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

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 (hGRF1-44NH2; 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 less than .07), the number of GH pulses (P less than .0001), the amplitude of the GH pulses (P less than .001), and area under the GH response curve (P less than .0002). Within GRF-infused calves baseline GH (P less than .0001) and area under the GH response curve (P less than .006) were greater on day 20 than on day 1 or 10 (day X treatment interaction, P less than .04). Area under the GH response curve was similar on each sampling day in saline-infused calves, but baseline GH was higher (P less than .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.(ABSTRACT TRUNCATED AT 250 WORDS)