Figure 6. Changes in MuRF1 and MAFbx expression and proteasome activity in adult and old rats after hindlimb unloading (HU) and reloading. mRNA expression of MuRF1 and MAFbx was assessed by quantitative PCR in the soleus (A,B) and tibialis anterior (TA) (C,D) muscles of adult (9 mo, open squares) and old (29 mo, filled squares) rats after 14 days of HU and following 1, 3, 7, and 14 days of reloading. Gene expression was normalized to tissue weight. Data are expressed as a fold change relative to the adult control group. Proteolytic activity of the β1, β2, and β5 subunits of the 20S and 26S proteasome was measured in the soleus (E) and TA (F) muscles of adult (open squares) and old (filled squares) rats after 14 days of HU and after 3, 7, and 14 days of reloading. Data are expressed as a percentage relative to the activity of the adult control group for each subunit (n=4-6/group). Values are mean ± SEM, *p<0.05 vs adult control, #p<0.05 vs old control, φp<0.05 vs adult at same time point.