Growth hormone increases whole-body protein turnover in growing pigs

Effects of porcine somatotropin administration on the responses to dietary lysine and a near-ideal blend of amino acids on the amino acid composition of whole-body protein and amino acid accretion rate in growing pigs

An experiment was conducted to assess the effects of porcine somatotropin (pST) on the responses to a near-ideal blend of AA on the AA composition of empty, whole-empty body (WEB) protein and WEB essential AA accretion rate in pigs from 22 to 60 kg BW. Forty Hampshire × Yorkshire gilts were individually penned and assigned to a 4 × 2 factorial arrangement of treatments consisting of four diets with and without pST injection. A fortified corn-soybean meal basal diet was formulated to contain 1.50% total Lys with Thr, Met, and Trp added to obtain a near-ideal blend of these AA relative to Lys. In three additional diets, Lys was reduced to 1.25%, 1.00%, and 0.75% by diluting the basal diet with cornstarch, cellulose, and sand such that the diets also contained the same ratios of AA. Pigs that received pST were administered a daily i.m. injection of 2 mg of pST. At 60 kg BW, the WEB (carcass, head, viscera, blood, nails, and hair) was ground and analyzed for proximate and AA composition. Administration of pST increased (P < 0.001) accretion rates of WEB protein and essential AA. Increasing dietary essential AA increased (quadratic, P < 0.03) accretion rate of WEB protein, His, Leu, Trp, and Val in pST-treated pigs, but not in untreated pigs. Lysine composition in the accreted WEB protein was not affected (P > 0.05) by dietary Lys. The efficiency of Lys utilization for WEB Lys accretion was linearly affected (P < 0.01) by dietary Lys. These results indicated that the dietary Lys needed to achieve maximum WEB Lys accretion is markedly increased by pST administration.

Effects of Dietary Coated Lysozyme on the Growth Performance, Antioxidant Activity, Immunity and Gut Health of Weaned Piglets

The purpose of this study was to evaluate the effects of dietary coated lysozyme on growth performance, serum biochemical indexes, antioxidant activity, digestive enzyme activity, intestinal permeability, and the cecal microbiota in weaned piglets. In total, 144 weaned Large White × Landrace piglets were divided into six treatment groups, with 3 replicates and 8 piglets per replicate: CN, a basal diet; CL-L, CL-M, and CL-H, basal diet supplemented with 100, 150, 500 mg/kg coated lysozyme; UL, basal diet supplemented with 150 mg/kg lysozyme; and Abs, basal diet supplemented with 150 mg/kg guitaromycin for 6 weeks. Compared with the CN and UL diets, dietary CL-H inclusion increased the average daily gain (ADG) and decreased the feed/gain (F/G) ratio of piglets (p < 0.05). The addition of 500 mg/kg coated lysozyme to the diet significantly increased the total protein (TP) and globulin (Glob) plasma levels of weaned piglets (p < 0.05). Supplementation with 500 mg/kg coated lysozyme significantly increased the serum IgM concentration and increased lipase activity in the duodenum (p < 0.05). The addition of coated lysozyme and lysozyme significantly decreased the malondialdehyde (MDA) content, while the superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and total antioxidant capacity (T-AOC) levels all increased (p < 0.05). High-throughput sequencing results showed that CL-H treatment effectively improved the intestinal microbiome. The relative abundance of Terrisporobacter in the CL-H and CL-M groups was significantly lower than that in the other groups (p < 0.05). LEfSe analysis results showed that the relative abundance of Coprococcus_3 was higher in the CL-M treatment group. The marker species added to the CL-H treatment group was Anaerofilum. In summary, as a potential substitute for feed antibiotics, lysozyme is directly used as a dietary additive, which is inefficient. Therefore, we used palm oil as the main coating material to coat lysozyme. Lysozyme after coating can more effectively improve the growth performance of piglets by improving the intestinal flora, improving the activity of digestive enzymes, reducing the damage to intestinal permeability and oxidative stress in piglets caused by weaning stress, and improving the immunity of piglets.

Nutritional principles, integration, modelling and research management to practical applications: an overview of John Langtree Black’s contribution to animal science

This is the introductory paper for a Festschrift to honour the contribution that John Langtree Black has made so far to animal science over his 50-year career. The paper outlines the extraordinary wide range of topics, disciplines and animal species covered by his research and intellectual thinking. It describes major contributions made to enhancing knowledge in areas of animal science and to the application of knowledge to rural industries through simulation models, risk-control management systems, methods for measuring ingredient and product quality and communication. An important contribution has been through the management, coordination and integration of findings in large interdisciplinary research programs, particularly with an emphasis on adherence to the scientific method and the role of statistics. The discipline of scientific integrity has been extended to his considerable involvement in community service.

The impact of growth promoters on muscle growth and the potential consequences for meat quality

To meet the demands of increased global meat consumption, animal production systems will have to become more efficient, or at least maintain the current efficiency utilizing feed ingredients that are not also used for human consumption. Use of growth promoters is a potential option for increasing production animal feed efficiency and increased muscle growth. The objective of this manuscript is to describe the mechanisms by which the growth promoters, beta-adrenergic agonists and growth hormone, mediate their effects, with specific consideration of the aspects which have implications for meat quality.

Lysine nutrition in swine and the related monogastric animals: muscle protein biosynthesis and beyond

Improving feed efficiency of pigs with dietary application of amino acids (AAs) is becoming increasingly important because this practice can not only secure the plasma AA supply for muscle growth but also protect the environment from nitrogen discharge with feces and urine. Lysine, the first limiting AA in typical swine diets, is a substrate for generating body proteins, peptides, and non-peptide molecules, while excess lysine is catabolized as an energy source. From a regulatory standpoint, lysine is at the top level in controlling AA metabolism, and lysine can also affect the metabolism of other nutrients. The effect of lysine on hormone production and activities is reflected by the change of plasma concentrations of insulin and insulin-like growth factor 1. Lysine residues in peptides are important sites for protein post-translational modification involved in epigenetic regulation of gene expression. An inborn error of a cationic AA transporter in humans can lead to a lysinuric protein intolerance condition. Dietary deficiency of lysine will impair animal immunity and elevate animal susceptibility to infectious diseases. Because lysine deficiency has negative impact on animal health and growth performance and it appears that dietary lysine is non-toxic even at a high dose of supplementation, nutritional emphasis should be put on lysine supplementation to avoid its deficiency rather than toxicity. Improvement of muscle growth of monogastric animals such as pigs via dietary lysine supply may be due to a greater increase in protein synthesis rather than a decrease in protein degradation. Nevertheless, the underlying metabolic and molecular mechanisms regarding lysine effect on muscle protein accretion merits further clarification. Future research undertaken to fully elucidate the metabolic and regulatory mechanisms of lysine nutrition could provide a sound scientific foundation necessary for developing novel nutritional strategies to enhance the muscle growth and development of meat animals.

Effects of porcine somatotropin administration on the responses to dietary lysine and a near-ideal blend of amino acids for growing pigs

Two experiments were conducted to assess the effects of porcine ST (pST) on the responses to a near-ideal blend of AA for pigs from 22 to 60 kg BW. Eighty Hampshire × Yorkshire gilts (40 gilts/experiment) were individually penned and assigned to a 4 × 2 factorial arrangement of treatments, consisting of 4 diets with and without pST injection. A fortified corn-soybean meal basal diet was formulated to contain 1.50% total Lys and Thr, Met, and Trp were added to obtain a near-ideal blend of these AA relative to Lys. In 3 additional diets, Lys was reduced to 1.25%, 1.00%, or 0.75% by diluting the basal diet with cornstarch, cellulose, and sand, such that the diets also contained the same ratios of AA. Pigs that received pST were administered a daily intramuscular injection of 2 mg of pST. Data from the 2 experiments were pooled. Administration of pST increased ADG (P < 0.01), G:F (P < 0.01), and LM area (P < 0.01), and decreased ADFI (P < 0.03), last rib backfat (P < 0.01), and 10th rib backfat (P < 0.01). Also, estimated carcass muscle and calculated lean gain increased (P < 0.01) in pST-treated pigs. Administration of pST also increased (P < 0.01) the percentage, total gain and accretion rate of water, protein, and ash in the carcass, and decreased (P < 0.01) the percentage, total gain, and accretion rate of carcass fat. Growth rate, G:F, and carcass traits improved (P < 0.01), percentage of carcass protein and water increased (P < 0.01), and carcass fat percentage decreased (P < 0.01) with increasing dietary Lys. The percentage, total gain, and accretion rate of carcass protein increased to a greater extent in pST-treated pigs than in untreated pigs, resulting in a pST × Lys interaction (P < 0.05). The results indicated that pST improves performance, leanness, and protein accretion in pigs from 22 to 60 kg BW, and that these responses to dietary Lys and a near-ideal blend of AA is greater in growing pigs treated with pST than untreated pigs.

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.

Effects of dietary factors and other metabolic modifiers on quality and nutritional value of meat

A number of technologies that increase feed efficiency and lean tissue deposition while decreasing fat deposition have been developed in an effort to improve profitability of animal production. In general, the mode of action of these metabolic modifiers is to increase muscle deposition while often simultaneously reducing fat deposition. However, there have been some concerns that the focus on increasing production efficiency and lean meat yield has been to the detriment of meat quality. The aim of this review is to collate data on the effects of these metabolic modifiers on meat quality, and then discuss these overall effects. When data from the literature are collated and subject to meta-analyses it appears that conservative use of each of these technologies will result in a 5-10% (0.3-0.5kg) increase in shear force with a similar reduction in perception of tenderness. However, it should be borne in mind that the magnitude of these increases are similar to those observed with similar increases in carcass leanness obtained through other means (e.g. nutritional, genetic selection) and may be an inherent consequence of the production of leaner meat. To counter this, there are some other metabolic factors and dietary additives that offer some potential to improve meat quality (for example immuncastration) and it is possible that these can be used on their own or in conjunction with somatotropin, approved β-agonists, anabolic implants and CLA to maintain or improve meat quality.

Modeling homeorhetic and homeostatic controls of pig growth

A dynamic mechanistic model of homeorhetic and homeostatic controls of pig growth was developed. The homeorhetic principles were based on changes in time of fractional rates of anabolism and catabolism of tissues. A minimum number of homeostatic principles integrated current data on plasma kinetics and the partitioning of nutrients between anabolism and catabolism of body tissues, and endogenous losses with integument and into the gut. The major features of the model are two levels of organization (tissue and plasma) and three body tissues (carcass proteins, visceral proteins, and body lipids). The protein tissues and plasma amino acids were subdivided into lysine, methionine and cystine, threonine, tryptophan, other essential AA, and nonessential AA compartments. Plasma glucose and fatty acids were also considered. Adenosine triphosphate and adenosine diphosphate were used to represent energy transformations, although these energy transformations were not included in the homeostatic control of pig growth. The mass variations within each of the 23 basic compartments were described with a specific deterministic, dynamic differential equation. The simulated metabolic rates of the protein and lipid tissues were similar to published data. The principal outputs from the model (protein and lipid gain, body weight, chemical body constituents, plasma parameters) showed that the proposed homeorhetic and homeostatic controls provide a mechanistic approach to modeling growth.

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.

Protein (lysine) restriction in primiparous lactating sows: effects on metabolic state, somatotropic axis, and reproductive performance after weaning

Low protein intake during lactation has been demonstrated to increase the loss of body protein and to reduce the reproductive performance of female pigs. The objectives of the current experiment were 1) to determine whether protein (lysine) restriction alters levels of somatotropic hormones, insulin, follicle-stimulating hormone, and leptin around weaning, and 2) to evaluate the relationships between these eventual alterations and postweaning reproductive performance. One day after farrowing, crossbred primiparous sows were randomly allocated to one of two diets containing 20% crude protein and 1.08% lysine (C, n = 12) or 10% crude protein and 0.50% lysine (L, n = 14) during a 28-d lactation. Diets provided similar amounts of metabolizable energy (3.1 Mcal/kg). Feed allowance was restricted to 4.2 kg/d throughout lactation, and litter size was standardized to 10 per sow within 5 d after farrowing. Catheters were fitted in the jugular vein of 21 sows around d 22 of lactation. Serial blood samples were collected 1 d before (day W - 1) and 1 d after (day W + 1) weaning, and single blood samples were collected daily from weaning until d 6 postweaning (day W + 6). Sows were monitored for estrus and inseminated. They were slaughtered at d 30 of gestation. During lactation, litter weight gain was similar among treatment groups. Reduced protein intake increased (P < 0.001) sow weight loss (-30 vs -19 kg) and estimated protein mobilization throughout lactation (-4.1 vs -2.0 kg). On day W - 1, L sows had higher (P < 0.02) plasma glutamine and alanine concentrations, but lower (P < 0.05) plasma tryptophan and urea than C sows. Mean and basal plasma GH were higher (P < 0.001), whereas plasma IGF-I and mean insulin were lower in L than in C sows on day W - 1. Preprandial leptin did not differ between treatments on day W - 1, but was higher (P < 0.01) in L sows than in C sows on day W + 1. Mean FSH concentrations were similar in both treatments on day W - 1 (1.3 ng/mL), but L sows had greater (P < 0.001) mean FSH on day W + 1 than C sows (1.6 vs 1.2 ng/mL). The weaning-to-estrus interval (5 +/- 1 d) was similar in both groups. Ovulation rate was lower in L than in C sows (20.0 +/- 1 vs 23.4 +/- 1, P < 0.05). No obvious relationships between reproductive traits and metabolic hormone data were observed. In conclusion, these results provide evidence that protein (lysine) restriction throughout lactation alters circulating concentrations of somatotropic hormones and insulin at the end of lactation and has a negative impact on postweaning ovulation rate.

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.

Growth hormone and insulin-like growth factor-I therapy promote protein deposition and growth in dexamethasone-treated piglets

BACKGROUND

Dexamethasone treatment facilitates the weaning of premature infants from mechanical ventilation but impairs protein homeostasis, lean tissue deposition, and growth. The current study was conducted to investigate whether dexamethasone mediates these effects by reducing protein synthesis or elevating protein breakdown, and whether adjuvant growth hormone+/-insulin-like growth factor-I therapy can attenuate such effects.

METHODS

Piglets (n = 24) were randomized to placebo, a tapered course of dexamethasone (0.5, 0.3, 0.2 mg/kg per day for 5, 5 and 4 days each, respectively), dexamethasone + growth hormone 0.1 mg/kg per day, or dexamethasone + growth hormone + insulin-like growth factor-I 0.1 mg/kg per day for 14 days. On day 13, 15N-glycine was administered as a single oral dose, and urine was collected at timed intervals during the subsequent 48 hours.

RESULTS

Total urinary N and cumulative 15N excretion were higher in all dexamethasone groups than in control subjects. Protein synthesis was suppressed, whereas protein breakdown was unaltered by dexamethasone. Adjunctive growth hormone+/-insulin-like growth factor-I therapy enhanced protein synthesis, but only combined therapy improved net protein gain compared with dexamethasone alone. Higher circulating insulin-like growth factor-I may have mediated the greater net protein gain. Blood urea nitrogen was elevated in all dexamethasone-treated groups at days 6 and 11 but was normalized by day 15 with adjunctive growth hormone+/-insulin-like growth factor-I. From a functional perspective, both adjunctive growth hormone and growth hormone+/-insulin-like growth factor-I partially attenuated the dexamethasone-induced reduction in weight and length gain but not in whole body lean and fat mass.

CONCLUSION

Adjunctive growth hormone+/-insulin-like growth factor-I therapy partially reverses the dexamethasone-induced reduction in protein synthesis, resulting in improved growth when given concurrently with a low tapering dose of dexamethasone.

Rates of lysine catabolism are inversely related to rates of protein synthesis when measured concurrently in adult female rats induced to grow at different rates

To test the effect of changes in the rate of protein synthesis on amino acid oxidation, both were studied concurrently in individual 200-g female Sprague-Dawley rats. In a growth trial (Experiment 1), recombinant bovine somatotropin (rbST) was injected subcutaneously (0, 2 or 12 mg/d) over 6 d (n = 4 rats per rbST level). Weight gain increased with rbST level (P < 0.01); 1.96 +/- 0.8, 4.24 +/- 0.8 and 8.67 +/- 0.8 g/d, respectively. After treatment with rbST (0 or 12 mg/d) for 4 d (Experiment 2), rats were injected via a tail vein catheter with valine (400 mmol, 4.07 mBq L-[3,4(n)-3H]valine) at 0, 4, 10, 13 or 16 h after the daily rbST injection and killed 20 min later. This flooding dose was 5 to 6 times, not 10 times, the free pool as hoped. Protein synthesis in rbST-treated rats increased 46% in muscle (P < 0.001) and 36% in liver (P < 0.01). The ks was unaltered with time after rbST injection (0-16 h, P > 0.05). When 600 mmol valine (4.4 mBq L-[3,4(n)-3H]valine) was used in Experiment 3, specific activity (SA) of free valine was constant over 20 min and was 94 +/- 4% of that injected. Finally, in Experiment 4, protein synthesis and amino acid oxidation rates measured in the same rat revealed a 35% increase (P < 0.01) in protein synthesis in hind leg muscle and a 29% increase in liver (P < 0.05) from rbST-injected (12 mg/d) rats (n = 6). Lysine oxidation was estimated by continuous (12 h) infusion of L-[1-14C]lysine via the opposite tail vein catheter. Expired CO2 was collected over 20-min intervals and SA at plateau was estimated by fitting an exponential model. Lysine oxidation was reduced (P < 0.05) by 44% in rbST-treated rats. The idea that an increase in protein synthesis results in decreased amino acid oxidation remains tenable.

Biology of somatotropin in growth and lactation of domestic animals

Impressive progress has been made during the past 15 years in our understanding of the biology of somatotropin (ST) in domestic animals. In part, this progress was sparked by advances in biotechnology that made feasible the production of large quantities of recombinant bovine ST (bST) and porcine ST (pST). The availability of recombinant bST and pST resulted in an exponential increase in investigations that explored their role in growth and lactation biology, as well as evaluated their potential for commercial use. Collectively, these studies established that administration of bST to lactating dairy cows increased milk yield, and treatment of growing pigs with pST markedly stimulated muscle growth and reduced fat deposition. In addition to these "efficacy" studies, a substantial number of investigations examined the mechanisms by which ST affects lactation and growth of domestic animals. This review summarizes the diverse physiological effects ST has on growth and lactation and discusses the underlying mechanisms that mediate these effects in domestic animals.