Divergent Effects of Estrogens on the Somatotropic Axis in Male and Female Calves

This study describes 24 h profiles of growth hormone (GH) and of cortisol in male and female prepubertal calves before, during and after treatment with estradiol-17/3 (E2, silastic implants, 45 mg). Plasma concentrations of estradiol, testosterone and of insulin-like growth factor-1 (lGF-1) were also recorded. No significant (P<0.05) sex differences in GH, IGF-1 and in cortisol secretion were observed in untreated animals before E2-treatment. GH means, baselines and peak amplitudes increased during the treatment and remained elevated 1 week after removal of E2 in female calves, but were unchanged in male calves. In contrast, IGF-1 plasma concentrations were increased in both sexes during and after E2-treatment. In males maximal IGF-1 concentrations were higher than in females. Cortisol profiles showed a high individual variability and no consistant changes related to sex or E2-stimulation. Based on the observation that the male calves already had a significant testosterone secretion, we conclude that the observed sex differences of GH-secretion in response to E2 are related to endogenous testosterone. The E2-stimulated increase of IGF-1 plasma concentrations in males appears as related to an altered GH responsiveness rather than to changes in GH secretory patterns.