Ontogeny of growth hormone, insulin-like growth factor-I, estradiol and cortisol in the growing lamb: effect of testosterone.

Exogenous sex steroids have altered growth hormone secretion in some domestic species. This study examined whether different physiological concentrations of testosterone alter muscle growth in sheep through modification of the somatotropic axis. The effects of testosterone on growth hormone (GH), insulin-like growth factor-I (IGF-I), estradiol (E2) and cortisol concentrations in growing lambs were evaluated in 20 rams, 20 wethers and 20 wethers receiving subcutaneous testosterone replacement therapy. Two animals from each of the three testosterone status groups were slaughtered at 14-day intervals from 49 to 133 days of age, and then at 28-day intervals until 217 days of age for a total of 10 slaughter ages. Animals were sampled every 10 min for an 8-h period 1 day prior to slaughter to characterize the episodic patterns of GH and testosterone. Immediately after slaughter, the semitendinosus, splenius and triceps brachii muscles were removed, trimmed of adhering fat and connective tissue, and weighed. Testosterone increased the combined muscle weight. GH concentrations decreased during the course of the experiment. However, there was no effect of testosterone on GH mean, baseline, amplitude or GH pulse frequency measured by PULSAR. IGF-I concentrations increased in response to testosterone treatment. Testosterone had no effect on cortisol levels while E2 levels were increased after 133 days. Increased muscle growth due to testosterone appeared to be caused either by a direct effect or by increased levels of IGF-I independent of circulating GH concentrations.