Contractile properties of clenbuterol-treated mdx muscle are enhanced by low-intensity swimming

The beta(2)-agonist clenbuterol has potent anabolic properties in normal and denervated muscle and, as such, may be of use in muscle wasting diseases such as muscular dystrophy. However, potential side effects such as the transformation of the fiber type pool toward increased proportions of fast-twitch fibers must be balanced with the beneficial anabolic properties. In the present report, we clearly show that extensor digitorum longus and soleus muscles from dystrophic mdx mice exposed to a combination of clenbuterol and low-intensity endurance swimming exercise did not undergo the slow- to fast-twitch fiber transformations caused by clenbuterol administration alone, yet increases in the force-generating capacity of the soleus (30-40%) that resulted fr om the clenbuterol treatment were maintained after the swimming program. The increased sensitivity of dystrophin-deficient dystrophic muscle to clenbuterol and low-intensity exercise that is evident in this study may have therapeutic implications in the treatment of muscle wasting diseases.