J Appl Physiol 102: 2056-2063, 2007.
First published January 11, 2007; doi:10.1152/japplphysiol.01138.2006
8750-7587/07 $8.00
INVITED REVIEW
HIGHLIGHTED TOPIC
Free Radical Biology in Skeletal Muscle
Free radical-mediated skeletal muscle dysfunction in inflammatory conditions
Gerald S. Supinski and
Leigh A. Callahan
Department of Medicine, Medical College of Georgia, Augusta, Georgia
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ABSTRACT
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Loss of functional capacity of skeletal muscle is a major cause of morbidity in patients with a number of acute and chronic clinical disorders, including sepsis, chronic obstructive pulmonary disease, heart failure, uremia, and cancer. Weakness in these patients can manifest as either severe limb muscle weakness (even to the point of virtual paralysis), respiratory muscle weakness requiring mechanical ventilatory support, and/or some combination of these phenomena. While factors such as nutritional deficiency and disuse may contribute to the development of muscle weakness in these conditions, systemic inflammation may be the major factor producing skeletal muscle dysfunction in these disorders. Importantly, studies conducted over the past 15 years indicate that free radical species (superoxide, hydroxyl radicals, nitric oxide, peroxynitrite, and the free radical-derived product hydrogen peroxide) play an key role in modulating inflammation and/or infection-induced alterations in skeletal muscle function. Substantial evidence exists indicating that several free radical species can directly alter contractile protein function, and evidence suggests that free radicals also have important effects on sarcoplasmic reticulum function, on mitochondrial function, and on sarcolemmal integrity. Free radicals also modulate activation of several proteolytic pathways, including proteosomally mediated protein degradation and, at least theoretically, may also influence pathways of protein synthesis. As a result, free radicals appear to play an important role in regulating a number of downstream processes that collectively act to impair muscle function and lead to reductions in muscle strength and mass in inflammatory conditions.
superoxide; nitric oxide; sepsis
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INFLAMMATION AND MUSCLE FUNCTION
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Loss of functional capacity of skeletal muscle is a major cause of morbidity in patients with a number of acute and chronic clinical disorders (20, 21, 32, 34, 37, 61, 79, 81). Acute medical problems associated with development of skeletal muscle weakness include pneumonia, generalized infections causing bacteremia, adult respiratory distress syndrome (ARDS), burns, uremia, and trauma (20, 34, 61, 81). Weakness in these patients can manifest as either severe limb muscle weakness (even to the point of virtual paralysis), respiratory muscle weakness requiring mechanical ventilatory support, and/or some combination of these phenomena. In general, clinicians tend to underestimate the degree to which skeletal muscle weakness contributes to patient morbidity, but recent clinical studies employing more objective measurements of muscle function have shown a surprisingly large incidence of relatively severe weakness in critically ill patients (32, 37, 79). As an example, one study examining the long-term consequences of hospitalization for ARDS found that these patients had significant impairment of exercise capacity long after discharge (e.g., 1 yr after release from the hospital) (32). More importantly, pulmonary function had returned to normal in the majority of these patients at an early point after discharge, and the reductions in long-term exercise capacity appeared to be directly linked to reductions in peripheral skeletal muscle function.
In addition, two recent studies found that critically ill patients requiring sustained mechanical ventilation had surprisingly severe reductions in diaphragm strength (37, 79). Both studies used bilateral magnetic stimulation to activate the phrenic nerves in the neck and recorded the transdiaphragmatic pressure evoked by supramaximal twitch stimuli to achieve an objective index of respiratory muscle strength. Laghi et al. (37) found that the average patient requiring mechanical ventilation had diaphragm twitch transdiaphragmatic pressures that were only 23% of the values measured in a healthy control population. Watson et al. (79) reported similar findings, with transdiaphragmatic pressures reduced on average to levels only 36% of controls. Interestingly, there was a fairly wide range of severities of weakness, with some patients found to have diaphragm pressure-generating capacities as low as 5% of normal levels.
Significant skeletal muscle weakness also appears to develop in a host of more chronic illnesses, including chronic obstructive pulmonary disease (COPD), congestive heart failure (CHF), uremia, cancer, and acquired immunodeficiency syndrome (AIDS) (4, 20, 29, 73, 49). In COPD, a correlation between exercise tolerance, long-term outcomes, and the development of peripheral muscle abnormalities is now apparent, with markedly reduced walking capacity observed in patients with decreases in peripheral muscle mass and strength (4). In heart failure, much of the exercise intolerance associated with this disorder appears to be independent of the level of cardiac output and more closely related to the level of peripheral muscle function (73). Uremia is associated with peripheral muscle wasting and weakness that cannot be explained on a nutritional basis (20). Cancer and AIDS are prototypical diseases associated with significant muscle wasting, and the development of muscle dysfunction in these patients is associated with a poor response to treatment (29, 49).
While factors such as nutritional deficiency and disuse may contribute to the development of muscle weakness in these common acute and chronic medical conditions, studies have found evidence of systemic inflammation in all of the disease processes listed above, and systemic inflammation may be the key factor in producing skeletal muscle dysfunction in all of these disorders. In keeping with this possibility, circulating levels of various cytokines and chemokines have been reported to increase in patients with sepsis, ARDS, burns, uremia, CHF, COPD, AIDS, and cancer (3, 15, 16, 25, 47, 56, 74, 82). Moreover, administration of cytokines or induction of cytokine production in animals has been shown to produce muscle wasting and muscle weakness (9). In addition, incubation of muscle cell line cultures with cytokines has been shown to produce loss of cellular proteins (43).
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SUBCELLULAR SITES OF MUSCLE DYSFUNCTION IN INFLAMMATORY STATES
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To further examine the relationship of inflammation and muscle function, investigators have studied in detail the effects of well-defined animal models of infection, i.e., cecal ligation-perforation, endotoxin administration, pneumonia, chronic infection with Leishmania, and cytokine administration on various aspects of muscle performance for both limb and respiratory skeletal muscle. Studies have shown that cecal ligation-perforation results in reductions in limb muscle protein content (30). Cecal ligation also appears to reduce muscle force generation per cross-sectional area (so-called specific force generation), albeit this latter phenomenon has been best studied in the respiratory muscles, and there is a paucity of data demonstrating that specific force falls in limb muscle in response to this model of inflammation (27). Endotoxin administration appears to rapidly induce severe reductions in muscle-specific force generation and, over a longer time frame, to produce reductions in muscle mass and protein content (59, 70, 71). Pneumonia also produces rapid reductions in the specific force generation of the respiratory muscles, but limb muscle function appears to be largely spared from the effects of this type of infection by a mechanism that remains unclear (17). Effects of experimental models of pneumonia on muscle mass and protein content have not been studied. There has also been little examination of the effects of more chronic infections on muscle, but one study by Drew et al. (18) found that protracted infection with Leishmania donovani resulted in steady reductions in specific force, muscle protein content, and muscle mass over time. In this latter study, both respiratory and limb muscle function and mass appeared to be affected over time.
Several investigators have examined the effects of acute and chronic administration of cytokines, with the largest number of studies examining the effects of administration of tumor necrosis factor-
(TNF-). Buck and Chojkier (9) found that implantation of tumors producing large amounts of TNF- resulted in significant atrophy of the limb musculature. In contrast, in a study in which a transgenic animal overexpressed TNF- in the heart, only diaphragm function was altered, and limb muscle function remained normal (44). Other studies have examined the effect of exposure of isolated, individual muscles or isolated muscle cell lines to TNF- in vitro. In isolated muscles, acute exposure (over hours) to TNF- induces a significant reduction in specific muscle force without a loss in muscle mass or protein levels (54). In contrast, exposure of isolated muscle cells to more prolonged TNF- (over days) induces a significant reduction in both cell size and protein content (43).
Further work has attempted to identify specific subcellular alterations within skeletal muscle that may contribute to overall reductions in contractile function and muscle protein content in response to inflammatory stimuli. The reductions in skeletal muscle-specific force generation that have been observed in animals models of acute inflammation could, in theory, be attributed to the following: 1) alterations in transmission of action potentials along the sarcolemmal membrane or t tubules, 2) alterations in sarcoplasmic reticulum function, 3) alterations in the calcium responsiveness of the contractile proteins per se, 4) alterations in cellular energetics such that optimal concentrations of ions and high-energy phosphate compounds cannot be maintained, and/or 5) due to disruption of cytoskeletal architecture or sarcolemmal membrane integrity. Evidence suggests that inflammation and/or systemic infections can result in alterations in most of these subcellular processes. Studies using isolated, permeabilized "skinned" single-muscle fibers have shown that endotoxin administration rapidly induces large reductions in the force-generating capacity of the contractile proteins in slow and fast fibers in both limb and respiratory muscles (10, 68). Some evidence suggests that inflammatory stress may also alter cellular calcium fluxes, with one study demonstrating an alteration in skeletal muscle ryanodine receptor function in an animal model of sepsis, and other work showing an increase in resting skeletal muscle cytosolic calcium concentrations in the cecal ligation-perforation model of sepsis (26, 46).
Several reports have shown that animal models of sepsis result in significant alterations in skeletal muscle mitochondrial function, with significant reductions in state 3 respiration rates, decreases in maximal skeletal muscle mitochondrial ATP generation rates, and profound reductions in the activity of one of the major transmitochondrial transporters of high-energy phosphate compounds, the sarcomeric mitochondrially specific creatine kinase (12, 13, 14). In theory, these changes in mitochondrial function could lead to marked alterations in cellular energy metabolism, reducing muscle function during periods of intense activity. In keeping with the potential importance of infection-induced alterations in skeletal muscle mitochondrial function, one recent study found that the level of skeletal muscle mitochondrial function in patients with sepsis was a powerful indicator of outcome, with markedly higher mortality observed in patients with reductions in mitochondrial functional capacity (8). Some work also indicates that infections and/or inflammation may result in disruption of the integrity of the sarcolemmal membrane of skeletal muscle, a response similar to that observed in muscular dystrophy (45).
The process by which reductions in skeletal muscle protein content develop in response to inflammation and/or infection has undergone considerable study, and it is clear that these stimuli both reduce muscle protein synthesis and increase muscle protein degradation. The precise mechanisms by which inflammation reduces protein synthesis are incompletely understood, but the process appears to involve alterations in circulating hormone levels that normally modulate protein synthesis (e.g., corticosteroid levels) and, in addition, organ-specific alterations in the activity of specific regulators of translation, such as alterations in eukaryotic initiation factor 2B
activity (72, 76). Reductions in protein synthesis by as much as 50% can occur within a few days in response to potent inflammatory stimuli, and this process alone may result in a significant reduction in muscle protein stores.
In addition, inflammation and/or infection is known to activate a number of proteolytic processes, including calpain, caspase, and the proteosome (24, 55, 64). Calpain and caspase have both been shown to become activated in skeletal muscle in inflammatory states (24, 64), and both proteases can cleave important components of skeletal muscle contractile apparatus and cytoskeleton, including actin, actinin, and spectrin in the case of caspase, and myosin, talin, and spectrin in the case of calpain. As a result, calpain and caspase activation may result in destabilization of the contractile protein matrix, leading to a disruption of force generation and facilitating release of contractile elements that can be subsequently degraded by other proteolytic processes (e.g., the proteosome) (19, 52). Recent studies demonstrate a marked increase in multiple elements of the proteosomal components in sepsis, including an increase in ubiquitin, the 20S proteosome component, and several E3 ligases [atrogin and muscle ring finger 1 (MuRF1)] (55, 80). In keeping with the importance of the proteosome in sepsis, administration of proteosome inhibitors in an animal model of sepsis has been shown to result in a marked reduction in protein degradation, as judged by use of the tyrosine release assay (36).
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FREE RADICALS AND INFLAMMATION-INDUCED MUSCLE DYSFUNCTION
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Importantly, studies conducted over the past 15 years indicate that free radical species (superoxide, hydroxyl radicals, nitric oxide, peroxynitrite, and the free radical-derived product hydrogen peroxide) play an important role in modulating many if not all of the inflammation and/or infection-induced alterations in muscle function (Fig. 1). In support of this contention, several reports have shown that administration of scavengers of free radicals or free radical products to intact animals blunts or prevents reductions in muscle-specific force generation in animal models of infection and/or systemic inflammation (28, 66, 75). This includes data by Fujimura et al. (27, 28), who found that administration of either polyethylene glycol adsorbed superoxide dismutase (PEG-SOD), a superoxide scavenger, catalase (this breaks down hydrogen peroxide), or DMSO (a hydroxyl radical scavenger) markedly attenuated reductions in diaphragm-specific force generation induced by the cecal ligation perforation model of sepsis. Similarly, PEG-SOD, catalase, and DMSO have been shown to prevent endotoxin-induced reductions in diaphragm and intercostal muscle force generation, while administration of N-acetylcysteine, a general antioxidant, prevents endotoxin-induced diaphragm dysfunction (66, 75). In other work, administration of free radical scavengers to animals implanted with TNF--expressing tumors was found to prevent muscle wasting, preserving muscle mass and protein content (9).
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Fig. 1. Theoretical relationship by which inflammation induces muscle dysfunction. Inflammation is associated with cytokine release, and cytokines, in turn, activate free radical generation by skeletal muscle NADPH oxidase, mitochondria, and nitric oxide synthase (NOS). Potential downstream targets of free radicals include the sarcolemmal membrane, sarcoplasmic reticulum (SR), contractile proteins, mitochondria, protein synthesis, and proteolytic pathways.
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Many previous studies have also found evidence of substantial increases in tissue markers of free radical-mediated protein and lipid modification in skeletal muscle taken from animal models of infection and/or inflammation. Specifically, infection and/or inflammation has been shown to result in increases in tissue levels of 1) malondialdehyde and 8-isoprostane, markers of free radical-induced lipid peroxidation; 2) 4-hydroxynonenol-protein conjugates, a marker of reaction of oxidatively modified lipids with proteins; 3) protein carbonyls, an index of free radical-induced protein modification; and 4) formation of nitrotyrosine side groups on proteins, a marker of reaction of the peroxynitrite radical with proteins (2, 23, 65, 67). Work by Barreiro et al. (2) has further examined the precise intracellular targets of free radical-induced alterations in muscle in an animal model of infection and found that modified proteins include contractile proteins (-actin), several proteins involved in energy production, creatine kinase, and carbonic anhydrase III. Of interest, several studies found that administration of free radical scavengers to animal models of infection prevents oxidative protein modifications in muscle, in parallel with an effect of these agents to preserve muscle function (59, 75).
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SUBCELLULAR ALTERATIONS INDUCED BY FREE RADICALS
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One of the mechanisms by which free radicals may be influencing muscle function in inflammatory states is by direct reaction with and modification of the contractile proteins. In support of this possibility, several studies have shown that incubation of isolated contractile proteins with either superoxide-generating solutions, hydroxyl radical-generating solution, or peroxynitrite rapidly leads to loss of contractile protein function, reducing the force generated in response to incubation of the muscle fiber with a given level of ionized calcium (11, 69). In addition, administration of either superoxide scavengers (i.e., PEG-SOD) or an inhibitor of nitric oxide synthesis (NG-nitro-L-arginine methyl ester) significantly attenuates endotoxin-induced reductions in diaphragm contractile protein force generation as assessed by measuring the force-pCa curves of individual skinned single muscle fibers from this muscle (see Fig. 2) (68). Moreover, studies examining the effect of incubation of TNF- with isolated intact single muscle fibers have shown that the early effects of this agent on muscle force generation (over a few hours) can be largely ascribed to effects on the contractile apparatus (54), while other data suggest that the effect of TNF- to reduce contractile force generation can be attenuated by administration of free radical scavengers (44). Taken together, these observations support the concept that superoxide and other free radical species can directly react with and modify the function of the contractile proteins and that this mechanism of action may play an important role in reducing muscle-specific force generation in inflammatory conditions.
Free radicals may also indirectly affect force generation by altering mitochondrial functional capacity and thereby altering high-energy phosphate compound (ATP, creatine phosphate) homeostasis during periods of intense muscle work. Several studies provide data suggesting that infection-induced mitochondrial damage may be free radical mediated. Two studies have found that administration of agents that prevent peroxynitrite formation (SOD and nitric oxide synthesis inhibitors) ablate sepsis-induced skeletal muscle mitochondrial dysfunction (see Fig. 3) (5, 12). These studies also found that sepsis induced nitrotyrosine side group formation in selected mitochondrial proteins and that SOD and nitric oxide synthase (NOS) inhibitors prevented modification of these mitochondrial proteins.
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Fig. 3. State 3 oxygen consumption values for skeletal muscle mitochondria. Data are presented for mitochondria isolated from (left to right) saline-treated control animals, animals given endotoxin, animals given superoxide dismutase (SOD) alone, animals given endotoxin plus SOD, and animals given endotoxin plus denatured SOD. *Value statistically different (P < 0.001) from the saline-treated control group. These data indicate that SOD administration prevents endotoxin-induced mitochondrial dysfunction. [Reprinted from Ref. 12, copyright 2001, with permission from Elsevier.]
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It is also plausible that free radical species may influence sarcoplasmic reticulum function in infections and/or inflammatory states based on the findings that superoxide, hydrogen peroxide, and nitric oxide all have well-recognized effects on ryanodine receptor function, that several studies demonstrate an increase in nitric oxide and superoxide generation in sepsis, and that inflammation appears to alter sarcoplasmic reticulum calcium handling in some models of sepsis (26, 31, 48, 62, 63). To date, however, there has been no direct demonstration that sepsis-induced skeletal muscle calcium perturbations are the direct result of the actions of either superoxide, nitric oxide, or reaction products of these molecules.
There is, however, substantial evidence that sepsis-induced diaphragm sarcolemmal injury is mediated by a free radical-mediated process. Specifically, Lin et al. (45) found that a significant percentage of diaphragm fibers had evidence of sarcolemmal injury, as indicated by hyperpermeability of fibers removed from septic animals when exposed in vitro to a low molecular weight tracer dye that is normally unable to penetrate muscle fibers. This form of sarcolemmal injury was observed following either endotoxin administration or cecal ligation-perforation. In both experimental models, administration of a NOS inhibitor, NG-monomethyl-L-arginine, markedly reduced sarcolemmal injury, arguing this process is linked to generation of free radical species by one or more NOS isoforms (see Fig. 4) (45). Of note, NOS isoforms either can generate nitric oxide when this enzyme is "coupled" (i.e., all substrates and cofactors are readily available) or can generate superoxide when the enzyme is "uncoupled" (when arginine or cofactors such as tetrahydrobiopterin are absent). As a result, the inhibition of injury by NG-monomethyl-L-arginine could be due to a reduction in production of either nitric oxide, superoxide, or a combination of these species.
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FREE RADICALS AND PROTEIN TURNOVER
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Recent work has also established a potential role for free radicals in triggering the activation of proteolytic pathways in skeletal muscle in inflammatory conditions. In vitro studies indicate that TNF- alone appears to be sufficient to initiate activation of the major proteolytic pathway in muscle, the proteosome, albeit other cytokines, especially IL-1β, appear to activate similar pathways (6, 42). Degradation of proteins by the proteosome requires several steps: 1) linkage of ubiquitin to a E1-activating enzyme, 2) transfer of the activated ubiquitin to a conjugating enzyme (E2) that acts as a carrier, 3) subsequent transfer of activated ubiquitin to a ligase protein (E3), which attaches ubiquitin to a protein targeted for degradation, and 4) degradation of ubiquitin-conjugated target proteins by the 26S proteosome complex (53). Studies performed using isolated muscle cells indicate that the effect of TNF- to activate the proteosome appears to first involve production of superoxide within minutes to hours following exposure to TNF-, followed by subsequent activation of MAPK signaling pathways, notably p38, followed by phosphorylation and degradation of the NF-
B inhibitory protein-B (I-B) (39, 41). Loss of I-B leads to translocation of NF-B to the nucleus and upregulation of E-proteins, including UbcH2/E2–20k (an E2 protein) (41, 43).
Expression of MuRF1 and atrogin, E3 ligases that are rate limiting for ubiquitin transfer to proteins targeted for degradation, also appears to be dependent on free radical mechanisms, as exposure of isolated muscle cells to hydrogen peroxide leads to rapid upregulation of both of these E3 ligases (see Fig. 5) (40). Since infection and/or inflammation is known to increase free radical formation in skeletal muscle and also increases both MuRF1 and atrogin in muscle, it, therefore, also seems likely that cytokine-induced increases in MuRF1 and atrogin may be free radical mediated, albeit this linkage has not been directly demonstrated.
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Fig. 5. H2O2 administration to C2C12 cells upregulates atrogin-1/MAFbx. C2C12 myotubes were incubated with 100 µM H2O2, and total RNA for cells was isolated for Northern blot analysis using full-length cDNA probes. A: autoradiograph, which demonstrates an increase in atrogin-1/MAFbx mRNA at 3 and 6 h after exposure to H2O2. B: photograph of an ethidium bromide-stained agarose gel, which serves as a loading control for A. C: averaged data quantified by densitometry. Means ± SE are shown. *Statistical difference from control (P < 0.05). [From Ref. 40.]
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Recent work argues, however, that proteolysis of contractile proteins in many catabolic conditions is a two-step process involving: 1) an initial step that disrupts the contractile matrix and involves activation of either calpain or caspase, followed by 2) a second step in which the proteosome is activated and disrupted proteins undergo ubiquitin conjugation followed by proteosomally mediated degradation (19, 52). As indicated earlier in this review, recent evidence demonstrates that both calpain and caspase can become activated in skeletal muscles in inflammatory conditions. It is possible that, like the proteosome, calpain and caspase activation may also be influenced by free radicals in skeletal muscle. While this link has not yet been studied in skeletal muscle, there is substantial evidence in other body tissues that activation of both calpain and caspase in response to many biological stresses is free radical dependent (1, 58, 78). For example, apoptosis of retinal photoreceptor cells is known to be dependent on both calpain and caspase activation, and studies have shown that this process can be completely blocked by administration of oxygen free radical scavengers (58). In addition, recent reports indicate that the free radical nitric oxide is a potent activator of calpain in brain, and several forms of excitotoxic brain injury are the result of this nitric oxide-mediated activation of calpain (1, 78).
Skeletal muscle protein synthesis is reduced in infections and/or inflammatory conditions, and this decrease is due, at least in part, to inhibition of key steps controlling translation of mRNA into protein. It is known that sepsis decreases the phosphorylation and activation of eucaryotic initiation factor 4E, an important regulator of translation, and recent data indicate that eucaryotic initiation factor 4E phosphorylation is regulated by mitogen-activated protein kinase-interacting kinase 1, which, in turn, is regulated by p38 (77). Other experiments have shown that muscle cell p38 activation in response to cytokines is dependent on free radical generation, arguing that free radicals may also play some role in the regulation of protein synthesis (39). This possibility has not yet been studied, however, and, in general, there has been little work elucidating the specific mechanism(s) by which infections and/or inflammation alter mRNA translation and protein synthesis.
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SOURCES OF FREE RADICALS IN SKELETAL MUSCLE
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As indicated above, marker studies have provided substantial evidence of a variety of free radical-mediated modifications in skeletal muscle as the result of infection and/or inflammation (2, 23, 65, 67). In addition, several papers have used fluorescent indicator techniques to demonstrate an increase in the production of free radicals by skeletal muscle removed from septic animals. As an example, Nethery et al. (51) infused hydroethidine into diaphragms taken from endotoxin-treated animals and observed marked formation of ethidium, the product of reaction of hydroethidine with free radical species, in the diaphragm. Studies have further identified at least three sources of free radical generation that appear to increase in response to an infection and/or inflammatory stimulus. First, studies have observed an increase in mitochondrial generation of superoxide/hydrogen peroxide, both for mitochondria isolated from skeletal muscles of septic animals and for mitochondria in isolated skeletal muscle cells exposed to TNF- (5, 50, 53). Second, skeletal muscle contains a superoxide-generating NADPH oxidase, consisting of an enzyme complex containing p47phox, gp91phox, p22phox, and p67phox subunits (22, 33, 35). Various reports have differed regarding the localization of this complex, with most recent evidence suggesting that this is localized to the transverse tubules where, under physiological conditions, it plays a role in regulation of ryanodine channel activity (33). The activity of this complex appears to be increased in skeletal muscle in animal models of sepsis (35).
Third, skeletal muscle contains a number of NOS isoforms, and reports have consistently indicated that upregulation of the inducible NOS isoform occurs in skeletal muscle in response to inflammatory and/or infectious stimuli (7, 38, 57). NOS, in turn, is capable of generating both nitric oxide or superoxide or a combination of these free radical species, depending upon the state of NOS coupling or uncoupling. While it seems fairly clear that it is possible that increased free radicals can be generated from all three of these enzyme systems in inflammatory and/or infectious conditions, the relative pathophysiological importance of these three sources is not clear, and it is also not known which source is responsible for which physiological derangements. In addition, no study has determined whether or not NOS produces exclusively nitric oxide in skeletal muscle or, under some conditions such as inflammation, can become uncoupled and produce a mixture of nitric oxide and superoxide.
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IMPLICATIONS AND FUTURE DIRECTIONS
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Limb skeletal muscle dysfunction is a major cause of morbidity in patients with a wide variety of acute and chronic inflammatory medical conditions (60). In addition, it is likely that inflammation-induced respiratory skeletal muscle dysfunction is a major factor contributing to the morbidity and mortality of patients admitted to intensive care units (37, 79). Currently, no pathophysiologically rational treatment is available to deal with this problem, and the finding of weakness is usually addressed clinically simply by attempting to improve nutrition.
Additional research is critical for progress in this area, and the development of specific therapies to prevent and reverse inflammation-induced muscle dysfunction is urgently needed. As indicated in this review, free radicals may play an important role in triggering activation of a number of downstream processes that collectively act to impair muscle function and lead to reductions in muscle strength and mass. Our understanding of many of these issues is still in its infancy (e.g., the mechanisms by which protein synthesis is reduced by inflammatory stimuli). A complete elucidation of the pathophysiological sequences by which cytokines and free radicals alter muscle function is needed. It is hoped that such information will lead to the development of rational therapies to reverse or prevent skeletal muscle dysfunction in this large patient population. One would expect such a treatment to have a significant impact on patient mortality, a huge impact on patient morbidity, and result in a major savings in health care costs.
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GRANTS
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This study was supported by National Heart, Lung, and Blood Institute Grants 69821, 63698, 80429, 80609, and 81525.
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FOOTNOTES
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Address for reprint requests and other correspondence: G. Supinski, Chandler Medical Center, University of Kentucky, 800 Rose St., Lexington, KY 40536. (e-mail: gsupi2{at}email.uky.edu)
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TOP
ABSTRACT
INFLAMMATION AND MUSCLE FUNCTION
), endotoxin-treated animals (
), animals given PEG-SOD (pegylated superoxide dismutase) and endotoxin (
), animals given PEG-SOD alone (
), and animals given both denatured PEG-SOD and endotoxin (
). Data points are mean findings for all fibers within a given experimental group; error bars represent ±1 SE. Force vs. pCa curves for the endotoxin and denatured PEG-SOD plus endotoxin groups are statistically different from the curve for the control group, indicating that administration of PEG-SOD prevents endotoxin-induced reductions in single-fiber force generation. [Reproduced with permission from Ref. 
P < 0.05 vs. CLP group; 
P < 0.05 vs. CLP + L-NMMA group. [Reproduced with permission from Ref.