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.

Differential effect of canagliflozin, a sodium-glucose cotransporter 2 (SGLT2) inhibitor, on slow and fast skeletal muscles from nondiabetic mice.

There has been a concern that sodium-glucose cotransporter 2 (SGLT2) inhibitors could reduce skeletal muscle mass and function. Here, we examine the effect of canagliflozin (CANA), an SGLT2 inhibitor, on slow and fast muscles from nondiabetic C57BL/6J mice. In this study, mice were fed with or without CANA under ad libitum feeding, and then evaluated for metabolic valuables as well as slow and fast muscle mass and function. We also examined the effect of CANA on gene expressions and metabolites in slow and fast muscles. During SGLT2 inhibition, fast muscle function is increased, as accompanied by increased food intake, whereas slow muscle function is unaffected, although slow and fast muscle mass is maintained. When the amount of food in CANA-treated mice is adjusted to that in vehicle-treated mice, fast muscle mass and function are reduced, but slow muscle was unaffected during SGLT2 inhibition. In metabolome analysis, glycolytic metabolites and ATP are increased in fast muscle, whereas glycolytic metabolites are reduced but ATP is maintained in slow muscle during SGLT2 inhibition. Amino acids and free fatty acids are increased in slow muscle, but unchanged in fast muscle during SGLT2 inhibition. The metabolic effects on slow and fast muscles are exaggerated when food intake is restricted. This study demonstrates the differential effects of an SGLT2 inhibitor on slow and fast muscles independent of impaired glucose metabolism, thereby providing new insights into how they should be used in patients with diabetes, who are at a high risk of sarcopenia.

Ryanodine receptors mediate high intracellular Ca2+ and some myocyte damage following eccentric contractions in rat fast-twitch skeletal muscle.

Eccentric contractions (ECC) facilitate cytosolic calcium ion (Ca2+) release from the sarcoplasmic reticulum (SR) and Ca2+ influx from the extracellular space. Ca2+ is a vital signaling messenger that regulates multiple cellular processes via its spatial and temporal concentration ([Ca2+]i) dynamics. We hypothesized that 1) a specific pattern of spatial/temporal intramyocyte Ca2+ dynamics portends muscle damage following ECC and 2) these dynamics would be regulated by the ryanodine receptor (RyR). [Ca2+]i in the tibialis anterior muscles of anesthetized adult Wistar rats was measured by ratiometric (i.e., ratio, R, 340/380 nm excitation) in vivo bioimaging with Fura-2 pre-ECC and at 5 and 24 h post-ECC (5 × 40 contractions). Separate groups of rats received RyR inhibitor dantrolene (DAN; 10 mg/kg ip) immediately post-ECC (+DAN). Muscle damage was evaluated by histological analysis on hematoxylin-eosin stained muscle sections. Compared with control (CONT, no ECC), [Ca2+]i distribution was heterogeneous with increased percent total area of high [Ca2+]i sites (operationally defined as R ≥ 1.39, i.e., ≥1 SD of mean control) 5 h post-ECC (CONT, 14.0 ± 8.0; ECC5h: 52.0 ± 7.4%, P < 0.01). DAN substantially reduced the high [Ca2+]i area 5 h post-ECC (ECC5h + DAN: 6.4 ± 3.1%, P < 0.01) and myocyte damage (ECC24h, 63.2 ± 1.0%; ECC24h + DAN: 29.1 ± 2.2%, P < 0.01). Temporal and spatially amplified [Ca2+]i fluctuations occurred regardless of DAN (ECC vs. ECC + DAN, P > 0.05). These results suggest that the RyR-mediated local high [Ca2+]i itself is related to the magnitude of muscle damage, whereas the [Ca2+]i fluctuation is an RyR-independent phenomenon.

Quercetin regulates skeletal muscle fiber type switching via adiponectin signaling.

This study aimed to investigate the role and underlying molecular mechanism of quercetin in regulating skeletal muscle fiber type transition. We found that dietary quercetin supplementation in mice significantly increased oxidative fiber-related gene expression, slow-twitch fiber percentage and succinic dehydrogenase (SDH) activity. By contrast, quercetin decreased lactate dehydrogenase (LDH) activity, fast MyHC protein expression, fast-twitch fiber percentage, and MyHC IIb mRNA expression. Furthermore, quercetin significantly increased serum adiponectin (AdipoQ) concentration, and the expression levels of AdipoQ and AdipoR1. However, inhibition of adiponectin signaling by AdipoR1 siRNA significantly attenuated the effects of quercetin on muscle fiber type-related gene expression, the percentages of slow MyHC-positive and fast MyHC-positive fibers, and metabolic enzyme activity in C2C12 myotubes. Together, our data indicated that quercetin could promote skeletal fiber switching from glycolytic type II to oxidative type I through AdipoQ signaling.

Orthograde signal of dihydropyridine receptor increases Ca2+ leakage after repeated contractions in rat fast-twitch muscles in vivo.

The purpose of this study was to investigate the mechanism underlying sarcoplasmic reticulum (SR) Ca2+ leakage after in vivo contractions. Rat gastrocnemius muscles were electrically stimulated in vivo, and then mechanically skinned fibers and SR microsomes were prepared from the muscles excised 30 min after repeated high-intensity contractions. The mechanically skinned fibers maintained the interaction between dihydropyridine receptors (DHPRs) and ryanodine receptors (RyRs), whereas the SR microsomes did not. Interestingly, skinned fibers from the stimulated muscles showed increased SR Ca2+ leakage, whereas Ca2+ leakage decreased in SR microsomes from the stimulated muscles. To enhance the orthograde signal of DHPRs, SR Ca2+ leakage in the skinned fiber was measured 1) under a continuously depolarized condition and 2) in the presence of nifedipine. As a result, in either of the two conditions, SR Ca2+ leakage in the rested fibers reached a level similar to that in the stimulated fibers. Furthermore, the increased SR Ca2+ leakage from the stimulated fibers was alleviated by treatment with 1 mM tetracaine (Tet) but not by treatment with 3 mM free Mg2+ (3 Mg). Tet exerted a greater inhibitory effect on the DHPR signal to RyR than 3 Mg, although their inhibitory effects on RyR were almost similar. These results suggest that the increased Ca2+ leakage after muscle contractions is mainly caused by the orthograde signal of DHPRs to RyRs.

Osteocytic Connexin43 Channels Regulate Bone-Muscle Crosstalk.

Bone-muscle crosstalk plays an important role in skeletal biomechanical function, the progression of numerous pathological conditions, and the modulation of local and distant cellular environments. Previous work has revealed that the deletion of connexin (Cx) 43 in osteoblasts, and consequently, osteocytes, indirectly compromises skeletal muscle formation and function. However, the respective roles of Cx43-formed gap junction channels (GJs) and hemichannels (HCs) in the bone-muscle crosstalk are poorly understood. To this end, we used two Cx43 osteocyte-specific transgenic mouse models expressing dominant negative mutants, Δ130-136 (GJs and HCs functions are inhibited), and R76W (only GJs function is blocked), to determine the effect of these two types of Cx43 channels on neighboring skeletal muscle. Blockage of osteocyte Cx43 GJs and HCs in Δ130-136 mice decreased fast-twitch muscle mass with reduced muscle protein synthesis and increased muscle protein degradation. Both R76W and Δ130-136 mice exhibited decreased muscle contractile force accompanied by a fast-to-slow fiber transition in typically fast-twitch muscles. In vitro results further showed that myotube formation of C2C12 myoblasts was inhibited after treatment with the primary osteocyte conditioned media (PO CM) from R76W and Δ130-136 mice. Additionally, prostaglandin E2 (PGE2) level was significantly reduced in both the circulation and PO CM of the transgenic mice. Interestingly, the injection of PGE2 to the transgenic mice rescued fast-twitch muscle mass and function; however, this had little effect on protein synthesis and degradation. These findings indicate a channel-specific response: inhibition of osteocytic Cx43 HCs decreases fast-twitch skeletal muscle mass alongside reduced protein synthesis and increased protein degradation. In contrast, blockage of Cx43 GJs results in decreased fast-twitch skeletal muscle contractile force and myogenesis, with PGE2 partially accounting for the measured differences.

Citrus hassaku Extract Powder Increases Mitochondrial Content and Oxidative Muscle Fibers by Upregulation of PGC-1α in Skeletal Muscle.

Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) is expressed in skeletal muscles and regulates systemic metabolism. Thus, nutraceuticals targeting skeletal muscle PGC-1α have attracted attention to modulate systemic metabolism. As auraptene contained in citrus fruits promotes lipid metabolism and improves mitochondrial respiration, it could increase mitochondrial function through PGC-1α. Therefore, we hypothesized that PGC-1α is activated by auraptene and investigated its effect using Citrus hassaku extract powder (CHEP) containing >80% of auraptene. C2C12 myotubes were incubated with vehicle or CHEP for 24 h; C57BL/6J mice were fed a control diet or a 0.25% (w/w) CHEP-containing diet for 5 weeks. PGC-1α protein level and mitochondrial content increased following CHEP treatment in cultured myotubes and skeletal muscles. In addition, the number of oxidative fibers increased in CHEP-fed mice. These findings suggest that CHEP-mediated PGC-1α upregulation induced mitochondrial biogenesis and fiber transformation to oxidative fibers. Furthermore, as CHEP increased the expression of the protein sirtuin 3 and of phosphorylated AMP-activated protein kinase (AMPK) and the transcriptional activity of PGC-1α, these molecules might be involved in CHEP-induced effects in skeletal muscles. Collectively, our findings indicate that CHEP mediates PGC-1α expression in skeletal muscles and may serve as a dietary supplement to prevent metabolic disorders.

Nicorandil decreases oxidative stress in slow- and fast-twitch muscle fibers of diabetic rats by improving the glutathione system functioning.

AIMS/INTRODUCTION: Myopathy is a common complication of any diabetes type, consisting in failure to preserve mass and muscular function. Oxidative stress has been considered one of the main causes for this condition. This study aimed to search if Nicorandil, a KATP channel opener, could protect slow- and fast-twitch diabetic rat muscles from oxidative stress, and to unveil its possible mechanisms. MATERIALS AND METHODS: Diabetes was induced in male Wistar rats by applying intraperitoneally streptozotocin (STZ) at 100 mg/kg doses. Nicorandil (3 mg/kg/day) was administered along 4 weeks. An insulin tolerance test and assessment of fasting blood glucose (FBG), TBARS, reduced (GSH), and disulfide (GSSG) glutathione levels, GSH/GSSG ratio, and mRNA expression of glutathione metabolism-related genes were performed at end of treatment in soleus and gastrocnemius muscles. RESULTS: Nicorandil significantly reduced FBG levels and enhanced insulin tolerance in diabetic rats. In gastrocnemius and soleus muscles, Nicorandil attenuated the oxidative stress by decreasing lipid peroxidation (TBARS), increasing total glutathione and modulating GPX1-mRNA expression in both muscle's types. Nicorandil also increased GSH and GSH/GSSG ratio and downregulated the GCLC- and GSR-mRNA in gastrocnemius, without significative effect on those enzymes' mRNA expression in diabetic soleus muscle. CONCLUSIONS: In diabetic rats, Nicorandil attenuates oxidative stress in slow- and fast-twitch skeletal muscles by improving the glutathione system functioning. The underlying mechanisms for the modulation of glutathione redox state and the transcriptional expression of glutathione metabolism-related genes seem to be fiber type-dependent.

Puerarin ameliorates skeletal muscle wasting and fiber type transformation in STZ-induced type 1 diabetic rats.

Puerarin is an isoflavonoid extracted from Pueraria lobate with extensive pharmacological effects in traditional Chinese medicine. The evidence implicates that puerarin mitigates hyperglycemia and various relevant complications. Here, the effect of puerarin on skeletal muscle wasting induced by type 1 diabetes (T1D) was explored. Streptozotocin (STZ)-induced T1D male Sprague Dawley (SD) rats were used in this study. Muscle strength, weight and size were measured. L6 rat skeletal muscle cells were applied for in vitro study. Our results showed that eight-week oral puerarin administration (100 mg/kg) increased muscle strengths and weights accompanied by enhanced skeletal muscle cross-sectional areas in diabetic rats. Simultaneously, puerarin also reduced expressions of several muscle wasting marker genes including F-box only protein 32 (Atrogin-1) and muscle-specific RING-finger 1 (Murf-1) in diabetic group both in vitro and in vivo. Transformation from type I fibers (slow muscle) to type II fibers (fast muscle) were also observed under puerarin administration in diabetic rats. Puerarin promoted Akt/mTOR while inhibited LC3/p62 signaling pathway in skeletal muscle cells. In conclusion, our study showed that puerarin mitigated skeletal muscle wasting in T1D rats and closely related with Akt/mTOR activation and autophagy inhibition. Whether this effect in murine applies to humans remains to be determined.

Eicosapentaenoic acid changes muscle transcriptome and intervenes in aging-related fiber type transition in male mice.

Age-related sarcopenia is associated with a variety of changes in skeletal muscle. These changes are interrelated with each other and associated with systemic metabolism, the details of which, however, are largely unknown. Eicosapentaenoic acid (EPA) is a promising nutrient against sarcopenia and has multifaceted effects on systemic metabolism. In this study, we hypothesized that the aging process in skeletal muscle can be intervened by the administration of EPA. Seventy-five-week-old male mice were assigned to groups fed an EPA-deprived diet (EPA-) or an EPA-enriched diet with 1 wt% EPA (EPA+) for 12 wk. Twenty-four-week-old male mice fed with normal chow were also analyzed. At baseline, the grip strength of the aging mice was lower than that of the young mice. After 12 wk, EPA+ showed similar muscle mass but increased grip strength compared with EPA-. EPA+ displayed higher insulin sensitivity than EPA-. Immunohistochemistry and gene expression analysis of myosin heavy chains (MyHCs) revealed fast-to-slow fiber type transition in aging muscle, which was partially inhibited by EPA. RNA sequencing (RNA-Seq) analysis suggested that EPA supplementation exerts pathway-specific effects in skeletal muscle including the signatures of slow-to-fast fiber type transition. In conclusion, we revealed that aging skeletal muscle in male mice shows lower grip strength and fiber type changes, both of which can be inhibited by EPA supplementation irrespective of muscle mass alteration.NEW & NOTEWORTHY This study demonstrated that the early phenotype of skeletal muscle in aging male mice is characterized by muscle weakness with fast-to-slow fiber type transition, which could be ameliorated by feeding with EPA-enriched diet. EPA induced metabolic changes such as an increase in systemic insulin sensitivity and altered muscle transcriptome in the aging mice. These changes may be related to the fiber type transition and influence muscle quality.

Thyroid hormone receptor α in skeletal muscle is essential for T3-mediated increase in energy expenditure.

Thyroid hormones are important for homeostatic control of energy metabolism and body temperature. Although skeletal muscle is considered a key site for thyroid action, the contribution of thyroid hormone receptor signaling in muscle to whole-body energy metabolism and body temperature has not been resolved. Here, we show that T3-induced increase in energy expenditure requires thyroid hormone receptor alpha 1 (TRα1 ) in skeletal muscle, but that T3-mediated elevation in body temperature is achieved in the absence of muscle-TRα1 . In slow-twitch soleus muscle, loss-of-function of TRα1 (TRαHSACre ) alters the fiber-type composition toward a more oxidative phenotype. The change in fiber-type composition, however, does not influence the running capacity or motivation to run. RNA-sequencing of soleus muscle from WT mice and TRαHSACre mice revealed differentiated transcriptional regulation of genes associated with muscle thermogenesis, such as sarcolipin and UCP3, providing molecular clues pertaining to the mechanistic underpinnings of TRα1 -linked control of whole-body metabolic rate. Together, this work establishes a fundamental role for skeletal muscle in T3-stimulated increase in whole-body energy expenditure.

Differential protective effects of Radix astragali, herbal medicine, on immobilization-induced atrophy of slow-twitch and fast-twitch muscles.

Radix astragali is a popular traditional herbal medicine that provides significant protection against tissue injury in various models of oxidative stress-related diseases. In this study, we aimed to investigate whether administration of Radix astragali prevented atrophy in both slow- and fast-twitch muscles following cast immobilization. Twenty-seven 12-week-old male F344 rats were divided into three experimental groups: control (CON), immobilized (IM), and immobilized with Radix astragali administration (IM+AR). Rats in the IM and IM+AR groups were subjected to immobilization of both lower extremities using casting-tape for 14 days. Rats in the IM+AR group were orally administered a decoction of Radix astragali daily for 21 days beginning 7 days before cast immobilization. As expected, rats in the IM group showed significant decreases (P < 0.05) in soleus and plantaris muscle-to-body weight ratios by 74.3% and 70.5%, respectively, compared with those in the CON group. Administration of Radix astragali significantly reversed (+35.5%) the weight reduction observed in soleus muscle, but not in the plantaris muscle, compared with that in the IM group. Furthermore, administration of Radix astragali inhibited MuRF1 mRNA expression only in the soleus muscle during cast immobilization. Our results demonstrated that administration of Radix astragali suppressed the immobilization-induced reductions in skeletal muscle mass and expression of MuRF1 mRNA in slow-twitch soleus muscles, but not in fast-twitch plantaris muscles.

A myosin-based mechanism for stretch activation and its possible role revealed by varying phosphate concentration in fast and slow mouse skeletal muscle fibers.

Stretch activation (SA) is a delayed increase in force following a rapid muscle length increase. SA is best known for its role in asynchronous insect flight muscle, where it has replaced calcium's typical role of modulating muscle force levels during a contraction cycle. SA also occurs in mammalian skeletal muscle but has previously been thought to be too low in magnitude, relative to calcium-activated (CA) force, to be a significant contributor to force generation during locomotion. To test this supposition, we compared SA and CA force at different Pi concentrations (0-16 mM) in skinned mouse soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscle fibers. CA isometric force decreased similarly in both muscles with increasing Pi, as expected. SA force decreased with Pi in EDL (40%), leaving the SA to CA force ratio relatively constant across Pi concentrations (17-25%). In contrast, SA force increased in soleus (42%), causing a quadrupling of the SA to CA force ratio, from 11% at 0 mM Pi to 43% at 16 mM Pi, showing that SA is a significant force modulator in slow-twitch mammalian fibers. This modulation would be most prominent during prolonged muscle use, which increases Pi concentration and impairs calcium cycling. Based upon our previous Drosophila myosin isoform studies and this work, we propose that in slow-twitch fibers a rapid stretch in the presence of Pi reverses myosin's power stroke, enabling quick rebinding to actin and enhanced force production, while in fast-twitch fibers, stretch and Pi cause myosin to detach from actin.

EGF receptor (EGFR) inhibition promotes a slow-twitch oxidative, over a fast-twitch, muscle phenotype.

A low quadriceps slow-twitch (ST), oxidative (relative to fast-twitch) fiber proportion is prevalent in chronic diseases such Chronic Obstructive Pulmonary Disease (COPD) and is associated with exercise limitation and poor outcomes. Benefits of an increased ST fiber proportion are demonstrated in genetically modified animals. Pathway analysis of published data of differentially expressed genes in mouse ST and FT fibers, mining of our microarray data and a qPCR analysis of quadriceps specimens from COPD patients and controls were performed. ST markers were quantified in C2C12 myotubes with EGF-neutralizing antibody, EGFR inhibitor or an EGFR-silencing RNA added. A zebrafish egfra mutant was generated by genome editing and ST fibers counted. EGF signaling was (negatively) associated with the ST muscle phenotype in mice and humans, and muscle EGF transcript levels were raised in COPD. In C2C12 myotubes, EGFR inhibition/silencing increased ST, including mitochondrial, markers. In zebrafish, egfra depletion increased ST fibers and mitochondrial content. EGF is negatively associated with ST muscle phenotype in mice, healthy humans and COPD patients. EGFR blockade promotes the ST phenotype in myotubes and zebrafish embryos. EGF signaling suppresses the ST phenotype, therefore EGFR inhibitors may be potential treatments for COPD-related muscle ST fiber loss.

Fiber type-selective exercise effects on AS160 phosphorylation.

Earlier research using muscle tissue demonstrated that postexercise elevation in insulin-stimulated glucose uptake (ISGU) occurs concomitant with greater insulin-stimulated Akt substrate of 160 kDa (AS160) phosphorylation (pAS160) on sites that regulate ISGU. Because skeletal muscle is a heterogeneous tissue, we previously isolated myofibers from rat epitrochlearis to assess fiber type-selective ISGU. Exercise induced greater ISGU in type I, IIA, IIB, and IIBX but not IIX fibers. This study tested if exercise effects on pAS160 correspond with previously published fiber type-selective exercise effects on ISGU. Rats were studied immediately postexercise (IPEX) or 3.5 h postexercise (3.5hPEX) with time-matched sedentary controls. Myofibers dissected from the IPEX experiment were analyzed for fiber type (myosin heavy chain isoform expression) and key phosphoproteins. Isolated muscles from the 3.5hPEX experiment were incubated with or without insulin. Myofibers (3.5hPEX) were analyzed for fiber type, key phosphoproteins, and GLUT4 protein abundance. We hypothesized that insulin-stimulated pAS160 at 3.5hPEX would exceed sedentary controls only in fiber types characterized by greater ISGU postexercise. Values for phosphorylation of AMP-activated kinase substrates (acetyl CoA carboxylaseSer79 and AS160Ser704) from IPEX muscles exceeded sedentary values in each fiber type, suggesting exercise recruitment of all fiber types. Values for pAS160Thr642 and pAS160Ser704 from insulin-stimulated muscles 3.5hPEX exceeded sedentary values for type I, IIA, IIB, and IIBX but not IIX fibers. GLUT4 abundance was unaltered 3.5hPEX in any fiber type. These results advanced understanding of exercise-induced insulin sensitization by providing compelling support for the hypothesis that enhanced insulin-stimulated phosphorylation of AS160 is linked to elevated ISGU postexercise at a fiber type-specific level independent of altered GLUT4 expression.

Effects of chronic Porphyromonas gingivalis lipopolysaccharide infusion on skeletal muscles in mice.

Periodontitis, which is caused by various oral organisms, predominantly affects adults, and is one of the main causes of tooth loss, as well as leading to progression of numerous systemic diseases. However, its relationship to sarcopenia (aging-associated degenerative loss of skeletal muscle mass and function) remains unclear. The aim of this study was to investigate the effects of Porphyromonas gingivalis lipopolysaccharide (PG-LPS) on skeletal muscle in mice, and to establish the underlying mechanisms. Mice (C57BL/6) were injected with PG-LPS (0.8 mg/kg/day) for 4 weeks. This treatment significantly decreased the weight of fast-twitch skeletal muscles (masseter and tibialis anterior muscles), but not that of slow-twitch skeletal muscle (soleus muscle). The area of fibrosis was significantly increased in masseter muscle, but remained unchanged in the other two muscles. The number of apoptotic myocytes was significantly increased (approximately eightfold) in masseter muscle. These data suggest that persistent subclinical exposure to PG-LPS might reduce the size of fast-twitch skeletal muscle, but not slow-twitch skeletal muscle. Masseter muscle appears to be especially susceptible to the adverse effects of PG-LPS, because muscle remodeling (muscle fibrosis and myocyte apoptosis) was induced solely in masseter muscle. Thus, periodontitis might be one of the major causes of oral sarcopenia.

Treatment with EUK-134 improves sarcoplasmic reticulum Ca2+ release but not myofibrillar Ca2+ sensitivity after fatiguing contraction of rat fast-twitch muscle.

Skeletal muscles undergoing vigorous activity can enter a state of prolonged low-frequency force depression (PLFFD). This study was conducted to examine whether antioxidant treatment is capable of accelerating the recovery from PLFFD, with a focus on the function of the sarcoplasmic reticulum (SR) and myofibril. One hour before fatiguing stimulation (FS) was administered, rats received an intraperitoneal injection of Eukarion (EUK-134), which mimics the activities of superoxide dismutase and catalase. Intact muscles of the hindlimbs were electrically stimulated via the sciatic nerve until the force was reduced to ~50% of the initial force (FS). Thirty minutes after cessation of FS, the superficial regions of gastrocnemius muscles were dissected and used for biochemical and skinned-fiber analyses. Whole muscle analyses revealed that antioxidant alleviated the FS-induced decrease in the reduced glutathione content. Skinned-fiber analyses showed that the antioxidant did not affect the FS-induced decrease in the ratio of force at 1 Hz to that at 50 Hz. However, the antioxidant partially inhibited the FS-mediated decrease in the ratio of depolarization-induced force to the maximum Ca2+-activated force. Furthermore, the antioxidant completely suppressed the FS-induced increase in myofibrillar Ca2+ sensitivity. These results suggest that antioxidant treatment is ineffective in facilitating the restoration of PLFFD, probably due to its negative effect on myofibrillar Ca2+ sensitivity, which supersedes its positive effect on SR Ca2+ release.

Skeletal muscle fiber type-selective effects of acute exercise on insulin-stimulated glucose uptake in insulin-resistant, high-fat-fed rats.

Insulin-stimulated glucose uptake (GU) by skeletal muscle is enhanced several hours after acute exercise in rats with normal or reduced insulin sensitivity. Skeletal muscle is composed of multiple fiber types, but exercise's effect on fiber type-specific insulin-stimulated GU in insulin-resistant muscle was previously unknown. Male rats were fed a high-fat diet (HFD; 2 wk) and were either sedentary (SED) or exercised (2-h exercise). Other, low-fat diet-fed (LFD) rats remained SED. Rats were studied immediately postexercise (IPEX) or 3 h postexercise (3hPEX). Epitrochlearis muscles from IPEX rats were incubated in 2-deoxy-[3H]glucose (2-[3H]DG) without insulin. Epitrochlearis muscles from 3hPEX rats were incubated with 2-[3H]DG ± 100 µU/ml insulin. After single fiber isolation, GU and fiber type were determined. Glycogen and lipid droplets (LDs) were assessed histochemically. GLUT4 abundance was determined by immunoblotting. In HFD-SED vs. LFD-SED rats, insulin-stimulated GU was decreased in type IIB, IIX, IIAX, and IIBX fibers. Insulin-independent GU IPEX was increased and glycogen content was decreased in all fiber types (types I, IIA, IIB, IIX, IIAX, and IIBX). Exercise by HFD-fed rats enhanced insulin-stimulated GU in all fiber types except type I. Single fiber analyses enabled discovery of striking fiber type-specific differences in HFD and exercise effects on insulin-stimulated GU. The fiber type-specific differences in insulin-stimulated GU postexercise in insulin-resistant muscle were not attributable to a lack of fiber recruitment, as indirectly evidenced by insulin-independent GU and glycogen IPEX, differences in multiple LD indexes, or altered GLUT4 abundance, implicating fiber type-selective differences in the cellular processes responsible for postexercise enhancement of insulin-mediated GLUT4 translocation.

The effects of ATP on the contractions of rat and mouse fast skeletal muscle.

INTRODUCTION: The aim of this study was to compare the effects of adenosine-5'-triphosphate (ATP) and adenosine on the contractility of rodent extensor digitorum longus (EDL) muscle at normal and low temperatures. METHODS: Contractions of rat and mouse isolated EDL were induced by either electrical stimulation (ES) or exogenous carbachol and recorded in the presence of ATP or adenosine (both at 100 µM). RESULTS: ATP at all temperatures caused a decrease of the contractions induced by carbachol in rat and mouse EDL and ES-induced contractions in rat EDL, while it potentiated the ES-induced contractions of mouse EDL. Adenosine reduced the contractility of rat and mouse EDL evoked by ES and did not affect the carbachol-induced contractions of rat and mouse EDL at any temperature. DISCUSSION: Under various temperature conditions, ATP inhibits pre- but potentiates postsynaptic processes in the mouse EDL; in the rat EDL ATP causes only inhibition of neuromuscular conduction. Muscle Nerve 59:509-516, 2019.

Nandrolone decanoate administration does not attenuate muscle atrophy during a short period of disuse.

BACKGROUND: A few days of bed rest or immobilization following injury, disease, or surgery can lead to considerable loss of skeletal muscle mass and strength. It has been speculated that such short, successive periods of muscle disuse may be largely responsible for the age-related loss of muscle mass throughout the lifespan. OBJECTIVE: To assess whether a single intramuscular injection of nandrolone decanoate prior to immobilization can attenuate the loss of muscle mass and strength in vivo in humans. DESIGN, SETTING AND PARTICIPANTS: Thirty healthy (22 ± 1 years) men were subjected to 7 days of one-legged knee immobilization by means of a full leg cast with (NAD, n = 15) or without (CON, n = 15) prior intramuscular nandrolone decanoate injection (200 mg). MEASURES: Before and immediately after immobilization, quadriceps muscle cross-sectional area (CSA) (by means of single-slice computed tomography (CT) scans of the upper leg) and one-legged knee extension strength (one-repetition maximum [1-RM]) were assessed for both legs. Furthermore, muscle biopsies from the immobilized leg were taken before and after immobilization to assess type I and type II muscle fiber cross-sectional area. RESULTS: Quadriceps muscle CSA decreased during immobilization in both CON and NAD (-6 ± 1% and -6 ± 1%, respectively; main effect of time P<0.01), with no differences between the groups (time × treatment interaction, P = 0.59). Leg muscle strength declined following immobilization (-6 ± 2% in CON and -7 ± 3% in NAD; main effect of time, P<0.05), with no differences between groups (time × treatment interaction, P = 0.55). CONCLUSIONS: This is the first study to report that nandrolone decanoate administration does not preserve skeletal muscle mass and strength during a short period of leg immobilization in vivo in humans.