Cellular mechanisms underlying temporal changes in skeletal muscle protein synthesis and breakdown during chronic β-adrenoceptor stimulation in mice

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Abstract
Chronic stimulation of β-adrenoceptors with β-adrenoceptor agonists (β-agonists) can induce substantial skeletal muscle hypertrophy, but the mechanisms mediating this muscle growth have yet to be elucidated. We investigated whether chronic β-adrenoceptor stimulation in mice with the β-agonist formoterol alters the muscle anabolic response following β-adrenoceptor stimulation. Twelve-week-old C57BL/6 mice were treated for up to 28 days with a once-daily injection of either saline (control, n = 9) or formoterol (100 μg kg⁻¹; n = 9). Rates of muscle protein synthesis were assessed at either 1, 7 or 28 days of treatment, 6 h after injection. Protein synthesis rates were higher in formoterol-treated mice at day 7 (∼1.5-fold, P < 0.05), but not at day 1 or 28. The increased muscle protein synthesis was associated with increased phosphorylation of S6K1 (r = 0.49, P < 0.01). Formoterol treatment acutely reduced maximal calpain activity by ∼25% (P < 0.05) but did not affect atrogin-1 protein levels and proteasome-mediated proteolytic activity, despite significantly enhanced phosphorylation of Akt (P < 0.05). Formoterol increased CREB phosphorylation by ∼30% (P < 0.05) and PPARγ coactivator-1α (PGC-1α) by 11-fold (P < 0.05) on day 1 only. These observations identify that formoterol treatment induces muscle anabolism, by reducing calpain activity and by enhancing protein synthesis via increased PI-3 kinase/Akt signalling.

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