Tissue-specific alterations in insulin-like growth factor-I concentrations in response to 3,3',5-triiodo-L-thyronine supplementation in the growth hormone receptor-deficient sex-linked dwarf chicken

Insulin-like growth factor-I (IGF-I) mediates many of the effects of growth hormone (GH). The regulation of IGF-I, independent of GH, is methodologically difficult to assess in vivo, as hypophysectomy results in derangement of many pituitary hormone axes in addition to GH, and a gene knockout model is not available. The recessive sex-linked dwarfing (SLD) gene (dw) in chickens results in a lack of functional target tissue GH receptors 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 3,3',5-triiodo-L-thyronine (T₃) on circulating and tissue IGF-I was determined in normal versus SLD birds. Adult, nonovulatory female normal and SLD chickens were restrict-fed 40 g of feed/kg bw/day containing 0, 0.5, or 1.0 ppm T₃, resulting in supplementation levels of 0 (control), 20 (low dose), or 40 (high dose) μg T₃/kg bw/day for 10 days. Samples of GH target tissues including liver, abdominal fat pad, skeletal muscle (pectoralis major), and spleen were extracted and assayed for IGF-I. Plasma T₃, T₄, GH, and IGF-I were determined by homologous RIA. Tissue GH binding was determined for hepatic membranes by radioreceptor assay. Under control conditions, dwarf chickens were markedly hypersomatotropic (33.3 ± 4.1 ng GH/ml plasma; mean ± SEM) compared to normals (2.4 ± 3.9 ng/ml), and T₃ supplementation reduced this to normal levels. Despite the high circulating level of GH in dwarfs, plasma IGF-I was low compared to normal controls (dwarfs 1.5 ± .9 ng/ml normals 5.3 ± .9 ng/ml; P = 0.004), but this difference was eliminated with low-dose T₃. In this study, tissue IGF-I was undetectable in liver and pectoralis muscle in adults (55 weeks of age) of both genotypes, under all treatments. In contrast, adipose tissue IGF-I was relatively high and did not differ (P = 0.84) between genotypes under control conditions (normals 776.5 ± 236.7 dwarfs 844.6 ± 236.7 pg/mg protein), but was increased in normals and decreased in dwarfs, resulting in higher levels (P = 0.02) in the normal (1249.9 ± 200.0 pg/mg protein) than in the dwarf genotype (558.4 ± 200.0 pg/mg protein) at the higher level of T₃ supplementation. This relationship was somewhat reversed in spleen where T₃ tended to decrease tissue IGF-I concentration in normals and increase it in dwarfs. The low level of plasma IGF-I despite nonmeasurable hepatic IGF-I tissue concentrations suggests that IGF-I synthesis by extrahepatic tissues contributes to the circulating pool of IGF-I. The relatively high control levels of adipose tissue IGF-I in the dwarf genotype further suggest that considerable IGF-I synthesis exists that is GH-independent in this extrahepatic tissue. The presence of GH action, however, may mediate the effects of other hormones that can influence local IGF-I production in this tissue, as reflected by the differential response to T₃ supplementation between genotypes. The tissue-specific nature of the effect of T₃ on IGF-I production supports an additional point of regulation of hormone action at the target tissue level.