Ca channels and skeletal muscle diseases

When observed under a microscope, skeletal muscle exhibits striations due to the highly organized arrangement of muscle proteins that interact with one another to induce muscle contraction. Muscle contraction requires transient increases in intracellular ‘Ca²⁺’ concentration. In this review, Ca²⁺ channels contributing to the functional integrity of intracellular Ca²⁺-release and extracellular Ca²⁺-entry during skeletal muscle contraction are reviewed in terms of their properties, newly emerging ancillary proteins to them, and their abnormalities related to human skeletal muscle diseases. Finally, the aim of this review is to show the big picture of the correlation among Ca²⁺ channels that participate in the Ca²⁺ homeostasis in skeletal muscle.     

Subclinical skeletal muscle involvement in long-QT syndrome

Deafness is said to be the only extracardiac manifestation of long-QT syndrome. Whether long-QT syndrome manifests in the skeletal muscle as well, has not been investigated so far. Six affected members of two families with long-QT syndrome without deafness (Romano–Ward syndrome) underwent a clinical neurological examination, nerve conduction studies and needle electromyography. The clinical neurological examination and nerve conduction studies were normal but abundant spontaneous activity (fibrillations and bursts of fibrillations) could be recorded from the right biceps brachii muscle (one patient) and the right abductor pollicis brevis muscle (all patients). Since all other causes were excluded, spontaneous discharges were interpreted to be related to the long-QT syndrome. In conclusion, long-QT syndrome does not seem to be confined to the heart but may involve the skeletal muscle subclinically as well.     

Involvement of cation transporting systems in myotonic diseases

Excitation-contraction coupling describes the series of events that beginning with propagated action potential on the muscle fiber surface membrane, leads to the twitch contraction of the fiber. Myotonic disorders are inherited diseases whose clinical manifestations include electrophysiological signs such as increased excitability and delayed relaxation of the muscles after voluntary contraction. All these disorders appear to be due to an abnormality of the muscle itself, since they persist after section or blocking of the motor nerve and after curarization. Most experimental and clinical data suggest that human myotonia arises from genetically-induced structural and functional alterations of the muscle cell membrane. The purpose of this review is to summarize the more recent developments in the molecular and pharmacological analysis of cation transporting systems such as ionic channels and (Na+,K+)-ATPase in myotonic disorders.     

Morphometric properties and innervation of muscle compartments in rat medial gastrocnemius

The rat medial gastrocnemius (MG) muscle is composed of the proximal and distal compartments. In this study, morphometric properties of the compartments and their muscle fibres at five levels of the muscle length and the innervation pattern of these compartments from lumbar segments were investigated. The size and number of muscle fibres in the compartments were different. The proximal compartment at the largest cross section (25% of the muscle length) had 34% smaller cross-sectional area but contained a slightly higher number of muscle fibres (max. 5521 vs. 5360) in comparison to data for the distal compartment which had the largest cross-sectional area at 40% of the muscle length. The muscle fibre diameters revealed a clear tendency within both compartments to increase along the muscle (from the knee to the Achilles tendon) up to 46.9?μm in the proximal compartment and 58.4?μm in the distal one. The maximal tetanic and single twitch force evoked by stimulation of L4, L5, and L6 ventral roots in whole muscle and compartments were measured. The MG was innervated from L4 and L5, only L5, or L5 and L6 segments. The proximal compartment was innervated by axons from L5 or L5 and L4, and the distal one from L5, L5 and L6, or L5 and L4 segments. The forces produced by the compartments summed non-linearly. The tetanic forces of the proximal and distal compartments amounted to 2.24 and 4.86?N, respectively, and their algebraic sums were 11% higher than the whole muscle force (6.37?N).     

A proteome map of murine heart and skeletal muscle

The balance of hypertrophy and atrophy is critical for the adaptation of cardiac and skeletal muscle mass to the demands of the environment and when deregulated can cause disease. Here we have used a proteomics approach to generate protein reference maps for the mouse heart and skeletal muscle, which provide a molecular basis for future functional and pathophysiological studies. The reference map provides information on molecular mass, pI, and literature data on function and localization, to facilitate the identification of proteins based on their migration in 2-D gels. In total, we have identified 351 cardiac and 284 skeletal muscle protein spots, representing 249 and 214 different proteins, respectively. In addition, we have visualized the protein pattern of mouse heart and skeletal muscle at defined conditions comparing knockout (KO) animals deficient in the sarcomeric protein titin (a genetic atrophy model) and control littermates. We found 20 proteins that were differently expressed linking titin's kinase region to the heat-shock- and proteasomal stress response. Taken together, the established reference maps should provide a suitable tool to relate protein expression and PTM to cardiovascular and skeletal muscle disease using the mouse as an animal model.     

Galectin-1 expression in innervated and denervated skeletal muscle

Galectin-1 is a soluble carbohydrate-binding protein with a particularly high expression in skeletal muscle. Galectin-1 has been implicated in skeletal muscle development and in adult muscle regeneration, but also in the degeneration of neuronal processes and/or in peripheral nerve regeneration. Exogenously supplied oxidized galectin-1, which lacks carbohydrate-binding properties, has been shown to promote neurite outgrowth after sciatic nerve sectioning. In this study, we compared the expression of galectin-1 mRNA and immunoreactivity in innervated and denervated mouse and rat hind-limb and hemidiaphragm muscles. The results show that galectin-1 mRNA expression and immunoreactivity are up-regulated following denervation. The galectin-1 mRNA is expressed in the extrasynaptic and perisynaptic regions of the muscle, and its immunoreactivity can be detected in both regions by Western blot analysis. The results are compatible with a role for galectin-1 in facilitating reinnervation of denervated skeletal muscle.     

Alanine metabolism in skeletal muscle in tissue culture

Alanine production by skeletal muscle in tissue culture was studied using an established myogenic line (L₆) of rat skeletal muscle cells. Correlation analyses were performed on rates of metabolism of alanine, glucose, lactate and pyruvate over incubation periods up to 96 h. Alanine production did not correlate significantly with glucose utilization (r = 0.24, P < 0.20). Alanine production, however, did correlate with lactate production (r = 0.72, P < 0.0005) as well as medium (r = 0.50, P < 0.025) and intracellular (r = 0.85, P < 0.0005) pyruvate concentrations. The intercepts of the latter two correlation analyses indicated that when medium or cell pyruvate fell below 0.28 mM or 1 nmol/mg protein, respectively, net alanine consumption occurred. Alanine synthesis also correlated (r = 0.71, P < 0.0005) with the percent change in the cell mass action ratio for the sum of the alanine and aspartate aminotransferase reactions, i.e., [alanine] [malate]/[aspartate] [lactate]. These results suggest that alanine production is not necessarily linked to the rate of glucose utilization but rather to pyruvate overflow above a critical intracellular level; under conditions of pyruvate overflow, alanine synthesis is driven by the tendency to establish equilibrium between metabolites of the linked amino acid transaminases in skeletal muscle.     

The innervation pattern of crustacean skeletal muscle

Summary The innervation pattern of distal muscle fibers of the opener muscle of walking legs of crayfish (Astacus leptodactylus) was investigated using methylene-blue staining, cobalt infiltration, and electron microscopy. A quantitative analysis of the entire innervation of single muscle fibers was attempted.It was found that instead of the generally assumed parallel array of numerous excitatory and inhibitory terminals, innervation consists of only a few branched terminals. The branches of excitatory and inhibitory terminals lie side-by-side. Both types are characterized by numerous varicosities (see Fig. 9B). The aggregate length of excitatory as well as inhibitory terminals on one muscle fiber is, on the average, about 1,500 m with a total of 152 varicosities spaced about 10 m apart. The average diameter of the varicosities is 4.26 m, that of the connecting thin segments about 0.5 m. Total terminal surface of motor or inhibitory terminals amounts to about 10,000 m² per muscle fiber. There are approximately 2,000 motor synapses on each muscle fiber, but their average total area is only about 6% of the terminal membrane area, or 0.06% of the (idealized) muscle fiber surface.There are conspicuous differences in the postsynaptic specializations associated with excitatory and inhibitory terminals; these are described in detail.The results are discussed in a functional context and with regard to design and results of electrophysiological experiments.Supported by Sonderforschungsbereich 138 of the Deutsche Forschungsgemeinschaft     

Mechanism of latency relaxation in frog skeletal muscle

The latency relaxation is a small drop of tension before skeletal muscle begins to develop active tension. This phenomenon was found nearly one century ago but its origin has not been clarified. In this review, the hypotheses for its mechanism are discussed in terms of the recent experimental results using X-ray diffraction. The latency relaxation takes place almost simultaneously as the structural change of the regulatory protein troponin, an unspecified structural change of the thick filament, and increase in stiffness. It seems difficult to associate all of these with the latency relaxation by assuming a simple mechanism.     

Omega-3 fatty acids differentially modulate enzymatic anti-oxidant systems in skeletal muscle cells

During physical activity, increased reactive oxygen species production occurs, which can lead to cell damage and in a decline of individual’s performance and health. The use of omega-3 polyunsaturated fatty acids as a supplement to protect the immune system has been increasing; however, their possible benefit to the anti-oxidant system is not well described. Thus, the aim of this study was to evaluate whether the omega-3 fatty acids (docosahexaenoic acid and eicosapentaenoic acid) can be beneficial to the anti-oxidant system in cultured skeletal muscle cells. C2C12 myocytes were differentiated and treated with either eicosapentaenoic acid or docosahexaenoic acid for 24 h. Superoxide content was quantified using the dihydroethidine oxidation method and superoxide dismutase, catalase, and glutathione peroxidase activity, and expression was quantified. We observed that the docosahexaenoic fatty acids caused an increase in superoxide production. Eicosapentaenoic acid induced catalase activity, while docosahexaenoic acid suppressed superoxide dismutase activity. In addition, we found an increased protein expression of the total manganese superoxide dismutase and catalase enzymes when cells were treated with eicosapentaenoic acid. Taken together, these data indicate that the use of eicosapentaenoic acid may present both acute and chronic benefits; however, the treatment with DHA may not be beneficial to muscle cells.     

Changes in skeletal muscle oxygenation during incremental exercise measured with near infrared spectroscopy

To determine the change in muscle oxygenation in response to progressively increasing work rate exercise, muscle oxyhemoglobin + oxymyoglobin saturation was measured transcutaneously with near infrared spectroscopy in the vastus lateralis muscle during cycle ergometry. Studies were done in 11 subjects while gas exchange was measured breath-by-breath. As work rate was increased, tissue oxygenation initially either remained constant near resting levels or, more usually, decreased. Near the work rate and metabolic rate where significant lactic acidosis was detected by excess CO₂ production (lactic acidosis threshold, LAT), muscle oxygenation decreased more steeply. As maximum oxygen uptake ( ) was approached, the rate of desaturation slowed. In 8 of the 11 subjects, tissue O₂ saturation reached a minimum which was sustained for 1–3 min before was reached. The LAT correlated with both the (r = 0.95,P < 0.0001) and the work rate (r = 0.94,P < 0.0001) at which the rate of tissue O₂ desaturation accelerated. These results describe a consistent pattern in the rate of decrease in muscle oxygenation, slowly decreasing over the lower work rate range, decreasing more rapidly in the work rate range of the LAT and then slowing at about 80% of , approaching or reaching a minimum saturation at .     

The skeletal muscles of rotifers and their innervation

The skeletal muscles of rotifers are monocellular or occasionally bicellular. They display great diversity of cytological features correlated to their functional differentiation. The cross-striated fibers of some retractors are fast contracting and relaxing, with A-band lengths of 0.7 µm to 1.6 µm, abundant sarcoplasmic reticulum and dyads. Other retractors and the circular muscles are tonic fibers (A band > 3 µm), stronger (large volume of myoplasm) or with greater endurance (superior volume of mitochondria/ myoplasm). All of these retractor muscles are coupled by gap junctions and are innervated at two symmetrical points; they constitute two motor units implicated in withdrawal behaviour.The muscles inserted on the ciliary roots of the cingulum control swimming. They are multi-innervated and each of them constitute one motor unit. They have characteristics of very fast fibers; the shortest A-band length is 0.5 µm in Asplanchna.All the skeletal muscles of bdelloids are smooth or obliquely striated as are some skeletal muscles of monogononts. These muscles are well suited for maximum shortening and are either phasic or tonic fibers.All rotifer skeletal muscles originate from ectoderm and contain thin and thick myofilaments whose diameters are identical to those of actin and myosin filaments in vertebrate fast muscles or in insect flight muscles. There are no paramyosinic features in the thick myofilaments. The insertion, innervation, coupling by gap junctions and other cytological differentiations of rotifer skeletal muscles are reviewed and their phylogeny discussed.     

Developmental aspects of the innervation of skeletal muscle fibers in the chick embryo

Summary The M. complexus in the chick, commonly called the hatching muscle, undergoes conspicuous growth during the latter stages of embryonic development. Myogenesis of this muscle was compared to that of M. biceps femoris with regard to development of types of muscle fiber and their innervation. In both muscles fibers are of relatively uniform size and show little growth in diameter between 12 days of development and hatching; fibers develop continuously and display a wide range of diameters at all stages.Initial thickenings on the sarcolemma of fibers where axons are closely approximate were first observed at 10 days of development in both muscles. In both muscles fibers are innervated prior to fibers. Terminal axon networks bridge intercellular spaces and contact fibers in different myogenic clusters, fibers that develop on the surface membrane of fibers exhibit focal thickenings of the membrane and some cell projections that are directed toward axon- fiber contacts. These changes occurred only in fibers of M. complexus.At 14 days of embryogenesis, the processes of synaptogenesis and of myelin formation are less advanced in M. biceps femoris than in M. complexus. At this stage a fibers were observed to be innervated in M. complexus, but not yet in M. biceps femoris. Each fiber was observed to be encircled by several preterminal axons.It is concluded that the earlier development of M. complexus is correlated with an equally early development of nerve-muscle interactions.This work was supported in part by a grant from the Muscular Dystrophy Association of America, Inc.Postdoctoral Fellow of the Muscular Dystrophy Association I would like to thank Professor Dr. H. Tamate for his valuable advise. I am also grateful to Dr. L. Doerr, H. Stokes and Judi K. Lund for their advice and skilled technical assistance     

Autophagy in skeletal muscle

Muscle mass represents 40-50% of the human body and, in mammals, is one of the most important sites for the control of metabolism. Moreover, during catabolic conditions, muscle proteins are mobilized to sustain gluconeogenesis in the liver and to provide alternative energy substrates for organs. However, excessive protein degradation in the skeletal muscle is detrimental for the economy of the body and it can lead to death. The ubiquitin-proteasome and autophagy-lysosome systems are the major proteolytic pathways of the cell and are coordinately activated in atrophying muscles. However, the role and regulation of the autophagic pathway in skeletal muscle is still largely unknown. This review will focus on autophagy and discuss its beneficial or detrimental role for the maintenance of muscle mass.     

Expression and localization of myotonic dystrophy protein kinase in human skeletal muscle cells determined with a novel antibody: possible role of the protein in cytoskeleton rearrangements during differentiation

Myotonic dystrophy is a multisystemic disorder, due to a CTG triplet expansion at the 3'UTR of the DM1 gene encoding for myotonic dystrophy protein kinase. Recent studies indicate that decreased DMPK levels could account for part of the symptoms suggesting a role of this protein in skeletal muscle differentiation. To investigate this aspect, polyclonal antibodies were raised against two peptides of the catalytic domain and against the human full-length DMPK (DMFL). In western blots, anti-hDMFL antibody was able to detect low amounts of purified human recombinant protein and recognized the splicing isoforms in heart and stomach of overexpressing mice. In human muscle extracts, this antibody specifically recognized a protein of apparent molecular weight of 85 kDa and it specifically stained neuromuscular junctions in skeletal muscle sections. In contrast, both anti-peptide antibodies demonstrated low specificity for either denatured or native DMPK, suggesting that these two epitopes are probably cryptic sites. Using anti-hDMFL, the expression and localization of DMPK was studied in human skeletal muscle cells (SkMC). Western blot analysis indicated that the antibody recognizes a main protein of apparent MW of 75 kDa, which appears to be expressed during differentiation into myotubes. Immunolocalization showed low levels of DMPK in the cytoplasm of undifferentiated cells; during differentiation the staining became more intense and was localized to the terminal part of the cells, suggesting that DMPK might have a role in cell elongation and fusion.     

The innervation of a ventral abdominal protractor muscle in Locusta

The innervation of the external ventral protractor muscle of the VIIth abdominal segment (M234) of Locusta migratoria is described using a combination of neurophysiological and neuroanatomical techniques. Cobalt backfills of the nerve innervating M234 revealed two neurons each with soma and fields of arborization in the VIIth abdominal ganglion. In addition, extracellular stimulation of the nerve while gradually increasing the stimulus amplitude resulted in a stepwise increase in both the excitatory junction potential amplitude and twitch amplitude so that two different amplitudes of each were observed indicating the sequential recruitment of two motor neurons. 4-Di-2-ASP stains of M234 revealed pre-synaptic boutons at M234 and a neurohaemal plexus covering the nerve the latter being corroborated by electron microscopic examination of nerve sections. Electron microscopic examination of M234 revealed two axon terminal types, one which is similar to the neurohaemal varicosities over the nerve, containing granules of high electron-density, and one which contains larger granules of medium electron-density. Both terminals types also contained small electron lucent vesicles. Finally, twitch contractions of M234 were modulated by glutamate, proctolin, octopamine, and SchistoF-LRFamide.Abbreviations EGAA Enhanced Graphics Acquisition and Analysis system by RC-Electronics, California - EJP excitatory junction potential - OA octopamine     

MicroRNAs in skeletal muscle biology and exercise adaptation

MicroRNAs (miRNAs) have emerged as important players in the regulation of gene expression, being involved in most biological processes examined to date. The proposal that miRNAs are primarily involved in the stress response of the cell makes miRNAs ideally suited to mediate the response of skeletal muscle to changes in contractile activity. Although the field is still in its infancy, the studies presented in this review highlight the promise that miRNAs will have an important role in mediating the response and adaptation of skeletal muscle to various modes of exercise. The roles of miRNAs in satellite cell biology, muscle regeneration, and various myopathies are also discussed.     

低剂量创伤弧菌感染小鼠心肌与骨骼肌电镜超微结构观察比较

目的:观察低剂量创伤弧菌感染小鼠心肌、骨骼肌的超微结构变化,比较创伤弧菌引起的特征性病变下肢水肿骨骼肌病变与心脏病变出现的次序,探讨下肢水肿是否存在与心肌病变有关.方法:16只6～8周ICR(清洁级)小鼠.实验组12只腹腔注射＜LD50(1.34×107个/ml)的菌量(4.45×105个/ml)0.2ml,4只注射生理盐水0.2 ml作为菌液对照.分别取1 h、3 h、6 h、12 h小鼠心肌、后肢骨骼肌肌肉组织0.1cm×0.1 cm×0.1 cm置电镜固定液,超薄切片观察超微结构.结果:引起的动物模型实验组小鼠主要的实质性病变在心肌.实验组3 h就发现肌原纤维间隙扩大,肌丝断裂,肌膜下水肿较明显.6 h肌丝排列紊乱,疏松,局灶性肌丝断裂溶解.12 h核固缩水肿,间隙水肿,肌丝断裂,线粒体肿胀.而骨骼肌肌肉超微结构变化不明显,3 h、6 h和12 h实验组未死亡小鼠只表现肌组织间质变化,肌浆网扩张,组织间质水肿,胶原纤维排列稀疏、溶解.结论:本实验心肌、骨骼肌水肿与临床下肢水肿的症状相吻合,并提示心肌的实质病变明显早于骨骼肌,比较而言创伤弧菌所致病变是以重要脏器心脏的损伤为主,早期创伤弧菌对骨骼肌肌肉的实质性损伤并不明显.由创伤弧菌所致的原发性败血症表现双下肢出血性水肿可能不是创伤弧菌初始病变.     

De novo purine synthesis in skeletal muscle

Myoblast and primary muscle cultures from rat were found to contain the complete pathway of de novo purine nucleotide synthesis. Quantitative assessment of the pathway in skeletal muscle in mice in vivo, revealed a more intensive purine production in muscle than in liver. Skeletal muscle is thus a major site of de novo purine production in the mammalian body.     

Role of innervation on the embryonic development of skeletal muscle

Summary The extent to which the motor innervation regulates the embryonic development of skeletal muscle was investigated by comparing changes in normal, aneural, and paralyzed superior oblique muscle of the duck embryo. The muscle was made aneural by permanently destroying the trochlear motor neurons with electrocautery on day 7 i.e., three days prior to innervation. Embryos were paralyzed by daily application of -bungarotoxin onto the chorioallantoic membrane from day 10 onwards. The differentiation of myoblasts and myotubes in the aneural muscle was severely affected and did not progress to the myofiber stage. A mass of dead cells in the aneural muscle was replaced by connective tissue. Although the differentiation of myoblasts and myotubes was also retarded in the paralyzed muscle, numerous muscle cells progressed to the myofiber stage. Neuromuscular junctions of normal ultrastructure were seen in all paralyzed muscles. Degeneration of some cells in the paralyzed muscle occurred but there was no evidence of a massive wave of cell death similar to that observed in the aneural muscle. These observations suggest that both the trophic factors from the nerve and the nerve-evoked muscle activity are essential for the execution of the developmental program of the muscle. Trophic factors may play a larger role in differentiation, and maintenance of the muscle than muscle activity.Supported by a grant from the Muscular dystrophy Association and a grant from NIHWe are grateful to Beth McBride and Greg Oblak for their technical assistance