Context: Growth hormone (GH) and testosterone are the major anabolic hormones that increase muscle mass and certain aspects of muscle function. GH stimulates connective tissue growth, an effect that is potentiated by testosterone co-administration. Decorin, a myokine and a connective tissue protein, is increased by exercise. It stimulates connective tissue accretion and muscle hypertrophy. The aim of this study was to determine whether GH and testosterone increase decorin concentrations.
Design and methods: In this randomized, placebo-controlled, double-blind study1,2, GH and/or testosterone, or placebo, were administered to healthy volunteers. 96 recreationally trained athletes (63 men, 33 women) received 8 weeks of treatment followed by a 6-week washout period. Men received either placebo, GH (2 mg/d SC), testosterone (250 mg/week IM), or combination. Women received either placebo or GH (2 mg/d). The main outcome measure was serum decorin levels.
Results: When compared to placebo, mean serum decorin concentration was significantly higher during GH in men (Δ 16.5 ± 5.3%; p<0.05), but not in women (Δ 9.4 ± 6.5%; p=0.16). Testosterone did not significantly change serum decorin concentration. Combined GH and testosterone treatment increased mean decorin concentration by 19.5 ± 3.7% (p<0.05). In both men and women, the increase in decorin concentration after 8 weeks of GH treatment significantly correlated with the increase in lean body mass (LBM; R=0.44, p<0.05), the functional component of LBM, body cell mass (R=0.44, p<0.05), and connective tissue markers, PINP (R=0.51, p<0.01) and PIIINP (R=0.47, p<0.01). Upon withdrawal of treatment for 6 weeks, decorin levels returned to baseline.
Conclusion: GH in men, but not in women, significantly increases serum decorin concentration. Testosterone did not significantly change circulating decorin levels. The GH-induced increase in decorin associates with changes in lean body mass and collagen markers. We conclude that decorin is stimulated by GH administration in a gender-dependent way.