Aims: The sources of cytosolic superoxide in skeletal muscle have not been defined. This study examined the subcellular sites that contribute to cytosolic superoxide in mature single muscle fibers at rest and during contractile activity. Results: Isolated fibers from mouse flexor digitorum brevis loaded with superoxide and nitricoxide-sensitive fluorescent probes, specific pathway inhibitors and immunolocalization techniques were used to identify subcellular sites contributing to cytosolic superoxide. Treatment with the electron transport chain complex III inhibitor, antimycin A, but not the complex I inhibitor, rotenone, caused increased cytosolic superoxide through release from the mitochondrial intermembrane space via voltage-dependent anion or Bax channels, but inhibition of these channels did not affect contraction-induced increases in cytosolic superoxide. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors decreased cytosolic superoxide at rest and following contractions. Protein and mRNA expression of NADPH oxidase subunits was demonstrated in single fibers. NOX2, NOX4, and p22 phox subunits localized to the sarcolemma and transverse tubules; NOX4 was additionally expressed in mitochondria. Regulatory p40 phox and p67 phox proteins were found in the cytoplasm of resting fibers, but following contractions, p40 phox appeared to translocate to the sarcolemma. Innovation: Superoxide and other reactive oxygen species generated by skeletal muscle are important regulators of muscle force production and adaptations to contractions. This study has defined the relative contribution of mitochondrial and cytosolic sources of superoxide within the cytosol of single muscle fibers at rest and during contractions. Conclusion: Muscle mitochondria do not modulate cytosolic superoxide in skeletal muscle but NADPH oxidase is a major contributor both at rest and during contractions. Antioxid. Redox Signal. 18, 603-621.

Skeletal muscle adapts to the stress of contractile activity via changes in gene expression to yield an increased content of a family of highly conserved cytoprotective proteins known as heat shock proteins (HSPs). These proteins function to maintain homeostasis, facilitate repair from injury and provide protection against future insults. The study of the exercise-induced production of HSPs in skeletal muscle is important for the exercise scientist as it may provide a valuable insight into the molecular mechanisms by which regular exercise can provide increased protection against related and non-related stressors. As molecular chaperones, HSPs are also fundamental in facilitating the cellular remodelling processes inherent to the training response. Whilst the exercise-induced stress response of rodent skeletal muscle is relatively well characterized, data from humans are more infrequent and less insightful. Data indicate that acute endurance- and resistance-type exercise protocols increase the muscle content of ubiquitin, alphaB-crystallin, HSP27, HSP60, HSC70 and HSP70. Although increased HSP transcription occurs during exercise, immediately post-exercise or several hours following exercise, time-course studies using western blotting techniques have typically demonstrated a significant increase in protein content is only detectable within 1-2 days following the exercise stress. However, comparison amongst studies is complicated by variations in exercise protocol (mode, intensity, duration, damaging, non-damaging), muscle group examined, predominant HSP measured and, perhaps most importantly, differences in subject characteristics both within and between studies (training status, recent activity levels, nutritional status, age, sex, etc.). Following 'non-damaging' endurance-type activities (exercise that induces no overt structural and functional damage to the muscle), the stress response is thought to be mediated by redox signalling (transient and reversible oxidation of muscle proteins) as opposed to increases in contracting muscle temperature per se. Following 'damaging' forms of exercise (exercise that induces overt structural and functional damage to the muscle), the stress response is likely initiated by mechanical damage to protein structure and further augmented by the secondary damage associated with inflammatory processes occurring several days following the initial insult. Exercise training induces an increase in baseline HSP levels, which is dependent on a sustained and currently unknown dose of training and also on the individual's initial training status. Furthermore, trained subjects display an attenuated or abolished stress response to customary exercise challenges, likely due to adaptations of baseline HSP levels and the antioxidant system. Whilst further fundamental work is needed to accurately characterize the exercise-induced stress response in specific populations following varying exercise protocols, exercise scientists should also focus their efforts on elucidating the precise biological si...

The exercise-induced expression of heat shock proteins (HSPs) in rodent models is relatively well defined. In contrast, comparable data from human studies are limited and the exercise-induced stress response of human skeletal muscle is far from understood. This study has characterized the time course and magnitude of the HSP response in the skeletal muscles of a healthy active, but untrained, young male population following a running exercise protocol. Eight subjects performed 45 min of treadmill running at a speed corresponding to their lactate threshold (11.7 +/- 0.5 km/h; 69.8 +/- 4.8% maximum O2 uptake). Muscle biopsies were obtained from the vastus lateralis muscle immediately before and at 24 h, 48 h, 72 h, and 7 days postexercise. Exercise induced a significant (P < 0.05) but variable increase in HSP70, heat shock cognate (HSC) 70, and HSP60 expression with peak increases (typically occurring at 48 h postexercise) to 210, 170, and 139% of preexercise levels, respectively. In contrast, exercise did not induce a significant increase in either HSP27, alphaB-crystallin, SOD 2 (MnSOD) protein content, or the activity of SOD and catalase. When examining baseline protein levels, HSC70, HSP27, and alphaB-crystallin appeared consistently expressed between subjects, whereas HSP70 and MnSOD displayed marked individual variation of up to 3- and 1.5-fold, respectively. These data are the first to define the time course and extent of HSP production in human skeletal muscle following a moderately demanding and nondamaging running exercise protocol. Data demonstrate a differential effect of aerobic exercise on specific HSPs.

Skeletal muscle aging is characterized by atrophy, a deficit in specific force generation, increased susceptibility to injury, and incomplete recovery after severe injury. The ability of muscles of old mice to produce heat shock proteins (HSPs) in response to stress is severely diminished. Studies in our laboratory using HSP70 overexpressor mice demonstrated that lifelong overexpression of HSP70 in skeletal muscle provided protection against damage and facilitated successful recovery after damage in muscles of old mice. The mechanisms by which HSP70 provides this protection are unclear. Aging is associated with the accumulation of oxidation products, and it has been proposed that this may play a major role in age-related muscle dysfunction. Muscles of old wild-type (WT) mice demonstrated increased lipid peroxidation, decreased glutathione content, increased catalase and superoxide dismutase (SOD) activities, and an inability to activate nuclear factor (NF)-kappaB after contractions in comparison with adult WT mice. In contrast, levels of lipid peroxidation, glutathione content, and the activities of catalase and SOD in muscles of old HSP70 overexpressor mice were similar to adult mice and these muscles also maintained the ability to activate NF-kappaB after contractions. These data provide an explanation for the preservation of muscle function in old HSP70 overexpressor mice.

We have previously shown that deletion of CuZnSOD in mice (Sod1(-/-) mice) leads to accelerated loss of muscle mass and contractile force during aging. To dissect the relative roles of skeletal muscle and motor neurons in this process, we used a Cre-Lox targeted approach to establish a skeletal muscle-specific Sod1-knockout (mKO) mouse to determine whether muscle-specific CuZnSOD deletion is sufficient to cause muscle atrophy. Surprisingly, mKO mice maintain muscle masses at or above those of wild-type control mice up to 18 mo of age. In contrast, maximum isometric specific force measured in gastrocnemius muscle is significantly reduced in the mKO mice. We found no detectable increases in global measures of oxidative stress or ROS production, no reduction in mitochondrial ATP production, and no induction of adaptive stress responses in muscle from mKO mice. However, Akt-mTOR signaling is elevated and the number of muscle fibers with centrally located nuclei is increased in skeletal muscle from mKO mice, which suggests elevated regenerative pathways. Our data demonstrate that lack of CuZnSOD restricted to skeletal muscle does not lead to muscle atrophy but does cause muscle weakness in adult mice and suggest loss of CuZnSOD may potentiate muscle regenerative pathways.

Skeletal muscle comprises the largest organ system in the human body and is essential for force generation and movement. Skeletal muscle is subjected to considerable stresses during everyday use. However, muscle has the unique ability to adapt and remodel to provide protection against such stresses. This adaptation occurs at the structural through to the cellular level, which includes changes in transcription of a range of protective proteins. Failure in such processes can be catastrophic. This failure in adaptation is particularly notable in older individuals. Our skeletal muscles become smaller and weaker as we age. This loss of muscle bulk results in a reduced capacity to generate force and results in a loss of the ability to undertake everyday tasks. This article describes the normal adaptive responses of muscle in younger individuals to the stress of various forms of exercise and the implications of a failure of these adaptive responses in the elderly.

Unlike muscles of young mice, skeletal muscles of old mice fail to recover completely following contractioninduced damage. The mechanisms by which this occurs are not fully understood. The ability of muscles of old mice to adapt following exercise by the increased production of heat shock proteins (HSPs) is blunted. Studies using transgenic mice have shown that this inability to produce HSPs has a major effect on muscle regeneration. Overexpression of HSP70 facilitated complete recovery of maximum tetanic force generation in muscles of old transgenic mice following contraction induced-damage in comparison with a deficit in muscles of old wild-type (WT) mice. We hypothesized that pharmacological induction of HSP70 in muscles of old WT mice would result in enhanced recovery from contraction-induced damage. A single dose of 40 mg/kg of 17-(allylamino)-17-demethoxygeldanamycin (17AAG) resulted in a significant increase in the HSP70 content of extensor digitorum longus muscles of adult C57BL6/J mice 3 days following treatment compared with vehicle-treated mice. Four weekly treatments of adult and old mice resulted in a two-to four-fold increase in muscle HSP70 content. Treatment of old mice with 17AAG at 3 days prior to and weekly for 4 weeks following a severely damaging contraction protocol resulted in enhanced recovery of force generation at 28 days postdamage compared with muscles of vehicle-treated mice. Data suggest that 17AAG overcomes the mechanism by which activation of the stress response fails in muscles of old mice and may have therapeutic benefit in the recovery following damage in muscles of older individuals.

These data demonstrate that increases in both systemic and local muscle temperature per se do not appear to mediate the exercise-induced production of HSPs in human skeletal muscle and suggest that non-heat stress factors associated with contractile activity are of more importance in mediating this response.