Sarcopenic obesity in cancer.

BACKGROUND: Sarcopenic obesity is a relatively new term. It is a clinical condition characterized by sarcopenia (loss of muscle mass and function) and obesity (increase in fat mass) that mainly affects older adults. As the incidence of sarcopenia and obesity increases worldwide, sarcopenic obesity is becoming a greater problem also in cancer patients. In fact, sarcopenic obesity is associated with poorer treatment outcomes, longer hospital stays, physical disability, and shorter survival in several cancers. Oxidative stress, lipotoxicity, and systemic inflammation, as well as altered expression of skeletal muscle anti-inflammatory myokines in sarcopenic obesity, are also associated with carcinogenesis. CONCLUSIONS: Reported prevalence of sarcopenic obesity in cancer varies because of heterogeneity in definitions and variability in diagnostic criteria used to estimate the prevalence of sarcopenia and obesity. Therefore, the aim of this review is to describe the definitions, prevalence, and diagnostic criteria as well as the mechanisms that cancer has in common with sarcopenic obesity.

Sarcopenic obesity and hypertension in elderly patients: a narrative review of pathophysiology and management strategies.

INTRODUCTION: Sarcopenic obesity and hypertension are a public health problem that is increasing worldwide due to the progressive aging of the population and the increasing prevalence of obesity and physical inactivity. Sarcopenic obesity is characterized by the simultaneous presence of sarcopenia (loss of muscle mass) and adiposity (increase in fat mass). Because symptoms are not specific, sarcopenic obesity remains largely undiagnosed. This review explores the latest research on sarcopenic obesity and its association with hypertension, with a focus on arterial stiffness. METHODS: A comprehensive narrative review was conducted by systematically searching PubMed and Scopus databases for relevant scientific literature. RESULTS: Sarcopenic obesity and hypertension are closely linked, sharing common factors such as inflammation, insulin resistance, and oxidative stress, with arterial stiffness playing a crucial role. DISCUSSION: Given the lack of specific symptoms for sarcopenic obesity, early diagnosis and management are crucial. Treatment strategies should prioritize weight loss, adequate protein intake, and regular physical activity. Further investigation is warranted for pharmacological interventions. CONCLUSION: Sarcopenic obesity and hypertension present significant challenges to global public health. Addressing arterial stiffness is paramount in managing these conditions effectively. Lifestyle modifications, including weight management and physical activity, remain central to the treatment of sarcopenic obesity, while additional research is needed to explore potential pharmacological options.

Effect of electrochemotherapy on myogenesis of mouse C2C12 cells in vitro.

Electrochemotherapy (ECT) is a local ablative therapy for the treatment of different skin and subcutaneous tumors and certain tumors in internal organs. Skeletal muscle represents a major tumor- surrounding tissue, exposed to side effects of ECT. At the cellular level, side-effects of ECT on skeletal muscle and underlying mechanisms have not been examined yet. Thus, we aimed to determine the effect of ECT in the mouse muscle cell line C2C12 during in vitro myogenesis. We evaluated the electroporation efficiency and viability of C2C12 myotubes at increasing voltages (200-1300 V/cm) using propidium iodide (PI). Permeabilization of PI into the cells was voltage-dependent accounting up to 97 % efficiency at the highest voltage. High cell viability and myotube integrity were maintained until 4 days after electroporation. ECT with the cytostatic drugs bleomycin and cisplatin decreased the viability of C2C12 myoblasts and myotubes in a dose-dependent manner. However, myoblasts were more sensitive to ECT than myotubes. Increased secretion of IL-6, observed 3 days after ECT, confirming its effects on early myogenesis. Only minor effects of ECT were observed in treated myotubes. These results contribute to the safety profile of ECT in tumor treatment.

Identification of sarcopenic obesity in adults undergoing orthopaedic surgery: Relationship between "a body shape index" (ABSI) and fat-free mass. A cross -sectional study.

BACKGROUND: Sarcopenic obesity is a condition characterised by the coexistence of low muscle mass and function (sarcopenia) and excessive fat mass (obesity). The aim of this study was to determine the prevalence of this condition in patients undergoing orthopaedic surgery by gender and type of orthopaedic surgery. In addition, this study investigated the suitability of a waist circumference-based anthropometric measure, body shape index (ABSI), for predicting sarcopenic obesity and the predictive power of ABSI for fat-free mass index (FFMI), a surrogate marker of lean body mass. METHODS AND FINDINGS: A cross-sectional study of overweight and obese orthopaedic patients undergoing knee or hip and spine surgery was conducted between October 2019 and March 2020 in Orthopaedic Hospital Valdoltra, Slovenia. General anthropometric parameters body mass index (BMI) and ABSI = (WC/(BMI2/3x height½) as well as body composition data (fat mass FM, fat-free mass FFM, FFMI, and the ratio FM/FFM as an index of sarcopenic obesity) were determined in 120 women (aged 66.5 ± 9.6 years) and 89 men (aged 65.5 ± 7.8 years) with overweight (25 kg/m2 ≤ BMI < 30 kg/m2) and obesity (BMI ≥ 30 kg/m2) by bioelectrical impedance analysis (BIA). Sarcopenic obesity phenotypes based on FM/FFM ratio > 0.80 was present in 15.3% of patients, mainly in female patients undergoing knee surgery. ABSI was significantly associated with age in all women and obese men and with waist circumference (WC) in all patients. ABSI did not correlate with BMI in women and men; however, multiple linear regression analysis showed that BMI independently predicted FFMI (R = 0.83 and 0.70, respectively, p < 0.001) in women and men (ß-coefficients: 0.801 and 0.686, respectively) and ABSI in women only (ß-coefficient: -0.104). Women with a lower ABSI had a significantly higher FFMI than the group with a higher ABSI. CONCLUSIONS: Sarcopenic obesity was most prevalent in obese women scheduled for knee surgery. In addition, ABSI, independently predicted FFMI in women and represents a significant predictor of sarcopenic obesity.

High-Protein Bar as a Meal Replacement in Elite Sports Nutrition: A Pilot Study.

This study was focused on the creation of high-protein bars formulated using whey protein isolate (24%) and soy protein isolate (6%) as the sources of proteins; oat flakes and inulin, both abundant in dietary fibres, and creatine monohydrate and other minor ingredients (vitamin and mineral mixture, potassium sorbate) to achieve the requirements for a meal replacement formula for physically active people. The nutritional profile of the high-protein bar was examined (energy 1215 kJ/288 kcal; protein 34.1 ± 0.20 g, fat 6.01 ± 0.13 g of which was saturated 3.12 ± 0.08 g, fibre 3.10 ± 0.17 g carbohydrate 23.0 ± 0.16 g of which sugars 1.50 ± 0.19 g and starch 21.5 ± 0.11 g in 100 g), and sensory properties with instrumental parameters (texture and colour) were determined and compared with bars commercially available on the market. The created high-protein bar was sensorily acceptable in comparison to other commercially available bars. The dietary intervention study was conducted on elite athletes (professional handball players) to evaluate effects of created versus control bar consumption on their metabolic parameters. The baseline characteristics (mean age, body mass index (BMI), fat mass, muscle mass, lean mass and fat percentage) of the athletes (8) were determined at the start of the study. The cross-over intervention study was organized in two successive phases (5 days each) with a seven-day long washout period between phases. Bars were consumed after the afternoon training unit. Blood samples were collected at the start and the end of the intervention study to analyse the metabolic profiles of the athletes. Serum levels of high-density cholesterol (HDL), low-density cholesterol (LDL) and total cholesterol (HOL), glucose, triacylglycerides (TAG), total and direct bilirubin, creatine kinase (CK), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were measured. The results showed that bar consumption significantly decreased serum aspartate transaminase (AST) and lactate dehydrogenase (LDH) and increased total and direct bilirubin levels, suggesting lower exercise-induced muscle damage and increased antioxidative response, respectively. Therefore, it can be concluded that the consumption of the created high-protein bar was able to improve physiological adaptation after training.

Physical activity and cancer risk. Actual knowledge and possible biological mechanisms.

BACKGROUND: Scientific evidence has shown that an increase in regular physical activity is associated with a decrease in the development of many types of cancer. Potential mechanisms that link physical activity to reduced cancer risk include a decrease in systemic inflammation, hyperinsulinemia, insulin-like growth factor (IGF-I), sex hormones, pro-inflammatory leptin and other obesity-related cytokines, and a significant increase in anti-inflammatory adiponectin levels. In addition, physical activity improves immune function and the composition and diversity of the gastrointestinal microbiota. Moderate physical activity is important for cancer protection, but the most significant changes in the inflammatory profile are conferred by physical activity performed at higher intensities. Thus, there is a need for further investigation into the type, intensity, and duration of physical activity for the prevention of some types of cancer and the development of effective recommendations. CONCLUSIONS: There is a strong evidence that physical activity of moderate to vigorous intensity protects against colon and breast cancer, and probably against cancer at all other sites.

SERUM BILIRUBIN CORRELATES WITH SERUM ADIPOKINES IN NORMAL WEIGHT AND OVERWEIGHT ASYMPTOMATIC ADULTS.

Overweight and obesity are considered as chronic low-grade inflammation accompanied by imbalanced production of adipokines. The aim of this study was to elucidate the relationship between serum bilirubin, which is an endogenous antioxidant with anti-inflammatory activity, and pro- and anti-inflammatory serum adipokines in asymptomatic normal weight and overweight individuals. Healthy men and women aged 25-49 participated in this cross-sectional study. All participants underwent fasting serological measurements of adipokines, interleukin-6, tumor necrosis factor alpha (TNF-α), C-reactive protein (CRP), total and direct serum bilirubin, and other biochemical parameters. Participants were divided into normal weight and overweight groups. We found a significant negative association between total bilirubin and CRP, TNF-α, visfatin and resistin values, and a significant positive association between total bilirubin and adiponectin values in both normal-weight and overweight groups. Importantly, after adjusting for body mass index, we also found a significant negative association between total serum bilirubin levels and both visfatin and CRP serum levels. Moreover, visfatin, resistin and CRP were predictors of the total serum bilirubin levels.

Total Serum Antioxidant Capacity in Healthy Normal Weight and Asymptomatic Overweight Adults.

Obesity and overweight are major contributors to the burden of chronic disease. Both are defined as abnormal or excessive fat accumulation and by increased production of free radicals leading to oxidative stress. The aim of the present study was to evaluate whether overweight and fat accumulation is associated with serum total antioxidant capacity (TAC) in men and women, irrespective of nutritional habits, nutrient intakes, physical activity, smoking, and other confounders, which may be responsible for modifying the association between serum TAC and overweight/obesity measures. This cross-sectional study was conducted on 60 normal weight and 60 overweight adults aged 25-49. All participants underwent standard anthromorphological measurements of body composition, blood pressure and biochemical measurements, aerobic capabilities assessment and dietary intake evaluation. TAC was measured by using the photochemioluminescence method. All data were analysed with SPSS software. Men had higher values of TAC than women and concentrations of TAC were significantly higher in overweight subjects compared to normal weight subjects. In the present study TAC tended to be increased by various metabolic risk factors, especially overweight/obesity parameters (body mass index, body fat), inflammation and increased serum levels of Cysteine, irrespective of nutritional habits, nutrient intakes, physical activity and smoking. Overweight and obesity at an early stage may stimulate TAC. Therefore, the elevation of TAC in overweight adults may be a compensatory response to oxidative stress, generated by reactive oxygen species.

Impact of 14-day bed rest on serum adipokines and low-grade inflammation in younger and older adults.

Ageing and inactivity both contribute to systemic inflammation, but the effects of inactivity on inflammation in healthy elderly individuals have not been elucidated. We hypothesised that 14-day bed rest could affect the pro- and anti-inflammatory markers in young subjects differently than in older adults. A short-term 14-day horizontal bed rest study (BR14) has been used as a model of inactivity in two groups of healthy male volunteers: 7 aged 18-30 years (young) and 16 aged 55-65 years (older adults). The effects of inactivity on inflammation were compared. Key low-grade inflammation mediators, tumour necrosis factor α (TNF-α), interleukin-6 (IL-6), visfatin, resistin, and anti-inflammatory adiponectin were measured in fasting serum samples, collected at baseline (BDC) and post BR14. Young responded to BR14 by increasing serum visfatin and resistin while older adults responded to BR14 by increasing IL-6 and TNF-α. In addition, serum adiponectin increased in all participants. Data from correlation analysis demonstrated positive association between Δ serum visfatin and Δ IL-6 in both groups, while Δ serum adiponectin was negatively associated with Δ TNF-α in young and positively associated with Δ resistin in the older adults. As little as 14 days of complete physical inactivity (BR14) negatively affected markers of low-grade inflammation in both groups, but the inflammation after BR14 was more pronounced in older adults. The effect of BR14 on IL-6 and resistin differed between young and older adults. Inflammatory responses to BR14 in older adults differed from those reported in the literature for obese or subjects in pathological states, suggesting potentially different mechanisms between inactivity- and obesity-induced inflammations.

Elevated serum levels of cysteine and tyrosine: early biomarkers in asymptomatic adults at increased risk of developing metabolic syndrome.

As there is effective intervention for delaying or preventing metabolic diseases, which are often present for years before becoming clinically apparent, novel biomarkers that would mark metabolic complications before the onset of metabolic disease should be identified. We investigated the role of fasting serum amino acids and their associations with inflammatory markers, adipokines, and metabolic syndrome (MetS) components in subjects prior to the onset of insulin resistance (IR). Anthropometric measurements, food records, adipokines, biochemical markers, and serum levels of amino acids were determined in 96 asymptomatic subjects aged 25-49 years divided into three groups according to the number of MetS components present. Cysteine and tyrosine were significantly higher already in group with one component of MetS present compared to subjects without MetS components. Serum amino acid levels correlated with markers of inflammation and adipokines. Alanine and glycine explained 10% of insulin resistance variability. The role of tyrosine and cysteine, that were higher already with 1 component of MetS present, should be further investigated as they might point to future insulin disturbances.

Inverse relationship between "a body shape index" (ABSI) and fat-free mass in women and men: Insights into mechanisms of sarcopenic obesity.

BACKGROUND & AIMS: Sarcopenic obesity may be defined by a high fat to fat-free mass (FM/FFM) ratio. Skeletal muscle may be negatively influenced by the pro-inflammatory milieu associated with visceral fat, while the loading effect induced by a heavier body mass index (BMI) may enhance muscle anabolism. Recently, a new anthropometric measure based on waist circumference (A Body Shape Index, ABSI) was developed. In this study we have assessed the predictive power of ABSI on the FFM index (FFMI), a surrogate marker of lean mass. METHODS: Standard anthropometric parameters and ABSI as well as body composition data (fat and fat-free mass determined by bioelectrical impedance analysis) were assessed in 111 female and 89 male overweight/obese subjects, with no clinically significant co-morbidities. Groups with higher- or lower-ABSI were identified according to median values of this index. RESULTS: In women and men, ABSI did not correlate with BMI, while multiple linear regression indicated that BMI (ß-coefficients: 0.62 and 0.77, respectively) and ABSI (ß-coefficients: -0.26 and -0.22, respectively) independently predicted FFMI (multiple R: 0.72 and 0.83, respectively, P < 0.001). Men and women with lower-ABSI exhibited significantly greater FFMI than the higher-ABSI groups for comparable values of BMI. In men, ABSI was correlated positively with C-reactive protein (CRP) (R = 0.30; P < 0.05) and negatively with the reciprocal of insulin (R = 0.28; P < 0.05), an index of insulin sensitivity. FM/FFM ratio significantly (P < 0.01) correlated with CRP (R = 0.31) in women only. CONCLUSIONS: ABSI, a recently introduced marker of abdominal adiposity, may contribute to define the risk of sarcopenia in overweight/obese individuals.

Low-grade inflammation in overweight and obese adults is affected by weight loss program.

PURPOSE: Low-grade systemic inflammation due to obesity is considered to be the key link between obesity and obesity-related disorders. The hypothesis was tested that significant alterations in inflammatory markers and adipokines would occur over a multidisciplinary intervention and that these changes might also be important for improvement of cardiovascular risk factors. METHODS: Thirty-tree overweight adults completed a 6-month multidisciplinary intervention program to evaluate the effects of a personalized dietary program based on the individual's resting metabolic rate (RMR) on anthropometric parameters, aerobic and anaerobic capabilities, metabolic profile, inflammation, and body image satisfaction. Body composition, physical activity, anaerobic capabilities, RMR, metabolic profile, and low-grade inflammation were measured. Diet composition and body image dissatisfaction were also assessed. RESULTS: After 6 months of multidisciplinary intervention the participants showed significantly decreased body weight, waist circumference (WC), and the inflammatory markers tumor necrosis factor-α, C-reactive protein, and visfatin. They also showed increased anti-inflammatory adiponectin and consequently decreased serum insulin, HOMA-IR, and total cholesterol. The important findings of the study were that reduction of sugars and saturated fatty acids in the diet, coupled with an increase in exercise, significantly correlated with reduction of WC and body mass index. In addition, positive correlations between ∆ BMI, ∆ WC, ∆ trunk fat, inflammation, and cardiovascular risk factors were demonstrated. CONCLUSIONS: Weight loss in combination with increased physical activity, a negative energy balance, and diet adjustment was associated with lower inflammation and consequently with lower cardiovascular risk factors.

Effect of ionizing radiation on human skeletal muscle precursor cells.

BACKGROUND: Long term effects of different doses of ionizing radiation on human skeletal muscle myoblast proliferation, cytokine signalling and stress response capacity were studied in primary cell cultures. MATERIALS AND METHODS: Human skeletal muscle myoblasts obtained from muscle biopsies were cultured and irradiated with a Darpac 2000 X-ray unit at doses of 4, 6 and 8 Gy. Acute effects of radiation were studied by interleukin - 6 (IL-6) release and stress response detected by the heat shock protein (HSP) level, while long term effects were followed by proliferation capacity and cell death. RESULTS: Compared with non-irradiated control and cells treated with inhibitor of cell proliferation Ara C, myoblast proliferation decreased 72 h post-irradiation, this effect was more pronounced with increasing doses. Post-irradiation myoblast survival determined by measurement of released LDH enzyme activity revealed increased activity after exposure to irradiation. The acute response of myoblasts to lower doses of irradiation (4 and 6 Gy) was decreased secretion of constitutive IL-6. Higher doses of irradiation triggered a stress response in myoblasts, determined by increased levels of stress markers (HSPs 27 and 70). CONCLUSIONS: Our results show that myoblasts are sensitive to irradiation in terms of their proliferation capacity and capacity to secret IL-6. Since myoblast proliferation and differentiation are a key stage in muscle regeneration, this effect of irradiation needs to be taken in account, particularly in certain clinical conditions.

Serum bilirubin levels are lower in overweight asymptomatic middle-aged adults: an early indicator of metabolic syndrome?

OBJECTIVE: Low levels of bilirubin have recently been associated with obesity, diabetes mellitus, and metabolic syndrome. Here, we hypothesized that serum bilirubin levels might be already altered in overweight asymptomatic middle-aged individuals before full development of the metabolic syndrome. METHODS: Healthy nonsmoking adults aged 25-49 (64 women and 32 men) participated in this cross-sectional study. All participants who reported stable weight within the last three months underwent standard anthropomorphological measurements of body composition, blood pressure measurements, aerobic and anaerobic capabilities assessment, dietary intake evaluation, and fasting serological measurements of total and direct bilirubin, glucose, insulin, triglycerides, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and C-reactive protein. Participants were divided into normal-weight and overweight groups. Linear correlation and multiple regression analyses were used to examine the association of serum bilirubin levels with all metabolic syndrome risk factor changes. RESULTS: Serum bilirubin levels were lower in overweight healthy individuals of both sexes, and were negatively associated with abdominal obesity, insulin resistance, fasting glucose, fasting insulin, fasting triglycerides, total cholesterol, low-density lipoprotein cholesterol, and C-reactive protein levels but positively associated with aerobic body capabilities. CONCLUSION: Our findings suggest that serum bilirubin levels have the potential to be employed as an early biomarker for indicating asymptomatic individuals at increased risk of developing metabolic syndrome.

Effects of inactivity on human muscle glutathione synthesis by a double-tracer and single-biopsy approach.

Oxidative stress is often associated to inactivity-mediated skeletal muscle atrophy. Glutathione is one of the major antioxidant systems stimulated, both at muscular and systemic level, by activation of oxidative processes. We measured changes in glutathione availability, oxidative stress induction and the extent of atrophy mediated by 35 days of experimental bed rest in vastus lateralis muscle of healthy human volunteers. To assess muscle glutathione synthesis, we applied a novel single-biopsy and double-tracer ([(2)H(2)]glycine and [(15)N]glycine) approach based on evaluation of steady-state precursor incorporation in product. The correlations between the traditional (multiple-samples, one-tracer) and new (one-sample, double-tracer infusion) methods were analysed in erythrocytes by Passing-Bablok and Altman-Bland tests. Muscle glutathione absolute synthesis rate increased following bed rest from 5.5 ± 1.1 to 11.0 ± 1.5 mmol (kg wet tissue)(-1) day(-1) (mean ± S.E.M.; n = 9; P = 0.02) while glutathione concentration failed to change significantly. Bed rest induced vastus lateralis muscle atrophy, as assessed by pennation angle changes measured by ultrasonography (from 18.6 ± 1.0 to 15.3 ± 0.9 deg; P = 0.01) and thickness changes (from 2.3 ± 0.2 to 1.9 ± 0.1 cm; P < 0.001). Moreover, bed rest increased protein oxidative stress, as measured by muscle protein carbonylation changes (from 0.6 ± 0.1 to 1.00 ± 0.1 Oxydized-to-total protein ratio; P < 0.04). In conclusion, we developed in erythrocytes a new minimally invasive method to determine peptide synthesis rate in human tissues. Application of the new method to skeletal muscle suggests that disuse atrophy is associated to oxidative stress induction as well as to compensatory activation of the glutathione system.

Hyperhomocysteinemia and the role of B vitamins in cancer.

BACKGROUND: Patients suffering from malignancies have increased complications due to corresponding cardiovascular diseases and risk factor for the development of venous thromboembolism. Epidemiological studies have shown that increased homocysteine plasma concentration (hyperhomocysteinemia) is related to a higher risk of coronary heart disease, stroke, peripheral vascular disease and malignancies. Homocysteine (tHcy) is an intermediate sulfur-containing amino acid produced from methionine during processing of dietary proteins. The plasma homocysteine levels are strongly influenced by diet, as well as by genetic factors. Folic acid, vitamins B6 and B12 are dietary components which influence the plasma homocysteine levels the most. Several studies have found that high blood levels of B vitamins are related to the integrity and function of DNA, and, are at least related to lower concentration of homocysteine. Folate depletion has been found to change DNA methylation and DNA synthesis in both animal and human studies. Because of this critical role of folate, most studies including homocysteine have focused on these two actions. CONCLUSIONS: Hyperhomocysteinemia proves to be the most common condition highly associated with both venous and arterial thrombosis in many cancer patients, while the associated pathophysiology has not been precisely established yet. Therefore, of current interest is the possible role of folate metabolism developing into a cancer initiating hyperhomocysteinemia. This review will discuss this possibility.

Neural agrin changes the electrical properties of developing human skeletal muscle cells.

Recent investigations suggest that the effects of neural agrin might not be limited to neuromuscular junction formation and maintenance and that other aspects of muscle development might be promoted by agrin. Here we tested the hypothesis that agrin induces a change in the excitability properties in primary cultures of non-innervated human myotubes. Electrical membrane properties of human myotubes were recorded using the whole-cell patch-clamp technique. Cell incubation with recombinant chick neural agrin (1 nM) led to a more negative membrane resting potential. Addition of strophanthidin, a blocker of the Na(+)/K(+) ATPase, depolarized agrin-treated myotubes stronger than control, indicating, in the presence of agrin, a higher contribution of the Na(+)/K(+) ATPase in establishing the resting membrane potential. Indeed, larger amounts of both the alpha1 and the alpha2 isoforms of the Na(+)/K(+) ATPase protein were expressed in agrin-treated cells. A slight but significant down-regulation of functional apamin-sensitive K(+) channels was observed after agrin treatment. These results indicate that neural agrin might act as a trophic factor promoting the maturation of membrane electrical properties during differentiation, confirming the role of agrin as a general promoter of muscle development.

Whole muscle contractile parameters and thickness loss during 35-day bed rest.

Extended exposure to microgravity leads to significant musculoskeletal adaptations. Contractile parameters of four skeletal muscles (biceps brachii-BB, vastus medialis-VM, biceps femoris-BF and gastrocnemius medialis-GM) were measured in ten healthy males (aged 22.3 +/- 2.2 years) during 35 days of horizontal bed rest by a mechanomyography-based method termed 'tensiomyography' (TMG). Two contractile parameters: contraction time (Tc) and maximal displacement (Dm) were individually measured from electrically evoked maximal single twitch TMG response of all four muscles before and after bed rest. Significant changes in Tc were found after bed rest, as shown by an increase in GM muscle Tc by 18% (p < 0.01). Dm values significantly increased (p < 0.01) after bed rest, by 24, 26 and 30% in the VM, BF and GM muscles, respectively. In the GM, the change in Dm significantly correlated with the decrease in muscle thickness (r = -0.70, p < 0.01). In conclusion, bed rest induced changes in both Dm and Tc of the TMG signal; changes in Dm being inversely related to those of muscle thickness. Amongst the investigated muscles, most affected, in terms of atrophy and mechanical alterations, were those of the lower limbs. The observed increase in Dm may be attributed to a decrease in muscle, as well as tendon stiffness, causing larger muscle fibre and non-contractile tissue oscillations following contraction.

Neural agrin controls maturation of the excitation-contraction coupling mechanism in human myotubes developing in vitro.

The aim of this study was to elucidate the mechanisms responsible for the effects of innervation on the maturation of excitation-contraction coupling apparatus in human skeletal muscle. For this purpose, we compared the establishment of the excitation-contraction coupling mechanism in myotubes differentiated in four different experimental paradigms: 1) aneurally cultured, 2) cocultured with fetal rat spinal cord explants, 3) aneurally cultured in medium conditioned by cocultures, and 4) aneurally cultured in medium supplemented with purified recombinant chick neural agrin. Ca(2+) imaging indicated that coculturing human muscle cells with rat spinal cord explants increased the fraction of cells showing a functional excitation-contraction coupling mechanism. The effect of spinal cord explants was mimicked by treatment with medium conditioned by cocultures or by addition of 1 nM of recombinant neural agrin to the medium. The treatment with neural agrin increased the number of human muscle cells in which functional ryanodine receptors (RyRs) and dihydropyridine-sensitive L-type Ca(2+) channels were detectable. Our data are consistent with the hypothesis that agrin, released from neurons, controls the maturation of the excitation-contraction coupling mechanism and that this effect is due to modulation of both RyRs and L-type Ca(2+) channels. Thus, a novel role for neural agrin in skeletal muscle maturation is proposed.

Intrinsic ionic conductances mediate the spontaneous electrical activity of cultured mouse myotubes.

Mouse skeletal myotubes differentiated in vitro exhibited spontaneous contractions associated with electrical activity. The ionic conductances responsible for the origin and modulation of the spontaneous activity were examined using the whole-cell patch-clamp technique and measuring [Ca(2+)](i) transients with the Ca(2+) indicator, fura 2-AM. Regular spontaneous activity was characterized by single TTX-sensitive action potentials, followed by transient increases in [Ca(2+)](i). Since the bath-application of Cd(2+) (300 microM) or Ni(2+) (50 muM) abolished the cell firing, T-type (I(Ca,T)) and L-type (I(Ca,L)) Ca(2+) currents were investigated in spontaneously contracting myotubes. The low activation threshold (around -60 mV) and the high density of I(Ca,T) observed in contracting myotubes suggested that I(Ca,T) initiated action potential firing, by bringing cells to the firing threshold. The results also suggested that the activity of I(Ca,L) could sustain the [Ca(2+)](i) transients associated with the action potential, leading to the activation of apamin-sensitive SK-type Ca(2+)-activated K(+) channels and the afterhyperpolarization (AHP) following single spikes. In conclusion, an interplay between voltage-dependent inward (Na(+) and Ca(2+)) and outward (SK) conductances is proposed to mediate the spontaneous pacemaker activity in cultured muscle myotubes during the process of myogenesis.