Muscle Plasticity and β(2)-Adrenergic Receptors: Adaptive Responses of β(2)-Adrenergic Receptor Expression to Muscle Hypertrophy and Atrophy

We discuss the functional roles of β(2)-adrenergic receptors in skeletal muscle hypertrophy and atrophy as well as the adaptive responses of β(2)-adrenergic receptor expression to anabolic and catabolic conditions. β(2)-Adrenergic receptor stimulation using anabolic drugs increases muscle mass by promoting muscle protein synthesis and/or attenuating protein degradation. These effects are prevented by the downregulation of the receptor. Endurance training improves oxidative performance partly by increasing β(2)-adrenergic receptor density in exercise-recruited slow-twitch muscles. However, excessive stimulation of β(2)-adrenergic receptors negates their beneficial effects. Although the preventive effects of β(2)-adrenergic receptor stimulation on atrophy induced by muscle disuse and catabolic hormones or drugs are observed, these catabolic conditions decrease β(2)-adrenergic receptor expression in slow-twitch muscles. These findings present evidence against the use of β(2)-adrenergic agonists in therapy for muscle wasting and weakness. Thus, β(2)-adrenergic receptors in the skeletal muscles play an important physiological role in the regulation of protein and energy balance.