Sex-linked Dwarf Research Articles

Decreased muscle cell proliferation in chicks with a deletion in the GH receptor gene

The increase in muscle weight in neonatal animals is a consequence of increased protein accretion and DNA content. GH increases protein accretion but direct effects of GH on myogenic cell proliferation have not been demonstrated. Sex-linked dwarfism in the chick is caused by mutation or deletion in the GH receptor gene and has provided a useful model to study the physiological consequences of GH insensitivity. This study determined the consequences of GH receptor gene mutation on muscle cell proliferation in vivo. Northern and Southern blotting and PCR analysis revealed restriction fragment length polymorphism patterns and a 1.7 kb deletion of the intracellular domain of the GH receptor gene in commercial dwarf broiler chicks, similar to the Connecticut strain in which there is a dysfunctional GH receptor. Cell proliferation was measured in muscle sections from normal and dwarf chicks after incorporation of 5-bromo-2'-deoxyuridine (BrdU; 25 mg/kg) in vivo at 2, 5 and 13 days of age. Incorporation of BrdU into nuclei was measured in frozen sections, counterstained with propidium iodide to estimate the total number of nuclei by quantitative image analysis, and the labelling index was calculated. Paraffin-embedded sections of breast muscle were stained using an anti-human IGF-I polyclonal antibody. Expression of IGF-I mRNA in muscle from each genotype at 5 days of age was measured by RNAse protection assay. The labelling index was similar in 2-day-old chicks from both genotypes (normal, 20.14 +/- 2.39%; dwarf, 19.79 +/- 5.83%). By day 5 the labelling index had decreased but was significantly higher (P < 0.02) in normal (12.53 +/- 3.36%) compared with the dwarf (6.25 +/- 1.39%). By 13 days of age, there was a further decrease in labelling index but no difference between the groups (normal, 4.92 +/- 1.28%; dwarf, 4.96 +/- 1.51%). IGF-I mRNA was expressed and IGF-I peptide was identified in muscle sections but there was no difference between genotypes. The results show that cell division in breast muscle in vivo is high in neonatal chicks but it declines with increasing age. The absence of a functional GH receptor in the dwarf is associated with a greater decline in DNA synthesis and suggests that GH may directly affect a proportion of cells, since there was no difference in IGF-I mRNA or peptide.

Growth hormone-independent expression of insulin-like growth factor I messenger ribonucleic acid in extrahepatic tissues of the chicken

The sex-linked dwarf (SLD) chicken, which lacks GH receptor (GHR), and its normal littermates provide a useful experimental system to investigate GH-dependent cellular responses. The GH dependence of insulin-like growth factor I (IGF-I) expression in tissues was examined in SLD and normal chickens of the Gifu 20 strain. Four weeks after hatching, the most abundant expression of IGF-I messenger RNA (mRNA) was observed in liver of normal chickens, whereas no IGF-mRNA expression was detected in that organ of dwarf chickens. On the contrary, in extrahepatic tissues such as spleen, lung, brain, kidney, heart, intestine, thymus, and muscle, IGF-I mRNA expression was equally observed in normal and GHR-lacking dwarf chickens. In the testis, expression of IGF-I mRNA was enhanced by about 5-fold in dwarf chickens showing an expression level comparable to that in normal liver. On day 16 in the embryonic stage, IGF-I mRNA was expressed in muscle, brain, eye, heart, and lung in both normal and SLD chick embryos. However, no IGF-I mRNA expression was observed in liver or kidney of normal and dwarf chick embryos. These results suggest that in chicken, IGF-I mRNA is expressed in liver in a GH-dependent manner after hatching, whereas in other tissues, mRNA expression is independent of GH and GHR before and after hatching, except for testis, in which GH seems to inhibit IGF-I mRNA expression.

Growth hormone-independent expression of insulin-like growth factor I messenger ribonucleic acid in extrahepatic tissues of the chicken.

The sex-linked dwarf (SLD) chicken, which lacks GH receptor (GHR), and its normal littermates provide a useful experimental system to investigate GH-dependent cellular responses. The GH dependence of insulin-like growth factor I (IGF-I) expression in tissues was examined in SLD and normal chickens of the Gifu 20 strain. Four weeks after hatching, the most abundant expression of IGF-I messenger RNA (mRNA) was observed in liver of normal chickens, whereas no IGF-mRNA expression was detected in that organ of dwarf chickens. On the contrary, in extrahepatic tissues such as spleen, lung, brain, kidney, heart, intestine, thymus, and muscle, IGF-I mRNA expression was equally observed in normal and GHR-lacking dwarf chickens. In the testis, expression of IGF-I mRNA was enhanced by about 5-fold in dwarf chickens showing an expression level comparable to that in normal liver. On day 16 in the embryonic stage, IGF-I mRNA was expressed in muscle, brain, eye, heart, and lung in both normal and SLD chick embryos. However, no IGF-I mRNA expression was observed in liver or kidney of normal and dwarf chick embryos. These results suggest that in chicken, IGF-I mRNA is expressed in liver in a GH-dependent manner after hatching, whereas in other tissues, mRNA expression is independent of GH and GHR before and after hatching, except for testis, in which GH seems to inhibit IGF-I mRNA expression.

Comparison of Gene Expression in Normal and Growth Hormone Receptor-Deficient Dwarf Chickens Reveals a Novel Growth Hormone-Regulated Gene

Because of a dysfunctional growth hormone (GH) receptor there is an absence of GH-dependent gene expression in the sex-linked dwarf chicken. Therefore, a comparison of mRNAs expressed in normal and dwarf chickens should lead to the identification of mRNAs that are regulated by GH action. We have compared gene expression in normal and dwarf chickens using the mRNA differential display technique. A combination of three anchored oligo dT primers and 15 random decamers were used to detect at least 75 differentially expressed mRNAs. One of these, designated GHRG-1,hybridizes to a 0.9 kb transcript found only in liver and in normal chickens shows a pattern of developmental expression which parallels the plasma GH profile. A GHRG-1 cDNA clone was isolated that encodes a 120 amino acid peptide with no homology to any known gene. Sequence of the promoter from a genomic clone shows a region with strong similarity to the GH response element identified in the serine protease inhibitor gene, Spi 2.1. These results suggest that GHRG-1 is a novel GH regulated gene.

Kinetic Parameters of Plasma Thyroid Hormone and Thyroid Hormone Receptors in a Dwarf and Control Line of Chicken

Sex-linked dwarf chickens have in plasma low triiodothyronine (T3) levels and slightly raised thyroxine (T4) concentrations and are functional hypothyroid. The kinetic parameters of T4 and T3 were investigated using 125I-labeled hormones. In addition the nuclear T3-receptors in the liver were examined using a radioreceptorassay and Scatchard analysis. Four-week-old dwarf (dw) and normal (Dw) chickens were injected with 125I-labeled T3 or T4 and blood samples taken 60, 120, 180, and 300 min after 125I-T3 injection and 120, 240, 360, and 480 minutes after 125I-T4 treatment. Labeled T4 and T3 and the degradation products were separated by paper chromatography. After the paper strips were dried, the iodinated compounds were visualized and counted in the gamma counter. The kinetic parameters, the half-life time (T1/2), the apparent distribution volume (Vd) and the metabolic clearance rate (MCR) were calculated using the natural base logarithm values of the measured radioactivity plotted against time and used for linear regression. T4 was cleared from circulation more slowly in dwarf than in control chicks and reflected a longer T1/2 (21.8%) and a reduced MCR (45%). The Vd tended to be lower (34.7%) in dwarfs. While the T1/2 of T3 was longer (28.1%) in dwarf chickens than in control animals, the MCR for T3 was considerably increased (31.8%). This results from an increased Vd (63.1%) in the dwarf chicks. The T3-receptor study in the liver of dwarf and non-dwarf chickens from Week 1 to Week 4 posthatching revealed that the total capacity and the affinity constant of the binding sites were comparable in dwarf and normal chickens. However, the occupancy of the receptors was higher in the dwarf animals.

Dysfunctional growth hormone receptor in a strain of sex-linked dwarf chicken: evidence for a mutation in the intracellular domain

The sex-linked dwarf (dwdw) chicken represents a valuable animal model for studying GH insensitivity and the consequence of mutations in the GH receptor (GHR) gene. We have recently reported undetectable hepatic GH-binding activity and an aberrantly sized transcript in a strain of dwdw chickens obtained from Arbor Acre Farms, Inc. (Glastonbury, CT, USA). Southern blot analysis of the chicken GHR (cGHR) gene revealed a restriction-fragment length polymorphism in HindIII and EcoRI digests of genomic DNA in this strain of dwdw chicken. In order to localize the molecular mutation, we analysed the gene structure and determined the complete sequence of the 3' untranslated region (3' UTR) of the normal cGHR. With the use of this information, we located a large deletion in the 3' end of the cGHR gene of the Connecticut (CT) strain of dwdw chicken. This deletion (1773 bp) contained 27 highly conserved amino acids of the 3' end of the coding region, the in-frame stop codon, a less frequently used poly(A) signal that is normally found 445 bp downstream of the stop codon, and a large portion of the 3' UTR. Because of this deletion, 27 novel amino acids were substituted and the open reading frame was extended for an additional 26 amino acids before reaching the transcriptional termination site. The predicted amino acid sequence of the novel carboxyl-terminus of the dwdw cGHR is largely hydrophobic with a polylysine tail, whereas the carboxyl-terminus of the wild-type (DwDw) cGHR is composed of hydrophilic amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth Hormone Secretory Characteristics of Sex-Linked Dwarf and Normal-Sized Chickens Reared on a Control or on a 3,3′,5-Triiodothyronine-Supplemented Diet

This study examined the pulsatile presence of growth hormone (GH) in the plasma—which reflects its pulsatile release—of 8-week-old male sex-linked dwarf (dwdw) or normal-sized (Dwdw) chickens from a brown-egg layer strain reared on a control or on a 0.5 ppm triiodothyronine(T 3)-supplemented diet. The overall GH mean, amplitude, and baseline levels of control dw chickens were significantly higher than those of control Dw chickens. No differences in peak length or peak frequency between genotypes were observed. Dietary T 3 supplementation abolished the pulsatile GH release in both genotypes. T 3 treatment depressed the mean GH levels of Dw but not of dw chickens.

Increase of egg weight with age in normal and dwarf, purebred and crossbred laying hens

Summary - An exponential curve, W = P - Q exp (-Rt) was fitted to egg weights (W) of individual hens from 8 genetic groups tested for egg production from 18-51 weeks of age (t). The groups were constituted of the combination of genotype at the sex-linked dwarfism locus (normal or dwarf) and line (White Leghorn, Brown Egg and both reciprocal crosses). The least-squares mean of the residual standard deviation about the curve was between 1.27 and 1.74 g in the 8 groups and estimated values of the initial P-Q and mature egg P weights were between 26.9 and 33.9 g and 56.9 and 65.8 g, respectively. Effects of genotype and line as well as heterosis were estimated for both egg weights and for rate of growth relative to the remaining expected growth (R). R was smaller for dwarf hens which thus reached mature egg weight later than normal females. Significant heterosis was found for both egg weights of dwarf hens (12.6 and 9.5%) and for the initial egg weight of normal ones (12.8%). Negative heterosis for R obtained for dwarf hens (-17.4%) corresponded to a flatter egg weight curve of these crossbreds.

Heterosis in Normal Versus Dwarf Laying Hens

The effect of genotype at the sex-linked dwarf locus on heterosis in crosses between a White Leghorn and a brown egg line for body weight, egg production, and related traits was studied. Heterozygous Dw/dw males were used to produce normal and dwarf pullets in each of the pure lines and their reciprocal crosses (eight genotype-line combinations). There were 54 pullets per combination. Line differences were significant for shank length, body weights at 8, 17, and 52 wk, age at first egg, egg number, clutch length, rate of lay, and egg weight. Heterosis was observed for all of these traits. Body weight as a covariate was not important in analyses of egg number, clutch length, and egg weight. The egg production reduction associated with the dw gene in pure lines was smaller in F1 hens. This discovery may be adequate to warrant use of dwarf crossbred hens for egg production.

Overexpression of a truncated growth hormone receptor in the sex-linked dwarf chicken: evidence for a splice mutation.

Sex-linked dwarfism in chickens is a form of GH resistance that resembles the Laron syndrome in humans. The dwarfism found in chickens is due to a mutant gene (dw) carried on the sex chromosome. The homozygous dwarf (dwdw) chicken is characterized by reductions in stature and plasma insulin-like growth factor-I (IGF-I) levels. Despite the absence of hepatic GH-binding activity, Southern blot analysis shows that there is no gross structural change in the gene for the GH receptor (GHR) in this strain of dwdw chicken. GH-dependent IGF-I production can be restored in cultured dwdw hepatocytes after transfection and transient expression of a chicken GHR (cGHR) cDNA, indicating that other factors that participate in GH-mediated IGF-I synthesis are intact. Northern blot analysis of liver, muscle, fat, and pituitary RNA from normal (DwDw) chickens shows a major transcript of 4.3 kilobases (kb) and three minor transcripts (0.8, 1.7, and 3.2 kb), which correspond to the cGHR. In contrast, the 0.8-kb transcript is the major cGHR transcript expressed in these tissues from dwdw chickens. Northern blot analysis with domain-specific probes shows that the 0.8-kb transcript in DwDw and dwdw liver contains only a small portion of the extracellular domain of the cGHR. A cDNA clone encoding this transcript has been isolated from a liver library prepared from a normal chicken.(ABSTRACT TRUNCATED AT 250 WORDS)

Overexpression of a truncated growth hormone receptor in the sex-linked dwarf chicken: evidence for a splice mutation

Overexpression of a truncated growth hormone receptor in the sex-linked dwarf chicken: evidence for a splice mutation

The Humoral Activity of the Avian Thymic Microenvironment

The thymic microenvironment (composed of the lymphoepithelial stroma and the secretory products of the thymic epithelium) provides the required milieu for the development of the thymus-derived lymphocytes (T cells). There is limited information characterizing or identifying the active secretory components of the avian thymus. The work discussed here has focused on examination of the presence, regulation, and activity of one of the thymic hormones (thymulin) in the chicken. A thymulin-like product has been shown to exist in chicken serum as assessed by the mammalian bioassay and an ELISA immunoassay; thymectomy removes this product from the serum. Serum thymulin activity has been shown to be directly related to the thyroid status of the chick with the functionally hypothyroid Cornell sex-linked dwarf strain having lower levels than the euthyroid K strain. Alterations in circulating thymulin concentrations produced by daily thymulin injections resulted in an altered profile of the major peripheral blood T cell subpopulations and produced significant changes in the autoimmune pathology present within the Obese strain chicken. These approaches represent preliminary attempts to study the role of thymulin in avian immune development and in immuneneuroendocrine interactions.

Growth hormone receptor gene expression in sex-linked dwarf Leghorn chickens: evidence against a gene deletion

GH receptor (GHR) mRNA has been identified in peripheral (liver and muscle) and central (brain and hypothalamus) tissues of sex-linked dwarf (SLD) Leghorn chickens. Total RNA was extracted from the tissues of immature (1 week, 4 week), pubertal (16 week) and adult (> 24 weeks) SLD and K (the normally growing strain) Leghorn chickens. In both groups and all tissues, an mRNA moiety of 4.4 kb hybridized with cRNA probes derived from the rabbit hepatic GHR sequence. An additional low-abundance transcript of 2.8 kb was also identified in some tissues. An age-related increase in expression was observed in K and SLD hepatic GHR mRNA, suggesting normal regulation of SLD GHR gene transcription. Amplification of cDNA from K and SLD tissues in the presence of oligonucleotide primers coding for the intracellular or extracellular domains of the chicken GHR generated electrophoretically separable fragments of expected size. Restriction enzyme digestion of the products with EcoRI, BstNI, HaeIII, NcoI or BamHI produced smaller moieties of expected sizes in both strains. These results demonstrate that, in contrast to broiler SLDs, a GHR gene deletion is not responsible for the GHR dysfunction in Leghorn SLDs. Although the actual defect in GHR gene expression in SLD Leghorns remains to be identified, this study demonstrates that sex-linked dwarfism, like Laron dwarfism, is due to a heterogeneity of lesions.

Abnormal growth hormone receptor gene expression in the sex-linked dwarf chicken

Sex-finked dwarfism is a recessive mutation that causes a reduction in body weight gain and long bone growth of chickens. We examined the effect of the dwarfing gene on body weight, hepatic GH-binding activity, and the structure and expression of the growth hormone receptor (GHR) gene in two different lines of sex-linked dwarf (SLD) broiler chickens. Liver samples from one line of dwarf chicken were obtained from Arbor Acres Farm, Inc. (Glastonbury, CT) and fertile eggs from the second line of SLD were obtained from the University of Georgia. In the GA line, the average body weight of homozygous ( dwdw) males at 1 1 weeks of age was 43% lower than that of normal ( DwDw) males, while heterozygous ( Dwdw) males were only 9% below normal. In the CT fine, hepatic GH-binding activity of 35-week-old chickens was high (20% specific binding) in normal ( DwDw) males and undetectable in liver membranes prepared from dwdw males. At 11 weeks of age, hepatic GH-binding activity of Dwdw males (3.9% specific binding) in the GA line was reduced by 44% and that of dwdw males was almost undetectable (0.34% specific binding) when compared to the average of normal GA males (7.l%-specific binding). Southern and Northern blot analyses revealed different abnormalities in the GHR.. gene from the two separate lines of SLD. A restriction fragment length polymorphism in DNA and an aberrantly sized transcript (mRNA) were detected in the CT line of SLD chickens. In the GA line, the DNA restriction pattern of dwdw males was identical to that found in normal GA chickens; however, there was no expression of the GHR gene in dwdw chickens since we did not detect an mRNA that corresponds to the GHR. These data suggest that the molecular defect of SLD chickens resides within the gene for the GHR and the dwdw phenotype results from a lack of synthesis of a functional GHR protein.

Effects of insulin-like growth factor-I (IGF-I) infusion and dietary tri-iodothyronine (T3) supplementation on growth, body composition and plasma hormone levels in sex-linked dwarf mutant and normal chickens.

This study used a sex-linked dwarf mutant (SLD) chicken to evaluate growth-promoting and metabolic effects of recombinant human insulin-like growth factor-I (rhIGF-I) treatment. The SLD chicken is characterized by a 30% reduction in body weight and by high plasma GH levels, low plasma IGF-I and triodothyronine (T3) levels and very low GH binding on liver membranes, suggesting reduced functional GH receptors compared with normal chickens. The effects of a continuous s.c. infusion by osmotic mini-pump of 0.1 mg rhIGF-I/kg per day from 4 to 8 weeks of age on body weight, bone growth and body composition were investigated in female SLD and normal chicks. In addition, half of the birds received a dietary supplement of T3 (0.1 parts per million). Plasma levels of IGF-I, GH, T3, thyroxine and insulin were followed during the treatment. In normal chicks, rhIGF-I infusion had no effect on growth and little effect on plasma hormone levels except for a decrease in plasma insulin. In dwarf chicks, rhIGF-I infusion slightly increased body weight but had no effect on longitudinal bone growth. In addition, plasma GH levels were decreased and T3 levels remained lower than in normal chicks. Normal and dwarf chicks showed a decrease in abdominal fat after both IGF-I administration and T3 supplementation, the treatments having additive effects in dwarf chicks only. The combined rhIGF-I and T3 treatment restored a quasi-normal hormonal pattern in dwarf chicks, except for insulin which remained lower than in normal chicks. These results suggest that IGF-I in the chicken has no direct endocrine effect on statural growth.

Effects of the Sex-linked Dwarf Gene (dw) on the Expression of the Muscular Dystrophy Gene (am) in Chicken

The sex-linked dwarf gene (dw) was introduced into companion muscular dystrophic (am) and nondystrophic (Am+) New Hampshire chicken lines to investigate influences of the dwarf gene on breast muscle weights, muscle fiber area, and the histological expression of muscular dystrophy. Dystrophic and nondystrophic chickens within dwarf or nondwarf genotypes were similar in body and carcass weights. Pectoralis and supracoracoideus muscle weights (as a percentage of adjusted carcass weight) were similar in nondystrophic dwarf and nondwarf males and females. In addition, pectoralis weight was similar in dystrophic dwarf males and dystrophic nondwarf males and females. However, pectoralis weight was significantly smaller in dystrophic dwarf females than in dystrophic nondwarf females, whereas supracoracoideus weight was significantly larger in dystrophic dwarf males than in dystrophic nondwarf males. Supracoracoideus weight was similar in dystrophic dwarf males and females and dystrophic nondwarf females. Pectoralis muscle fiber area was influenced by sex and by dwarf and dystrophy genotype. Muscle fiber area was larger in females than in males, smaller in dwarfs than in nondwarfs, and smaller in dystrophic than in nondystrophic muscles. Muscle fiber degeneration and adipose infiltration was more extensive in dystrophic than in nondystrophic females and males, and it was more advanced in dwarfs than in nondwarfs. Excessive acetylcholinesterase staining patterns were characteristic of dystrophic muscle in both dwarf and nondwarf genotypes. Nondystrophic and dystrophic dwarf male and female chickens are comparable substitutes for nondwarfs as biomedical models with respect to pectoralis histology, acetylcholinesterase staining pattern, and pectoralis muscle hypertrophy.

Effect of triiodothyronine supplementation on thyrotropin-releasing hormone-induced growth hormone secretion in sex-linked dwarf and normal chicks

The effect of a dietary triiodothyronine (T 3) supplement, of either 0.1 or 0.5 μg/g of feed, was studied on the thyrotropin-releasing hormone (TRH)-induced growth hormone (GH) secretion in sex-linked dwarf ( dw) or normal ( Dw) chicks of both sexes. In normal chicks, 0.1 μg/g T 3 decreased plasma GH levels before TRH as well as the GH increase after TRH, and 0.5 μg/g T3 totally suppressed any response to TRH, either at 4 or at 7 weeks of age. Dwarf chicks were more sensitive to TRH than normals when receiving either 0 or 0.1 μg/g T 3; 0.5 μg/g T 3 abolished the difference between genotypes at 4 weeks of age but not so clearly at 7 weeks of age, where dwarf females showed a slight but still significant GH increase after TRH. Interactions between genotype, TRH injection, and T 3 treatments were often significant at 4 weeks of age and even more at 7 weeks of age. Dwarf chicks receiving 0.1 μg/g T 3, expected to have normal plasma T 3 levels, showed a higher GH response after TRH. This suggests that other hormones may be involved in the regulation of this response, particularly IGF-I, which is known to remain at a low level in T 3-treated dwarf chicks.

Molecular Cloning of the Chicken Growth Hormone Receptor Complementary Deoxyribonucleic Acid: Mutation of the Gene in Sex-Linked Dwarf Chickens*

A novel complementary DNA (cDNA) encoding the chicken GH receptor was isolated from a chicken liver cDNA library, using polymerase chain reaction with primers derived from highly conserved sequences of the mammalian GH receptor. The nucleotide sequence predicts a mature protein of 592 amino acids and a 16 amino acid signal peptide that are partially homologous to the sequence reported for the rabbit (53%), rat (58%), and human (50%) GH receptors. Despite this low level of homology, a number of structural features of the GH receptor are conserved, including 7 cysteine residues in the extracellular domain and 5 in the intracellular region. Three transcripts of approximately 4.7, 4.0, and 1.0 kilobases are present on Northern blots of total RNA prepared from the livers of 35-week-old male chickens. Expression of the GH receptor was also detected in a wide range of tissues. The chicken GH receptor cDNA was then used as a probe in Southern and Northern blot analyses of DNA and RNA prepared from livers of sex-linked dwarf chickens, which have undetectable levels of hepatic GH-binding activity, in addition to other endocrine abnormalities. A restriction fragment length polymorphism was found in DNA, and an aberrantly-sized transcript was found in hepatic RNA of the dwarf chicken. These results indicate that a mutation in the GH receptor gene is responsible for the phenotype of the sex-linked dwarf chicken. This type of dwarfism resembles Laron-type dwarfism in humans, where a defect in the GH receptor gene has recently been identified. These receptor-deficient chickens should serve as a unique model system for studying the role of the GH receptor in growth and development.

Central somatostatinergic regulation of growth hormone secretion in dwarf chickens

1. 1. Basal circulating growth hormone (GH) concentrations in sex-linked-dwarf (SLD) chickens were unaffected by the intracerebroventricular (icv) injection of 10, 50 or 100 μg somatostatin (SRIF). 2. 2. The GH response to systemic thyrotropin-releasing hormone (TRH; 10μg/kg, iv) was, however, ‘paradoxically’ enhanced 20 min after icv SRIF administration. 3. 3. A lower dose (1.0μg) of SRIF had no effect on basal or TRH-induced GH release. 4. 4. High-litre SRIF antisera (4μl) also had no acute effect on basal plasma GH concentrations, but augmented the GH response to TRH challenge. 5. 5. SRIF would appear to act at central sites to modulate stimulated GH secretion in SLD chickens.

Effects of dietary T 3 on growth parameters and hormone levels in normal and sex-linked dwarf chickens

Tri-iodothyronine (T 3) has been administered in the diet, from day of hatch until 8 weeks of age, to sex-linked dwarf and normal chicks of both sexes from a brown-egg slow-growing strain. Feed was supplemented with either 0.1 ppm or 0.5 ppm T 3. A significant genotype by treatment interaction was observed on body weight: the effect of T 3 in males was significantly positive for dwarfs and null for normals, the effect in females was null for dwarfs and significantly negative for normals. Feed efficiency was rather decreased by the treatment in both genotypes. Abdominal fatness was decreased in a dose-dependent manner in both genotypes, while rectal temperature was raised in dwarf chicks only. Plasma T 3 was raised to normal levels in dwarfs receiving 0.1 ppm exogenous T 3, while the 0.5 ppm dose yielded hyperthyroid levels. Plasma GH levels were decreased in a dose-dependent manner by the T 3 treatment, the effect being larger in dwarfs. Surprisingly, plasma IGF-I was unchanged in spite of the GH decrease, whatever the genotype or the sex. It was concluded that exogenous T 3 alone can have a stimulatory effect on growth in dwarfs but can not fully restore a normal growth rate. Both T 3 and IGF-I are important for a normal growth and the relationships between T 3 and IGF-I production should be further investigated in order to better understand the physiological modifications due to the sex-linked dwarf gene.