Nuclear-localized androgen receptor content following resistance exercise training is associated with hypertrophy in males but not females

Bioavailable testosterone and androgen receptor activation, but not total testosterone, are associated with muscle mass and strength in females

Testosterone, the major androgen, influences the reproductive and non-reproductive systems in males and females via binding to the androgen receptor (AR). Both circulating endogenous testosterone and muscle AR protein content are positively associated with muscle mass and strength in males, but there is no such evidence in females. Here, we tested whether circulating testosterone levels were associated with muscle mass, function, or the muscle anabolic response to resistance training in pre-menopausal females. Twenty-seven pre-menopausal, untrained females (aged 23.5 ± 4.8 years) underwent a 12-week resistance training programme. Muscle strength, size, power, and plasma and urine androgen hormone levels were measured. Skeletal muscle biopsies were collected before and after the training programme to quantify the effect of resistance training on AR content and nuclear localisation. Primary muscle cell lines were cultured from a subset (n = 6) of the participants' biopsies and treated with testosterone to investigate its effect on myotube diameter, markers of muscle protein synthesis and AR cellular localisation. Physiological levels of total testosterone were not associated with muscle mass or strength at baseline or with the changes in muscle mass and strength that occurred in response to resistance training in our cohort of pre-menopausal females. In contrast, bioavailable testosterone and the proportion of nuclear-localised AR were positively associated with skeletal muscle mass and strength in pre-menopausal females. In vitro, supra-physiological doses of testosterone increased myocyte diameter, but this did not occur via the Akt/mTOR pathway as previously suggested. Instead, we show a marked increase in AR nuclear localisation with testosterone administration in vitro. KEY POINTS: Total circulating testosterone was not related to muscle mass or strength before or after resistance training in pre-menopausal females. Bioavailable testosterone was positively related to exercise-induced muscle hypertrophy in pre-menopausal females. In vivo nuclear localisation of the androgen receptor was positively related to muscle mass in pre-menopausal females at baseline, but not to resistance training-induced hypertrophy. Testosterone treatment induced androgen receptor nuclear translocation but did not induce mTOR signalling in primary skeletal myocytes cultured from pre-menopausal female muscle.

Hormones, Hypertrophy, and Hype: An Evidence-Guided Primer on Endogenous Endocrine Influences on Exercise-Induced Muscle Hypertrophy

We review the evidence indicating that endogenous changes in these hormones, including testosterone, growth hormone, insulin growth factor-1, and estrogen, and their proposed anabolic effects contribute to and augment resistance exercise training (RET)-induced hypertrophy. Additionally, we provide recommendations for gold-standard methodological rigor to establish best practices for verifying menstrual phases as part of their research, ultimately enhancing our understanding of the impact of ovarian hormones on RET-induced adaptations.

Androgen receptor markers do not differ between nonresponders and responders to resistance training-induced muscle hypertrophy

The aim of this study was to investigate whether baseline values and acute and chronic changes in androgen receptors (AR) markers, including total AR, cytoplasmic (cAR), and nuclear (nAR) fractions, as well as DNA-binding activity (AR-DNA), are involved in muscle hypertrophy responsiveness by comparing young nonresponder and responder individuals. After 10 wk of resistance training (RT), participants were identified as nonresponders using two typical errors (TE) obtained through two muscle cross-sectional area (mCSA) ultrasound measurements (2 × TE; 4.94%), and the highest responders within our sample were numerically matched. Muscle biopsies were performed at baseline, 24 h after the first RT session (acute responses), and 96 h after the last session (chronic responses). AR, cAR, and nAR were analyzed using Western blotting, and AR-DNA was analyzed using an ELISA-oligonucleotide assay. Twelve participants were identified as nonresponders (ΔmCSA: -1.32%) and 12 as responders (ΔmCSA: 21.35%). There were no baseline differences between groups in mCSA, AR, cAR, nAR, or AR-DNA (P > 0.05). For acute responses, there was a significant difference between nonresponders (+19.5%) and responders (-14.4%) in AR-DNA [effect size (ES) = -1.39; 95% confidence interval (CI): -2.53 to -0.16; P = 0.015]. There were no acute between-group differences in any other AR markers (P > 0.05). No significant differences between groups were observed in chronic responses across any AR markers (P > 0.05). Nonresponders and responders presented similar baseline, acute, and chronic results for the majority of the AR markers. Thus, our findings do not support the influence of AR markers on muscle hypertrophy responsiveness to RT in untrained individuals.NEW & NOTEWORTHY We explored, for the first time, the influence of androgen receptor (AR) through the separation of cytoplasmic and nuclear cell fractions [i.e., cytoplasmic androgen receptor (cAR), nuclear androgen receptor (nAR), and androgen receptor DNA-binding activity (AR-DNA)] on muscle hypertrophy responsiveness to resistance training. The absence of muscle hypertrophy in naïve individuals does not seem to be explained by baseline values, and acute or chronic changes in AR markers.

Sex differences in human performance

Sex as a biological variable is an underappreciated aspect of biomedical research, with its importance emerging in more recent years. This review assesses the current understanding of sex differences in human physical performance. Males outperform females in many physical capacities because they are faster, stronger and more powerful, particularly after male puberty. This review highlights key sex differences in physiological and anatomical systems (generally conferred via sex steroids and puberty) that contribute to these sex differences in human physical performance. Specifically, we address the effects of the primary sex steroids that affect human physical development, discuss insight gained from an observational study of 'real-world data' and elite athletes, and highlight the key physiological mechanisms that contribute to sex differences in several aspects of physical performance. Physiological mechanisms discussed include those for the varying magnitude of the sex differences in performance involving: (1) absolute muscular strength and power; (2) fatigability of limb muscles as a measure of relative performance; and (3) maximal aerobic power and endurance. The profound sex-based differences in human performance involving strength, power, speed and endurance, and that are largely attributable to the direct and indirect effects of sex-steroid hormones, sex chromosomes and epigenetics, provide a scientific rationale and framework for policy decisions on sex-based categories in sports during puberty and adulthood. Finally, we highlight the sex bias and problem in human performance research of insufficient studies and information on females across many areas of biology and physiology, creating knowledge gaps and opportunities for high-impact studies.

Lifespan Effects of Muscle-Specific Androgen Receptor Overexpression on Body Composition of Male and Female Rats

Androgenic actions of gonadal testosterone are thought to be a major mechanism promoting sex differences in body composition across the lifespan. However, this inference is based on studies of androgen receptor (AR) function in late adolescent or emerging adult rodents. Here we assess body composition and AR expression in skeletal muscle of rats at defined ages, comparing wild-type (WT) to transgenic human skeletal actin-driven AR overexpression (HSAAR) rats which overexpress AR in skeletal muscle. Male and female HSAAR and WT Sprague Dawley rats (N = 288) underwent dual-energy x-ray absorptiometry (DXA) scanning and tissue collection at postnatal day (PND) 1, 10, 21, 42, 70, 183, 243, and 365. Expected sex differences in body composition and muscle mass largely onset with puberty (PND-21), with no associated changes to skeletal muscle AR protein. In adulthood, HSAAR increased tibialis anterior (TA) and extensor digitorum longus mass in males, and reduced the expected gain in gonadal fat mass in both sexes. In WT rats, AR protein was reduced in soleus, but not TA, throughout life. Nonetheless, soleus AR protein expression was greater in male rats than female rats at all ages of sexual development, yet only at PND-70 in TA. Overall, despite muscle AR overexpression effects, results are inconsistent with major sex differences in body composition during sexual development being driven by changes in muscle AR, rather suggesting that changes in ligand promote sexual differentiation of body composition during pubertal timing. Nonetheless, increased skeletal muscle AR in adulthood can be sufficient to increase muscle mass in males, and reduce adipose in both sexes.