Temporal response of circulating metabolites and hormones during somatotropin treatment of growing pigs

Exogenous porcine somatotropin stimulates mammary development in late-pregnant gilts

The goal of this project was to determine if increasing insulin-like growth factor-1 (IGF-1) concentrations in late pregnancy can stimulate mammogenesis in gilts. Yorkshire × Landrace gilts of a similar body weight (BW; 196.2 ± 6.2 kg) on day 89 of gestation were separated in 2 groups, namely, controls (CTL, n = 17) that were injected with sterile water, and porcine somatotropin-treated (pST, n = 20) that received injections of 5 mg of pST (Reporcin). Injections were given daily from days 90 to 109 of gestation and gilts were slaughtered on day 110 to collect mammary glands for compositional analyses. Blood samples were obtained on days 89, 96, 103, and 109 of gestation to measure IGF-1, free fatty acids (FFA), urea, glucose, and insulin concentrations. Treated gilts gained more BW (22.7 vs. 18.2 kg, P < 0.05) and lost more backfat (P < 0.05) than CTL gilts during the treatment period. There was a treatment × day effect (P < 0.01) on IGF-1, glucose, and urea concentrations. Concentrations of IGF-1 increased 4-fold (P < 0.01) in pST compared with CTL gilts on days 96, 103, and 109 of gestation. Insulin values were also greater on days 96 (P < 0.01) and 103 (P = 0.01), and tended to be greater (P < 0.10) on day 109 of gestation in pST gilts. Glucose was greater in pST than CTL gilts on days 96 (P < 0.01), 103 (P < 0.01), and 109 (P = 0.01). Concentrations of urea were lower (P < 0.01) on days 96, 103, and 109 of gestation in gilts receiving pST injections, and FFA was not altered by treatment on any sampling day (P > 0.10). Injections of pST did not affect mammary extraparenchymal tissue weight (P > 0.10) but increased mammary parenchymal mass (1922 vs. 1576 ± 124 g, P < 0.05). The composition of parenchymal tissue was also altered by treatment. Mammary parenchyma from pST gilts contained more (P < 0.05) protein, DNA and RNA and less fat (P < 0.05) and dry matter (P < 0.01) than that from CTL gilts. These findings provide a clear demonstration that increasing circulating IGF-1 in late-pregnant gilts can stimulate mammary development both in terms of total parenchymal mass and of parenchymal tissue composition.

Dietary stimulation of the endogenous somatotropic axis in weaner and grower-finisher pigs using medium chain triglycerides and cysteamine hydrochloride

BACKGROUND

Three experiments were conducted to examine the overall hypothesis that addition of medium chain triglycerides (MCT) and cysteamine hydrochloride (CSH) into the diets of young and growing pigs would stimulate the endogenous somatotropic axis to improve growth and performance.

RESULTS

In Experiment 1, weaner pigs were given either a 5 d dietary supplement of 5 % MCT (n = 8) or a control diet (n = 8). MCT increased the plasma concentration of growth hormone (GH; P < 0.05) and the GH secretagogue, ghrelin (P < 0.05). Additionally, the MCT treatment reduced scouring (P < 0.05), maintained villous height in the small intestine (P < 0.05) and stabilised daily weight gain (P < 0.05), compared to the controls. Experiment 2 compared the effects of 4 levels (0, 1, 3 and 6 % v/v) of MCT supplementation in grower-finisher male pigs, of approximately 35 kg live weight (n = 15 per treatment). Blood samples taken after 7 wk of treatment showed that the MCT supplementation increased circulating ghrelin (P < 0.001), GH (P < 0.01) and insulin (P < 0.05) concentrations in a dose-dependent manner. Daily weight gain, feed intake and feed conversion ratio were not affected by the MCT diets. In Experiment 3, 64 female pigs of approximately 60 kg live weight were allocated to one of three dietary treatments: control (n = 20); 6 % MCT (n = 21); and 70 mg/kg CSH (n = 21). After 3 wk of supplementation, the MCT treated pigs had a higher plasma concentration of ghrelin compared to the control and CSH pigs (P < 0.05). Plasma concentrations of GH and weight were not affected by any of the dietary treatments.

CONCLUSIONS

Evidence is provided in Experiment 1 to support the use of dietary MCT supplements, perhaps acting via stimulation of somatotropic endocrine pathways, to minimise weaning-associated disorders such as slowing of growth and diarrhoea. In Experiments 2 and 3, although there was no effect on weight gain or feed conversion ratio in either experiment, MCT and CSH increased endocrine components of the somatotropic axis.

Porcine somatotropin alters insulin response in growing pigs by reducing insulin sensitivity rather than changing responsiveness

Exogenous porcine somatotropin (pST) treatment consistently improves growth performance and reduces fat deposition in pigs, and it is hypothesized that one component of the mechanism is through altering the sensitivity and/or responsiveness to insulin. Therefore, a study was conducted to investigate the effect of pST treatment on whole-body glucose metabolism in response to varying doses of insulin. Eight barrows were surgically prepared with indwelling catheters and randomly assigned to one of two treatment groups (0 or 120 μg pST/kg BW · d) for 13 d. Whole-body glucose kinetics were measured during infusion of [6-(3)H]-glucose under basal conditions and during hyperinsulinemic-euglycemic clamps at various insulin infusion rates (7, 28, and 140, and 14, 70, and 280 ng insulin/kg BW · min) and alterations in the dose-response parameters were calculated with nonlinear regression. Treatment with pST increased basal plasma concentrations of glucose (36%; P = 0.005), insulin (276%; P = 0.001), and NEFAs (177%; P = 0.01) and decreased the rate of glucose disappearance (-59%; P = 0.001). The responsiveness (maximum response) for steady state glucose infusion rate to maintain glycemia was not altered by pST (112 vs 106 μmol/min · kg; P = 0.78), whereas the sensitivity (effective dose at 50% of maximum response) was increased almost 7-fold (1.3 vs 8.7 ng/mL; P = 0.027). Similar responses were observed for rate of glucose disappearance and insulin-dependent glucose utilization. Therefore, pST-induced insulin resistance with regard to whole-body glucose uptake is due to a reduced sensitivity to insulin, rather than a change in responsiveness.

Consumption of pasteurized human lysozyme transgenic goats' milk alters serum metabolite profile in young pigs

Nutrition, bacterial composition of the gastrointestinal tract, and general health status can all influence the metabolic profile of an organism. We previously demonstrated that feeding pasteurized transgenic goats' milk expressing human lysozyme (hLZ) can positively impact intestinal morphology and modulate intestinal microbiota composition in young pigs. The objective of this study was to further examine the effect of consuming hLZ-containing milk on young pigs by profiling serum metabolites. Pigs were placed into two groups and fed a diet of solid food and either control (non-transgenic) goats' milk or milk from hLZ-transgenic goats for 6 weeks. Serum samples were collected at the end of the feeding period and global metabolite profiling was performed. For a total of 225 metabolites (160 known, 65 unknown) semi-quantitative data was obtained. Levels of 18 known and 4 unknown metabolites differed significantly between the two groups with the direction of change in 13 of the 18 known metabolites being almost entirely congruent with improved health status, particularly in terms of the gastrointestinal tract health and immune response, with the effects of the other five being neutral or unknown. These results further support our hypothesis that consumption of hLZ-containing milk is beneficial to health.

Fed levels of amino acids are required for the somatotropin-induced increase in muscle protein synthesis

Chronic somatotropin (pST) treatment in pigs increases muscle protein synthesis and circulating insulin, a known promoter of protein synthesis. Previously, we showed that the pST-mediated rise in insulin could not account for the pST-induced increase in muscle protein synthesis when amino acids were maintained at fasting levels. This study aimed to determine whether the pST-induced increase in insulin promotes skeletal muscle protein synthesis when amino acids are provided at fed levels and whether the response is associated with enhanced translation initiation factor activation. Growing pigs were treated with pST (0 or 180 microg x kg(-1) x day(-1)) for 7 days, and then pancreatic-glucose-amino acid clamps were performed. Amino acids were raised to fed levels in the presence of either fasted or fed insulin concentrations; glucose was maintained at fasting throughout. Muscle protein synthesis was increased by pST treatment and by amino acids (with or without insulin) (P<0.001). In pST-treated pigs, fed, but not fasting, amino acid concentrations further increased muscle protein synthesis rates irrespective of insulin level (P<0.02). Fed amino acids, with or without raised insulin concentrations, increased the phosphorylation of S6 kinase (S6K1) and eukaryotic initiation factor (eIF) 4E-binding protein 1 (4EBP1), decreased inactive 4EBP1.eIF4E complex association, and increased active eIF4E.eIF4G complex formation (P<0.02). pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of muscle protein synthesis requires fed amino acid levels, but not fed insulin levels. However, under the current conditions, the response to amino acids is not mediated by the activation of translation initiation factors that regulate mRNA binding to the ribosomal complex.

Stimulation of muscle protein synthesis by somatotropin in pigs is independent of the somatotropin-induced increase in circulating insulin

Chronic treatment of growing pigs with porcine somatotropin (pST) promotes protein synthesis and doubles postprandial levels of insulin, a hormone that stimulates translation initiation. This study aimed to determine whether the pST-induced increase in skeletal muscle protein synthesis was mediated through an insulin-induced stimulation of translation initiation. After 7-10 days of pST (150 microg x kg(-1) x day(-1)) or control saline treatment, pancreatic glucose-amino acid clamps were performed in overnight-fasted pigs to reproduce 1) fasted (5 microU/ml), 2) fed control (25 microU/ml), and 3) fed pST-treated (50 microU/ml) insulin levels while glucose and amino acids were maintained at baseline fasting levels. Fractional protein synthesis rates and indexes of translation initiation were examined in skeletal muscle. Effectiveness of pST treatment was confirmed by reduced urea nitrogen and elevated insulin-like growth factor I levels in plasma. Skeletal muscle protein synthesis was independently increased by both insulin and pST. Insulin increased the phosphorylation of protein kinase B and the downstream effectors of the mammalian target of rapamycin, ribosomal protein S6 kinase, and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1). Furthermore, insulin reduced inactive 4E-BP1.eIF4E complex association and increased active eIF4E.eIF4G complex formation, indicating enhanced eIF4F complex assembly. However, pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of skeletal muscle protein synthesis in growing pigs is independent of the insulin-associated activation of translation initiation.

Porcine somatotropin alters body composition and the distribution of fat and lean tissue in the finisher gilt

The present study was designed to determine whether porcine somatotropin (pST) reduces whole animal and belly fat using dual energy X-ray absorptiometry and manual dissection. The study utilised 24 Large White × Landrace gilts selected at 16 weeks of age with an approximate liveweight of 80 kg and housed in individual pens. Gilts were stratified on liveweight into 8 blocks and 1 pig from each block was assigned to either 0, 5 or 10 mg per day of pST. Pigs were fed ad libitum a wheat-based diet containing 200 g crude protein, 10.2 g available lysine and 14.6 MJ DE/kg, to ensure that responses to pST were expressed. Feed intake and liveweight were measured on a weekly basis. An Hologic QDR4500A dual energy X-ray absorptiometer was used to determine lean, fat and ash composition of pigs initially and again 4 weeks later at the end of the experiment. After slaughter, the composition of the whole half carcass as well as the shoulder, ham, belly and loin primal cuts was determined with dual energy X-ray absorptiometry and verified with manual dissection. Daily pST treatment decreased feed intake (3440, 2710 and 2537 g/day for 0, 5 or 10 mg pST per day, respectively; P<0.001) and decreased feed conversion ratio (2.95, 2.18 and 2.03 g/g; P<0.001) even though there was no significant effect on daily gain. Pigs treated with pST deposited more lean tissue (620, 839 and 873 g/day; P<0.05) and less fat (384, 218 and 176 g/day; P<0.001) than control animals, but there was no effect of pST on ash deposition. As a consequence, pigs treated with 5 and 10 mg pST/day contained 5 and 9 kg less dissectible fat than control gilts, respectively. A dose dependent decrease in belly, loin, ham and shoulder fat was also observed, although the decrease in belly fat was more pronounced than for the whole carcass and other primal cuts. Overall, pST treatment has the potential to decrease carcass and especially belly fat in pigs and increase consumer acceptance of pork in markets that place a premium on carcass fat and lean yield in the belly region. The results determined with dual energy X-ray absorptiometry were confirmed by manual dissection.

Exogenous porcine somatotropin administered to neonatal pigs at high doses can alter lifetime fat but not lean tissue deposition

The growth rate of the young pig is generally much less than its potential and may be constrained by endocrine status as well as nutrient intake. The aim of the present study was to determine whether porcine (p) somatotropin (ST) treatment of the sucking pig could alter subsequent body composition. Twelve mixed-parity cross-bred sows with an average litter size of ten piglets were used to nurse pigs for the present study. On day 1 of lactation, the median two male pigs (by weight) from each litter were randomly allocated to one of two doses of pST (0 or 1 mg/kg per d) until weaning on day 21. Pigs were weaned and offered feed ad libitum until slaughter at 134 d of age. Body composition was measured using dual-energy X-ray absorptiometry (DXA) at 21, 49, 77, 105 and 133 d of age. There was no significant difference in growth rates between day 1 and 21 of lactation in pigs injected with either saline (9 g/l NaCl/l) or pST (258 v. 246 g/d for control and pST-treated pigs respectively, P=0.61), and as a consequence there was no significant difference in liveweight at weaning (7.13 v. 6.84 kg, P=0.59). However, fat mass at weaning tended to be decreased (1.18 v. 0.96 kg, P=0.064), while the % fat in the body at weaning was significantly (16.7 v. 13.9 %, P=0.008) decreased by exogenous pST treatment. In the immediate post-weaning period there was a reduction in lean tissue deposition (347 v. 300 g/d, P=0.021) but no effect on fat deposition (35 v. 33 g/d, P=0.72). Over the entire weaning-to-slaughter period, pST treatment of neonatal pigs decreased the rate of fat deposition (130 v. 112 g/d, P=0.033), but had no effect on lean tissue deposition (550 v. 538 g/d, P=0.49). Therefore, treatment of nursing pigs with high doses of pST for a short period before weaning may provide a means of reducing the fat content of pork and pork products.

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.

Long-term recombinant porcine somatotropin (PST) treatment mitigates the responses to subchronic lipopolysaccharide in swine

The effect of multiple lipopolysaccharide (LPS) challenges in swine undergoing long-term treatment with porcine somatotropin (PST) was determined. Changes in aspartate serine transaminase (AST) occurred only at 24h following the first LPS challenge dose (P<0.05), while PST treatment moderated any change from occurring. Nonesterified free fatty acid (NEFA) levels were elevated in PST treated animals for the first 3 days following daily LPS treatment (P<0.05), while LPS treatment alone had no effect on plasma NEFA levels. Plasma urea nitrogen (PUN) levels were unchanged by LPS following the initial LPS challenge, but were decreased following the second challenge dose (P=0.014). These changes were long lasting, with a return to normal PUN levels not evident until Day 6. The PST treatment mitigated changes in PUN (P<0.05) when LPS was administered. Haptoglobin plasma levels, along with lipid peroxide production were not affected by LPS challenge or PST administration. LPS challenge reduced the levels of immunoreactive heat shock protein 70 (HSP70) throughout the entire challenge period (P<0.001). PST-LPS animals had normal levels of this protein. The results of the present study demonstrate that long-term PST treatment mitigates the adverse effects of subchronic LPS administration.

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.

Effects of dietary fat and conjugated linoleic acid on plasma metabolite concentrations and metabolic responses to homeostatic signals in pigs

Sixteen female cross-bred (Large White x Landrace) pigs (initial weight 65 kg) with venous catheters were randomly allocated to four treatment groups in a factorial design. The respective factors were dietary fat (25 or 100 g/kg) and dietary conjugated linoleic acid (CLA; 0 or 10 g CLA-55/kg). Pigs were fed every 3 h (close to ad libitum digestible energy intake) for 8 d and were bled frequently. Plasma glucose and non-esterified fatty acid (NEFA) responses to insulin and adrenaline challenges were determined on day 8. Plasma concentrations of NEFA were significantly increased (10.5 and 5.4 % for low- and high-fat diets respectively, P=0.015) throughout the experiment, suggesting that there was a possible increase in fat mobilisation. The increase in lipolysis, an indicator of ss-adrenergic stimulated lipolysis, was also evident in the NEFA response to adrenaline. However, the increase in plasma triacylglycerol (11.0 and 7.1 % for low- and high-fat diets respectively, P=0.008) indicated that CLA could have reduced fat accretion via decreased adipose tissue triacylglycerol synthesis from preformed fatty acids, possibly through reduced lipoprotein lipase activity. Plasma glucose, the primary substrate for de novo lipid synthesis, and plasma insulin levels were unaffected by dietary CLA suggesting that de novo lipid synthesis was largely unaffected (P=0.24 and P=0.30 respectively). In addition, the dietary CLA had no effect upon the ability of insulin to stimulate glucose removal.

Short-term infusion of LongR(3) insulin-like growth factor (IGF)-I decreases hepatic IGF-I mRNA but not IGF binding protein-3 mRNA expression in pigs

Infusion of pigs with an insulin-like growth factor-I (IGF-I) analogue (LongR(3)IGF-I) that does not bind to IGF-binding proteins decreases growth rate and the plasma concentration of growth hormone (GH), IGF-I, IGFBP-3, and insulin. This study was designed to determine whether the decrease is due to changes in IGF-I and IGFBP-3 gene expression. IGF-I or LongR(3)IGF-I (180 microg/kg/day) was infused into 55-kg finisher pigs for 4 days using Travenol infuser pumps. Plasma IGF-I concentration was measured by radioimmunoassay and plasma IGFBP-3 and IGFBP-2 were estimated by Western ligand blotting. Steady-state levels of IGF-I and IGFBP-3 mRNA were measured by RNase protection assay. Neither IGF-I nor LongR(3)IGF-I had a significant effect on hepatic IGF-I class 1 mRNA expression, whereas hepatic IGF-I class 2 mRNA expression was significantly reduced by both peptides. Plasma IGFBP-3 levels were unaffected by IGF-I treatment but were reduced by LongR(3)IGF-I treatment. The decrease in IGFBP-3 was not due to decreased gene expression in porcine liver or kidney, since neither IGF-I nor LongR(3)IGF-I treatment altered IGFBP-3 mRNA. This study infers a direct effect of the IGF analogue LongR(3)IGF-I on GH through its inhibition of plasma IGF-I concentration and class 2 IGF-I mRNA. The decrease in plasma IGFBP-3 was not accompanied by a decrease in hepatic or renal IGFBP-3 mRNA, suggesting that in this case, plasma IGFBP-3 protein levels are posttranslationally regulated or are derived from tissues other than liver or kidney.

Somatotropin-induced amino acid conservation in pigs involves differential regulation of liver and gut urea cycle enzyme activity

Somatotropin (ST) treatment promotes animal growth and allows for the conservation of amino acids by increasing nitrogen retention and reducing ureagenesis and amino acid oxidation. To determine whether the improvement in amino acid conservation with ST treatment involves regulation of urea cycle enzyme activities in both liver and intestine, growing swine were treated with either ST (150 microg x kg(-1) x d(-1)) or saline for 7 d. Fully fed pigs (n = 20) were infused intravenously for 2 h with NaH(13)CO(3) followed by a 4-h intraduodenal infusion of [1-(13)C]phenylalanine. Arterial and portal venous blood and breath samples were obtained at baseline and steady-state conditions for measurement of amino acid and blood urea nitrogen (BUN) concentrations and whole-body phenylalanine oxidation. Urea cycle enzyme activities were determined in liver and jejunum. ST decreased BUN (-46%), arterial (-34%) and portal venous (-43%) amino acid concentrations and whole-body phenylalanine oxidation (-30%). The activities of carbamoylphosphate synthase-I (-45%), argininosuccinate synthase (-38%), argininosuccinate lyase (-23%), arginase (-27%), and glutaminase (-18%), but not of ornithine carbamoyltransferase, ornithine aminotransferase, or glutamate dehydrogenase were reduced in liver of ST-treated pigs. ST slightly increased intestinal activity of glutaminase (+9%) but did not affect that of any other enzymes. ST decreased hepatic, but increased jejunal, N-acetylglutamate (an essential allosteric activator of carbamoylphosphate synthase-I; -26% and +32%, respectively) and carbamoylphosphate (a substrate for ornithine carbamoyltransferase; -20% and +28%, respectively) content. These results demonstrate that the reduced amino acid catabolism with ST treatment in growing pigs involves a reduction in hepatic urea cycle enzyme activities. The effect of ST treatment on porcine urea cycle enzymes is tissue-specific and is associated with a reduction in substrate availability for hepatic ureagenesis.

Somatotropin increases protein balance independent of insulin's effects on protein metabolism in growing pigs

Somatotropin (ST) administration enhances protein deposition and elicits profound metabolic responses, including hyperinsulinemia. To determine whether the anabolic effect of ST is due to hyperinsulinemia, pair-fed weight-matched growing swine were treated with porcine ST (150 microg x kg body wt(-1) x day(-1)) or diluent for 7 days (n = 6/group, approximately 20 kg). Then pancreatic glucose-amino acid clamps were performed after an overnight fast. The objective was to reproduce the insulin levels of 1) fasted control and ST pigs (basal insulin, 5 microU/ml), 2) fed control pigs (low insulin, 20 microU/ml), and 3) fed ST pigs (high insulin, 50 microU/ml). Amino acid and glucose disposal rates were determined from the infusion rates necessary to maintain preclamp blood levels of these substrates. Whole body nonoxidative leucine disposal (NOLD), leucine appearance (R(a)), and leucine oxidation were determined with primed, continuous infusions of [(13)C]leucine and [(14)C]bicarbonate. ST treatment was associated with higher NOLD and protein balance and lower leucine oxidation and amino acid and glucose disposals. Insulin lowered R(a) and increased leucine oxidation, protein balance, and amino acid and glucose disposals. These effects of insulin were suppressed by ST treatment; however, the protein balance remained higher in ST pigs. The results show that ST treatment inhibits insulin's effects on protein metabolism and indicate that the stimulation of protein deposition by ST treatment is not mediated by insulin. Comparison of the protein metabolic responses to ST treatment during the basal fasting period with those in the fully fed state from a previous study suggests that the mechanism by which ST treatment enhances protein deposition is influenced by feeding status.

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.

The role of the somatotrophic axis in the metabolism of the chicken

As it is for mammalian species, growth hormone (GH) is indispensable for normal growth and development of avian species. In contrast to mammals, exogenous GH administration has little, if any, potential for improving the growth rate and feed efficiency of rapidly growing broilers; it is more likely to do so in older birds. This is at least partly because of age-related changes in tissue GH-binding activity and GH-receptor mRNA expression. The effects of GH on lipid deposition depends on the age of the bird and pattern of GH administration. Pulsatile, but not continuous, GH administration to older broilers seems to reduce fat deposition. As in rats, the bioactivity of GH might also depend on the pulse-induced cyclicity in GH receptors and GH-binding proteins. In chickens, GH is also a very potent lipolytic hormone, but seems to have no diabetogenic effect, which is reported in mammalian species. Both insulin-like growth factors have apparently no growth-promoting effects in normal growing broilers, but seem to have opposite effects on fat deposition. In contrast to GH, both insulin-like growth factors have a marked hypoglycemic effect. Whether all these effects are direct effects, or are mediated by secondary mechanisms, awaits further investigations.

Decreased glucose response to insulin is maximal within 24 hours of somatotropin injection in growing pigs

Porcine somatotropin (pST) administration reduces the sensitivity of pigs to insulin, and after several days of treatment, it alters baseline circulating concentrations of pST, insulin, glucose, and other metabolites. The length of time required to develop this reduction in whole-body insulin sensitivity has not been established. To investigate this, eight castrate pigs received daily injections of either recombinant pST (120 micrograms/kg BW) or exicipient. Intravenous insulin tolerance tests (1.0 microgram/kg BW) were done before the first pST injection (Day 0) and on Day 1, 3, 5, and 8 of pST treatment. Control animals had insulin tolerance tests on Day 0 and again on Day 8 of treatment. By Day 8, the glucose response to the insulin tolerance tests in pST-treated pigs was 45% of the response of the control group (P < 0.01). Glucose response areas between Day 1, 3, 5, and 8 of pST treatment were not different (P > 0.10). The maximum effect of pST on glucose response to insulin was achieved by Day 1. Therefore, factors responsible for the development of reduced whole-body insulin sensitivity in pST-treated pigs are fully expressed within 21 hr of the initial pST treatment.

The interaction between nutritional status and growth hormone in young cattle: differential responsiveness of fat and protein metabolism

The effect of dietary intake level on in vivo plasma leucine and plasma palmitate flux rates and on the response to a bolus injection of bovine growth hormone (GH) was investigated in six young steers. Animals were fed on a pelleted diet of dried grass-barley (0.7:0.3, w/w) in quantities sufficient to supply 0.8, 1.2, 1.6, 2.0, 2.4 or 2.65 x maintenance energy requirement, offered in hourly portions. Continuous intravenous infusions of [1-13C]leucine or [1-13C]palmitate were used to determine the flux of amino acid and fatty acid through the plasma pool before, immediately (1-3 h) after and 22-24 h after a subcutaneous injection of bovine GH (0.55 mg/kg body weight). Hourly blood samples were taken for 27 h to monitor the temporal responses of circulating hormones and metabolites following GH administration. The animal on the lowest plane of nutrition had elevated plasma GH and reduced insulin-like growth factor-1 concentrations compared with those fed on higher intake levels. Plasma leucine flux and leucine concentration increased with intake while palmitate flux and plasma non-esterified fatty acid (NEFA) concentrations were inversely related to intake. Leucine flux rate decreased in the animals fed on the two highest intake levels in response to GH 22-24 h after administration, but plasma leucine concentrations were reduced in all animals at this time. Only the animal fed on the lowest intake level showed an immediate response to GH (within 3 h of administration) with increased palmitate flux and plasma NEFA concentrations but a lipolytic response was apparent in other animals 22-24 h post-administration although the magnitude of the response was markedly reduced at high intakes. We conclude that lipid and protein metabolism are differentially responsive to GH and nutritional status.

Nutrition, development and efficacy of growth modifiers in livestock species

Somatotropin (ST) and synthetic beta-adrenergic agonists (beta-AA) are growth-modifying agents that increase the rate and sometimes, the efficiency of protein deposition in lean tissues of livestock species. The ST-induced increase in muscle protein deposition is effected by a relatively modest increase in protein synthetic rate. This is possibly mediated by the endocrine influence of marked increases in circulating IGF (insulin-like growth factor)-I, and other ST-dependent components of the IGF system; mediation by locally expressed IGF-I may also occur. Increased muscle protein accretion in animals treated with beta-AA seems to be directly mediated by binding of the synthetic agonist to muscle beta-1 or beta-2 receptors, leading to increased muscle protein synthesis, possibly accompanied or followed by decreased protein degradation. This response is transient, due to down-regulation of beta-adrenergic receptors. Maximal responses of muscle protein accretion to both ST and beta-AA are attenuated by feeding inadequate levels of total protein or specific, limiting amino acids. For ST, but not beta-AA, this effect in growing pigs is partially offset by increased efficiency of utilization of absorbed amino acids for protein deposition, with predictable consequences for dietary protein and amino acid requirements. Both ST and beta-AA are less efficacious in promoting muscle protein deposition in very young animals. For ST, this is related to postnatal development of the somatotropic axis; a mechanistic explanation for the similar lack of effect of beta-AA is lacking. In both cases, this phenomenon must be considered against the very high inherent capacity and efficiency of lean tissue protein accretion in the neonate.

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.

Enhancement of the insulin antagonistic effect of porcine somatotropin in swine using a monoclonal antibody

A monoclonal antibody (mAb), PS-7.6, to porcine somatotropin (pST) significantly enhanced the growth responses to pST injections in hypophysectomized (hypox) rats but could not be tested in pigs because of the large quantity of antibody required for a growth trial. Because pST inhibits the hypoglycemic effects of insulin, an insulin tolerance test procedure was established to measure pST activity in jugular-catheterized pigs. Doses of 0, 30, 100, and 300 micrograms/kg per day of pST were split and administered subcutaneously (sc) in equal portions twice daily for 2 d. After a 17-hr fast, plasma samples were obtained at 10-min intervals for 30 min before an intravenous injection of insulin (0.08 IU/kg) and then for an additional 50 min. Because pST increased fasting plasma glucose concentrations, preinsulin glucose values were used as a covariate to adjust the postinsulin concentrations. pST caused a dose-dependent increase in resistance to the insulin injection in these pigs. The areas under the curves (AUC), for plasma glucose were 22.1, 29.0, 39.0, and 47.2 mg/dl per min for the 0, 30, 100, and 300 micrograms/kg pST doses, respectively. Because different doses of pST could be detected, the PS-7.6 enhancement of pST treatment was evaluated. In the first experiment, five pigs/group each received sc injections of either vehicle, pST (75 micrograms/kg; approximately 3.0 mg/d), pST (75 micrograms/kg) + PS-7.6 at 3.75 mg/kg, or pST (75 micrograms/kg) + PS-7.6 at 15 mg/kg for 2 d before the insulin test. The pST and PS-7.6 were combined and incubated for at least 1 hr at room temperature before being injected. The injection of pST alone did not significantly change insulin tolerance activity (23.1 vs. 21.1, AUC), but insulin resistance was enhanced when this dose of pST also included PS-7.6 (27.4 and 29.5, AUC, respectively; P < 0.05). In a second experiment, the effects of PS-7.6 and PS-4.2, a mAb that did not potentiate the pST-stimulated growth of hypox rats, were compared. The five pigs/treatment received either vehicle, pST (75 micrograms/kg), pST (75 micrograms/kg) + PS-7.6 (3.75 mg/kg), or pST (75 micrograms/kg) + PS-4.2 (3.75 mg/kg) for 2 d. The administration of pST increased the resistance to insulin (26.7 vs. 18.8, AUC; P < 0.01), which was markedly potentiated by PS-7.6 (54.3, AUC, P < 0.001) but not affected by PS-4.2 (27.6 AUC). The injection of PS-7.6 at 7.5 mg/kg without exogenous pST did not alter the sensitivity to insulin. These results indicate that PS-7.6, but not PS-4.2, enhanced the insulin antagonistic activity of pST in swine, suggesting that an enhancement of pST-stimulated growth would also occur in PS-7.6-treated pigs.

Effect of porcine somatotropin on the response of growing pigs to acute challenges of glucose, insulin and epinephrine and during a hyperinsulinemic-euglycemic clamp

Response of tissues to homeostatic signals may play a role in the mediation of nutrient partitioning effects of somatotropin. To investigate this, the effects of exogenous porcine somatotropin (pST) on the metabolic responses to a series of intravenous challenges with dextrose, insulin and epinephrine were examined in twelve crossbred barrows (65 kg). In addition, the hyperinsulinemic-euglycemic clamp technique was used to further explore effects of pST on insulin resistance in eight of these animals. Pigs received daily sc injections of either pituitary-derived pST (120 micrograms/kg bw) or an equivalent volume of excipient for 28 d. Treatment with pST resulted in a chronic elevation of plasma glucose, insulin and non-esterified fatty acid concentrations and lowered glucagon concentrations. Acute iv challenges of dextrose (100 mg/kg bw), insulin (1.0 micrograms/kg bw), and epinephrine (2.2 micrograms/kg bw) were administered on days 21, 22, and 23 of the treatment period, respectively. Hyperinsulinemic-euglycemic clamps were carried out on day 28. Effects of pST were most dramatic for responses associated with insulin. In pST-treated pigs, insulin response to dextrose infusion was enhanced, while glucose response to insulin was attenuated and glucose clearance rate was reduced. During the hyperinsulinemic-euglycemic clamp, dextrose infusion rate required to maintain euglycemia during physiologic elevations of insulin was reduced in pST-treated pigs to 28% of control. In pST-treated pigs, glucose response to epinephrine challenge was halved, while insulin response was increased three-fold. Therefore, one mechanism by which pST shifts the nutrient partition is by altering metabolic responses to homeostatic signals. In growing pigs, this is especially evident for glucose response to insulin.