Regulation of growth hormone secretion in ruminants

Regulation of growth hormone (GH) in ruminants was studied by infusing exogenous GH-releasing factor (GHRF), somatostatin (SRIH), and GH intravenously into animals and monitoring the levels of plasma hormones. Exogenous GHRF increased GH secretion in a dose-dependent manner in sheep and cattle. Intravenous infusion of SRIH in cattle suppressed the GH response to injections of GHRF. Intravenous injections of GHRF six times per day in older sheep caused a conservation of urinary nitrogen. In a similar study with young calves, changes in nitrogen metabolism were not significant;Plasma hormone concentrations and body composition were measured in intact and castrated, male and female cattle at 5, 8, 12, and 15 mo of age. As the animals aged, plasma somatomedin-C (Sm-C), testosterone (T), and insulin (I) increased while GH half-life, GH secretion rate (SR)/kg body weight (bwt), GH baseline concentrations, number and amplitude of GH secretion periods, and GH response to exogenous GHRF declined. The rate of fat, protein, water, and ash deposition was different between 8 to 12 mo nd 12 to 15 mo of age. Bulls differed from steers, heifers and ovariectomized heifers by higher baseline concentrations of GH; increased periods of secretion of GH with greater amplitude; increased GH SR; higher concentrations of T, total estrogens, and Sm-C; and higher protein, water and ash and lower fat in empty body gain. Females in comparison with males had higher concentrations of thyroid hormones, lower GH concentrations and GH SR/kg bwt, fewer GH secretory periods with less amplitude, and greater percent body fat. Castrates had less total estrogens, T, and Sm-C with greater levels of triiodothyronine and thyroxine. Overall GH concentrations, GH SR/kg bwt and GH:I ratio in the blood were negatively associated with percent empty body fat and positively associated with percent empty body protein