Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 21, 1996
Publication Date: N/A
Interpretive Summary: Extensive research is being conducted on the potential use of growth hormone (GH) in livestock production because of its positive effects on growth and leanness. One of the effects of GH injection is an inhibition of natural GH secretion. It is important that we know how GH treatment affects the natural ability of the pituitary gland to produce this hormone, ,particularly during critical periods of early neonatal development. We have found that GH treatment of neonatal pigs reduces the amount of GH within the pituitary gland, and thereby inhibits the secretion of this hormone. The long-term effects of this disturbance of pituitary function in baby pigs are not yet known. A sub-normal state of natural GH production would exist following the cessation of GH treatment, perhaps providing an explanation for the recognized rapid loss of beneficial effects post-treatment. This information will be useful to individuals in academia, government, industry, and regulatory agencies who have an interest in growth promotant biotechnology.
Technical Abstract: The effect of recombinant porcine growth hormone (pGH) treatment on pituitary function was evaluated in neonatal pigs. Piglets received i.p. recombinant pGH (42-d, .5 mg/day) or vehicle implants beginning at 3 days of age. Ten piglets were sacrificed at 4 and 6 weeks of age (5/treatment group) for the collection of pituitary glands, blood, and liver tissue. Blood samples also were drawn at 3 and 12 days 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 determined 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). Cultured pituitary cells from each animal were challenged with .1, 1, and 10 nM growth hormone-releasing hormone (GHRH); 2 mM 8-Br- cAMP; or 100 nM phorbol myristate acetate (PMA). The release of GH from cultured pituitary cells was stimulated by all secretagogues (p < 0.001). The secretion of GH, but not PRL or TSH, in culture was inhibited by previous in vivo GH treatment (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-week-old piglets secreted more GH, but not PRL or TSH, than cultures from 4-week-old piglets (p < 0.05). Likewise, cellular GH, but not PRL or TSH, content was greatest in cultures from 6-week-old animals (p = 0.002). Piglet growth was not affected by exogenous GH treatment (p = 0.67). These results demonstrate that exogenous pGH treatment selectively down-regulates somatotroph function in neonatal pigs.