Changes in voltage activation of contraction in frog skeletal muscle fibres as a result of sarcoplasmic reticulum Ca2+-ATPase activity

Recent advances and long-standing problems in detecting oxidative damage and reactive oxygen species in skeletal muscle

An increasingly sophisticated array of approaches are now available for the study of the activities of reactive oxygen species and oxidative modifications in skeletal muscle, but the most up-to-date techniques are not readily available to many researchers in this field due to their requirement for sophisticated mass spectrometry, imaging or other high cost technologies. Most papers published therefore rely on a number of established approaches although the choice of approach is also clearly dependent upon the experimental model and access to skeletal muscle that is available to the investigator, how much detail is required and the overall question to be addressed. Numerous reports have described the problems associated with some of the popular approaches that are widely followed, including measurement of thiobarbituric acid substances and the sole use of fluorescence-based probes such as dichlorodihydrofluorescein. This brief review reports the areas in which methods are improving to allow valid assessments to made in this area and indicates some of the more recent developments that provide alternative ways to assess the activity of individual species and endpoints in the various experimental models that may be examined.

Interactions between mitochondrial reactive oxygen species and cellular glucose metabolism

Mitochondrial reactive oxygen species (ROS) production and detoxification are tightly balanced. Shifting this balance enables ROS to activate intracellular signaling and/or induce cellular damage and cell death. Increased mitochondrial ROS production is observed in a number of pathological conditions characterized by mitochondrial dysfunction. One important hallmark of these diseases is enhanced glycolytic activity and low or impaired oxidative phosphorylation. This suggests that ROS is involved in glycolysis (dys)regulation and vice versa. Here we focus on the bidirectional link between ROS and the regulation of glucose metabolism. To this end, we provide a basic introduction into mitochondrial energy metabolism, ROS generation and redox homeostasis. Next, we discuss the interactions between cellular glucose metabolism and ROS. ROS-stimulated cellular glucose uptake can stimulate both ROS production and scavenging. When glucose-stimulated ROS production, leading to further glucose uptake, is not adequately counterbalanced by (glucose-stimulated) ROS scavenging systems, a toxic cycle is triggered, ultimately leading to cell death. Here we inventoried the various cellular regulatory mechanisms and negative feedback loops that prevent this cycle from occurring. It is concluded that more insight in these processes is required to understand why they are (un)able to prevent excessive ROS production during various pathological conditions in humans.

Characterization of reactive oxygen species in diaphragm

Reactive oxygen species (ROS) exist as natural mediators of metabolism to maintain cellular homeostasis. However, ROS production may significantly increase in response to environmental stressors, resulting in extensive cellular damage. Although several potential sources of increased ROS have been proposed, exact mechanisms of their generation have not been completely elucidated. This is particularly true for diaphragmatic skeletal muscle, the key muscle used for respiration. Several experimental models have focused on detection of ROS generation in rodent diaphragm tissue under stressful conditions, including hypoxia, exercise, and heat, as well as ROS formation in single myofibres. Identification methods include direct detection of ROS with confocal or fluorescent microscopy and indirect detection of ROS through end product analysis. This article explores implications of ROS generation and oxidative stress, and also evaluates potential mechanisms of cellular ROS formation in diaphragmatic skeletal muscle.

Skeletal muscle contractions induce acute changes in cytosolic superoxide, but slower responses in mitochondrial superoxide and cellular hydrogen peroxide

Skeletal muscle generation of reactive oxygen species (ROS) is increased following contractile activity and these species interact with multiple signaling pathways to mediate adaptations to contractions. The sources and time course of the increase in ROS during contractions remain undefined. Confocal microscopy with specific fluorescent probes was used to compare the activities of superoxide in mitochondria and cytosol and the hydrogen peroxide content of the cytosol in isolated single mature skeletal muscle (flexor digitorum brevis) fibers prior to, during, and after electrically stimulated contractions. Superoxide in mitochondria and cytoplasm were assessed using MitoSox red and dihydroethidium (DHE) respectively. The product of superoxide with DHE, 2-hydroxyethidium (2-HE) was acutely increased in the fiber cytosol by contractions, whereas hydroxy-MitoSox showed a slow cumulative increase. Inhibition of nitric oxide synthases increased the contraction-induced formation of hydroxy-MitoSox only with no effect on 2-HE formation. These data indicate that the acute increases in cytosolic superoxide induced by contractions are not derived from mitochondria. Data also indicate that, in muscle mitochondria, nitric oxide (NO) reduces the availability of superoxide, but no effect of NO on cytosolic superoxide availability was detected. To determine the relationship of changes in superoxide to hydrogen peroxide, an alternative specific approach was used where fibers were transduced using an adeno-associated viral vector to express the hydrogen peroxide probe, HyPer within the cytoplasmic compartment. HyPer fluorescence was significantly increased in fibers following contractions, but surprisingly followed a relatively slow time course that did not appear directly related to cytosolic superoxide. These data demonstrate for the first time temporal and site specific differences in specific ROS that occur in skeletal muscle fibers during and after contractile activity.

Monitoring of hydrogen peroxide and other reactive oxygen and nitrogen species generated by skeletal muscle

Understanding the roles and functions of reactive oxygen and nitrogen species in skeletal muscle requires the ability to monitor specific species at rest and during muscle use. These species are generated at a variety of sites in muscle fibers, and approaches to their analysis are becoming available. We utilize microdialysis approaches to sample the interstitial space of skeletal muscle in vivo to allow continuous monitoring of nitric oxide and some reactive oxygen species. The approach to monitor intracellular species that we currently favor utilizes isolated single muscle fibers to allow the use of fluorescent probes and epifluorescence microscopy. Methods are described that illustrate these approaches.

TNF signals via neuronal-type nitric oxide synthase and reactive oxygen species to depress specific force of skeletal muscle

TNF promotes skeletal muscle weakness, in part, by depressing specific force of muscle fibers. This is a rapid, receptor-mediated response, in which TNF stimulates cellular oxidant production, causing myofilament dysfunction. The oxidants appear to include nitric oxide (NO); otherwise, the redox mechanisms that underlie this response remain undefined. The current study tested the hypotheses that 1) TNF signals via neuronal-type NO synthase (nNOS) to depress specific force, and 2) muscle-derived reactive oxygen species (ROS) are essential co-mediators of this response. Mouse diaphragm fiber bundles were studied using live cell assays. TNF exposure increased general oxidant activity (P < 0.05; 2',7'-dichlorodihydrofluorescein diacetate assay) and NO activity (P < 0.05; 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate assay) and depressed specific force across the full range of stimulus frequencies (1-300 Hz; P < 0.05). These responses were abolished by pretreatment with N(ω)-nitro-L-arginine methyl ester (L-NAME; a nonspecific inhibitor of NOS activity), confirming NO involvement. Genetic nNOS deficiency replicated L-NAME effects on TNF-treated muscle, diminishing NO activity (-80%; P < 0.05) and preventing the decrement in specific force (P < 0.05). Comparable protection was achieved by selective depletion of muscle-derived ROS. Pretreatment with either SOD (degrades superoxide anion) or catalase (degrades hydrogen peroxide) depressed oxidant activity in TNF-treated muscle and abolished the decrement in specific force. These findings indicate that TNF signals via nNOS to depress contractile function, a response that requires ROS and NO as obligate co-mediators.

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.

NOX2-dependent ROS is required for HDAC5 nuclear efflux and contributes to HDAC4 nuclear efflux during intense repetitive activity of fast skeletal muscle fibers

Reactive oxygen species (ROS) have been linked to oxidation and nuclear efflux of class IIa histone deacetylase 4 (HDAC4) in cardiac muscle. Here we use HDAC-GFP fusion proteins expressed in isolated adult mouse flexor digitorum brevis muscle fibers to study ROS mediation of HDAC localization in skeletal muscle. H(2)O(2) causes nuclear efflux of HDAC4-GFP or HDAC5-GFP, which is blocked by the ROS scavenger N-acetyl-l-cysteine (NAC). Repetitive stimulation with 100-ms trains at 50 Hz, 2/s ("50-Hz trains") increased ROS production and caused HDAC4-GFP or HDAC5-GFP nuclear efflux. During 50-Hz trains, HDAC5-GFP nuclear efflux was completely blocked by NAC, but HDAC4-GFP nuclear efflux was only partially blocked by NAC and partially blocked by the calcium-dependent protein kinase (CaMK) inhibitor KN-62. Thus, during intense activity both ROS and CaMK play roles in nuclear efflux of HDAC4, but only ROS mediates HDAC5 nuclear efflux. The 10-Hz continuous stimulation did not increase the rate of ROS production and did not cause HDAC5-GFP nuclear efflux but promoted HDAC4-GFP nuclear efflux that was sensitive to KN-62 but not NAC and thus mediated by CaMK but not by ROS. Fibers from NOX2 knockout mice lacked ROS production and ROS-dependent nuclear efflux of HDAC5-GFP or HDAC4-GFP during 50-Hz trains but had unmodified Ca(2+) transients. Our results demonstrate that ROS generated by NOX2 could play important roles in muscle remodeling due to intense muscle activity and that the nuclear effluxes of HDAC4 and HDAC5 are differentially regulated by Ca(2+) and ROS during muscle activity.

Prion protein expression and functional importance in skeletal muscle

Skeletal muscle expresses prion protein (PrP) that buffers oxidant activity in neurons.

AIMS

We hypothesize that PrP deficiency would increase oxidant activity in skeletal muscle and alter redox-sensitive functions, including contraction and glucose uptake. We used real-time polymerase chain reaction and Western blot analysis to measure PrP mRNA and protein in human diaphragm, five murine muscles, and muscle-derived C2C12 cells. Effects of PrP deficiency were tested by comparing PrP-deficient mice versus wild-type mice and morpholino-knockdown versus vehicle-treated myotubes. Oxidant activity (dichlorofluorescin oxidation) and specific force were measured in murine diaphragm fiber bundles.

RESULTS

PrP content differs among mouse muscles (gastrocnemius>extensor digitorum longus, EDL>tibialis anterior, TA; soleus>diaphragm) as does glycosylation (di-, mono-, nonglycosylated; gastrocnemius, EDL, TA=60%, 30%, 10%; soleus, 30%, 40%, 30%; diaphragm, 30%, 30%, 40%). PrP is predominantly di-glycosylated in human diaphragm. PrP deficiency decreases body weight (15%) and EDL mass (9%); increases cytosolic oxidant activity (fiber bundles, 36%; C2C12 myotubes, 7%); and depresses specific force (12%) in adult (8-12 mos) but not adolescent (2 mos) mice.

INNOVATION

This study is the first to directly assess a role of prion protein in skeletal muscle function.

CONCLUSIONS

PrP content varies among murine skeletal muscles and is essential for maintaining normal redox homeostasis, muscle size, and contractile function in adult animals.

Control of reactive oxygen species production in contracting skeletal muscle

SIGNIFICANCE

The increased activities of free radicals or reactive oxygen species in tissues of exercising humans and animals were first reported ∼30 years ago. A great deal has been learned about the processes that can generate these molecules, but there is little agreement on which are important, how they are controlled, and there are virtually no quantitative data. Superoxide and nitric oxide are generated by skeletal muscle and their reactions lead to formation of secondary species. A considerable amount is known about control of superoxide generation by xanthine oxidase activity, but similar information for other generation systems is lacking.

RECENT ADVANCES

Re-evaluation of published data indicates potential approaches to quantification of the hydrogen peroxide concentration in resting and contracting muscle cells. Such calculations reveal that, during contractions, intracellular hydrogen peroxide concentrations in skeletal muscle may only increase by ∼100 nM. The primary effects of this modest increase appear to be in "redox" signaling processes that mediate some of the responses and adaptations of muscle to exercise. These act, in part, to increase the expression of cytoprotective proteins (e.g., heat shock proteins and antioxidant enzymes) that help maintain cell viability. During aging, these redox-mediated adaptations fail and this contributes to age-related loss of skeletal muscle.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Understanding the control of ROS generation in muscle and the effect of aging and some disease states will aid design of interventions to maintain muscle mass and function, but is dependent upon development of new analytical approaches. The final part of this review indicates areas where such developments are occurring.

Downstream mechanisms of nitric oxide-mediated skeletal muscle glucose uptake during contraction

There is evidence that nitric oxide (NO) is required for the normal increases in skeletal muscle glucose uptake during contraction, but the mechanisms involved have not been elucidated. We examined whether NO regulates glucose uptake during skeletal muscle contractions via cGMP-dependent or cGMP-independent pathways. Isolated extensor digitorum longus (EDL) muscles from mice were stimulated to contract ex vivo, and potential NO signaling pathways were blocked by the addition of inhibitors to the incubation medium. Contraction increased (P < 0.05) NO synthase (NOS) activity (∼40%) and dichlorofluorescein (DCF) fluorescence (a marker of oxidant levels; ∼95%), which was prevented with a NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA), and antioxidants [nonspecific antioxidant, N-acetylcysteine (NAC); thiol-reducing agent, DTT], respectively. L-NMMA and NAC both attenuated glucose uptake during contraction by ∼50% (P < 0.05), and their effects were not additive. Neither the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one, which prevents the formation of cGMP, the cGMP-dependent protein (PKG) inhibitor Rp-8-bromo-β-phenyl-1,N2-ethenoguanosine 3',5'-cyclic monophosphorothioate sodium salt nor white light, which breaks S-nitrosylated bonds, affects glucose uptake during contraction; however, DTT attenuated (P < 0.05) contraction-stimulated glucose uptake (by 70%). NOS inhibition and antioxidant treatment reduced contraction-stimulated increases in protein S-glutathionylation and tyrosine nitration (P < 0.05), without affecting AMPK or p38 MAPK phosphorylation. In conclusion, we provide evidence to suggest that NOS-derived oxidants regulate skeletal muscle glucose uptake during ex vivo contractions via a cGMP/PKG-, AMPK-, and p38 MAPK-independent pathway. In addition, it appears that NO and ROS may regulate skeletal muscle glucose uptake during contraction through a similar pathway.

Nitric oxide bioavailability modulates the dynamics of microvascular oxygen exchange during recovery from contractions

AIM

lowered microvascular PO(2) (PO(2) mv) during the exercise off-transient likely impairs muscle metabolic recovery and limits the capacity to perform repetitive tasks. The current investigation explored the impact of altered nitric oxide (NO) bioavailability on PO(2) mv during recovery from contractions in healthy skeletal muscle. We hypothesized that increased NO bioavailability (sodium nitroprusside: SNP) would enhance PO(2) mv and speed its recovery kinetics while decreased NO bioavailability (l-nitro arginine methyl ester: l-NAME) would reduce PO(2) mv and slow its recovery kinetics.

METHODS

  PO(2) mv was measured by phosphorescence quenching during transitions (rest-1 Hz twitch-contractions for 3 min-recovery) in the spinotrapezius muscle of Sprague-Dawley rats under SNP (300 microm), Krebs-Henseleit (CONTROL) and l-NAME (1.5 mm) superfusion conditions.

RESULTS

relative to recovery in CONTROL, SNP resulted in greater overall microvascular oxygenation as assessed by the area under the PO(2) mv curve (PO(2 AREA) ;

CONTROL

3471 ± 292 mmHg s; SNP: 4307 ± 282 mmHg s; P < 0.05) and faster off-kinetics as evidenced by the mean response time (MRToff;

CONTROL

60.2 ± 6.9 s; SNP: 34.8 ± 5.7 s; P < 0.05), whereas l-NAME produced lower PO(2 AREA) (2339 ± 444 mmHg s; P < 0.05) and slower MRToff (86.6 ± 14.5s; P < 0.05).

CONCLUSION

no bioavailability plays a key role in determining the matching of O(2) delivery-to-O(2) uptake and thus the upstream O(2) pressure driving capillary-myocyte O(2) flux (i.e. PO(2) mv) following cessation of contractions in healthy skeletal muscle. Additionally, these data support a mechanistic link between reduced NO bioavailability and prolonged muscle metabolic recovery commonly observed in ageing and diseased populations.

Skeletal muscle glucose uptake during contraction is regulated by nitric oxide and ROS independently of AMPK

Reactive oxygen species (ROS) and nitric oxide (NO) have been implicated in the regulation of skeletal muscle glucose uptake during contraction, and there is evidence that they do so via interaction with AMP-activated protein kinase (AMPK). In this study, we tested the hypothesis that ROS and NO regulate skeletal muscle glucose uptake during contraction via an AMPK-independent mechanism. Isolated extensor digitorum longus (EDL) and soleus muscles from mice that expressed a muscle-specific kinase dead AMPKalpha2 isoform (AMPK-KD) and wild-type litter mates (WT) were stimulated to contract, and glucose uptake was measured in the presence or absence of the antioxidant N-acetyl-l-cysteine (NAC) or the nitric oxide synthase (NOS) inhibitor N(G)-monomethyl-l-arginine (l-NMMA). Contraction increased AMPKalpha2 activity in WT but not AMPK-KD EDL muscles. However, contraction increased glucose uptake in the EDL and soleus muscles of AMPK-KD and WT mice to a similar extent. In EDL muscles, NAC and l-NMMA prevented contraction-stimulated increases in oxidant levels (dichloroflourescein fluorescence) and NOS activity, respectively, and attenuated contraction-stimulated glucose uptake in both genotypes to a similar extent. In soleus muscles of AMPK-KD and WT mice, NAC prevented contraction-stimulated glucose uptake and l-NMMA had no effect. This is likely attributed to the relative lack of neuronal NOS in the soleus muscles compared with EDL muscles. Contraction increased AMPKalpha Thr(172) phosphorylation in EDL and soleus muscles of WT but not AMPK-KD mice, and this was not affected by NAC or l-NMMA treatment. In conclusion, ROS and NO are involved in regulating skeletal muscle glucose uptake during contraction via an AMPK-independent mechanism.

Redox regulation of adaptive responses in skeletal muscle to contractile activity

Skeletal muscle is a highly malleable tissue that responds to changes in its pattern of activity or the mechanical and environmental stresses placed upon it. The signaling pathways involved in these multiple adaptations are increasingly well described, but there is a lack of information on the factors responsible for initiating these processes. Reactive oxygen species (ROS) are produced at various sites in skeletal muscle and there is increasing evidence that these species play targeted roles in modulating redox-sensitive signaling pathways that are important to the muscle for making adaptations. This review will outline some of the processes involved and the types of experimental approaches that seem necessary to fully evaluate these redox signaling systems in muscle. To understand how labile, highly reactive ROS can play a role in cell signaling that is discrete and yet regulated to prevent oxidative damage, an increased knowledge of the subcellular localization and compartmentalization of both ROS generation and the redox-sensitive targets of ROS activity is required. It seems likely that application of this increased knowledge will lead to new approaches to manipulating muscle metabolism to maintain health and prevent loss of muscle function in age-related diseases.

Stretch-stimulated glucose uptake in skeletal muscle is mediated by reactive oxygen species and p38 MAP-kinase

Alternatives to the canonical insulin-stimulated pathway for glucose uptake are exercise- and exogenous reactive oxygen species (ROS)-stimulated glucose uptake. We proposed a model wherein mechanical loading, i.e. stretch, stimulates production of ROS to activate AMP-activated kinase (AMPK) to increase glucose uptake. Immunoblotting was used to measure protein phosphorylation; the fluorochrome probe 2'7'-dichlorofluorescin diacetate was used to measure cytosolic oxidant activity and 2-deoxy-d[1,2-(3)H]glucose was used to measure glucose uptake. The current studies demonstrate that stretch increases ROS, AMPKalpha phosphorylation and glucose transport in murine extensor digitorum longus (EDL) muscle (+121%, +164% and +184%, respectively; P < 0.05). We also demonstrate that stretch-induced glucose uptake persists in transgenic mice expressing an inactive form of the AMPKalpha2 catalytic subunit in skeletal muscle (+173%; P < 0.05). MnTBAP, a superoxide dismutase (SOD) mimetic, N-acteyl cysteine (NAC), a non-specific antioxidant, ebselen, a glutathione mimetic, or combined SOD plus catalase (ROS-selective scavengers) all decrease stretch-stimulated glucose uptake (P < 0.05) without changing basal uptake (P > 0.16). We also demonstrate that stretch-stimulated glucose uptake persists in the presence of the phosphatidylinositol 3-kinase (PI3-K) inhibitors wortmannin and LY294001 (P < 0.05) but is diminished by the p38-MAPK inhibitors SB203580 and A304000 (P > 0.99). These data indicate that stretch-stimulated glucose uptake in skeletal muscle is mediated by a ROS- and p38 MAPK-dependent mechanism that appears to be AMPKalpha2- and PI3-K-independent.

Aging-dependent regulation of antioxidant enzymes and redox status in chronically loaded rat dorsiflexor muscles

This study compares changes in the pro-oxidant production and buffering capacity in young and aged skeletal muscle after exposure to chronic repetitive loading (RL). The dorsiflexors from one limb of young and aged rats were loaded 3 times/week for 4.5 weeks using 80 maximal stretch-shortening contractions per session. RL increased H2O2 in tibialis anterior muscles of young and aged rats and decreased the ratio of reduced/oxidized glutathione and lipid peroxidation in aged but not young adult animals. Glutathione peroxidase (GPx) activity decreased whereas catalase activity increased with RL in muscles from young and aged rats. RL increased CuZn superoxide disumutase (SOD) and Mn SOD protein concentration and CuZn SOD activity in muscles from young but not aged animals. There were no changes in protein content for GPx-1 and catalase or messenger RNA for any of the enzymes studied. These data show that aging reduces the adaptive capacity of muscles to buffer increased pro-oxidants imposed by chronic RL.

In situ detection and measurement of intracellular reactive oxygen species in single isolated mature skeletal muscle fibers by real time fluorescence microscopy

Reactive oxygen species (ROS) produced by skeletal muscle stimulate adaptive responses to activity and mediate some degenerative processes. ROS activity is usually studied by measuring indirect end-points of their reactions with various biomolecules. In order to develop a method to measure the intracellular ROS generation in real-time in mature skeletal muscle fibers, these were isolated from the flexor digitorum brevis (FDB) muscle of mice and cultured on collagen-coated plates. Fibers were loaded with 5- (and 6-) chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-DCFH DA) and measurements of 5- (and 6-) chloromethyl-2',7'-dichlorofluorescin (CM-DCF) fluorescence from individual fibers obtained by microscopy over 45 min. The sensitivity of this approach was demonstrated by addition of 1 microM H(2)O(2) to the extracellular medium. Contractions of isolated fibers induced by field electrical stimulation caused a significant increase in CM-DCF fluorescence that was abolished by pre-treatment of fibers with glutathione ethyl ester. Thus, CM-DCF fluorescence microscopy can detect physiologically relevant changes in intracellular ROS activity in single isolated mature skeletal muscle fibers in real-time, and contractions generated a net increase that was abolished when the intracellular glutathione content was enhanced. This technique has advantages over previous approaches because of the maturity of the fibers and the analysis of single cells, which prevent contributions from nonmuscle cells.

Free radicals generated by contracting muscle: by-products of metabolism or key regulators of muscle function?

Skeletal muscle fibers generate reactive oxygen species (ROS) at a number of subcellular sites and this generation is increased by contractile activity. Early studies suggested that generation of superoxide as a by-product of mitochondrial oxygen consumption was the major source of muscle ROS generation and that the species produced were inevitably damaging to muscle, but recent data argue against both of these possibilities. Developments in analytical approaches have shown that specific ROS are generated in a controlled manner by skeletal muscle fibers in response to physiological stimuli and play important roles in the physiological adaptations of muscle to contractions. These include optimization of contractile performance and initiation of key adaptive changes in gene expression to the stresses of contractions. These positive benefits of the ROS that are induced by contractile activity contrast starkly with the increasing evidence that ROS-induced degenerative pathways are fundamental to aging processes in skeletal muscle. A fuller understanding of these contrasting roles is recognized to be important in the design of strategies to maintain and optimize skeletal muscle function during exercise and to help prevent the devastating effects of sarcopenia and other muscle-wasting conditions.

Real-time measurement of nitric oxide in single mature mouse skeletal muscle fibres during contractions

Nitric oxide (NO) is thought to play multiple roles in skeletal muscle including regulation of some adaptations to contractile activity, but appropriate methods for the analysis of intracellular NO activity are lacking. In this study we have examined the intracellular generation of NO in isolated single mature mouse skeletal muscle fibres at rest and following a period of contractile activity. Muscle fibres were isolated from the flexor digitorum brevis muscle of mice and intracellular NO production was visualized in real-time using the fluorescent NO probe 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM DA). Some leakage of DAF-FM was apparent from fibres loaded with the probe, but they retained sufficient probe to respond to changes in intracellular NO following addition of the NO donor 3-(2-hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-1-propanamine (NOC-7) up to 30 min after loading. Electrically stimulated contractions in isolated fibres increased the rate of change in DAF-FM fluorescence by approximately 48% compared to non-stimulated fibres (P < 0.05) and the rate of change in DAF-FM fluorescence in the stimulated fibres returned to control values by 5 min after contractions. Treatment of isolated fibres with the NO synthase inhibitors NG-nitro-L-arginine methyl ester hydrochloride (L-NAME) or NG-monomethyl-L-arginine (L-NMMA) reduced the increase in DAF-FM fluorescence observed in response to contractions of untreated fibres. Treatment of fibres with the cell-permeable superoxide scavenger 4,5-dihydroxy-1,3-benzenedisulphonic acid (Tiron) also reduced the increase in fluorescence observed during contractions suggesting that superoxide, or more probably peroxynitrite, contributes to the fluorescence observed. Thus this technique can be used to examine NO generation in quiescent and contracting skeletal muscle fibres in real time, although peroxynitrite and other reactive nitrogen species may potentially contribute to the fluorescence values observed.

Genetic modification of the manganese superoxide dismutase/glutathione peroxidase 1 pathway influences intracellular ROS generation in quiescent, but not contracting, skeletal muscle cells

Increased amounts of reactive oxygen species (ROS) are generated by skeletal muscle during contractile activity, but their intracellular source is unclear. The oxidation of 2',7'-dichlorodihydrofluorescein (DCFH) was examined as an intracellular probe for reactive oxygen species in skeletal muscle myotubes derived from muscles of wild-type mice and mice that were heterozygous knockout for manganese superoxide dismutase (Sod2(+/-)), homozygous knockout for glutathione peroxidase 1 (GPx1(-/-)), or MnSOD transgenic overexpressors (Sod2-Tg). Myoblasts were stimulated to fuse and loaded with DCFH 5-7 days later. Intracellular DCF epifluorescence was measured and myotubes were electrically stimulated to contract for 15 min. Quiescent myotubes with decreased MnSOD or GPx1 showed a significant increase in the rate of DCFH oxidation whereas those with increased MnSOD did not differ from wild type. Following contractions, myotubes from all groups showed an equivalent increase in DCF fluorescence. Thus the oxidation of DCFH in quiescent skeletal muscle myotubes is influenced by the content of enzymes that regulate mitochondrial superoxide and hydrogen peroxide content. In contrast, the increase in DCFH oxidation following contractions was unaffected by reduced or enhanced MnSOD or absent GPx1, indicating that reactive oxygen species produced by contractions were predominantly generated by nonmitochondrial sources.

The contraction induced increase in gene expression of peroxisome proliferator-activated receptor (PPAR)-γ coactivator 1α (PGC-1α), mitochondrial uncoupling protein 3 (UCP3) and hexokinase II (HKII) in primary rat skeletal muscle cells is dependent on reactive oxygen species

We evaluated the role of reactive oxygen species (ROS) for the contraction induced increase in expression of PGC-1alpha, HKII and UCP3 mRNA. Rat skeletal muscle cells were subjected to acute or repeated electrostimulation in the presence and absence of antioxidants. Contraction of muscle cells lead to an increased H2O2 formation, as measured by oxidation of H2HFF. Acute contraction of the muscle cells lead to a transient increase in PGC-1alpha and UCP3 mRNA by 172 and 65%, respectively (p<0.05), whereas this increase was absent in the presence of antioxidants. Repeated contraction sessions induced a sustained elevation in PGC-1alpha and UCP3 mRNA and a transient increase in HKII (p<0.05) and this effect was not present with treatment of cells with either an antioxidant cocktail or with GPX+GSH. Incubation of cells for 10 days with ROS produced by xanthine oxidase/xanthine increased the level of PGC-1alpha, HKII and UCP3 mRNA by 175, 58 and 115%, respectively (p<0.05). A 10-day incubation of cells with antioxidants was found to have no effect on the basal mRNA content (p>0.05). The present data demonstrate that contraction of skeletal muscle cells leads to an enhanced formation of ROS and an elevation in PGC-1alpha, UCP3 and HKII mRNA content which is abolished in the presence of antioxidants, suggesting that ROS are of importance for the contraction induced increase in expression of these genes in skeletal muscle.

Mechanical ventilation promotes redox status alterations in the diaphragm

Oxidative stress is an important mediator of diaphragm muscle atrophy and contractile dysfunction during prolonged periods of controlled mechanical ventilation (MV). To date, specific details related to the impact of MV on diaphragmatic redox status remain unknown. To fill this void, we tested the hypothesis that MV-induced diaphragmatic oxidative stress is the consequence of both an elevation in intracellular oxidant production in conjunction with a decrease in the antioxidant buffering capacity. Adult rats were assigned to one of two experimental groups: 1) control or 2) 12 h of MV. Compared with controls, diaphragms from MV animals demonstrated increased oxidant production, diminished total antioxidant capacity, and decreased glutathione levels. Heme oxygenase-1 (HO-1) mRNA and protein levels increased (23.0- and 5.1-fold, respectively) following MV. Thioredoxin reductase-1 and manganese superoxide dismutase mRNA levels were also increased in the diaphragm following MV (2.4- and 1.6-fold, respectively), although no change was detected in the levels of either protein. Furthermore, copper-zinc superoxide dismutase and glutathione peroxidase mRNA were not altered following MV, although protein content decreased -1.3- and -1.7-fold, respectively. We conclude that MV promotes increased oxidant production and impairment of key antioxidant defenses in the diaphragm; collectively, these changes contribute to the MV-induced oxidative stress in this key inspiratory muscle.

Intracellular generation of reactive oxygen species by contracting skeletal muscle cells

The aim of this work was to examine the intracellular generation of reactive oxygen species in skeletal muscle cells at rest and during and following a period of contractile activity. Intracellular generation of reactive oxygen species was examined directly in skeletal muscle myotubes using 2',7'-dichlorodihydrofluorescein (DCFH) as an intracellular probe. Preliminary experiments confirmed that DCFH located to the myotubes but was readily photoxidizable during repeated intracellular fluorescence measurements and strategies to minimize this were developed. The rate of oxidation of DCFH did not change significantly over 30 min in resting myotubes, but was increased by approximately 4-fold during 10 min of repetitive, electrically stimulated contractile activity. This increased rate was maintained over 10 min following the end of the contraction protocol. DCF fluorescence was distributed evenly throughout the myotube with no evidence of accumulation at any specific intracellular sites or localization to mitochondria. The rise in DCF fluorescence was effectively abolished by treatment of the myotubes with the intracellular superoxide scavenger, Tiron. Thus these data appear to represent the first direct demonstration of a rise in intracellular oxidant activity during contractile activity in skeletal muscle myotubes and indicate that superoxide, generated from intracellular sites, is the ultimate source of oxidant(s) responsible for the DCFH oxidation.

Reactive oxygen species formation in the transition to hypoxia in skeletal muscle

Many tissues produce reactive oxygen species (ROS) during reoxygenation after hypoxia or ischemia; however, whether ROS are formed during hypoxia is controversial. We tested the hypothesis that ROS are generated in skeletal muscle during exposure to acute hypoxia before reoxygenation. Isolated rat diaphragm strips were loaded with dihydrofluorescein-DA (Hfluor-DA), a probe that is oxidized to fluorescein (Fluor) by intracellular ROS. Changes in fluorescence due to Fluor, NADH, and FAD were measured using a tissue fluorometer. The system had a detection limit of 1 μM H2O2 applied to the muscle superfusate. When the superfusion buffer was changed rapidly from 95% O2 to 0%, 5%, 21%, or 40% O2, transient elevations in Fluor were observed that were proportional to the rise in NADH fluorescence and inversely proportional to the level of O2 exposure. This signal could be inhibited completely with 40 μM ebselen, a glutathione peroxidase mimic. After brief hypoxia exposure (10 min) or exposure to brief periods of H2O2, the fluorescence signal returned to baseline. Furthermore, tissues loaded with the oxidized form of the probe (Fluor-DA) showed a similar pattern of response that could be inhibited with ebselen. These results suggest that Fluor exists in a partially reversible redox state within the tissue. When Hfluor-loaded tissues were contracted with low-frequency twitches, Fluor emission and NADH emission were significantly elevated in a way that resembled the hypoxia-induced signal. We conclude that in the transition to low intracellular Po2, a burst of intracellular ROS is formed that may have functional implications regarding skeletal muscle O2-sensing systems and responses to acute metabolic stress.

Reactive oxygen species reduce myofibrillar Ca2+ sensitivity in fatiguing mouse skeletal muscle at 37 degrees C

The mechanisms of muscle fatigue were studied in small muscle bundles and single fibres isolated from the flexor digitorum brevis of the mouse. Fatigue caused by repeated isometric tetani was accelerated at body temperature (37 degrees C) when compared to room temperature (22 degrees C). The membrane-permeant reactive oxygen species (ROS) scavenger, Tiron (5 mM), had no effect on the rate of fatigue at 22 degrees C but slowed the rate of fatigue at 37 degrees C to that observed at 22 degrees C. Single fibres were microinjected with indo-1 to measure intracellular calcium. In the accelerated fatigue at 37 degrees C the tetanic [Ca2+](i) did not change significantly and the decline of maximum Ca2+-activated force was similar to that observed at 22 degrees C. The cause of the greater rate of fatigue at 37 degrees C was a large fall in myofibrillar Ca2+ sensitivity. In the presence of Tiron, the large fall in Ca2+ sensitivity was abolished and the usual decline in tetanic [Ca2+](i) was observed. This study confirms the importance of ROS in fatigue at 37 degrees C and shows that the mechanism of action of ROS is a decline in myofibrillar Ca2+ sensitivity.

Role of hydrogen peroxide in ACh-induced dilation of human submucosal intestinal microvessels

The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several mediators of vasodilation, which include prostacyclin (PGI(2)), nitric oxide, and endothelium-derived hyperpolarizing factor (EDHF). We have recently defined the role of nitric oxide and PGI(2) in the dilation of submucosal intestinal arterioles from patients with normal bowel function. However, significant endothelium-dependent dilator capacity to ACh remained after inhibiting both these mediators. The current study was designed to examine the potential role of EDHF in human intestinal submucosal arterioles. ACh elicited endothelium-dependent relaxation in the presence of inhibitors of nitric oxide synthase and cyclooxygenase (23 +/- 10%, n = 6). This ACh-induced relaxation was inhibited and converted to constriction by catalase (-53 +/- 10%, n = 6) or KCl (-30 +/- 3%, n = 7), whereas 17-octadecynoic acid and 6-(2-propargylloxyphenyl) hexanoic acid, two inhibitors of cytochrome P450 monooxygenase, had no significant effect (3 +/- 1% and 20 +/- 8%, n = 5, respectively). Exogenous H(2)O(2) elicited dose-dependent relaxation of intact microvessels (52 +/- 10%, n = 7) but caused frank vasoconstriction in arterioles denuded of endothelium (-73 +/- 8%, n = 7). ACh markedly increased the dichlorofluorescein fluorescence in intact arterioles in the presence of nitric oxide synthase and cyclooxygenase inhibitors compared with control and compared with catalase-treated microvessels (363.6 +/- 49, 218.8 +/- 10.6, 221.9 +/- 27.9, respectively, P < 0.05 ANOVA, n = 5 arbitrary units). No changes in the dichlorofluorescein fluorescence were recorded in vessels treated with ACh alone. These results indicate that endothelial production of H(2)O(2) occurs in response to ACh in human gut mucosal arterioles but that H(2)O(2) is not an EDHF in this tissue. Rather, we speculate that it stimulates the release of a chemically distinct EDHF.

Possible involvement of the alpha1 isoform of 5'AMP-activated protein kinase in oxidative stress-stimulated glucose transport in skeletal muscle

Recent studies have suggested that 5'AMP-activated protein kinase (AMPK) is activated in response to metabolic stresses, such as contraction, hypoxia, and the inhibition of oxidative phosphorylation, which leads to insulin-independent glucose transport in skeletal muscle. In the present study, we hypothesized that acute oxidative stress increases the rate of glucose transport via an AMPK-mediated mechanism. When rat epitrochlearis muscles were isolated and incubated in vitro in Krebs buffer containing the oxidative agent H(2)O(2), AMPKalpha1 activity increased in a time- and dose-dependent manner, whereas AMPKalpha2 activity remained unchanged. The activation of AMPKalpha1 was associated with phosphorylation of AMPK Thr(172), suggesting that an upstream kinase is involved in the activation process. H(2)O(2)-induced AMPKalpha1 activation was blocked in the presence of the antioxidant N-acetyl-l-cysteine (NAC), and H(2)O(2) significantly increased the ratio of oxidized glutathione to glutathione (GSSG/GSH) concentrations, a sensitive marker of oxidative stress. H(2)O(2) did not cause an increase in the conventional parameters of AMPK activation, such as AMP and AMP/ATP. H(2)O(2) increased 3-O-methyl-d-glucose transport, and this increase was partially, but significantly, blocked in the presence of NAC. Results were similar when the muscles were incubated in a superoxide-generating system using hypoxanthine and xanthine oxidase. Taken together, our data suggest that acute oxidative stress activates AMPKalpha1 in skeletal muscle via an AMP-independent mechanism and leads to an increase in the rate of glucose transport, at least in part, via an AMPKalpha1-mediated mechanism.

Oxidant activity in skeletal muscle fibers is influenced by temperature, CO2 level, and muscle-derived nitric oxide

Free radicals are produced continuously by skeletal muscle fibers. Extracellular release of reactive oxygen species (ROS) and nitric oxide (NO) derivatives has been demonstrated, but little is known about intracellular oxidant regulation. We used a fluorescent oxidant probe, 2',7'-dichlorofluorescin (DCFH), to assess net oxidant activity in passive muscle fiber bundles isolated from mouse diaphragm and studied in vitro. We tested the following three hypotheses. 1) Net oxidant activity is decreased by muscle cooling. 2) CO(2) exposure depresses intracellular oxidant activity. 3) Muscle-derived ROS and NO both contribute to overall oxidant activity. Our results indicate that DCFH oxidation was diminished by cooling muscle fibers from 37 degrees C to 23 degrees C (P < 0.001). The rate of DCFH oxidation correlated positively with CO(2) exposure (0-10%; P < 0.05) and negatively with concurrent changes in pH (7.0-8.5; P < 0.05). Separate exposures to anti-ROS enzymes (superoxide dismutase, 1 kU/ml; catalase, 1 kU/ml), a glutathione peroxidase mimetic (ebselen, 30 microM), NO synthase inhibitors (N(omega)-nitro-l-arginine methyl ester, 1 mM; N(omega)-monomethyl-l-arginine, 1 mM), or an NO scavenger (hemoglobin, 1 microM) each inhibited DCFH oxidation (P < 0.05). Oxidation was increased by hydrogen peroxide, 100 microM, an NO donor (NOC-22, 400 microM), or the substrate for NO synthase (l-arginine, 5 mM). We conclude that net oxidant activity in resting muscle fibers is 1) decreased at subphysiological temperatures, 2) increased by CO(2) exposure, and 3) influenced by muscle-derived ROS and NO derivatives to similar degrees.

Coronary vasospasm and the regulation of coronary blood flow

Under physiologic conditions, epicardial arteries contribute minimally to coronary vascular resistance. However, in the presence of endothelial dysfunction, stimuli that normally produce vasodilation may instead cause constriction. Examples include neural release of acetylcholine or norepinephrine, platelet activation and production of serotonin and thrombin, and release of local factors such as bradykinin. This shift from a primary endothelial-mediated vasodilator influence to one of endothelial dysfunction and unchecked vasoconstriction is precisely the milieu in which coronary vasospasm is observed. This condition, which typically occurs during periods of relatively sedentary activity, is associated with focal and transient obstruction of an epicardial arterial segment resulting in characteristic echocardiographic changes and symptoms of myocardial ischemia. This review highlights the current understanding of mechanisms regulating the coronary circulation during health and examines the pathophysiologic changes that occur with coronary spasm. Genetic and other predisposing conditions are addressed, as well as novel therapies based on recent mechanistic insights of the coronary contractile dysfunction associated with coronary spasm.

Mechanism of dilation to reactive oxygen species in human coronary arterioles

We tested whether reactive oxygen species (ROS) generated from treatment with xanthine (XA) and xanthine oxidase (XO) alter vascular tone of human coronary arterioles (HCA). Fresh human coronary arterioles (HCA) from right atrial appendages were cannulated for video microscopy. ROS generated by XA (10–4 M) + XO (10 mU/ml) dilated HCA (99 ± 1%, 20 min after application of XA/XO). This dilation was not affected by denudation or superoxide dismutase (150 U/ml). Catalase (500 U/ml or 5,000 U/ml) attenuated the dilation early on, but a significant latent vasodilation appeared after 5 min peaking at 20 min (51 ± 1%, 20 min after application of XA/XO + 500 U/ml catalase, P < 0.01 vs. control). KCl (40 mM) reduced the early and sustained vasodilation to XA/XO in the absence of catalase but 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 × 10–5 M), diethyldithiocarbamate trihydrate (DDC, 10–2 M), and deferoxamine (DFX, 10–3 M) had no effect. In contrast, the catalase-resistant vasodilation was significantly attenuated by DDC, ODQ, and DFX as well as polyethylene-glycolated catalase (5,000 U/ml), but KCl had no effect. Confocal microscopy revealed that even in the presence of catalase, 2′,7′-dichlorodihydrofluoresein diacetate fluorescence was observed in the vascular smooth muscle, but this was abolished by DDC. These data indicate that the exogenously generated superoxide anion ([Formula: see text]) by XA/XO is spontaneously converted to H2O2, which dilates HCA through vascular smooth muscle hyperpolarization. [Formula: see text] is also converted to H2O2 likely by superoxide dismustase within vascular cells and dilates HCA through a different pathway involving the activation of guanylate cyclase. These findings suggest that exogenously and endogenously produced H2O2 may elicit vasodilation by different mechanisms.

Effects of disulfiram on excitation-contraction coupling in rat soleus muscle

The aim of this study was to analyze whether disulfiram could affect excitation-contraction coupling in rat slow-twitch ( soleus) muscle.In small bundles of intact fibers, the amplitude and the time constant of relaxation of twitch and potassium contractures were dose-dependently and reversibly reduced by disulfiram at concentrations up to 27 microM. At larger concentrations (up to 67.5 microM) these effects were still present but less pronounced. In the presence of disulfiram (27 microM), the relationship between the amplitude of potassium contractures and membrane potential was shifted to more positive potentials whereas, the steady state inactivation curve was unchanged. These observations suggest that disulfiram has no effect on voltage sensors. In saponin-skinned fibers, the amount of Ca(2+) taken up, estimated by using the amplitude of 10 mM of caffeine contracture, was increased by disulfiram (27 microM). By contrast no significant modification was observed in the sensitivity of the ryanodine receptors to caffeine (contractures generated at 5 mM of caffeine) and in the myofibrillar Ca(2+) sensitivity (Triton X-100 skinned fibers). These results indicate that disulfiram induces a dose-dependent reversible effect on the contractile responses of soleus mammalian skeletal muscle by acting mainly on the sarcoplasmic reticulum Ca(2+)-ATPase activity.

Nandrolone decanoate treatment affects sarcoplasmic reticulum Ca(2+) ATPase function in skinned rat slow- and fast-twitch fibres

The effects of anabolic-androgenic steroid administration on the function of the sarcoplasmic reticulum (SR) pump were investigated in chemically skinned fibres from the extensor digitorum longus (EDL) and soleus muscles of sedentary rats. Twenty male rats were divided into two groups, one group received an intramuscular injection of nandrolone decanoate (15 mg x kg(-1)) weekly for 8 weeks, the second received similar weekly doses of vehicle (sterile peanut oil). Compared with control muscles, nandrolone decanoate treatment reduced SR Ca(2+) loading in EDL and soleus fibres by 49% and 29%, respectively. In control and treated muscles, the rate of Ca(2+) leakage depended on the quantity of Ca(2+) loaded. Furthermore, for similar SR Ca(2+) contents, the Ca(2+) leakage rate was not significantly modified by nandrolone decanoate treatment. Nandrolone decanoate treatment thus affects Ca (2+) uptake by the SR in a fibre-type dependent manner.

Formation of hydrogen peroxide and nitric oxide in rat skeletal muscle cells during contractions

We examined intra- and extracellular H(2)O(2) and NO formation during contractions in primary rat skeletal muscle cell culture. The fluorescent probes DCFH-DA/DCFH (2,7-dichlorofluorescein-diacetate/2,7-dichlorofluorescein) and DAF-2-DA/DAF-2 (4,5-diaminofluorescein-diacetate/4,5-diaminofluorescein) were used to detect H(2)O(2) and NO, respectively. Intense electrical stimulation of muscle cells increased the intra- and extracellular DCF fluorescence by 171% and 105%, respectively, compared with control nonstimulated cells (p <.05). The addition of glutathione (GSH) or Tiron prior to electrical stimulation inhibited the intracellular DCFH oxidation (p <.05), whereas the addition of GSH-PX + GSH inhibited the extracellular DCFH oxidation (p <.05). Intense electrical stimulation also increased (p <.05) the intra- and extracellular DAF-2 fluorescence signal by 56% and 20%, respectively. The addition of N(G)-nitro-L-arginine (L-NA) completely removed the intra- and extracellular DAF-2 fluorescent signal. Our results show that H(2)O(2) and NO are formed in skeletal muscle cells during contractions and suggest that a rapid release of H(2)O(2) and NO may constitute an important defense mechanism against the formation of intracellular (*)OH and (*)ONOO. Furthermore, our data show that DCFH and DAF-2 are suitable probes for the detection of ROS and NO both intra- and extracellularly in skeletal muscle cell cultures.

Method to overcome photoreaction, a serious drawback to the use of dichlorofluorescin in evaluation of reactive oxygen species

Non-fluorescent dichlorofluorescin (DCFH) was converted to fluorescent products by photo-irradiation during observations with spectrofluorometer and fluorescence microscopy. Photo-irradiation of DCFH at 250, 300, 330, 400, 500, or 600 nm generated fluorescent dichlorofluorescein (DCF), an oxidation product of DCFH, and an unrecognized fluorescent product. The ratio of the unknown product to DCF varied from 0.15 to 8.21 depending on wavelength. Although reactive oxygen species scavengers, such as catalase, superoxide dismutase, and sodium azide, did not suppress the increase in non-specified fluorescence, reagents such as ascorbic acid, mercaptopropionyl glycine, and methoxycinnamic acid, in a cell-free system, almost completely suppressed it with little effect on the fluorescence of DCF. Meanwhile, ascorbic acid also suppressed non-specified fluorescence in cells, but not completely. At low concentrations of DCFH, the speed of increasing fluorescence was considerably retarded, to such a degree that the fluorescence increase in cells during fluorescence microscopic observation was negligible. The addition, at the time of evaluation, of the above reagents to cell-free systems and, in cell systems, reducing the concentration of DCFH, effectively suppressed the photoreaction of DCFH.

N-acetylcysteine infusion alters blood redox status but not time to fatigue during intense exercise in humans

Infusion of the antioxidant N-acetylcysteine (NAC) reduces fatigability in electrically evoked human muscle contraction, but due to reported adverse reactions, no studies have investigated NAC infusion effects during voluntary exercise in humans. We investigated whether a modified NAC-infusion protocol (125 mg. kg(-1). h(-1) for 15 min, then 25 mg. kg(-1). h(-1)) altered blood redox status and enhanced performance during intense, intermittent exercise. Eight untrained men participated in a counterbalanced, double-blind, crossover study in which they received NAC or saline (control) before and during cycling exercise, which comprised three 45-s bouts and a fourth bout that continued to fatigue, at 130% peak oxygen consumption. Arterialized venous blood was analyzed for glutathione status, hematology, and plasma electrolytes. NAC infusion induced no severe adverse reactions. Exercise decreased the reduced glutathione (P < 0.005) and increased oxidized glutathione concentrations (P < 0.005); NAC attenuated both effects (P < 0.05). NAC increased the rise in plasma K(+) concentration-to-work ratio (P < 0.05), indicating impaired K(+) regulation, although time to fatigue was unchanged (NAC 102 +/- 45 s; saline 107 +/- 53 s). Thus NAC infusion altered blood redox status during intense, intermittent exercise but did not attenuate fatigue.

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.

Mitochondrial reactive oxygen species regulate spatial profile of proinflammatory responses in lung venular capillaries

Cytokine-induced lung expression of the endothelial cell (EC) leukocyte receptor P-selectin initiates leukocyte rolling. To understand the early EC signaling that induces the expression, we conducted real-time digital imaging studies in lung venular capillaries. To compare receptor- vs nonreceptor-mediated effects, we infused capillaries with respectively, TNF-alpha and arachidonate. At concentrations adjusted to give equipotent increases in the cytosolic Ca(2+), both agents increased reactive oxygen species (ROS) production and EC P-selectin expression. Blocking the cytosolic Ca(2+) increases abolished ROS production; blocking ROS production abrogated P-selectin expression. TNF-alpha, but not arachidonate, released Ca(2+) from endoplasmic stores and increased mitochondrial Ca(2+). Furthermore, Ca(2+) depletion abrogated TNF-alpha responses partially, but arachidonate responses completely. These differences in Ca(2+) mobilization by TNF-alpha and arachidonate were reflected in spatial patterning in the capillary in that the TNF-alpha effects were localized at branch points, while the arachidonate effects were nonlocalized and extensive. Furthermore, mitochondrial blockers inhibited the TNF-alpha- but not the arachidonate-induced responses. These findings indicate that the different modes of Ca(2+) mobilization determined the spatial patterning of the proinflammatory response in lung capillaries. Responses to TNF-alpha revealed that EC mitochondria regulate the proinflammatory process by generating ROS that activate P-selectin expression.

Detection of reactive oxygen and nitrogen species in tissues using redox-sensitive fluorescent probes

The take-home message of this chapter is that the fluorescent probes for ROS and RNS have great potential in improving our understanding of redox behavior within cells and tissues. However, data obtained from studies using these probes must be expressed in the context of the limitations of the chemistry of the probes in the cellular microenvironment, which may change under different conditions, such as cell stress or injury. In most cases, as suggested, results should be described in a general context of reflecting an increase in oxidizing reactions within the cell and not as a quantitative measure of the production of a specific oxidant species. It is highly recommended that results be verified, when possible, with alternative fluorescent probes or preferably using alternative methods, such as electron spin resonance or other newly emerging technology.

Detection of reactive oxygen and reactive nitrogen species in skeletal muscle

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are usually identified with pathological states and mediators of cellular injury. However, over the last decade ROS and RNS have been identified in skeletal muscle under physiological conditions. Detection of ROS and RNS production by skeletal muscle cells is fundamental to the problem of differentiating between physiological and pathological levels. The goal of this paper is to review the techniques that have been used to detect ROS and RNS in skeletal muscle. Electron spin resonance, fluorescent assays, cyotchrome c reduction, chemiluminescence, hydroxylation of salicylate, and nitration of phenylalanine are some of the assay systems that have been used thus far. A large body of evidence now indicates that ROS and RNS are continually produced by many different skeletal muscle types studied in vivo, in situ, and in vitro. Under resting conditions, ROS and RNS are detectable in both intracellular and extracellular compartments. Production increases during both non-fatiguing and fatiguing muscle contractions. In the absence of disease, the individual molecular species detected in skeletal muscle include parent radicals for the ROS and RNS cascades: superoxide anions and nitric oxide. Both are generated at rates estimated to range from pmol-to-nmol/mg muscle/minute. Evidence indicates that hydrogen peroxide, hydroxyl radicals, and peroxynitrite are also present under physiological conditions. However, the molecular species that mediate specific biological effects remains largely undetermined, as do the sources of ROS and RNS within muscle fibers. Eventual delineation of the mechanisms whereby ROS and RNS regulate cellular function will hinge on our understanding of the production and distribution of ROS and RNS within skeletal muscle.

alpha(4)-integrin mediates neutrophil-induced free radical injury to cardiac myocytes

Previous work has demonstrated that circulating neutrophils (polymorphonuclear leukocytes [PMNs]) adhere to cardiac myocytes via beta(2)-integrins and cause cellular injury via the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase enzyme system. Since PMNs induced to leave the vasculature (emigrated PMNs) express the alpha(4)-integrin, we asked whether (a) these PMNs also induce myocyte injury via NADPH oxidase; (b) beta(2)-integrins (CD18) still signal oxidant production, or if this process is now coupled to the alpha(4)-integrin; and (c) dysfunction is superoxide dependent within the myocyte or at the myocyte-PMN interface. Emigrated PMNs exposed to cardiac myocytes quickly induced significant changes in myocyte function. Myocyte shortening was decreased by 30-50% and rates of contraction and relaxation were reduced by 30% within the first 10 min. Both alpha(4)-integrin antibody (Ab)-treated PMNs and NADPH oxidase-deficient PMNs were unable to reduce myocyte shortening. An increased level of oxidative stress was detected in myocytes within 5 min of PMN adhesion. Addition of an anti-alpha(4)-integrin Ab, but not an anti-CD18 Ab, prevented oxidant production, suggesting that in emigrated PMNs the NADPH oxidase system is uncoupled from CD18 and can be activated via the alpha(4)-integrin. Addition of exogenous superoxide dismutase (SOD) inhibited all parameters of dysfunction measured, whereas overexpression of intracellular SOD within the myocytes did not inhibit the oxidative stress or the myocyte dysfunction caused by the emigrated PMNs. These findings demonstrate that profound molecular changes occur within PMNs as they emigrate, such that CD18 and associated intracellular signaling pathways leading to oxidant production are uncoupled and newly expressed alpha(4)-integrin functions as the ligand that signals oxidant production. The results also provide pathological relevance as the emigrated PMNs have the capacity to injure cardiac myocytes through the alpha(4)-integrin-coupled NADPH oxidase pathway that can be inhibited by extracellular, but not intracellular SOD.

Cytokines and oxidative signalling in skeletal muscle

A growing body of literature indicates that cytokines regulate skeletal muscle function, including gene expression and adaptive responses. Tumour necrosis factor-alpha (TNF-alpha) is the cytokine most prominently linked to muscle pathophysiology and, therefore, has been studied most extensively in muscle-based systems. TNF-alpha is associated with muscle catabolism and loss of muscle function in human diseases that range from cancer to heart failure, from arthritis to AIDS. Recent advances have established that TNF-alpha causes muscle weakness via at least two mechanisms, accelerated protein loss and contractile dysfunction. Protein loss is a chronic response that occurs over days to weeks. Changes in gene expression required for TNF-alpha induced catabolism are regulated by the transcription factor nuclear factor-kappaB which is essential for the net loss of muscle protein caused by chronic TNF-alpha exposure. Contractile dysfunction is an acute response to TNF-alpha stimulation, developing over hours and resulting in decreased force production. Both actions of TNF-alpha involve a rapid rise in endogenous oxidants as an essential step in post-receptor signal transduction. These oxidants appear to include reactive oxygen species derived from mitochondrial electron transport. Such information provides insight into the cellular and molecular mechanisms of TNF-alpha action in skeletal muscle and establishes a scientific basis for continued research into cytokine signalling.

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.

Cardiac-specific overexpression of tumor necrosis factor-alpha causes oxidative stress and contractile dysfunction in mouse diaphragm

BACKGROUND

We have developed a transgenic mouse with cardiac-restricted overexpression of tumor necrosis factor-alpha (TNF-alpha). These mice develop a heart failure phenotype characterized by left ventricular dysfunction and remodeling, pulmonary edema, and elevated levels of TNF-alpha in the peripheral circulation from cardiac spillover. Given that TNF-alpha causes atrophy and loss of function in respiratory muscle, we asked whether transgenic mice developed diaphragm dysfunction and whether contractile losses were caused by oxidative stress or tissue remodeling.

METHODS AND RESULTS

muscles excised from transgenic mice and littermate controls were studied in vitro with direct electrical stimulation. Cytosolic oxidant levels were measured with 2', 7'-dichlorofluorescin diacetate; emissions of the oxidized product were detected by fluorescence microscopy. Force generation by the diaphragm of transgenic animals was 47% less than control (13.2+/-0. 8 [+/-SEM] versus 25.1+/-0.6 N/cm(2); P:<0.001); this weakness was associated with greater intracellular oxidant levels (P:<0.025) and was partially reversed by 30-minute incubation with the antioxidant N:-acetylcysteine 10 mmol/L (P:<0.01). Exogenous TNF-alpha 500 micromol/L increased oxidant production in diaphragm of wild-type mice and caused weakness that was inhibited by N:-acetylcysteine, suggesting that changes observed in the diaphragm of transgenic animals were mediated by TNF-alpha. There were no differences in body or diaphragm weights between transgenic and control animals, nor was there evidence of muscle injury or apoptosis.

CONCLUSIONS

Elevated circulating levels of TNF-alpha provoke contractile dysfunction in the diaphragm through an endocrine mechanism thought to be mediated by oxidative stress.

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.