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1 Division of Exercise Physiology, West Virginia University School of Medicine, Morgantown, WV, USA
* To whom correspondence should be addressed. E-mail: salway{at}hsc.wvu.edu.
The influence of aging on skeletal myocyte apoptosis is not well understood. In this study we examined apoptosis and apoptotic regulatory factor responses to muscle atrophy induced via limb unloading following loading-induced hypertrophy. Muscle hypertrophy was induced by attaching a weight to one wing of young and aged Japanese quails for 14 days. Removing the weight for 7 or 14 days following the initial 14 days of loading induced muscle atrophy. The contralateral wing served as the intra-animal control. A time-released bromodeoxyuridine (BrdU) pellet was implanted subcutaneously with wing weighting to identify activated satellite cells/muscle precussor cells throughout the experimental period. Bcl-2 mRNA and protein levels decreased after 7-days of unloading but they were unchanged after 14-days of unloading in young muscles. Bcl-2 protein but not mRNA level decreased after 7-days of unloading in muscles of aged birds. 7-days of unloading increased the mRNA level of Bax in muscles from both young and aged birds. 14-days of unloading increased mRNA and protein levels of Bcl-2, decreased mRNA and protein levels of Bax, and decreased nuclear AIF protein level in muscles of old birds. BrdU-positive nuclei were found in all unloaded muscles from both age groups but the number of BrdU-positive nuclei relative to the total nuclei decreased after 14-days of unloading as compared with 7-days of unloading. The TUNEL index was higher after 7-days of unloading in both young and aged muscles and after 14-days of unloading in aged muscles. Immunofluorescent staining revealed that almost all of the TUNEL-positive nuclei were also BrdU-immunopositive suggesting, that activated satellite cell nuclei (both fused and unfused) underwent nuclear-apoptosis during unloading. There were significant correlations among Bcl-2, Bax, AIF, and TUNEL index. Our data are consistent with the hypothesis that apoptosis regulates, at least in part, unloading-induced muscle atrophy and loss of activated satellite cell nuclei in previously loaded muscles. Moreover, these data suggest that aging influences the apoptotic responses to prolonged unloading following hypertrophy in skeletal myocytes.
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