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

Energy (and Reactive Oxygen Species Generation) Saving Distribution of Mitochondria for the Activation of ATP Production in Skeletal Muscle

Exercise produces oxidants from a variety of intracellular sources, including NADPH oxidases (NOX) and mitochondria. Exercise-derived reactive oxygen species (ROS) are beneficial, and the amount and location of these ROS is important to avoid muscle damage associated with oxidative stress. We discuss here some of the evidence that involves ROS production associated with skeletal muscle contraction and the potential oxidative stress associated with muscle contraction. We also discuss the potential role of H2O2 produced after NOX activation in the regulation of glucose transport in skeletal muscle. Finally, we propose a model based on evidence for the role of different populations of mitochondria in skeletal muscle in the regulation of ATP production upon exercise. The subsarcolemmal population of mitochondria has the enzymatic and metabolic components to establish a high mitochondrial membrane potential when fissioned at rest but lacks the capacity to produce ATP. Calcium entry into the mitochondria will further increase the metabolic input. Upon exercise, subsarcolemmal mitochondria will fuse to intermyofibrillar mitochondria and will transfer the mitochondria membrane potential to them. These mitochondria are rich in ATP synthase and will subsequentially produce the ATP needed for muscle contraction in long-term exercise. These events will optimize energy use and minimize mitochondria ROS production.

Calcium and Reactive Oxygen Species Signaling Interplays in Cardiac Physiology and Pathologies

Mitochondria are key players in energy production, critical activity for the smooth functioning of energy-demanding organs such as the muscles, brain, and heart. Therefore, dysregulation or alterations in mitochondrial bioenergetics primarily perturb these organs. Within the cell, mitochondria are the major site of reactive oxygen species (ROS) production through the activity of different enzymes since it is one of the organelles with the major availability of oxygen. ROS can act as signaling molecules in a number of different pathways by modulating calcium (Ca2+) signaling. Interactions among ROS and calcium signaling can be considered bidirectional, with ROS regulating cellular Ca2+ signaling, whereas Ca2+ signaling is essential for ROS production. In particular, we will discuss how alterations in the crosstalk between ROS and Ca2+ can lead to mitochondrial bioenergetics dysfunctions and the consequent damage to tissues at high energy demand, such as the heart. Changes in Ca2+ can induce mitochondrial alterations associated with reduced ATP production and increased production of ROS. These changes in Ca2+ levels and ROS generation completely paralyze cardiac contractility. Thus, ROS can hinder the excitation-contraction coupling, inducing arrhythmias, hypertrophy, apoptosis, or necrosis of cardiac cells. These interplays in the cardiovascular system are the focus of this review.

Effects of high-intensity interval exercise on muscle fatigue and SR function in rats: a comparison with moderate-intensity continuous exercise

Over the past decade, high-intensity interval exercise (HIIE) training has received attention as a more efficient training to improve endurance capacity. It is unclear, however, whether the extent of acute exercise-related muscle fatigue differs between HIIE and moderate-intensity continuous exercise, traditional endurance training. Here we provide evidence that restoration of force production takes a longer time after HIIE, which is ascribable to long-lasting depressions in Ca2+ release of the sarcoplasmic reticulum.

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.

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.

Mitochondrial redox state and Ca2+ sparks in permeabilized mammalian skeletal muscle

Intact skeletal muscle fibres from adult mammals exhibit neither spontaneous nor stimulated Ca(2+) sparks. Mechanical or chemical skinning procedures have been reported to unmask sparks. The present study investigates the mechanisms that determine the development of Ca(2+) spark activity in permeabilized fibres dissected from muscles with different metabolic capacity. Spontaneous Ca(2+) sparks were detected with fluo-3 and single photon confocal microscopy; mitochondrial redox potential was evaluated from mitochondrial NADH signals recorded with two-photon confocal microscopy, and Ca(2+) load of the sarcoplasmic reticulum (SR) was estimated from the amplitude of caffeine-induced Ca(2+) transients recorded with fura-2 and digital photometry. In three fibre types studied, there was a time lag between permeabilization and spark development. Under all experimental conditions, the delay was the longest in slow-twitch oxidative fibres, intermediate in fast-twitch glycolytic-oxidative fibres, and the shortest in fast-twitch glycolytic cells. The temporal evolution of Ca(2+) spark frequencies was bell-shaped, and the maximal spark frequency was reached slowly in mitochondria-rich oxidative cells but quickly in mitochondria-poor glycolytic fibres. The development of spontaneous Ca(2+) sparks did not correlate with the SR Ca(2+) content of the fibre, but did correlate with the redox potential of their mitochondria. Treatment of fibres with scavengers of reactive oxygen species (ROS), such as superoxide dismutase (SOD) and catalase, dramatically and reversibly reduced the spark frequency and also delayed their appearance. In contrast, incubation of fibres with 50 microm H(2)O(2) sped up the development of Ca(2+) sparks and increased their frequency. These results indicate that the appearance of Ca(2+) sparks in permeabilized skeletal muscle cells depends on the fibre's oxidative strength and that misbalance between mitochondrial ROS production and the fibre's ability to fight oxidative stress is likely to be responsible for unmasking Ca(2+) sparks in skinned preparations. They also suggest that under physiological and pathophysiological conditions the appearance of Ca(2+) sparks may be, at least in part, limited by the fine-tuned equilibrium between mitochondrial ROS production and cellular ROS scavenging mechanisms.

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.

MHC-based fiber type and E-C coupling characteristics in mechanically skinned muscle fibers of the rat

In this study, we investigated whether the previously established differences between fast- and slow-twitch single skeletal muscle fibers of the rat, in terms of myosin heavy chain (MHC) isoform composition and contractile function, are also detectable in excitation-contraction (E-C) coupling. We compared the contractile responsiveness of electrophoretically typed, mechanically skinned single fibers from the soleus (Sol), the extensor digitorum longus (EDL), and the white region of the sternomastoid (SM) muscle to t-system depolarization-induced activation. The quantitative parameters assessed were the amplitude of the maximum depolarization-induced force response (DIFR(max); normalized to the maximum Ca(2+)-activated force in that fiber) and the number of responses elicited until the force declined by 75% of DIFR(max) (R-D(75%)). The mean DIFR(max) values for type IIB EDL and type IIB SM fibers were not statistically different, and both were greater than the mean DIFR(max) for type I Sol fibers. The mean R-D(75%) for type IIB EDL fibers was greater than that for type I Sol fibers as well as type IIB SM fibers. These data suggest that E-C coupling characteristics of mechanically skinned rat single muscle fibers are related to MHC-based fiber type and the muscle of origin.

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.

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.

Excitation-contraction coupling and sarcoplasmic reticulum function in mechanically skinned fibres from fast skeletal muscles of aged mice

Ageing is generally associated with a decline in skeletal muscle mass and strength, and a slowing of muscle contraction, factors that impact upon the quality of life for the elderly. Alterations in Ca2+ handling are thought to contribute to these age-related changes in muscle contractility, yet the effects of ageing on sarcoplasmic reticulum (SR) Ca2+ handling and the Ca2+ transport system remain unresolved. We used mechanically skinned single fibres from the fast twitch extensor digitorum longus (EDL) muscles from young (4-month-old) and old (27- to 28-month-old) mice to test the hypothesis that the age-related changes in skeletal muscle contractility, especially the slower rate of contraction, are due to changes intrinsic to the muscle fibres. There were no age-related differences in the peak height of depolarization-induced contractile response (DICR) or the number of DICRs elicited before rundown (DICR < 50 % of initial). The time taken to reach peak DICR (TPDICR) was approximately12 % slower in single muscle fibres from old compared with young mice (P < 0.05). The rate of relaxation following DICR was not different in young and old mice. Examination of SR function demonstrated that SR Ca2+ reloading in Ca2+ -depleted skinned fibres was not different in young and old mice, nor was there any age-related difference in Ca2+ leak from the SR. However, low [caffeine] contracture in fibres from old mice was only half of that observed in fibres from young mice (P < 0.05), indicating a lower sensitivity of the SR Ca2+ release channel (CRC) to caffeine. We found no difference in maximum Ca2+ -activated force (P(o)) or specific force (sP(o); P(o) corrected for cross-sectional area) in EDL muscle fibres from young and old mice. Impaired excitation-contraction (E-C) coupling and a decrease in SR CRC function are mechanisms which are likely to contribute to the overall slowing of muscle contraction with age.