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1 Applied Physiology and Kinesiology, University of Florida, Gainesville, Florida, USA
2 Applied Physiology and Kinesiology, University of Florida, Gainesville, Florida, USA; Geriatric, Research, Education and Clinical Center, Malcom Randall VA Medical Center, Gainesville, Florida, USA
* To whom correspondence should be addressed. E-mail: sdodd{at}hhp.ufl.edu.
The ubiquitin-proteasome pathway is primarily responsible for myofibrillar protein degradation during hindlimb unweighting (HU). Beta-adrenergic agonists such as clenbuterol (CB) induce muscle hypertrophy and attenuate muscle atrophy due to disuse or inactivity. However, the molecular mechanism by which CB exerts these effects remains poorly understood. The aims of this study were to investigate whether CB attenuates HU-induced muscle atrophy through an inhibition of the ubiquitin-proteasome pathway, and whether insulin-like growth factor I (IGF-1) mediates this inhibition. Rats were randomized to the following groups: weight-bearing control (CON), 14-day CB-treated (CB), 14-day HU (HU), CB+HU. HU-induced atrophy was associated with increased proteolysis and up-regulation of components of the ubiquitin-proteasome pathway (ubiquitin conjugates, ubiquitin conjugating enzyme E2-14kDa, and 20S proteasome activity). Up-regulation of the ubiquitin-proteasome occurred in all muscles tested, but was more pronounced in muscles composed primarily of slow twitch fibers (soleus = SOL) than in fast twitch muscles (plantaris = PA and tibialis anterior = TA). Although CB induced hypertrophy in all muscles, CB attenuated the HU-induced atrophy and reduced ubiquitin conjugates only in the fast PA and TA, and not in the slow SOL muscle. CB did not elevate IGF-I protein content in either of the muscles examined. These results suggest that CB induces hypertrophy and alleviates HU-induced atrophy, particularly in the fast muscles, at least in part, through a muscle-specific inhibition of the ubiquitin-proteasome pathway, and that these effects are not mediated by the local production of IGF-I in skeletal muscle.
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