Growth hormone-releasing factor on growth hormone secretion in prepubertal calves

The effects of iv administration of growth hormone-releasing factor (GRF) on growth hormone (GH) release and on nitrogen metabolism were measured in prepubertal calves. Crossbred beef heifers (111 kg) were used in a Latin square design to test the effects of 0, 0.01, 0.033, 0.067, and 0.1 microgram human pancreatic (hp) GRF [hpGRF (1,40)OH]/kg body wt on plasma GH concentrations. When they were given doses of 0.067 and 0.1 microgram hpGRF/kg body wt, plasma GH increased (P less than 0.05) within 5-15 min, compared with injections of control buffer, and then returned to preinjection concentrations. The response to 0.067 microgram hpGRF/kg body wt every 3 hr for 42 hr was studied in five heifers (137 kg body wt). The animals responded to 50% of the GRF injections with an increase in plasma GH during every 6-hr period measured. Nitrogen retention, hormone concentrations, and weight gain were measured in five bull calves (90 kg body wt) administered 0 or 0.067 microgram Nle rat hypothalamic GRF (1,29)NH2/kg body wt every 4 hr for 10 days. Metabolic parameters were interpreted to indicate an anabolic response to GRF even though increases of 16% in nitrogen retention, 23% in plasma somatomedin C concentrations, and 36% in weight gain with pulsatile GRF treatment were variable and statistically similar to those of controls. These results indicate that GRF induces peak GH secretion within 15 min in prepubertal calves and that calves can respond to multiple injections of GRF with an increase in plasma GH.