Am. Arnold et al., ONTOGENY OF GROWTH-HORMONE, INSULIN-LIKE GROWTH-FACTOR-I, ESTRADIOL AND CORTISOL IN THE GROWING LAMB - EFFECT OF TESTOSTERONE, Journal of Endocrinology, 150(3), 1996, pp. 391-399
Exogenous sex steroids have altered growth hormone secretion in some d
omestic species. This study examined whether different physiological c
oncentrations of testosterone alter muscle growth in sheep through mod
ification of the somatotropic axis. The effects of testosterone on gro
wth hormone (GH), insulin-like growth factor-I (IGF-I), estradiol (E(2
)) and cortisol concentrations in growing lambs were evaluated in 20 r
ams, 20 wethers and 20 wethers receiving subcutaneous testosterone rep
lacement therapy. Two animals from each of the three testosterone stat
us 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 perio
d 1 day prior to slaughter to characterize the episodic patterns of GH
and testosterone. Immediately after slaughter, the semi-tendinosus, s
plenius and triceps brachii muscles were removed, trimmed of adhering
fat and connective tissue, and weighed. Testosterone increased the com
bined muscle weight. GH concentrations decreased during the course of
the experiment. However, there was no effect of testosterone on GH mea
n, baseline, amplitude or GH pulse frequency measured by PULSAR. IGF-I
concentrations increased in response to testosterone treatment. Testo
sterone had no effect on cortisol levels while E(2) levels were increa
sed after 133 days. Increased muscle growth due to testosterone appear
ed to be caused either by a direct effect or by increased levels of IG
F-I independent of circulating GH concentrations.