Features of the response of human striated muscle fibers to hypokinesia combined with physical loading

Antiorthostatic hypokinesia for 30 days produces in human beings certain atrophic changes and decreasing metabolic level in both types of the skeletal muscle fibers, the metabolic activity being more decreased in the fibers of the II type. The physical loading of speed-power character does not prevent atrophy phenomena, nevertheless produces an increase of the metabolic activity in both types of the fibers. Changes of adaptive character, resulted from the physical loading, do not ensure a complete compensation of the negative hypoxic effect, especially in relation to contractile structures of the fiber.     

Changes in the respiratory muscles and their microcirculatory bed during exposure of the body to chronic hypoxia and its aftereffects

Changes of the respiratory muscles have been studied under the effect of a rarefied atmosphere when the pressure is 560 mm Hg (2,500 m), 405 mm Hg (5,000 m), 286 mm Hg (7,500 m) and during the period of their aftereffect. The experimental group consists of 260 and the control group--of 130 white rats. Adaptation of the respiratory muscles to the effect of the rarefied atmosphere at the pressure of 560 mm Hg takes place mainly at the expense of certain functional changes of the microcirculatory bed and hypertrophy of the muscle fibers. The period of aftereffect is characterized with normalization of these phenomena. Adaptation of the respiratory muscles at the pressure of 405 mm Hg takes place at the expense of hypertrophy of the muscle fibers, small destructive changes and a complex rearrangement of the microcirulatory bed. During the period of aftereffect, by the 40th day no destructive changes are noted in the muscle tissue. The capillary bed undergoes some rearrangements by the 42d -56th day with increasing quantitative indices per area unit. This results in improvement of the muscles nutrition. In the respiratory muscles at the pressure 286 mm Hg, atrophic changes in the muscle fibers take place at certain stages of the experiment. Essential destructive changes are observed, rearrangement of the microcirculatory bed with decreasing convolution of the longitudinal capillaries and a decreasing number of the transversal capillaries are noted by the end of the experiment. During the period of aftereffect, by the 56th day, the external and internal intercostal muscles completely restore their structure, and in the diaphragm the destructive changes remain. By the same time, the microcirculatory bed becomes more rare at the expense of decreasing capillarization of the muscle fibers and a decreasing number of the transversal capillaries.     

Changes of the NADP-linked malic enzyme in the developing rat skeletal muscle

The activities of NADP-linked malic enzyme, hexose monophosphate shunt dehydrogenases and NADP-linked isocitrate dehydrogenase were studied during development of skeletal muscle and compared with those in the liver. The variation patterns of malic enzyme activity in the liver and in the skeletal muscle were very similar, however the amplitude of the changes was different. The enzyme activity increased approx 16-fold in the liver and about 2-fold in skeletal muscle at the same stage of development. In skeletal muscle the increase of the malic enzyme activity was only slightly higher than of lactic dehydrogenase and citrate synthase. Studies on the intracellular distribution of malic enzyme in skeletal muscle showed that both mitochondrial and extramitochondrial enzymes increased between 20th and 37th day of life, the increase of the extramitochondrial enzyme being more pronounced. The hexose monophosphate shunt dehydrogenases activity showed an increase in the liver but no change was observed in the skeletal muscle at the weaning time. Changes in the activity of the liver and skeletal muscle isocitrate dehydrogenase were not significant between 10th and 80th day of life. The results suggest that the malic enzyme in the liver is playing a different physiological role than in the skeletal muscle.     

Structural and metabolic characteristics of human soleus fibers after long duration spaceflight

The fiber size decline, alterations in fiber metabolic potential and increase of connective tissue component were shown in human m. vastus lateralis after short and long-duration space flights and in m.soleus and m.vastus lateralis after 120 day head down tilt bed rest. It is known from rat and monkey studies that the exposure to weightlessness leads to the most pronounced changes in postural muscles, e.g. m.soleus. It was shown that 17 day space flight induced significant decrease of fiber cross-sectional area and slow-to-fast fiber type transformation in human soleus. But in the cited work the fiber population under study was limited like in most single fiber technique analyses. The present study was purposed to investigate the structural and metabolic properties of soleus muscle in Russian cosmonauts exposed to 129-day space flight on board of the International Space Station.     

miR-222 is involved in the regulation of genistein on skeletal muscle fiber type

In skeletal muscle, the composition of the fiber types has a profound impact on athletic performance, such as endurance or strength output. The proportions of muscle fiber types have also been associated with certain diseases, including dyskinesia, obesity and insulin resistance. Genistein, a natural estrogen, has been demonstrated to regulate fatty acid oxidation and insulin sensitivity in skeletal muscle. However, it is unknown whether genistein can regulate skeletal muscle fiber types. Furthermore, the mechanism of its effect on skeletal muscle energy metabolism is not entirely clear. In this study, in vivo and in vitro experiments were used to explore the effect of genistein on the muscle fiber-type transitions and muscle metabolism. The results indicated that genistein not only promotes skeletal muscle development but increases the expression of slow muscle fibers in mice as well. It was also demonstrated that genistein altered the ratios of fiber type and promoted mitochondrial biogenesis in C2C12 myoblasts. Interestingly, the expression of miR-222 was decreased by genistein, and it was demonstrated that this microRNA targets the PGC1α gene. In C2C12 myoblasts, miR-222 appears to regulate fiber type conversion and mitochondrial biogenesis. However, this function was significantly reduced following genistein treatment. These results suggest that miR-222 may be involved in the regulation of genistein on skeletal muscle fiber and muscle metabolism, and genistein may be used to improve muscle health.     

Role of afferent control in maintaining structural and metabolic characteristics of stretched soleus in rats exposed to hindlimb suspension

It is known that gravitational unloading (GU) induces atrophy of skeletal muscles and slow-to-fast muscle fiber (MF) transformation. Stretching of m. soleus prevents those changes, probably afferent information from the stretched muscle acting as triggering mechanism. It was shown that EMG of suspended animals or of stretched muscle is similar to that of control animals. Our study was aimed at revealing contribution of the afferent information from stretched m.soleus exposed to GU in maintenance of cross-sectional area (CSA) of MF and of myosine heavy chains and oxidative potential of skeletal muscles.     

Effects of long-duration bed rest on structural compartments of m. soleus in man

Magnetic resonance imaging (MRI), histomorphometry and electron microscopy of muscle demonstrate that long-term exposure to actual or simulated weightlessness (including head down bed rest) leads to decreased volume of antigravity muscles in mammals. In muscles interbundle space is occupied by the connective tissue. Rat studies show that hindlimb unloading induces muscle fiber atrophy along with increase in muscle non-fiber connective tissue compartment. Beside that, usually 20% of the muscle fiber volume is comprised by non-contractile (non-myofibrillar) compartment. The aim of the present study was to compare changes in muscle volume, and in muscle fiber size with alterations in myofibrillar apparatus, and in connective tissue compartment in human m. soleus under conditions of 120 day long head down bed rest (HDBR).     

Structural metabolic processes in the skeletal muscle tissue of chick embryos administered actinomycin D

The development of the skeletal muscle tissue has been studied cytophotometrically, electron microscopically and radioautographically at administration of actinomycin-D (0.2 mcg/g) to the 11- and 15-day-old chick embryos). Different character of restorative processes under the conditions when RNA synthesis is disturbed by actinomycin-D administration is noted: before morphologically distinguished myosatellites appear (before the 13th-14th day of embryogenesis) and after myosatellites appearance (from the 14th-15th day of development). Evidently, the myosatellites are the muscle cells resistive to certain external factors, they ensure an effective adaptation of the skeletal muscle tissue to unfavourable effects. When the satellite cells appear, the skeletal muscle tissue acquires a new quality as a dynamically stable cambial system.     

Are cultured human myotubes far from home?

Satellite cells can be isolated from skeletal muscle biopsies, activated to proliferating myoblasts and differentiated into multinuclear myotubes in culture. These cell cultures represent a model system for intact human skeletal muscle and can be modulated ex vivo. The advantages of this system are that the most relevant genetic background is available for the investigation of human disease (as opposed to rodent cell cultures), the extracellular environment can be precisely controlled and the cells are not immortalized, thereby offering the possibility of studying innate characteristics of the donor. Limitations in differentiation status (fiber type) of the cells and energy metabolism can be improved by proper treatment, such as electrical pulse stimulation to mimic exercise. This review focuses on the way that human myotubes can be employed as a tool for studying metabolism in skeletal muscles, with special attention to changes in muscle energy metabolism in obesity and type 2 diabetes.     

Spinal cord-muscle relations: their role in neuro-muscular development in birds

In the present study we focused our attention on the role of spinal cord-muscle interactions in the development of muscle and spinal cord cells. Four experimental approaches were used: 1) muscle fiber-spinal cord co-culture; 2) chronic spinal cord stimulation in chick embryos; 3) direct electrical stimulation of the denervated chick muscle; 4) skeletal muscle transplantation in close apposition to the spinal cord in chick embryos. The characteristics of mATPase and energetic metabolism enzyme activities and of myosin isoform expression were used as markers for fiber types in two peculiar muscles, the fast-twitch PLD and the slow-tonic ALD. In vitro, in the absence of neurons, myoblasts can express some characteristics of either slow or fast muscle types according to their origin, while in the presence of neurons, muscle fiber differentiation seems to be related to the spontaneous rhythm delivered by the neurons. The in ovo experiments of chronic spinal cord stimulation demonstrate that the differentiation of the fast and slow muscle features appears to be rhythm dependent. In the chick, direct stimulation of denervated muscles shows that the rhythm of the muscle activity is also involved in the control of muscle properties. In chick embryos developing ALD, the changes induced by modifications of muscle tension demonstrate that this factor also influences muscle development. Other experiments show that muscle back-transplantation can alter the early spinal cord development.     

Lymphatic bed of the heart in the post-resuscitation period

In the experiment performed on 96 rabbits, by means of silver nitrate impregnation and Gerota mass injection methods, changes in the epicardial lymphatic bed have been investigated after 5-minutes' clinical death caused by hemorrhage. During first hours after resuscitation certain signs of the lymphatic drenage activation are revealed. In most of the animals the specific density of the lymphatic bed increases, capillaries and postcapillaries dilate, hyperargirophilia of cytoplasm and nuclei of endotheliocytes appears. In the animals sacrificed against the background of a severe state (with certain, signs of a pronounced hypoxia), there are dystrophically and degeneratively altered endotheliocytes. This is accompanied with hyper- hypoargyrophilia of endothelium, fragmentation of its cell borders, with deterioration of injection ability of the lymph outflow pathways. In the rabbits survived, by the 7th-14th days a gradual normalization of the parameters studied takes place.     

Invited Review: plasticity and energetic demands of contraction in skeletal and cardiac muscle.

Numerous studies have explored the energetic properties of skeletal and cardiac muscle fibers. In this mini-review, we specifically explore the interactions between actin and myosin during cross-bridge cycling and provide a conceptual framework for the chemomechanical transduction that drives muscle fiber energetic demands. Because the myosin heavy chain (MHC) is the site of ATP hydrolysis and actin binding, we focus on the mechanical and energetic properties of different MHC isoforms. Based on the conceptual framework that is provided, we discuss possible sites where muscle remodeling may impact the energetic demands of contraction in skeletal and cardiac muscle.     

Cessation of biomechanical stretch model of C2C12 cells models myocyte atrophy and anaplerotic changes in metabolism using non-targeted metabolomics analysis

Studies of skeletal muscle disuse, either in patients on bed rest or experimentally in animals (immobilization), have demonstrated that decreased protein synthesis is common, with transient parallel increases in protein degradation. Muscle disuse atrophy involves a process of transition from slow to fast myosin fiber types. A shift toward glycolysis, decreased capacity for fat oxidation, and substrate accumulation in atrophied muscles have been reported, as has accommodation of the liver with an increased gluconeogenic capacity. Recent studies have modeled skeletal muscle disuse by using cyclic stretch of differentiated myotubes (C2C12), which mimics the loading pattern of mature skeletal muscle, followed by cessation of stretch. We utilized this model to determine the metabolic changes using non-targeted metabolomics analysis of the media. We identified increases in amino acids resulting from muscle atrophy-induced protein degradation (largely sarcomere) that occurs with muscle atrophy that are involved in feeding the Kreb’s cycle through anaplerosis. Specifically, we identified increased alanine/proline metabolism (significantly elevated proline, alanine, glutamine, and asparagine) and increased α-ketoglutaric acid, the proposed Kreb’s cycle intermediate being fed by the alanine/proline metabolic anaplerotic mechanism. Additionally, several unique pathways not clearly delineated in previous studies of muscle unloading were seen, including: (1) elevated keto-acids derived from branched chain amino acids (i.e. 2-ketoleucine and 2-keovaline), which feed into a metabolic pathway supplying acetyl-CoA and 2-hydroxybutyrate (also significantly increased); and (2) elevated guanine, an intermediate of purine metabolism, was seen at 12 h unloading. Given the interest in targeting different aspects of the ubiquitin proteasome system to inhibit protein degradation, this C2C12 system may allow the identification of direct and indirect alterations in metabolism due to anaplerosis or through other yet to be identified mechanisms using a non-targeted metabolomics approach.     

Resistance training affects GLUT-4 content in skeletal muscle of humans after 19 days of head-down bed rest.

This study assessed the effects of inactivity on GLUT-4 content of human skeletal muscle and evaluated resistance training as a countermeasure to inactivity-related changes in GLUT-4 content in skeletal muscle. Nine young men participated in the study. For 19 days, four control subjects remained in a -6 degrees head-down tilt at all times throughout bed rest, except for showering every other day. Five training group subjects also remained at bed rest, except during resistance training once in the morning. The resistance training consisted of 30 isometric maximal voluntary contractions for 3 s each; leg-press exercise was used to recruit the extensor muscles of the ankle, knee, and hip. Pauses (3 s) were allowed between bouts of maximal contraction. Muscle biopsy samples were obtained from the lateral aspect of vastus lateralis (VL) muscle before and after the bed rest. GLUT-4 content in VL muscle of the control group was significantly decreased after bed rest (473 +/- 48 vs. 398 +/- 66 counts. min-1. microgram membrane protein-1, before and after bed rest, respectively), whereas GLUT-4 significantly increased in the training group with bed rest (510 +/- 158 vs. 663 +/- 189 counts. min-1. microgram membrane protein-1, before and after bed rest, respectively). The present study demonstrated that GLUT-4 in VL muscle decreased by approximately 16% after 19 days of bed rest, and isometric resistance training during bed rest induced a 30% increase above the value of GLUT-4 before bed rest.     

Experimental Evaluation of Fiber Orientation Based Material Properties of Skeletal Muscle in Tension

Biomechanical researches are essential to develop new techniques to improve the clinical relevance. Skeletal muscle generates the force which results in the motion of human body, so it is essential to study the mechanical and structural properties of skeletal muscle. Many researchers have carried out mechanical study of skeletal muscle with in-vivo testing. This work aims to examine anisotropic mechanical behavior of skeletal muscle with in vitro test (tensile test). It is important to understand the mechanical and structural behavior of skeletal muscle when it is subjected to external loading; the research aims to determine the structural properties of skeletal muscle by tensile testing. Tensile testing is performed on 5 samples of skeletal muscle of a goat at the rate of 1mm/min with fiber orientation along the length and 45° inclined to the length. It is found that muscle is stiffer in the direction parallel to the muscle fiber than at 45° to the muscle fibers. The tensile strength of the skeletal muscle along the fiber direction is 0.44 MPa at maximum load of 110 N and for direction 45° inclined to the muscle fibers, the strength is 0.234 MPa at max load 43 N. The displacement of Muscle sample against the maximum load is small along the length of the muscle fiber i.e. under longitudinal elongation [15.257 mm] as compared to 45° inclined to the length of skeletal muscle [17.775 mm] and under cross fiber elongation [19.7291mm by FEA]. The testing is not performed for 90° fiber orientation due to unavailability of soft tissue in cross fiber direction of the required specification, but finite element analysis is done on the skeletal muscle for the cross fiber orientation. As the fiber orientation within skeletal muscle differs with respect to the length of the muscle, the stiffness of skeletal muscle is also changing effectively. Hence skeletal muscle exhibits the anisotropic mechanical behavior.     

Dietary Fat Influences the Expression of Contractile and Metabolic Genes in Rat Skeletal Muscle

Dietary fat plays a major role in obesity, lipid metabolism, and cardiovascular diseases. To determine whether the intake of different types of dietary fats affect the muscle fiber types that govern the metabolic and contractile properties of the skeletal muscle, we fed male Wistar rats with a 15% fat diet derived from different fat sources. Diets composed of soybean oil (n-6 polyunsaturated fatty acids (PUFA)-rich), fish oil (n-3 PUFA-rich), or lard (low in PUFAs) were administered to the rats for 4 weeks. Myosin heavy chain (MyHC) isoforms were used as biomarkers to delineate the skeletal muscle fiber types. Compared with soybean oil intake, fish oil intake showed significantly lower levels of the fast-type MyHC2B and higher levels of the intermediate-type MyHC2X composition in the extensor digitorum longus (EDL) muscle, which is a fast-type dominant muscle. Concomitantly, MyHC2X mRNA levels in fish oil-fed rats were significantly higher than those observed in the soybean oil-fed rats. The MyHC isoform composition in the lard-fed rats was an intermediate between that of the fish oil and soybean oil-fed rats. Mitochondrial uncoupling protein 3, pyruvate dehydrogenase kinase 4, and porin mRNA showed significantly upregulated levels in the EDL of fish oil-fed rats compared to those observed in soybean oil-fed and lard-fed rats, implying an activation of oxidative metabolism. In contrast, no changes in the composition of MyHC isoforms was observed in the soleus muscle, which is a slow-type dominant muscle. Fatty acid composition in the serum and the muscle was significantly influenced by the type of dietary fat consumed. In conclusion, dietary fat affects the expression of genes related to the contractile and metabolic properties in the fast-type dominant skeletal muscle, where the activation of oxidative metabolism is more pronounced after fish oil intake than that after soybean oil intake.     

Human muscle signaling responses to 3-day head-out dry immersion

To date little is known about catabolic NO-dependent signaling systems in human skeletal muscle during early stages of gravitational unloading. The goal of the study was to analyze signaling pathways that determine the initial development of proteolytic events in human soleus muscle during short-term gravitational unloading (simulated microgravity). Gravitational unloading was simulated by 3-day head-out dry immersion. Before and after the immersion the samples of soleus muscle were taken under local anesthesia, using biopsy technique. The content of desmin, IRS-1, phospho-AMPK, total and phospho-nNOS in soleus of 6 healthy men was determined using Western-blotting before and after the dry-immersion. Three days of the dry immersion resulted in a significant decrease in desmin, phospho-nNOS and phospho-AMPK as compared to the pre-immersion values. The results of the study suggest that proteolytic processes in human soleus at the early stage of gravitational unloading are associated with inactivation of nNOS. Reduction in AMPK phosphorylation could serve as a trigger event for the development of primary atrophic changes in skeletal muscle.     

Succinate dehydrogenase activity of brachial muscle antagonists after decrease in the force load at various times of the day in rats

In 140 male rats of Wistar strain the distal part of one thoracic extremity has been amputated in such a way that places of the brachial muscle fixation remained intact. This resulted in decrease of the force component of muscle contractions (more intensive in the musculus triceps brachii) without essential swing and speed of movements. The operation was performed during one day at three series: at 7 a. m. 3 p. m. and 11 p. m. During following 45 days the muscles developing various contraction force were studied: the medial head of the musculus triceps brachii and brachial muscle. Succinate dehydrogenase (SDG) activity in muscle fibers (MF) was estimated photometrically. In MF initially characterized with various SDG activity, attention was payed to its dynamics and also relation of the fibers number, possessing various level of the enzymatic activity was taken into consideration. SDG activity in MF and relation of various MF change wavy-like, fluctuating from the control meaning and returning to them again. MF with different SDG activity do not similarly react to a sharp decrease of the force loading. Fibers with high enzymatic activity react to changes of the force loading immediately, intensity of the reaction at this stage slightly depends on degree of biomechanical changes; at later stages the response is differentiated: if the loading is decreased considerably (musculus triceps brachii), the process continues developing and increasing uninterruptedly, if it decreases moderately (brachial muscle)--it stabilizes and even has tendency to adaptation and returning towards the control state.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphological changes in rat skeletal muscle following reinnervation

Morphological changes appearing in the course of muscle regeneration after reinnervation of denervated M. soleus (slow) and M. tibialis anterior (fast) rat skeletal muscle were investigated. It was found that pathological changes typical for denervation atrophy (seen on the 10th day after crushing the sciatic nerve) and symptoms of regeneration (beginning about the 15th day) were much more pronounced in the soleus than in the tibialis muscle. Some stages of regeneration in the soleus muscle could be distinguished. The contractile material destructions were the first pathological changes that disappeared after the beginning of regeneration. In the second stage other denervation changes disappeared and intensive regeneration of muscle fibres was observed. In the next stage regeneration slowed down, and the reduction of the excess of muscle nuclei was visible. Four months after crushing the nerve, regeneration proceeded to completion with only some traces of the passed processes: in the soleus muscle, chains of sarcolemmal nuclei, satellite cells and newly formed muscle fibres were more often seen than in contralateral muscle; in the tibialis, collagen depots were present around the vessels and between muscle fascicles.