Administration of porcine somatotropin by sustained-release implant: growth factor and metabolic responses in crossbred white and genetically lean and obese boars and gilts.

Differences in endocrine and metabolic responses to porcine somatotropin administered by daily injection or sustained-release implant (pST-SR) have been previously observed in genetically lean and obese, gilts and barrows. The current study extended those findings by examining responses to pST-SR in gilts and boars of a contemporary crossbred line, as well as lean and obese lines. Pigs were treated with 0, 1, or 2 pST-SR implants inserted subcutaneously behind the ear. The osmotically driven pST-SR implants delivered 2 mg of recombinant pST/d. Pigs were bled on d 0, 7, 14, 28, and 42 after implantation. Sera were assayed for pST, insulin-like growth factor (IGF)-I, IGF-II, IGF-binding protein-2 (IGFBP-2), insulin, glucose, and blood urea nitrogen (BUN). Circulating pST concentrations were increased in a dose-dependent manner (P < .001) in the pST-SR treated pigs, but remained elevated (P < .05) only in the 4 mg of pST-SR/d group on d 42. Significant effects of line, dose, time, line x dose, and time x dose were noted for IGF-I. Serum IGF-I was elevated in a dose-dependent manner over the 42-d period in all pST-treated swine. Examination of the line x dose x time interaction indicated that the IGF-I response to pST-SR was greatest in the obese line compared with the lean and crossbred lines. Conversely, serum IGF-II responded to pST-SR to the least extent in the obese pigs. Circulating IGFBP-2 concentrations were reduced by pST-SR, but were not affected by line. The BUN concentrations were reduced by pST-SR. An interaction of line x dose x time (P < .001) indicated that the response was greater in the obese line. Line x dose x time interactions were also noted for insulin and glucose concentrations, which were elevated by pST-SR in a dose-response manner in all lines, but to a much greater extent in the obese pigs.(ABSTRACT TRUNCATED AT 250 WORDS)