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Growth hormone and testosterone interact positively to enhance protein and energy metabolism in hypopituitary men

¹Pituitary Research Unit, Garvan Institute of Medical Research and Department of Endocrinology, St. Vincent’s Hospital, Sydney, Australia; and ²Dept. of Diabetes and Endocrinology, Guy’s, King’s and St. Thomas’ School of Medicine, St. Thomas' Hospital, London, United Kingdom

Submitted 11 October 2004 ; accepted in final form 18 February 2005

We investigated the impact of growth hormone (GH) alone, testosterone (T) alone, and combined GH and T on whole body protein metabolism. Twelve hypopituitary men participated in two studies. Study 1 compared the effects of GH alone with GH plus T, and study 2 compared the effects of T alone with GH plus T. IGF-I, resting energy expenditure (REE), and fat oxidation (F_ox) and rates of whole body leucine appearance (R_a), oxidation (L_ox), and nonoxidative leucine disposal (NOLD) were measured. In study 1, GH treatment increased mean plasma IGF-I (P < 0.001). GH did not change leucine R_a but reduced L_ox (P < 0.02) and increased NOLD (P < 0.02). Addition of T resulted in an additional increase in IGF-I (P < 0.05), reduction in Lox (P < 0.002), and increase in NOLD (P < 0.002). In study 2, T alone did not alter IGF-I levels. T alone did not change leucine R_a but reduced L_ox (P < 0.01) and increased NOLD (P < 0.01). Addition of GH further reduced L_ox (P < 0.05) and increased NOLD (P < 0.05). In both studies, combined treatments on REE and F_ox were greater than either alone. In summary, GH-induced increase of circulating IGF-I is augmented by T, which does not increase IGF-I in the absence of GH. T and GH exerted independent and additive effects on protein metabolism, F_ox and REE. The anabolic effects of T are independent of circulating IGF-I.

insulin-like growth factor I; protein turnover

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