| Abstrakt: |
Four Holstein steers (312 kg) in a 4 × 4 Latin-square design were injected iv with 0, 100, 300 and 1,000 µg of a 44 amino acid growth hormone-releasing factor, hpGRF-44 NH2in Exp. 1. Blood was collected at 20-min intervals from −1 to +6 h. All doses of hpGRF-44 NH2stimulated an increase in serum growth hormone (GH) concentrations, whereas prolactin and luteinizing hormone concentrations were unaffected. Steers injected with the 1,000-µg 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-µg dose tended to be higher (P<.15) than the peak caused by either the 300-µg or 100-µg dose, and the latter two were similar. Area under the GH response curve increased (P<.05) with increasing doses of hpGRF-44 NH2. Endogenous episodic secretion of GH resumed within 6 h after injection of 100 µg hpGRF-44 NH2, but not after either the 300-µg or 1,000-µg dose. In Exp. 2, six Holstein steers (352 kg) in a 6 × 6 Latin-square design received an iv injection of 0, 10, 25, 50 and 100 µg hpGRF-44 NH2and 100 µg 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 µg hpGRF-44 NH2, but all of the steers responded to all other doses of hpGRF-44 NH2and to hpGRF-40 OH. Steers at each dose of hpGRF-44 NH2had a GH release detectable in the serum by 5 min after injection. Amplitude of the GH peak and area under the GH response curve increased (P<.05) with increasing doses of hpGRF-44 NH2. At the 100-µg dose, hpGRF-44 NH2and hpGRF-40 OH were equipotent. In conclusion, the hpGRF peptides were potent secretagogues that specifically stimulated GH release in cattle. |
