Endocrine and metabolite responses to porcine growth hormone administered by sustained release implant for different lengths of time in male pigs.

The effects of long term administration of GH on serum concentrations of hormones and metabolites was investigated in intact and castrate male swine. At 10 weeks of age, male swine were assigned to six treatments (n = 10/group): nonimplanted intact and castrate males; intact males implanted for 6 weeks, from 22-28 weeks of age; intact males implanted for 12 weeks, from 16-28 weeks of age; and intact and castrate males implanted for 18 weeks, from 10-28 weeks of age. Recombinant porcine GH was administered with sustained release implants designed to deliver a dose of 4 mg/day for 6 weeks. Throughout the study, blood samples were collected, and serum was harvested to quantitate circulating concentrations of glucose, urea nitrogen, GH, insulin, insulin-like growth factor I (IGF-I), IGF-II, and PRL. The pattern of administered GH in the serum suggests that the presence of testes and prior treatment with GH influence GH clearance. Somatotropin treatment elevated serum concentrations of GH and increased serum levels of glucose, insulin, IGF-I, and IGF-II in both intact and castrate animals. However, during the prepubertal period of 10-16 weeks, GH-treated intact males were resistant to the diabetogenic actions of GH, whereas significantly increased serum levels of glucose and insulin occurred in GH-treated castrates during this period. Changes in serum levels of IGF-I throughout the study and in insulin after the first 6 weeks followed the pattern of circulating GH concentrations in the treated animals. Serum concentrations of IGF-II were increased after GH administration, but, in contrast to the IGF-I response, IGF-II levels remained elevated as GH concentrations waned in the latter portion of the implant period. The maintenance of higher serum levels of IGF-II may be less dependent upon GH than are insulin and IGF-I. Administration of GH to intact males is more efficacious in altering metabolites and hormones, with the exception of IGF-I, during the peripubertal and postpubertal periods than during the prepubertal period.