4 DAYS CLENBUTEROL TREATMENT INCREASES PROTEASOME ACTIVITY IN RAT SOLEUS MUSCLE.

Abstract

1287 Four to eight weeks of clenbuterol administration is known to result in an increase in muscle mass of the extensor digitorum longus (EDL) and to induce a slow-to-fast fiber type shift in the soleus, which is associated with an increase in the total adenine nucleotide pool (TAN). However, the mechanisms by which these effects are achieved are unknown. The 26S proteasome is known to modulate myofibrillar protein degradation and an increase in 26S activity is known to precede this process. Therefore, male Sprague-Dawley rats (300-350 g) were administered a single daily bolus of CLEN (250μg/kg/day, n=9) or saline (0.1ml, n=8) s.c. for 4 days. At the end of day 4, EDL and soleus muscles were removed, snap frozen and used to determine TAN and proteasome activity. Changes in proteasome chymotrypsin-like activity were measured by fluorogenic substrates and changes in ATPase sub-unit expression were investigated by densitometry on representative Western-blots. Following CLEN treatment there was no difference in the TAN of EDL (33.3±0.7 mmol/kg dry muscle) and soleus (20.3±1.0 mmol/kg dry muscle) compared with corresponding controls (33.0±0.7 mmol/kg dry muscle, p>0.05; 21.2±0.8 mmol/kg dry muscle, p>0.05, respectively). Proteasome chymotrypsin-like activity was increased in soleus (28±1%, p<0.05) but unaffected in EDL. Densitometry on Western-blots of the soleus revealed an increase in the expression of p42, MSS1 and TBP1 subunits (142%, 84% and 39% respectively). These results demonstrate that 4 days CLEN treatment increases proteasome activity and sub-unit expression in the soleus muscle but not in the EDL. It is suggested that these increases are linked with the slow-to-fast fiber type conversion seen with longer administration.