Skeletal muscle reloading following disuse is characterized by profound oxidative damage. This study tested the hypothesis that intermittent hyperthermia during reloading attenuates oxidative damage and augments skeletal muscle regrowth following immobilization. Forty animals were randomly divided into four groups: control (Con), immobilized (Im), reloaded (RC), and reloaded and heated (RH). All groups but Con were immobilized for 7 days. Animals in the RC and RH groups were then reloaded for 7 days with (RH) or without (RC) hyperthermia (4141.5°C for 30 min on alternating days) during reloading. Heating resulted in ?25% elevation in heat shock protein expression (P < 0.05) and an ?30% greater soleus regrowth (P < 0.05) in RH compared with RC. Furthermore, oxidant damage was lower in the RH group compared with RC because nitrotyrosine and 4-hydroxy-2-nonenol were returned to near baseline when heating was combined with reloading. Reduced oxidant damage was independent of antioxidant enzymes (manganese superoxide dismutase, copper-zinc superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase). In summary, these data suggest that intermittent hyperthermia during reloading attenuates oxidative stress and improves the rate of skeletal muscle regrowth during reloading after immobilization.
muscle atrophy is a critical health concern in several disease states and impacts the quality of life and ability to live independently in the elderly (28,43). Often, atrophy is a necessary consequence of clinical interventions, such as bed confinement or limb immobilization. Independent of events leading to atrophy, reloading of the muscle is necessary to return to a preatrophy functional level. Much research has been dedicated to the determination of atrophic mechanisms. Comparatively little attention has been dedicated to the search for mechanisms and interventions that enhance regrowth during subsequent muscle reloading.
Reloading is marked by oxidative damage because cellular macromolecules are modified by free radicals (17). Furthermore, there is a cytosolic Ca2+ overload (2, 13, 37) contributing to a loss of Ca2+ homeostatic balance that likely exacerbates the oxidative stress response via xanthine oxidase-mediated superoxide anion radical (O2?) production (25, 29). Specifically, elevated intracellular Ca2+ activates the protease calpain, which in turn converts xanthine dehydrogenase into the O2?-producing xanthine oxidase. This oxidative stress response is also associated with contraction induced injury. Superoxide dismutase (SOD) supplementation attenuates the reduction in force associated with contraction-induced injury, indicating that loss in force generation is not solely mechanical in nature but that it is also related to production of reactive oxygen species (5,47). Regardless of the source of free radicals, increased oxidant production during reloading results in increased lipid oxidation as well as oxidized glutathione (17). Furthermore, vitamin E administered concurrently with reloading results in a reduction in lipid oxidation and muscle mass recovers at a significantly faster rate (17).
In addition to chaperone qualities, heat shock proteins (HSPs) have repeatedly been shown to possess antioxidant properties. Recently, we used heating to induce HSPs and reduce oxidant damage and attenuate the muscle atrophy seen during immobilization (38). Studies in other tissues, designed to clarify the antioxidant roles of HSP 25 (HSP25) and HSP 72 (HSP72), have used molecular biology techniques to induce expression of these HSPs. HSP overexpression improved cell survivability and reduced cell damage in renal tubular cells and human neuroblastoma cells following H2O2 exposure (16, 46). Moreover, HSP72-overexpressing mutants recovered force significantly faster and had less damage following a lengthening contraction protocol than wild-type controls (24).
Thus it seems clear that HSPs can augment muscle regrowth by preventing oxidant stress. Based on this rationale, we hypothesized that intermittent hyperthermia during 1 wk of reloading after immobilization, will increase the rate of muscle regrowth and decrease oxidative damage. Findings from this study support these hypotheses and demonstrate, for the first time, that intermittent heating augments muscle regrowth and attenuates oxidative damage during reloading. Our data suggest a link between the enhanced regrowth rate, decreased oxidative damage and overexpression of HSPs.