Neurovascular alignment in adult mouse skeletal muscles

OBJECTIVE: Muscle blood flow increases with motor unit recruitment. The physical relationships between somatic motor nerves, which control muscle fiber contraction, and arterioles, which control microvascular perfusion, are unexplored. The authors tested the hypothesis that motor axons align with arterioles in adult skeletal muscle. METHODS: Transgenic mice (C57BL/6 background, n = 5; 10 months of age) expressing yellow fluorescent protein in all motor nerves underwent vascular casting (Microfil). Excised epitrochlearis, gracilis, gluteus maximus, and spinotrapezius muscles were imaged at 380x and 760x and a computer-integrated tracing system (Neurolucida) was used to acquire 3-dimensional digital renderings of entire arteriolar and neural networks within each muscle. RESULTS: Arteriolar networks were typically approximately 3-fold longer than neural networks. Nerves coursed with arterioles until terminating at motor endplates. Across muscles, proximity analyses revealed that approximately 75% of total nerve length (9.8-48.8 mm) lay within 200 microm of the nearest arteriole (diameters of 15-60 microm). CONCLUSIONS: Somatic motor nerves and arterioles align closely within adult mammalian skeletal muscle. Understanding the signals governing neurovascular alignment may hold important clues for the advancement of tissue engineering and regeneration.     

Determination of potassium in the skeletal muscles by whole-body counting

Changes in muscular function are related to nutritional status, disease and physical activity. To study these relationships, it is desirable to be able to determine the whole body potassium content, which is characteristic to the muscular tissue. This can be achieved by measurements in a whole-body counter, identifying contributions from the upper and lower parts of the body. In a whole-body counter with large plastic scintillators, a special measuring methodology is required. Such a method of measuring (40)K in the leg muscles, extracting the part of the detector signal originating from the lower part of the body, has been developed and tested by independent phantom measurements. The results suggest that it is suited to perform regional measurements of body potassium but validation and implementation into clinical research are still necessary.     

Age-related loss of muscle fibres is highly variable amongst mouse skeletal muscles

Sarcopenia is the age-related loss of skeletal muscle mass and strength, attributable in part to muscle fibre loss. We are currently unable to prevent fibre loss because we do not know what causes it. To provide a platform from which to better understand the causes of muscle fibre death we have quantified fibre loss in several muscles of aged C57Bl/6J mice. Comparison of muscle fibre numbers on dystrophin-immunostained transverse tissue sections at 6 months of age with those at 24 months shows a significant fibre loss in extensor digitorum longus and soleus, but not in sternomastoid or cleidomastoid muscles. The muscles of the elderly mice were mostly lighter than their younger counterparts, but fibres in the elderly muscles were of about the same cross-sectional area. This study shows that the contribution of fibre death to sarcopenia is highly variable and that there is no consistent pattern of age-related fibre loss between skeletal muscles.     

The psychology of appearance in the elderly

Appearance counts heavily in human affairs. The good-looking have many advantages. These benefits also extend to old age. Elderly persons who preserve a youthful appearance (look young for their age) are likely to be more optimistic, more outgoing, and more social. They rate themselves more highly on many psychologic dimensions. Those who look older than their stated age die earlier and are not as healthy physiologically. Cosmetics can help the elderly attain some of the benefits enjoyed by the physically attractive.     

Accumulation of severely atrophic myofibers marks the acceleration of sarcopenia in slow and fast twitch muscles

The age-related decline in muscle mass, known as sarcopenia, exhibits a marked acceleration in advanced age. Although many studies have remarked upon the accumulation of very small myofibers, particularly at advanced stages of sarcopenia, the significance of this phenomenon in the acceleration of sarcopenia has never been examined. Furthermore, although mitochondrial dysfunction characterized by a lack of cytochrome oxidase (COX) activity has been implicated in myofiber atrophy in sarcopenia, the contribution of this phenotype to the accumulation of severely atrophied fibers in aged muscles has never been determined. To this end, we examined the fiber size distribution in the slow twitch soleus (Sol) and fast twitch gastrocnemius (Gas) muscles between young adulthood (YA) and senescence (SEN). We also quantified the abundance of COX deficient myocytes and their size attributes to gain insight into the contribution of this phenotype to myofiber atrophy with aging. Our data showed that the progression of muscle atrophy, particularly its striking acceleration between late middle age and SEN, was paralleled by an accumulation of severely atrophic myofibers (≤ 1000 μm² in size) in both Sol and Gas. On the other hand, we observed no COX deficient myofibers in Sol, despite nearly 20% of the myofibers being severely atrophic. Similarly, only 0.17 ± 0.06% of all fibers in Gas were COX deficient, and their size was generally larger (2375 ± 319 μm²) than the severely atrophied myofibers noted above. Collectively, our results suggest that similar processes likely contribute to the acceleration of sarcopenia in both slow twitch and fast twitch muscles, and that COX deficiency is not a major contributor to this phenomenon.     

缺氧预处理预防老年大鼠肢体制动后骨骼肌萎缩的研究

目的了解缺氧预处理对肢体制动老年大鼠骨骼肌萎缩的作用。方法将老龄SD大鼠随机分为对照组、单纯制动组和缺氧预处理组。缺氧预处理完成后,将单纯制动组和缺氧预处理组大鼠右后肢石膏固定3w。取比目鱼肌,称重、测定肌细胞直径、截面积。并取肌肉匀浆上清液检测SOD及MDA。结果后肢固定3w后,单纯制动组比目鱼肌湿重、肌细胞直径及肌纤维截面积低于对照组及缺氧预处理组。与单纯制动组相比,缺氧预处理组肌肉SOD活性明显升高,MDA水平明显降低。结论缺氧预处理可提高大鼠骨骼肌的抗氧化能力,对老年大鼠肢体制动后引起的骨骼肌萎缩有预防作用。     

Ectopic expression of glucagon receptor in skeletal muscles improves glucose homeostasis in a mouse model of diabetes

Aims/hypothesis  

Excessive secretion of glucagon partially contributes to the development of diabetic hyperglycaemia. However, complete blocking of glucagon action will lead to adverse effects, since glucagon exerts certain beneficial effects via its receptor in many organs. We aimed to study the effects of a ‘decoy receptor’ for circulating glucagon on modulating beta cell function and glucose homeostasis in mice by over-producing the glucagon receptor (GCGR) in skeletal muscles.     

Type 2 iodothyronine deiodinase levels are higher in slow-twitch than fast-twitch mouse skeletal muscle and are increased in hypothyroidism

Because of its large mass, relatively high metabolic activity and responsiveness to thyroid hormone, skeletal muscle contributes significantly to energy expenditure. Despite the presence of mRNA encoding the type 2 iodothyronine-deiodinase (D2), an enzyme that activates T(4) to T3, very low or undetectable activity has been reported in muscle homogenates of adult humans and mice. With a modified D2 assay, using microsomal protein, overnight incubation and protein from D2 knockout mouse muscle as a tissue-specific blank, we examined slow- and fast-twitch mouse skeletal muscles for D2 activity and its response to physiological stimuli. D2 activity was detectable in all hind limb muscles of 8- to 12-wk old C57/BL6 mice. Interestingly, it was higher in the slow-twitch soleus than in fast-twitch muscles (0.40 ± 0.06 vs. 0.076 ± 0.01 fmol/min · mg microsomal protein, respectively, P < 0.001). These levels are greater than those previously reported. Hypothyroidism caused a 40% (P < 0.01) and 300% (P < 0.001) increase in D2 activity after 4 and 8 wk treatment with antithyroid drugs, respectively, with no changes in D2 mRNA. Neither D2 mRNA nor activity increased after an overnight 4 C exposure despite a 10-fold increase in D2 activity in brown adipose tissue in the same mice. The magnitude of the activity, the fiber specificity, and the robust posttranslational response to hypothyroidism argue for a more important role for D2-generated T(3) in skeletal muscle physiology than previously assumed.     

Characteristics of muscle fiber type are predictive of skeletal muscle mass and strength in elderly men

OBJECTIVES: To investigate the relationship between skeletal muscle fiber type‐specific characteristics, circulating hormone concentrations, and skeletal muscle mass and strength in older men. DESIGN: Cross‐sectional analyses. SETTING: University research center. PARTICIPANTS: Forty‐one community dwelling elderly men (≥65). MEASUREMENTS: Leg strength (1‐repetition maximum, 1RM) and whole‐body and limb muscle mass were determined, and muscle fiber type composition, cross‐sectional area (CSA), myonuclear content, and satellite cell (SC) content were assessed in skeletal muscle biopsy samples. In addition, blood samples were collected to determine serum testosterone, sex hormone–binding globulin, insulinlike growth factor (IGF)‐1, and IGF binding protein‐3 concentrations. RESULTS: Muscle mass correlated with muscle strength (0.41 ≤ correlation coefficient (r)≤0.72; P<.01). Muscle fiber CSA, myonuclear content, and SC content were significantly lower in type II than in type I muscle fibers. Myonuclear and SC content were positively correlated with muscle fiber CSA. Furthermore, greater muscle fiber CSA (type I and II) was associated with greater thigh muscle area and muscle strength (0.30 ≤ r ≤ 0.45; P<.05). Testosterone concentration was positively correlated with muscle mass and muscle fiber CSA. Regression analysis showed that SC content, myonuclear content, and testosterone concentration are predictive of muscle fiber CSA. Furthermore, muscle mass and type II muscle fiber CSA are predictive of muscle strength. CONCLUSION: Skeletal muscle mass and strength in elderly men are positively correlated with muscle fiber type–specific CSA, myonuclear content, and SC content. These findings support the assumption that a decline in SC content plays an important role in age‐related decline in muscle mass and strength.     

Catabolites of chlorophyll in senescing barley leaves are localized in the vacuoles of mesophyll cells

Senescing barley leaves accumulate a series of pink pigments with the chemical properties of catabolites derived from chlorophyll. Levels of the major component of this group of pigments were quantified by HPLC and shown to be maximal in tissues exhibiting maximal rates of chlorophyll degradation. Protoplasts were isolated from senescent leaf tissue and fractionated to yield intact vacuoles and plastids. Although small but significant proportions both of total catabolites and of the dominant component of the series were recovered from the plastid fraction, the vast bulk of these compounds could be assigned to the vacuole. These observations suggest a role for the vacuole in the later stages of chlorophyll breakdown during senescence.     

Distribution of the nuclear thyroid-hormone receptor in extraocular and skeletal muscles.

The correlation between the occurrence of Graves' ophthalmopathy and Graves' hyperthyroidism may indicate a role for tri-iodothyronine (T3) hormone in the pathogenesis of Graves' ophthalmopathy. In Graves' ophthalmopathy the recti eye muscles are greatly enlarged whereas skeletal muscles seem unaffected. The distribution of the nuclear T3 receptor was studied in normal human and rat eye and skeletal muscles with immunohistochemistry using mouse (monoclonal) antibodies, and by in-situ hybridization for the detection of mRNA encoding the T3-receptor protein. Nuclear staining with T3-receptor antibodies was found in all types of tissues studied. Cytoplasmic staining occurred predominantly in the muscle fibres of the orbital layer of the eye muscles and was generally absent or very low in skeletal muscle fibres and hepatocytes. Immunostaining could be inhibited by preabsorbing the antibodies with bacterially expressed T3-receptor protein, implying specificity. The presence of nuclear and cytoplasmic hormone-free T3 receptor sites was indicated after preincubation of sections with T3 hormone; T3-receptor immunostaining decreased and T3-hormone staining increased. In-situ hybridization clearly revealed the presence of alpha-1 and beta-1 forms of the T3-receptor mRNA in liver, skeletal muscles, and orbital and intermediate layers of the eye muscles. The data demonstrate the presence of T3 hormone-receptor molecules in the extraocular and skeletal muscles. The different susceptibilities of these muscles to Graves' hyperthyroidism may relate to the quantitative differences in T3 hormone-receptor distribution.     

Disorders of laminins in diseases of myocardial and skeletal muscles

The Emery-Dreifuss muscular dystrophy is caused by muscular lesions and disorders of cardial rhythm and/or by cardiomyopathy. An autosomal dominant form is related to mutations of genes, which are coding for lamins A/C. GROUP AND METHODS: In the group A the authors examined 37 patients with the diagnosis of dilatation cardiomyopathy (DKMP) and the mean ejection fraction 28.4; 8.8%. In the group B of 13 patients a cardiac stimulator was implanted for a rhythm disorder. Both groups were subjected to cardiological, neurological, clinical and electromyographic (EMG) examinations. A muscle biopsy from m. vastus lateralis was made and the sample was evaluated by histology, histochemistry and immunohistochemistry. The coding sequences of genes for lamins were amplified by polymerase chain reaction and the products were analyzed by the DHPLC method (denaturing higher performance liquid chromatography). RESULTS: In the group A there was a clinically myopathic picture in three patients, while EMG examination revealed a myogenic finding in 12 patients and a marginally myogenic one in five patients. The histological finding in 12 patients was evaluated as myogenic and marginally myogenic in six. In one patient the mutation analysis revealed mutation in the gene for lamin A/C. A myogenic finding in this patient was determined by EMG as well as by histological examination and the autosomal dominant form of the Emery-Dreifuss muscular dystrophy was therefore diagnosed. In the group B one patient displayed a myopathic neurological finding and a myogenic finding during EMG. A subsequent mutation analysis revealed a mutation in the gene for lamin A/C. The case was therefore the autosomal dominant form of the Emery-Dreifuss muscular dystrophy. In the other patients the clinically marginal myopathic finding was observed once, a marginally myogenic finding during EMG was seen five times, histology and immunochemistry revealed a myogenic finding once and a marginally myogenic finding also once. The other findings were within normal range. CONCLUSIONS: A careful neurological examination including EMG determined symptoms of skeletal muscle myopathies in a surprisingly high percentage of our cardiological patients. This observation draws attention to the need of neurological examination in patients with DKMP in order to discovered disorder in this area in time. In two patients mutations in genes coding lamins A/C were detected. It would be useful to analyze also genes coding for other cytoskeletal proteins in the future.