Abstract
Maintenance of skeletal muscle mass throughout the life course is key for the regulation of health, with physical activity a critical component of this, in part, due to its influence upon key hormones such as testosterone, estrogen, growth hormone (GH), and insulin-like growth factor (IGF). Despite the importance of these hormones for the regulation of skeletal muscle mass in response to different types of exercise, their interaction with the processes controlling muscle mass remain unclear. This review presents evidence on the importance of these hormones in the regulation of skeletal muscle mass and their responses, and involvement in muscle adaptation to resistance exercise. Highlighting the key role testosterone plays as a primary anabolic hormone in muscle adaptation following exercise training, through its interaction with anabolic signaling pathways and other hormones via the androgen receptor (AR), this review also describes the potential importance of fluctuations in other hormones such as GH and IGF-1 in concert with dietary amino acid availability; and the role of estrogen, under the influence of the menstrual cycle and menopause, being especially important in adaptive exercise responses in women. Finally, the downstream mechanisms by which these hormones impact regulation of muscle protein turnover (synthesis and breakdown), and thus muscle mass are discussed. Advances in our understanding of hormones that impact protein turnover throughout life offers great relevance, not just for athletes, but also for the general and clinical populations alike.
Highlights
Skeletal muscle accounts for ∼40–45% of total body mass (Romagnoli et al, 2020)
Activation of these androgenresponse elements (ARE) stimulates the transcription of protein targets and other anabolic systems, such as the local production of insulin-like growth factor (IGF)-1 which is related to muscle protein accretion through a decrease in IGFBP-4 mRNA concentration coupled with an increase in insulin-like growth factor-1 (IGF-1) mRNA (Bamman et al, 2001; West et al, 2010)
resistance exercise (RE)-induced increases in key endogenous steroid and peptide hormone responses are likely to be an integral part of the integrated response to acute exercise and exercise-induced muscle growth
Summary
Skeletal muscle accounts for ∼40–45% of total body mass (Romagnoli et al, 2020). Following a rapid post-natal growth phase, skeletal muscle mass is typically maintained at a steady state in adulthood through a controlled balance between muscle protein synthesis (MPS) and breakdown (MPB)—unless in the presence of physiological (exercise) or pathological (age or disease) stimuli. Exercise-Induced Testosterone Release, and Links With Muscle Adaptation by Sex and Age. The relationship between RE and testosterone responses have been extensively reviewed in young men (Ahtiainen et al, 2003, 2005; West et al, 2009; West and Phillips, 2012), with the majority of studies suggesting that it is the acute transient elevations in testosterone that likely drive the proposed hormonal adaptations associated with muscle growth. Immediately following RE, serum testosterone levels peak [∼from 13 (resting levels) to 38 (at ∼30 mins) nmol.L−1] with a concomitant upregulation of AR mRNA and protein content within the muscle (Willoughby and Taylor, 2004; Hooper et al, 2017) It seems high intensity RE stimulates basophilic cells of the anterior pituitary to release luteinizing hormone (LH) from gonadotrophs in the anterior pituitary which acts as the primary regulator of testosterone secretion from the Leydig cells of the testes (Fry and Kraemer, 1997). With provision of exogenous testosterone helping to restore this blunting somewhat (Gharahdaghi et al, 2019), the influence of testosterone on muscle may be small and permissive in the young, but the need for hormonal input for the control of muscle mass may be more important as we age to overcome age-related deficits in the responsiveness of older muscle to exercise training
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