Effects of Moderate-Intensity Physical Training on Skeletal Muscle Substrate Transporters and Metabolic Parameters of Ovariectomized Rats.

A deficit of estrogen is associated with energy substrate imbalance, raising the risk of metabolic diseases. Physical training (PT) is a potent metabolic regulator through oxidation and storage of substrates transported by GLUT4 and FAT CD36 in skeletal muscle. However, little is known about the effects of PT on these carriers in an estrogen-deficit scenario. Thus, the aim of this study was to determine the influence of 12 weeks of PT on metabolic variables and GLUT4 and FAT CD36 expression in the skeletal muscle of animals energetically impaired by ovariectomy (OVX). The trained animals swam 30 min/day, 5 days/week, at 80% of the critical load intensity. Spontaneous physical activity was measured biweekly. After training, FAT CD36 and GLUT4 expressions were quantified by immunofluorescence in the soleus, as well as muscular glycogen and triglyceride of the soleus, gluteus maximus and gastrocnemius. OVX significantly reduced FAT CD36, GLUT4 and spontaneous physical activity (p < 0.01), while PT significantly increased FAT CD36, GLUT4 and spontaneous physical activity (p < 0.01). PT increased soleus glycogen, and OVX decreased muscular triglyceride of gluteus maximus. Therefore, OVX can cause energy disarray through reduction in GLUT4 and FAT CD36 and their muscle substrates and PT prevented these metabolic consequences, masking ovarian estrogen's absence.

Effects of Low-Intensity and Long-Term Aerobic Exercise on the Psoas Muscle of mdx Mice: An Experimental Model of Duchenne Muscular Dystrophy.

Duchenne muscular dystrophy (DMD) is a muscle disease characterized by the absence of the protein dystrophin, which causes a loss of sarcolemma integrity, determining recurrent muscle injuries, decrease in muscle function, and progressive degeneration. Currently, there is a need for therapeutic treatments to improve the quality of life of DMD patients. Here, we investigated the effects of a low-intensity aerobic training (37 sessions) on satellite cells, peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α protein (PGC-1α), and different types of fibers of the psoas muscle from mdx mice (DMD experimental model). Wildtype and mdx mice were randomly divided into sedentary and trained groups (n = 24). Trained animals were subjected to 37 sessions of low-intensity running on a motorized treadmill. Subsequently, the psoas muscle was excised and analyzed by immunofluorescence for dystrophin, satellite cells, myosin heavy chain (MHC), and PGC-1α content. The minimal Feret's diameters of the fibers were measured, and light microscopy was applied to observe general morphological features of the muscles. The training (37 sessions) improved morphological features in muscles from mdx mice and caused an increase in the number of quiescent/activated satellite cells. It also increased the content of PGC-1α in the mdx group. We concluded that low-intensity aerobic exercise (37 sessions) was able to reverse deleterious changes determined by DMD.

An increase in force after stretch of diaphragm fibers and myofibrils is accompanied by an increase in sarcomere length nonuniformities and Ca2+ sensitivity.

When muscle fibers from limb muscles are stretched while activated, the force increases to a steady-state level that is higher than that produced during isometric contractions at a corresponding sarcomere length, a phenomenon known as residual force enhancement (RFE). The mechanisms responsible for the RFE are an increased stiffness of titin molecules that may lead to an increased Ca2+ sensitivity of the contractile apparatus, and the development of sarcomere length nonuniformities. RFE is not observed in cardiac myofibrils, which makes this phenomenon specific to certain preparations. The aim of this study was to investigate whether the RFE is present in the diaphragm, and its potential association with an increased Ca2+ sensitivity and the development of sarcomere length nonuniformities. We used two preparations: single intact fibers and myofibrils isolated from the diaphragm of mice. We investigated RFE in a variety of lengths across the force-length relationship. RFE was observed in both preparations at all lengths investigated and was larger with increasing magnitudes of stretch. RFE was accompanied by an increased Ca2+ sensitivity as shown by a change in the force-pCa2+ curve, and increased sarcomere length nonuniformities. Therefore, RFE is a phenomenon commonly observed in skeletal muscles, with mechanisms that are similar across preparations.

The Musculoskeletal Involvement After Mild to Moderate COVID-19 Infection.

COVID-19, a disease caused by the novel coronavirus SARS-CoV-2, has been drastically affecting the daily lives of millions of people. COVID-19 is described as a multiorgan disease that affects not only the respiratory tract of infected individuals, but it has considerable effects on the musculoskeletal system, causing excessive fatigue, myalgia, arthralgia, muscle weakness and skeletal muscle damage. These symptoms can persist for months, decreasing the quality of life of numerous individuals. Curiously, most studies in the scientific literature focus on patients who were hospitalized due to SARS-CoV-2 infection and little is known about the mechanism of action of COVID-19 on skeletal muscles, especially of individuals who had the mild to moderate forms of the disease (non-hospitalized patients). In this review, we focus on the current knowledge about the musculoskeletal system in COVID-19, highlighting the lack of researches investigating the mild to moderate cases of infection and pointing out why it is essential to care for these patients. Also, we will comment about the need of more experimental data to assess the musculoskeletal manifestations on COVID-19-positive individuals.

Effect of 12-wk Training in Ovariectomised Rats on PGC-1α, NRF-1 and Energy Substrates.

Metabolic diseases are associated with hypoestrogenism owing to their lower energy expenditure and consequent imbalance. Physical training promotes energy expenditure through PGC-1α and NRF-1, which are muscle proteins of the oxidative metabolism. However, the influence of physical training on protein expression in individuals with hypoestrogenism remains uncertain. Thus, the aim of this study is to determine the effect of 12 weeks of moderate-intensity swimming training on the muscle expression of PGC-1α, NRF-1, glycogen and triglyceride in ovariectomised rats. OVX and OVX+TR rats were subjected to ovariectomy. The trained animals swam for 30 minutes, 5 days/week, at 80% of the critical load intensity. Soleus was collected to quantify PGC-1α and NRF-1 expressions, while gastrocnemius and gluteus maximus were collected to measure glycogen and triglyceride. Blood glucose was also evaluated. Whereas ovariectomy decreased PGC-1α expression (p<0.05) without altering NRF-1 (p=0.48), physical training increased PGC-1α (p<0.01) and NRF-1 (p<0.05). Ovariectomy reduced glycogen (p<0.05) and triglyceride (p<0.05), whereas physical training increased glycogen (p<0.05) but did not change triglyceride (p=0.06). Ovariectomy increased blood glucose (p<0.01), while physical training reduced it (p<0.01). In summary, 12 weeks of individualized and moderate-intensity training were capable of preventing muscle metabolic consequences caused by ovariectomy.

Resistance training variable manipulations are less relevant than intrinsic biology in affecting muscle fiber hypertrophy.

We aimed to investigate whether muscle fiber cross-sectional area (fCSA) and associated molecular processes could be differently affected at the group and individual level by manipulating resistance training (RT) variables. Twenty resistance-trained subjects had each leg randomly allocated to either a standard RT (RT-CON: without specific variables manipulations) or a variable RT (RT-VAR: manipulation of load, volume, muscle action, and rest interval at each RT session). Muscle fCSA, satellite cell (SC) pool, myonuclei content, and gene expression were assessed before and after training (chronic effect). Gene expression was assessed 24 h after the last training session (acute effect). RT-CON and RT-VAR increased fCSA and myonuclei domain in type I and II fibers after training (p < 0.05). SC and myonuclei content did not change for both conditions (p > 0.05). Pax-7, MyoD, MMP-2 and COL3A1 (chronic) and MGF, Pax-7, and MMP-9 (acute) increased similar for RT-CON and RT-VAR (p < 0.05). The increase in acute MyoG expression was significantly higher for the RT-VAR than RT-CON (p < 0.05). We found significant correlation between RT-CON and RT-VAR for the fCSA changes (r = 0.89). fCSA changes were also correlated to satellite cells (r = 0.42) and myonuclei (r = 0.50) changes. Heatmap analyses showed coupled changes in fCSA, SC, and myonuclei responses at the individual level, regardless of the RT protocol. The high between and low within-subject variability regardless of RT protocol suggests that the intrinsic biological factors seem to be more important to explain the magnitude of fCSA gains in resistance-trained subjects.

Acute melatonin administration improves exercise tolerance and the metabolic recovery after exhaustive effort.

The present study investigated the effects of acute melatonin administration on the biomarkers of energy substrates, GLUT4, and FAT/CD36 of skeletal muscle and its performance in rats subjected to exhaustive swimming exercise at an intensity corresponding to the maximal aerobic capacity (tlim). The incremental test was performed to individually determine the exercise intensity prescription and 48 h after, the animals received melatonin (10 mg·kg-1) or vehicles 30 min prior to tlim. Afterwards, the animals were euthanized 1 or 3 h after the exhaustion for blood and muscles storage. The experiment 1 found that melatonin increased the content of glycogen and GLUT4 in skeletal muscles of the animals that were euthanized 1 (p < 0.05; 22.33% and 41.87%) and 3 h (p < 0.05; 37.62% and 57.87%) after the last procedures. In experiment 2, melatonin enhanced the tlim (p = 0.01; 49.42%), the glycogen content (p < 0.05; 40.03%), GLUT4 and FAT/CD36 in exercised skeletal muscles (F = 26.83 and F = 25.28, p < 0.01). In summary, melatonin increased energy substrate availability prior to exercise, improved the exercise tolerance, and accelerated the recovery of muscle energy substrates after the tlim, possibly through GLUT4 and FAT/CD36.

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 MyBP-C isoforms tune the molecular contractility of divergent skeletal muscle systems.

Skeletal muscle myosin-binding protein C (MyBP-C) is a myosin thick filament-associated protein, localized through its C terminus to distinct regions (C-zones) of the sarcomere. MyBP-C modulates muscle contractility, presumably through its N terminus extending from the thick filament and interacting with either the myosin head region and/or the actin thin filament. Two isoforms of MyBP-C (fast- and slow-type) are expressed in a muscle type-specific manner. Are the expression, localization, and Ca2+-dependent modulatory capacities of these isoforms different in fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles derived from Sprague-Dawley rats? By mass spectrometry, 4 MyBP-C isoforms (1 fast-type MyBP-C and 3 N-terminally spliced slow-type MyBP-C) were expressed in EDL, but only the 3 slow-type MyBP-C isoforms in SOL. Using EDL and SOL native thick filaments in which the MyBP-C stoichiometry and localization are preserved, native thin filament sliding over these thick filaments showed that, only in the C-zone, MyBP-C Ca2+ sensitizes the thin filament and slows thin filament velocity. These modulatory properties depended on MyBP-C's N terminus as N-terminal proteolysis attenuated MyBP-C's functional capacities. To determine each MyBP-C isoform's contribution to thin filament Ca2+ sensitization and slowing in the C-zone, we used a combination of in vitro motility assays using expressed recombinant N-terminal fragments and in silico mechanistic modeling. Our results suggest that each skeletal MyBP-C isoform's N terminus is functionally distinct and has modulatory capacities that depend on the muscle type in which they are expressed, providing the potential for molecular tuning of skeletal muscle performance through differential MyBP-C expression.

The effects of fatigue and oxidation on contractile function of intact muscle fibers and myofibrils isolated from the mouse diaphragm.

The goal of this study was to investigate the effects of repetitive stimulation and the oxidant H2O2 on fatigue of diaphragm intact fibers and in myofibrils measured with different Ca2+ concentrations. Intact fibers were isolated from mice diaphragm, and twitch and tetanic contractions (500 ms duration) were performed at different frequencies of stimulation ranging from 15 Hz to 150 Hz to establish a force-frequency relation before and after a fatigue and recovery protocol, without or after a treatment with H2O2. Fatigue was induced with isometric contractions (500 ms, 40 Hz) evoked every 0.8 seconds, with a total of 625 tetani. After the fatigue, the force recovery was followed by invoking tetanic contractions (500 ms, 40 Hz) every 1 min, with a total duration of 30 min. Individual myofibrils were also isolated from the mouse diaphragm and were tested for isometric contractions before and after treatment with H2O2 and NAC. In a second series of experiments, myofibrils were activated at different pCa (pCa = -log10 [Ca2+]), before and after H2O2 treatment. After 15 minutes of H2O2 treatment, the myofibrillar force was decreased to 54 ± 12% of its control, maximal value, and a result that was reversed by NAC treatment. The force was also decreased after myofibrils were treated with H2O2 and activated in pCa ranging between 4.5 and 5.7. These results suggest that fatigue in diaphragm intact fibers and at the myofibrils level is caused partially by oxidation of the contractile proteins that may be responsible for changing the force in various levels of Ca2+ activation.

Hypertrophic cardiomyopathy R403Q mutation in rabbit ß-myosin reduces contractile function at the molecular and myofibrillar levels.

In 1990, the Seidmans showed that a single point mutation, R403Q, in the human ß-myosin heavy chain (MHC) of heart muscle caused a particularly malignant form of familial hypertrophic cardiomyopathy (HCM) [Geisterfer-Lowrance AA, et al. (1990) Cell 62:999-1006.]. Since then, more than 300 mutations in the ß-MHC have been reported, and yet there remains a poor understanding of how a single missense mutation in the MYH7 gene can lead to heart disease. Previous studies with a transgenic mouse model showed that the myosin phenotype depended on whether the mutation was in an α- or ß-MHC backbone. This led to the generation of a transgenic rabbit model with the R403Q mutation in a ß-MHC backbone. We find that the in vitro motility of heterodimeric R403Q myosin is markedly reduced, whereas the actin-activated ATPase activity of R403Q subfragment-1 is about the same as myosin from a nontransgenic littermate. Single myofibrils isolated from the ventricles of R403Q transgenic rabbits and analyzed by atomic force microscopy showed reduced rates of force development and relaxation, and achieved a significantly lower steady-state level of isometric force compared with nontransgenic myofibrils. Myofibrils isolated from the soleus gave similar results. The force-velocity relationship determined for R403Q ventricular myofibrils showed a decrease in the velocity of shortening under load, resulting in a diminished power output. We conclude that independent of whether experiments are performed with isolated molecules or with ordered molecules in the native thick filament of a myofibril, there is a loss-of-function induced by the R403Q mutation in ß-cardiac myosin.

Residual force enhancement is regulated by titin in skeletal and cardiac myofibrils.

KEY POINTS: When a skeletal muscle is stretched while it contracts, the muscle produces a relatively higher force than the force from an isometric contraction at the same length: a phenomenon referred to as residual force enhancement. Residual force enhancement is puzzling because it cannot be directly explained by the classical force-length relationship and the sliding filament theory of contraction, the main paradigms in the muscle field. We used custom-built instruments to measure residual force enhancement in skeletal myofibrils, and, for the first time, in cardiac myofibrils. Our data report that residual force enhancement is present in skeletal muscles, but not cardiac muscles, and is regulated by the different isoforms of the titin protein filaments. ABSTRACT: When a skeletal muscle contracts isometrically, the muscle produces a force that is relative to the final isometric sarcomere length (SL). However, when the same final SL is reached by stretching the muscle while it contracts, the muscle produces a relatively higher force: a phenomenon commonly referred to as residual force enhancement. In this study, we investigated residual force enhancement in rabbit skeletal psoas myofibrils and, for the first time, cardiac papillary myofibrils. A custom-built atomic force microscope was used in experiments that stretched myofibrils before and after inhibiting myosin and actin interactions to determine whether the different cardiac and skeletal titin isoforms regulate residual force enhancement. At SLs ranging from 2.24 to 3.13 µm, the skeletal myofibrils enhanced the force by an average of 9.0%, and by 29.5% after hindering myosin and actin interactions. At SLs ranging from 1.80 to 2.29 µm, the cardiac myofibrils did not enhance the force before or after hindering myosin and actin interactions. We conclude that residual force enhancement is present only in skeletal muscles and is dependent on the titin isoforms.

Prolonged controlled mechanical ventilation in humans triggers myofibrillar contractile dysfunction and myofilament protein loss in the diaphragm.

BACKGROUND: Prolonged controlled mechanical ventilation (CMV) in humans and experimental animals results in diaphragm fibre atrophy and injury. In animals, prolonged CMV also triggers significant declines in diaphragm myofibril contractility. In humans, the impact of prolonged CMV on myofibril contractility remains unknown. The objective of this study was to evaluate the effects of prolonged CMV on active and passive human diaphragm myofibrillar force generation and myofilament protein levels. METHODS AND RESULTS: Diaphragm biopsies were obtained from 13 subjects undergoing cardiac surgery (control group) and 12 brain-dead organ donors (CMV group). Subjects in each group had been mechanically ventilated for 2-4 and 12-74 h, respectively. Specific force generation of diaphragm myofibrils was measured with atomic force cantilevers. Rates of force development (Kact), force redevelopment after a shortening protocol (Ktr) and relaxation (Krel) in fully activated myofibrils (pCa(2+)=4.5) were calculated to assess myosin cross-bridge kinetics. Myofilament protein levels were measured with immunoblotting and specific antibodies. Prolonged CMV significantly decreased active and passive diaphragm myofibrillar force generation, Kact, Ktr and Krel. Myosin heavy chain (slow), troponin-C, troponin-I, troponin-T, tropomyosin and titin protein levels significantly decreased in response to prolonged CMV, but no effects on α-actin, α-actinin or nebulin levels were observed. CONCLUSIONS: Prolonged CMV in humans triggers significant decreases in active and passive diaphragm myofibrillar force generation. This response is mediated, in part, by impaired myosin cross-bridge kinetics and decreased myofibrillar protein levels.

The increase in non-cross-bridge forces after stretch of activated striated muscle is related to titin isoforms.

Skeletal muscles present a non-cross-bridge increase in sarcomere stiffness and tension on Ca(2+) activation, referred to as static stiffness and static tension, respectively. It has been hypothesized that this increase in tension is caused by Ca(2+)-dependent changes in the properties of titin molecules. To verify this hypothesis, we investigated the static tension in muscles containing different titin isoforms. Permeabilized myofibrils were isolated from the psoas, soleus, and heart ventricle from the rabbit, and tested in pCa 9.0 and pCa 4.5, before and after extraction of troponin C, thin filaments, and treatment with the actomyosin inhibitor blebbistatin. The myofibrils were tested with stretches of different amplitudes in sarcomere lengths varying between 1.93 and 3.37 µm for the psoas, 2.68 and 4.21 µm for the soleus, and 1.51 and 2.86 µm for the ventricle. Using gel electrophoresis, we confirmed that the three muscles tested have different titin isoforms. The static tension was present in psoas and soleus myofibrils, but not in ventricle myofibrils, and higher in psoas myofibrils than in soleus myofibrils. These results suggest that the increase in the static tension is directly associated with Ca(2+)-dependent change in titin properties and not associated with changes in titin-actin interactions.

Reduced passive force in skeletal muscles lacking protein arginylation.

Arginylation is a posttranslational modification that plays a global role in mammals. Mice lacking the enzyme arginyltransferase in skeletal muscles exhibit reduced contractile forces that have been linked to a reduction in myosin cross-bridge formation. The role of arginylation in passive skeletal myofibril forces has never been investigated. In this study, we used single sarcomere and myofibril measurements and observed that lack of arginylation leads to a pronounced reduction in passive forces in skeletal muscles. Mass spectrometry indicated that skeletal muscle titin, the protein primarily linked to passive force generation, is arginylated on five sites located within the A band, an important area for protein-protein interactions. We propose a mechanism for passive force regulation by arginylation through modulation of protein-protein binding between the titin molecule and the thick filament. Key points are as follows: 1) active and passive forces were decreased in myofibrils and single sarcomeres isolated from muscles lacking arginyl-tRNA-protein transferase (ATE1). 2) Mass spectrometry revealed five sites for arginylation within titin molecules. All sites are located within the A-band portion of titin, an important region for protein-protein interactions. 3) Our data suggest that arginylation of titin is required for proper passive force development in skeletal muscles.

Non-crossbridge forces in activated striated muscles: a titin dependent mechanism of regulation?

When skeletal muscles are stretched during activation in the absence of myosin-actin interactions, the force increases significantly. The force remains elevated throughout the activation period. The mechanism behind this non-crossbridge force, referred to as static tension, is unknown and generates debate in the literature. It has been suggested that the static tension is caused by Ca(2+)-induced changes in the properties of titin molecules that happens during activation and stretch, but a comprehensive evaluation of such possibility is still lacking. This paper reviews the general characteristics of the static tension, and evaluates the proposed mechanism by which titin may change the force upon stretch. Evidence is presented suggesting that an increase in intracellular Ca(2+) concentration leads to Ca(2+) binding to the PEVK region of titin. Such binding increases titin stiffness, which increases the overall sarcomere stiffness and causes the static tension. If this form of Ca(2+)-induced increase in titin stiffness is confirmed in future studies, it may have large implications for understating of the basic mechanisms of muscle contraction.

Nitrosative modifications of the Ca2+ release complex and actin underlie arthritis-induced muscle weakness.

OBJECTIVE: Skeletal muscle weakness is a prominent clinical feature in patients with rheumatoid arthritis (RA), but the underlying mechanism(s) is unknown. Here we investigate the mechanisms behind arthritis-induced skeletal muscle weakness with special focus on the role of nitrosative stress on intracellular Ca(2+) handling and specific force production. METHODS: Nitric oxide synthase (NOS) expression, degree of nitrosative stress and composition of the major intracellular Ca(2+) release channel (ryanodine receptor 1, RyR1) complex were measured in muscle. Changes in cytosolic free Ca(2+) concentration ([Ca(2+)]i) and force production were assessed in single-muscle fibres and isolated myofibrils using atomic force cantilevers. RESULTS: The total neuronal NOS (nNOS) levels were increased in muscles both from collagen-induced arthritis (CIA) mice and patients with RA. The nNOS associated with RyR1 was increased and accompanied by increased [Ca(2+)]i during contractions of muscles from CIA mice. A marker of peroxynitrite-derived nitrosative stress (3-nitrotyrosine, 3-NT) was increased on the RyR1 complex and on actin of muscles from CIA mice. Despite increased [Ca(2+)]i, individual CIA muscle fibres were weaker than in healthy controls, that is, force per cross-sectional area was decreased. Furthermore, force and kinetics were impaired in CIA myofibrils, hence actin and myosin showed decreased ability to interact, which could be a result of increased 3-NT content on actin. CONCLUSIONS: Arthritis-induced muscle weakness is linked to nitrosative modifications of the RyR1 protein complex and actin, which are driven by increased nNOS associated with RyR1 and progressively increasing Ca(2+) activation.

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.

Arginylation of myosin heavy chain regulates skeletal muscle strength.

Protein arginylation is a posttranslational modification with an emerging global role in the regulation of actin cytoskeleton. To test the role of arginylation in the skeletal muscle, we generated a mouse model with Ate1 deletion driven by the skeletal muscle-specific creatine kinase (Ckmm) promoter. Ckmm-Ate1 mice were viable and outwardly normal; however, their skeletal muscle strength was significantly reduced in comparison to controls. Mass spectrometry of isolated skeletal myofibrils showed a limited set of proteins, including myosin heavy chain, arginylated on specific sites. Atomic force microscopy measurements of contractile strength in individual myofibrils and isolated myosin filaments from these mice showed a significant reduction of contractile forces, which, in the case of myosin filaments, could be fully rescued by rearginylation with purified Ate1. Our results demonstrate that arginylation regulates force production in muscle and exerts a direct effect on muscle strength through arginylation of myosin.

Post-immobilization eccentric training promotes greater hypertrophic and angiogenic responses than passive stretching in muscles of weanling rats.

This study investigated how different types of remobilization after hind limb immobilization, eccentric exercise and passive static stretching, influenced the adaptive responses of muscles with similar function and fascicle size, but differing in their contractile characteristics. Female Wistar weanling rats (21 days old) were divided into 8 groups: immobilized for 10 days, maintaining the ankle in maximum plantar flexion; immobilized and submitted to eccentric training for 10 or 21 days on a declining treadmill for 40min; immobilized and submitted to passive stretching for 10 or 21 days for 40min by maintaining the ankle in maximum dorsiflexion; control of immobilized; and control of 10 or 21 days. The soleus and plantaris muscles were analyzed using fiber distribution, lesser diameter, capillary/fiber ratio, and morphology. Results showed that the immobilization reduced the diameter of all fiber types, caused changes in fiber distribution and decreased the number of transverse capillaries in both muscles. The recovery period of the soleus muscle is longer than that of the plantaris after detraining. Moreover, eccentric training induced greater hypertrophic and angiogenic responses than passive stretching, especially after 21 days of rehabilitation. Both techniques demonstrated positive effects for muscle rehabilitation with the eccentric exercise being more effective.