Year-long clenbuterol treatment of mice increases mass, but not specific force or normalized power, of skeletal muscles.

1. Clenbuterol has been proposed for the treatment of muscle wasting disorders, but its long-term effects on skeletal muscle function have not been tested rigorously. We tested the hypothesis that year-long treatment of young (6 months) mice with clenbuterol would increase skeletal muscle mass and in vitro measurements of specific force (Po) and power output. 2. Male mice (C57BL/10ScSn) were divided into treated (n = 6) or untreated (n = 8) groups. Treated mice received clenbuterol (1.5-2 mg/kg per day) in their drinking water for 52 weeks, following a staggered 3 day on/3 day off schedule to attenuate the response to clenbuterol. 3. Clenbuterol treatment increased the absolute mass of each muscle tested: the heart by 28%, extensor digitorum longus (EDL) by 16%, soleus by 22% and tibialis anterior by 17%. For treated compared with untreated mice, absolute Po (mN) was greater in soleus muscles but not different in EDL muscles. Absolute power output (mW) of the EDL and soleus muscles was not different and no differences were observed for the specific Po (kN/m2) or normalized power output (W/kg) of EDL muscles, soleus muscles or diaphragm muscle strips. 4. We conclude that, following year-long treatment of mice with clenbuterol, the mass of the heart and both fast and slow skeletal muscles is increased, but the lack of any change in normalized Po or power output indicates that clenbuterol has little therapeutic effect on the functional properties of skeletal muscle.