EFFECT OF GROWTH HORMONE-RELEASING FACTOR (1–44)NH2 ADMINISTRATION ON SOMATOTROPIN IN PIGS

Two experiments were conducted to examine the dose response and optimum pattern of subcutaneous (s.c.) administration of human growth hormone-releasing factor [hGRF (1–44)NH2]on serum somatotropin (ST) in barrows. In exp. 1, 10 barrows (79.9 ± 3.3 kg) were used in a replicated 5 × 5 Latin square and blood samples were collected for 300 min from a single injection of hGRF. Injecting 0, 1, 10, 40 or 100 μg hGRF kg−1 resulted in dose related increase (P < 0.01) in peak concentrations of pST (3.93, 4.41, 10.61, 16.01 and 23.56 ng mL−1), and area under the ST response curve (AUC) (565.4, 546.2, 757.5, 1048.0 and 1314.0 ng min mL−1, respectively). The 40 μg kg−1 dose resulted in a higher (P < 0.05) peak pST and AUC than 0 or 1 μg hGRF kg−1. Peak concentration and AUC were greater (P < 0.05) for the 100 μg hGRF kg−1 dose compared with the 0, 1 and 10 μg hGRF kg−1 doses, but there was no difference between the 40 and 100 μg kg−1 doses. In exp. 2, eight barrows (85.4 ± 8.0 kg) were used in a replicated 4 × 4 Latin square to examine the relationship between pattern of hGRF administration and serum ST. Blood samples were collected −15, 0 and 15 min from initiation of administration and then hourly for 24 h. Treatments were designed to deliver 160 μg hGRF kg−1 every 24 h and were administered as 40 μg kg−1 injected four times daily, 80 μg kg−1 injected twice daily or by continuous s.c. infusion. There was no difference (P > 0.10) between treatments for peak ST concentration, AUC or area above the ST baseline. These data indicated a dose response to s.c. hGRF injections and ST in barrows. Further, ST response was not altered by the pattern of s.c. administration when barrows received 160 μg hGRF kg−1 daily by either 4 or 2 discrete injections or by continuous infusion. Key words: Swine, somatotropin, growth hormone-releasing factor