Growth, body composition, hormonal and metabolic status in lambs treated long-term with growth hormone

Parity influences the utilization of exogenous glucose in suckler anoestrous Charolais beef cows

This work was designed to study the influence of parity (multiparous (MP, no. = 9) or primiparous (PP, no. = 10)) and feeding level (control diet (CD; 1·00) or restricted diet (RD; proportionately 0·70 of requirements) in a 2×2 factorial design on the utilization of exogenous glucose in the suckler anoestrous Charolais beef cow. Lack of ovarian activity was confirmed by low circulating progesterone concentrations measured in weekly blood samples taken after parturition. An injection of glucose (1·5 mmol/kg live weight) lasting 20 min was given 30 days and 50 days post partum. Blood samples were collected before and for 120 min after the start of the injection and plasma insulin, glucose, non-esterified fatty acids (NEFA), β-hydroxybutyrate and urea were measured. Although plasma glucose concentrations were not significantly different between treatment groups in the period immediately after the injection of glucose, CD PP animals had higher plasma insulin levels than CD MP animals (at 25 min: 596·4, 283·8, 435·1 and 309·6 pmol/l, P < 0·05 for groups PP CD, MP CD, PP RD and MP RD respectively). Plasma NEFA concentrations were initially higher in the RD groups compared with the CD groups but were reduced by the glucose injection (P < 0·001). However, the PP RD cows took longer to respond to the exogenous glucose compared with the MP RD cows (at 25 min: PP RD: 133·2 and MP RD: 57·5 eq/l, P < 0·05). In conclusion, suckler anoestrous PP cows appear to require higher plasma insulin levels than MP cows to metabolize exogenous glucose and underfed PP cows were slower to reduce plasma NEFA after glucose injection than underfed MP cows.

The effect of immunization against somatostatin and β-agonist administration alone and in combination on growth and carcass composition in young steers

The effect of active immunization against somatostatin and β-agonist administration alone and in combination on growth, carcass composition and endocrine responses was investigated in young steers given a pelleted diet of dried grass and barley (0·7: 0·3) ad libitum for 16 weeks. Immunization alone increased growth rate by proportionately 0·10 but had no effect on plasma growth hormone (GH) or insulin-like growth factor-1 (IGF-1) concentrations (P > 0·10). Carcass protein content was unchanged but fat content tended to be increased. This was possibly associated with increased (P < 0·10) plasma insulin concentrations observed in these animals. Cimaterol administration significantly increased growth rate (P < 0·05) and carcass weight (P < 0·01). Carcass protein content was significantly (P < 0·01) increased and carcass fat was significantly (P < 0·001) reduced. Administration of cimaterol to immunized animals resulted in an enhancement of the repartitioning effect of the pagonist although growth rates were intermediate between the separate treatments. The mechanism by which this was achieved is unclear. Plasma IGF-1 concentrations tended (P < 0·10) to be lower in these animals compared with controls. Plasma insulin concentrations were also reduced (P < 0·05) in these animals. Both immunized animals and those given cimaterol alone showed an increased GH response to a single intravenous injection of growth hormone releasing factor (GRF) (P < 0·05). Animals given the combined treatment however showed a reduced response suggesting that the effects were not mediated through direct changes in GH secretion. The improved lean:fat ratio in the carcass was observed to be mediated through an enhancement of the lipolytic response to the /J-agonist with only small effects on protein accretion. It is suggested that immunoneutralization of somatostatin may increase the sensitivity of adipose tissue to lipolytic stimuli. This could occur through changes in the secretion of gastrointestinal or pancreatic hormones as a result of neutralization of the inhibitory influences of somatostatin. Further work is required to clarify these effects.

The effect of active immunization against gonadotropin-hormone-releasing-hormone on growth performance and sample joint composition of bulls

Forty-six Simmental × British Friesian bull calves were allocated to six treatment groups. In four groups (each of eight animals) half the animals were given a prime injection of gonadotropin-hormone-releasing-hormone (GnRH) either as the decapeptide or as an octapeptide (residues 3 to 10) conjugated to egg albumen. Prime injection times were at 3, 4, 5 or 6 months of age. All animals were boosted with a GnRH conjugate, similar to that used for the prime injection, at 8 months. At 8 months, six other animals were surgically castrated while the remaining eight were left as untreated bull controls. Weight gain and consumption of a barley-based diet offered ad libitum were recorded for individual animals. Blood samples were taken at a minimum of fortnightly intervals and the serum analysed for antibody titre against GnRH, testosterone and insulin-like growth factor 1 (IGF-1). Animals were slaughtered at 12 months and chemical analyses performed on the dissectible material of the 10th rib for protein, lipid, ash and water content. Greater antibody titres and a longer period of low serum testosterone were achieved with the octapeptide conjugate compared with the decapeptide. Serum IGF-1 slowly decreased following both surgical- and effective immuno-castration. There were no significant differences in food intake between the groups. Both steers and the more responsive immunocastrates had higher fat (P < 0·01), lower protein (P < 0·05) and water concentrations (P < 0·01) in tissues from a rib sample joint compared with untreated bulls. As immuno-responsiveness decreased there were indications of compensatory changes in body composition. The technique may be applicable during periods of, for example, mixed grazing for bulls.

Translational control of protein synthesis in muscle and liver of growth hormone-treated pigs

GH treatment increases protein deposition and the efficiency of dietary protein used for growth. To identify the mechanisms that regulate tissue protein synthesis in response to exogenous GH treatment, fully fed, growing swine were treated with GH for 7 d. Fasted and fed pigs were infused with [1-(13)C]leucine to determine protein synthesis rates, and translation initiation factor activity levels were measured in skeletal muscle and liver. Feeding increased protein synthesis and translational efficiency in both muscle and liver of control and GH-treated pigs, and this was associated with increased 4E-BP1 and S6 kinase 1 phosphorylation, decreased association of eukaryotic initiation factor (eIF) 4E with 4E-BP1, and increased association of eIF4E with eIF4G. GH increased muscle protein synthesis and translational efficiency in fed pigs. GH increased liver protein synthesis of fasted and fed pigs in association with increased ribosome number. In muscle, but not liver, GH increased eIF2B activity and 4E-BP1 phosphorylation in both the fasted and fed state and increased the association of eIF4E with eIF4G in the fed state. We conclude that GH increases muscle protein synthesis in the fed state, in part, via mechanisms that enhance the binding of mRNA and methionyl-tRNA to the 40S ribosomal subunit, whereas GH increases liver protein synthesis in the fasted and fed states by increasing ribosome number. The results further indicate that the GH-induced protein synthetic response is dependent upon nutritional state and is tissue specific.

Somatotropin-induced protein anabolism in hindquarters and portal-drained viscera of growing pigs

To differentiate the effect of somatotropin (ST) treatment on protein metabolism in the hindquarter (HQ) and portal-drained viscera (PDV), growing swine (n = 20) treated with ST (0 or 150 microg x kg(-1) x day(-1)) for 7 days were infused intravenously with NaH(13)CO(3) and [(2)H(5)]phenylalanine and enterally with [1-(13)C]phenylalanine while in the fed state. Arterial, portal venous, and vena cava whole blood samples, breath samples, and blood flow measurements were obtained for determination of tissue and whole body phenylalanine kinetics under steady-state conditions. In the fed state, ST treatment decreased whole body phenylalanine flux, oxidation, and protein degradation without altering protein synthesis, resulting in an improvement in whole body net protein balance. Blood flow to the HQ (+80%), but not to the PDV, was increased with ST treatment. In the HQ and PDV, ST increased phenylalanine uptake (+44 and +23%, respectively) and protein synthesis (+43 and +41%, respectively), with no effect on protein degradation. In ST-treated and control pigs, phenylalanine was oxidized in the PDV (34-43% of enteral and arterial sources) but not the HQ. In both treatment groups, dietary (40%) rather than arterial (10%) extraction of phenylalanine predominated in gut amino acid metabolism, whereas localized blood flow influenced HQ amino acid metabolism. The results indicate that ST increases protein anabolism in young, growing swine by increasing protein synthesis in the HQ and PDV, with no effect on protein degradation. Differing results between the whole body and the HQ and PDV suggest that the effect of ST treatment on protein metabolism is tissue specific.

Regulation of IGF-I mRNA by GH: putative functions for class 1 and 2 message

This study investigated mechanisms regulating hepatic insulin-like growth factor (IGF)-I class 1 and 2 mRNA levels. Lambs were treated with growth hormone (GH) either as an acute, single dose or over a longer term. Total hepatic unspliced, pre-mRNA levels increased after the single dose of GH but were attenuated after 8 days of GH, with exon 1- and 2-derived pre-mRNA levels displaying coordinate responses. Surprisingly, changes in total spliced, mature mRNA levels did not reflect those for pre-mRNA, instead being augmented after 8 days of GH. GH also induced a differential increase in the ratio of mature class 2-to-class 1 IGF-I mRNA; therefore, this must be predominantly via posttranscriptional mechanisms. Increases in the ratio of class 2-to-class 1 mRNA were observed in polysomal vs. total RNA preparations derived from GH-treated but not control lambs, indicating an increased proportion of class 2 transcripts engaged in translation. Our findings indicate that GH may stabilize mature class 2 transcripts or destabilize mature class 1 transcripts and that class 2 mRNA may have a greater translational potential. The following two main functions of hepatic class 2 IGF-I mRNA are suggested: an efficient "monitor" of GH status via providing a rapid negative feedback mechanism and a coordinator of endocrine-regulated tissue growth.

Somatotropin increases protein balance by lowering body protein degradation in fed, growing pigs

Somatotropin (ST) administration enhances protein deposition in well-nourished, growing animals. To determine whether the anabolic effect is due to an increase in protein synthesis or a decrease in proteolysis, pair-fed, weight-matched ( approximately 20 kg) growing swine were treated with porcine ST (150 microg. kg(-1). day(-1), n = 6) or diluent (n = 6) for 7 days. Whole body leucine appearance (R(a)), nonoxidative leucine disposal (NOLD), urea production, and leucine oxidation, as well as tissue protein synthesis (K(s)), were determined in the fed steady state using primed continuous infusions of [(13)C]leucine, [(13)C]bicarbonate, and [(15)N(2)]urea. ST treatment increased the efficiency with which the diet was used for growth. ST treatment also increased plasma insulin-like growth factor I (+100%) and insulin (+125%) concentrations and decreased plasma urea nitrogen concentrations (-53%). ST-treated pigs had lower leucine R(a) (-33%), leucine oxidation (-63%), and urea production (-70%). However, ST treatment altered neither NOLD nor K(s) in the longissimus dorsi, semitendinosus, or gastrocnemius muscles, liver, or jejunum. The results suggest that in the fed state, ST treatment of growing swine increases protein deposition primarily through a suppression of protein degradation and amino acid catabolism rather than a stimulation of protein synthesis.

Ontogeny of IGF-I responsiveness to bGH in young lambs

The ontogeny of hepatic growth hormone (GH) receptors (GHR), as measured by responses of both plasma insulin-like growth factor-I (IGF-I) and hepatic GHR to an exogenous bGH stimulus, was examined using sheep of different ages (Days 1-7, 14-21, 28-35, and 56-63 of life, and yearlings). The IGF-I response to bGH was first examined in yearling sheep using two doses of bGH (0.1 and 0.2 mg/kg LW/d). Based on these results, lambs in four groups up to Day 63 of life were treated for 5 d with bGH (n = 10) at a dose of 0.15 mg/kg LW/d or with saline (n = 10). Jugular blood samples were taken once daily on Days - 1, 4, and 5 of treatment. bGH treatment in lambs up to Day 63 of life had little effect on plasma concentrations of GH, insulin, glucose or urea, but significantly (P < 0.05) increased circulating concentrations of IGF-I at all ages and of NEFA at Day 62/63 of life. In contrast, bGH treatment at either dose in yearlings significantly increased these parameters, except for plasma urea concentrations which were decreased in bGH-treated yearlings. However, the responses of plasma IGF-I concentration to bGH stimulus in lambs up to Day 63 of life were small compared to those in yearling sheep. Consistent with this, bGH treatment failed to affect hepatic GH binding in young lambs, but up-regulated it in yearling sheep. Furthermore, basal (unstimulated) GH binding did not differ between sheep of 7 vs. 63 vs. 365 d of age, despite the greater IGF-I responses to bGH in the latter group. It is suggested that hepatic GHR in lambs up to Day 63 of life are not fully functional compared to the situation in yearlings.

Action of long(R3)-insulin-like growth factor-1 on protein metabolism in beef heifers

Insulin-like growth factor-1 (IGF-1) is perhaps the most important endogenous factor controlling growth. Most studies to date in livestock have shown that IGF-1 has greatest efficacy when animals are in a catabolic state. We have determined the effects of an i.v. infusion of the IGF-1 analog Long(R3)-IGF-1 on protein metabolism in beef heifers that were slowly losing liveweight because of restricted feeding. There was a tendency for both whole-body protein and skeletal muscle protein to be conserved in Long(R3)-IGF-1-treated heifers. Long(R3)-IGF-1 administration markedly reduced the plasma concentrations of all amino acids measured and glucose. There was a significant change in the profile differences of endogenous plasma IGF-1 concentrations during the 8-hr infusion period, with plasma IGF-1 decreasing sharply in the test group. There was a significant difference in mean profiles for plasma IGF-2 between the test and control groups. Overall, plasma IGF-2 for the control group decreased only slightly over time (about 40 ng/ml), whereas the test group decreased dramatically (by about 140 ng/ml). Increased plasma concentrations of a 31-32-kDa IGF-binding protein (possibly IGF-binding protein-1) in the treated group was detected by radioligand blot. We found that Long(R3)-IGF-1 infusion tended to preserve whole-body and muscle protein in beef heifers on a low-quality diet, and suggest that further investigation of this treatment may provide an alternative approach to reducing weight loss during the dry season.

Nutritional management and growth hormone treatment of preterm infants born small for gestational age

In severe cases of intrauterine growth retardation, elective preterm delivery may provide the possibility for nutritional intervention to prevent some of the long-term consequences of the catabolic condition in utero. Neonatal nutritional management is aimed at providing a high protein intake of up to 4 g/kg/day in order to obtain the rapid increase in protein that is seen in normally growing infants during the early postnatal period. Unfortunately, due to impaired production of urea, high plasma levels of ammonia, which may be rate limiting with respect to an optimal gain in protein, are often observed in the preterm infant born small for gestational age (SGA). In an attempt to stimulate protein synthesis in preterm infants born SGA, growth hormone (GH) treatment was given to seven such infants during the early postnatal period. The infants received daily subcutaneous injections of GH (1.0 IU/kg/day) from postnatal day 7 until a body weight of 2000 g was reached (postnatal week 7-8). A further seven preterm infants born SGA were studied as controls. GH treatment had no significant effects on growth, body composition, net protein gain and glucose metabolism. Furthermore, plasma levels of insulin-like growth factor I (IGF-1) and IGF-binding protein-3 revealed a normal developmental increase and were not significantly altered by GH treatment. These results may be explained by a relative GH insensitivity or resistance during this period of early preterm life.

Paradoxical increases of circulating nonesterified fatty acids in somatotropin treated cattle undergoing mild disturbances

Effects of various doses of bovine somatotropin (bST) on plasma concentrations of nonesterified fatty acids (NEFA) were studied in a 14-d Latin square with six Holstein heifers. Animals were given daily injections of excipient or bST at 12:00 p.m. and fed twice daily at 7:00 a.m. and 7:00 p.m. On Day 14, plasma NEFA remained low through the day except around the 7:00 p.m. feeding when they were substantially elevated. The elevation was significantly greater in bST-treated animals and corresponded to the excitement of the animals in anticipation of the evening feeding. To further investigate this phenomenon, a second experiment was conducted in which nine growing Holstein steers were fed hourly and received either daily intramuscular (i.m.) injection of excipient or bST (120 mg/kg BW) for 15 d in a crossover design. Daily profiles of NEFA were obtained under undisturbed conditions or concurrently with intensive handling. Although no elevations could be detected in any case in control animals, bST caused a substantial rise in NEFA concentration only when animals were subjected to intensive handling. This suggested that NEFA peaks noted in bST-treated heifers in the first experiment resulted from increased ability of adipose tissue to respond to adrenergic stimulation associated with the anticipation of feeding. Consistent with this hypothesis, plasma NEFA concentrations in bST-treated steers were increased to a greater extent during a challenge involving i.v. injection of epinephrine. This amplification of adipose tissue response by bST must be considered when conducting intensive studies. Even the minimal excitement associated with blood sampling can confound the results regarding lipid mobilization, and this may have contributed to the notion that ST is a lipolytic hormone.

Chronic administration of growth hormone (GH) to adult chickens exerts marked effects on circulating concentrations of insulin-like growth factor-I (IGF-I), IGF binding proteins, hepatic GH regulated gene I, and hepatic GH receptor mRNA

In young birds, growth hormone (GH) administration has been found to have only a small or even no effect on circulating concentrations of insulin-like growth factor-I (IGF-I). This is in obvious contrast to the situation in mammals. The present study examines the effect of continuous administration of GH in adult male chickens. Plasma concentrations of IGF-I were markedly elevated (2.5-3.0-fold, p < 0.001) in GH-treated chickens. There were also some transient increases in the circulating levels of IGF binding proteins. Adult chickens showed other manifestations of increased responsiveness to GH, including elevated hepatic expression of GH-regulated gene-I (mRNA) with GH treatment (p < 0.05), and a tendency (p < 0.08) for decreased GH-receptor mRNA. In contrast to the changes in circulating concentrations of GH and IGF-I with GH treatment, no changes in plasma concentrations of thyroid hormones, reproductive hormones, glucose, or nonesterified fatty acids were evident.

Effects of exogenous somatotropin on whole-body glycemic response to insulin in young preruminant and ruminant lambs

The purpose of this study was to determine the effects of exogenous recombinant bovine somatotropin (bST) treatment on whole-body glycemic responsiveness and sensitivity to exogenous insulin in preruminant and ruminant lambs. Twelve milk-fed (MF) and 12 ruminating (RUM) wether lambs weighing 20 +/- 0.6 kg were assigned to one of four treatment groups: MF control, MF plus bST, RUM control, and RUM plus bST. Lambs received a daily subcutaneous injection of 160 micrograms of sometribove (recombinant methionyl bST) bST/kg live weight or the equivalent volume of sterile water (control) for 10 d. The MF lambs had higher plasma insulin and nonessential fatty acids and lower acetate concentrations than RUM lambs (all P < 0.05). Plasma insulin-like growth factor concentrations were similar in both. The administration of bST raised plasma insulin-like growth factor-1 (P < 0.001) and insulin (P < 0.05) in MF and RUM lambs, but with greater effect in MF lambs (P < 0.01 and P < 0.1, respectively). Six successive dose-incremented insulin challenges (50, 100, 200, 300, 500, and 700 mU/kg body weight) were performed two per day on Days 8, 9, and 10 of treatment. Dose-response curves for absolute decline in glucose concentration from preinjection baseline to nadir were used to characterize whole-body responsiveness and sensitivity (ED50) to insulin. Somatotropin treatment increased insulin ED50 values 64 and 70% (P < 0.07) in RUM and MF lambs, respectively, suggesting that sensitivity to insulin was reduced. Insulin ED50 values were 40% higher in MF than in RUM lambs (P < 0.05). Insulin clearance rates increased with each dose increment to 300 mU/kg body weight (P = 0.001) and were 50% lower in bST-treated MF lambs than in all other treatment groups (P < 0.05). Results suggest that somatotropin modulates the insulin control of glucose homeostasis similarly in preruminant and ruminant lambs by decreasing sensitivity but not maximum responsiveness.

Differential regulation of IGF-I leader exon transcription

We have shown that there is differential expression of the two IGF-I leader exons during physiological changes in GH and nutritional status in liver, with exon 2 being more responsive than exon 1. It may be that this exon is responsible for endocrine IGF-I synthesised in response to specific anabolic stimuli inducing subsequent whole body and skeletal muscle growth.

Influence of diet and beta-agonist administration on plasma concentrations of growth hormone and insulin-like growth factor-1 in young steers

The effect of feeding grass silage or a forage-concentrate (dried grass-barley) diet ad lib. to young cattle on growth rate, plasma growth hormone (GH) and insulin-like growth factor-1 (IGF-1) concentrations was examined. The effect of including the beta-adrenergic agonist cimaterol in the forage-concentrate diet was also investigated. Significantly higher growth rates were observed in animals fed on the forage-concentrate diet than in those fed on the silage diet (P < 0.001), and these were further enhanced by dietary inclusion of cimaterol (P < 0.05). Plasma GH levels were higher in the silage-fed animals (P < 0.001) but IGF-1 levels were significantly lower (P < 0.001) than in the forage-concentrate-fed animals. Cimaterol had no effect on mean plasma GH and IGF-1 concentrations. Nevertheless, plasma IGF-1 levels correlated positively with growth rate across all three treatments (r 0.84, P < 0.001, n 17). Spectral analysis of the GH data obtained for each animal was used to determine whether regular periodicities could be detected in the 24 h profile and whether they differed between the three treatment groups. Regular periodicities of four-five cycles/d were detected in the averaged GH profiles of the silage-fed and the control forage-concentrate-fed animals. In contrast, in animals given cimaterol regular GH cycles were not detectable at any of the frequencies tested. This suggests that cimaterol disrupts the rhythm of GH secretion without altering the overall mean concentrations. The data also suggest that due to an inadequate nutrient supply, the GH-IGF-1 regulatory mechanism was uncoupled in the cattle fed on silage, which may have contributed to the poor growth response of these animals.

Homeorhetic actions on tissue protein metabolism after the administration of rat growth hormone to normal rats

An acute treatment with rat growth hormone (1 microgram/g) to intact female rats produced immediate changes in muscle and bone protein synthesis as well as in muscle protein breakdown, while glucose and glutamate-piruvate transaminase plasma levels were not altered. These effects, apparently, are not mediated by systemic insulin-like growth factor I. Also a long-term treatment with somatotropin (0.1 microgram/g/d) for 22 days was performed, in which protein synthesis rates in muscle, liver and bone remained unchanged. However, the growth hormone long-term treatment induced a decrease in muscle proteolytic activity and an increase in tibia weight. In this context, this experiment describes, apparently for the first time, the systemic effect of growth hormone in entire female rats. Data suggest that a single dose of rat growth hormone produces immediate changes in tissue protein metabolism, through a direct effect of growth hormone. These effects were not observed after the long-term growth hormone treatment, although these animals showed an increased in tibia proportions.

Response of young broiler chickens to chronic injection of recombinant-derived human insulin-like growth factor-I

The purpose of this study was to determine if exogenous insulin-like growth factor-I (IGF-I) would improve growth rate or body composition of young broiler chickens. Broiler cockerels were given a daily intramuscular (im) injection of sodium acetate buffer (buffer control), 100 or 200 micrograms recombinant-derived human IGF-I (rhIGF-I) per kg body weight from 11 to 24 days of age. Exogenous IGF-I did not affect the average daily gain, average daily feed consumption, or the gain-to-feed ratio of broiler chickens. Although daily injection of 200 micrograms/kg of rhIGF-I reduced (P less than 0.05) body ash content, there was no significant effect of IGF-I treatment on either body fat or protein content. Plasma GH levels were depressed (P less than 0.05) by chronic treatment with rhIGF-I. In contrast, plasma levels of T3 and T4 were not affected by rhIGF-I treatment. The half-life of rhIGF-I in plasma was determined at 25 days of age in naive control or chronically-injected chickens after a single intravenous dose of 50 micrograms rhIGF-I/kg. We found a single compartment, first-order disappearance pattern of rhIGF-I from chicken plasma. The half-life (t1/2) of rhIGF-I in plasma was similar (t1/2 = 32.5 min) for naive controls (injected once) or chronically-treated chickens which had received a daily injection of rhIGF-I (100 or 200 micrograms/kg) for 14 d. These data indicate that daily injection of IGF-I cannot be used to enhance growth performance or body composition of broiler chickens when given during the early growth period. The depression of plasma GH levels in rhIGF-I-injected chickens supports a negative-feedback role of IGF-I on pituitary GH secretion.

Evidence of a short-term effect of growth hormone on in vivo bone protein synthesis in normal rats

The growth of skeletal tissue is influenced by growth hormone (GH), whose effects are believed to be mediated, at least in part, by somatomedins or insulin-like growth factors (IGFs). This report describes an action of GH by increasing bone protein synthesis. This effect was not accompanied by changes in circulating IGFs. It is concluded that this experiment shows apparently for the first time in intact animals a short-term effect on bone turnover of GH.