Pituitary responsiveness to gonadotropin- and thyrotropin-releasing hormones in goats treated with recombinant bovine somatotropin

Abstract

Non-lactating, crossbred Alpine goats, of similar age and body weight were used to evaluate the effects of bovine somatotropin (bST) administration on pituitary function. Following ovarian synchronization, treated goats ( n=4) received a subcutaneous injection of 160 mg rbST, three times, at 14-day intervals, while control goats ( n=4) remained uninjected. The day after the last rbST injection ovarian synchronization was repeated and the following day all animals were challenged with 50 μg gonadotropin-releasing hormone (GnRH) and 100 μg thyrotropin-releasing hormone (TRH), intravenously. Blood samples were collected at −10, 0, 30, 60, 90, 120, 180, 270 and 360 min after challenge. Additionally, blood sampling was conducted three times per week throughout the experimental period for the detection of peripheral somatotropin, progesterone, T 3 and T 4 concentrations. Treatment with rbST significantly increased somatotropin concentrations in treated animals (21.2±1.3 ng/ml) as compared to controls (5.2±0.4 ng/ml). GnRH and TRH infusions resulted in similar LH, TSH, T 3 and T 4 responses in both groups, in terms of the area under the curve (AUC), the peak value (PV) and the time to peak (PT). Plasma progesterone concentrations during the treatment period did not differ between groups. Treatment with rbST did not affect T 4 levels during the experimental period, but a highly significant effect of treatment was observed for T 3 levels, particularly after the second rbST administration (1.47±0.06 ng/ml versus 2.01±0.1 ng/ml) for the control and treated group. In conclusion, it has been shown that rbST treatment did not affect pituitary responsiveness to GnRH or TRH challenge, demonstrating that the pituitary control of ovarian and thyroid function is not influenced by bST administration.