Changes in growth hormone status related to body weight of growing cattle

The growth hormone (GH) status of 24 castrated male cattle was studied over a range of body weights by measuring GH in the blood plasma and anterior pituitary and by calculating metabolic clearance (MCR) and secretion rates after injection of GH. As the cattle increased in size, pituitary weight and GH content, MCR of GH and secretion of GH decreased relative to body weight. Plasma concentrations of GH did not decrease as markedly as secretion because MCR/body weight was also lower in the larger animals. At all sizes, more than 99% of the total GH in the body was located in the anterior pituitary. It was calculated that the pituitary releases 0.58% of its GH content per hour. This rate of release did not significantly change with body weight. It is suggested that basal secretion of GH by the anterior pituitary is related to GH content and that, as animals grow, there is gradually less GH available per unit of body mass.