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Vorozole, a selective aromatase inhibitor, was administered in ovo to test the specific embryonic role of estrogen in conferring the sex distinction in GH release and body phenotype in broilers. On Day 6 of incubation, eggs were injected with saline or with different concentrations of vorozole. Postnatal blood samples were analyzed for T3, T4, GH, estradiol (E2), and testosterone (T). At the age of 4 wk, control and vorozole-treated birds were cannulated, and serial blood samples were withdrawn every 10 min for 5 hr, wherein GH pulsatility characteristics were determined using deconvolution analysis. The proportional abdominal fat pad weight was reduced significantly in the treated groups, especially in female birds. The vorozole treatment increased plasma T3, E2, T, and GH concentrations, and decreased T4. The frequency of the GH pulses was lower and the interval between the bursts (min) was higher in the vorozole-treated group, as were the mass secreted per burst (ng/ml), the amplitude (ng/ml/min) and the production rate (ng/ml/5 hr). In conclusion, early in ovo treatment with a potent aromatase inhibitor is able to increase the mean serum T3 and GH concentration and masculinize the GH pulse pattern, resulting in an economically favorable decrease in abdominal fat pad content in male and female broilers at slaughter age.

An experiment with 840 day-old male broiler chicks (Ross) was initiated to investigate the effect of intermittent lighting schedules, combined with two isocaloric feeds differing in protein content, on ascites mortality. At 9 d of age, chicks were randomly distributed over two rooms: one room with a 23 h light(L):1 h dark (D) lighting schedule (CL), and another room with an intermittent lighting schedule (IL, 1L:3D). In each room, two isocaloric feeds differing in CP content, supplemented or not supplemented with 1.5 ppm triiodothyronine (T3), were provided. The experiment was repeated under identical conditions except that lighting schedules were changed between rooms. From Day 14 onwards, biweekly growth and feed intake were determined and feed conversion (FCR) was calculated. Daily mortality was recorded and necropsies were performed. Weekly blood samples were taken from 10 randomly chosen birds per experimental group for measurements of growth hormone (GH), thyroid hormones [thyroxine (T4) and triiodothyronine (T3)], and hematocrit values. Birds reared in IL manifested a more concave growth trajectory than those in CL, which was associated with a lower FCR and higher plasma GH levels during the finisher period. Mortality due to ascites was markedly increased by dietary T3 supplementation. In hyperthyroid chickens, ascites mortality was lower in IL than in CL and lower in birds fed normal protein than in those fed subnormal protein levels. Dietary T3 decreased plasma GH and T4 levels, whereas T3 levels were increased. It is concluded that IL or a higher protein content of the feed reduces the incidence of T3-induced ascites mortality. Possible causal mechanisms are discussed.

1. The role of both insulin-like growth factors (IGF)-I and -II in regulation of growth hormone (GH) secretion in chickens was examined. Seven-week-old male broiler chickens were injected intravenously (i.v.) with recombinant human IGF-I or IGF-II or specific anti-IGF-I or IGF-II immunoglobulins. Blood samples were taken before treatment and at 15 min intervals afterwards for 1 h. Controls received saline i.v. 2. Both IGF-I and IGF-II administration resulted in a rapid, significant decrease in plasma GH concentrations, but the concentrations of both triiodothyronine and thyroxine remained unchanged. 3. Immunisation against both IGF-I and IGF-II produced a significant elevation in plasma GH. 4. These data show that both IGFs can regulate GH concentrations in birds. Furthermore, the immunoneutralisation data suggest that these hormones have a physiological role in the regulation of GH secretion.

1. The present study was conducted to establish the effect of compensatory growth of broiler chickens on pulsatile growth hormone (GH) secretion. 2. Exposing male broiler chickens to intermittent lighting (IL) at 10 d of age was associated with a transient reduction in body weight gain which was followed by compensatory growth from 4 weeks of age onwards. At 34 d of age, cannulated IL broiler chickens manifesting compensatory growth and control chickens reared under continuous illumination (CL) were serially sampled at 10 min intervals over 5 consecutive hours and plasma GH concentrations measured. The resultant GH time series were analysed by deconvolution analysis. 3. The overall mean GH concentration was higher for IL than for CL broilers. The burst frequency did not differ between lighting treatments, but during each GH surge, IL broilers released a higher GH mass which resulted in higher GH amplitude values. As a consequence, GH production rates of IL broilers during the entire sampling session were markedly elevated compared to those of their age-matched CL broilers. There were no differences in the monoexponential GH half-life. 4. Compensatory growth in broiler chickens is associated with an amplification of GH secretory burst mass. The underlying causal mechanisms remain to be elucidated.

Protein assimilation and metabolism studies are hindered by the lack of an adequate oral tracer, i.e., labeled proteins. We present a new and easily reproducible methodology for producing large amounts of egg proteins labeled with L-leucine-13C1. Laying hens were fed a 0.2% leucine-deficient food supplemented with 0.2% L-leucine-13C1 (99 atom %). At plateau, eggs containing highly enriched proteins were obtained. The 13C content of egg white relative to the total C content was 1.3371 atom %, corresponding to delta = 206%. The overall tracer recovery in egg proteins was high (40.2%), making this method financially attractive as well. Accurately measurable levels of 13CO2 in breath were obtained after ingestion of a physiological load of labeled egg white proteins. Thus, egg proteins with sufficient 13C enrichment and applicable for human protein assimilation and metabolism kinetic studies were produced in an easily reproducible and highly efficient manner.

Plasma concentrations of thyroid hormones (TH) and growth hormone (GH) during postnatal development in the precocial Japanese quail (Coturnix japonica), and the altricial European starling (Sturnus vulgaris) were measured. Developmental changes in plasma TH found in the starling, differ considerably from those detected in quail. In quail rather high triiodothyronine (T3) values were measured during the first few days after hatching. Subsequently a steady decrease of plasma T3, to adult levels, was observed. Thyroxine (T4) concentrations did not change significantly during the period studied. In starlings, on day 1, plasma T3 was low and T4 was at or below the assay detection limit. An increase in concentrations of both hormones occurred between day 1 and day 5 after hatching. A gradual increase of T4 occurred afterwards, while plasma T3 levels remained nearly constant during this growing period. Plasma GH concentrations showed a similar pattern in both species. There was an increase during first days after hatching and GH concentrations peaked at day 7 in quail and day 5 in starlings. A subsequent decrease was observed in both species. Although the GH profiles in species studied were comparable, there were noticeable differences in their growth rates. Thus different developmental strategies have been reflected in different developmental patterns of TH in precocial Japanese quail and altricial European starling whereas developmental changes of GH levels were similar in both species.

The purpose of the present experiments was to compare the effects of the intermittent lighting schedule (IL) consisting of repeated cycles of 1 h light (L):3 h darkness (D) with a nearly continuous lighting schedule 23L:1D (CL) on the performance of floor-reared male and female broiler chickens. In addition, N balance trials were performed on caged broilers in order to assess their efficiency of dietary N retention. Changing from CL to IL at Day 7 was followed by a temporary reduction in feed intake and BW gain. However, IL males exhibited compensatory growth during the finisher period in such a way that at Day 41 (market age), a similar BW as that of their CL counterparts was attained. In contrast, IL females did not completely attain a BW equal to that of CL females by 41 d of age. Imposing IL reduced feed intake and improved feed conversion. Abdominal fat pad contents of broilers reared under IL were significantly smaller than those of CL broilers. From the N balance trials, it can be inferred that broilers reared under IL have an improved efficiency of dietary N retention, especially during the period of increased growth velocity. Imposing IL at Day 7 is associated with a temporary reduction in BW gain, followed by complete growth compensation of males by Day 41. Female broilers did not restore BW by this age. As a consequence of the altered growth trajectory, the manifestation of the fat growth wave is shifted to an older age together with an enhanced efficiency of N retention.

Male and female broiler chicks were raised separately in nearly continuous lighting [23 h light (L):1 h dark (D), CL] and consumed feed ad libitum. At 7 d of age, the intermittent lighting schedule (1L:3D, IL) was imposed on half of the chicks, whereas the other chicks remained under CL. In addition to performance characteristics, several parameters of the somatotrophic and thyrotrophic axes were studied together with plasma concentrations of testosterone. Males had a higher growth rate than females regardless of the imposed lighting schedule and this pronounced growth difference is reflected by higher plasma concentrations of growth hormone (GH), and a better GH receptor occupancy. Differences in growth rate between sexes could not be attributed to differences in circulating 3,3',5-triiodothyronine (T3) levels or to hepatic deiodination activities. However, from 3 wk of age onwards, males had significantly higher plasma testosterone levels than females. Plasma GH and T3 levels decreased whereas plasma insulin-like growth factor-I and thyroxine levels increased with age in all experimental groups. The age-related decline in plasma GH levels were less pronounced for males than for females. No major changes in other hormonal parameters or deiodination activities could be observed as a result of imposing IL, except for the higher plasma GH levels of IL chickens, and for plasma testosterone concentrations in IL males at Day 41, which were twice the levels found in their CL counterparts. These results therefore suggest that the somatotrophic axis as well as circulating testosterone levels mediate the sex-related differences in growth rate and the compensatory growth as present in males.

Osmotic minipumps containing either saline or recombinant human insulin-like growth factor-II (IGF-II) were implanted into 4-wk-old female broiler chickens such that the treated chickens received 0.5 mg IGF-II/kg body weight per d. At the end of the trial, no differences in body weight gain or bone length were detected between the treated and control groups. Similarly, there were no differences between the two treatments with respect to heart, spleen, liver, or bursa of Fabricius weight. The relative weight of the abdominal fat pads was greater (P < 0.05) in the birds treated with IGF-II than in the controls, whereas the weight of breast muscle was reduced (P = 0.06) in the birds treated with IGF-II. There was no effect of IGF-II treatment on feed intake or feed conversion efficiency. Plasma growth hormone (GH) levels were acutely depressed by 15 min after IGF-II administration; and also after 2 wk of IGF-II treatment. Plasma triiodothyronine (T3) concentrations were significantly depressed by IGF-II treatment. These results suggest that IGF-II may not stimulate growth in chickens, but can act as a nutrient partitioning agent, either directly or indirectly through altering plasma GH or T3 concentrations.

The present study examined the effects of 3 days and 7 days of feed deprivation-as used in artificial moulting programs -on several metabolic criteria in adult Warren SSL hens. Body weight decreased gradually during the fasting period. Abdominal fat pad content decreased only during the first 3 days of fasting and no further during the following 4 days of feed deprivation. The fasting-induced changes in plasma glycerol and uric acid levels confirmed these proportional changes in body composition. In both fed and prolonged fasting (7 days) conditions, glucagon stimulated the in vitro lipolytic activity of abdominal adipose tissue explants. Chicken growth hormone stimulated the in vitro lipolytic activity of adipose tissue from fed but not from hens fasted for 7 days. The relationship between these physiological changes during progressive fasting in the adult hen and energetic and amino acid requirements is discussed.

The effects of passive immunization against IGF-1 either alone, or together with immunization against IGF-2, on growth and metabolism were examined in chickens. Immunization against IGF-1 alone had no effect upon any aspect of growth, carcass composition, efficiency of energy utilization or hormone concentrations studied. Immunization against both IGF-1 and IGF-2 together resulted in a lighter final body weight (P < 0.05) compared with controls. Immunization against both IGFs together decreased abdominal fat content (P < 0.05) and resulted in a heavier mean spleen weight (P < 0.01). The joint immunization was also associated with elevated plasma T3 concentrations. These data may indicate a role for IGF-2, but not for IGF-1, in fat metabolism in chickens.

The effect of a high (33 degrees C) compared to a normal (23 degrees C) ambient temperature on the pulsatile release of growth hormone (GH) has been studied in 4-week-old, cannulated male meat-type chickens selected for high body weight (GL line) or for improved feed efficiency (FC line). When reared in the normal temperature environment, overall circulating mean and amplitude GH values of FC chickens were significantly higher than those of GL chickens. There were no differences in GH pulse length or frequency between genotypes. The high rearing temperature completely abolished the pulsatility of GH in the plasma in both lines, but without affecting overall mean GH levels.

The disappearance rates for glycosylated (GcGH) and non-glycosylated chicken growth hormone (NGcGH) were compared following their intravenous injection into anaesthetized adult laying hens. The metabolic clearance rate of GcGH was about 20% lower (P = 0.13) compared to NGcGH. Similarly, there was a tendency towards a longer half-life for GcGH. No major physiological significance could be attributed to the glycosylation state of the preparations as far as their effects on circulating insulin-like growth factor I and iodohormone levels were concerned.

1. In the studies reported here, broiler lines divergently selected for susceptibility to ascites under low temperature conditions were tested for their sensitivity to 3,3',5-triiodothyronine (T3) with respect to growth rate, rate of mortality, plasma concentrations of T3, right ventricular hypertrophy (RVH) and incidence of ascites. 2. Mean body weight of the ascites-susceptible line (BC-line) was higher than that of the ascites-resistant line (A-line). Adding 0.5 mg T3/kg of the diet depressed growth rate to the same extent in both lines. The effect of T3 on growth was more pronounced for males than for females. 3. T3-supplementation increased the relative weight of the heart and the incidence of RVH to the same extent in both lines. More of the T3-treated BC-line chickens had fluid accumulation in the abdominal cavity than the T3-treated A-line chickens. 4. Dietary T3-treatment depressed the plasma concentration of growth hormone (GH) profoundly and insulin-like growth factor (IGF-I) slightly but to the same extent in both lines. The coefficient of variation of GH concentrations indicate that T3 treatment mainly decreased GH-pulsatility in young growing broilers. 5. Higher doses of dietary T3 (1 and 2 mg/kg) increased mortality in a dose-dependent manner. With 2 mg T3/kg, mortality in the BC-line was almost double that in the A-line. 6. These studies indicate that the development of ascites could be linked with thyroid function. Moreover, dietary T3 supplementation could be used to help identify ascites-inducing factors or genetic lines with differential sensitivity for ascites.

1. Plasma growth hormone (GH) pulsatility and hepatic GH receptor characteristics were compared in experimental lines of meat-type chickens selected for high (HF) or low (LF) abdominal fat content. 2. Mean GH concentration, baseline and amplitude of pulses were slightly, but not significantly, greater in LF chickens. Length and frequency of pulses were similar. 3. LF chickens exhibited higher plasma triiodothyronine (T3) concentrations. This difference between genotypes disappeared when the diet was supplemented with 1 mg/kg T3. 4. Specific binding of GH to liver membranes was higher for the fat line but was depressed by T3 supplementation to the same level in both lines. No difference was observed between lines for affinity constants. 5. It is concluded that direct selection for leanness has a less pronounced, if any, effect on GH pulsatility as compared with selection for food conversion efficiency; therefore, different physiological mechanisms are triggered to achieve leanness.

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(T3)-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 T3 supplementation abolished the pulsatile GH release in both genotypes. T3 treatment depressed the mean GH levels of Dw but not of dw chickens.

The production and characterization of monoclonal antibodies (mAbs) toward the chicken pituitary glycoproteins are described. MAbs were first subdivided on an immunocytochemical (ICC) basis: one category of mAbs reacts with both thyrotropic and gonadotropic cells whereas another stains exclusively gonadotropic cells. The hypothesis that the former are anti-glycoprotein alpha and that the latter are anti-beta LH and/or anti-beta FSH mAbs was further elaborated in a homologous tracer binding assay. The putative anti-beta gonadotropin mAbs exclusively recognize iodinated USDA-cLH-I-1, whereas the supposed anti-alpha mAbs react with both USDA-cLH-I-1 and USDA-cFSH-I-1. The ICC localization of pituitary thyrotropes has been achieved in a double-staining protocol using an anti-LH mAb to subtract specifically the gonadotropic cells from the complete alpha-subunit-containing cell population. The result is a strictly cephalic cell population that is densely granulated in hyperthyroid and hardly detectable in hypothyroid birds. A similar subtractive approach was used to develop a thyroid-stimulating hormone (TSH)-indicator in RIA. The total titer of alpha-containing molecules is first quantified with an anti-alpha mAb. The contribution of both LH and FSH to this titer is then measured by homologous USDA-RIA and subtracted from the total alpha-immunoreactivity to give an indication of the TSH level of the sample. The direct effect of thyroid-releasing hormone on pituitary TSH release in vitro in juvenile chicks and the effect of hypo- and hyperthyroidism on circulating TSH levels in vivo in juvenile broilers have been demonstrated.

1. Diurnal-nocturnal changes in food intake, gut storage of ingesta, food transit time and heat production were studied in male broiler chickens reared under a 14L:10D lighting schedule (lights on from 06.00 to 20.00 h). 2. Food consumption during the scotophase was negligible. Peak food consumption during the photoperiod occurred at the beginning of the photoperiod and in the late afternoon. 3. During the photoperiod, the crop and proventriculus/gizzard contained only small quantities of ingesta. However, at the beginning of the scotoperiod, dried ingesta content of crop and proventriculus/gizzard increased by 10.5- and 2.76-fold respectively. This increase was followed by a gradual decrease towards the end of the scotoperiod. 4. Food transit time during the scotoperiod was significantly longer than that during the photoperiod. 5. The daily pattern of heat production closely followed the 14L:10D lighting schedule. Total heat production during darkness averaged 53% of total heat production during the photoperiod. 6. It was estimated that the storage of energy (as ingesta) in the crop and proventriculus/gizzard, followed by its gradual release and the increased food transit time during the night, contributed 75.5% of nocturnal energy needs. It must be recognised that these mechanisms play a major role in the energy balance of the growing chicken during periods without food intake.

The present studies evaluated the ability of jojoba meal (JO) to inhibit feed intake of broiler breeder pullets to limit body weight gain as recommended by the breeder company. A first experiment, using graded levels of JO supplementation (0 to 12%), was conducted to establish appropriate JO supplementation. Adequate reduction of growth rate was obtained with 4% JO supplementation. However, notwithstanding their similar growth rate, 4% JO chickens consumed considerably more feed compared with feed-restricted chickens. The dose-dependent impairment of feed intake with increasing levels of JO supplementation was also associated with increased plasma growth hormone and thyroxine and with decreased plasma insulin-like growth factor-I and triiodothyronine concentrations compared with 0% JO chickens. A second experiment included a pair-fed group. Notwithstanding their similar feed intake, 4% JO chickens gained significantly less body weight compared with their pair-fed counterparts. The 4% JO chickens also had a longer feed transit time per kilogram body weight. Again, circulating levels of the somatotrophic and thyrotrophic hormones were altered according to the dietary treatment. From all these observations, it was concluded that the growth retardation caused by JO supplementation was provoked by an inhibition of appetite linked with the simmondsin content of JO as well as by other antinutritional compounds affecting digestibility.

The influence of genetic selection for improved feed conversion (FC) and fast growth (GL) in broiler chickens, on the hepatic growth hormone receptor was investigated as a function of age. Selection for improved feed conversion resulted in lean chickens, whereas selection for fast growth resulted in fat chickens. Growth hormone receptor characteristics were determined in liver microsomal fractions and plasma GH concentrations were measured by radioimmunoassay. Embryos from the FC line showed higher specific GH binding than GL embryos at day 16 of embryonic development only (P < 0.05). On day 18 of embryonic development, the FC line had slightly more GH receptors than the GL line (P < 0.05), whereas simultaneous plasma GH concentrations were greater in the GL line. The effect of selection on hepatic growth hormone binding was greatest in 4 week old growing chicks at which age the GL line showed a 2- to 3-fold higher % of specific binding than the FC line (P < 0.001). The difference at this stage is probably due to down regulation of the receptor by the significantly higher GH concentrations in the FC line. The adult GL chickens also showed higher specific binding than FC, but the difference was less pronounced than in the growing stage (P < 0.05). This was due to a higher number of GH receptors, while receptor affinity was unchanged. Because no line difference in plasma GH concentrations was found in adult hens, other unknown mechanisms probably play a role in determining differences in GH receptor binding between these selected lines at older ages.

Disappearance of growth hormone from blood (plasma) was studied in young broiler chickens in two experiments: a) following single injection of GH (50 micrograms or 250 micrograms/kg) to anesthetized chicks; b) injection of GH (50 micrograms/kg) (under anesthesia) to two lines of broilers selected for either rapid growth or feed efficiency. Clearance of GH never followed a single negative exponential curve. Characterization of the disappearance by a single metabolic clearance rate and half-life time were found to be inaccurate and inappropriate on methodological grounds. High doses of hormone might disturb receptor or catabolism equilibria and result in aberrant values of MCR which could probably be because of the very dynamic nature of the GH clearance system, which appeared to be influenced by dose, line (or genetics of the animal) and by some indices of clearance. Therefore, physiological significance of such data must be interpreted carefully.

The aim of this study was to compare the effect of high (33 degrees C) and normal (33-20 degrees C) rearing temperature on growth and plasma levels of the somatotrophic hormones of 2 genetic lines of broiler chickens selected for 6-wk body weight (GL-line) or for feed conversion between 3 and 6 wk of age (FC-line) or for feed conversion between 3 and 6 wk of age (FC-line). Blood samples were collected weekly and analysed for growth hormone (GH) and insulin-like growth factors (IGF)-I and -II levels by RIA. The growth-depressing effect of the HT-treatment was more pronounced in the heavier GL-line and in males. A similar age-related pattern for all hormones studied was observed with the highest levels between 2 and 4 wk of age. FC-line chickens and males had consistently higher plasma GH levels than GL-line chickens and females respectively. No consistent effect of rearing temperature on plasma GH levels were observed. At 2 wk of age, HT-treatment resulted in higher plasma IGF-I levels while this was reversed from 3 wk of age onwards. GL-line chickens had significantly higher plasma IGF-I levels at 2, 3 and 4 wk of age. No consistent effect of sex on plasma IGF-I levels could be observed. For the whole period studied, GL-line chickens had significantly higher plasma IGF-II levels than FC-line chickens. No consistent effect of sex or temperature treatment on plasma IGF-II levels was observed.

1. Differences in responses to lipolytic agents have been investigated in vitro in abdominal adipose tissue from lines of broiler chickens selected for body weight (GL, a 'fat' line) or for food efficiency (FC, a 'lean' line). 2. Dibutyryl cyclic adenosine monophosphate stimulated in vitro lipolysis, as measured by the glycerol release, by adipose tissue from GL or from FC chickens to the same extent. 3. Glucagon stimulated glycerol release from adipose tissue from FC chickens, but not from GL chickens. 4. Adipose tissue from GL chickens was much more sensitive to chicken growth hormone (GH) compared to FC chickens. 5. It is concluded that the selection criteria applied influenced the number of adipose GH and glucagon receptors, the number of adipose GH receptors being lower and of glucagon receptors being higher in FC chickens compared to GL chickens.

1. The effect of the crude protein content (200 and 150 g/kg) of isoenergetic diets on episodic growth hormone (GH) release and on heat production was investigated in male broiler chickens. 2. Decreasing the crude protein content of isoenergetic diets from 200 g/kg (HP diet) to 150 g/kg (LP diet) resulted in depressed body weight gain, impaired food conversion efficiency and increased abdominal fat deposition. 3. The pattern of growth hormone secretion was markedly affected by dietary treatment. Broiler chickens fed on the LP diet had higher overall mean, amplitude, baseline and peak frequency than the HP chickens. 4. The LP chickens produced more heat per unit of metabolic body weight than the HP chickens. 5. The hypothesis relating the pattern of GH secretion to protein conversion efficiency was corroborated.

The aim of the present experiment was to study the growth hormone (GH) response upon thyrotropin releasing hormone (TRH) challenge (2 micrograms/kg body weight) in broiler chickens selected for body weight gain (GL line: fat line) or for feed efficiency (FC line: lean line) reared at either a moderate (33-23 degrees C) or high (33 degrees C) ambient temperature. A higher plasma GH level at 5 min after TRH administration was observed in the high temperature conditioned chickens of both lines. Also at high ambient temperature, an enhanced GH decrease between 15 min and 30 min post-injection and a higher acute elimination rate was calculated compared to moderate ambient temperature. A significantly higher GH secretory response was observed in the leaner FC line chickens, which was probably related to the more pronounced pulsatory GH secretion rate in these chickens. There was no difference in GH acute elimination rate between both lines in both environments. No interactions between line and rearing temperature for these parameters of GH dynamics were observed.

Osmotic minipumps were implanted in 4-wk-old female broiler chickens to supply a 2-wk continuous infusion of recombinant human insulin-like growth factor-I (rhIGF-I) in three doses (.03, .1, and .3 mg/kg BW per day). At the end of the experimental period no differences in BW were detected, although abdominal fat was significantly reduced in the highest dose group. Measurement of fat content in both breast and thigh muscle indicated a different effect of IGF-I treatment on these parameters, as no reduction was observed. Determination of circulating IGF-I levels revealed a twofold increase in the .3-mg group whereas the lowest dose did not increase circulating plasma levels. The changes in IGF-I levels did not influence growth hormone levels whereas thyroxine levels were significantly decreased both in the .03- and .3-mg groups after 1 wk of treatment. At the same time plasma triiodothyronine levels were increased in the .1- and .3-mg/kg groups. These results indicate that a continuous infusion of IGF-I did not increase weight gain but may play a role as a fat repartitioning agent.

1. The pronounced episodic release of chicken growth hormone (cGH) in male broiler chicks, as well as its peak interval of about 90 min was confirmed in this study. 2. Lines selected for 6-week body weight (GL line) or for food conversion between 3 and 6 weeks of age (FC line) not only differ in the selection criteria but also in percentage abdominal fat. 3. The overall mean plasma GH concentration was higher in the FC line in both experiments, and this was only caused by peak amplitudes which were almost twice as high as in the FC line. Peak intervals were identical in both selected lines. 4. These observations suggest that decreased abdominal fat and increased protein efficiency may be genetically linked to increased plasma GH peak amplitude. This, and earlier observations on the effectiveness of pulsatile administration of cGH on the growth of broilers suggest that the biological function of GH may depend on its secretory pattern rather than its mean concentration in plasma.

1. The concentrations of growth hormone (GH), insulin-like growth factor I (IGF-I), thyroxine (T4), triiodothyronine (T3), reverse-triiodothyronine (rT3), triglycerides (Tri), free fatty acids (FFA) and glucose (Glu) were determined at 2, 4 and 6 weeks of age in blood plasma of male and female chickens of broiler lines selected for body weight (GL) or food conversion (FC). 2. Plasma concentrations measured in the same animal over a 24 h or a 2 week interval were not significantly correlated with each other. For different traits measured in the same plasma sample only the correlation between T4 and rT3 differed significantly from zero. 3. All traits were dependent on age. Line and sex effects were significant (P less than 0.05) for GH, T4, Tri, FFA and Glu. Additionally, line significantly influenced the plasma T3/T4 ratio and sex influenced plasma rT3. Interactions between line, sex and/or age were seldom significant. 4. Within line and sex, GH (at 6 weeks of age) and T3 (at 4 weeks of age) were negatively, and IGF-I and Tri (both at 6 weeks of age) positively correlated with the amount of abdominal fat at 6 weeks of age. No significant correlation between body weight at 2, 4 or 6 weeks of age and any of the plasma traits was found.

In the present experiment, clenbuterol was supplemented (.42 ppm) from Day 1 or from 2 or 4 wk of age until slaughter age (6 wk). The effects on growth performance and on plasma hormone and metabolite profiles were investigated at 2, 4, and 6 wk of age in male and female broilers. There were no consistent or cumulative effects of clenbuterol feeding on growth and feed efficiency. Clenbuterol feeding from Day 1, but not later, depressed subsequent feed intake. Relative abdominal fat pad weight was reduced profoundly and was even more pronounced after prolonged supplementation and for females. Fat content of thigh meat (including skin), but not of breast meat (without skin), was reduced by clenbuterol feeding. No consistent effects of clenbuterol supplementation on plasma thyroid hormones, growth hormone, insulin-like growth factor-I, and corticosterone levels were detected. Very low density lipoprotein (VLDL) concentrations were depressed in male broilers fed clenbuterol for 6 wk, but the plasma triglyceride level did not follow this pattern. There was no evidence for a consistent effect of clenbuterol on lipolysis in vivo, measured by plasma glycerol level. Between 4 and 6 wk of age, plasma VLDL and glycerol levels decreased in females but increased in males. This corresponds to the higher fat deposition in females. However, the most consistent effects were age-related changes in the plasma levels of most hormones and metabolites. From the present study, it seems that clenbuterol acts primarily on fat deposition, the extent being dependent on sex, location of fat, and duration of beta-adrenergic agonist supplementation.

The aim of the present experiments was to study the plasma glucose-insulin relationship in eu-, hypo- or hyperthyroid broiler chickens. None of the thyroid states modified the fasting plasma glucose and insulin levels. Hypothyroid chickens exhibited a normal glucose tolerance and a normal glucose-induced insulin release after oral glucose (2 g/kg body weight) administration compared to euthyroid chickens. In contrast, hyperthyroid chickens exhibited an improved glucose tolerance accompanied by a lower insulin release. Insulin injections at a concentration of 0.1 U/kg body weight was only hypoglycemic in hyperthyroid chickens, which confirms the observation that these chickens are more sensitive to insulin. From this study it can be suggested that alterations in body composition according to thyroid status are at least partly mediated by changes in the control of carbohydrate metabolism by pancreatic hormones.

Daily changes in the concentrations of plasma corticosterone, prolactin, thyroid hormones and somatomedin C were measured in 28-day-old fat and lean lines of broilers produced by selection for high and low concentrations of very low density lipoproteins (VLDL). The effects of daily injections of corticosterone on the concentrations of these hormones and on fattening were compared in the two lines. The selection procedure had no effect on the concentrations of any of the hormones. However, daily rhythms in concentrations of plasma corticosterone, tri-iodothyronine (T3) and prolactin were less often observed in the fat line than in the lean line. No differences were seen between lines in the daily rhythms in plasma thyroxine (T4) and somatomedin C. Daily injections of 2500 micrograms corticosterone/kg body weight, in both lines, depressed mean concentrations of plasma prolactin, T3 and somatomedin C and body weight. This dose of corticosterone also increased abdominal fat pad and liver weights expressed as a percentage of body weight. The liver and fat pad responses to 2500 micrograms corticosterone in both lines were greater when the steroid was injected at the end rather than towards the beginning of the 14-h daily photoperiod. There was no difference between the lines in the fattening response to corticosterone. Lower doses of 100 and 500 micrograms corticosterone per day did not induce fattening or affect concentrations of plasma prolactin. They did, however, depress concentrations of plasma T3. Concentrations of plasma T4 were increased in both lines treated with 2500, but not with 100 or 500 micrograms corticosterone, towards the beginning of the daily photoperiod.(ABSTRACT TRUNCATED AT 250 WORDS)

Relatively few methods involving dietary manipulation of thyroid status have been used to study the effect of that status on growth and body composition as well as on changes in hormone levels and metabolism. The present work examines the influence of either a hyperthyroid status (induced by feeding triiodothyronine: T3 or thyroxine: T4) or a hypothyroid status (induced by feeding methimazole: MMI) on growth, feed consumption and body composition as well as on changes in thyroid hormone levels and metabolism and in hormone levels of the somatotroph axis of broiler chickens. MMI depressed growth but increased fatness. Long-term administration of thyroid hormones decreased both growth and fat deposition, T3 being more effective than T4. The reduced growth of MMI-treated birds was not only related to the hypothyroid state but also to decreased somatomedin C (Sm-C) production, although growth hormone (GH) levels remained high. Thyroid hormone-treated birds showed slightly depressed GH levels but unchanged Sm-C levels. MMI induced an increase in hepatic 5'-monodeiodination activity, while T3 and T4 reduced that activity. The T4 administered was largely eliminated as inactive reverse T3. Both of the statuses (hypothyroid and hyperthyroid) profoundly changed peripheral thyroid hormone metabolism and influenced GH/Sm-C relationships, but in the opposite way. Hormonal changes could be related to the observed changes in growth and adiposity of broiler chickens and illustrate the negative correlation between T3 and body fat.