Submitted to: Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/1996
Publication Date: N/A
Citation: Interpretive Summary: It is fairly well documented that the pituitary growth hormone is important to growth and metabolism in birds. The insulin-like growth factors are thought to be the main effectors of growth hormone via the growth hormone receptor. To better understand the growth hormone:insulin-like growth factor-I relationship a study was conducted using dwarf chickens in which the growth hormone receptor is non-functional. In this study, dwarf and normal chickens were fed increasing amounts of thyroid hormone and various tissues removed and analyzed for IGF-I content. Surprisingly, IGF-I content was the greatest in fat compared to muscle and other tissues. This study demonstrates that other hormones can regulate IGF-I synthesis and secretion, independent of growth hormone. The results of this study will be of interest to other scientists.
Technical Abstract: Insulin-like growth factor-I (IGF-I) mediates many of the effects of growth hormone (GH). The regulation of IGF-I is methodologically difficult to assess in vivo, as hypophysectomy results in derangement of many pituitary hormone axes. The recessive sex-linked dwarfing (SLD) gene (dw) in chickens results in a lack of functional target tissue GH receptors (GHR) due to a variety of molecular defects, which provides a unique model for evaluating GH independent regulation of IGF-I. In the present study, the impact of triiodothyronine (T3) on circulating and tissue IGF-I was determined in normal versus SLD birds. Adult, non-ovulatory female normal and SLD chickens were restrict-fed 40 g of feed/kg bwt/d containing 0, 0.5 or 1.0 ppm T3, resulting in supplementation levels of 0 (control), 20 (low dose) or 40 (high dose) ug T3/kg bwt/d for 10 d. Samples of liver, abdominal fat pad, skeletal muscle and spleen were extracted and assayed for IGF-I. Dwarf birds were markedly hypersomatotropic compared to normals, and T3 supplementation reduced this to control values. Despite the high GH levels in dwarfs, plasma IGF-I was low. This difference was eliminated with low dose of T3, due to non-significant but opposite changes in each genotype that were not observed at the high dose level. Tissue IGF-I was undetectable in liver and muscle. In contrast, adipose tissue IGF-I was high and was increased in controls and decreased in dwarfs when treated with T3. The low levels of plasma IGF-I in view of the fact that nonmeasurable hepatic IGF-I tissue concentrations suggests IGF-I synthesis by extrahepatic tissues contributes to the circulating pool of IGF-I.