atrophy

Skeletal muscle is highly responsive to use. In particular, muscle atrophy attributable to decreased activity is a common problem among the elderly and injured/immobile. However, each muscle does not respond the same way. We developed an agent-based model that generates a tissue-level skeletal muscle response to disuse/immobilization. The model incorporates tissue-specific muscle fiber architecture parameters and simulates changes in muscle fiber size as a result of disuse-induced atrophy that are consistent with published experiments. We created simulations of 49 forelimb and hindlimb muscles of the rat by incorporating eight fiber-type and size parameters to explore how these parameters, which vary widely across muscles, influence sensitivity to disuse-induced atrophy. Of the 49 muscles modeled, the soleus exhibited the greatest atrophy after 14 days of simulated immobilization (51% decrease in fiber size), whereas the extensor digitorum communis atrophied the least (32%). Analysis of these simulations revealed that both fiber-type distribution and fiber-size distribution influence the sensitivity to disuse atrophy even though no single tissue architecture parameter correlated with atrophy rate. Additionally, software agents representing fibroblasts were incorporated into the model to investigate cellular interactions during atrophy. Sensitivity analyses revealed that fibroblast agents have the potential to affect disuse-induced atrophy, albeit with a lesser effect than fiber type and size. In particular, muscle atrophy elevated slightly with increased initial fibroblast population and increased production of TNF-α. Overall, the agent-based model provides a novel framework for investigating both tissue adaptations and cellular interactions in skeletal muscle during atrophy.

Effects of eccentric exercise in rehabilitation of phasic and tonic muscles after leg immobilization in rats

Eccentric exercise is an essential resource for skeletal muscle rehabilitation following muscle disuse however, abnormalities linked to the tissue recuperation require further research. Our aim was analyze the adaptation ability of rehabilitated muscular tissue in rats during different periods of eccentric training after 10 days of limb immobilization. Twenty-seven Wistar rats were divided into six groups: immobilized 10 days, immobilized and eccentric trained for 10 days, immobilized and eccentric trained for 21 days, and three age-matched control groups. After sacrifice, soleus and plantaris muscles were frozen, cut and stained for general histology using hematoxylin and eosin and Gomori trichrome methods and immunohistochemical methods for fiber typing (mATPase, NADH2-TR), for capillaries (CD31) and intermediate filaments (desmin, vimentin) and high resolution microscopy of resin embedded material. Immobilization resulted in more intense morphological alterations in soleus muscles such as formation of target fibers, nuclear centralization, a reduction in the number of type I fibers, diameter of type I, IIA, IIAD fibers, and capillaries. After 10 days of eccentric training, increases in the nuclear centralization and the number of lobulated fibers were observed. This period was insufficient to reestablish the capillary/fiber (C/F) ratio and distribution of fiber types as that observed in the control group. However, 21 days of rehabilitation allowed the reversal of all morphological and quantitative abnormalities. For the plantaris muscles, 10-days of training restored their basic characteristics. Despite the fact that immobilization affected soleus and plantaris muscles, 10 days of eccentric training was insufficient to restore the morphological characteristics of soleus muscles, which was not the case observed in plantaris muscle.

Adaptive responses of TRPC1 and TRPC3 during skeletal muscle atrophy and regrowth

INTRODUCTION

We assessed the time-dependent changes of transient receptor potential canonical type 1 (TRPC1) and TRPC3 expression and localization associated with muscle atrophy and regrowth in vivo.

METHODS

Mice were subjected to hindlimb unloading for 7 or 14 days (7U, 14U) followed by 3, 7, or 14 days of reloading (3R, 7R, 14R).

RESULTS

Soleus muscle mass and tetanic force were reduced significantly at 7U and 14U and recovered by 14R. Recovery of muscle fiber cross-sectional area was observed by 28R. TRPC1 mRNA was unaltered during the unloading-reloading period. However, protein expression remained depressed through 14R. Decreased localization of TRPC1 to the sarcolemma was observed. TRPC3 mRNA and protein expression levels were decreased significantly during the early phase of reloading.

CONCLUSIONS

Given the known role of these channels in muscle development, changes observed in TRPC1 and TRPC3 may relate closely to muscle atrophy and remodeling processes.

Time course of diaphragm function recovery after controlled mechanical ventilation in rats

Controlled mechanical ventilation (CMV) is known to result in rapid and severe diaphragmatic dysfunction, but the recovery response of the diaphragm to normal function after CMV is unknown. Therefore, we examined the time course of diaphragm function recovery in an animal model of CMV. Healthy rats were submitted to CMV for 24-27 h (n = 16), or to 24-h CMV followed by either 1 h (CMV + 1 h SB, n = 9), 2 h (CMV + 2 h SB, n = 9), 3 h (CMV + 3 h SB, n = 9), or 4-7 h (CMV + 4-7 h SB, n = 9) of spontaneous breathing (SB). At the end of the experiment, the diaphragm muscle was excised for functional and biochemical analysis. The in vitro diaphragm force was significantly improved in the CMV + 3 h SB and CMV + 4-7 h SB groups compared with CMV (maximal tetanic force: +27%, P < 0.05, and +59%, P < 0.001, respectively). This was associated with an increase in the type IIx/b fiber dimensions (P < 0.05). Neutrophil influx was increased in the CMV + 4-7 h SB group (P < 0.05), while macrophage numbers remained unchanged. Markers of protein synthesis (phosphorylated Akt and eukaryotic initiation factor 4E binding protein 1) were significantly increased (±40%, P < 0.001, and ±52%, P < 0.01, respectively) in the CMV + 3 h SB and CMV + 4-7 h SB groups and were positively correlated with diaphragm force (P < 0.05). Finally, also the maximal specific force generation of skinned single diaphragm fibers was increased in the CMV + 4-7 h SB group compared with CMV (+45%, P < 0.05). In rats, reloading the diaphragm for 3 h after CMV is sufficient to improve diaphragm function, while complete recovery occurs after longer periods of reloading. Enhanced muscle fiber dimensions, increased protein synthesis, and improved intrinsic contractile properties of diaphragm muscle fibers may have contributed to diaphragm function recovery.

Effects of 40min of maintained stretch on the soleus and plantaris muscles of rats applied for different periods of time after hindlimb immobilization

Stretching is frequently used in physiotherapy to minimize or even reverse the alterations that occur after muscle disuse. Alterations that occur after 10 and 21 days of maintained stretch in soleus and plantaris muscles post-disuse were evaluated in the present study in experimental rats. Thirty adult female Wistar rats were divided into seven groups: hindlimb immobilization for 10 days; immobilization and 10 days stretched; immobilized and 21 days stretched; three control groups consisting of animals of different ages and anesthetized group. The right hindlimb was immobilized using a lightweight apparatus composed of two sections: (i) upper part: a small cotton T-shirt and, and (ii) lower part: a steel mesh to fix the ankle in plantar-flexion-shortened position. Fragments of the soleus and plantaris muscle were frozen and processed using histochemical and immunohistochemical methods. Limb immobilization caused important morphological alterations in skeletal muscle including: reduction in the number of type I fibers and an increase in type IIC fibers, reduction in the lesser diameter of type I, IIA and IIAD fibers and in the number of capillaries in soleus muscle. The stretching program applied for 10 days was insufficient to allow recovery from the disuse alterations in both muscles. However, after 21 days there were improved morphological characteristics, size and distribution of the different fibers.

Contribution of social isolation, restraint, and hindlimb unloading to changes in hemodynamic parameters and motion activity in rats

The most accepted animal model for simulation of the physiological and morphological consequences of microgravity on the cardiovascular system is one of head-down hindlimb unloading. Experimental conditions surrounding this model include not only head-down tilting of rats, but also social and restraint stresses that have their own influences on cardiovascular system function. Here, we studied levels of spontaneous locomotor activity, blood pressure, and heart rate during 14 days under the following experimental conditions: cage control, social isolation in standard rat housing, social isolation in special cages for hindlimb unloading, horizontal attachment (restraint), and head-down hindlimb unloading. General activity and hemodynamic parameters were continuously monitored in conscious rats by telemetry. Heart rate and blood pressure were both evaluated during treadmill running to reveal cardiovascular deconditioning development as a result of unloading. The main findings of our work are that: social isolation and restraint induced persistent physical inactivity, while unloading in rats resulted in initial inactivity followed by normalization and increased locomotion after one week. Moreover, 14 days of hindlimb unloading showed significant elevation of blood pressure and slight elevation of heart rate. Hemodynamic changes in isolated and restrained rats largely reproduced the trends observed during unloading. Finally, we detected no augmentation of tachycardia during moderate exercise in rats after 14 days of unloading. Thus, we concluded that both social isolation and restraint, as an integral part of the model conditions, contribute essentially to cardiovascular reactions during head-down hindlimb unloading, compared to the little changes in the hydrostatic gradient.

Role of exercise therapy in prevention of decline in aging muscle function: glucocorticoid myopathy and unloading

Changes in skeletal muscle quantity and quality lead to disability in the aging population. Physiological changes in aging skeletal muscle are associated with a decline in mass, strength, and inability to maintain balance. Glucocorticoids, which are in wide exploitation in various clinical scenarios, lead to the loss of the myofibrillar apparatus, changes in the extracellular matrix, and a decrease in muscle strength and motor activity, particularly in the elderly. Exercise therapy has shown to be a useful tool for the prevention of different diseases, including glucocorticoid myopathy and muscle unloading in the elderly. The purpose of the paper is to discuss the possibilities of using exercise therapy in the prevention of glucocorticoid caused myopathy and unloading in the elderly and to describe relationships between the muscle contractile apparatus and the extracellular matrix in different types of aging muscles.

Muscle weakness in the elderly: role of sarcopenia, dynapenia, and possibilities for rehabilitation

Aging is a multifactorial process leading to changes in skeletal muscle quantity and quality, which cause muscle weakness and disability in the aging population. This paper discusses the reasons for muscle weakness—and its biological and physiological mechanisms—in the elderly and describes the role of sarcopenia and dynapenia, and the possibilities to modify the age-associated decline in muscle function and decelerate the development of muscle weakness and disability. Resistance and endurance training are effective measures of exercise therapy in the elderly, which improve muscle metabolism and thereby muscle function and life quality.

Oxidative capacity and fatigability in run-trained malignant hyperthermia-susceptible mice

INTRODUCTION

The purpose of this study was to test the hypothesis that malignant hyperthermia model mice (RyR1Y522S/wt) are more vulnerable to exercise-induced muscle injury and fatigability and adapt less to run training.

METHODS

After 6 weeks of voluntary wheel running, we measured anterior crural muscle fatigability, muscle injury, and cytochrome oxidase (COX) and citrate synthase (CS).

RESULTS

Although RyR1Y522S/wt mice ran without undergoing MH episodes, they ran 42% less distance than wild-type (WT) mice. Muscles from WT mice exhibited increased fatigue resistance and COX content after training. Muscles from RyR1Y522S/wt mice demonstrated no significant change in fatigability or COX and CS after training. However, muscles from RyR1Y522S/wt mice displayed less intrinsic fatigability and greater COX/CS content and muscle damage than WT mice.

CONCLUSIONS

RyR1Y522S/wt mice can run without having rhabdomyolysis, and their inability to adapt to training appears to stem from intrinsic enhancement of mitochondrial enzymes and fatigue resistance.

Skeletal muscle mass recovery from atrophy in IL-6 knockout mice

AIM

Skeletal muscle interleukin-6 (IL-6) expression is induced by continuous contraction, overload-induced hypertrophy and during muscle regeneration. The loss of IL-6 can alter skeletal muscle's growth and extracellular matrix remodelling response to overload-induced hypertrophy. Insulin-like growth factor-1 (IGF-1) gene expression and related signalling through Akt/mTOR is a critical regulator of muscle mass. The significance of IL-6 expression during the recovery from muscle atrophy is unclear. This study's purpose was to determine the effect of IL-6 loss on mouse gastrocnemius (GAS) muscle mass during recovery from hindlimb suspension (HS)-induced atrophy.

METHODS

Female C57BL/6 [wild type (WT)] and IL-6 knockout (IL-6 KO) mice at 10 weeks of age were assigned to control, HS or HS followed by normal cage ambulation groups.

RESULTS

GAS muscle atrophy was induced by 10 days of HS. HS induced a 20% loss of GAS mass in both WT and IL-6 KO mice. HS+7 days of recovery restored WT GAS mass to cage-control values. GAS mass from IL-6 KO mice did not return to cage-control values until HS+14 days of recovery. Both IGF-1 mRNA expression and Akt/mTOR signalling were increased in WT muscle after 1 day of recovery. In IL-6 KO muscle, IGF-1 mRNA expression was decreased and Akt/mTOR signalling was not induced after 1 day of recovery. MyoD and myogenin mRNA expression were both induced in WT muscle after 1 day of recovery, but not in IL-6 KO muscle.

CONCLUSION

  Muscle IL-6 expression appears important for the initial growth response during the recovery from disuse.

Alterações morfométricas em músculo respiratório de ratos submetidos à imobilização de pata

A imobilização é um recurso frequentemente utilizado na prática clínica, sendo comum em patologias álgicas e nas fraturas. O objetivo deste estudo foi analisar a influência do processo de imobilização em músculo do sistema respiratório, o diafragma. O experimento foi efetuado com 12 ratos Wistar machos divididos em dois grupos, controle e imobilizado. O procedimento de imobilização foi realizado através de um método alternativo de imobilização por fita adesiva, sendo mantida por duas semanas. Analisou-se a morfometria das fibras do diafragma com coloração de hematoxilina e eosina. Ao compararmos o diâmetro médio das fibras musculares do diafragma dos animais imobilizados (47,15μm ± 0,329μm) em relação ao controle (54,67μm ± 0,396μm), encontramos diferença estatística entre os grupos (p < 0,0001). Considerando os dados encontrados, foi possível concluir que a imobilização de pata, no modelo utilizado, foi capaz de gerar hipotrofia da musculatura respiratória, assim como um quadro geral de redução de massa corporal do animal.

Alterations of protein turnover underlying disuse atrophy in human skeletal muscle

Unloading-induced atrophy is a relatively uncomplicated form of muscle loss, dependent almost solely on the loss of mechanical input, whereas in disease states associated with inflammation (cancer cachexia, AIDS, burns, sepsis, and uremia), there is a procatabolic hormonal and cytokine environment. It is therefore predictable that muscle loss mainly due to disuse alone would be governed by mechanisms somewhat differently from those in inflammatory states. We suggest that in vivo measurements made in human subjects using arterial-venous balance, tracer dilution, and tracer incorporation are dynamic and thus robust by comparison with static measurements of mRNA abundance and protein expression and/or phosphorylation in human muscle. In addition, measurements made with cultured cells or in animal models, all of which have often been used to infer alterations of protein turnover, appear to be different from results obtained in immobilized human muscle in vivo. In vivo measurements of human muscle protein turnover in disuse show that the primary variable that changes facilitating the loss of muscle mass is protein synthesis, which is reduced in both the postabsorptive and postprandial states; muscle proteolysis itself appears not to be elevated. The depressed postprandial protein synthetic response (a phenomenon we term "anabolic resistance") may even be accompanied by a diminished suppression of proteolysis. We therefore propose that most of the loss of muscle mass during disuse atrophy can be accounted for by a depression in the rate of protein synthesis. Thus the normal diurnal fasted-to-fed cycle of protein balance is disrupted and, by default, proteolysis becomes dominant but is not enhanced.

Eccentric contractions do not induce rhabdomyolysis in malignant hyperthermia susceptible mice

Recent studies suggest a link between exercise-induced rhabdomyolysis and mutations of the ryanodine receptor (RYR1) associated with malignant hyperthermia (MH). We hypothesized that MH-susceptible mice (RYR1Y522S/wt) would exhibit greater anterior crural muscle [tibialis anterior (TA) and extensor digitorum longus (EDL) muscles] damage and strength deficits following the performance of a single or repeated bouts of eccentric contractions compared with wild-type (WT) mice. After a single injury bout, RYR1Y522S/wt mice produced more isometric torque than WT mice immediately and 3 and 7 days postinjury. Moreover, EDL muscle isometric specific force deficits were fully recovered for RYR1Y522S/wt but not WT mice 14 days postinjury. The percentage of fibers in TA muscle exhibiting signs of muscle damage 7 and 14 days postinjury were at least three times less in RYR1Y522S/wt than in WT mice. Uninjured and injured EDL muscle from RYR1Y522S/wt mice also displayed greater S-glutathionylation of RYR1 than that from WT mice. During the weekly injury bouts, torque production by RYR1Y522S/wt mice was fully recovered before the third and fourth injury bouts, whereas torque was still reduced for WT mice. Three days after multiple injury bouts, there were approximately 50% fewer fibers exhibiting signs of muscle damage in RYR1Y522S/wt than in WT TA muscle. These findings indicate that the RYR1Y522S/wt mutation protects skeletal muscle from exercise-induced muscle injury and do not support a direct association between MH susceptibility and contraction-induced rhabdomyolysis when core temperature is maintained at lower physiological temperatures during exercise.

Detrimental effects of reloading recovery on force, shortening velocity, and power of soleus muscles from hindlimb-unloaded rats

To better understand how atrophied muscles recover from prolonged nonweight-bearing, we studied soleus muscles (in vitro at optimal length) from female rats subjected to normal weight bearing (WB), 15 days of hindlimb unloading (HU), or 15 days HU followed by 9 days of weight bearing reloading (HU-R). HU reduced peak tetanic force (P(o)), increased maximal shortening velocity (V(max)), and lowered peak power/muscle volume. Nine days of reloading failed to improve P(o), while depressing V(max) and intrinsic power below WB levels. These functional changes appeared intracellular in origin as HU-induced reductions in soleus mass, fiber cross-sectional area, and physiological cross-sectional area were partially or completely restored by reloading. We calculated that HU-induced reductions in soleus fiber length were of sufficient magnitude to overextend sarcomeres onto the descending limb of their length-tension relationship upon the resumption of WB activity. In conclusion, the force, shortening velocity, and power deficits observed after 9 days of reloading are consistent with contraction-induced damage to the soleus. HU-induced reductions in fiber length indicate that sarcomere hyperextension upon the resumption of weight-bearing activity may be an important mechanism underlying this response.

Staging of disuse atrophy of skeletal muscles on immunofluorescence microscopy

The Japanese population is rapidly aging, thereby causing excess demand for facilities for elderly invalids. It is imperative that social measures and scientific studies be carried out to enable better care of bedridden elderly people. The purpose of the present study was to review the histological changes that occur in disuse atrophy of skeletal muscles, the primary pathophysiology of bedridden invalids, with the object of developing a staging standard to be used by researchers and clinicians. Rat hindlimb suspension was used as an experimental model. Atrophy of the soleus muscle was evaluated qualitatively and quantitatively on immunofluorescence microscopy. The myofibrils decreased significantly in the first 2-3 weeks of disuse atrophy. The earliest morphological change was fan-shaped multistep forking of sarcomeres, which appeared by the first week. This type of muscular lesion, designated here as 'sarcomeric disarray', was first described in the present study. Central-core lesions appeared mainly in slow muscle fibers by the second week. These lesions disappeared by the fourth or fifth week. Nerves remained intact and no inflammation or regeneration occurred up to the fifth week. Methods and criteria were compiled for staging of disuse atrophy based on the present results and a diagnosis kit designed for studies on disuse atrophy of skeletal muscles.

Morphological comparison of different protocols of skeletal muscle remobilization in rats after hindlimb suspension

The aim of this study was to evaluate and compare the efficacy of different remobilization protocols in different skeletal muscles considering the changes induced by hindlimb suspension of the tail. Thirty-six female Wistar rats were divided into six groups: control I, control II, suspended, suspended free, suspended trained on a declined treadmill and suspended trained on a flat treadmill. Fragments of soleus and tibialis anterior (TA) muscle were frozen and processed by different histochemical methods. The suspended soleus showed a significant increase in the proportional number of intermediate/hybrid fibers and a decrease in the number of type I fibers. Some of these changes proved to be reversible after remobilization. The three remobilization programs led to the recovery of both the proportional number of fibers and their size. The TA muscle presented a significant increase in the number and size of type I fibers and a cell size reduction of type IIB fibers, which were recovered after training on a declined treadmill and free movement. Especially regarding the soleus, the present findings indicate that, among the protocols, training on a declined treadmill was found to induce changes of a more regenerative nature, seemingly indicating a better tissue restructuring after the suspension procedure.

[Cellular regulation of anabolism and catabolism in skeletal muscle during immobilisation, aging and critical illness]

Skeletal muscle atrophy is associated with situations of acute and chronical illness, such as sepsis, surgery, trauma and immobility. Additionally, it is a common problem during the physiological process of aging. The myofibrillar proteins myosin and actin, which are essential for muscle contraction, are the major targets during the process of protein degradation. This leads to a general loss of muscle mass, muscle strength and to increased muscle fatigue. In critically ill or immobile patients skeletal muscle atrophy is accompanied by enhanced inflammation, reduced wound healing, weaning complications and difficulties in mobilisation. During aging it results in falls, fractures, physical injuries and loss of mobility. Relating to the primary stimulators - hormones, muscle lengthening, stress, inflammation, neuronal activity - research is now focusing on the investigation of the signal transduction pathways, which influence protein synthesis and protein degradation during skeletal muscle atrophy.

Cellular patterns of the atrophic response in murine soleus and gastrocnemius muscles submitted to simulated weightlessness

The purpose of the present study was to investigate the mechanisms of cell death (apoptosis vs. necrosis) during muscle atrophy induced by 1 week of hindlimb suspension. Biochemical and morphological parameters were examined in murine soleus and gastrocnemius muscles. A total of 70 male Charles River CD1 mice were randomly assigned to seven groups (n = 10/group): Cont (loading control conditions) and 6HS, 12HS, 24HS, 48HS, 72HS and 1wkHS with respect to the period of hindlimb suspension (HS). Compared to the Cont, skeletal muscle atrophy was confirmed by a significant decrease of 44 and of 17% in fiber cross-sectional areas in the gastrocnemius and soleus, respectively. A significant increase in caspase-3 activity was noticed in 6HS (196%, P < 0.05) and in 12HS (201%, P < 0.05), as well as the amount of cytosolic mono- and oligonucleosomes at 12HS (142%, P < 0.05) and 24HS (203%, P < 0.05) in the gastrocnemius and soleus, respectively. The profile of necrotic markers showed a peak of myeloperoxidase activity at 24HS (170%, P < 0.05) and at 72HS (114%, P < 0.05) in the gastrocnemius and soleus, respectively. The analysis of N-acetylglucosaminidase activity evidenced more increment in the soleus at 72HS (60%, P < 0.05). The analysis of the basal values of these parameters suggested that apoptosis prevailed in the slow-twitch muscle analyzed, whereas lysosomic activity seemed to be more pronounced in the gastrocnemius. The morphological data supported the biochemical results pointing towards a shift from apoptosis to necrosis, which seems to corroborate the aponecrosis theory.

Estrogen status and skeletal muscle recovery from disuse atrophy

Although estrogen loss can alter skeletal muscle recovery from disuse, the specific components of muscle regrowth that are estrogen sensitive have not been described. The primary purpose of this study was to determine the components of skeletal muscle mass recovery that are biological targets of estrogen. Intact, ovariectomized (OVX), and ovariectomized with 17β-estradiol replacement (OVX+E2) female rats were subjected to hindlimb suspension for 10 days and then returned to normal cage ambulation for the duration of recovery. Soleus muscle mass returned to control levels by day 7 of recovery in the intact animals, whereas OVX soleus mass did not recover until day 14. Intact rats recovered soleus mean myofiber cross-sectional area (CSA) by day 14 of recovery, whereas the OVX soleus remained decreased (42%) at day 14. OVX mean fiber CSA did return to control levels by day 28 of recovery. The OVX+E2 treatment group recovered mean CSA at day 14, as in the intact animals. Myofibers demonstrating central nuclei were increased at day 14 in the OVX group, but not in intact or OVX+E2 animals. The percent noncontractile tissue was also increased 29% in OVX muscle at day 14, but not in either intact or OVX+E2 groups. In addition, collagen 1a mRNA was increased 45% in OVX muscle at day 14 of recovery. These results suggest that myofiber growth, myofiber regeneration, and extracellular matrix remodeling are estrogen-sensitive components of soleus muscle mass recovery from disuse atrophy.

Noninvasive monitoring of muscle damage during reloading following limb disuse

Cast immobilization causes skeletal muscle disuse atrophy and an increased susceptibility to muscle damage. The objective of this study was to explore the utility of noninvasive magnetic resonance (MR) imaging to monitor muscle damage in the lower hindlimb muscles of the mouse during reloading following cast immobilization and to compare the findings in different muscles. The hindlimbs of C57BL6 mice were immobilized for 2 weeks in plantarflexion using a bilateral casting model. Following immobilization the mice were allowed to reambulate and muscle damage was monitored at different times. Cage-restricted reloading following cast immobilization induced a significant shift (P < 0.0001) in the transverse (T2) relaxation characteristics of the postural slow-twitch soleus muscle, but not in the neighboring gastrocnemius. Soleus T2 values peaked at 2 days of reloading. Muscle-specific changes in MR T2 relaxation properties correlated with uptake of Evans blue dye, a histological marker of muscle damage. This study demonstrates that T2 MR imaging can be implemented to monitor noninvasively and sequentially muscle-specific damage during reloading following limb disuse.

Transcriptional reprogramming during reloading of atrophied rat soleus muscle

The hypothesis was tested that differential, coregulated transcriptional adaptations of various cellular pathways would occur early with increased mechanical loading of atrophied skeletal muscle and relate to concurrent damage of muscle fibers. Atrophy and slow-to-fast fiber transformation of rat soleus muscle was provoked by 14 days of hindlimb suspension (HS). Subsequent reloading of hindlimbs caused a fourfold increase in the percentage of muscle fibers, demonstrating endomysial tenascin-C staining. Five days after reloading, when 10% of the fibers were damaged, the normal muscle weight and slow-type fiber percentage were reestablished. Microarray analysis revealed major, biphasic patterns of gene expressional alterations with reloading that distinguish between treatments and gene ontologies. Transcript levels of factors involved in protein synthesis and certain proteasomal mRNAs were increased after 1 day of reloading and correlated to the percentage of fibers surrounded by tenascin-C. By contrast, levels of gene messages for fatty acid transporters, respiratory chain constituents, and voltage-gated cation channels were transiently reduced after 1 day of muscle loading and associated with the number of damaged fibers and the regain in muscle weight. This coregulation points toward important retooling of oxidative metabolism and the T- and SR-tubular systems with rebuilding of slow fibers. The observations demonstrate that early nuclear reprogramming with reloading of atrophic soleus muscle is coordinated and links to the processes involved in mechanical damage and regeneration of muscle fibers.