Molecular responsiveness to testosterone is uncoupled from skeletal muscle mass and strength in males aged 18 - 80 years

Objective Testosterone exerts wide-ranging physiological effects, including the regulation of anabolic processes in skeletal muscle. However, its role in skeletal muscle ageing remains unclear. This study aimed to establish the association between endogenous testosterone levels and skeletal muscle phenotype across the adult lifespan and determine whether ageing modulates this relationship. Methods Eighty-one healthy males aged 18-80 underwent comprehensive phenotypic assessment. Body composition and muscle morphology were assessed using dual-energy X-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT). Muscle strength was estimated leg press one-repetition maximum (e1RM). Vastus lateralis biopsies were used to assess fibre type, sequence the transcriptome and establish primary myocytes. Cell lines from young (n=5) and older (n=7) donors were cultured and compared under control and testosterone-treated conditions for cellular phenotype. Results Ageing was associated with lower free testosterone, muscle mass and strength, altered type IIa fibre characteristics, and widespread transcriptional changes in skeletal muscle. Free, but not total, testosterone was strongly associated with the muscle transcriptome, although neither fraction was associated with muscle mass or strength. In primary myocytes, testosterone increased androgen receptor abundance and nuclear localisation and enriched pathways involved in protein translation. Conclusion Testosterone is biologically active in male skeletal muscle, with its free fraction strongly associated with the transcriptome; however, these molecular effects do not translate into differences in muscle mass or strength. Testosterone may therefore play a primary role in establishing the male muscle phenotype during puberty, before transitioning to a more ancillary role in adult skeletal muscle.