Growth hormone secretion and clearance rates in growing beef steers implanted with estrogenic anabolic compounds

The effect of estrogenic anabolic compounds on the kinetic parameters of metabolism of growth hormone (GH) was studied in growing beef steers. Twenty-four beef steers were randomly placed into four groups and assigned to one of the following four treatment groups: zeranol, diethylstilbestrol (DES), Synovex-S and an unimplanted control. GH metabolism was studied from eight steers on day 20 following the implantation of anabolic compounds. The animals were rapidly injected with a solution of bGH (NIH-GH-B18) and the disappearance of injected GH from the plasma was monitored up to 120 min following the injection. The plasma GH disappearance curve displayed an initial rapid phase lasting 5 min and a slow disappearance phase lasting 42 min; the fractional turnover rate from the two compartments were 0.167 and 0.017 min-1, respectively. The average volume of distribution of GH in steers was 6% of the body weight. Mean values of metabolic clearance and secretion rates of GH in steers were 21 liters/h and 252 micrograms/h or 74.5 ml/kg/h and 0.91 microgram/kg/h, respectively. Steers implanted with anabolic compounds gained more rapidly (P less than 0.05) than the controls. Plasma basal GH concentration appeared to be higher in all the implanted than in the control steers. The secretion rate of GH was increased (P less than 0.05) in steers implanted with anabolic compounds when compared to control steers. The secretion rate (microgram/kg/h) was about 96% (P less than 0.05), 107% (P less than 0.05) and 81% (P less than 0.05) higher in steers implanted with DES, zeranol and Synovex-S, respectively, than in the control steers. All the compounds studied were equally effective in increasing the secretion of GH on day 20 following their implantation. Metabolic clearance rate of GH was not affected by anabolic compound implantation in steers. There was, however, a slight reduction in metabolic clearance rate due to DES and a slight elevation due to zeranol and Synovex-S when compared to control steers. There was a positive correlation between growth rate and GH secretion rate (r = 0.78; P less than 0.05). The results indicate that the implantation of estrogenic anabolic compounds increases the secretion rate of GH from the pituitary gland and suggest that their mechanism of action is mediated through increased GH secretion.