Effect of bovine growth hormone administration on metabolism of growing Hereford heifers: protein and lipid metabolism and plasma concentrations of metabolites and hormones.

The objectives were to determine the effects of daily injection of bovine growth hormone (bGH) on the metabolism of [1-14C]leucine and [1-14C]palmitate and on hormone and metabolite concentrations in growing Hereford heifers. The experimental design was a 28-d single reversal with two 14-d injection periods of placebo or bGH. Energy intake was restricted to a level slightly above maintenance. Injection of bGH did not affect circulating concentrations of glucose, beta-hydroxybutyrate, urea nitrogen, prolactin, triiodothyronine or thyroxine. Plasma concentrations of insulin and nonesterified fatty acids (NEFA) were chronically elevated whereas leucine concentration was chronically decreased after 1 wk of bGH injection compared to placebo injection. Leucine oxidation was lower and whole-body protein synthesis was higher during bGH injection than during placebo injection. There were increases in both total irreversible loss and oxidation of NEFA during bGH injection compared to placebo injection. These results suggest mobilization of stored fatty acids and increased reliance on NEFA to provide energy for cellular processes. The dual and reciprocal effects of bGH on nitrogen and NEFA metabolism demonstrate its role as a homeorhetic regulator, affecting metabolism of several body tissues to support lean body accretion in Hereford heifers at near-maintenance intake of metabolizable energy.