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 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.