H2O2 modulates twitch tension and increases Po of Ca2+ release channel in frog skeletal muscle

In vivo cytosolic H2O2 changes and Ca2+ homeostasis in mouse skeletal muscle

Hydrogen peroxide (H2O2) and calcium ions (Ca2+) are functional regulators of skeletal muscle contraction and metabolism. Although H2O2 is one of the activators of the type-1 ryanodine receptor (RyR1) in the Ca2+ release channel, the interdependence between H2O2 and Ca2+ dynamics remains unclear. This study tested the following hypotheses using an in vivo model of mouse tibialis anterior (TA) skeletal muscle. 1) Under resting conditions, elevated cytosolic H2O2 concentration ([H2O2]cyto) leads to a concentration-dependent increase in cytosolic Ca2+ concentration ([Ca2+]cyto) through its effect on RyR1; and 2) in hypoxia (cardiac arrest) and muscle contractions (electrical stimulation), increased [H2O2]cyto induces Ca2+ accumulation. Cytosolic H2O2 (HyPer7) and Ca2+ (Fura-2) dynamics were resolved by TA bioimaging in young C57BL/6J male mice under four conditions: 1) elevated exogenous H2O2; 2) cardiac arrest; 3) twitch (1 Hz, 60 s) contractions; and 4) tetanic (30 s) contractions. Exogenous H2O2 (0.1-100 mM) induced a concentration-dependent increase in [H2O2]cyto (+55% at 0.1 mM; +280% at 100 mM) and an increase in [Ca2+]cyto (+3% at 1.0 mM; +8% at 10 mM). This increase in [Ca2+]cyto was inhibited by pharmacological inhibition of RyR1 by dantrolene. Cardiac arrest-induced hypoxia increased [H2O2]cyto (+33%) and [Ca2+]cyto (+20%) 50 min postcardiac arrest. Compared with the exogenous 1.0 mM H2O2 condition, [H2O2]cyto after tetanic muscle contractions rose less than one-tenth as much, whereas [Ca2+]cyto was 4.7-fold higher. In conclusion, substantial increases in [H2O2]cyto levels evoke only modest Ca2+ accumulation via their effect on the sarcoplasmic reticulum RyR1. On the other hand, contrary to hypoxia secondary to cardiac arrest, increases in [H2O2]cyto from muscle contractions are small, indicating that H2O2 generation is unlikely to be a primary factor driving the significant Ca2+ accumulation after, especially tetanic, muscle contractions.NEW & NOTEWORTHY We developed an in vivo mouse myocyte H2O2 imaging model during exogenous H2O2 loading, ischemic hypoxia induced by cardiac arrest, and muscle contractions. In this study, the interrelationship between cytosolic H2O2 levels and Ca2+ homeostasis during muscle contraction and hypoxic conditions was revealed. These results contribute to the elucidation of the mechanisms of muscle fatigue and exercise adaptation.

Anti-Fibrotic Effect of Human Wharton's Jelly-Derived Mesenchymal Stem Cells on Skeletal Muscle Cells, Mediated by Secretion of MMP-1

Extracellular matrix (ECM) components play an important role in maintaining skeletal muscle function, but excessive accumulation of ECM components interferes with skeletal muscle regeneration after injury, eventually inducing fibrosis. Increased oxidative stress level caused by dystrophin deficiency is a key factor in fibrosis in Duchenne muscular dystrophy (DMD) patients. Mesenchymal stem cells (MSCs) are considered a promising therapeutic agent for various diseases involving fibrosis. In particular, the paracrine factors secreted by MSCs play an important role in the therapeutic effects of MSCs. In this study, we investigated the effects of MSCs on skeletal muscle fibrosis. In 2–5-month-old mdx mice intravenously injected with 1 × 10⁵ Wharton’s jelly (WJ)-derived MSCs (WJ-MSCs), fibrosis intensity and accumulation of calcium/necrotic fibers were significantly decreased. To elucidate the mechanism of this effect, we verified the effect of WJ-MSCs in a hydrogen peroxide-induced fibrosis myotubes model. In addition, we demonstrated that matrix metalloproteinase-1 (MMP-1), a paracrine factor, is critical for this anti-fibrotic effect of WJ-MSCs. These findings demonstrate that WJ-MSCs exert anti-fibrotic effects against skeletal muscle fibrosis, primarily via MMP-1, indicating a novel target for the treatment of muscle diseases, such as DMD.

Redox regulation of the actin cytoskeleton and its role in the vascular system

The actin cytoskeleton is critical for form and function of vascular cells, serving mechanical, organizational and signaling roles. Because many cytoskeletal proteins are sensitive to reactive oxygen species, redox regulation has emerged as a pivotal modulator of the actin cytoskeleton and its associated proteins. Here, we summarize work implicating oxidants in altering actin cytoskeletal proteins and focus on how these alterations affect cell migration, proliferation and contraction of vascular cells. Finally, we discuss the role of oxidative modification of the actin cytoskeleton in vivo and highlight its importance for vascular diseases.

[Direct mechanism of action in toxic myopathies]

Toxic myopathies are a large group of disorders generated by surrounding agents and characterized by structural and/or functional disturbances of muscles. The most recurrent are those induced by commonly used medications. Illicit drugs, environmental toxins from animals, vegetables, or produced by micro-organisms as well as chemical products commonly used are significant causes of such disorders. The muscle toxicity results from multiple mechanisms at different biological levels. Many agents can induce myotoxicity through a direct mechanism in which statins, glucocorticoids and ethyl alcohol are the most representative. Diverse mechanisms were highlighted as interaction with macromolecules and induction of metabolic and cellular dysfunctions. Muscle damage can be related to amphiphilic properties of some drugs (chloroquine, hydroxychloroquine, etc.) leading to specific lysosomal disruptions and autophagic dysfunctions. Some agents affect the whole muscle fiber by inducing oxidative stress (ethyl alcohol and some statins) or triggering cell death pathways (apoptosis or necrosis) resulting in extensive alterations. More studies on these mechanisms are needed. They would allow a better knowledge of the intracellular mediators involved in these pathologies in order to develop targeted therapies of high efficiency.

Mitochondria-derived ROS bursts disturb Ca²⁺ cycling and induce abnormal automaticity in guinea pig cardiomyocytes: a theoretical study

Mitochondria are in close proximity to the redox-sensitive sarcoplasmic reticulum (SR) Ca(2+) release [ryanodine receptors (RyRs)] and uptake [Ca(2+)-ATPase (SERCA)] channels. Thus mitochondria-derived reactive oxygen species (mdROS) could play a crucial role in modulating Ca(2+) cycling in the cardiomyocytes. However, whether mdROS-mediated Ca(2+) dysregulation translates to abnormal electrical activities under pathological conditions, and if yes what are the underlying ionic mechanisms, have not been fully elucidated. We hypothesize that pathological mdROS induce Ca(2+) elevation by modulating SR Ca(2+) handling, which activates other Ca(2+) channels and further exacerbates Ca(2+) dysregulation, leading to abnormal action potential (AP). We also propose that the morphologies of elicited AP abnormality rely on the time of mdROS induction, interaction between mitochondria and SR, and intensity of mitochondrial oxidative stress. To test the hypotheses, we developed a multiscale guinea pig cardiomyocyte model that incorporates excitation-contraction coupling, local Ca(2+) control, mitochondrial energetics, and ROS-induced ROS release. This model, for the first time, includes mitochondria-SR microdomain and modulations of mdROS on RyR and SERCA activities. Simulations show that mdROS bursts increase cytosolic Ca(2+) by stimulating RyRs and inhibiting SERCA, which activates the Na(+)/Ca(2+) exchanger, Ca(2+)-sensitive nonspecific cationic channels, and Ca(2+)-induced Ca(2+) release, eliciting abnormal AP. The morphologies of AP abnormality are largely influenced by the time interval among mdROS burst induction and AP firing, dosage and diffusion of mdROS, and SR-mitochondria distance. This study defines the role of mdROS in Ca(2+) overload-mediated cardiac arrhythmogenesis and underscores the importance of considering mitochondrial targets in designing new antiarrhythmic therapies.

An olive oil-derived antioxidant mixture ameliorates the age-related decline of skeletal muscle function

Age-related skeletal muscle decline is characterized by the modification of sarcolemma ion channels important to sustain fiber excitability and to prevent metabolic dysfunction. Also, calcium homeostasis and contractile function are impaired. In the aim to understand whether these modifications are related to oxidative damage and can be reverted by antioxidant treatment, we examined the effects of in vivo treatment with an waste water polyphenolic mixture (LACHI MIX HT) supplied by LACHIFARMA S.r.l. Italy containing hydroxytirosol (HT), gallic acid, and homovanillic acid on the skeletal muscles of 27-month-old rats. After 6-week treatment, we found an improvement of chloride ClC-1 channel conductance, pivotal for membrane electrical stability, and of ATP-dependent potassium channel activity, important in coupling excitability with fiber metabolism. Both of them were analyzed using electrophysiological techniques. The treatment also restored the resting cytosolic calcium concentration, the sarcoplasmic reticulum calcium release, and the mechanical threshold for contraction, an index of excitation-contraction coupling mechanism. Muscle weight and blood creatine kinase levels were preserved in LACHI MIX HT-treated aged rats. The antioxidant activity of LACHI MIX HT was confirmed by the reduction of malondialdehyde levels in the brain of the LACHI MIX HT-treated aged rats. In comparison, the administration of purified HT was less effective on all the parameters studied. Although muscle function was not completely recovered, the present study provides evidence of the beneficial effects of LACHI MIX HT, a natural compound, to ameliorate skeletal muscle functional decline due to aging-associated oxidative stress.

Hydrogen peroxide alters sternohyoid muscle function

Upper airway (UA) dilator muscles are critical for the maintenance of airway patency. Injury or fatigue to this group of muscles, as observed in patients with obstructive sleep apnoea (OSA) and animal models of OSA, may leave the UA susceptible to collapse. Although the mechanisms underlying respiratory muscle dysfunction are not completely understood, there is strong evidence suggesting a link between increased production of reactive oxygen species and altered muscle function. The aim of this study was to examine the effects of H2O2 on rat sternohyoid muscle function in vitro. Sternohyoid contractile and endurance properties were examined at 35 °C under control or hypoxic conditions. Studies were conducted in the presence of varying concentrations of H2O2 (0, 0.01, 0.1 and 1 mM). Muscle function was also examined in the presence of antioxidants [desferoxamine (DFX), catalase] and the reducing agent dithiothreitol (DTT). H2O2 decreased muscle endurance in a concentration-dependent manner. This was partially reversed by catalase, DFX and DTT. Our results suggest that oxidants may contribute to UA respiratory muscle dysfunction with implications for the control of UA patency in vivo.

Redox control of cardiac excitability

Reactive oxygen species (ROS) have been associated with various human diseases, and considerable attention has been paid to investigate their physiological effects. Various ROS are synthesized in the mitochondria and accumulate in the cytoplasm if the cellular antioxidant defense mechanism fails. The critical balance of this ROS synthesis and antioxidant defense systems is termed the redox system of the cell. Various cardiovascular diseases have also been affected by redox to different degrees. ROS have been indicated as both detrimental and protective, via different cellular pathways, for cardiac myocyte functions, electrophysiology, and pharmacology. Mostly, the ROS functions depend on the type and amount of ROS synthesized. While the literature clearly indicates ROS effects on cardiac contractility, their effects on cardiac excitability are relatively under appreciated. Cardiac excitability depends on the functions of various cardiac sarcolemal or mitochondrial ion channels carrying various depolarizing or repolarizing currents that also maintain cellular ionic homeostasis. ROS alter the functions of these ion channels to various degrees to determine excitability by affecting the cellular resting potential and the morphology of the cardiac action potential. Thus, redox balance regulates cardiac excitability, and under pathological regulation, may alter action potential propagation to cause arrhythmia. Understanding how redox affects cellular excitability may lead to potential prophylaxis or treatment for various arrhythmias. This review will focus on the studies of redox and cardiac excitation.

Redox biology of exercise: an integrative and comparative consideration of some overlooked issues

The central aim of this review is to address the highly multidisciplinary topic of redox biology as related to exercise using an integrative and comparative approach rather than focusing on blood, skeletal muscle or humans. An attempt is also made to re-define ‘oxidative stress’ as well as to introduce the term ‘alterations in redox homeostasis’ to describe changes in redox homeostasis indicating oxidative stress, reductive stress or both. The literature analysis shows that the effects of non-muscle-damaging exercise and muscle-damaging exercise on redox homeostasis are completely different. Non-muscle-damaging exercise induces alterations in redox homeostasis that last a few hours post exercise, whereas muscle-damaging exercise causes alterations in redox homeostasis that may persist for and/or appear several days post exercise. Both exhaustive maximal exercise lasting only 30 s and isometric exercise lasting 1–3 min (the latter activating in addition a small muscle mass) induce systemic oxidative stress. With the necessary modifications, exercise is capable of inducing redox homeostasis alterations in all fluids, cells, tissues and organs studied so far, irrespective of strains and species. More importantly, ‘exercise-induced oxidative stress’ is not an ‘oddity’ associated with a particular type of exercise, tissue or species. Rather, oxidative stress constitutes a ubiquitous fundamental biological response to the alteration of redox homeostasis imposed by exercise. The hormesis concept could provide an interpretative framework to reconcile differences that emerge among studies in the field of exercise redox biology. Integrative and comparative approaches can help determine the interactions of key redox responses at multiple levels of biological organization.

Redox modulation of diaphragm contractility: Interaction between DHPR and RyR channels

Previous reports indicate that reactive oxygen species (ROS) may modulate contractility in skeletal muscle. Although Ca(2+)-sensitivity of the contractile apparatus appears to be a primary site of regulation, dihydropyridine receptor (DHPR or L-type Ca(2+) channels) and calcium efflux in isolated sarcoplasmic reticulum (SR) vesicles appear to be redox sensitive as well. However, DHPR as a target is poorly understood in intact muscles at body temperature, particularly in the diaphragm, a muscle more dependent on external Ca(2+) than locomotor muscles. Previously, we reported that oxidant challenge via xanthine oxidase (XO) alters the K(+) contractures in diaphragm fiber bundles, suggestive of a role of L-type Ca(2+) channels. Contractility of isolated rat diaphragm fiber bundles revealed a biphasic response to ROS challenge that was dose and time dependent. Potentiation of twitch and low-frequency diaphragm fiber bundle contractility with 0.02 U•ml(-1) XO was reversible or partially preventable with washout, dithiothreitol, and the SOD/catalase mimetic EUK-134. The RyR antagonist ruthenium red inhibited xanthine oxidase-induced potentiation, while the RyR agonist caffeine elevated diaphragm twitch and low-frequency tension in a non-additive manner by 55% when introduced simultaneously with ROS challenge. The DHPR antagonist nitrendipine (15 μM) inhibited elevation in low-frequency diaphragm tension produced by ROS challenge. Caffeine threshold tension curves were shifted to the left with 0.02 U•ml(-1) XO, but this effect was partially reversed with 15 μM nitrendipine. These results are consistent with the hypothesis that DHPR redox state and RyR function are modulated in an interactive manner, affecting contractility in intact diaphragm fiber bundles.

Greater susceptibility of the sarcoplasmic reticulum to H2O2 injuries in diaphragm muscle from mdx mice

The aim of the present study was to investigate the direct effects of a reactive oxygen species, H(2)O(2), on the contractile function and sarcoplasmic reticulum properties of dystrophin-deficient diaphragm using chemically skinned fibers and sarcoplasmic reticulum vesicle preparations. The results obtained using Triton X-100-skinned fibers demonstrate that exposure to 1 mM H(2)O(2) had similar effects on the maximal Ca(2+)-activated tension and on the Ca(2+) sensitivity of the contractile apparatus of diaphragm fibers in Bl10 and mdx mice. The effects of H(2)O(2) were also assessed on sarcoplasmic reticulum function using saponin-skinned fibers and sarcoplasmic reticulum vesicle preparations. We found that H(2)O(2) induced changes in sarcoplasmic reticulum properties, particularly in the Ca(2+) pump function. The most important finding was that diaphragm muscle from mdx mice displayed increased sensitivity to the oxidant. Furthermore, in isolated superfused diaphragm muscle from mdx mice, the data demonstrate that the amount of superoxide anion produced under fatiguing conditions was increased. Our study shows that the sarcoplasmic reticulum, and the Ca(2+) pump in particular, in dystrophin-deficient muscles display increased susceptibility to H(2)O(2) injuries. This suggests that free radicals might, therefore, be involved in the pathophysiological pathway and dysregulation of Ca(2+) homeostasis of muscular dystrophy.

Exercise-induced oxidative stress:myths, realities and physiological relevance

Although assays for the most popular markers of exercise-induced oxidative stress may experience methodological flaws, there is sufficient credible evidence to suggest that exercise is accompanied by an increased generation of free radicals, resulting in a measurable degree of oxidative modifications to various molecules. However, the mechanisms responsible are unclear. A common assumption that increased mitochondrial oxygen consumption leads per se to increased reactive oxygen species (ROS) production is not supported by in vitro and in vivo data. The specific contributions of other systems (xanthine oxidase, inflammation, haem protein auto-oxidation) are poorly characterised. It has been demonstrated that ROS have the capacity to contribute to the development of muscle fatigue in situ, but there is still a lack of convincing direct evidence that ROS impair exercise performance in vivo in humans. It remains unclear whether exercise-induced oxidative modifications have little significance, induce harmful oxidative damage, or are an integral part of redox regulation. It is clear that ROS play important roles in numerous physiological processes at rest; however, the detailed physiological functions of ROS in exercise remain to be elucidated.

Considerações críticas e metodológicas na determinação de espécies reativas de oxigênio e nitrogênio em células musculares durante contrações

Uma excessiva produção de espécies reativas pode ser prejudicial, superando a capacidade antioxidante e conduzindo a um desequilíbrio redox. A maioria das evidências da formação de espécies reativas em células musculares são "indiretas", ao passo que as evidências "diretas" ainda são escassas. As razões para este fato são múltiplas. Esta revisão sugere a utilização de sondas fluorescentes como DCFH (reativa ao H2O2), DAF-2 (reativa ao NO) e fluoróforo nitróxido (reativa ao O2·-) para determinação dessas espécies. Em adição, o presente estudo sugere que: 1) as medidas "indiretas" de ataque oxidativo em amostras sangüíneas não necessariamente refletem o ataque oxidativo ocorrido nas células musculares; 2) amostras de músculos isolados e homogenatos podem apresentar uma grande quantidade de tecido vascular contendo células endoteliais, hemácias e leucócitos, os quais podem gerar EROs e NO, dificultando a interpretação dos resultados; 3) as sondas fluorescentes DCFH-DA/DCFH, DAF-2-DA/DAF-2 e nitróxido são sensíveis na detecção do H2O2, NO e O2·- respectivamente, em tecido muscular durante contrações; 4) como método alternativo no estudo da produção de EROs e NO em músculo esquelético, culturas de células musculares e fibra muscular isolada são indicados como modelos experimentais.

N-acetylcysteine enhances muscle cysteine and glutathione availability and attenuates fatigue during prolonged exercise in endurance-trained individuals

The production of reactive oxygen species in skeletal muscle is linked with muscle fatigue. This study investigated the effects of the antioxidant compound N-acetylcysteine (NAC) on muscle cysteine, cystine, and glutathione and on time to fatigue during prolonged, submaximal exercise in endurance athletes. Eight men completed a double-blind, crossover study, receiving NAC or placebo before and during cycling for 45 min at 71% peak oxygen consumption (VO2 peak) and then to fatigue at 92% VO2 peak. NAC was intravenously infused at 125 mg.kg(-1).h(-1) for 15 min and then at 25 mg.kg(-1).h(-1) for 20 min before and throughout exercise. Arterialized venous blood was analyzed for NAC, glutathione status, and cysteine concentration. A vastus lateralis biopsy was taken preinfusion, at 45 min of exercise, and at fatigue and was analyzed for NAC, total glutathione (TGSH), reduced glutathione (GSH), cysteine, and cystine. Time to fatigue at 92% VO2 peak was reproducible in preliminary trials (coefficient of variation 5.6 +/- 0.6%) and with NAC was enhanced by 26.3 +/- 9.1% (NAC 6.4 +/- 0.6 min vs. Con 5.3 +/- 0.7 min; P <0.05). NAC increased muscle total and reduced NAC at both 45 min and fatigue (P <0.005). Muscle cysteine and cystine were unchanged during Con, but were elevated above preinfusion levels with NAC (P <0.001). Muscle TGSH (P <0.05) declined and muscle GSH tended to decline (P=0.06) during exercise. Both were greater with NAC (P <0.05). Neither exercise nor NAC affected whole blood TGSH. Whereas blood GSH was decreased and calculated oxidized glutathione increased with exercise (P <0.05), both were unaffected by NAC. In conclusion, NAC improved performance in well-trained individuals, with enhanced muscle cysteine and GSH availability a likely mechanism.

The Myofibrillar Complex and Fatigue: A Review

The basis for all biological movement is the conversion of chemical energy to mechanical energy by different classes of motor proteins. In skeletal muscle this motor protein is myosin II, a thick filament-based molecule that harnesses the free energy furnished by ATP hydrolysis to perform mechanical work against actin proteins of the thin filament. The cyclic attachment and detachment of myosin with actin that generates muscle force and shortening is Ca2+ regulated. Intense muscle activity may lead to metabolically induced inhibitions to the function of these myofibrillar proteins when Ca2+ regulation is normal, a phenomenon referred to as myofibrillar fatigue. Studies using single muscle fibers at room temperature or lower have shown that myosin motor function is inhibited by the accumulation of the ATP-hydrolysis products ADP, Pi, and H+ as well as by excess generation of reactive oxygen species (ROS). These metabolically induced impairments to myosin motor function reduce muscle work and power output by impairing maximal Ca2+ activated force, the Ca2+ sensitivity of force, and/or unloaded shortening velocity. Based on uncertainties about their inhibitory effect on muscle function at more physiological temperatures, the influence of ATP-hydrolysis product and ROS accumulation on myofibrillar protein function of human skeletal muscle remains to be clarified. Key words: actin, myosin, muscle contraction

Gold sodium thiomalate improves membrane potential impaired by high-frequency stimulation

Effects of gold sodium thiomalate (GSTM) on membrane potential and tetanus tension were examined to elucidate whether the gold compound improves mechanical and electrical muscle dysfunction produced by continuous repeated stimulation of frog skeletal muscles. Continuous stimulation (50 Hz for 2 min, 0.05 ms pulse duration) to the sartorius muscle depolarized the membrane, decreased action potential amplitude, and prolonged action potential duration. GSTM (0.1 mM), unlike thiomalic acid (0.1 mM), markedly decreased impairment of these electrical parameters produced during the stimulation period. In the presence of 500 units/mL of catalase, fatigue stimulation still lengthened by 1.5-fold the half-duration of the action potential after a 5-min rest. The prolongation was, however, smaller than that in controls (no catalase). Application of both catalase and GSTM led to no further changes in action potential compared with the application of catalase alone. GSTM did not affect resting tension of single toe muscle fibers though it suppressed the maximum tension after continuous stimulation. These findings suggest that GSTM can inhibit excitable dysfunction of skeletal muscles subjected to continuous stimulation and that such protective effects of GSTM may be partially mediated by H2O2.Key words: gold sodium thiomalate, catalase, continuous stimulation, resting and action potentials, force, frog skeletal muscle.

Dual action of hydrogen peroxide on synaptic transmission at the frog neuromuscular junction

There is evidence that reactive oxygen species (ROS) are produced and released during neuromuscular activity, but their role in synaptic transmission is not known. Using a two-electrode voltage-clamp technique, at frog neuromuscular junctions, the action H2O2 on end-plate currents (EPC) was studied to determine the targets for this membrane-permeable ROS. In curarized or cut muscles, micromolar concentrations of H2O2 increased the amplitude of EPCs. Higher (> 30 microM) doses inhibited EPCs and prolonged current decay. These effects were presynaptic since H2O2 did not change the amplitude or duration of miniature EPCs (although it reduced the rate of spontaneous release at high concentrations). Quantal analysis and deconvolution methods showed that facilitation of EPCs was due to increased quantal release, while depression was accompanied by temporal dispersion of evoked release. Extracellular recordings revealed prolonged presynaptic Ca2+ entry in the presence of high H2O2. Both low and high H2O2 increased presynaptic potentiation during high-frequency stimulation. Pro-oxidant Fe2+ did not affect facilitation by low doses of H2O2 but augmented the inhibition of EPCs by high H2O2, indicating involvement of hydroxyl radicals. High Mg2+ and the ROS scavenger N-acetylcysteine eliminated both the facilitatory and depressant effects of H2O2. The facilitatory effect of H2O2 was prevented by protein kinase C (PKC) inhibitors and 4beta-phorbol 12-myristate, 13-acetate (PMA), an activator of PKC. PKC inhibitors but not PMA also abolished the depressant effect of H2O2. Our data suggest complex presynaptic actions of H2O2, which could serve as a fast feedback modulator of intense neuromuscular transmission.

Excitation-contraction coupling and fatigue mechanisms in skeletal muscle: studies with mechanically skinned fibres

This review attempts to give an insight into the key aspects of excitation-contraction (E-C) coupling and fatigue in skeletal muscle, in particular summarizing the results and perspectives obtained from studies with mechanically skinned muscle fibres. These skinned fibre studies have provided many novel insights, such as the role of intracellular Mg2+ and ATP in the coupling mechanism, as well as how the accumulation of metabolic products, precipitation of inorganic phosphate in the sarcoplasmic reticulum (SR) and disruption of the coupling mechanism by high intracellular [Ca2+], may contribute to different types of muscle fatigue. The recent demonstration of action potential (AP)-induced Ca2+ release in skinned fibres [G.S. Posterino et al. (2000) J Physiol 527: 131-137] showed unequivocally that the normal E-C coupling mechanism [W. Melzer et al. (1995) Biochim Biophys Acta 1241: 59-116] was retained in this preparation and indicated the considerable potential of this technique. Among other things, it has been possible to show that AP activation of the voltage-sensors in the transverse-tubular (T-) system is normally sufficient to give maximal activation of the Ca2+ release channels (ryanodine receptors) in the SR and that increasing the sensitivity of the release channels to Ca2+, such as by oxidation or other means, does not increase the amount of Ca2+ released by an AP. In contrast, when the voltage-sensors are not fully activated, modulating the responsiveness of the Ca2+ release channels does affect the amount of Ca2+ release. It is suggested that some forms of muscle fatigue are caused by inadequate activation of the Ca2+ release channels due both to (a) inactivation or dysfunction of the voltage-sensors and (b) inhibitory effects on the release channels caused by local changes in the cytoplasmic environment (in particular by low [ATP] and raised concentrations of Mg2+, ATP metabolites and other factors) and by a decrease in the pool of releasable Ca2+ within the SR.

Effects of oxidation and cytosolic redox conditions on excitation-contraction coupling in rat skeletal muscle

In this study the effects of oxidation and reduction on various steps in the excitation-contraction (E-C) coupling sequence was examined in mammalian skeletal muscle. In mechanically skinned fast-twitch fibres, electric field stimulation was used to generate action potentials in the sealed transverse-tubular (T-) system, thereby eliciting twitch responses, which are a sensitive measure of Ca2+ release. Treatment of fibres with the oxidant H2O2 (200 microM and 10 mM) for 2-5 min markedly potentiated caffeine-induced Ca2+ release and the force response to partial depolarisation of the T-system (by solution substitution). Importantly, such H2O2 treatment had no effect at all on any aspect of the twitch response (peak amplitude, rate of rise, decay rate constant and half-width), except in cases where it interfered with the T-system potential or voltage-sensor activation, resulting in a reduction or abolition of the twitch response. Exposure to strong thiol reductants, dithiothreitol (DTT, 10 mM) and reduced glutathione (GSH, 5 mM), did not affect the twitch response over 5 min, nor did varying the glutathione ratio (reduced to oxidised glutathione) from the level present endogenously in the cytosol of a rested fibre (30:1) to the comparatively oxidised level of 3:1. In fibres that had been oxidised by H2O2 (10 mM) (or by 2,2'-dithiodipyridine, 100 microM), exposure to GSH (5 mM) caused potentiation of twitch force (by approximately 20 % for H2O2); this effect was due to the increase in the Ca2+ sensitivity of the contractile apparatus that occurs under such circumstances and was fully reversed by subsequent exposure to 10 mM DTT. We conclude that: (a) the redox potential across the sarcomplamsic reticulum has no noticeable direct effect on normal E-C coupling in mammalian skeletal muscle, (b) oxidising the Ca2+-release channels and greatly increasing their sensitivity to Ca2+-induced Ca2+ release does not alter the amount of Ca2+ released by an action potential and (c) oxidation potentiates twitches by a GSH-mediated increase in the Ca2+ sensitivity of the contractile apparatus.

Effects of oxidation and reduction on contractile function in skeletal muscle fibres of the rat

This study investigated the effects of the oxidants hydrogen peroxide (H(2)O(2)) and 2,2'-dithiodipyridine (DTDP), and reductants, glutathione (GSH) and dithiothreitol (DTT), on the properties of the contractile apparatus of rat fast- and slow-twitch skeletal muscle fibres, in order to assess how oxidation affects muscle function. Skinned muscle fibres were activated in heavily-buffered Ca(2+) solutions. The force-[Ca(2+)] relationship before and after various treatments was fitted by a Hill curve described by the maximum Ca(2+)-activated force, pCa(50) (-log(10)[Ca(2+)] giving half-maximum force) and n(H) (the Hill coefficient). Exposing freshly skinned fibres to strong reducing conditions (i.e. 10 mM DTT or 5 mM GSH) had little if any effect on Ca(2+) sensitivity (pCa(50) or n(H)). The effect of oxidants H(2)O(2) and DTDP depended on whether the fibre was relaxed (in pCa > 9) or activated during the exposure. In both fast- and slow-twitch fibres a 5 min exposure to 10 mM H(2)O(2) at pCa > 9 had no effect on pCa(50), causing only a reduction in n(H). In contrast, when fast-twitch fibres were activated in the presence of 10 mM H(2)O(2) (or 100 microM DTDP) there was a substantial increase in pCa(50) (by approximately 0.06 and 0.1, respectively), as well as larger decreases in n(H) than occurred in relaxed fibres, with all effects being reversed by DTT (10 mM, 10 min). In slow-twitch soleus fibres, the activation-dependent effect of DTDP was even greater (pCa(50) increased by ~0.35), and it was found that the rate of reversal in DTT was also increased by activation. A separate important phenomenon was that fast-twitch fibres that had been oxidised with H(2)O(2) or DTDP (while either relaxed or activated) showed a paradoxical increase in Ca(2+) sensitivity (~0.04 and 0.25 increase in pCa(50), respectively) when briefly exposed to the endogenous reductant GSH (5 mM, 2 min). This effect was reversed by DTT or longer (> 20 min) exposure to GSH, did not occur in slow-twitch soleus fibres, and may contribute to post-tetanic potentiation in fast-twitch muscle. Maximum force was not affected by any of the above treatments, whereas exposure to a high concentration of DTDP (1 mM) did greatly reduce force production. These findings reveal a number of novel and probably important effects of oxidation on the contractile apparatus in skeletal muscle fibres.

Endurance training adaptations modulate the redox-force relationship of rat isolated slow-twitch skeletal muscles

1. Studies have shown that, in isolated skeletal muscles, maximum isometric force production (Po) is dependent on muscle redox state. Endurance training increases the anti-oxidant capacity of skeletal muscles, a factor that could impact on the force-producing capacity following exogenous exposure to an oxidant. We tested the hypothesis that 12 weeks treadmill training would increase anti-oxidant capacity in rat skeletal muscles and alter their response to exogenous oxidant exposure. 2. At the conclusion of the 12 week endurance-training programme, soleus (slow-twitch) muscles from trained rats had greater citrate synthase (CS) and catalase (CAT) activity compared with soleus muscles from untrained rats (P < 0.05). In contrast, CAT activity of extensor digitorum longus (EDL; fast-twitch) muscles from trained rats was not different to EDL muscles of untrained rats. The CS activity was lower in EDL muscles from trained compared with untrained rats (P < 0.05). 3. Equilibration with exogenous hydrogen peroxide (H2O2, 5 mmol/L) increased the Po of soleus muscles from untrained rats for the duration of treatment (30 min), whereas the Po of EDL muscles was affected biphasically, with a small increase initially (after 5 min), followed by a more marked decrease in Po (after 30 min). The H2O2-induced increase in Po of soleus muscles from trained rats was less than that in untrained rats (P < 0.05), but no differences were observed in the Po of EDL muscles following training. 4. The results indicate that 12 weeks endurance running training conferred adaptations in soleus but not EDL muscles. These adaptations were associated with an attenuation of the oxidant-induced increase in Po of soleus muscles from trained compared with untrained rats. We conclude that endurance training-adapted soleus muscles have a slightly altered redox-force relationship.

H2O2 activates ryanodine receptor but has little effect on recovery of releasable Ca2+ content after fatigue

We studied whether hydrogen peroxide (H(2)O(2)) at </=10 microM activates the ryanodine receptor and decreases releasable Ca(2+) content in the sarcoplasmic reticulum after fatigue. Exposure of rabbit or frog skeletal muscle ryanodine receptors to 10 microM H(2)O(2) enhanced channel activity in lipid bilayers when the redox potential was defined at cis = -220 mV and trans = -180 mV. Channel activation by 10 microM H(2)O(2) was also observed when cis potential was set at -220 mV without defining trans potential, but the effect was less. Reduction of trans redox potential from -180 to -220 mV did not alter channel activity. H(2)O(2) at 500 microM failed to activate the channel when the redox potential was not controlled. Stimulation of the frog muscle fiber for 2 min (50 Hz, a duty cycle of 200 ms/s) decreased tetanus tension by approximately 50%. After 1 min, tetanus recovered rapidly to approximately 70% of control and thereafter slowly approached the control level. Amplitudes of caffeine- and 4-chloro-m-cresol-induced contractures were decreased after a 60-min rest. The decrease is not enhanced by exposure to 10 microM H(2)O(2). These results suggest that H(2)O(2) markedly activates the ryanodine receptor under the redox control in vitro, but externally applied H(2)O(2) may not play an important role in the postfatigue recovery process.

Phenylethylamine induces an increase in cytosolic Ca2+ in yeast

Beta-phenylethylamine (PEA) induced an increase in cytosolic free calcium ion concentration ([Ca2+]c) in Saccharomyces cerevisiae cells monitored with transgenic aequorin, a Ca2+-dependent photoprotein. The PEA-induced [Ca2+]c increase was dependent on the concentrations of PEA applied, and the Ca2+ mostly originated from an extracellular source. Preceding the Ca2+ influx, H2O2 was generated in the cells by the addition of PEA. Externally added H2O2 also induced a [Ca2+]c increase. These results suggest that PEA induces the [Ca2+]c increase via H2O2 generation. The PEA-induced [Ca2+]c increase occurred in the mid1 mutant with a slightly smaller peak than in the wild-type strain, indicating that Mid1, a stretch-activated nonselective cation channel, may not be mainly involved in the PEA-induced Ca2+ influx. When PEA was applied, the MATa mid1 mutant was rescued from alpha-factor-induced death in a Ca2+-limited medium, suggesting that the PEA-induced [Ca2+]c increase can reinforce calcium signaling in the mating pheromone response pathway.

Redox states of type 1 ryanodine receptor alter Ca(2+) release channel response to modulators

The type 1 ryanodine receptor (RyR1) from rabbit skeletal muscle displayed two distinct degrees of response to cytoplasmic Ca(2+) [high- and low-open probability (P(o)) channels]. Here, we examined the effects of adenine nucleotides and caffeine on these channels and their modulations by sulfhydryl reagents. High-P(o) channels showed biphasic Ca(2+) dependence and were activated by adenine nucleotides and caffeine. Unexpectedly, low-P(o) channels did not respond to either modulator. The addition of a reducing reagent, dithiothreitol, to the cis side converted the high-P(o) channel to a state similar to that of the low-P(o) channel. Treatment with p-chloromercuriphenylsulfonic acid (pCMPS) transformed low-P(o) channels to a high-P(o) channel-like state with stimulation by beta,gamma-methylene-ATP and caffeine. In experiments under redox control using glutathione buffers, shift of the cis potential toward the oxidative state activated the low-P(o) channel, similar to that of the high-P(o) or the pCMPS-treated channel, whereas reductive changes inactivated the high-P(o) channel. Changes in trans redox potential, in contrast, did not affect channel activity of either channel. In all experiments, channels with higher P(o) were stimulated to a great extent by modulators, but ones with lower P(o) were unresponsive. These results suggest that redox states of critical sulfhydryls located on the cytoplasmic side of the RyR1 may alter both gating properties of the channel and responsiveness to channel modulators.

Hydrogen peroxide increases depolarization-induced contraction of mechanically skinned slow twitch fibres from rat skeletal muscles

The effect of exogenous hydrogen peroxide (H(2)O(2)) on excitation-contraction (E-C) coupling and sarcoplasmic reticulum (SR) function was compared in mechanically skinned slow twitch fibres (prepared from the soleus muscles) and fast twitch fibres (prepared from the extensor digitorum longus; EDL muscles) of adult rats. Equilibration (5 min) with 1 mM H(2)O(2) diminished the ability of the Ca(2+)-depleted SR to reload Ca(2+) in both slow (P < 0.01) and fast twitch fibres (P < 0.05) compared to control. Under conditions when all Ca(2+) uptake was prevented, 1 mM H(2)O(2) increased SR Ca(2+) "leak" in fast twitch fibres by 24 +/- 5 % (P < 0.05), but leak was not altered in slow twitch fibres. Treatment with 1 mM H(2)O(2) also increased the peak force of low [caffeine] contracture by approximately 45% in both fibre types compared to control (P < 0.01), which could be partly reversed following treatment with 10 mM dithiothreitol (DTT). The changes in SR function caused by 1 mM H(2)O(2) were associated with an approximately 65% increase in the peak height of depolarization-induced contractile response (DICR) in slow twitch fibres, compared to control (no H(2)O(2); P < 0.05). In contrast, peak contractile force of fast twitch fibres was not altered by 1 mM H(2)O(2) treatment. Equilibration with 5 mM H(2)O(2) induced a spontaneous force response in both slow and fast twitch fibres, which could be partly reversed by 2 min treatment with 10 mM DTT. Peak DICR was also increased approximately 40% by 5 mM H(2)O(2) in slow twitch fibres compared to control (no H(2)O(2); P < 0.05). Our results indicate that exogenous H(2)O(2) increases depolarization-induced contraction of mechanically skinned slow but not fast twitch fibres. The increase in depolarization-induced contraction in slow twitch fibres might be mediated by an increased SR Ca(2+) release during contraction and/or an increase in Ca(2+) sensitivity.

Ca(2+)-activated K+ channel inhibition by reactive oxygen species

We studied the effect of H(2)O(2) on the gating behavior of large-conductance Ca(2+)-sensitive voltage-dependent K(+) (K(V,Ca)) channels. We recorded potassium currents from single skeletal muscle channels incorporated into bilayers or using macropatches of Xenopus laevis oocytes membranes expressing the human Slowpoke (hSlo) alpha-subunit. Exposure of the intracellular side of K(V,Ca) channels to H(2)O(2) (4-23 mM) leads to a time-dependent decrease of the open probability (P(o)) without affecting the unitary conductance. H(2)O(2) did not affect channel activity when added to the extracellular side. These results provide evidence for an intracellular site(s) of H(2)O(2) action. Desferrioxamine (60 microM) and cysteine (1 mM) completely inhibited the effect of H(2)O(2), indicating that the decrease in P(o) was mediated by hydroxyl radicals. The reducing agent dithiothreitol (DTT) could not fully reverse the effect of H(2)O(2). However, DTT did completely reverse the decrease in P(o) induced by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid). The incomplete recovery of K(V,Ca) channel activity promoted by DTT suggests that H(2)O(2) treatment must be modifying other amino acid residues, e.g., as methionine or tryptophan, besides cysteine. Noise analysis of macroscopic currents in Xenopus oocytes expressing hSlo channels showed that H(2)O(2) induced a decrease in current mediated by a decrease both in the number of active channels and P(o).

Hydrogen peroxide modulates Ca2+-activation of single permeabilized fibres from fast- and slow-twitch skeletal muscles of rats

We examined the effects of redox modulation on single membrane-permeabilized fibre segments from the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of adult rats to determine whether the contractile apparatus was the redox target responsible for the increased contractility of muscles exposed to low concentrations of H2O2. The effects of H2O2 on maximum Ca2+-activated force were dose-dependent with 30 min exposure to 5 mM H2O2 causing a progressive decrease by 22+/-3 and 13+/-2% in soleus and EDL permeabilized muscle fibres, respectively. Lower concentrations of exogenous H2O2 (100 microM and 1 mM) had no effect on maximum Ca2+-activated force. Subsequent exposure to the reductant dithiothreitol (DTT, 10 mM, 10 min) fully reversed the H2O2-induced depression of force in EDL, but not in soleus muscle fibres. Incubation with DTT alone for 10 min did not alter Ca2+-activated force in either soleus or EDL muscle fibres. The sensitivity of the contractile filaments to Ca2+ (pCa50) was not altered by exposure to any concentration of exogenous H2O2. However, all concentrations of H2O2 diminished the Hill coefficient in permeabilized fibres from the EDL muscle, indicating that the cooperativity of Ca2+ binding to troponin is altered. H2O2 (5 mM) did not affect rigor force, which indicates that the number of crossbridges participating in contraction was not reduced. In conclusion, H2O2 may reduce the maximum Ca2+ activated force production in skinned muscle fibres by decreasing the force per crossbridge.

Properties and modulation of alpha human atrial natriuretic peptide (α-hANP)-formed ion channels

Using the lipid bilayer technique we have optimized recording conditions and confirmed that alpha human atrial natriuretic peptide [α-hANP(1–28)] forms single ion channels. The single channel currents recorded in 250/50 mM KCl cis/trans chambers show that the ANP-formed channels were heterogeneous, and differed in their conductance, kinetic, and pharmacological properties. The ANP-formed single channels were grouped as: (i) H2O2- and Ba2+-sensitive channel with fast kinetics; the nonlinear current-voltage (I-V) relationship of this channel had a reversal potential (Erev) of –28.2 mV, which is close to the equilibrium potential for K+ (EK = –35 mV) and a maximal slope conductance (gmax) of 68 pS at positive potentials. Sequential ionic substitution (KCl, K gluconate and choline Cl) of the cis solution suggests that the current was carried by cations. The fast channel had three modes (spike mode, burst mode, and open mode) that differed in their kinetics but not in their conductance properties. (ii) A large conductance channel possessing several subconductance levels that showed time-dependent inactivation at positive and negative membrane potentials (Vm). The inactivation ratio of the current at the end of the voltage step (Iss) to the initial current (Ii) activated immediately after the voltage step, (Iss/Ii), was voltage dependent and described by a bell-shaped curve. The maximal current-voltage (I-V) relationship of this channel, which had an Erev of +17.2 mV, was nonlinear and the value of gmax was 273 pS at negative voltages. (iii) A transiently-activated channel: the nonlinear I-V relationship of this channel had an Erev of –29.8 mV and the value of gmax was 160 pS at positive voltages. We propose that the voltage-dependence of the ionic currents and the kinetic parameters of these channel types indicate that if they were formed in vivo and activated by cytosolic factors they could change the membrane potential and the electrolyte homeostasis of the cell.Key words: natriuretic peptides, channel forming peptides, heterogeneous channels, signal transduction.

Intracellular calcium dynamics--sparks of insight

Calcium ions are of key importance in a large number of cellular functions. In the past decade a large variety of cells have been found to show localized increases in the intracellular calcium concentration named calcium sparks. In this brief review, the methodology of detecting calcium sparks by confocal microscopy is summarized. Some of the properties of calcium sparks in muscle (cardiac, skeletal and smooth muscles), neurons, nerve terminals and oocytes aredescribed. Speculations are put forward regarding their possible role in microcontrol of cell function.

Redox modulation of maximum force production of fast-and slow-twitch skeletal muscles of rats and mice

We used intact fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles from rats and mice to test the hypothesis that exogenous application of an oxidant would increase maximum isometric force production (P(o)) of slow-twitch muscles to a greater extent than fast-twitch skeletal muscles. Exposure to an oxidant, hydrogen peroxide (H(2)O(2); 100 microM to 5 mM, 30 min), affected P(o) of rat muscles in a time- and dose-dependent manner. P(o) of rat soleus muscles was increased by 8 +/- 1 (SE) and 14 +/- 1% (P < 0.01) after incubation with 1 and 5 mM H(2)O(2), respectively, whereas in mouse soleus muscles P(o) was only increased after incubation with 500 microM H(2)O(2). P(o) of rat EDL muscles was affected by H(2)O(2) biphasically; initially there was a small increase (3 +/- 1%), but then P(o) diminished significantly after 30 min of treatment. In contrast, all concentrations of H(2)O(2) tested decreased P(o) of mouse EDL muscles. A reductant, dithiothreitol (DTT; rat = 10 mM, mouse = 1 mM), was added to quench H(2)O(2), and it reversed the potentiation in P(o) in rat soleus but not in rat EDL muscles or in any H(2)O(2)-treated mouse muscles. After prolonged equilibration (30 min) with 5 mM H(2)O(2) without prior activation, P(o) was potentiated in rat soleus but not EDL muscles, demonstrating that the effect of oxidation in the soleus muscles was also dependent on the activation history of the muscle. The results of these experiments demonstrate that P(o) of both slow- and fast-twitch muscles from rats and mice is modified by redox modulation, indicating that maximum P(o) of mammalian skeletal muscles is dependent on oxidation.

Contractile response of skeletal muscle to low peroxide concentrations: myofibrillar calcium sensitivity as a likely target for redox-modulation

Endogenous peroxides and related reactive oxygen species may influence various steps in the contractile process. Single mouse skeletal muscle fibers were used to study the effects of hydrogen peroxide (H2O2) and t-butyl hydroperoxide (t-BOOH) on force and myoplasmic Ca2+ concentration ([Ca2+]i). Both peroxides (1010 to 105 M) decreased tetanic [Ca2+]i and increased force during submaximal tetani. Catalase (1 kU/ml) blocked the effect of H2O2, but not of t-BOOH. The decrease in tetanic [Ca2+]i was constant, while the effect on force was biphasic: A transitory increase was followed by a steady decline to the initial level. Myofibrillar Ca2+ sensitivity remained increased during incubation with either peroxide. Only the highest peroxide concentration (10 mM) increased resting [Ca2+]i and slowed the return of [Ca2+]i to its resting level after a contraction, evidence of impaired sarcoplasmic reticulum Ca2+ re-uptake. The peroxides increased maximal force production and the rate of force redevelopment, and decreased maximum shortening velocity. N-ethylmaleimide (25 mM, thiol-alkylating agent) prevented the response to 1 mM H2O2. These results show that myofibrillar Ca2+ sensitivity and cross-bridge kinetics are influenced by H2O2 and t-BOOH concentrations that approach those found physiologically, and these findings indicate a role for endogenous oxidants in the regulation of skeletal muscle function.

H(2)O(2) and ethanol act synergistically to gate ryanodine receptor/calcium-release channel

We examined the effect of low concentrations of H(2)O(2) on the Ca(2+)-release channel/ryanodine receptor (RyR) to determine if H(2)O(2) plays a physiological role in skeletal muscle function. Sarcoplasmic reticulum vesicles from frog skeletal muscle and type 1 RyRs (RyR1) purified from rabbit skeletal muscle were incorporated into lipid bilayers. Channel activity of the frog RyR was not affected by application of 4.4 mM (0.02%) ethanol. Open probability (P(o)) of such ethanol-treated RyR channels was markedly increased on subsequent addition of 10 microM H(2)O(2). Increase of H(2)O(2) to 100 microM caused a further increase in channel activity. Application of 4.4 mM ethanol to 10 microM H(2)O(2)-treated RyRs activated channel activity. Exposure to 10 or 100 microM H(2)O(2) alone, however, failed to increase P(o). Synergistic action of ethanol and H(2)O(2) was also observed on the purified RyR1 channel, which was free from FK506 binding protein (FKBP12). H(2)O(2) at 100-500 microM had no effect on purified channel activity. Application of FKBP12 to the purified RyR1 drastically decreased channel activity but did not alter the effects of ethanol and H(2)O(2). These results suggest that H(2)O(2) may play a pathophysiological, but probably not a physiological, role by directly acting on skeletal muscle RyRs in the presence of ethanol.

Hyperbaric oxygen improves contractile function of regenerating rat skeletal muscle after myotoxic injury

There is growing interest in hyperbaric oxygen (HBO) as an adjunctive treatment for muscle injuries. This experiment tested the hypothesis that periodic inhalation of HBO hastens the functional recovery and myofiber regeneration of skeletal muscle after myotoxic injury. Injection of the rat extensor digitorum longus (EDL) muscle with bupivacaine hydrochloride causes muscle degeneration. After injection, rats breathed air with or without periodic HBO [100% O(2) at either 2 or 3 atmospheres absolute (ATA)]. In vitro maximum isometric tetanic force of injured EDL muscles and regenerating myofiber size were unchanged between 2 ATA HBO-treated and untreated rats at 14 days postinjury but were approximately 11 and approximately 19% greater, respectively, in HBO-treated rats at 25 days postinjury. Maximum isometric tetanic force of injured muscles was approximately 27% greater, and regenerating myofibers were approximately 41% larger, in 3 ATA HBO-treated rats compared with untreated rats at 14 days postinjury. These findings demonstrate that periodic HBO inhalation increases maximum force-producing capacity and enhances myofiber growth in regenerating skeletal muscle after myotoxic injury with greater effect at 3 than at 2 ATA.

Hydrogen peroxide decelerates recovery of action potential after high-frequency fatigue in skeletal muscle

Effects of reactive oxygen species (ROS), especially hydrogen peroxide (H(2)O(2)), on recovery of action potential by resting for 30 min after high-frequency fatigue were studied using frog skeletal muscle fibers. After stimulation at a frequency of 50 HZ for 2 min, the action potential amplitude was decreased by 14.5 mV from controls, and resting membrane was depolarized by 15.4 mV. Action potential duration was also prolonged by high-frequency stimulation (1.5 ms in controls to 2.6 ms). The high-frequency stimulation used here caused no muscle damage. The action potential was partially improved after a 30-min rest. Addition of catalase at 500 units/ml or H(2)O(2) at 0.5 mM to sartorius muscle did not alter any of the parameters of the action potential after high-frequency stimulation. Treatment with catalase accelerated post-fatigue recovery of the action potential. Application of H(2)O(2) delayed post-fatigue recovery of resting and action potentials. When added to detubulated toe muscle fibers, catalase no longer improved the attenuation of action potential induced by high-frequency stimulation, even after a 30-min rest. These findings suggest that removal of H(2)O(2) from transverse tubules is effective for post-fatigue recovery of action potential in skeletal muscle.

Interaction of nitric oxide and reactive oxygen species on rat diaphragm contractility

Growing evidence indicates that reactive oxygen species (ROS) as well as nitric oxide (NO) have a profound influence on contractile function of skeletal muscle possibly through modulation of excitation-contraction coupling. We hypothesized that if NO and xanthine oxidase (XO) interact at key sites in excitation-contraction coupling, the effects of XO with nitric oxide synthase (NOS) inhibitors and NO donors on contractile function of the unfatigued diaphragm would not be additive. Diaphragm fibre bundles were extracted from 4-month Fischer-344 rats and placed in Krebs solution bubbled with 95% O2, 5% CO2. Baseline twitch tension, tension at 20 Hz (low-frequency), and maximal tetanic tension (Po) at 120 Hz were then measured (PRE). In Experiment 1 diaphragm fibre bundles were exposed to Krebs with 200 microM hypoxanthine as a control (CON); 0.02 U mL-1 XO + 200 microM hypoxanthine; 1 mM of the NOS inhibitor N-nitro-L-arginine (L-NNA) or L-NNA + XO. Five minutes were allowed for equilibration, and a second set of contractile measures was taken (POST). In Experiment 2 we exposed diaphragm fibre bundles to one of the following four solutions: CON, XO, 100 microM of the NO donor sodium nitroprusside (SNP) and XO + SNP, and evaluated contractile function as described above. In Experiment 3 we tested to determine if peroxynitrite production from the reaction of superoxide anion and NO affected the above results for SNP using 30 microM ebselen as a peroxynitrite quencher. Xanthine oxidase resulted in a significant potentiation of diaphragm twitch tension and tension at 20 Hz (+29%) without affecting Po. L-NNA also significantly increased 20 Hz tension but did not alter Po. However, the combination of XO + L-NNA did not further increase low-frequency contractility. Sodium nitroprusside alone did not affect diaphragm contractility, but did attenuate XO-induced potentiation in the XO + SNP group. Ebselen did not alter the impact of SNP on XO in the diaphragm. These data support the hypothesis that XO and NO interact or compete at similar sites of action that modulate contractility of the unfatigued diaphragm.

Induction of skeletal muscle contracture and calcium release from isolated sarcoplasmic reticulum vesicles by sanguinarine

The benzophenanthrine alkaloid, sanguinarine, was studied for its effects on isolated mouse phrenic-nerve diaphragm preparations. Sanguinarine induced direct, dose-dependent effects on muscle contractility. Sanguinarine-induced contracture was partially inhibited when the extracellular Ca(2+) was removed or when the diaphragm was pretreated with nifedipine. Depletion of sarcoplasmic reticulum (SR) internal calcium stores completely blocked the contracture. Sanguinarine induced Ca(2+) release from the actively loaded SR vesicles was blocked by ruthenium red and dithiothreitol (DTT), consistent with the ryanodine receptor (RyR) as the site of sanguinarine action. Sanguinarine altered [(3)H]-ryanodine binding to the RyR of isolated SR vesicles, potentiating [(3)H]-ryanodine binding at lower concentrations and inhibiting binding at higher concentrations. All of these effects were reversed by DTT, suggesting that sanguinarine-induced Ca(2+) release from SR occurs through oxidation of critical SH groups of the RyR SR calcium release channel.

How many cysteine residues regulate ryanodine receptor channel activity?

RyRs contain 80-100 cysteine residues per subunit, of which approximately 25% are free for covalent modification, while the remainder are either modified or form intraprotein disulfides. Oxidizing and nitrosylating reagents have several effects on single RyR channel activity, which depend on the type of modifying reagent, the isoform of the RyR, and ligands bound to the channel. We present evidence here for four major classes of functional cysteine residues associated with RyR channels, i.e., two classes with free -SH groups that either activate or inhibit channels when covalently modified and two classes, with endogenous modification, that either inhibit or activate. Single-channel characteristics provide evidence for four discrete responses within the first activating class, two responses within the second inhibiting class and two types of response within the third endogenously modified class. All but one of these changes in channel properties depend on residues located on the cytoplasmic or membrane-associated domains of the RyR; the remaining response is confined to the luminal domain. If it is assumed that each type of response depends on a separate subclass of cysteine residue and that each subclass contains a minimum of one cysteine per subunit, our results suggest that there are at least nine cysteine residues per subunit with functional connections to the gating mechanism of RyR channels. These cysteine residues may be selectively modified under physiological and pathological conditions to regulate Ca2+ release from the sarcoplasmic reticulum and contraction.

Sarcoplasmic reticulum Ca(2+) release and muscle fatigue

Efforts to examine the relevant mechanisms involved in skeletal muscle fatigue are focusing on Ca(2+) handling within the active muscle cell. It has been demonstrated time and again that reductions in sarcoplasmic reticulum (SR) Ca(2+) release resulting from increased or intense muscle contraction will compromise tension development. This review seeks to accomplish two related goals: 1) to provide an up-to-date molecular understanding of the Ca(2+)-release process, with considerable attention devoted to the SR Ca(2+) channel, including its associated proteins and their regulation by endogenous compounds; and 2) to examine several putative mechanisms by which cellular alterations resulting from intense and/or prolonged contractile activity will modify SR Ca(2+) release. The mechanisms that are likely candidates to explain the reductions in SR Ca(2+) channel function following contractile activity include elevated Ca(2+) concentrations, alterations in metabolic homeostasis within the "microcompartmentalized" triadic space, and modification by reactive oxygen species.

Different effects of two gold compounds on muscle contraction, membrane potential and ryanodine receptor

Effects of gold sodium thiomalate and NaAuCl4 on skeletal muscle function were studied using intact single fibres of frog skeletal muscle and fragmented sarcoplasmic reticulum prepared from frog and rabbit skeletal muscles. Gold sodium thiomalate at a concentration of 500 microM decreased tension amplitude by 27% and resting membrane potential by 5.3% after 30 and 22 min, respectively. The duration of tetanus tension was markedly shortened by 500 microM gold sodium thiomalate. When 10 microM NaAuCl4 was applied to gold sodium thiomalate-pretreated fibres, the fibres lost the ability to contract upon electrical stimulation, similar to the effects of 10 microM NaAuCl4 alone. In the presence of thiomalic acid, on the other hand, NaAuCl4 did not completely block tetanus tension even at 50 microM. Thiomalic acid also inhibited NaAuCl4-induced membrane depolarization. These findings suggest that thiomalate masks the effects of gold ion on muscle function. When sarcoplasmic reticulum vesicles were incorporated into lipid bilayers, exposure of the cis side of the Ca2+-release channel to 100 microM gold sodium thiomalate rapidly increased the open probability of the channel 3.3-fold, from 0.032 in controls to 0.105, with an increase in number of open events and a decrease in mean closed time. The ability of NaAuCl4 to activate the Ca2+-release channel was much stronger than that of gold sodium thiomalate. Only 1 microM NaAuCl4 was enough to activate the channel and this gold was effective from either side of the channel. These results suggest that gold sodium thiomalate could be used as an antirheumatic drug without considering severe side-effects on skeletal muscle. Coexistent thiomalate probably contributes to protection of muscle function from side-effects of gold ion.

Hydrogen peroxide relaxes porcine coronary arteries by stimulating BKCa channel activity

It has been known for a number of years that neutrophils and macrophages secrete H2O2 while fighting disease, and the levels obtained within the vasculature under these conditions can reach several hundred micromolar. Because the effect of H2O2 on vascular smooth muscle is not fully understood, the present study examined the cellular effects of H2O2 on coronary arteries. Under normal ionic conditions, H2O2 relaxed arteries that were precontracted with prostaglandin F2alpha or histamine (EC50 = 252 +/- 22 microM). The effect of H2O2 was concentration dependent and endothelium independent. In contrast, H2O2 did not relax arteries contracted with 80 mM KCl, suggesting involvement of K+ channels. Single-channel patch-clamp recordings revealed that H2O2 increased the activity of the large-conductance (119 pS), Ca2+- and voltage-activated K+ (BKCa) channel. This response was mimicked by arachidonic acid and inhibited by eicosatriynoic acid, a lipoxygenase blocker, suggesting involvement of leukotrienes. Further studies on intact arteries demonstrated that eicosatriynoic acid not only blocked the vasodilatory response to H2O2 but unmasked a vasoconstrictor effect that was reversed by blocking cyclooxygenase activity with indomethacin. These findings identify a novel effector molecule, the BKCa channel, which appears to mediate the vasodilatory effect of H2O2, and suggest that a single signaling pathway, arachidonic acid metabolism, can mediate the vasodilatory and vasoconstrictor effects of H2O2 and possibly other reactive oxygen species.

Interaction of reactive oxygen species with ion transport mechanisms

The use of electrophysiological and molecular biology techniques has shed light on reactive oxygen species (ROS)-induced impairment of surface and internal membranes that control cellular signaling. These deleterious effects of ROS are due to their interaction with various ion transport proteins underlying the transmembrane signal transduction, namely, 1) ion channels, such as Ca2+ channels (including voltage-sensitive L-type Ca2+ currents, dihydropyridine receptor voltage sensors, ryanodine receptor Ca2+-release channels, and D-myo-inositol 1,4,5-trisphosphate receptor Ca2+-release channels), K+ channels (such as Ca2+-activated K+ channels, inward and outward K+ currents, and ATP-sensitive K+ channels), Na+ channels, and Cl- channels; 2) ion pumps, such as sarcoplasmic reticulum and sarcolemmal Ca2+ pumps, Na+-K+-ATPase (Na+ pump), and H+-ATPase (H+ pump); 3) ion exchangers such as the Na+/Ca2+ exchanger and Na+/H+ exchanger; and 4) ion cotransporters such as K+-Cl-, Na+-K+-Cl-, and Pi-Na+ cotransporters. The mechanism of ROS-induced modifications in ion transport pathways involves 1) oxidation of sulfhydryl groups located on the ion transport proteins, 2) peroxidation of membrane phospholipids, and 3) inhibition of membrane-bound regulatory enzymes and modification of the oxidative phosphorylation and ATP levels. Alterations in the ion transport mechanisms lead to changes in a second messenger system, primarily Ca2+ homeostasis, which further augment the abnormal electrical activity and distortion of signal transduction, causing cell dysfunction, which underlies pathological conditions.

Effect of hydrogen peroxide and dithiothreitol on contractile function of single skeletal muscle fibres from the mouse

1. We used intact single fibres from a mouse foot muscle to study the role of oxidation-reduction in the modulation of contractile function. 2. The oxidant hydrogen peroxide (H2O2, 100-300 microM) for brief periods did not change myoplasmic Ca2+ concentrations ([Ca2+]i) during submaximal tetani. However, force increased by 27 % during the same contractions. 3. The effects of H2O2 were time dependent. Prolonged exposures resulted in increased resting and tetanic [Ca2+]i, while force was significantly diminished. The force decline was mainly due to reduced myofibrillar Ca2+ sensitivity. There was also evidence of altered sarcoplasmic reticulum (SR) function: passive Ca2+ leak was increased and Ca2+ uptake was decreased. 4. The reductant dithiothreitol (DTT, 0.5-1 mM) did not change tetanic [Ca2+]i, but decreased force by over 40 %. This was completely reversed by subsequent incubations with H2O2. The force decline induced by prolonged exposure to H2O2 was reversed by subsequent exposure to DTT. 5. These results show that the elements of the contractile machinery are differentially responsive to changes in the oxidation-reduction balance of the muscle fibres. Myofibrillar Ca2+ sensitivity appears to be especially susceptible, while the SR functions (Ca2+ leak and uptake) are less so.

Sulfhydryls associated with H2O2-induced channel activation are on luminal side of ryanodine receptors

The mechanism underlying H2O2-induced activation of frog skeletal muscle ryanodine receptors was studied using skinned fibers and by measuring single Ca(2+)-release channel current. Exposure of skinned fibers to 3-10 mM H2O2 elicited spontaneous contractures. H2O2 at 1 mM potentiated caffeine contracture. When the Ca(2+)-release channels were incorporated into lipid bilayers, open probability (Po) and open time constants were increased on intraluminal addition of H2O2 in the presence of cis catalase, but unitary conductance and reversal potential were not affected. Exposure to cis H2O2 at 1.5 mM failed to activate the channel in the presence of trans catalase. Application of 1.5 mM H2O2 to the trans side of a channel that had been oxidized by cis p-chloromercuriphenylsulfonic acid (pCMPS; 50 microM) still led to an increase in Po, comparable to that elicited by trans 1.5 mM H2O2 without pCMPS. Addition of cis pCMPS to channels that had been treated with or without trans H2O2 rapidly resulted in high Po followed by closure of the channel. These results suggest that oxidation of luminal sulfhydryls in the Ca(2+)-release channel may contribute to H2O2-induced channel activation and muscle contracture.

Emodin-induced muscle contraction of mouse diaphragm and the involvement of Ca2+ influx and Ca2+ release from sarcoplasmic reticulum

1. The effects on skeletal muscle of emodin, an anthraquinone, were studied in the mouse isolated diaphragm and sarcoplasmic reticulum (SR) membrane vesicles. 2. Emodin dose-dependently caused muscle contracture, simultaneously depressing twitch amplitude. Neither tubocurarine nor tetrodotoxin blocked the contraction suggesting that it was caused myogenically. 3. The contraction induced by emodin persisted in a Ca2+ free medium with a slight reduction in the maximal force of contraction. The contraction induced by emodin in the Ca2+ free medium was completely blocked when the internal Ca2+ pool of the muscle was depleted by ryanodine. These data suggest that the contraction caused by emodin is due to the release of Ca2+ from the intracellular ryanodine-sensitive pool. 4. In contrast to the effect seen in the Ca2+ free medium, emodin induced a small but consisted contraction in the ryanodine-treated muscle in Krebs medium. The contraction was blocked in the presence of dithiothreitol and was partially blocked by nifedipine, suggesting that oxidation of a sulphhydryl group on the external site of dihydropyridine receptor is involved. 5. Emodin dose-dependently increased Ca2+ release from actively loaded SR vesicles and this effect was blocked by ruthenium red, a specific Ca2+ release channel blocker, and the thiol reducing agent, DTT, suggesting that emodin induced Ca2+ release through oxidation of the critical SH of the ryanodine receptor. 6. [3H]-ryanodine binding was dose-dependently potentiated by emodin in a biphasic manner. The degree of potentiation of ryanodine binding by emodin increased dose-dependently at concentrations up to 10 microM but decreased at higher concentrations of 10-100 microM. 7. These data suggest that muscle contraction induced by emodin is due to Ca2+ release from the SR of skeletal muscle, as a result of oxidation of the ryanodine receptor and influx of extracellular Ca2+ through voltage-dependent Ca2+ channels of the plasma membrane.

Effect of reactive oxygen species on K+ contractures in the rat diaphragm

Reactive oxygen species (ROS) are postulated to alter low-frequency contractility of the unfatigued and fatigued diaphragm. It has been proposed that ROS affect contractility through changes in membrane excitability and excitation-contraction coupling. If this hypothesis is true, then ROS should alter depolarization-dependent K+ contractures. Xanthine oxidase (0.01 U/ml) + hypoxanthine (1 mM) were used as a source of superoxide anion eliciting oxidative stress on diaphragm fiber bundles in vitro. Diaphragm fiber bundles from 4-mo-old Fischer 344 rats were extracted and immediately placed in Krebs solution bubbled with 95% O2-5% CO2. After 10 min of equilibration, a K+ contracture (Pre; 135 mM KCl) was induced. Fiber bundles were assigned to the following treatment groups: normal Krebs-Ringer (KR; Con) and the xanthine oxidase system (XO) in KR solution. After 15 min of treatment exposure, a second (Post) K+ contracture was elicited. Mean time-to-peak tension for contractures was significantly decreased in Post vs. Pre (16.0 +/- 0.7 vs. 19.8 +/- 1.0 s) with XO; no change was noted with Con. Furthermore, peak contracture tension was significantly higher (31.5%) in the XO group Post compared with Pre; again, no significant change was found with KR. The relaxation phase was also altered with XO but not with KR. Additional experiments were conducted with application of 1 mM hypoxanthine, with results similar to the Con group. We conclude that the application of ROS altered the dynamics of K+ contractures in the rat diaphragm, indicating changes in voltage-dependent excitation-contraction coupling.

Cation pumps in skeletal muscle: potential role in muscle fatigue

Two membrane bound pumps in skeletal muscle, the sarcolemma Na+-K+ adenosine triphosphatase (ATPase) and the sarcoplasmic reticulum Ca2+-ATPase, provide for the maintenance of transmembrane ionic gradients necessary for excitation and activation of the myofibrillar apparatus. The rate at which the pumps are capable of establishing ionic homeostasis depends on the maximal activity of the enzyme and the potential of the metabolic pathways for supplying adenosine triphosphate (ATP). The activity of the Ca2+-ATPase appears to be expressed in a fibre type specific manner with both the amount of the enzyme and the isoform type related to the speed of contraction. In contrast, only minimal differences exist between slow-twitch and fast-twitch fibres in Na+-K+ ATPase activity. Evidence is accumulating that both active transport of Na+ and K+ across the sarcolemma and Ca2+-uptake by the sarcoplasmic reticulum may be impaired in vivo in a task specific manner resulting in loss of contractile function. In contrast to the Ca2+-ATPase, the Na+-K+ ATPase can be rapidly upregulated soon after the onset of a sustained pattern of activity. Similar programmes of activity result in a downregulation of Ca2+-ATPase but at a much later time point. The manner in which the metabolic pathways reorganize following chronic activity to meet the changes in ATP demand by the cation pumps and the degree to which these adaptations are compartmentalized is uncertain.

Signal transduction: activation of the guanylate cyclase-cyclic guanosine-3'-5' monophosphate system by hormones and free radicals

Intracellular communication and transmission of messages for many hormones and free radicals occur after the hormones and free radicals bind to their receptors by enhancing the activity of guanylate cyclase, the enzyme that catalyzes the conversion of guanosine triphosphate to the intracellular messenger cyclic guanosine-3'-5' monophosphate (cyclic GMP). The guanylate cyclase-linked receptors exist intracellularly (ie, cytoplasmic) and in membrane-bound forms. Enhancement of guanylate cyclase by hormones or free radicals increases intracellular cyclic GMP, which closes cation channels in the kidney while activating cation channels in the retina and olfactory cilia, either directly or by cyclic GMP-dependent protein kinase. Cyclic GMP also has potent blood pressure lowering properties. Cyclic GMP promotes growth by increasing DNA, RNA, and protein synthesis. Overactivity of this system is observed in Traveler's diarrhea, whereas underactivity occurs in Chediak-Higashi syndrome in which lysosomal enzyme release and chemotaxis are defective and can be corrected in vitro by addition of cyclic GMP.

Niflumic acid differentially modulates two types of skeletal ryanodine-sensitive Ca(2+)-release channels

The effects of niflumic acid on ryanodine receptors (RyRs) of frog skeletal muscle were studied by incorporating sarcoplasmic reticulum (SR) vesicles into planar lipid bilayers. Frog muscle had two distinct types of RyRs in the SR: one showed a bell-shaped channel activation curve against cytoplasmic Ca2+ or niflumic acid, and its mean open probability (Po) was increased by perchlorate at 20-30 mM (termed "alpha-like" RyR); the other showed a sigmoidal activation curve against Ca2+ or niflumic acid, with no effect on perchlorate (termed "beta-like" RyR). The unitary conductance and reversal potential of both channel types were unaffected after exposure to niflumic acid when clamped at 0 mV. When clamped at more positive potentials, the beta-like RyR channel rectified this, increasing the unitary current. Treatment with niflumic acid did not inhibit the response of both channels to Ca2+ release channel modulators such as caffeine, ryanodine, and ruthenium red. The different effects of niflumic acid on Po and the unitary current amplitude in both types of channels may be attributable to the lack or the presence of inactivation sites and/or distinct responses to agonists.