Differential effects of peroxynitrite on contractile protein properties in fast- and slow-twitch skeletal muscle fibers of rat

Arginine ingestion inhibits phagocyte invasion in eccentrically contracted rat fast-twitch muscle

Eccentric contraction (ECC) has been shown to induce leukocyte invasion into skeletal muscle, resulting in muscle inflammation. This study aimed to investigate whether prior ingestion of L-arginine (ARG), a nitric oxide precursor, inhibits ECC-induced macrophage invasion. Male Wistar rats received ARG in water for 7 days, beginning 3 days prior to ECC. ECCs were induced in the anterior crural muscles for 200 cycles. Three days later, the tibialis anterior and extensor digitorum longus muscles were excised for biochemical analysis and force measurement, respectively. ARG ingestion increased nitrite and nitrate levels in plasma and muscle, inhibiting force depression and reducing CD68 content in muscles subjected to ECC. ARG ingestion also ameliorated an ECC-induced increase in protein nitration, although neither ARG ingestion nor ECC induction affected protein carbonyl levels. The present results suggest that ingestion of ARG or ARG-rich foods may alleviate inflammation by attenuating phagocyte invasion in eccentrically contracted skeletal muscles.

From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle

Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.

Endogenous and Exogenous Antioxidants in Skeletal Muscle Fatigue Development during Exercise

Muscle fatigue is defined as a decrease in maximal force or power generated in response to contractile activity, and it is a risk factor for the development of musculoskeletal injuries. One of the many stressors imposed on skeletal muscle through exercise is the increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which intensifies as a function of exercise intensity and duration. Exposure to ROS/RNS can affect Na⁺/K⁺-ATPase activity, intramyofibrillar calcium turnover and sensitivity, and actin-myosin kinetics to reduce muscle force production. On the other hand, low ROS/RNS concentrations can likely upregulate an array of cellular adaptative responses related to mitochondrial biogenesis, glucose transport and muscle hypertrophy. Consequently, growing evidence suggests that exogenous antioxidant supplementation might hamper exercise-engendering upregulation in the signaling pathways of mitogen-activated protein kinases (MAPKs), peroxisome-proliferator activated co-activator 1α (PGC-1α), or mammalian target of rapamycin (mTOR). Ultimately, both high (exercise-induced) and low (antioxidant intervention) ROS concentrations can trigger beneficial responses as long as they do not override the threshold range for redox balance. The mechanisms underlying the two faces of ROS/RNS in exercise, as well as the role of antioxidants in muscle fatigue, are presented in detail in this review.

Rosiglitazone Ameliorates Cardiac and Skeletal Muscle Dysfunction by Correction of Energetics in Huntington’s Disease

Huntington’s disease (HD) is a rare neurodegenerative disease that is accompanied by skeletal muscle atrophy and cardiomyopathy. Tissues affected by HD (central nervous system [CNS], skeletal muscle, and heart) are known to suffer from deteriorated cellular energy metabolism that manifests already at presymptomatic stages. This work aimed to test the effects of peroxisome proliferator-activated receptor (PPAR)-γ agonist—rosiglitazone on grip strength and heart function in an experimental HD model—on R6/1 mice and to address the mechanisms. We noted that rosiglitazone treatment lead to improvement of R6/1 mice grip strength and cardiac mechanical function. It was accompanied by an enhancement of the total adenine nucleotides pool, increased glucose oxidation, changes in mitochondrial number (indicated as increased citric synthase activity), and reduction in mitochondrial complex I activity. These metabolic changes were supported by increased total antioxidant status in HD mice injected with rosiglitazone. Correction of energy deficits with rosiglitazone was further indicated by decreased accumulation of nucleotide catabolites in HD mice serum. Thus, rosiglitazone treatment may not only delay neurodegeneration but also may ameliorate cardio- and myopathy linked to HD by improvement of cellular energetics.

Role of nitration in control of phosphorylase and glycogenolysis in mouse skeletal muscle

Phosphorylase is one of the most carefully studied proteins in history, but knowledge of its regulation during intense muscle contraction is incomplete. Tyrosine nitration of purified preparations of skeletal muscle phosphorylase results in inactivation of the enzyme and this is prevented by antioxidants. Whether an altered redox state affects phosphorylase activity and glycogenolysis in contracting muscle is not known. Here, we investigate the role of the redox state in control of phosphorylase and glycogenolysis in isolated mouse fast-twitch (extensor digitorum longus, EDL) and slow-twitch (soleus) muscle preparations during repeated contractions. Exposure of crude muscle extracts to H₂O₂ had little effect on phosphorylase activity. However, exposure of extracts to peroxynitrite (ONOO^-), a nitrating/oxidizing agent, resulted in complete inactivation of phosphorylase (half-maximal inhibition at ∼200 µM ONOO^-), which was fully reversed by the presence of an ONOO^- scavanger, dithiothreitol (DTT). Incubation of isolated muscles with ONOO^- resulted in nitration of phosphorylase and marked inhibition of glycogenolysis during repeated contractions. ONOO^- also resulted in large decreases in high-energy phosphates (ATP and phosphocreatine) in the rested state and following repeated contractions. These metabolic changes were associated with decreased force production during repeated contractions (to ∼60% of control). In contrast, repeated contractions did not result in nitration of phosphorylase, nor did DTT or the general antioxidant N-acetylcysteine alter glycogenolysis during repeated contractions. These findings demonstrate that ONOO^- inhibits phosphorylase and glycogenolysis in living muscle under extreme conditions. However, nitration does not play a significant role in control of phosphorylase and glycogenolysis during repeated contractions.NEW & NOTEWORTHY Here we show that exogenous peroxynitrite results in nitration of phosphorylase as well as inhibition of glycogenolysis in isolated intact mouse skeletal muscle during short-term repeated contractions. However, repeated contractions in the absence of exogenous peroxynitrite do not result in nitration of phosphorylase or affect glycogenolysis, nor does the addition of antioxidants alter glycogenolysis during repeated contractions. Thus phosphorylase is not subject to redox control during repeated contractions.

Current perspectives on obesity and skeletal muscle contractile function in older adults

Obesity has become one of the most pressing public health issues of the 21st century and currently affects a substantial proportion of the older adult population. Although the cardiometabolic complications are well documented, research from the past 20 years has drawn attention to the detrimental effects of obesity on physical performance in older adults. Obesity-related declines in physical performance are due, in part, to compromised muscle strength and power. Recent evidence suggests there are a number of mechanisms potentially underlying reduced whole muscle function, including alterations in myofilament protein function and cellular contractile properties, and these may be related to morphological adaptations, such as shifts in fiber type composition and increased intramyocellular lipid content within skeletal muscle. To date, even less research has focused on how exercise and weight loss interventions for obese older adults affect these mechanisms. In light of this work, we provide an update on the current knowledge related to obesity and skeletal muscle contractile function and highlight a number of questions to address potential etiologic mechanisms as well as intervention strategies, which may help advance our understanding of how physical performance can be improved among obese older adults.

Effect of icosapent ethyl on susceptibility to ventricular arrhythmias in postinfarcted rat hearts: Role of GPR120-mediated connexin43 phosphorylation

The ω‐3 fatty acids exert as an antioxidant via the G protein‐coupled receptor 120 (GPR120). Icosapent ethyl, a purified eicosapentaenoic acid, showed a marked reduction in sudden cardiac death. Connexin43 is sensitive to redox status. We assessed whether icosapent ethyl attenuates fatal arrhythmias after myocardial infarction, a status of high oxidative stress, through increased connexin43 expression and whether the GPR120 signalling is involved in the protection. Male Wistar rats after ligating coronary artery were assigned to either vehicle or icosapent ethyl for 4 weeks. The postinfarction period was associated with increased oxidative‐nitrosative stress. In concert, myocardial connexin43 levels revealed a significant decrease in vehicle‐treated infarcted rats compared with sham. These changes of oxidative‐nitrosative stress and connexin43 levels were blunted after icosapent ethyl administration. Provocative arrhythmias in the infarcted rats treated with icosapent ethyl were significantly improved than vehicle. Icosapent ethyl significantly increased GPR120 compared to vehicle after infarction. The effects of icosapent ethyl on superoxide and connexin43 were similar to GPR120 agonist GW9508. Besides, the effects of icosapent ethyl on oxidative‐nitrosative stress and connexin43 phosphorylation were abolished by administering AH‐7614, an inhibitor of GPR120. SIN‐1 abolished the Cx43 phosphorylation of icosapent ethyl without affecting GPR120 levels. Taken together, chronic use of icosapent ethyl after infarction is associated with up‐regulation of connexin43 phosphorylation through a GPR120‐dependent antioxidant pathway and thus plays a beneficial effect on arrhythmogenic response to programmed electrical stimulation.

Quadriceps Lipid Content Has Sex-Specific Associations With Whole-Muscle, Cellular, and Molecular Contractile Function in Older Adults

Increased adiposity is associated with reduced skeletal muscle function in older adults, but the mechanisms underlying this relationship remain unclear. To explore whether skeletal muscle properties track with adiposity, whole-muscle, cellular, and molecular function were examined in relation to adiposity measured at various anatomical levels in healthy older (60-80 years) men and women. Although women had greater absolute and relative body and thigh fat than men, quadriceps muscle attenuation, an index of intramuscular lipid content, was similar between sexes. At the whole-muscle level, greater quadriceps attenuation was associated with reduced knee extensor function in women, but not men. In women, decreased myosin heavy chain I and IIA fiber-specific force was associated with higher intramuscular lipid content, which may be explained, in part, by the reduced myofilament lattice stiffness found in myosin heavy chain IIA fibers. Longer myosin attachment times in myosin heavy chain I fibers from men and women were associated with greater amounts of adipose tissue, suggesting that fat deposits lead to slower myosin-actin cross-bridge kinetics. Our results indicate greater quantities of adipose tissue alter myofilament properties and cross-bridge kinetics, which may partially explain the adiposity-induced decrements in single-fiber and whole-muscle function of older adults, especially women.

Targeting reactive oxygen species (ROS) to combat the age-related loss of muscle mass and function

The loss of muscle mass and function with age, termed sarcopenia, is an inevitable process, which has a significant impact on quality of life. During ageing we observe a progressive loss of total muscle fibres and a reduction in cross-sectional area of the remaining fibres, resulting in a significant reduction in force output. The mechanisms which underpin sarcopenia are complex and poorly understood, ranging from inflammation, dysregulation of protein metabolism and denervation. However, there is significant evidence to demonstrate that modified ROS generation, redox dis-homeostasis and mitochondrial dysfunction may have an important role to play. Based on this, significant interest and research has interrogated potential ROS-targeted therapies, ranging from nutritional-based interventions such as vitamin E/C, polyphenols (resveratrol) and targeted pharmacological compounds, using molecules such as SS-31 and MitoQ. In this review we evaluate these approaches to target aberrant age-related ROS generation and the impact on muscle mass and function.

Effects of high-intensity intermittent exercise on the contractile properties of human type I and type II skeletal muscle fibers

In vitro studies have shown that alterations in redox state can cause a range of opposing effects on the properties of the contractile apparatus in skeletal muscle fibers. To test whether and how redox changes occurring in vivo affect the contractile properties, vastus lateralis muscle fibers from seven healthy young adults were examined at rest (PRE) and following (POST) high-intensity intermittent cycling exercise. Individual mechanically skinned muscle fibers were exposed to heavily buffered solutions at progressively higher free [Ca²⁺] to determine their force-Ca²⁺ relationship. Following acute exercise, Ca²⁺ sensitivity was significantly decreased in type I fibers (by 0.06 pCa unit) but not in type II fibers (0.01 pCa unit). Specific force decreased after the exercise in type II fibers (-18%) but was unchanged in type I fibers. Treatment with the reducing agent dithiothreitol (DTT) caused a small decrease in Ca²⁺-sensitivity in type II fibers at PRE (by ∼0.014 pCa units) and a significantly larger decrease at POST (∼0.035 pCa units), indicating that the exercise had increased S-glutathionylation of fast troponin I. DTT treatment also increased specific force (by ∼4%), but only at POST. In contrast, DTT treatment had no effect on either parameter in type I fibers at either PRE or POST. In type I fibers, the decreased Ca²⁺ sensitivity was not due to reversible oxidative changes and may have contributed to a decrease in power production during vigorous exercises. In type II fibers, exercise-induced redox changes help counter the decline in Ca²⁺-sensitivity while causing a small decline in maximum force.NEW & NOTEWORTHY This study identified important cellular changes occurring in human skeletal muscle fibers following high-intensity intermittent exercise: 1) a decrease in contractile apparatus Ca²⁺ sensitivity in type I but not type II fibers, 2) a decrease in specific force only in type II muscle fibers, and 3) a redox-dependent increase in Ca²⁺ sensitivity occurring only in type II fibers, which would help maintain muscle performance by countering the normal metabolite-induced decline in Ca²⁺ sensitivity.

Redox basis of exercise physiology

Redox reactions control fundamental processes of human biology. Therefore, it is safe to assume that the responses and adaptations to exercise are, at least in part, mediated by redox reactions. In this review, we are trying to show that redox reactions are the basis of exercise physiology by outlining the redox signaling pathways that regulate four characteristic acute exercise-induced responses (muscle contractile function, glucose uptake, blood flow and bioenergetics) and four chronic exercise-induced adaptations (mitochondrial biogenesis, muscle hypertrophy, angiogenesis and redox homeostasis). Based on our analysis, we argue that redox regulation should be acknowledged as central to exercise physiology.

Force generated by myosin cross-bridges is reduced in myofibrils exposed to ROS/RNS

Skeletal muscle weakness is associated with oxidative stress and oxidative posttranslational modifications on contractile proteins. There is indirect evidence that reactive oxygen/nitrogen species (ROS/RNS) affect skeletal muscle myofibrillar function, although the details of the acute effects of ROS/RNS on myosin-actin interactions are not known. In this study, we examined the effects of peroxynitrite (ONOO−) on the contractile properties of individual skeletal muscle myofibrils by monitoring myofibril-induced displacements of an atomic force cantilever upon activation and relaxation. The isometric force decreased by ~50% in myofibrils treated with the ONOO− donor (SIN-1) or directly with ONOO−, which was independent of the cross-bridge abundancy condition (i.e., rigor or relaxing condition) during SIN-1 or ONOO− treatment. The force decrease was attributed to an increase in the cross-bridge detachment rate ( gapp) in combination with a conservation of the force redevelopment rate (kTr) and hence, an increase in the population of cross-bridges transitioning from force-generating to non-force-generating cross-bridges during steady-state. Taken together, the results of this study provide important information on how ROS/RNS affect myofibrillar force production which may be of importance for conditions where increased oxidative stress is part of the pathophysiology.

Eccentric training enhances the αB-crystallin binding to the myofibrils and prevents skeletal muscle weakness in adjuvant-induced arthritis rat

Patients with rheumatoid arthritis (RA) frequently suffer from muscle weakness. We examined whether eccentric training prevents skeletal muscle weakness in adjuvant-induced arthritis (AIA) rat, a widely used animal model for RA. AIA was induced in the knees of Wistar rats by injection of complete Freund’s adjuvant. To induce eccentric contractions (ECCs), neuromuscular electrical stimulation (45 V) was applied to the plantar flexor muscles simultaneously with forced dorsiflexion of the ankle joint (0–40°) and was given every 6 s. ECC exercise was applied every other day for a total of 11 sessions and consisted of 4 sets of 5 contractions. There was a significant reduction in in vitro maximum Ca2+-activated force in skinned fibers in gastrocnemius muscle from AIA rats. These changes were associated with reduced expression levels of contractile proteins (i.e., myosin and actin), increased levels of inflammation redox stress-related biomarkers (i.e., TNF-α, malondialdehyde-protein adducts, NADPH oxidase 2, and neuronal nitric oxide synthase), and autolyzed active calpain-1 in AIA muscles. ECC training markedly enhanced the steady-state levels of αB-crystallin, a small heat shock protein, and its binding to the myofibrils and prevented the AIA-induced myofibrillar dysfunction, reduction in contractile proteins, and inflammation-oxidative stress insults. Our findings demonstrate that ECC training preserves myofibrillar function without muscle damage in AIA rats, which is at least partially attributable to the protective effect of αB-crystallin on the myofibrils against oxidative stress-mediated protein degeneration. Thus ECC training can be a safe and effective intervention, counteracting the loss of muscle strength in RA patients.

NEW & NOTEWORTHY Eccentric contractions (ECCs) are regarded as an effective way to increase muscle strength. No studies, however, assess safety and effectiveness of ECC training on muscle weakness associated with rheumatoid arthritis. Here, we used adjuvant-induced arthritis (AIA) rats to demonstrate that ECC training prevents intrinsic contractile dysfunction without muscle damage in AIA rats, which may be attributed to the protective effect of αB-crystallin on the myofibrils against inflammation-oxidative stress insults.

Remote transplantation of human adipose-derived stem cells induces regression of cardiac hypertrophy by regulating the macrophage polarization in spontaneously hypertensive rats

Left ventricular hypertrophy (LVH) in hypertension has prognostic significance on cardiovascular mortality and morbidity. Recently, we have shown that n-butylidenephthalide (BP) improves human adipose-derived stem cell (hADSC) engraftment via attenuated reactive oxygen species (ROS) production. This prompted us to investigate whether remote transplantation of BP-pretreated hADSCs confers attenuated LVH at an established phase of hypertension. Male spontaneously hypertensive rats (SHRs) aged 12 weeks were randomly allocated to receive right hamstring injection of vehicle, clinical-grade hADSCs, and BP-preconditioned hADSCs for 8 weeks. As compared with untreated SHRs, naïve hADSCs decreased the ratio of LV weight to tibia, cardiomyocyte cell size, and collagen deposition independent of hemodynamic changes. These changes were accompanied by attenuated myocardial ROS production and increased p-STAT3 levels. Compared with naïve hADSCs, BP-preconditioned hADSCs provided a further decrease of ROS and LVH and an increase of local hADSC engraftment, STAT3 phosphorylation, STAT3 activity, STAT3 nuclear translocation, myocardial IL-10 levels, and the percentage of M2 macrophage infiltration. SIN-1 or S3I-201 reversed the effects of BP-preconditioned ADSCs increase on myocardial IL-10 levels. Furthermore, SIN-1 abolished the phosphorylation of STAT3, whereas superoxide levels were not affected following the inhibition of STAT3. Our results highlighted the feasibility of remote transplantation of hADSCs can be considered as an alternative procedure to reverse cardiac hypertrophy even at an established phase of hypertension. BP-pretreated hADSCs polarize macrophages into M2 immunoregulatory cells more efficiently than naïve hADSCs via ROS/STAT3 pathway.

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Hypertension was associated with left ventricular hypertrophy.

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Compared with untreated SHRs, naïve hADSCs injected at the right hamstring decreased LV mass and cardiomyocyte cell size.

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BP-preconditioned ADSCs provided a further increase of the M2 macrophage infiltration.

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The beneficial effects of BP-preconditioned stem cell administration can be abolished by exogenous SIN-1 or 3SI-201.

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Remote transplantation of hADSCs can be considered as an alternative procedure to reverse cardiac hypertrophy.

l-arginine ingestion inhibits eccentric contraction-induced proteolysis and force deficit via S-nitrosylation of calpain

It has been shown that calpains are involved in the proteolysis of muscle proteins that occurs with eccentric contraction (ECC) and that exogenously applied nitric oxide decreases the calpain‐mediated proteolysis. The aim of this study was to examine the effects of ingestion of l‐arginine (ARG), a nitric oxide precursor, on ECC‐related calpain activation. In the first and second experiments, male Wistar rats were given ARG in water for 7 days starting from 3 days before the ECC protocol (average ingestion, ~600 mg kg‐body wt⁻¹ day⁻¹). Tibialis anterior muscles underwent 200 repeated ECCs and, subsequently, were excised 3 days later. Whole muscle analyses (the first experiment) revealed that ARG attenuated ECC‐induced force deficit and autolysis of calpain‐1, and increased the amounts of S‐nitrosylated calpain‐1. Regarding ryanodine receptor (RyR) and dihydropyridine receptor (DHPR), ECC‐induced proteolysis was completely inhibited by ARG, whereas the inhibition was partial for junctophilin‐1 (JP1). Skinned fiber analyses (the second experiment) showed that ARG also inhibited ECC‐elicited reductions in the ratio of depolarization‐induced to maximum Ca²⁺‐activated force. In the third experiment, homogenates of rested muscles were treated with S‐nitrosylating agent, S‐nitrosoglutathione (GSNO), and/or high Ca²⁺ concentration ([Ca²⁺]). Treatment with high [Ca²⁺] and without GSNO produced proteolysis of RyR, DHPR, and JP1. On the other hand, treatment with high [Ca²⁺] and GSNO caused complete inhibition of RyR and DHPR proteolysis and partial inhibition of JP1 proteolysis. These results indicate that ARG ingestion can attenuate ECC‐induced proteolysis of Ca²⁺ regulatory proteins and force deficit by decreasing calpain activation via S‐nitrosylation.

Muscle Weakness in Rheumatoid Arthritis: The Role of Ca2+ and Free Radical Signaling

In addition to the primary symptoms arising from inflammatory processes in the joints, muscle weakness is commonly reported by patients with rheumatoid arthritis (RA). Muscle weakness not only reduces the quality of life for the affected patients, but also dramatically increases the burden on society since patients' work ability decreases. A 25–70% reduction in muscular strength has been observed in pateints with RA when compared with age-matched healthy controls. The reduction in muscle strength is often larger than what could be explained by the reduction in muscle size in patients with RA, which indicates that intracellular (intrinsic) muscle dysfunction plays an important role in the underlying mechanism of muscle weakness associated with RA. In this review, we highlight the present understanding of RA-associated muscle weakness with special focus on how enhanced Ca^2 + release from the ryanodine receptor and free radicals (reactive oxygen/nitrogen species) contributes to muscle weakness, and recent developments of novel therapeutic interventions.

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Muscle weakness is commonly reported by patients with rheumatoid arthritis (RA).

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Intrinsic muscle weakness is important in the underlying mechanisms of muscle weakness associated with rheumatoid arthritis.

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Enhanced Ca^2 + release and peroxynitrite-induced stress contributes to RA-induced muscle weakness.

Inhibition of infarction-induced sympathetic innervation with endothelin receptor antagonism via a PI3K/GSK-3β-dependent pathway

Although endothelin (ET)-1 has been shown to upregulate nerve growth factor (NGF) expression, the molecular mechanisms are largely unknown. Phosphatidylinositol 3-kinase (PI3K)/Akt/glycogen synthase kinase (GSK)-3β signal has been implicated in the regulation of NGF. We investigated whether selective ET receptor blockers attenuated cardiac sympathetic reinnervation through restoring PI3K/Akt/GSK-3β activity. After ligation of the left anterior descending artery, male Wistar rats were randomized to either vehicle, atrasentan (an ET_A receptor antagonist) or A-192621 (an ET_B receptor antagonist) for 4 weeks. Sympathetic hyperinnervation after infarction was confirmed by myocardial norepinephrine measurement and immunofluorescent analysis. Post infarction was associated with increased reactive oxygen species (ROS), as measured by myocardial superoxide levels and dihydroethidine fluorescence staining. This was paralleled by a significant upregulation of NGF expression on mRNA and protein levels in the vehicle-treated rats, which reduced after administering atrasentan, not A-192621. Arrhythmic scores in the vehicle-treated rats were significantly higher than those treated with atrasentan. In an in vivo study atrasentan-induced decreased NGF was associated with activation of PI3K/Akt signaling pathway, which was further confirmed by the ex vivo study showing the restoration of NGF levels after coadministration of PI3K inhibitors (wortmannin and LY294002). Lithium chloride, an inhibitor of GSK-3β, did not provide additional attenuated NGF levels compared with atrasentan alone. Finally, atrasentan-attenuated NGF levels were reversed in the presence of peroxynitrite generator. ET_A receptor antagonism is a mediator to attenuate sympathetic hyperinnervation probably through restoration of PI3K/Akt/GSK-3β/ROS signaling pathway, a potential pharmacological target for arrhythmias after infarction.

S-nitrosylation and S-glutathionylation of Cys134 on troponin I have opposing competitive actions on Ca2+ sensitivity in rat fast-twitch muscle fibers

Nitric oxide is generated in skeletal muscle with activity and decreases Ca2+ sensitivity of the contractile apparatus, putatively by S-nitrosylation of an unidentified protein. We investigated the mechanistic basis of this effect and its relationship to the oxidation-induced increase in Ca2+ sensitivity in mammalian fast-twitch (FT) fibers mediated by S-glutathionylation of Cys134 on fast troponin I (TnIf). Force-[Ca2+] characteristics of the contractile apparatus in mechanically skinned fibers were assessed by direct activation with heavily Ca2+-buffered solutions. Treatment with S-nitrosylating agents, S-nitrosoglutathione (GSNO) or S-nitroso- N-acetyl-penicillamine (SNAP), decreased pCa50 ( = −log10 [Ca2+] at half-maximal activation) by ~−0.07 pCa units in rat and human FT fibers without affecting maximum force, but had no effect on rat and human slow-twitch fibers or toad or chicken FT fibers, which all lack Cys134. The Ca2+ sensitivity decrease was 1) fully reversed with dithiothreitol or reduced glutathione, 2) at least partially reversed with ascorbate, indicative of involvement of S-nitrosylation, and 3) irreversibly blocked by low concentration of the alkylating agent, N-ethylmaleimide (NEM). The biotin-switch assay showed that both GSNO and SNAP treatments caused S-nitrosylation of TnIf. S-glutathionylation pretreatment blocked the effects of S-nitrosylation on Ca2+ sensitivity, and vice-versa. S-nitrosylation pretreatment prevented NEM from irreversibly blocking S-glutathionylation of TnIf and its effects on Ca2+ sensitivity, and likewise S-glutathionylation pretreatment prevented NEM block of S-nitrosylation. Following substitution of TnIf into rat slow-twitch fibers, S-nitrosylation treatment caused decreased Ca2+ sensitivity. These findings demonstrate that S-nitrosylation and S-glutathionylation exert opposing effects on Ca2+ sensitivity in mammalian FT muscle fibers, mediated by competitive actions on Cys134 of TnIf.

Dapagliflozin, a selective SGLT2 Inhibitor, attenuated cardiac fibrosis by regulating the macrophage polarization via STAT3 signaling in infarcted rat hearts

During myocardial infarction, infiltrated macrophages have pivotal roles in cardiac remodeling and delayed M1 toward M2 macrophage phenotype transition is considered one of the major factors for adverse ventricular remodeling. We investigated whether dapagliflozin, a sodium-glucose cotransporter 2 (SGLT2) inhibitor, attenuates cardiac fibrosis via regulating macrophage phenotype by a reactive oxygen and nitrogen species (RONS)/STAT3-dependent pathway in postinfarcted rats. Normoglycemic male Wistar rats were subjected to coronary ligation and then randomized to either saline, dapagliflozin (a specific SGLT2 inhibitor), phlorizin (a nonspecific SGLT1/2 inhibitor), dapagliflozin + S3I-201 (a STAT3 inhibitor), or phlorizin + S3I-201 for 4 weeks. There were similar infarct sizes among the infarcted groups at the acute and chronic stages of infarction. At day 3 after infarction, post-infarction was associated with increased levels of superoxide and nitrotyrosine, which can be inhibited by administering either dapagliflozin or phlorizin. SGLT2 inhibitors significantly increased STAT3 activity, STAT3 nuclear translocation, myocardial IL-10 levels and the percentage of M2 macrophage infiltration. At day 28 after infarction, SGLT2 inhibitors were associated with attenuated myofibroblast infiltration and cardiac fibrosis. Although phlorizin decreased myofibroblast infiltration, the effect of dapagliflozin on attenuated myofibroblast infiltration was significantly higher than phlorizin. The effects of SGLT2 inhibitors on cardiac fibrosis were nullified by adding S3I-201. Furthermore, the effects of dapagliflozin on STAT3 activity and myocardial IL-10 levels can be reversed by 3-morpholinosydnonimine, a peroxynitrite generator. Taken together, these observations provide a novel mechanism of SGLT2 inhibitors-mediated M2 polarization through a RONS-dependent STAT3-mediated pathway and selective SGLT2 inhibitors are more effective in attenuating myofibroblast infiltration during postinfarction remodeling.

Superoxide dismutase/catalase mimetic EUK-134 prevents diaphragm muscle weakness in monocrotalin-induced pulmonary hypertension

Patients with pulmonary hypertension (PH) suffer from inspiratory insufficiency, which has been associated with intrinsic contractile dysfunction in diaphragm muscle. Here, we examined the role of redox stress in PH-induced diaphragm weakness by using the novel antioxidant, EUK-134. Male Wistar rats were randomly divided into control (CNT), CNT + EUK-134 (CNT + EUK), monocrotaline-induced PH (PH), and PH + EUK groups. PH was induced by a single intraperitoneal injection of monocrotaline (60 mg/kg body weight). EUK-134 (3 mg/kg body weight/day), a cell permeable mimetic of superoxide dismutase (SOD) and catalase, was daily intraperitoneally administered starting one day after induction of PH. After four weeks, diaphragm muscles were excised for mechanical and biochemical analyses. There was a decrease in specific tetanic force in diaphragm bundles from the PH group, which was accompanied by increases in: protein expression of NADPH oxidase 2/gp91phox, SOD2, and catalase; 3-nitrotyrosine content and aggregation of actin; glutathione oxidation. Treatment with EUK-134 prevented the force decrease and the actin modifications in PH diaphragm bundles. These data show that redox stress plays a pivotal role in PH-induced diaphragm weakness. Thus, antioxidant treatment can be a promising strategy for PH patients with inspiratory failure.

Muscle redox signalling pathways in exercise. Role of antioxidants

Recent research highlights the importance of redox signalling pathway activation by contraction-induced reactive oxygen species (ROS) and nitric oxide (NO) in normal exercise-related cellular and molecular adaptations in skeletal muscle. In this review, we discuss some potentially important redox signalling pathways in skeletal muscle that are involved in acute and chronic responses to contraction and exercise. Specifically, we discuss redox signalling implicated in skeletal muscle contraction force, mitochondrial biogenesis and antioxidant enzyme induction, glucose uptake and muscle hypertrophy. Furthermore, we review evidence investigating the impact of major exogenous antioxidants on these acute and chronic responses to exercise. Redox signalling pathways involved in adaptive responses in skeletal muscle to exercise are not clearly elucidated at present, and further research is required to better define important signalling pathways involved. Evidence of beneficial or detrimental effects of specific antioxidant compounds on exercise adaptations in muscle is similarly limited, particularly in human subjects. Future research is required to not only investigate effects of specific antioxidant compounds on skeletal muscle exercise adaptations, but also to better establish mechanisms of action of specific antioxidants in vivo. Although we feel it remains somewhat premature to make clear recommendations in relation to application of specific antioxidant compounds in different exercise settings, a bulk of evidence suggests that N-acetylcysteine (NAC) is ergogenic through its effects on maintenance of muscle force production during sustained fatiguing events. Nevertheless, a current lack of evidence from studies using performance tests representative of athletic competition and a potential for adverse effects with high doses (>70mg/kg body mass) warrants caution in its use for performance enhancement. In addition, evidence implicates high dose vitamin C (1g/day) and E (≥260 IU/day) supplementation in impairments to some skeletal muscle cellular adaptations to chronic exercise training. Thus, determining the utility of antioxidant supplementation in athletes likely requires a consideration of training and competition periodization cycles of athletes in addition to type, dose and duration of antioxidant supplementation.

Effects of urate-lowering agents on arrhythmia vulnerability in post-infarcted rat hearts

Hyperuricemia has been shown to be associated with ventricular arrhythmias. However, the mechanisms remained unknown. We assessed whether different urate-lowering agents can attenuate arrhythmias through lowering urate itself or inhibiting xanthenes oxidize (XO) activity in infarcted rats. Male Wistar rats after ligating coronary artery were randomized to either allopurinol, or febuxostat, chemically unrelated inhibitors of XO, benzbromarone or vehicle for 4 weeks. Post-infarction was associated with increased oxidant stress, as measured by myocardial superoxide, isoprostane, XO activity and dihydroethidine fluorescence staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham-operated rats. Sympathetic hyperinnervation was blunted after administering both XO inhibitors, assessed by immunofluorescent analysis, Western blotting and real-time quantitative RT-PCR. Besides, the XO inhibitors-attenuated nerve growth factor levels were reversed in the presence of peroxynitrite generator. Arrhythmic scores in the XO inhibitors-treated infarcted rats were significantly lower than that in vehicle. For similar levels of urate lowering, the uricosuric agent benzbromarone had no beneficial effects on oxidative stress, sympathetic hyperinnervation or arrhythmia vulnerability. Chronic use of XO inhibitors, but not uricosuric agent, down-regulated sympathetic innervation probably through a superoxide-dependent pathway and plays a role in the beneficial effect on arrhythmogenic response.

Reactive oxygen/nitrogen species and contractile function in skeletal muscle during fatigue and recovery

The production of reactive oxygen/nitrogen species (ROS/RNS) is generally considered to increase during physical exercise. Nevertheless, direct measurements of ROS/RNS often show modest increases in ROS/RNS in muscle fibres even during intensive fatiguing stimulation, and the major source(s) of ROS/RNS during exercise is still being debated. In rested muscle fibres, mild and acute exposure to exogenous ROS/RNS generally increases myofibrillar submaximal force, whereas stronger or prolonged exposure has the opposite effect. Endogenous production of ROS/RNS seems to preferentially decrease submaximal force and positive effects of antioxidants are mainly observed during fatigue induced by submaximal contractions. Fatigued muscle fibres frequently enter a prolonged state of reduced submaximal force, which is caused by a ROS/RNS-dependent decrease in sarcoplasmic reticulum Ca(2+) release and/or myofibrillar Ca(2+) sensitivity. Increased ROS/RNS production during exercise can also be beneficial and recent human and animal studies show that antioxidant supplementation can hamper the beneficial effects of endurance training. In conclusion, increased ROS/RNS production have both beneficial and detrimental effects on skeletal muscle function and the outcome depends on a combination of factors: the type of ROS/RNS; the magnitude, duration and location of ROS/RNS production; and the defence systems, including both endogenous and exogenous antioxidants.

Dipeptidyl peptidase-4 inhibition attenuates arrhythmias via a protein kinase A-dependent pathway in infarcted hearts

BACKGROUND

The effect of dipeptidyl peptidase-4 (DPP-4) inhibitors on arrhythmias remains unknown. The aim of this study was to investigate whether sitagliptin attenuates arrhythmias through inhibiting nerve growth factor (NGF) expression, focusing on cyclic adenosine monophosphate (cAMP) downstream signaling such as protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac).

METHODS AND RESULTS

Male Wistar rats were randomized to either vehicle or sitagliptin for 4 weeks starting 24 h after ligating the coronary artery. Post-infarction was associated with increased oxidative stress. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated rats compared with sham. Compared with the vehicle, infarcted rats treated with sitagliptin had significantly increased cAMP levels, decreased DPP-4 activity, oxidative stress, NGF levels and immunofluorescence-stained sympathetic hyperinnervation. Arrhythmic scores were significantly lower in the sitagliptin-treated infarcted rats than in vehicle. Ex vivo studies showed that sitagliptin increased the phosphorylated cAMP response element-binding protein (CREB), which can be reversed by H-89 (a PKA inhibitor), not brefeldin A (an Epac inhibitor).Heme oxygenase-1(HO-1) expression was increased by a PKA agonist but not by an Epac agonist.HO-1expression was attenuated in KG-501 (a CREB inhibitor)-treated infarcted rats in the presence of a PKA agonist.

CONCLUSIONS

Sitagliptin protects ventricular arrhythmias by attenuating NGF-induced sympathetic innervation via upregulation ofHO-1expression in a cAMP/PKA/CREB-dependent antioxidant pathway in non-diabetic infarcted rats.

Improved tolerance of acute severe hypoxic stress in chronic hypoxic diaphragm is nitric oxide-dependent

The effects of chronic hypoxia (CH) on respiratory muscle performance have hardly been investigated, despite clinical relevance. Results from recent studies are indicative of unique adaptive strategies in hypoxic diaphragm. Respiratory muscle tolerance of acute severe hypoxic stress was examined in normoxic and CH diaphragm in the presence and absence of a nitric oxide (NO) synthase inhibitor. We tested the hypothesis that improved tolerance of severe hypoxic stress in CH diaphragm is NO-dependent. Wistar rats were exposed to normoxia (sea-level, n = 6) or CH (ambient pressure = 380 mmHg, n = 6) for 6 weeks. Diaphragm muscle functional properties were determined ex vivo under severe hypoxic conditions (gassed with 95%N2/5% CO2) with and without 1 mM L-N(G)-nitroarginine (L-NNA, nNOS inhibitor). Fatigue tolerance, but not force, was significantly improved in CH diaphragm (p = 0.008). CH exposure did not affect diaphragm muscle fibre oxidative capacity determined from cluster analysis of area-density plots of muscle fibre succinate dehydrogenase activity. Acute NOS inhibition reduced diaphragm peak tetanic force (p = 0.018), irrespective of gas treatment, and completely reversed improved fatigue tolerance of the CH diaphragm. We conclude that CH exposure improves fatigue tolerance during acute severe hypoxic stress in an NO-dependent manner, independent of muscle fibre oxidative capacity.

Potential molecular mechanisms underlying muscle fatigue mediated by reactive oxygen and nitrogen species

Intense contractile activity causes a dramatic decline in the force and velocity generating capacity of skeletal muscle within a few minutes, a phenomenon that characterizes fatigue. Much of the research effort has focused on how elevated levels of the metabolites of ATP hydrolysis might inhibit the function of the contractile proteins. However, there is now growing evidence that elevated levels of reactive oxygen and nitrogen species (ROS/RNS), which also accumulate in the myoplasm during fatigue, also play a causative role in this type of fatigue. The most compelling evidence comes from observations demonstrating that pre-treatment of intact muscle with a ROS scavenger can significantly attenuate the development of fatigue. A clear advantage of this line of inquiry is that the molecular targets and protein modifications of some of the ROS scavengers are well-characterized enabling researchers to begin to identify potential regions and even specific amino acid residues modified during fatigue. Combining this knowledge with assessments of contractile properties from the whole muscle level down to the dynamic motions within specific contractile proteins enable the linking of the structural modifications to the functional impacts, using advanced chemical and biophysical techniques. Based on this approach at least two areas are beginning emerge as potentially important sites, the regulatory protein troponin and the actin binding region of myosin. This review highlights some of these recent efforts which have the potential to offer uniquely precise information on the underlying molecular basis of fatigue. This work may also have implications beyond muscle fatigue as ROS/RNS mediated protein modifications are also thought to play a role in the loss of muscle function with aging and in some acute pathologies like cardiac arrest and ischemia.

Muscle dysfunction associated with adjuvant-induced arthritis is prevented by antioxidant treatment

Background

In addition to the primary symptoms arising from inflamed joints, muscle weakness is prominent and frequent in patients with rheumatoid arthritis (RA). Here, we investigated the mechanisms of arthritis-induced muscle dysfunction in rats with adjuvant-induced arthritis (AIA).

Methods

AIA was induced in the knees of rats by injection of complete Freund’s adjuvant and was allowed to develop for 21 days. Muscle contractile function was assessed in isolated extensor digitorum longus (EDL) muscles. To assess mechanisms underlying contractile dysfunction, we measured redox modifications, redox enzymes and inflammatory mediators, and activity of actomyosin ATPase and sarcoplasmic reticulum (SR) Ca²⁺-ATPase.

Results

EDL muscles from AIA rats showed decreased tetanic force per cross-sectional area and slowed twitch contraction and relaxation. These contractile dysfunctions in AIA muscles were accompanied by marked decreases in actomyosin ATPase and SR Ca²⁺-ATPase activities. Actin aggregates were observed in AIA muscles, and these contained high levels of 3-nitrotyrosine and malondialdehyde-protein adducts. AIA muscles showed increased protein expression of NADPH oxidase 2/gp91^phox, neuronal nitric oxide synthase, tumor necrosis factor α (TNF-α), and high-mobility group box 1 (HMGB1). Treatment of AIA rats with EUK-134 (3 mg/kg/day), a superoxide dismutase/catalase mimetic, prevented both the decrease in tetanic force and the formation of actin aggregates in EDL muscles without having any beneficial effect on the arthritis development.

Conclusions

Antioxidant treatment prevented the development of oxidant-induced actin aggregates and contractile dysfunction in the skeletal muscle of AIA rats. This implies that antioxidant treatment can be used to effectively counteract muscle weakness in inflammatory conditions.

A change of heart: oxidative stress in governing muscle function?

Redox/cysteine modification of proteins that regulate calcium cycling can affect contraction in striated muscles. Understanding the nature of these modifications would present the possibility of enhancing cardiac function through reversible cysteine modification of proteins, with potential therapeutic value in heart failure with diastolic dysfunction. Both heart failure and muscular dystrophy are characterized by abnormal redox balance and nitrosative stress. Recent evidence supports the synergistic role of oxidative stress and inflammation in the progression of heart failure with preserved ejection fraction, in concert with endothelial dysfunction and impaired nitric oxide-cyclic guanosine monophosphate-protein kinase G signalling via modification of the giant protein titin. Although antioxidant therapeutics in heart failure with diastolic dysfunction have no marked beneficial effects on the outcome of patients, it, however, remains critical to the understanding of the complex interactions of oxidative/nitrosative stress with pro-inflammatory mechanisms, metabolic dysfunction, and the redox modification of proteins characteristic of heart failure. These may highlight novel approaches to therapeutic strategies for heart failure with diastolic dysfunction. In this review, we provide an overview of oxidative stress and its effects on pathophysiological pathways. We describe the molecular mechanisms driving oxidative modification of proteins and subsequent effects on contractile function, and, finally, we discuss potential therapeutic opportunities for heart failure with diastolic dysfunction.

Contractile properties and sarcoplasmic reticulum calcium content in type I and type II skeletal muscle fibres in active aged humans

KEY POINTS

Muscle weakness in old age is due in large part to an overall loss of skeletal muscle tissue, but it remains uncertain how much also stems from alterations in the properties of the individual muscle fibres. This study examined the contractile properties and amount of stored intracellular calcium in single muscle fibres of Old (70 ± 4 years) and Young (22 ± 3 years) adults. The maximum level of force production (per unit cross-sectional area) in fast twitch fibres in Old subjects was lower than in Young subjects, and the fibres were also less sensitive to activation by calcium. The amount of calcium stored inside muscle fibres and available to trigger contraction was also lower in both fast- and slow-twitch muscle fibres in the Old subjects. These findings indicate that muscle weakness in old age stems in part from an impaired capacity for force production in the individual muscle fibres.

ABSTRACT

This study examined the contractile properties and sarcoplasmic reticulum (SR) Ca(2+) content in mechanically skinned vastus lateralis muscle fibres of Old (70 ± 4 years) and Young (22 ± 3 years) humans to investigate whether changes in muscle fibre properties contribute to muscle weakness in old age. In type II fibres of Old subjects, specific force was reduced by ∼17% and Ca(2+) sensitivity was also reduced (pCa50 decreased ∼0.05 pCa units) relative to that in Young. S-Glutathionylation of fast troponin I (TnIf ) markedly increased Ca(2+) sensitivity in type II fibres, but the increase was significantly smaller in Old versus Young (+0.136 and +0.164 pCa unit increases, respectively). Endogenous and maximal SR Ca(2+) content were significantly smaller in both type I and type II fibres in Old subjects. In fibres of Young, the SR could be nearly fully depleted of Ca(2+) by a combined caffeine and low Mg(2+) stimulus, whereas in fibres of Old the amount of non-releasable Ca(2+) was significantly increased (by > 12% of endogenous Ca(2+) content). Western blotting showed an increased proportion of type I fibres in Old subjects, and increased amounts of calsequestrin-2 and calsequestrin-like protein. The findings suggest that muscle weakness in old age is probably attributable in part to (i) an increased proportion of type I fibres, (ii) a reduction in both maximum specific force and Ca(2+) sensitivity in type II fibres, and also a decreased ability of S-glutathionylation of TnIf to counter the fatiguing effects of metabolites on Ca(2+) sensitivity, and (iii) a reduction in the amount of releasable SR Ca(2+) in both fibre types.

Sitagliptin attenuates sympathetic innervation via modulating reactive oxygen species and interstitial adenosine in infarcted rat hearts

We investigated whether sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, attenuates arrhythmias through inhibiting nerve growth factor (NGF) expression in post-infarcted normoglycemic rats, focusing on adenosine and reactive oxygen species production. DPP-4 bound adenosine deaminase has been shown to catalyse extracellular adenosine to inosine. DPP-4 inhibitors increased adenosine levels by inhibiting the complex formation. Normoglycemic male Wistar rats were subjected to coronary ligation and then randomized to either saline or sitagliptin in in vivo and ex vivo studies. Post-infarction was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Compared with vehicle, infarcted rats treated with sitagliptin significantly increased interstitial adenosine levels and attenuated oxidative stress. Sympathetic hyperinnervation was blunted after administering sitagliptin, as assessed by immunofluorescent analysis and western blotting and real-time quantitative RT-PCR of NGF. Arrhythmic scores in the sitagliptin-treated infarcted rats were significantly lower than those in vehicle. Ex vivo studies showed a similar effect of erythro-9-(2-hydroxy-3-nonyl) adenine (an adenosine deaminase inhibitor) to sitagliptin on attenuated levels of superoxide and NGF. Furthermore, the beneficial effects of sitagliptin on superoxide anion production and NGF levels can be reversed by 8-cyclopentyl-1,3-dipropulxanthine (adenosine A₁ receptor antagonist) and exogenous hypoxanthine. Sitagliptin protects ventricular arrhythmias by attenuating sympathetic innervation via adenosine A₁ receptor and xanthine oxidase-dependent pathways, which converge through the attenuated formation of superoxide in the non-diabetic infarcted rats.

Acute effects of taurine on sarcoplasmic reticulum Ca2+ accumulation and contractility in human type I and type II skeletal muscle fibers

Taurine occurs in high concentrations in muscle and is implicated in numerous physiological processes, yet its effects on many aspects of contractility remain unclear. Using mechanically skinned segments of human vastus lateralis muscle fibers, we characterized the effects of taurine on sarcoplasmic reticulum (SR) Ca2+ accumulation and contractile apparatus properties in type I and type II fibers. Prolonged myoplasmic exposure (>10 min) to taurine substantially increased the rate of accumulation of Ca2+ by the SR in both fiber types, with no change in the maximum amount accumulated; no such effect was found with carnosine. SR Ca2+ accumulation was similar with 10 or 20 mM taurine, but was significantly slower at 5 mM taurine. Cytoplasmic taurine (20 mM) had no detectable effects on the responsiveness of the Ca2+ release channels in either fiber type. Taurine caused a small increase in Ca2+ sensitivity of the contractile apparatus in type I fibers, but type II fibers were unaffected; maximum Ca2+-activated force was unchanged in both cases. The effects of taurine on SR Ca2+ accumulation 1) only became apparent after prolonged cytoplasmic exposure, and 2) persisted for some minutes after complete removal of taurine from the cytoplasm, consistent with the hypothesis that the effects were due to an action of taurine from inside the SR. In summary, taurine potentiates the rate of SR Ca2+ uptake in both type I and type II human fibers, possibly via an action from within the SR lumen, with the degree of potentiation being significantly reduced at low physiological taurine levels.

Muscle disuse alters skeletal muscle contractile function at the molecular and cellular levels in older adult humans in a sex-specific manner

Physical inactivity that accompanies ageing and disease may hasten disability by reducing skeletal muscle contractility. To characterize skeletal muscle functional adaptations to muscle disuse, we compared contractile performance at the molecular, cellular and whole‐muscle levels in healthy active older men and women (n = 15) and inactive older men and women with advanced‐stage, symptomatic knee osteoarthritis (OA) (n = 16). OA patients showed reduced (P < 0.01) knee extensor function. At the cellular level, single muscle fibre force production was reduced in OA patients in myosin heavy chain (MHC) I and IIA fibres (both P < 0.05) and differences in IIA fibres persisted after adjustments for fibre cross‐sectional area (P < 0.05). Although no group differences in contractile velocity or power output were found for any fibre type, sex was found to modify the effect of OA, with a reduction in MHC IIA power output and a trend towards reduced shortening velocity in women, but increases in both variables in men (P < 0.05 and P = 0.07, respectively). At the molecular level, these adaptations in MHC IIA fibre function were explained by sex‐specific differences (P ≤ 0.05) in myosin–actin cross‐bridge kinetics. Additionally, cross‐bridge kinetics were slowed in MHC I fibres in OA patients (P < 0.01), attributable entirely to reductions in women with knee OA (P < 0.05), a phenotype that could be reproduced in vitro by chemical modification of protein thiol residues. Our results identify molecular and cellular functional adaptations in skeletal muscle that may contribute to reduced physical function with knee OA‐associated muscle disuse, with sex‐specific differences that may explain a greater disposition towards disability in women.

Nitrosative modifications of the Ca2+ release complex and actin underlie arthritis-induced muscle weakness

OBJECTIVE

Skeletal muscle weakness is a prominent clinical feature in patients with rheumatoid arthritis (RA), but the underlying mechanism(s) is unknown. Here we investigate the mechanisms behind arthritis-induced skeletal muscle weakness with special focus on the role of nitrosative stress on intracellular Ca(2+) handling and specific force production.

METHODS

Nitric oxide synthase (NOS) expression, degree of nitrosative stress and composition of the major intracellular Ca(2+) release channel (ryanodine receptor 1, RyR1) complex were measured in muscle. Changes in cytosolic free Ca(2+) concentration ([Ca(2+)]i) and force production were assessed in single-muscle fibres and isolated myofibrils using atomic force cantilevers.

RESULTS

The total neuronal NOS (nNOS) levels were increased in muscles both from collagen-induced arthritis (CIA) mice and patients with RA. The nNOS associated with RyR1 was increased and accompanied by increased [Ca(2+)]i during contractions of muscles from CIA mice. A marker of peroxynitrite-derived nitrosative stress (3-nitrotyrosine, 3-NT) was increased on the RyR1 complex and on actin of muscles from CIA mice. Despite increased [Ca(2+)]i, individual CIA muscle fibres were weaker than in healthy controls, that is, force per cross-sectional area was decreased. Furthermore, force and kinetics were impaired in CIA myofibrils, hence actin and myosin showed decreased ability to interact, which could be a result of increased 3-NT content on actin.

CONCLUSIONS

Arthritis-induced muscle weakness is linked to nitrosative modifications of the RyR1 protein complex and actin, which are driven by increased nNOS associated with RyR1 and progressively increasing Ca(2+) activation.

Peroxynitrite induces degradation of myosin heavy chain via p38 MAPK and muscle-specific E3 ubiquitin ligases in C2 skeletal myotubes

Oxidative stress and inflammation play an important role in the catabolism of skeletal muscles. Recently, cigarette smoke (CS) was shown to stimulate muscle catabolism by activation of p38 MAPK and up-regulation of the muscle-specific E3 ubiquitin ligases (E3s) atrogin-1 and MuRF1 which are over-expressed during muscle atrophy. Peroxynitrite (ONOO-), an oxidative ingredient of CS, also produced during oxidative stress and inflammation, was previously shown to induce ubiquitination and degradation of muscle proteins. To investigate the involvement of p38 MAPK and the muscle-specific E3s in ONOO--induced muscle catabolism, C2 myotubes, differentiated from a myoblast cell line, were exposed to ONOO- (25 μM) in a time-dependent manner. Following exposure, degradation of myosin heavy chain (MyHC) and actin, activation of p38 MAPK, and levels of atrogin-1 and MuRF1 were studied by Western blotting. Peak phosphorylation of p38 MAPK was observed at 1 h of ONOO- exposure. ONOO- caused a significant increase in the levels of atrogin-1 and MuRF1. In accordance, a significant decrease in MyHC levels was observed in a time-dependent manner. These findings support previous studies in which the catabolic effects of ONOO- were shown. In addition, ONOO- was demonstrated to induce degradation of muscle proteins by activation of p38 MAPK and up-regulation of the muscle-specific E3s atrogin-1 and MuRF1.

Weakness of whole muscles in mice deficient in Cu, Zn superoxide dismutase is not explained by defects at the level of the contractile apparatus

Mice deficient in Cu,Zn superoxide dismutase (Sod1 (-/-) mice) demonstrate elevated oxidative stress associated with rapid age-related declines in muscle mass and force. The decline in mass for muscles of Sod1 (-/-) mice is explained by a loss of muscle fibers, but the mechanism underlying the weakness is not clear. We hypothesized that the reduced maximum isometric force (F o) normalized by cross-sectional area (specific F o) for whole muscles of Sod1 (-/-) compared with wild-type (WT) mice is due to decreased specific F o of individual fibers. Force generation was measured for permeabilized fibers from muscles of Sod1 (-/-) and WT mice at 8 and 20 months of age. WT mice were also studied at 28 months to determine whether any deficits observed for fibers from Sod1 (-/-) mice were similar to those observed in old WT mice. No effects of genotype were observed for F o or specific F o at either 8 or 20 months, and no age-associated decrease in specific F o was observed for fibers from Sod1 (-/-) mice, whereas specific F o for fibers of WT mice decreased by 20 % by 28 months. Oxidative stress has also been associated with decreased maximum velocity of shortening (V max), and we found a 10 % lower V max for fibers from Sod1 (-/-) compared with WT mice at 20 months. We conclude that the low specific F o of muscles of Sod1 (-/-) mice is not explained by damage to contractile proteins. Moreover, the properties of fibers of Sod1 (-/-) mice do not recapitulate those observed with aging in WT animals.

Accessibility of myofilament cysteines and effects on ATPase depend on the activation state during exposure to oxidants

Signaling by reactive oxygen species has emerged as a major physiological process. Due to its high metabolic rate, striated muscle is especially subject to oxidative stress, and there are multiple examples in cardiac and skeletal muscle where oxidative stress modulates contractile function. Here we assessed the potential of cysteine oxidation as a mechanism for modulating contractile function in skeletal and cardiac muscle. Analyzing the cysteine content of the myofilament proteins in striated muscle, we found that cysteine residues are relatively rare, but are very similar between different muscle types and different vertebrate species. To refine this list of cysteines to those that may modulate function, we estimated the accessibility of oxidants to cysteine residues using protein crystal structures, and then sharpened these estimates using fluorescent labeling of cysteines in cardiac and skeletal myofibrils. We demonstrate that cysteine accessibility to oxidants and ATPase rates depend on the contractile state in which preparations are exposed. Oxidant exposure of skeletal and cardiac myofibrils in relaxing solution exposes myosin cysteines not accessible in rigor solution, and these modifications correspond to a decrease in maximum ATPase. Oxidant exposure under rigor conditions produces modifications that increase basal ATPase and calcium sensitivity in ventricular myofibrils, but these effects were muted in fast twitch muscle. These experiments reveal how structural and sequence variations can lead to divergent effects from oxidants in different muscle types.

Systemic vascular function is associated with muscular power in older adults

Age-associated loss of muscular strength and muscular power is a critical determinant of loss of physical function and progression to disability in older adults. In this study, we examined the association of systemic vascular function and measures of muscle strength and power in older adults. Measures of vascular endothelial function included brachial artery flow-mediated dilation (FMD) and the pulse wave amplitude reactive hyperemia index (PWA-RHI). Augmentation index (AIx) was taken as a measure of systemic vascular function related to arterial stiffness and wave reflection. Measures of muscular strength included one repetition maximum (1RM) for a bilateral leg press. Peak muscular power was measured during 5 repetitions performed as fast as possible for bilateral leg press at 40% 1RM. Muscular power was associated with brachial FMD (r = 0.43, P < 0.05), PWA-RHI (r = 0.42, P < 0.05), and AIx (r = -0.54, P < 0.05). Muscular strength was not associated with any measure of vascular function. In conclusion, systemic vascular function is associated with lower-limb muscular power but not muscular strength in older adults. Whether loss of muscular power with aging contributes to systemic vascular deconditioning or vascular dysfunction contributes to decrements in muscular power remains to be determined.

ROS-mediated decline in maximum Ca2+-activated force in rat skeletal muscle fibers following in vitro and in vivo stimulation

We hypothesised that normal skeletal muscle stimulated intensely either in vitro or in situ would exhibit reactive oxygen species (ROS)-mediated contractile apparatus changes common to many pathophysiological conditions. Isolated soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat were bubbled with 95% O₂ and stimulated in vitro at 31°C to give isometric tetani (50 Hz for 0.5 s every 2 s) until maximum force declined to ≤30%. Skinned superficial slow-twitch fibers from the SOL muscles displayed a large reduction (∼41%) in maximum Ca²⁺-activated specific force (F_max), with Ca²⁺-sensitivity unchanged. Fibers from EDL muscles were less affected. The decrease in F_max in SOL fibers was evidently due to oxidation effects on cysteine residues because it was reversed if the reducing agent DTT was applied prior to activating the fiber. The GSH∶GSSG ratio was ∼3-fold lower in the cytoplasm of superficial fibers from stimulated muscle compared to control, confirming increased oxidant levels. The presence of Tempol and L-NAME during in vitro stimulation prevented reduction in F_max. Skinned fibers from SOL muscles stimulated in vivo at 37°C with intact blood supply also displayed reduction in F_max, though to a much smaller extent (∼12%). Thus, fibers from muscles stimulated even with putatively adequate O₂ supply display a reversible oxidation-induced decrease in F_max without change in Ca²⁺-sensitivity, consistent with action of peroxynitrite (or possibly superoxide) on cysteine residues of the contractile apparatus. Significantly, the changes closely resemble the contractile deficits observed in a range of pathophysiological conditions. These findings highlight how readily muscle experiences ROS-related deficits, and also point to potential difficulties when defining muscle performance and fatigue.

Sphingomyelinase depresses force and calcium sensitivity of the contractile apparatus in mouse diaphragm muscle fibers

Diseases that result in muscle weakness, e.g., heart failure, are characterized by elevated sphingomyelinase (SMase) activity. In intact muscle, SMase increases oxidants that contribute to diminished muscle force. However, the source of oxidants, specific processes of muscle contraction that are dysfunctional, and biochemical changes underlying the weakness elicited by SMase remain unknown. We tested three hypotheses: 1) SMase-induced depression of muscle force is mediated by mitochondrial reactive oxygen species (ROS), 2) SMase depresses force and calcium sensitivity of the contractile apparatus, and 3) SMase promotes oxidation and phosphorylation of myofibrillar proteins. Our experiments included intact muscle bundles, permeabilized single fibers, and isolated myofibrillar proteins. The mitochondrial-targeted antioxidant d-Arg-2',6'-dimethyl-Tyr-Lys-Phe-NH(2), decreased cytosolic oxidants and protected intact muscle bundles from weakness stimulated by SMase. SMase depressed maximal calcium-activated force by 20% in permeabilized single fibers (in kN/m(2): control 117 ± 6; SMase 93 ± 8; P < 0.05). Calcium sensitivity of permeabilized single fibers decreased from 5.98 ± 0.03 (control) to 5.91 ± 0.02 (SMase; P < 0.05). Myofibrillar protein nitrotyrosines, carbonyls, and phosphorylation were unaltered by SMase. Our study shows that the fall in specific force of intact muscle elicited by SMase is mediated by mitochondrial ROS and can be attributed largely to dysfunction of the contractile apparatus.

S-glutathionylation of troponin I (fast) increases contractile apparatus Ca2+ sensitivity in fast-twitch muscle fibres of rats and humans

Oxidation can decrease or increase the Ca2+ sensitivity of the contractile apparatus in rodent fast-twitch (type II) skeletal muscle fibres, but the reactions and molecular targets involved are unknown. This study examined whether increased Ca2+ sensitivity is due to S-glutathionylation of particular cysteine residues. Skinned muscle fibres were directly activated in heavily buffered Ca2+ solutions to assess contractile apparatus Ca2+ sensitivity. Rat type II fibres were subjected to S-glutathionylation by successive treatments with 2,2′-dithiodipyridine (DTDP) and glutathione (GSH), and displayed a maximal increase in pCa50 (−log10 [Ca2+] at half-maximal force) of ∼0.24 pCa units, with little or no effect on maximum force or Hill coefficient. Partial similar effect was produced by exposure to oxidized gluthathione (GSSG, 10 mM) for 10 min at pH 7.1, and near-maximal effect by GSSG treatment at pH 8.5. None of these treatments significantly altered Ca2+ sensitivity in rat type I fibres. Western blotting showed that both the DTDP–GSH and GSSG–pH 8.5 treatments caused marked S-glutathionylation of the fast troponin I isoform (TnI(f)) present in type II fibres, but not of troponin C (TnC) or myosin light chain 2. Both the increased Ca2+ sensitivity and glutathionylation of TnI(f) were blocked by N-ethylmaleimide (NEM). S-nitrosoglutathione (GSNO) also increased Ca2+ sensitivity, but only in conditions where it caused S-glutathionylation of TnI(f). In human type II fibres from vastus lateralis muscle, DTDP–GSH treatment also caused similar increased Ca2+ sensitivity and S-glutathionylation of TnI(f). When the slow isoform of TnI in type I fibres of rat was partially substituted (∼30%) with TnI(f), DTDP–GSH treatment caused a significant increase in Ca2+ sensitivity (∼0.08 pCa units). TnIf in type II fibres from toad and chicken muscle lack Cys133 present in mammalian TnIf, and such fibres showed no change in Ca2+ sensitivity with DTDP–GSH nor any S-glutathionylation of TnI(f) (latter examined only in toad). Following 40 min of cycling exercise in human subjects (at ∼60% peak oxygen consumption), TnI(f) in vastus lateralis muscle displayed a marked increase in S-glutathionylation (∼4-fold). These findings show that S-glutathionylation of TnI(f), most probably at Cys133, increases the Ca2+ sensitivity of the contractile apparatus, and that this occurs in exercising humans, with likely beneficial effects on performance.

Differential effects of NADPH oxidase and xanthine oxidase inhibition on sympathetic reinnervation in postinfarct rat hearts

Superoxide has been shown to play a major role in ventricular remodeling and arrhythmias after myocardial infarction. However, the source of increased myocardial superoxide production and the role of superoxide in sympathetic innervation remain to be further characterized. Male Wistar rats, after coronary artery ligation, were randomized to vehicle, allopurinol, or apocynin for 4weeks. To determine the role of peroxynitrite in sympathetic reinnervation, we also used 3-morpholinosydnonimine (a peroxynitrite generator). The postinfarction period was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine, xanthine oxidase activity, NADPH oxidase activity, and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Sympathetic hyperinnervation was blunted after administration of allopurinol. Arrhythmic scores in the allopurinol-treated infarcted rats were significantly lower than those in vehicle. For similar levels of ventricular remodeling, apocynin had no beneficial effects on oxidative stress, sympathetic hyperinnervation, or arrhythmia vulnerability. Allopurinol-treated hearts had significantly decreased nerve growth factor expression, which was substantially increased after coadministration of 3-morpholinosydnonimine. These results indicate that xanthine oxidase but not NADPH oxidase largely mediates superoxide production after myocardial infarction. Xanthine oxidase inhibition ameliorates sympathetic innervation and arrhythmias possibly via inhibition of the peroxynitrite-mediated nerve growth factor pathway.

Acute effects of reactive oxygen and nitrogen species on the contractile function of skeletal muscle

Reactive oxygen and nitrogen species (ROS/RNS) are important for skeletal muscle function under both physiological and pathological conditions. ROS/RNS induce long-term and acute effects and the latter are the focus of the present review. Upon repeated muscle activation both oxygen and nitrogen free radicals likely increase and acutely affect contractile function. Although fluorescent indicators often detect only modest increases in ROS during repeated activation, there are numerous studies showing that manipulations of ROS can affect muscle fatigue development and recovery. Exposure of intact muscle fibres to the oxidant hydrogen peroxide (H(2)O(2)) affects mainly the myofibrillar function, where an initial increase in Ca(2+) sensitivity is followed by a decrease. Experiments on skinned fibres show that these effects can be attributed to H(2)O(2) interacting with glutathione and myoglobin, respectively. The primary RNS, nitric oxide (NO()), may also acutely affect myofibrillar function and decrease the Ca(2+) sensitivity. H(2)O(2) can oxidize the sarcoplasmic reticulum Ca(2+) release channels. This oxidation has a large stimulatory effect on Ca(2+)-induced Ca(2+) release of isolated channels, whereas it has little or no effect on the physiological, action potential-induced Ca(2+) release in skinned and intact muscle fibres. Thus, acute effects of ROS/RNS on muscle function are likely to be mediated by changes in myofibrillar Ca(2+) sensitivity, which can contribute to the development of muscle fatigue or alternatively help counter it.