Total testosterone is not associated with muscle mass, function or exercise adaptations in pre-menopausal females

Testosterone, the major androgen hormone, 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) underwent a 12-week resistance training program. Muscle strength, size, power and plasma and urine androgen hormone levels were measured. Skeletal muscle biopsies were collected before and after the training program to quantify the effect of resistance training on AR protein and mRNA 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. Total testosterone was 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 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 conclusion, we found that, bioavailable testosterone and the proportion of nuclear-localised AR, but not total testosterone, with skeletal muscle mass and strength in pre-menopausal females.