RhoA expression during recovery from skeletal muscle disuse

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Abstract
Functional overload and anabolic steroid administration induce signaling pathways that regulate skeletal muscle RhoA expression. The purpose of this study was to determine RhoA and associated protein expression at the onset of disuse and after a brief period of reloading. Male Sprague-Dawley rats were randomly assigned to cage control (Con), 3 days of hindlimb suspension (Sus), or 3 days of hindlimb suspension with 12 h of reloading (12-h Reload). The reloading stimuli consisted of 12 h of resumed normal locomotion after 3 days of hindlimb suspension. Plantaris muscle-to-body weight (mg/g) ratio decreased 17% from Con with Sus but returned to Con with 12-h Reload, increasing 13% from Sus. Sus decreased RhoA protein concentration 46%, whereas 12-h Reload induced a 24% increase compared with Sus. The ratio of cytosolic- to membrane-associated RhoA protein was not changed with either Sus or 12-h Reload. RhoA mRNA concentration was decreased 48% by Sus, and 12-h Reload induced a 170% increase from Sus. β1-Integrin protein, a transmembrane protein associated with RhoA activation, was not altered by Sus but increased 155% with 12-h Reload. Although β1-integrin mRNA was not altered by Sus, it increased 70% from Con with 12-h Reload. Rho family member Cdc42 protein associated with the muscle membrane was decreased 60% with Sus, and 12-h Reload induced a 172% increase compared with Sus. In conclusion, decreased RhoA protein expression and mRNA abundance are early adaptations to disuse but recover rapidly after normal locomotion is resumed.