Muscle Fiber Cross-sectional Area Research Articles

Standardized Boesenbergia pandurata Extract Prevents Dexamethasone-Induced Muscle Atrophy and Dysfunction in C57BL/6 Mice.

Muscle atrophy, characterized by diminished muscle mass and impaired function, poses a substantial global health concern. Boesenbergia pandurata (Roxb.) Schltr., commonly known as fingerroot, possesses a variety of advantageous activities, including anti-inflammatory, antioxidant, antibacterial, and anticancer effects. However, there are currently no preclinical studies available that explore the potential of B. pandurata extract (BPE) to mitigate muscle atrophy. In this study, we aimed to explore the protective effects of BPE, standardized to panduratin A content, against muscle atrophy and its underlying molecular mechanisms in a dexamethasone-induced muscle atrophy mouse model. Compared with the dexamethasone group, BPE significantly restored muscle mass, muscle volume, muscle fiber cross-sectional area, grip strength, and exercise endurance. Additionally, BPE suppressed inflammatory responses by downregulating the expressions of nuclear factor kappa B and inflammatory cytokines while also enhancing antioxidant effects by increasing the expressions of antioxidant enzymes. Moreover, BPE promoted protein synthesis and muscle differentiation by stimulating the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin signaling pathway. Furthermore, it suppressed myostatin expression and inhibited the expressions of E3 ubiquitin ligases by preventing the nuclear translocation of forkhead box O3a, thereby alleviating proteolysis. Overall, BPE effectively regulates unbalanced protein metabolism, suggesting its potential as a functional food ingredient for preventing muscle wasting diseases.

Mitochondrial quality control measures, systemic inflammation, and lower-limb muscle power in older adults: a PROMPT secondary analysis

ObjectivesThe study was conducted to explore associations between markers of mitochondrial quality control (MQC) from vastus lateralis muscle biopsies, serum inflammatory markers, and measures of muscle power assessed by two different tools in a sample of older adults. DesignSecondary analysis of data collected in the PeppeR develOpMental ProjecT (PROMPT) at the University of Florida (Gainesville, FL, USA). MethodsForty-three older adults (n = 20 women) were included in the study. Muscle volume of the calf and thigh was quantified by three-dimensional magnetic resonance imaging. Lower-limb muscle power was estimated using 5-time sit-to-stand (5STS) muscle power equations and isokinetic test. Protein markers of MQC were measured in muscle samples by Western immoblotting (n = 12–23), while type I and II fiber cross-sectional area (CSA) and their proportion were quantified using immunohistochemistry (n = 12). Cytochrome C oxidase enzyme activity was measured spectrophotometrically. Finally, inflammatory markers were quantified in the serum using a multiplex immunoassay (n = 39). ResultsMean age of participants was 78.1 ± 5.5 years, and the average body mass index was 26.2 ± 4.5 kg/m2. Markers of mitochondrial biogenesis (i.e., PGC-1α), mitochondrial import proteins (i.e., cHsp70 and mtHsp70), and type I fiber CSA were significantly associated with muscle power estimated via both 5STS muscle power equations and isokinetic test (p < 0.05). Specific associations were also found according to the muscle power assessment method. 5STS muscle power measures were negatively correlated with ClvCasp3, P-AMPK, T-AMPK, P-p38, GM-CSF, INF-γ, IL1b, IL6, IL8, and TNF-α, whereas positive associations were found with BAX (p < 0.05). In contrast, isokinetic measures were significantly and positively correlated with RIP140, Hsp60, and type II muscle fiber CSA (p < 0.05). ConclusionsMarkers of mitochondrial biogenesis (PGC-1α), mitochondrial import proteins (cHsp70 and mtHsp70), and type I muscle fiber CSA were significantly linked to lower-limb muscle power in older adults. These results suggest that muscle power is influenced by mitochondrial signaling. We also found that the relationship between mitochondrial mediators, inflammatory markers, and muscle power varied according to the assessment tool used.

Transcriptomic Profiling Reveals Differences in Slow-Twitch and Fast-Twitch Muscles of a Cigarette Smoke-Exposed Rat Model.

Cigarette smoking is known to affect muscle function and exercise capacity, including muscle fatigue resistance. Most studies showed diminished cross-sectional area and fibre type shifting in slow-twitch muscles such as the soleus, while effects on fast-twitch muscles were seldom reported and the differential responses between muscle types in response to exposureto cigarette smoke (CS) were largely unknown. This study aimed to elucidate the histomorphological, biochemical and transcriptomic changes induced by CS on both slow-twitch and fast-twitch muscles. Male Sprague-Dawley rats were randomly divided into two groups: sham air (SA) and CS. The rats were exposed to CS for 8 weeks using an exposure chamber system to mimic smoking conditions. Histomorphological analyses on muscle fibre type and cross-sectional area were determined in soleus and extensor digitorum longus (EDL). Transcriptomic profiles were investigated for identifying differentially expressed genes (DEGs) and potential mechanistic pathways involved. Inflammatory responses in terms of the macrophage population and the level of inflammatory cytokines were measured. Markers for muscle-specific proteolysis were also examined. Soleus muscle, but not in EDL, exhibited a significant increase in Type IIa fibres (SA: 9.0 ± 3.3%; CS: 19.8 ± 2.4%, p = 0.002) and decrease in Type I fibres (SA: 90.1 ± 3.6%; CS: 77.9 ± 3.3%, p = 0.003) after CS exposure. RNA sequencing revealed 165 identified DEGs in soleus including upregulation of 'Cd68', 'Ccl2' and 'Ucp2' as well as downregulation of 'Ucp3', etc. Pathways enrichment analysis revealed that the upregulated pathways in soleus were related to immune system and cellular response, while the downregulated pathways were related to oxidative metabolism. Only 10 DEGs were identified in EDL with less enriched pathways. The soleus also showed elevated pro-inflammatory cytokines, and the total macrophage marker CD68 was significantly higher in soleus of CS compared to the SA group (CD68+/no. of fibre: SA = 60.3 ± 39.3%; CS = 106.5 ± 27.2%, p = 0.0039), while the two groups in EDL muscle showed no significant difference. The expression of E3 ubiquitin ligase atrogin-1 associated with muscle degradation pathways was 1.63-fold higher in the soleus after CS, while no significant differences were observed in the EDL. The CS-induced inflammatory responses on soleus muscle are likely mediated via targeting mitochondrial-related signalling, resulting in mitochondrial dysfunction and impaired oxidative capacity. The presumably less active mitochondrial-related signalling in EDL renders it less susceptible to changes towards CS, accounting for differential impacts between muscle types.

Melatonin attenuates sepsis-induced muscle atrophy by regulating the PI3K/Akt signaling pathway

BackgroundIn intensive care units, sepsis-related muscle atrophy is a severe complication of numerous diseases, yet the underlying mechanism and potential therapeutic options remain elusive. Recent research has identified melatonin as a promising candidate for attenuating organ dysfunction triggered by sepsis. MethodsWe used in vitro and in vivo models to simulate sepsis, C2C12 myotubes were treated with LPS, and the mice underwent cecal ligation and puncture (CLP) surgery. Following a pretreatment regimen involving melatonin and the AKT inhibitor MK-2206 2HCl, we analyzed changes in p-Akt and MuRF1 protein levels, fiber cross-sectional areas, and myotube diameters. The analyses included RNA sequencing, Western blotting, qRT–PCR, and immunofluorescence staining. ResultsActivation of the PI3K/Akt pathway in skeletal muscle occurred 24 h post-CLP surgery in mice. This was accompanied by upregulated MuRF1 expression and reduced muscle fiber cross-sectional area, which culminated in muscle atrophy. However, these detrimental effects were attenuated when the mice were pretreated with melatonin via intraperitoneal injection for seven consecutive days. Similarly, LPS treatment of C2C12 myotubes activated the PI3K/Akt pathway, elevated MuRF1 expression, and markedly reduced myotube diameter after 48 h, leading to muscle atrophy. Pretreatment of C2C12 myotubes with melatonin 24 h in advance mitigated these adverse effects. However, cotreatment of C2C12 myotubes with melatonin and MK-2206 2HCl attenuated the beneficial effects of melatonin. ConclusionMelatonin can attenuate sepsis-induced muscle atrophy by regulating the PI3K/Akt pathway.

Influence of Bovine Myosin Heavy Chain Isoforms and Muscle Fiber Cross-Sectional Are&lt;em&gt;a o&lt;/em&gt;n the Eating Quality and Connective Tissue Characteristics of 11 Different Beef Muscles

The objective of this study was to determine the impact of muscle fiber type, cross-sectional area (CSA), and diameter on the eating quality of 11 different beef muscles. Eleven different beef muscles were utilized in 2 separate studies. In the 2 studies, triceps brachii, rectus abdominus, rectus femoris, supraspinatus, gluteus medias, pectoralis profundi, semitendinosus, longissimus thoracis, longissimus lumborum, tensor fascia latae, and gastrocnemius were collected from 10 USDA Choice carcasses (N = 110). To determine muscle fiber type, myofibrillar proteins were extracted and separated via gel electrophoresis and immunoblot, while muscle fiber CSA and diameter were determined using a dystrophin antibody stain via fluorescence microscopy. Pearson correlation analysis was performed to determine the relationship between muscle fiber type, CSA, diameter, and the eating quality of the 11 beef cuts from previously reported studies. Muscles from both studies showed distinct differences in the relative percentage of type I and type IIA muscle fiber types, CSA, and diameter (P &amp;lt; 0.05). Correlation analysis from study 1 demonstrated positive correlations between type I fibers and many positive attributes of eating quality such as tenderness, juiciness, and lipid flavor intensity, while negative correlations were found between type IIA fibers and those attributes (P &amp;lt; 0.01). Interestingly, results from study 2 showed that increasing type I fiber percentage may also contribute to greater connective tissue content and collagen crosslink density (P &amp;lt; 0.01). Finally, a negative correlation was found between muscle fiber CSA and diameter with connective tissue amount (P &amp;lt; 0.05), and a positive correlation was found between muscle fiber CSA and diameter with tenderness measurements (P &amp;lt; 0.05) in both studies. Overall, muscles with greater type I fiber % delivered a more favorable eating experience than those with more glycolytic metabolism. Notably, increased CSA and fiber diameter did not diminish eating quality and were found to have a muscle-specific relationship with tenderness.

Targeting Metastasis in Head and Neck Squamous Cell Carcinoma Using Follistatin mRNA Lipid Nanoparticles.

Metastatic progression significantly reduces survival rates and complicates treatment strategies in various cancers. Our study introduces an mRNA therapy for metastasis inhibition by targeting activin A overexpression, a pivotal driver of metastasis and cachexia. Utilizing follistatin mRNA lipid nanoparticles, we effectively downregulated activin A both locally in the tumor environment and systemically. This led to a reduction in tumor burden and suppression of metastatic spread in a murine head and neck squamous cell carcinoma model. Treated mice exhibited minimal metastatic occurrence compared to controls. Additionally, our therapy preserved the cross-sectional area of muscle fibers and adipose tissues, combating the muscle and fat wasting typically observed in cancer-associated cachexia. The therapy also demonstrated a favorable safety profile, underscoring its potential for clinical translation. By integrating metastasis-suppressing and cachexia-alleviating mechanisms, our approach represents a promising advancement in comprehensive cancer management. Considering the widespread upregulation of activin A in many cancer types, our therapy holds considerable potential for application across a broad spectrum of oncologic treatments.

Glycyrrhiza uralensis extract improves dexamethasone-induced sarcopenia via regulating myostatin- and FoxO3a-mediated E3 ubiquitin ligase

Sarcopenia affects the skeletal muscle system and impairs physical activity. This study investigated the effects of Glycyrrhiza uralensis (GU) extract on ameliorating dexamethasone (DEX)-induced sarcopenia in C2C12 myoblasts and C57BL/6J mice. Initially, network pharmacology analysis determined the association between GU and DEX-induced sarcopenia and predicted potentially related signaling pathways. Subsequently, the protective effects of different GU extracts on DEX-induced damage in C2C12 myotubes were evaluated by cell viability assays and Jenner and Giemsa (J&G) staining. Finally, the GU extract screened in vitro was evaluated by body composition analysis, grip strength and endurance tests, serum analysis, histological analysis, and protein expression level analysis in DEX-induced C57BL/6J mice. Network pharmacology confirmed the correlation between GU and DEX-induced sarcopenia involving TNF, MSTN, and FoxO signaling pathways. In DEX-induced damaged myotubes, only GU water extract (GW) dose-dependently increased cell viability. J&G staining of GW-treated myotubes revealed that GW increased the cell fusion index, maturation index and myotube diameter, attenuating DEX-induced cell damage. Oral administration of GW at doses above 300 mg/kg/day restored lean body mass lost due to DEX and subsequently reduced body fat. GW also enhanced grip strength and endurance, reduced muscle tissue injury, and increased the cross-sectional area of muscle fibers in DEX-induced mice. GW reduced the inflammatory response by alleviating serum TNF-α levels and ultimately decreased E3 ubiquitin ligase (MAFbx and MuRF-1) by downregulating myostatin and promoting FoxO3a phosphorylation. These results suggest that GU inhibits muscle protein degradation and is a potential food and medicinal homolog for preventing or treating sarcopenia.

Muscle type-specific effects of bilateral abobotulinumtoxinA injection on muscle growth and contractile function in spastic mice.

Intramuscular injection of botulinum neurotoxin type A (BoNT-A) is commonly used to improve or maintain the joint range of motion in young children with spasticity. However, the effectiveness of BoNT-A treatment is variable and movement limitations are recurrent. Here we show long-term effects of a single, bilateral abobotulinumtoxinA (aboBoNT-A) injection in the gastrocnemius medialis and soleus muscles of wild-type and spastic (B6.Cg-Glrbspa/J with a mutation in the glycine receptor) mice at a young age (6-7 days). Specifically, we evaluated the impact of aboBoNT-A-A on gait, physical performance, and spontaneous physical behavior, as well as on contractile force characteristics, morphology, and histological phenotype of soleus and gastrocnemius muscles by comparing their results to those of saline-injected controls up to 9 weeks after the injection. The detailed time course of the study specifies the timing of the aboBoNT-A injection at 1 week, the period of behavioral studies from 4-9 weeks, and the age of the mice (10 weeks) at the time of contractile force characteristics and histology assessments. In spastic mice, aboBoNT-A injection had a minor and very specific effect on physical performance, by only modestly increasing stride length as a function of age. aboBoNT-A injection caused a reduction in the force-generating capacity and a slightly smaller physiological cross-sectional area in gastrocnemius medialis, but not in soleus. Reduced physiological cross-sectional area in aboBoNT-A-injected muscles was due to a lower number of muscle fibers, rather than reduced muscle fiber cross-sectional area. The percentage of slow-type muscle fibers and mitochondrial succinate dehydrogenase activity were increased, which was associated with an improved muscle endurance capacity. In conclusion, aboBoNT-A injection reduced the number of muscle fibers, causing muscle hypertrophy in remaining fibers and a shift towards more oxidative fibers, resulting in an improved endurance capacity and gait. This study proposed potential cellular mechanisms for the therapeutic efficacy of aboBoNT-A in spasticity.

Effects of time-restricted feeding and weight-loaded swimming test on androgen levels and androgen receptor expression in orchiectomized male Wistar rats

Background & AimsTestosterone, vital for reproductive health and muscle development, declines with age, increasing susceptibility to conditions like diabetes, obesity and sarcopenia. Conventional hormone therapy carries risks, including elevated prostate-specific antigens and prostate cancer risk, prompting exploration of safer options like intermittent fasting (IF) and physical training (PT) which potentially boost androgen in certain cases. However, their combined impacts on testosterone remain underexplored. This study aimed to assess the individual and combined effects of IF and PT on androgen and androgen receptor (AR) levels. MethodsForty male Wistar rats were divided into five groups (n = 8 each): negative control (NC) receiving food ad libitum without orchiectomized, positive control (PC) receiving daily testosterone enanthate injections, IF with 16/8 time-restricted feeding, PT with 1-hour forced swimming sessions, and combined IF+PT. After 8 weeks, DHEA and testosterone levels, AR expression, gastrocnemius muscle histology, and body weight were assessed. ResultsIn comparison to the NC group (429.40 ± 26.86 g), body weight in the IF (348.90 ± 15.94 g, PT (391.40 ± 16.35 g), and IF+PT groups, (360.90 ± 29.90 g) was significantly lowered (p < 0.05) after 8 weeks of study. The muscle fiber cross-sectional area in the IF (2968 μm2; IQR 1995-4053 μm2), PT (2956 μm2; IQR 2089-4371 μm2), and IF+PT groups, (3389 μm2; IQR 2260-4596 μm2) was significantly greater than the NC group (2508 μm2; IQR 1800-3567 μm2. p < 0.05) after the study. DHEA levels significantly increased in the PT and IF+PT groups (375.01 ±32.55 ng/μL and 420.00 ± 24.50 ng/μL, respectively) compared to the NC group (257.09 ± 67.79 ng/μL, p < 0.05). However, neither IF nor PT, alone or in combination, resulted in improvements in testosterone levels or AR expression in the gastrocnemius muscle. with testosterone levels remained unchanged. ConclusionIF, PT, and IF+PT demonstrated potential effects on improving androgen levels, managing weight, and enhancing muscle growth, with IF+PT emerging as the most effective intervention. Despite these positive outcomes, the lack of impact on AR expression and testosterone levels suggests the need for further research to elucidate the underlying mechanisms and potential clinical applications for managing androgen deficiency through these interventions.

Asparagine synthetase regulates the proliferation and differentiation of chicken skeletal muscle satellite cells.

Asparagine synthetase (ASNS) is an aminotransferase responsible for the biosynthesis of aspartate by using aspartic acid and glutamine. ASNS is highly expressed in fast-growing broilers, but few studies have reported the regulatory role of ASNS in muscle development. To explore the function of ASNS in chicken muscle development, the expression of ASNS in different chicken breeds and tissues were first performed by real-time quantitative reverse transcription polymerase chain reaction (RT-PCR). Then, using real-time quantitative RT-PCR, western blot, EdU assay, cell cycle assay and immunofluorescence, the effects of ASNS on the proliferation and differentiation of chicken skeletal muscle satellite cell (SMSC) were investigated. Finally, potential mechanisms by which ASNS influences chicken muscle fiber differentiation were identified through RNA-Seq. The mRNA expression pattern of ASNS in muscles mirrors trends in muscle fiber cross-sectional area, average daily weight gain, and muscle weight across different breeds. ASNS knockdown inhibited SMSC proliferation, while overexpression showed the opposite. Moreover, ASNS attenuated SMSC differentiation by activating the adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) pathway. Additionally, 5-aminoimidazole4-carboxamide1-β-D-ribofuranoside (AICAR) treatment suppressed the cell differentiation induced by siRNA-ASNS. RNA-Seq identified 1,968 differentially expressed genes (DEGs) during chicken SMSC differentiation when overexpression ASNS. Gene ontology (GO) enrichment analysis revealed that these DEGs primarily participated in 8 biological processes, 8 cellular components, and 4 molecular functions. Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis identified several significantly enriched signaling pathways, such as the JAK-STAT signaling pathway, tumor necrosis factor signaling pathway, toll-like receptor signaling pathway, and PI3K-Akt signaling pathway. ASNS promotes proliferation while inhibits the differentiation of chicken SMSCs. This study provides a theoretical basis for studying the role of ASNS in muscle development.

Astragaloside IV Improves Muscle Atrophy by Modulating the Activity of UPS and ALP via Suppressing Oxidative Stress and Inflammation in Denervated Mice.

Peripheral nerve injury is common clinically and can lead to neuronal degeneration and atrophy and fibrosis of the target muscle. The molecular mechanisms of muscle atrophy induced by denervation are complex and not fully understood. Inflammation and oxidative stress play an important triggering role in denervated muscle atrophy. Astragaloside IV (ASIV), a monomeric compound purified from astragalus membranaceus, has antioxidant and anti-inflammatory properties. The aim of this study was to investigate the effect of ASIV on denervated muscle atrophy and its molecular mechanism, so as to provide a new potential therapeutic target for the prevention and treatment of denervated muscle atrophy. In this study, an ICR mouse model of muscle atrophy was generated through sciatic nerve dissection. We found that ASIV significantly inhibited the reduction of tibialis anterior muscle mass and muscle fiber cross-sectional area in denervated mice, reducing ROS and oxidative stress-related protein levels. Furthermore, ASIV inhibits the increase in inflammation-associated proteins and infiltration of inflammatory cells, protecting the denervated microvessels in skeletal muscle. We also found that ASIV reduced the expression levels of MAFbx, MuRF1 and FoxO3a, while decreasing the expression levels of autophagy-related proteins, it inhibited the activation of ubiquitin-proteasome and autophagy-lysosome hydrolysis systems and the slow-to-fast myofiber shift. Our results show that ASIV inhibits oxidative stress and inflammatory responses in skeletal muscle due to denervation, inhibits mitophagy and proteolysis, improves microvascular circulation and reverses the transition of muscle fiber types; Therefore, the process of skeletal muscle atrophy caused by denervation can be effectively delayed.

Effect of Photobiomodulation on an Experimental Model ofLower Limb Ischemia.

Photobiomodulation (PBM) has been shown to be promising for the promotion of angiogenesis. The present study investigated the effects of PBM on vascularization in an animal model of peripheral artery disease. Wistar rats were divided into three groups. The control group received no procedures. The ischemia group was submitted to ligation of the femoral artery of the hindleg. The ischemia + PBM group was submitted to ligation of the femoral artery followed by PBM (660 and 808 nm, 100 mW, 4 J) over the site. Animals with ischemia treated with PBM exhibited comparable results to the control group with regards to the diameter of the α-SMA+ vessels, cross-sectional area of muscle fibers, percentage of collagen and serum concentration of IL-17A, as well as similarities in terms of vertical mobility, temperature of the hindleg, number of acts of grooming, and percentage of movement, indicating a condition like that of limbs unaffected by ischemia.

Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression.

Skeletal muscle mitochondria and capillaries are crucial for aerobic fitness, and suppressed levels are associated with chronic and age-related diseases. Currently, evidence-based exercise training recommendations to enhance these characteristics are limited. It is essential to explore how factors, such as fitness level, age, sex, and disease affect mitochondrial and capillary adaptations to different exercise stimuli. The main aim of this study was to compare the effects of low- or moderate intensity continuous endurance training (ET), high-intensity interval or continuous training (HIT), and sprint interval training (SIT) on changes in skeletal muscle mitochondrial content and capillarization. Secondarily, the effects on maximal oxygen consumption (VO2max), muscle fiber cross-sectional area, and fiber type proportion were investigated. A systematic literature search was conducted in PubMed, Web of Science, and SPORTDiscus databases, with no data restrictions, up to 2 February 2022. Exercise training intervention studies of ET, HIT, and SIT were included if they had baseline and follow-up measures of at least one marker of mitochondrial content or capillarization. In total, data from 5973 participants in 353 and 131 research articles were included for the mitochondrial and capillary quantitative synthesis of this review, respectively. Additionally, measures of VO2max, muscle fiber cross-sectional area, and fiber type proportion were extracted from these studies. After adjusting for relevant covariates, such as training frequency, number of intervention weeks, and initial fitness level, percentage increases in mitochondrial content in response to exercise training increased to a similar extent with ET (23 ± 5%), HIT (27 ± 5%), and SIT (27 ± 7%) (P > 0.138), and were not influenced by age, sex, menopause, disease, or the amount of muscle mass engaged. Higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in mitochondrial content. Per total hour of exercise, SIT was ~ 2.3 times more efficient in increasing mitochondrial content than HIT and ~ 3.9 times more efficient than ET, while HIT was ~ 1.7 times more efficient than ET. Capillaries per fiber increased similarly with ET (15 ± 3%), HIT (13 ± 4%) and SIT (10 ± 11%) (P = 0.556) after adjustments for number of intervention weeks and initial fitness level. Capillaries per mm2 only increased after ET (13 ± 3%) and HIT (7 ± 4%), with increases being larger after ET compared with HIT and SIT (P < 0.05). This difference coincided with increases in fiber cross-sectional area after ET (6.5 ± 3.5%), HIT (8.9 ± 4.9%), and SIT (11.9 ± 15.1%). Gains in capillarization occurred primarily in the early stages of training (< 4weeks) and were only observed in untrained to moderately trained participants. The proportion of type I muscle fibers remained unaltered by exercise training (P > 0.116), but ET and SIT exhibited opposing effects (P = 0.041). VO2max increased similarly with ET, HIT, and SIT, although HIT showed a tendency for greater improvement compared with both ET and SIT (P = 0.082), while SIT displayed the largest increase per hour of exercise. Higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in VO2max. Women displayed greater percentage gains in VO2max compared with men (P = 0.008). Generally, lower initial fitness levels were associated with greater percentage improvements in mitochondrial content, capillarization, and VO2max. SIT was particularly effective in improving mitochondrial content and VO2max in the early stages of training, while ET and HIT showed slower but steady improvements over a greater number of training weeks. The magnitude of change in mitochondrial content, capillarization, and VO2max to exercise training is largely determined by the initial fitness level, with greater changes observed in individuals with lower initial fitness. The ability to adapt to exercise training is maintained throughout life, irrespective of sex and presence of disease. While training load (volume × intensity) is a suitable predictor of changes in mitochondrial content and VO2max, this relationship is less clear for capillary adaptations.

Repeated passive heat treatment increases muscle tissue capillarization, but does not affect postprandial muscle protein synthesis rates in healthy older adults.

Prolonged passive heat treatment (PHT) has been suggested to trigger skeletal muscle adaptations that may improve muscle maintenance in older individuals. To assess the effects of PHT on skeletal muscle tissue capillarization, perfusion capacity, protein synthesis rates, hypertrophy and leg strength, 14 older adults (9males, 5 females; 73 ± 6years) underwent 8weeks of PHT (infrared sauna: 3× per week, 45min at ∼60°C). Before and after PHT we collected muscle biopsies to assess skeletal muscle capillarization and fibre cross-sectional area (CSA). Basal and postprandial muscle tissue perfusion kinetics and protein synthesis rates were assessed using contrast-enhanced ultrasound and primed continuous l-[ring-13C6]phenylalanine infusions, respectively. One-repetition maximum (1RM) leg strength and vastus lateralis muscle CSA were assessed. Type I and type II muscle fibre capillarization strongly increased following PHT (capillary-to-fibre perimeter exchange index: +31 ± 18 and +33 ± 30%, respectively; P<0.001). No changes were observed in basal (0.24 ± 0.27 vs. 0.18 ± 0.11 AU; P=0.266) or postprandial (0.20 ± 0.12 vs. 0.18 ± 0.14 AU; P=0.717) microvascular blood flow following PHT. Basal (0.048 ± 0.014 vs. 0.051 ± 0.019%/h; P=0.630) and postprandial (0.041 ± 0.012 vs. 0.051 ± 0.024%/h; P=0.199) muscle protein synthesis rates did not change in response to prolonged PHT. Furthermore, no changes in vastus lateralis muscle CSA (15.3 ± 4.6 vs. 15.2 ± 4.6 cm2; P=0.768) or 1RM leg strength (46 ± 12 vs. 47 ± 12 kg; P=0.087) were observed over time. In conclusion, prolonged PHT increases muscle tissue capillarization but this does not improve muscle microvascular blood flow or increase muscle protein synthesis rates in healthy, older adults. Prolonged PHT does not induce skeletal muscle hypertrophy or increase leg strength in healthy, older adults. KEY POINTS: Repeated exposure to heat has been suggested to trigger skeletal muscle adaptive responses. We investigated the effect of 8weeks of whole-body passive heat treatment (PHT; infrared sauna: 3× per week for 45min at ∼60°C) on skeletal muscle tissue capillarization, perfusion capacity, basal, and postprandial muscle protein synthesis rates, muscle (fibre) hypertrophy, and leg strength in healthy, older adults. Prolonged PHT increases muscle tissue capillarization, but this does not improve muscle microvascular blood flow or increase muscle protein synthesis rates. Despite increases in muscle tissue capillarization, prolonged PHT does not suffice to induce skeletal muscle hypertrophy or increase leg strength in healthy, older adults.

Study on the relationship between NF-kB pathway and skeletal muscle dopamine receptors in muscle-attenuated mice

Background: To investigate the relationship between the NF-kB signaling pathway and muscle-attenuated skeletal muscle dopaminereceptor (DR) in mice. Methodology: 40 specific pathogens free (SPF) C57BL/6 mice aged 6 to 7 months were randomly divided into a model group and control group by random number table method, with 20 mice in each group. Muscle-attenuated mice were established in model group, and the expression of NF-kB protein was detected by Western-blot, and the expressions of IL-β1, TNF-a, DRD1, and DRD2 were detected by enzyme linked immunosorbent assay (ELISA). Meanwhile, the muscle fiber cross-sectional area was determined. Results: The relative expression of NF-kB protein, IL-,1 and TNF-a in model group were significantly higher than those in control group (P &lt; 0.05). The levels of DRD1 and DRD2 in skeletal muscle in model group were significantly lower than those in control group (P &lt; 0.05). The cross-sectional are the model group was significantly lower than in the control group (P &lt; 0.05). The relative expression of NF-kB protein was negatively correlated with DRD1, DRD2 and muscle fiber cross-sectional area. DRD1 and DRD2 were positively correlated with muscle fiber cross-sectional area. Conclusion: In muscle-atrophied mice, NF-kB protein, an indicator of the NF-kB signaling pathway, was negatively correlated with DR, and both had an important role in muscle atrophy. Keywords: Nuclear factor kB signaling pathway; Muscle attenuation; Mice; Skeletal muscle; Dopamine receptor.

Specific tension of human muscle in vivo: a systematic review.

The intrinsic force production capability of human muscle can be expressed as "Specific Tension," or, the maximum force generated per cross-sectional area of muscle fibers. This value can be used to determine, for example, whether muscle quality changes during exercise, atrophy, disease, or hypertrophy. A value of 22.5 N/cm2 for mammalian muscle has generally become accepted based on detailed studies of small mammals. Determining the specific tension of human muscle is much more challenging as almost all determinations are indirect. Calculation of human muscle specific tension requires an understanding of that muscle's contribution to joint torque, its activation magnitude, tendon compliance, and joint moment arm. Determining any of these parameters is technically challenging in humans and thus, it is no surprise that human specific tension values reported vary from 2 to 73 N/cm2. In this systematic review, we screened 1,506 published papers and identified the 30 studies published between 1983 and 2023 that used appropriate methods and which reported 96 human specific tension values. We weighted each parameter based on whether it was directly measured, estimated, or calculated based on the literature, with decreasing weighting used, the more indirect the methods. Based on this exhaustive review of the relevant human literature, we suggest that the most accurate value that should be used for human muscle specific tension is 26.8 N/cm2.

Reconsidering assumptions in the analysis of muscle fibre cross-sectional area.

Cross-sectional area (CSA) is a fundamental variable in characterizing muscle mechanical properties. Typically, the CSA of a single muscle fibre is assessed by measuring either one or two diameters, and assuming the cross-section is either circular or elliptical in shape. However, fibre cross-sections have irregular shapes. The accuracy and precision of CSAs determined using circular and elliptical shape assumptions are unclear for mammalian skinned muscle fibres. Second harmonic generation imaging of skinned rabbit soleus fibres revealed that the circular assumption overstated real CSA by 5.3±25.9% whereas the elliptical assumption overstated real CSA by 2.8±6.9%. A preferred rotational alignment can bias the circular assumption, as real CSA was overstated by 22.1±24.8% when using the larger fibre diameter and understated by 11.4±13% when using the smaller fibre diameter. With 73% lower variable error and reduced bias, the elliptical assumption is superior to the circular assumption when assessing the CSA of skinned mammalian fibres.

Role of Irisin in exercise training-regulated endoplasmic reticulum stress, autophagy and myogenesis in the skeletal muscle after myocardial infarction.

Patients with heart failure (HF) are often accompanied by skeletal muscle abnormalities, which can lead to exercise intolerance and compromise daily activities. Irisin, an exercise training (ET) -induced myokine, regulates energy metabolism and skeletal muscle homeostasis. However, the precise role of Irisin in the benefits of ET on inhibiting skeletal muscle atrophy, particularly on endoplasmic reticulum (ER) stress, autophagy, and myogenesis following myocardial infarction (MI) remains unclear. In this study, we investigated the expression of Irisin protein in wild-type mice with MI, and assessed its role in the beneficial effects of ET using an Fndc5 knockout mice. Our findings revealed that MI reduced muscle fiber cross-sectional area (CSA), while downregulating the expression of Irisin, PGC-1α and SOD1. Concurrently, MI elevated the levels of ER stress and apoptosis, and inhibited autophagy in skeletal muscle. Conversely, ET mitigated ER stress and apoptosis in the skeletal muscle of infarcted mice. Notably, Fndc5 knockout worsened MI-induced ER stress and apoptosis, suppressed autophagy and myogenesis, and abrogated the beneficial effects of ET. In conclusion, our findings highlight the role of Irisin in the ET-mediated alleviation of skeletal muscle abnormalities. This study provides valuable insights into MI-induced muscle abnormalities and enhances our understanding of exercise rehabilitation mechanisms in clinical MI patients.

Melatonin protects against sarcopenia in middle-aged mice.

Sarcopenia is a common age-related disease. Melatonin (MEL) is an age-related endocrine hormone, which displays a crucial role in resisting oxidative stress during aging. Importantly, the antioxidant properties of MEL can be mediated by mitochondria. Therefore, we wondered whether MEL could mitigate oxidative stress caused by mitochondria in sarcopenia. The middle-aged mice were administered 5 mg/kg/d and 10 mg/kg/d of MEL for 2 months. Young mice were used as the control group. After treatment with MEL, the grip strength of the fore/hind limbs, running time, and distance were elevated, and the weights of the gastrocnemius (GA), tibialis anterior (TA), extensor digitorum longus (EDL), and soleus (SOL) were enhanced in middle-aged mice. Additionally, MEL was observed to alleviate histological damage and increase the cross-sectional area of muscle fibers in GA tissues of middle-aged mice. Furthermore, following MEL treatment, there was an increase in the percentage and size of normal mitochondria as well as mtDNA copy number but a reduction in the levels of malondialdehyde (MDA), protein carbonyl, and reactive oxygen species (ROS) in the GA tissues of middle-aged mice. At the molecular level, MEL repressed the levels of ATROGIN-1, muscle RING-finger protein-1 (MURF-1), and the ratio of p-P38/P38, but elevated the expression of cytochrome c oxidase subunit 4 (COX4), cystatin C (CYTC), nuclear respiratory factor 1 (NRF-1), mitochondrial transcription factor A (TFAM), and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) in the GA tissues of middle-aged mice. Importantly, 10 mg/kg MEL was more efficacious in the treatment of sarcopenia than 5 mg/kg MEL. MEL attenuates sarcopenia in middle-aged mice, and the mechanism may relate to mitochondria-induced oxidative stress and the PGC-1α/TFAM pathway.

Hydrogen sulfide inhibits skeletal muscle ageing by up-regulating autophagy through promoting deubiquitination of adenosine 5'-monophosphate (AMP)-activated protein kinase α1 via ubiquitin specific peptidase 5.

Hydrogen sulfide (H2S), the third gasotransmitter discovered, regulates a variety of physiological functions. Whether H2S alleviates skeletal muscle ageing by regulating autophagy has not been reported. Mice were administered 150mg/kg/day of D-galactose (D-gal), and C2C12 myotubes were cultured in 20g/L D-gal to induce ageing. Sodium hydrosulfide (NaHS) was employed as an exogenous donor in the treatment group. The intracellular concentration of H2S was quantified by the 7-azido-4-methylcoumarin fluorescence probe. The proteins involved in the ubiquitin-mediated degradation of AMPKα1 were detected by liquid chromatography tandem mass spectrometry (LC-MS/MS) and co-immunoprecipitation (Co-IP). S-sulfhydration of USP5 was tested by a biotin-switch assay. Associated proteins were analysed by western blot. NaHS was found to effectively restore the H2S content in both ageing gastrocnemius (+91.89%, P<0.001) and C2C12 myotubes (+27.55%, P<0.001). In comparison to theD-gal group, NaHS was observed to increase the mean cross-sectional area of muscle fibres (+44.91%, P<0.001), to decrease the collagen volume fraction of gastrocnemius (-81.32%, P=0.001) and to reduce the β-galactosidase-positive area of C2C12 myotubes (-28.74%, P<0.001). NaHS was also found to reverse the expression of muscle atrophy F box protein (MAFbx), muscle-specific RING finger protein 1 (MuRF1), Cyclin D1 and p21 in the ageing gastrocnemius tissue (MAFbx: -31.73%, P=0.008; MuRF1: -32.37%, P=0.003; Cyclin D1: +45.34%, P=0.010; p21: -25.53%, P=0.022) and C2C12 myotubes (MAFbx: -16.38%, P<0.001; MuRF1: -16.45%, P=0.003; Cyclin D1: +40.23%, P<0.001; p21: -35.85%, P=0.026). The AMPKα1-ULK1 pathway was activated and autophagy was up-regulated in NaHS-treated gastrocnemius tissue (p-AMPKα1: +61.61%, P=0.018; AMPKα1: +30.64%, P=0.010; p-ULK1/ULK1: +85.87%, P=0.005; p62: -29.07%, P<0.001; Beclin1: +24.75%, P=0.007; light chain 3 II/I [LC3 II/I]: +55.78%, P=0.004) and C2C12 myotubes (p-AMPKα1: +77.49%, P=0.018; AMPKα1: +26.18%, P=0.022; p-ULK1/ULK1: +38.34%, P=0.012; p62: -9.02%, P=0.014; Beclin1: +13.36%, P<0.001; LC3 II/I: +79.38%, P=0.017; autophagy flux: +24.88%, P=0.034) compared with the D-gal group. The effects of NaHS on autophagy were comparable to those of acadesine and LYN-1604, and chloroquine could reverse its effects on ageing. LC-MS/MS and Co-IP experiments demonstrated that USP5 is a deubiquitinating enzyme of AMPKα1. Following the knockdown of USP5, the activation of AMPKα1 was decreased (p-AMPKα1: -42.10%, P<0.001; AMPKα1: -43.93%, P<0.001), autophagy was inhibited (p-ULK1/ULK1: -27.51, P=0.001; p62: +36.00, P<0.001; Beclin1: -22.15%, P<0.001) and NaHS lost its ability to up-regulate autophagy. NaHS was observed to restore the expression (gastrocnemius: +62.17%, P<0.001; C2C12 myotubes: +37.51%, P=0.003) and S-sulfhydration (+53.07%, P=0.009) of USP5 and reduce the ubiquitination of AMPKα1. H2S promotes the deubiquitination of AMPKα1 by increasing the expression and S-sulfhydration of USP5, thereby up-regulating autophagy and alleviating skeletal muscle ageing.