Testosterone on myostatin

testosterone is known to suppress myostatin levels in skeletal muscle (33,34) and inhibition of myostatin promotes muscle growth and reduces sarcopenia (35,36,37), we examined the effects of testosterone supplementation on myostatin levels in the gastrocnemius muscle.

Context: Fibroblast growth factor (FGF)2 is an important stimulatory modulator of satellite cells in skeletal muscle. Satellite cells play a cardinal role in muscle growth and repair.

Objective: We evaluated whether skeletal muscle expression of FGF2 and muscle growth and differentiation factors are reduced in patients with hypogonadotropic hypogonadism (HH) and whether testosterone replacement therapy results in their restoration.

Design: This is a secondary analysis of a previously completed trial of testosterone replacement in men with type 2 diabetes and HH.

Setting: Clinical Research Center at a university.

Patients: Twenty-two men with HH and 20 eugonadal men were compared at baseline.

Interventions: Twelve men with HH were treated with intramuscular injections of 250 mg testosterone every 2 weeks for 22 weeks, and 10 men received placebo injections. Quadriceps muscle biopsies and blood samples were obtained before and after testosterone therapy.

Outcome measures and results: The expression of FGF2 and FGF receptor (FGFR)2 in skeletal muscle of men with HH was significantly lower than that in eugonadal men by 57% and 39%, respectively (P < 0.05). After 22 weeks of testosterone, the expression of FGF2 increased, whereas that of myogenic regulatory factor (MRF)4 and myostatin decreased significantly. There was no change in expression of FGFR2, myogenin, or myogenic differentiation protein in the skeletal muscle. Plasma FGF2 and IGF-1 concentrations increased after testosterone therapy.

Conclusions: These data show that testosterone is a major modulator of FGF2, MRF4, and myostatin expression in skeletal muscle. These effects may contribute to the increase in muscle mass after testosterone therapy.