The relationship between age and genotype and circulating concentrations of triiodothyronine (T3), thyroxine (T4), and growth hormone in commercial meat strain chickens

The presence of the sex-linked dwarf gene (dw) in homozygous male (dw/dw) and female (dw/-) meat strain chickens is associated with a significant reduction in circulating levels of triiodothyronine (T3). Heterozygous (Dw/dw) male broiler strain chickens have T3 concentrations similar to those in homozygous (Dw/Dw) male broilers. Genetically normal (Dw/Dw) but significantly slower growing roaster strain male meat chickens had consistently higher T3 than the faster growing broilers at all ages in one experiment but only at 8 weeks in a second experiment. Age and not growth rate appears to have a greater influence on serum T3 concentrations in the slow- and fast-growing normal strains. Growth hormone levels were significantly higher in the dwarf chickens at all ages and in all three experiments. The heterozygous and homozygous broilers had similar GH levels and the slow-growing, genetically normal roasters had intermediate concentrations between the broiler and dwarf lines. GH was influenced to a greater extent by the rate of body weight gain than by increasing age in the genetically normal fast and slow growing strains.     

Differences in hepatic growth hormone receptor binding during development of normal and dwarf chickens

The aim of this study was to compare the growth hormone (GH) receptor in liver microsomal fractions of normal chickens (Dw) and chickens carrying the dwarf gene (dw). Specific binding of GH to its hepatic receptor was significantly higher for Dw embryos from d 14 till d 20 of incubation than for dw embryos. The difference in binding was due to a decreased binding capacity but not affinity in the livers of the dwarf embryos. The same binding pattern was found in livers of adult chickens: lower binding was again caused by a lower number of GH receptors and at this stage the difference was even clearer than during embryonic development. Binding studies on livers of growing chicks demonstrated that binding was low for both genotypes, but a small though significant difference between them remained. The cause of this decrease in number of GH receptors in dwarf birds has yet to be determined but may be due to the primary action of the dwarf gene.     

The effect of the dw gene on the muscle protein turnover rate in chickens

Fractional rates (%/day) of muscle protein synthesis and degradation of the genotypes Dw/Dw and dw/dw of male White Plymouth Rock chickens were determined by measuring the output of N-methylhistidine (N-MH) in the excreta at 2, 4, and 8 weeks of age. The fractional growth rate of dw/dw was significantly lower (P<0.05) than that of Dw/Dw at 2 weeks of age but not at 4 and 8 weeks of age. No significant differences in the degradation rate (K _d; %/day) were found at any age. A significant difference (P<0.05) between genotypes in the rate of synthesis (K _s; %/day) was found at 2 weeks of age (Dw/Dw=11.8, dw/dw=9.9) but not at 4 and 8 weeks of age. These results suggest that the dw gene has a depressing effect on the synthesis rate of muscle protein, and the difference between genotypes in the growth rate at the early stage is a reflection of this effect.     

Deficiency of immunoreactive somatostatin in the median eminence of Snell dwarf mice

Snell dwarf mice (dw/dw) are characterized by a genetically determined, congenital lack of pituitary GH, TSH and prolactin. Given that hypothalamic somatostatin is involved in the regulation of pituitary GH and TSH release, it was decided to investigate the content of immunoreactive somatostatin (IRS) in the median eminence of dw/dw and phenotypically normal mice of the same strain. The content of IRS in the pyloric antrum and pineal gland of these animals was also examined. The effects of ovariectomy and of hyperprolactinemia (induced by a pituitary graft under the kidney capsule) on the median eminence content of IRS were also studied in both normal and dwarf mice. Median eminence IRS content was significantly lower in the dw/dw (23.6 +/- 1.8 ng) than in normal mice (57.4 +/- 7.1 ng); no difference was found in the pyloric IRS content of dw/dw (16.9 +/- 1.6 ng/mg of protein) and normal animals (13.8 +/- 1.9 ng/mg of protein), nor in the pineal content of IRS (639.4 +/- 64.4 pg/gland in the dw/dw; 732 +/- 265 pg/gland in normals). Neither ovariectomy nor hyperprolactinemia were found to affect the IRS content in the tissues studied in normal or dwarf mice. Treatment of an additional group of 9 dwarf mice with L-thyroxine (L-T4 2 micrograms/48 h. s.c. for 2 weeks) significantly increased the animals weight (10.2 +/- 0.4 g versus 7.4 +/- 0.3 g) and produced maturation of facial features; however, it did not change the IRS content in any of the tissues studied. It is concluded that the content of IRS in the median eminence of mice with a congenital lack of GH, TSH and prolactin is significantly reduced and that this is unlikely to be related to the deficiency of thyroid hormones in these animals.     

Muscle growth and protein degradation during early development in chicks of fast and slow growing strains

1. Growth of breast and leg muscles and excretion of N tau methyl histidine in layer (slow growing) and broiler (fast growing) chicks were measured at five time intervals between 2 and 33 days of age. 2. The results indicate that muscles of the broiler chick grow faster than in layer chicks and that breast muscles of both strains grow faster than leg muscles in the first 2 weeks after hatching. 3. N tau methyl histidine excretion by layer chicks is higher than that by broilers relative to body weight, musculature and relative maturity at all ages examined. 4. The results suggest that faster growth of muscles is accompanied by a lower rate of protein degradation although at ages of less than 2 weeks differences in protein synthesis rates may also contribute to muscle growth.     

Impaired peripheral T3 production but normal induced thyroid hormone secretion in the sex-linked dwarf chick embryo

Plasma concentrations of thyroxine (T4), triiodothyronine (T3) and chicken GH (cGH), together with hepatic 5'-monodeiodination (5'-D) activity, were measured in normal (Dw) and dwarf chick (dw) embryos at incubation d 18. An injection of 10 micrograms of ovine GH (oGH) raised plasma concentrations of T3 in Dw embryos after 1 and 2 h and stimulated hepatic 5'-D activity after 2 h. A non-specific increase in T4 was also observed after 1 h in Dw animals probably due to the heterologous nature of the injection. These effects were not observed in dw embryos. An injection of 1 microgram of TRH was able to increase cGH levels after 15 min in Dw embryos, whereas the the observed increase in the dw group was not significant. In Dw embryos, 0.01, 0.1 and 1 microgram of TRH increased plasma concentrations of T3 in a dose-dependent way, whereas in dw embryos, no reaction to the TRH injections was seen, except for the highest dose used. Contrary to this observation, T4 was increased to the same level in both Dw and dw embryos following TRH injections. An injection of 1 microgram of ovine CRH increased corticosterone after 0.5 h and elevated T3 and T4 after 2 h to the same extent in Dw and dw embryos. It is concluded that the thyrotrophic activities of TRH and oCRH and the corticotropic activity of oCRH do not differ between normal and sex-linked dwarf embryos. However TRH and GH were unable to stimulate the T4-T3 conversion in the liver of dw embryos, presumably due to the lack of hepatic GH receptors in these animals.     

Induction of pregnancy-associated murine protein-1 in dwarf mice by human growth hormone

Pregnancy-associated murine protein-1 (PAMP-1) could not be detected in peripheral blood of female dwarf mice (genotype dw/dw of the DW strain). By contrast the normal size females of the DW strain (genotypes +/+ and +/dw) had PAMP-1 serum levels of 18.9 AU +/- 15.7 AU/ml. Following administration of biosynthetic human growth hormone (hGH) every 2 h for 52 h PAMP-1 was detected in all dwarf females at concentrations of 16.0 AU +/- 3.3 AU/ml. The albumin levels in the circulation of DW females of normal size were significantly higher (P less than 0.05) than those of DW dwarfs, and the hGH administration did not change the serum albumin levels. The present experiment adds weight to the suggestion that the PAMP-1 serum level is regulated by GH.     

Photoperiodic requirements for LH release in juvenile broiler and egg-laying strains of domestic chickens fed ad libitum or restricted diets

Photoperiodic response curves for LH release were obtained for juvenile female domestic chickens at 8 weeks of age by measuring changes in plasma LH concentrations after increasing the daily photoperiod from 8 to 10.5, 12.75, 15.25, 17.75 or 20 h. The birds were bred either for meat production (broiler) or commercial egg-laying and were fed ad libitum or a restricted diet, similar to that used under commercial conditions. Ovarian and oviduct growth was stimulated by 2 weeks after transfer to 20 h light/day in the dwarf broiler strain, irrespective of the dietary treatment, but not in birds of the egg-laying strain. Baseline concentrations of plasma LH were higher in the egg-laying than in the dwarf broiler strain birds. A significant effect of dietary treatment was observed on the changes in concentration of plasma LH in the nonphotostimulated dwarf broiler, but not in the egg-laying bird. There was no significant interaction between dietary treatment and photoinduced LH release in birds of either strain. The shortest photoperiod needed to stimulate LH release (critical daylength) was less than 10.5 h in the dwarf broilers and between 10.5 and 12.75 h in the egg-laying birds. The shortest photoperiod needed to stimulate the maximum release of LH (saturation daylength) was between 10.5 and 12.75 h in the dwarf broiler strain. The saturation daylength in birds of the egg-laying strain was longer, being between 12.75 and 15.25 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of gender and functional overload on plantaris muscle morphology in the dwarf (HsdOla:dw-4) Lewis rat

To investigate relationships between pituitary function and gender on skeletal muscle growth and hypertrophy, fiber cross sectional area (CSA) and type were assessed in the plantaris muscle of normal and dwarf (Dw) male and female Lewis rats after 6 weeks of functional overload (FO). Serum growth hormone levels were 70-80% less in Dw rats of both genders, and body mass was 62% greater in normal rats when compared to their Dw counterparts. Muscle weight was affected by gender, dwarfism, and FO as well as a significant gender*Dw*FO interaction. FO increased Type I, IIA, and IIX/B fiber CSA 120%, 102%, and 75%, respectively. Only type 1H fibers exhibited a reduction in CSA as a function of gender or dwarfism. Both type IIA and IIX/B fibers were affected by a significant gender*Dw*FO interaction. Our results suggest that the growth of type II fibers is sensitive to gender and pituitary function, while hypertrophy of type II muscle fibers is a function of the interaction between mechanical load, gender, and pituitary function.     

Motivation and ability to walk for a food reward in fast- and slow-growing broilers to 12 weeks of age

Poor physical abilities of broilers may prevent them from performing behaviours for which they are motivated. The aim of this study was to measure the influence of physical ability and motivation on the performance of broilers in short physical tasks. We tested birds from a fast- and a slow-growing broiler strain in a runway to 12 weeks of age. To manipulate motivation, half of the birds of each strain was feed deprived for 3h and the other half for 24h before testing. Each bird was tested in a control and a slalom runway test once a week. With a similar motivation, slow growers had a shorter latency to start walking and walked faster through the runway than fast growers in both tests. In fast growers walking speed decreased faster with age than in slow growers. Slow growers vocalised more in both tests. In the slalom test, 24h deprived birds vocalised more than 3h deprived birds. Although the fast and slow growers have a different genetic background, the results indicated that motivation is the dominant determinative factor for walking in birds with a low body weight, while physical ability is the dominant determinative factor for walking in birds with a high body weight.     

De novo lipogenesis and lipolysis activities in normal (Dw) and dwarf (dw) White Leghorn laying hens

1. In vitro activities of glucose oxidation, de novo lipogenesis and lipolysis were compared in normal (Dw) and dwarf (dw) laying hens. 2. Dwarfism reduced the hepatic glucose oxidation while de novo lipogenesis was not altered. As liver weight was depressed, total liver lipogenesis capacity was probably reduced by dwarfism. 3. As compared to normal hens, de novo lipogenesis and basal or stimulated lipolysis were lower in dwarf adipose tissue while its lipid content was enhanced in dwarfs. 4. Results suggest that in laying hens dwarfism reduces the adipose tissue lipid mobilization but probably also the liver de novo lipogenesis.     

A decreased capacity of hepatic growth hormone (GH) receptors and failure of thyrotrophin-releasing hormone to stimulate the peripheral conversion of thyroxine into triiodothyronine in sex-linked dwarf broiler hens

The effect of two different doses of thyrotrophic releasing hormone (TRH) upon the plasma levels of growth (GH) and thyroid hormones in both sex-linked dwarf (dw) and normal (Dw) broiler hens was determined. In normal hens, 1.5 and 24 microg TRH/kg increased the GH plasma concentrations after 15 min. Plasma concentrations of T3 increased significantly 1 h after TRH injection, whereas T4 concentration decreased after 2 following injection of 24 microg/kg TRH. In dwarf hens both doses of TRH increased the plasma concentrations of GH and the GH response lasted longer. However, TRH was ineffective in raising T3 and T4 levels. Saline-injected dwarf birds showed no differences in plasma T4 and T3 levels in comparison with normal hens. A smaller number of hepatic cGH receptors was found in dwarf hens, whereas the affinity of the hepatic GH receptor was not influenced by the genotype. It is concluded that the sex-linked dwarf broiler hen is unable to respond to a TRH-induced GH stimulus probably because of a deficiency in hepatic GH receptors resulting in a failure to stimulate the T4 to T3 converting activity.     

Biochemical correlates of selection for weight-for-age in chickens: twenty-fold higher muscle ornithine decarboxylase levels in modern broilers

Summary Little is known about the biochemical correlates of selection for growth in farm or laboratory animals, or the identity of the gene products affected or produced by trait-genes. Modern broiler chickens have about 8-fold greater breast muscle mass than layer chickens at 7 weeks of age and over 2-fold greater breast muscle mass than their 1972 counterparts. This increase in muscle mass is associated with over 20-fold higher levels of ornithine decarboxylase (ODC) in broiler chickens at 1 week of age as compared with layer strain chickens; there is a comparable increase in a relaxed-selection strain of broilers. The increase in ODC levels is larger than the differences in muscle or body weight between broilers and layers at 7 weeks of age, occurs at an age when there is no difference in weights between the strains and precedes the major growth spurt. Increases in ODC levels and hence polyamine synthesis have been associated with, and usually precede, rapid growth and cell proliferation in a wide range of cell types and organisms in response to many different stimuli. Therefore, the correlation of ODC levels with genetic differences in muscle growth make it worth investigating the control of ODC gene expression in these strains.     

Variation among chicken stocks in the fractional rates of muscle protein synthesis and degradation

Fractional rates (% · day^–1) of synthesis and degradation were determined by measuring the output of N-methylhistidine (MeHis) in the excreta at 4 and 8 weeks of age in the chicken. At 4 weeks of age, the fractional rate of synthesis of the meat-type stock was twice that of the egg-type stock (White Leghorn), but the fractional rates of synthesis at 8 weeks of age were similar (4.1–5.1% · day^–1) among stocks. The fractional rate of degradation (1.3–1.5% · day^–1) of the meat-type stock at 8 weeks of age was less than half the rate of the egg-type stock (2.9% · day^–1). The fractional rates of synthesis and degradation at 4 weeks of age in the Satsuma native fowl were relatively high compared with those in the other stocks. In particular, the rate of degradation (8.6% · day^–1) at 4 weeks of age was approximately twice that of other stocks. These results show that fractional rates of synthesis and degradation of muscle protein in the chicken differ among genetically diverse groups. The effect of changes in rates of synthesis and degradation on the change in fractional growth rate also differed. From regression coefficients (b_{K s · FGR} and b_{K d · FGR}) of these rates in skeletal muscle protein on the fractional growth rate, it was recognized that the change in growth rate accompanies the changes in both synthesis and degradation in White Leghorn and commercial broilers but only the change in synthesis in White Plymouth Rock (dw) and Satsuma native fowl.     

Developmental study of digestive motor response to dietary pattern in young broilers

1. Gastrointestinal (GI) morphometry and motility were measured in young broilers (3, 8 and 15 days old) when they were submitted to three dietary patterns (ad libitum food, acute 48-hr fast and cumulative semi-starvation). 2. All GI regions were hypertrophied when acute or intermittent starvation were applied; lightly when acute starvation was applied and strongly with intermittent starvation. 3. All the 3-, 8- and 15-day-old acutely and intermittently starved chickens increased GI motility at the shortest times (0.5 and 1 hr) after the marker administration, but decreased GI motility at the longest times (2 and 4 hr). 4. The GI motor response of intermittently starved chicks was faster than the observed one in acutely starved chicks. The GI tract of intermittent starved chicks increased its motor response proportionally to the broilers age, while acutely starved chicks decreased their GI motility when the broilers age was increased. 5. A relationship between morphometrical and motor GI responses to dietary pattern has been inferred. From the gizzard and small bowel hypertrophy caused by intermittent starvation, an increased reflex GI motor response can be expected. Acute starvation only caused a light increase of GI motility, because it only increased the gizzard morphometry. 6. We concluded that young chicks respond to acute starvation by means of a short-term mechanism (an increase of GI motor reflexes) and responds to intermittent starvation by means of a medium- and long-term mechanism (a hypertrophy of GI tract and its subsequent increase of GI motor reflexes).     

Regulation of arachidonate-induced platelet aggregation by the lipoxygenase product, 12-hydroperoxyeicosatetraenoic acid

Two lines of genetically involved and control chickens were compared with regard to the onset of muscle dystrophy during the early stages of growth ex ovo. Definite structural and functional involvement of pectoralis muscle developed within the first 4-5 weeks. In parallel experiments, microsomal membranes were obtained weekly from pectoralis muscle during the first 14 weeks ex ovo. The microsomes were studied with respect to ultrastructural features, protein composition, Ca2+ uptake and ATPase activity. Microsomal preparations obtained from all newborn chickens contain two types of vesicles: one type reveals an asymmetric distribution and 'high density' of particles on freeze-fracture faces which is characteristic of sarcoplasmic reticulum (SR) membrane; the other type reveals a symmetric distribution and 'low density' of particles. The yield of 'low density' microsomes from muscle of normal birds is very much reduced as the chicks grow from 1 to 4-5 weeks ex ovo. On the contrary, it remains high in chicks developing muscle dystrophy. Ca2+ uptake and coupled ATPase activity are found to be of nearly identical specific activity in control and genetically involved newborn chicks. The specific activity of the control birds, however, increases as the chicks grow from 1 to 4-5 weeks of age, while the specific activity of the dystrophic birds remains low. Such a difference appears to be related to the relative representation of sarcoplasmic reticulum and 'low density' vesicles in the microsomal preparations. It is concluded that failure to obtain a normal differentiation of muscle cell membranes is a basic defect noted in the early growth of genetically involved chickens. This defect appears along with the earliest signs of the dystrophic process.     

Effects of Dietary Selenium, Vitamin E, and Their Combination on Growth, Serum Metabolites, and Antioxidant Defense System in Skeletal Muscle of Broilers Under Heat Stress

This experiment was conducted to evaluate the effects of dietary vitamin E, selenium (Se), and a combination of the two, on the performance, serum metabolites and oxidative stability of skeletal muscle of broilers during heat stress. The broilers raised in either a thermoneutral (23.9°C constant) or heat stress (23.9°C to 37°C cycling) environment were assigned to 6 dietary treatments (0, 0.5, or 1 mg/kg Se; 125 and 250 mg/kg vitamin E; or 0.5 mg/kg Se plus 125 mg/kg vitamin E) from 1 to 49 days of age. At the end of the experiment, blood samples were collected from chicks, the chicks sacrificed, and pectoralis superficialis muscle was used for measurement of malondialdehyde (MDA) concentration and enzyme activities of glutathione peroxidase (GPx) and superoxide dismutase (SOD). The heat-stressed chicks consumed less feed, gained less weight, and had higher feed conversion ratio when compared to thermoneutral chicks (P<0.05). Serum concentrations of iron (Fe) and zinc (Zn) were decreased by heat stress (P<0.05), whereas the serum concentrations of copper (Cu), glucose, and uric acid were significantly increased under heat stress (P<0.05). The chicks that received supplemental of vitamin E exhibited significantly higher serum concentrations of Zn (P<0.05) and significantly lower concentrations of Cu, glucose, and uric acid (P<0.05) when exposed to heat stress. Dietary Se also caused a significant decrease in serum glucose, uric acid, and Cu concentrations of heat-stressed broilers (P<0.05), but had no significant effect on Zn concentration (P>0.05). The GPx activity remained relatively constant (P>0.05), though SOD activity and MDA levels in skeletal muscle were enhanced on exposure to heat stress (P<0.05). The heat-stressed chicks that received the combined supplementary level of vitamin E and Se had the lowest concentration of MDA and the highest activity of SOD in the skeletal muscle (P<0.05). Dietary Se also caused a significant increase in enzyme activity of GPx in the skeletal muscle (P<0.05). These results indicate that the derangement of blood parameters and oxidative stability in broilers under heat stress are improved by supplemental vitamin E and Se.     

Effects of ambient temperature and early open-field response on the behaviour,feed intake and growth of fast- and slow-growing broiler strains

Increased activity improves broiler leg health, but also increases the heat production of the bird. This experiment investigated the effects of early open-field activity and ambient temperature on the growth and feed intake of two strains of broiler chickens. On the basis of the level of activity in an open-field test on day 3 after hatching, fast-growing Ross 208 and slow-growing i657 chickens were allocated on day 13 to one of the 48 groups. Each group included either six active or six passive birds from each strain and the groups were housed in floor-pens littered with wood chips and fitted with two heat lamps. Each group was fed ad libitum and subjected to one of the three temperature treatments: two (HH; 26°C), one (HC; 16°C to 26°C) or no (CC; 16°C) heat lamps turned on. Production and behavioural data were collected every 2 weeks until day 57. For both strains, early open-field activity had no significant effects on their subsequent behaviour or on any of the production parameters measured, and overall, the slow-growing strain was more active than the fast-growing strain. Ambient temperature had significant effects on production measures for i657 broilers, with CC chickens eating and weighing more, and with a less efficient feed conversion than HH chickens, with HC birds intermediate. A similar effect was found for Ross 208 only for feed intake from 27 to 41 days of age. Ross 208 chickens distributed themselves in the pen with a preference for cooler areas in the hottest ambient temperature treatments. In contrast, the behaviour of the slow-growing strain appeared to be relatively unaffected by the ambient temperature. In conclusion, fast-growing broilers use behavioural changes when trying to adapt to warm environments, whereas slow-growing broilers use metabolic changes to adapt to cooler ambient temperatures.     

Response of pituitary thyrotrophs to thyrotrophin-releasing hormone in Snell dwarf mice

Summary The effect of thyrotrophin-releasing hormone (TRH) on pituitary thyrotrophs was investigated in Snell dwarf mice (dw/dw) that are genetically deficient in thyrotrophin (TSH) and in normal animals of the same strain. The normal animals were treated with either saline or 10 g TRH per day for 2 weeks, while the dwarf mice were given daily injections of saline, 10 g TRH for 2 weeks or 10 g for 6 weeks. At the end of each experimental period, the pituitary glands were removed and fixed for light-microscopic analysis using immunocytochemistry, or for transmission electron-microscopic study. Compared to thyrotrophs observed in the pituitary glands of untreated normal mice, thyrotrophs in TRH-treated normal mice appeared to be more numerous by immunocytochemistry and showed signs of stimulation by electron microscopy. In contrast, immunostainable thyrotrophs could not be identified in the pituitary glands of untreated or TRH-treated dwarfs. However, a few cells exhibiting ultrastructural features of stimulated thyrotrophs, were noticeable in the dwarfs following TRH administration. Thus, while failing to induce the synthesis of immunoreactive TSH under the applied experimental conditions, exogenous TRH appeared to elicit differentiation of thyrotroph precursors into ultrastructurally recognizable thyrotrophs. The discrepancy between the immunocytochemical and ultrastructural findings remains unresolved; more work is required to clarify the question as to why ultrastructural maturation of thyrotrophs was unaccompanied by the production of immunoreactive TSH.