Quercetin induces a slow myofiber phenotype in engineered human skeletal muscle tissues.

Skeletal muscle comprises slow and fast myofibers, with slow myofibers excelling in aerobic metabolism and endurance. Quercetin, a polyphenol, is reported to induce slow myofibers in rodent skeletal muscle both in vitro and in vivo. However, its effect on human myofiber types remains unexplored. In this study, we evaluated quercetin's impact on slow myofiber induction using human skeletal muscle satellite cells. In a two-dimensional culture, quercetin enhanced gene expression, contributing to muscle differentiation, and significantly expanded the area of slow-type myosin heavy chain positive cells. It also elevated the gene expression of Pgc1α, an inducer of slow myofibers. Conversely, quercetin did not affect mitochondrial abundance, fission, or fusion, but it did increase the gene expression of Cox7A2L, which aids in promoting mitochondrial supercomplexity and endurance, and Mb, which contributes to oxidative phosphorylation. In a three-dimensional culture, quercetin significantly extended the time to peak tension and half relaxation time of the engineered human skeletal muscle tissues constructed on microdevices. Moreover, quercetin enhanced the muscle endurance of the tissues and curbed the rise in lactate secretion from the exercised tissues. These findings suggest that quercetin may induce slow myofibers in human skeletal muscle.

Piperine enhances contractile force in slow- and fast-twitch muscle.

Piperine has been shown to bind to myosin and shift the distribution of conformational states of myosin molecules from the super-relaxed state to the disordered relaxed state. However, little is known about the implications for muscle force production and potential underlying mechanisms. Muscle contractility experiments were performed using isolated muscles and single fibres from rats and mice. The dose-response effect of piperine on muscle force was assessed at several stimulation frequencies. The potentiation of muscle force was also tested in muscles fatigued by eccentric contractions. Potential mechanisms of force potentiation were assessed by measuring Ca2+ levels during stimulation in enzymatically dissociated muscle fibres, while myofibrillar Ca2+ sensitivity was assessed in chemically skinned muscle fibres. Piperine caused a dose-dependent increase in low-frequency force with no effect on high-frequency force in both slow- and fast-twitch muscle, with similar relative increases in twitch force, rate of force development and relaxation rate. The potentiating effect of piperine on low-frequency force was reversible, and piperine partially recovered low-frequency force in fatigued muscle. Piperine had no effect on myoplasmic free [Ca2+] levels in mouse muscle fibres, whereas piperine substantially augmented the force response to submaximal levels of [Ca2+] in rat MyHCII fibres and MyHCI fibres along with a minor increase in maximum Ca2+-activated force. Piperine enhances low-frequency force production in both fast- and slow-twitch muscle. The effects are reversible and can counteract muscle fatigue. The primary underlying mechanism appears to be an increase in Ca2+ sensitivity. KEY POINTS: Piperine is a plant alkaloid derived from black pepper. It is known to bind to skeletal muscle myosin and enhance resting ATP turnover but its effects on contractility are not well known. We showed for the first time a piperine-induced force potentiation that was pronounced during low-frequency electrical stimulation of isolated muscles. The effect of piperine was observed in both slow and fast muscle types, was reversible, and could counteract the force decrements observed after fatiguing muscle contractions. Piperine treatment caused an increase in myofibrillar Ca2+ sensitivity in chemically skinned muscle fibres, while we observed no effect on intracellular Ca2+ concentrations during electrical stimulation in enzymatically dissociated muscle fibres.

Azilsartan prevents muscle loss and fast- to slow-twitch muscle fiber shift in natural ageing sarcopenic rats.

Sarcopenia is a musculoskeletal disease that reduces muscle mass and strength in older individuals. The study investigates the effects of azilsartan (AZL) on skeletal muscle loss in natural sarcopenic rats. Male Sprague-Dawley rats aged 4-6 months and 18-21 months were selected as young-matched control and natural-aged (sarcopenic) rats, respectively. Rats were allocated into young and old control (YC and OC) and young and old AZL treatment (YT and OT) groups, which received vehicles and AZL (8 mg/kg, orally) for 6 weeks. Rats were then sacrificed after muscle function analysis. Serum and gastrocnemius (GN) muscles were isolated for further endpoints. AZL significantly improved muscle grip strength and antioxidant levels in sarcopenic rats. AZL also restored the levels of insulin, testosterone, and muscle biomarkers such as myostatin and creatinine kinase in sarcopenic rats. Furthermore, AZL treatment improved the cellular and ultrastructure of GN muscle and prevented the shift of type II (glycolytic) myofibers to type I (oxidative) myofibers. The results showed that AZL intervention restored protein synthesis in natural sarcopenic rats by increasing p-Akt-1 and decreasing muscle RING-finger protein-1 and tumor necrosis factor alpha immunoexpressions. In conclusion, the present findings showed that AZL could be an effective intervention in treating age-related muscle impairments.

Arginine Regulates Skeletal Muscle Fiber Type Formation via mTOR Signaling Pathway.

The composition of skeletal muscle fiber types affects the quality of livestock meat and human athletic performance and health. L-arginine (Arg), a semi-essential amino acid, has been observed to promote the formation of slow-twitch muscle fibers in animal models. However, the precise molecular mechanisms are still unclear. This study investigates the role of Arg in skeletal muscle fiber composition and mitochondrial function through the mTOR signaling pathway. In vivo, 4-week C56BL/6J male mice were divided into three treatment groups and fed a basal diet supplemented with different concentrations of Arg in their drinking water. The trial lasted 7 weeks. The results show that Arg supplementation significantly improved endurance exercise performance, along with increased SDH enzyme activity and upregulated expression of the MyHC I, MyHC IIA, PGC-1α, and NRF1 genes in the gastrocnemius (GAS) and quadriceps (QUA) muscles compared to the control group. In addition, Arg activated the mTOR signaling pathway in the skeletal muscle of mice. In vitro experiments using cultured C2C12 myotubes demonstrated that Arg elevated the expression of slow-fiber genes (MyHC I and Tnnt1) as well as mitochondrial genes (PGC-1α, TFAM, MEF2C, and NRF1), whereas the effects of Arg were inhibited by the mTOR inhibitor rapamycin. In conclusion, these findings suggest that Arg modulates skeletal muscle fiber type towards slow-twitch fibers and enhances mitochondrial functions by upregulating gene expression through the mTOR signaling pathway.

Leucine promotes energy metabolism and stimulates slow-twitch muscle fibers expression through AMPK/mTOR signaling in equine skeletal muscle satellite cells.

Previous research has shown that leucine (Leu) can stimulate and enhance the proliferation of equine skeletal muscle satellite cells (SCs). The gene expression profile associated with Leu-induced proliferation of equine SCs has also been documented. However, the specific role of Leu in regulating the expression of slow-twitch muscle fibers (slow-MyHC) and mitochondrial function in equine SCs, as well as the underlying mechanism, remains unclear. During this investigation, equine SCs underwent culturing in differentiation medium and were subjected to varying concentrations of Leu (0 mM, 0.5 mM, 1 mM, 2 mM, 5 mM, and 10 mM) over a span of 3 days. AMP-activated protein kinase (AMPK) inhibitor Compound C and mammalian target of rapamycin complex (mTOR) inhibitor Rapamycin were utilized to explore its underlying mechanism. Here we showed that the expression of slow-MyHC at 2 mM Leu level was significantly higher than the concentration levels of 0 mM,0.5 mM and 10 mM (P <0.01), and there was no significant difference compared to other groups (P > 0.05); the basal respiration, maximum respiration, standby respiration and the expression of slow-MyHC, PGC-1α, Cytc, ND1, TFAM, and COX1 were significantly increased with Leu supplementation (P < 0.01). We also found that Leu up-regulated the expression of key proteins on AMPK and mTOR signaling pathways, including LKB1, p-LKB1, AMPK, p-AMPK, S6, p-S6, 4EBP1, p-4EBP1, mTOR and p-mTOR (P < 0.05 or P < 0.01). Notably, when we treated the equine SCs with the AMPK inhibitor Compound C and the mTOR inhibitor Rapamycin, we observed a reduction in the beneficial effects of Leu on the expression of genes related to slow-MyHC and signaling pathway-related gene expressions. This study provides novel evidence that Leu promotes slow-MyHC expression and enhances mitochondrial function in equine SCs through the AMPK/mTOR signaling pathways, shedding light on the underlying mechanisms involved in these processes for the first time.

Grape seed proanthocyanidin extract promotes skeletal muscle fiber type transformation through modulation of cecal microbiota and enhanced butyric acid production.

The conversion of fast-twitch fibers into slow-twitch fibers within skeletal muscle plays a crucial role in improving physical stamina and safeguarding against metabolic disorders in individuals. Grape seed proanthocyanidin extract (GSPE) possesses numerous pharmacological and health advantages, effectively inhibiting the onset of chronic illnesses. However, there is a lack of research on the specific mechanisms by which GSPE influences muscle physiology and gut microbiota. This study aims to investigate the role of gut microbiota and their metabolites in GSPE regulation of skeletal muscle fiber type conversion. In this experiment, 54 male BALB/c mice were randomly divided into three groups: basal diet, basal diet supplemented with GSPE, and basal diet supplemented with GSPE and antibiotics. During the feeding period, glucose tolerance and forced swimming tests were performed. After euthanasia, samples of muscle and feces were collected for analysis. The results showed that GSPE increased the muscle mass and anti-fatigue capacity of the mice, as well as the expression of slow-twitch fibers. However, the beneficial effects of GSPE on skeletal muscle fibers disappeared after adding antibiotics to eliminate intestinal microorganisms, suggesting that GSPE may play a role by regulating intestinal microbial structure. In addition, GSPE increased the relative abundance of Blautia, Muribaculaceae, and Enterorhabdus, as well as butyrate production. Importantly, these gut microbes exhibited a significant positive correlation with the expression of slow-twitch muscle fibers. In conclusion, supplementation with GSPE can increase the levels of slow-twitch fibers by modulating the gut microbiota, consequently prolonging the duration of exercise before exhaustion. PRACTICAL APPLICATION: This research suggests that grape seed proanthocyanidin extract (GSPE) has potential applications in improving physical stamina and preventing metabolic disorders. By influencing the gut microbiota and increasing butyric acid production, GSPE contributes to the conversion of fast-twitch muscle fibers into slow-twitch fibers, thereby enhancing anti-fatigue capacity and exercise endurance. While further studies are needed, incorporating GSPE into dietary supplements or functional foods could support individuals seeking to optimize their exercise performance and overall metabolic health.

Effect of DHEA on Recovery of Muscle Atrophy Induced by Parkinson's Disease

PURPOSE: The purpose of this study was to determine the effect of dehydroepiandrosterone (DHEA) on recovery of muscle atrophy induced by Parkinson's disease. METHODS: The rat model was established by direct injection of 6-hydroxydopamine (6-OHDA, 20 microg) into the left striatum using stereotaxic surgery. Rats were divided into two groups; the Parkinson's disease group with vehicle treatment (Vehicle; n=12) or DHEA treatment group (DHEA; n=22). DHEA or vehicle was administrated intraperitoneally daily at a dose of 0.34 mmol/kg for 21 days. At 22-days after DHEA treatment, soleus, plantaris, and striatum were dissected. RESULTS: The DHEA group showed significant increase (p<.01) in the number of tyrosine hydroxylase (TH) positive neurons in the lesioned side substantia nigra compared to the vehicle group. Weights and Type I fiber cross-sectional areas of the contralateral soleus of the DHEA group were significantly greater than those of the vehicle group (p=.02, p=.00). Moreover, extracellular signal-regulated kinase (ERK) phosphorylation significantly decreased in the lesioned striatum, but was recovered with DHEA and also in the contralateral soleus muscle, Akt and ERK phosphorylation recovered significantly and the expression level of myosin heavy chain also recovered by DHEA treatment. CONCLUSION: Our results suggest that DHEA treatment recovers Parkinson's disease induced contralateral soleus muscle atrophy through Akt and ERK phosphorylation.

Cyclosporin A preferentially attenuates skeletal slow-twitch muscle regeneration

Calcineurin, a Ca2+/calmodulin-dependent phosphatase, is associated with muscle regeneration via NFATc1/GATA2-dependent pathways. However, it is not clear whether calcineurin preferentially affects the regeneration of slow- or fast-twitch muscles. We investigated the effect of a calcineurin inhibitor, cyclosporin A (CsA), on the morphology and fiber diameter of regenerating slow- and fast-twitch muscles. Adult Wistar rats (259.5 ± 9 g) maintained under standard conditions were treated with CsA (20 mg/kg body weight, ip) for 5 days, submitted to cryolesion of soleus and tibialis anterior (TA) muscles on the 6th day, and then treated with CsA for an additional 21 days. The muscles were removed, weighed, frozen, and stored in liquid nitrogen. Cryolesion did not alter the body weight gain of the animals after 21 days of regeneration (P = 0.001) and CsA significantly reduced the body weight gain (15.5 percent; P = 0.01) during the same period. All treated TA and soleus muscles showed decreased weights (17 and 29 percent, respectively, P < 0.05). CsA treatment decreased the cross-sectional area of both soleus and TA muscles of cryoinjured animals (TA: 2108 ± 930 vs 792 ± 640 µm²; soleus: 2209 ± 322 vs 764 ± 439 m²; P < 0.001). Histological sections of both muscles stained with Toluidine blue revealed similar regenerative responses after cryolesion. In addition, CsA was able to minimize these responses, i.e., centralized nuclei and split fibers, more efficiently so in TA muscle. These results indicate that calcineurin preferentially plays a role in regeneration of slow-twitch muscle.