Serum growth hormone and nitrogen metabolism responses in young bull calves infused with growth hormone-releasing factor for 20 days

Abstract

Intravenous infusion of growth hormone (GH)-releasing factor (GRF) sustains elevated serum GH for at least 5 days in young Holstein steers, but the effects of extended infusion of GRF on serum GH and nitrogen (N) metabolism have not been determined. Thirteen Dutch-Friesian bull calves (148 ± 1.5 kg) were assigned randomly to receive daily either 0 or 3.6 mg GRF (hGRF 1–44NH 2; U-68420) in saline as a continuous infusion for 20 days. Calves were fed milk replacer twice daily while housed indoors in wooden-slatted floor box crates (metabolism cages). Nitrogen determinations were made on daily feed, feces, and urine samples which were pooled for days 9 to 14 of treatment. Concentrations of GH were quantified in blood samples collected at 20 min intervals for 8 hr on day 1, 10 and 20. The infusion of GRF increased baseline GH (P<.07), the number of GH pulses (P<.0001), the amplitude of the GH pulses (P<.001), and area under the GH response curve (P<.0002). Within GRF-infused calves baseline GH (P<.0001) and area under the GH response curve (P<.006) were greater on day 20 than on day 1 or 10 (day × treatment interaction, P<.04). Area under the GH response curve was similar on each sampling day in saline-infused calves, but baseline GH was higher (P<.03) on day 20 than either day 1 or 10. Infusion of GRF increased episodic GH secretion in spite of limited pulsatile activity in saline-infused calves. On days 9–14, infusion of GRF decreased the excretion of urinary N (P<.02) and improved N balance (P<.04) without affecting N intake, fecal N, or the coefficient of N digestibility compared to saline-infused calves. In the present study, GRF administered as a continuous iv infusion for 20 days maintained elevated serum GH concentrations, conserved the episodic GH pattern, and improved N balance in young bull calves. The findings that (1) the pituitary does not become refractory to continued exposure to GRF and (2) a positive effect on N metabolism clearly make GRF or an analog an attractive alternative to exogenous GH as a means to improve animal production.