Functional roles of β₂-adrenergic receptors in skeletal muscle hypertrophy and atrophy

We discuss the functional roles of β₂-adrenergic receptors in skeletal muscle hypertrophy and atrophy, as well as the adaptive responses of β₂-adrenergic receptor expression to anabolic and catabolic conditions. Stimulation of the β₂-adrenergic receptor 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 β₂-adrenergic receptor density in exercise- recruited slow-twitch muscles. However, excessive stimulation of β₂- adrenergic receptors negates their beneficial effects. Although preventive effects of β₂- adrenergic receptor stimulation on atrophy induced by muscle disuse and catabolic hormones or drugs were observed, these catabolic conditions decreased β₂- adrenergic receptor expression in slow-twitch muscles. These findings present evidence against the use of β₂-adrenergic agonists in therapy for muscle wasting and weakness. Thus, β₂-adrenergic receptors in the skeletal muscles play an important physiological role in the regulation of protein and energy balance.