Administration of synthetic human pancreatic growth hormone-releasing factor for five days sustains raised serum concentrations of growth hormone in steers

Serum GH concentrations in steers were examined during extended treatment with synthetic GH-releasing factor(1-44)NH2 (GRF). The results indicate that GRF given as frequent microinjections stimulate and sustain raised serum GH concentrations for at least 5 days in steers. The GH secretory pattern remained episodic and was characterized by a significant increase in the amplitude of the GH pulses without a change in the number of GH pulses per day. In the first of two experiments, young Holstein steers received 0, 0.05, 0.5 or 5.0 mg GRF during a 24-h period as microinjections every 3.75 min. The 5.0 mg GRF/24h dose significantly increased baseline GH, amplitude of GH pulses and area under the GH curve compared with the other treatments. The number of GH pulses/24 h was similar for all doses of GRF. In a second experiment with Holstein steers, administration of 3.6 mg GRF/day for 5 days increased serum GH concentrations throughout the duration of the treatment without altering the temporal GH secretory pattern. The GH response to GRF did not diminish from days 1 to 5 of treatment suggesting that there was no pituitary desensitization.