SUPPRESSION OF SOMATOTROPH FUNCTION INDUCED BY GROWTH-HORMONE TREATMENT IN NEONATAL PIGS

The effect of recombinant porcine growth hormone (pGH) treatment on pi tuitary function was evaluated in young pigs. Piglets received intrape ritoneal recombinant pGH implants (0.5 mg/d sustained release) or vehi cle implants beginning at 3 d of age. Ten piglets were sacrificed at 4 and 6 wk of age (five piglets/treatment group) for the collection of pituitary glands, blood, and liver tissue. Blood samples also were dra wn at 3 and 12 d of age. Serum concentrations of GH, prolactin (PRL), thyroid-stimulating hormone (TSH), insulin-like growth factor-1 (IGF-1 ) and IGF-2 were evaluated. Levels of IGF-1 and IGF-2 mRNA were determ ined in liver samples. Treatment with GH increased circulating levels of GH and IGF-1 (P < 0.01), but not PRL, TSH, or IGF-2. Hepatic IGF-1, but not IGF-2, mRNA levels were increased by pGH (P < 0.001). Culture d pituitary cells from each animal were challenged with 0.1, 1, and 10 nM GH-releasing hormone (GHRH); 2 mM 8-Br-cAMP; or 100 nM phorbol myr istate acetate. The release of GH from cultured pituitary cells was st imulated by all secretagogues (P < 0.001). The secretion of GH, but no t PRL or TSH, in culture was inhibited by previous in vivo GH treatmen t (P < 0.001). Similarly, cellular GH, but not PRL or TSH, content was lower in the GH-implant group (P = 0.005). Cell cultures from 6-wk-ol d piglets secreted more GH, but not PRL or TSH, than cultures from 4-w k-old piglets (P < 0.05). Likewise, cellular GH, but not PRL or TSH, c ontent was greatest in cultures from 6-wk-old animals (P = 0.002). Pig let growth was not affected by exogenous GH treatment (P = 0.67). Thes e results demonstrate that exogenous pGH treatment selectively down-re gulates somatotroph function in young pigs. (C) Elsevier Science Inc. 1997.