Growth hormone response of steers injected with synthetic human pancreatic growth hormone-releasing factors

Four Holstein steers (312 kg) in a 4 X 4 Latin-square design were injected iv with 0, 100, 300 and 1,000 micrograms of a 44 amino acid growth hormone-releasing factor, hpGRF-44 NH2 in Exp. 1. Blood was collected at 20-min intervals from -1 to +6 h. All doses of hpGRF-44 NH2 stimulated an increase in serum growth hormone (GH) concentrations, whereas prolactin and luteinizing hormone concentrations were unaffected. Steers injected with the 1,000-micrograms dose appeared to have a biphasic release of GH that was not observed at the other doses. Amplitude of the GH peak after the 1,000-micrograms dose tended to be higher (P less than .15) than the peak caused by either the 300-micrograms or 100-micrograms dose, and the latter two were similar. Area under the GH response curve increased (P less than .05) with increasing doses of hpGRF-44 NH2. Endogenous episodic secretion of GH resumed within 6 h after injection of 100 micrograms hpGRF-44 NH2, but not after either the 300-micrograms or 1,000-micrograms dose. In Exp. 2, six Holstein steers (352 kg) in a 6 X 6 Latin-square design received an iv injection of 0, 10, 25, 50 and 100 micrograms hpGRF-44 NH2 and 100 micrograms hpGRF-40 OH. Blood was collected at 20-min intervals from -1 to +4 h, with additional samples at 5, 10 and 15 min. Four of six steers responded to 10 micrograms hpGRF-44 NH2, but all of the steers responded to all other doses of hpGRF-44 NH2 and to hpGRF-40 OH.