Muscle-specific effects of hindlimb suspension and clenbuterol in mature male rats.

Anabolic agents are useful tools for probing the mechanisms by which muscle fibers perceive and respond to disuse. beta(2)-Adrenergic agonists exert protective, and/or reparative, effects on atrophying muscle tissue. The effects of one such agent, clenbuterol (Cb), were examined on muscle mass, total protein content, and myofibrillar protein content in selected hindlimb muscles [adductor longus (ADL), extensor digitorum longus (EDL), plantaris (PLAN), soleus (SOL)] of mature male rats, under different loading conditions. Pair-fed rats were divided into four experimental groups: vehicle- and Cb-treated nonsuspended, vehicle- and Cb-treated hindlimb suspended (HLS). Experiments lasted 14 days, during which the rats received subcutaneous injections of 1 mg/kg Cb or 1 ml/kg vehicle. HLS induced significant atrophy in all muscles, except the EDL, in a generally fiber type-related pattern. However, myofibrillar protein content was affected in a more regional pattern. Cb treatment of nonsuspended rats induced hypertrophy in all muscles, in a generally uniform pattern. However, myofibrillar protein content was affected in a more fiber type-related pattern. Cb treatment of HLS rats reduced or eliminated HLS-induced atrophy in all muscles, in a muscle-specific pattern. Overall, the ADL and SOL were most susceptible to HLS-induced atrophy. The PLAN had the greatest magnitude of Cb-induced sparing of atrophy. The results show that, in mature male rats, Cb exerts anabolic effects that are load-dependent and muscle-specific. Responses to this drug cannot be reliably predicted by fiber-type composition alone. Copyright 2002 S. Karger AG, Basel