Growth hormone effects on in vitro metabolism of avian adipose and liver tissue

Perspectives on the endocrinology of poultry growth and metabolism

Birds have rapid pre- and post-hatching growth rates. The major hormones required to support normal growth are growth hormone (GH), triiodothyronine (T(3)) and insulin-like growth factor-I (IGF-I). Optimal growth requires a "set-point" concentration of both IGF-I and T(3) in the circulation. Pituitary GH plays a role in controlling the circulating concentrations of both IGF-I and T(3). Nutritional restriction (energy, protein) leads to reductions in circulating concentrations of both IGF-I and T(3) with increased GH secretion due removal of negative feedback. Similarly, there is un-coupling of the GH-IGF-I axis in stunting disease. A critical control point is at the level of the liver and GH receptor/signal transduction. The major hormones controlling metabolism include glucagon, insulin, adrenal glucocorticoid hormone, corticosterone and potentially somatostatin. Chickens and turkeys have higher circulating concentrations of glucose than those of livestock mammals. What are not known include the following: the biological basis for the high basal glucose concentrations; the quantitative fluxes of key metabolites in the fed and fasted state through growth and development; the relative contribution of different organs to gluconeogenesis; the relative importance of insulin and somatostatin in controlling lipolysis and the role of gastro-intestinal hormones in the control of metabolism.

In ovo treatment with an aromatase inhibitor masculinizes postnatal hormone levels, abdominal fat pad content, and GH pulsatility in broiler chickens

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 At the age of 4 wk, control and vorozole-treated birds were cannulated, and serial blood samples were withdrawn every 10 min for 5 hr, wherein GH pulsatility characteristics were determined using deconvolution analysis.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.

Influence of adenosine or adrenergic agonists on growth hormone stimulated lipolysis by chicken adipose tissue in vitro

In vitro lipolysis by chicken adipose explants was stimulated by growth hormone (GH) or glucagon. Adenosine or the adenosine agonist, N6-phenylisopropyladenosine (PIA), inhibited GH stimulated lipolysis, the effect of adenosine not being observed in the presence or adenosine deaminase. Glucagon induced lipolysis was also reduced by PIA. It is suggested that adenosine may act by Gi linked to either adenylate cyclase (for glucagon) or the signal transduction mechanism for GH. Lipolysis was not stimulated by GH in the presence of phenylephrine (alpha 1 adrenergic agonist), isoproterenol (beta adrenergic agonist), adrenaline or glucagon. Although the presence of p-amino clonidine (alpha 2 adrenergic agonist) depressed basal lipolysis, a response to GH was still present. Either glucagon or beta-adrenergic agonists (isoproterenol, adrenaline) stimulated lipolysis. In both cases, GH attenuated the lipolytic response to these hormones, which act via a cyclic adenosine monophosphate signal transduction mechanism.

Identification of growth hormone DNA polymorphisms which respond to divergent selection for abdominal fat content in chickens

Two strains of meat-type chickens which had been derived from the same genetic base, but were selected for high or low abdominal fat content, respectively, were analyzed for polymorphisms in the growth hormone gene (GH). A total of four DNA polymorphisms were identified, one at a SacI restriction site and three at MspI restriction sites. Restriction mapping indicated that all polymorphisms were in exons and/or introns and not in flanking regions of the gene. The incidence of GH polymorphisms was determined in 20 chickens from each strain and significant differences were observed for two of the four polymorphisms. Analysis by DNA fingerprinting using (CAC)5 as a probe indicated that the inbreeding coefficient was 0.1 in both strains and that random genetic drift was minimal. Thus, the selection for abdominal fat appears to have affected the frequency of alleles of the growth hormone gene. Whether this is the direct consequence of an altered growth hormone gene on fat metabolism or reflects linkage to an allele of a neighbouring gene remains to be determined.

Ascites in broilers. 2. Disturbances in the hormonal regulation of metabolic rate and fat metabolism

Effects of different broiler stocks, ambient temperatures (Ta), dietary energy content (AME), and dietary levels of unsaturated fat on plasma thyroid and growth hormone concentrations and energy metabolism were studied. An experiment with a 2 x 2 x 2 x 2 factorial split-plot arrangement of treatments with 96 groups of 12 male broilers each was performed. Blood samples were taken at 3, 4, and 5 wk of age. Energy metabolism parameters were determined over an entire period from 1 to 5 wk of age. Chickens from a line selected for fast growth rate and low feed conversion ratio but also more sensitive to heart failure syndrome (HFS) and ascites (Line SS) than commercial birds (Line BC) exhibited the greatest responses to experimental factors. Differences in levels of plasma thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3), and growth hormone (GH) between stocks at different ages were highly dependent on Ta and dietary fat content. Differences in heat production per metabolic weight, percentage of retained fat energy in retained energy, and efficiency of AME intake for retained energy between stocks corresponded to differences in hormone levels. High-fat diets (polyunsaturated fatty acids) inhibited the extra thyroidal conversion of T4 to T3 in both stocks. Differences between stocks in T3 and rT3 levels in plasma indicated that BC birds (in contrast to SS birds) were better able to compensate for an inhibited T4 conversion to T3 by producing more T4. Overall results suggest that the occurrence of HFS and ascites in SS birds could be initiated independently by different factors. These factors might be a limited thyroid hormone production and a lower capacity for oxygen consumption. An inverse relationship between T3 and GH levels found in particular combinations of experimental factors, together with changes in fat deposition, support published concepts about the positive effects of T3 on lipogenesis and GH on lipolysis.

Abdominal adipose tissue from broiler chickens selected for body weight or for food efficiency differ in in vitro lipolytic sensitivity to glucagon and to chicken growth hormone, but not to dibutyryl cAMP

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.

Lipolytic and diabetogenic effects of native and biosynthetic growth hormone in the chicken: A re-evaluation

1. There is controversy as to whether growth hormone (GH) is lipolytic and/or diabetogenic in vivo in chickens. The ability of GH to influence circulating concentrations of free fatty acids was examined in anaesthetized (suppressing endogenous GH secretion) adult and young chickens using three preparations of GH. 2. Plasma concentrations of free fatty acids were increased following the intravenous injection of native bovine GH (50 micrograms/kg to either young or adult chickens), recombinant chicken GH (American Cyanamid) (50 micrograms/kg to adult chickens) and recombinant chicken GH (Amgen) (50 micrograms/kg to young chickens). 3. Similarly, infusion of recombinant chicken GH was accompanied by a gradual increase (P less than 0.05, 90 min following start of infusion) in plasma concentrations of free fatty acids in both anaesthetized hypophysectomized and sham operated young chickens. 4. These data support an acute lipolytic role for GH in the chicken. 5. The injection of neither bovine nor chicken GH had any consistent effect on circulating concentrations of glucose. Moreover, if GH was administered in the presence of glucose, GH had no effect on plasma concentrations of glucose. 6. Further evidence for a lack of a diabetogenic role for GH comes from the inability of chronic administration of GH to influence the decline in plasma concentrations of glucose following challenge with the insulin secretagogue, tolabutamide.

Effect of pulsatile or continuous administration of pituitary-derived chicken growth hormone (p-cGH) on lipid metabolism in broiler pullets

1. The effects of pulsatile and continuous intravenous administration of exogenous, pituitary-derived chicken growth hormone (p-cGH) on lipid metabolism and endocrine/metabolite levels of broiler-strain pullets were studied. 2. Eight-week-old pullets were administered p-cGH or vehicle over a 10 min period every 90 min for 7 days. 3. Pullets were also administered the same daily amount (123 micrograms/kg of body weight/day) continuously for 7 days. 4. Feed intake, body weight gain, in vitro lipogenesis and hepatic enzyme activities were determined with certain hormones identified with the control of growth. 5. Pulsatile p-cGH administration for 7 days lacked effect on weight gain, feed efficiency, muscle or bone development. 6. Abdominal fat pad size was decreased (P less than 0.05) by pulsatile but not continuous administration of p-cGH. Pulsatile p-cGH administration also decreased (P less than 0.05) in vitro lipogenesis. Liver malic enzyme and isocitrate dehydrogenase activities were increased (P less than 0.05) by pulsatile but not continuous administration of p-cGH. In contrast, glutamic oxaloacetic transaminase activity was increased by a continuous infusion of p-cGH. 7. Plasma concentrations of T4 corticosterone and triglycerides were decreased (P less than 0.05) by a pulsatile but not a constant infusion of p-cGH. 8. Plasma T3 and GH were increased (P less than 0.05) by pulsatile p-cGH compared to both a continuous infusion of p-cGH and the saline controls. 9. This study is the first to prove that in the broiler chicken, the pattern of exogenous p-cGH administration is a factor influencing in vitro responses to the hormone.

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.

Lipid synthesis and lipoprotein secretion by chick liver cells in culture: influence of growth hormone and insulin-like growth factor-I

1. Chick liver cells were incubated in unsupplemented medium (control), or medium supplanted with either 1 microgram/ml pituitary derived chicken growth hormone (GH), 50 ng/ml recombinant human insulin like growth factor-I (IGF-I), or 1 microgram growth hormone/ml and 50 ng insulin like growth factor-I/ml (GH + IGF-I). 2. GH supplementation stimulated acetate incorporation into liver cell lipid. Low density lipoprotein (LDL) lipid secretion was increased quantitatively by GH. 3. Cells incubated with IGF-I incorporated more acetate into lipid and secreted more lipid as VLDL and HDL than controls. 4. A metabolic antagonism between GH and IGF-I was evident with respect to lipogenesis. 5. Neither GH nor IGF-I altered, quantitatively, cell protein synthesis or apoprotein secretion.

Recombinant bovine somatotropin stimulates short term increases in growth rate and insulin-like growth factor I (IGF-I) in chickens

Recombinant bovine somatotropin (rbGH) was administered by subcutaneous injection at daily doses of 0.5 or 2.5 mg/kg for a two week period in female broiler chicks between 4 and 6 weeks of age. Half of the chicks received dietary corticosterone at a 1 ppm level. Growth rate was significantly increased 6.1% and 6.9% following one week of treatment with 0.5 or 2.5 mg/kg rbGH respectively. Treatment with the same respective doses of rbGH in the presence of 1 ppm corticosterone, supplied to suppress any possible immune response elicited by the heterologous somatotropin, resulted in an 8.0% and 7.8% increase (P less than .05) in growth rate during the first week of treatment. The rbGH-associated increase in growth rate was accompanied by a significant increase in food intake, higher circulating levels of IGF-I, and lower plasma T4 concentrations, while plasma T3 levels were unchanged. All effects were attenuated during the second week of treatment, concomitant with the development of high antibody titer against rbGH regardless of dietary corticosterone administration. Carcass parameters relating to bone, muscle and fat were not different between rbGH-treated and control chickens at the end of the two week treatment period. Thus rbGH is capable of stimulating a short-term improvement in growth rate, which is related to increased feed consumption and is of limited duration.

Thyroid function, growth hormone, and organ growth in broilers deficient in phosphorus

The effects of a dietary P deficiency on thyroid function, serum growth hormone, and growth parameters in 10 to 29-day-old broiler cockerels was determined. Chicks fed severely P-deficient diets (.05% or .10% available P) grew more slowly and ate less feed than controls fed .65% P. The deficiency was also accompanied by hypercalcemia, hypophosphatemia, and decreases in percent bone ash, fat-free tibial weight, and tibial length and width. Increases in the relative weights of kidneys, hearts, and pituitary glands (.05% P only) occurred as well. Most of these changes occurred to a lesser extent or not at all in pair-fed controls, showing that they resulted specifically from the P deficiency and were not simply a result of reductions in feed intake. Phosphorus deficiency also was accompanied by peripheral edema and hydropericardium. Relative thyroid weight was unaffected. Serum triiodothyronine was consistently lower in the P-deficient chicks, although effects were significant only in one experiment. Thyroxine levels tended to be low also, but not consistently so. Serum growth hormone in P-deficient chicks in both studies was consistently lower than that in pair-fed controls, but this was significant only when .10% but not .05% available P was fed. The findings suggest that serum levels of both thyroid hormone and growth hormone are altered by P deficiency, but the results were not clearly definitive.

Pulsatile secretion pattern of growth hormone in turkeys: effects of age and sex

Male and female turkeys were cannulated through the jugular vein, and blood samples were withdrawn remotely at 10-min intervals for a period of 8 hr at two points in the post-hatch growth phase (4 and 14 weeks of age). Growth hormone (GH) concentration was determined for each sample by radioimmunoassay using a recombinant chicken growth hormone preparation as standard. Data were evaluated for age- and sex-related differences. Four-week-old male and female turkeys displayed a pulsatile pattern of GH secretion. Growth hormone secretory profile characteristics differed significantly between ages with regard to overall mean, number of peaks, amplitude of peaks, interval between peaks, baseline, and total GH detected. Male four-week-old turkeys had a peak amplitude significantly greater than that of females of the same age. Older (14-week-old) male turkeys demonstrated a significantly greater number of GH secretory peaks than females during the 8-hr sampling period; however, overall it did not appear that the older birds had an organized pattern of GH secretion above baseline levels.

Effect of exogenous corticosterone in genetically fat and lean chickens

1. The effects of daily injections of corticosterone (1 or 5 mg/bird) on growth, fat deposition, liver lipid and plasma concentrations of uric acid, glucose, insulin and growth hormone were studied using genetically selected lines of fat (FL) and lean (LL) chickens. 2. Both doses of corticosterone depressed body weight gain and increased the liver lipid and the abdominal fat to the same extent in both lines. 3. In both lines, corticosterone caused a dose-dependent increase in the plasma concentrations of uric acid, glucose and insulin in the fasted and refed states. 4. In untreated birds, plasma concentrations of growth hormone (GH) were slightly higher in FL than in LL chickens and slightly decreased during refeeding. The response was not modified by injection of 1 mg corticosterone. Injections of 5 mg decreased plasma GH in both lines in the fasting state and in LL chickens during refeeding. In contrast, the same dose increased GH in FL chickens during refeeding. This contradiction remains unexplained. 5. The results suggest that corticosterone sensitivity is not involved in difference of fattening between FL and LL chickens.

Some biological activities of recombinant DNA-derived growth hormone on plasma metabolite concentrations in domestic fowl

The biological activity of recombinant-DNA-derived chicken growth hormone (rcGH) has been examined in young broiler cockerels, by determining its effects on plasma concentrations of glucose, free fatty acids and alpha-amino nitrogen. A single injection of rcGH increased plasma glucose, which remained high for several hours, whereas daily treatment with rcGH for 1 week had no effect on basal plasma glucose concentrations but blunted the glucose response to a further rcGH challenge. Plasma free fatty acids were also promptly increased following acute rcGH treatment, and chronic exposure to rcGH again attenuated this response. The effects of rcGH on plasma alpha-amino nitrogen were more variable. The stress of repeated blood sampling tended to reduce alpha-amino nitrogen, and after rcGH, an increase relative to vehicle-injected controls was seen in both acute and chronically-treated birds. These data suggest that rcGH has both hyperglycaemic and lipolytic activity in chickens, and may also increase amino acid availability.

Plasma levels of luteinizing hormone, growth hormone, and estradiol from six weeks of age to sexual maturity in two lines of chickens selected for low or high abdominal fat content

An experiment was conducted to investigate possible differences in plasma hormone levels between females of two lines of chickens selected for high (FL) or low (LL) abdominal fat content. Blood was taken at weekly intervals from 6 weeks of age to sexual maturity, and assays were performed for luteinizing hormone (LH), growth hormone (GH), and estradiol (E2). Age-related increases in the plasma titers of LH and estradiol were found while GH remained low apart from initial and final fluctuations. There was no difference between the strains in plasma E2, only minor differences in plasma GH, but a clear-cut difference was observed in plasma LH during the prepubertal period. Plasma LH tended to be higher in the LL, but the reason for this remains unclear.

Arginine vasotocin induces free fatty acid release from avian adipose tissue in vitro

The effect of arginine vasotocin (AVT) on free fatty acid (FFA) release from pigeon adipose tissue slices incubated in vitro was studied. AVT at concentrations of 0.5 microgram/ml and 5.0 micrograms/ml was found to produce significant increases in the release of FFA from the adipose tissue. This augmentation of FFA release did not require the presence of hydrocortisone.

Metabolic and anatomical adaptations of heavy-bodied chicks to intermittent feeding. I. Food intake, growth rate, organ weight, and body composition

Chicks of a heavy-bodied strain were deprived of food on alternate days from 14 to 83 days of age. Relative food intake was depressed by intermittent feeding to 25 days of age, after which the deprived chicks resumed the relative intake of the ad libitum-fed controls, i.e., the relative intake on repletion day was twice that of the control chicks. Following an adaptation period of 2 weeks, the relative growth rate of the intermittently fed chicks exceeded that of the control group, the excessive growth being due to an increase on the day of repletion which was more than twice that of the control birds. Body fat concentration was depressed by intermittent feeding throughout the experimental period. Although, in ad libitum-fed chicks, periodical deposition of body protein and fat was parallel throughout the experimental period, in the treated chicks, protein deposition prevailed initially and fat deposition prevailed at the end of the experimental period. Intermittent feeding was accompanied by a consistent increase in the relative weight of the liver, pancreas, and gastrointestinal tract, including the small intestine. The differences in food intake, growth, body composition, and gastro-intestinal tract between heavy- and light-breed chicks exposed to intermittent feeding are discussed.

Characterization of hepatic growth hormone binding sites in two fish species, Gillichthys mirabilis (Teleostei) and Acipenser transmontanus (Chondrostei)

To obtain information on the presence of growth hormone (GH) receptors in liver of nonmammalian vertebrates the specific binding of 125I-bovine growth hormone (bGH) to liver membranes of seven species representing the major groups was studied by radioreceptor assay. A substantial degree of specific binding was detected with sturgeon (Acipenser transmontanus) liver membranes and a much lower amount was detected on hepatic membranes of Gillichthys mirabilis. No significant specific binding was detected on liver membranes of pigeon, turtle, bullfrog, tilapia, or leopard shark. Gillichthys and sturgeon liver membranes were further characterized and compared with hepatic membranes from male rabbits. The sturgeon and Gillichthys membranes showed binding that was dependent upon time, temperature, pH, and membrane concentration. Scatchard analysis of the binding of 125I-bGH to sturgeon and rabbit membranes revealed both high and low affinity binding sites. The high affinity sites had KA values of 3.1 X 10(11) and 1.0 X 10(11) M-1, and capacities of 12 and 50 fmol/mg protein, respectively. Membranes from Gillichthys liver contained only a single class of binding sites with a KA of 6.7 X 10(9) M-1 and a binding capacity of 49 fmol/mg. Hormonal specificity of the sturgeon and Gillichthys hepatic binding sites was studied using methionyl-human GH (met-hGH), ovine prolactin (oPRL), and a crude preparation of sturgeon (st)GH. The met-hGH and stGH inhibited the binding of 125I-bGH to sturgeon liver membranes while only met-hGH displaced labeled bGH from Gillichthys liver membranes. One microgram of oPRL did not significantly inhibit 125I-bGH binding in either membrane assay. Based on these studies, sturgeon hepatic GH receptors seem to be more like those of nonprimate mammals than those of teleosts. Our results, in conjunction with the data of J. N. Fryer (Gen. Comp. Endocrinol. 39, 123-130 (1979)), indicate that considerable evolutionary divergence has occurred among teleost hepatic GH receptors. Thus, vertebrate GH receptors seem to have undergone at least as much evolution as has the hormone itself.

Lipogenesis and lipolysis in fed and fasted chicks from high and low body weight lines

Lipogenic and lipolytic capacities were examined in fasted and nonfasted 28-day-old chicks from high-weight (HW) and low-weight (LW) selected lines. Lipogenesis was assessed in liver and bone (sternum) tissues through the activities of malic enzyme (EC 1.1.1.40), citrate cleavage enzyme (EC 4.1.3.8), and acetyl coenzyme A carboxylase (EC 6.4.1.2), as well as through the in vitro incorporation of acetate-1-14C into liver and bone lipid fractions. Lipolysis was estimated through the in vitro release of free fatty acids (FFA) from abdominal adipose tissue and through plasma FFA. Fasting depressed lipogenesis and increased lipolysis. Regardless of the feeding state, LW chicks exhibited higher lipogenic and lipolytic capacities than their HW counterparts, suggesting that lipid degradation may be relatively more important than synthesis in determining net fat deposition. In addition, the incorporation of radioactive acetate into bone lipid was associated with detectable activity of lipogenic enzymes, providing further evidence that the skeleton is an important site of lipogenesis in the chicken.

Endocrine changes associated with fat deposition and mobilization in svalbard ptarmigan (Lagopus mutus hyperboreus)

Plasma concentrations of Triiodothyronine (T3), thyroxine (T4) and growth hormone (GH) have been measured in blood samples taken from Svalbard ptarmigans (Lagopus mutus hyperboreus), shot throughout 1 whole year at Svalbard (79 degrees N). Plasma T3 levels varied in a monophasic pattern with low levels in winter and a peak in August, whereas plasma T4 levels remained constant throughout the year. High plasma T3 levels coincide with molt and a large food intake while low plasma levels of T3 coincide with molt arrest and a low food intake. Plasma GH levels were highest in winter and lowest in May and September. The low plasma GH levels in early autumn coincide with elevated liver weights and maximum rate of fat deposition. High GH levels in midwinter coincide with low liver weights and the mobilization of fat stores. A possible relationship between molt, food intake, fat deposition/mobilization, and plasma levels of T3 and GH is discussed.

Adrenergic control of lipogenesis and lipolysis in the chicken in vitro

The adrenergic inhibition of lipogenesis and stimulation of lipolysis in the avian has been examined using chicken hepatocytes and adipose tissue explants in vitro. Lipogenesis was inhibited by adrenergic agonists: epinephrine (alpha + beta) greater than isoproterenol (beta 1/beta 2) greater than norepinephrine (alpha 1/alpha 2, beta 1) greater than metaproterenol (beta 2), phenylephrine (alpha 1). Dobutamine (beta 1 agonist) and dopamine (dopaminergic agonist) did not significantly affect [14C]acetate incorporation into lipid, while clonidine and para-aminoclonidine (alpha 2 agonists) were slightly stimulatory. Lipolysis in young and adult chicken adipose tissue was stimulated by epinephrine, isoproterenol, phenylephrine, dobutamine and metaproterenol, but was inhibited by clonidine and para-aminoclonidine. Both the antilipogenic and lipolytic effects of epinephrine were partially blocked by phentolamine (alpha 1 = alpha 2 antagonist) or propranolol (beta 1 = beta 2 antagonist), but completely inhibited by phentolamine and propranolol administered together.

Growth hormone: its physiology and control

Growth hormone (GH) is a protein hormone produced by the somatotrophs of the anterior pituitary gland of birds and other vertebrates. The secretion of GH in birds is under hypothalamic control; it involves three peptidergic releasing factors: growth hormone-releasing factor (GRF) (stimulatory); thyrotropin-releasing hormone (TRH) (stimulatory); and somatostatin (SRIF) (inhibitory). In addition, there is evidence for effects of biogenic amines (including serotonin and norepinephrine) and prostaglandins at the level of the hypothalamus and possibly also the pituitary gland. In all avian species examined, plasma concentrations of GH are high in young posthatching chicks but low in the adult and embryo. The difference in plasma concentrations of GH between young and adult birds is due to both greater GH secretion and reduced clearance. The lower secretion of GH in adult birds reflects fewer somatotrophs in the pituitary, changes in somatotroph structure, and reduced GH responses to TRH or GRF administration. There is only limited data on the role of GH in birds. GH appears to be required for normal growth; acting at least in part by increasing somatomedin production. However, plasma concentrations of GH do not necessarily correlate with growth rate. For instance, in chicks with reduced growth rate owing to either goitrogen or protein deficiency in the diet, plasma concentrations of GH are elevated. GH also can influence lipid metabolism by increasing lipolysis, decreasing lipogenesis, and stimulating the uptake of glucose by adipose tissue. The physiological significance of these actions is, however, not established. In addition, GH affects the secretion of other hormones, the immune system, and perhaps also the reproductive system.

Adipose tissue metabolism and its control in birds

At any age, with increases in body weight, the elevation in total lipid or abdominal fat is more than proportional to body weight. This observation explains why selection of broilers for rapid growth rate leads to excessive fat accumulation. Although adipocyte hyperplasia continues until 12 or 14 weeks of age, hypertrophia becomes increasingly important with age and the degree of fattening. The number of adipocytes appears to be correlated with body size, and the size of the adipocyte is closely related to the fat content of the live bird. The hormonal control of lipolysis involves a number of hormones and appears to be very complex. The increase in fat in adipose tissue is mainly due to hepatic lipogenesis. This review discusses the relative role of hormones in fat metabolism, some pecularities of insulin activity in birds, and the important functions of plasma lipoprotein and adipose lipoprotein lipase. A comparison of a fat line of chickens and mammalian models of obesity is also made.

Influences of growth hormone on glucose uptake by avian adipose tissue

The uptake of glucose by chicken adipose tissue was determined in vitro using adipocytes or adipose explants using 14C orthomethyl glucose (OMG). The net uptake of 14C-OMG was enhanced by preparations of bovine and chicken GH and by insulin. Insulin and growth hormone (GH) (chicken or bovine) stimulated the uptake of 14C-OMG occurring in 1 hr by adipose tissue (both adipocytes and explants) from chicks hypophysectomized for 5 days but not from sham-operated birds. Tissue preincubated with GH became refractory to GH stimulation of 14C-OMG uptake. The short-term (2 to 15 min) uptake of 14C-OMG by explants was, however, stimulated by the bovine GH and insulin in tissue from sham-operated but not hypophysectomized chicks. Surprisingly, net decreases in 14C-OMG uptake, in the presence of bovine GH, were observed with adipose tissue (adipocytes and explants) from hypophysectomized birds, when incubated in the presence of epinephrine, and from hypophysectomized birds immediately following surgery.

Lipogenesis and lipolysis in normal and dwarf chickens from lines selected for high and low body weight

Lipogenic and lipolytic comparisons were made among normal and dwarf adult males from both high-weight (HW) and low-weight (LW) selected lines. Six nonfasted birds from each of the four populations were sacrificed, and in vitro lipogenesis and lipolysis were measured in liver, adipose, and bone tissue. Also determined were activities of acetyl CoA carboxylase (E.C.6.4.1.2), NADP-malate dehydrogenase (E.C.1.1.1.40), ATP-citrate lyase (E.C.4.1.3.8), and plasma-free fatty acids (FFA). In comparison with the HW chickens, the LW males tended to exhibit higher activities of lipogenic enzymes, a greater capacity to incorporate acetate-1-14C into liver slices, an increased mobilization of FFA from adipose tissue, and high concentrations of FFA in plasma. The results indicated that the maintenance of increased postmaturational body fat associated with selection for high body weight was primarily the result of decreased lipolysis rather than enhanced lipogenesis. Effects of the dwarf allele on lipogenesis and lipolysis were not clear. Acetate incorporation into bone tissue was substantially higher than for adipose tissue, suggesting that bone may be an important site of lipogenesis in the fowl.

Seasonal changes in circulating levels of luteinizing hormone and growth hormone in the migratory Canada goose

Seasonal changes in plasma levels of LH and GH in the Canada goose (Branta canadensis interior) have been studied. The population examined breeds in the Canadian subarctic region (Fort Churchill, Manitoba) and winters in the southern United States (Swan Lake Refuge, Missouri). Peak levels of LH were observed during spring postmigratory (nestbuilding, early May) and nesting (incubating, early June) periods, while no significant differences were observed during the other periods studied. The highest levels of GH were observed during molting (late July) and the immediately following fall premigratory (early September) period.

Effect of intermittent feeding on blood plasma growth hormone and prolactin in chickens of a heavy breed

1. Variations in plasma growth hormone (GH) and prolactin concentrations were determined during growth (at 20, 33, 56 and 83 d of age) in ad libitum (control) and intermittently (alternate days) fed chicks. 2. In each group of birds the concentration of plasma GH was inversely related to age. The mean prolactin concentration was highest in the youngest (20-d-old) birds. 3. The concentration of plasma GH in the intermittently-fed birds deprived of food for 24 h (depleted birds) was significantly higher than that in the controls at 33, 56 and 83 d of age. The mean GH concentration in the intermittently-fed birds 24 h after refeeding (repleted birds) was less than that in the depleted ones. 4. The overall mean concentration of plasma prolactin in the depleted birds was significantly less than that in the control and repleted birds. 5. These results are consistent with the effects of fasting on GH and prolactin secretion and demonstrate that growth retardation in the intermittently-fed birds was not due to impaired GH secretion.

Effect of feed or water restriction on basal and TRH-stimulated growth hormone secretion in the growing turkey poult

Both feed and water restriction of growing turkeys resulted in an increase in the basal levels of plasma growth hormone (GH). Restricted birds also showed an increased response to thyrotropin-releasing hormone (TRH) stimulation. These changes were significantly greater in feed-restricted birds than in the water-restricted birds. After return to ad lib conditions, basal plasma GH levels remained elevated above control levels in both previously restricted groups. However, the GH response to TRH stimulation returned to normal during this period. The increase in GH secretion during undernutrition is a metabolic adjustment consistent with the known role of GH in glucose, protein and fat metabolism. The elevated GH levels present after return to adequate nutrition may aid compensatory growth. Feed and water restriction were equally effective in limiting the growth of young turkey poults. Body weight gain and feed efficiency were severely affected during the first 3 weeks of restriction, but improved during the 4th week. Return to ad lib conditions resulted in compensatory growth and markedly improved feed efficiency in both restricted groups. Feed-restricted birds showed a significant increase in water consumption after 8 days of restriction. This polydipsia may result from intermittent feeding of hungary animals. Water consumption returned to normal after 1 week of ad lip feeding. Birds restricted in water consumption voluntarily limited their feed intake to a level only slightly higher than that of the feed-restricted group. When water was supplied ad lib, these birds immediately resumed normal feed consumption.

Variations in plasma growth hormone concentrations in laying hens

1. Variations in plasma concentrations of growth hormone (GH) and luteinising hormone (LH) were measured by radioimmunoassay in blood samples taken during the ovulatory cycle of the domestic hen. 2. A peak in LH concentrationas observed 4 h prior to ovulation. 3. Plasma GH concentrations were quite variable but a significant increase was observed between 2 h before and 2 h after ovulation and the concentration remained elevated for about 4 h thereafter.