Pulsatile delivery of a leucine supplement during long-term continuous enteral feeding enhances lean growth in term neonatal pigs

Pulsatile Leucine Administration during Continuous Enteral Feeding Enhances Skeletal Muscle Mechanistic Target of Rapamycin Complex 1 Signaling and Protein Synthesis in a Preterm Piglet Model

BACKGROUND

Continuous feeding does not elicit an optimal anabolic response in skeletal muscle but is required for some preterm infants. We reported previously that intermittent intravenous pulses of leucine (Leu; 800 μmol Leu·kg-1·h-1 every 4 h) to continuously fed pigs born at term promoted mechanistic target of rapamycin complex 1 (mTORC1) activation and protein synthesis in skeletal muscle.

OBJECTIVES

The aim was to determine the extent to which intravenous Leu pulses activate mTORC1 and enhance protein synthesis in the skeletal muscle of continuously fed pigs born preterm.

METHODS

Pigs delivered 10 d preterm was advanced to full oral feeding >4 d and then assigned to 1 of the following 4 treatments for 28 h: 1) ALA (continuous feeding; pulsed with 800 μmol alanine·kg-1·h-1 every 4 h; n = 8); 2) L1× (continuous feeding; pulsed with 800 μmol Leu·kg-1·h-1 every 4 h; n = 7); 3) L2× (continuous feeding; pulsed with 1600 μmol Leu·kg-1·h-1 every 4 h; n = 8); and 4) INT (intermittent feeding every 4 h; supplied with 800 μmol alanine·kg-1 per feeding; n = 7). Muscle protein synthesis rates were determined with L-[2H5-ring]Phenylalanine. The activation of insulin, amino acid, and translation initiation signaling pathways were assessed by Western blot.

RESULTS

Peak plasma Leu concentrations were 134% and 420% greater in the L2× compared to the L1× and ALA groups, respectively (P < 0.01). Protein synthesis was greater in the L2× than in the ALA and L1× groups in both the longissimus dorsi and gastrocnemius muscles (P < 0.05) but not different from the INT group (P > 0.10). Amino acid signaling upstream and translation initiation signaling downstream of mTORC1 largely corresponded to the differences in protein synthesis.

CONCLUSIONS

Intravenous Leu pulses potentiate mTORC1 activity and protein synthesis in the skeletal muscles of continuously fed preterm pigs, but the amount required is greater than in pigs born at term.

Branched-chain amino acid supplementation does not enhance lean tissue accretion in low birth weight neonatal pigs, despite lower Sestrin2 expression in skeletal muscle

Postnatal muscle growth is impaired in low birth weight (L) neonatal pigs. Leucine supplementation has been established as a dietary intervention to enhance muscle growth in growing animals. The aim of this study was to investigate the efficacy of supplementing L neonatal pig formulas with branched-chain amino acids (B) to enhance the rate of protein accretion. Twenty-four 3-day old pigs were divided into two groups low (L) and normal birth weight (N) based on weight at birth. Pigs were assigned to a control (C) or 1% branched-chain amino acids (B) formulas, and fed at 250 mL·kg body weight -1·d-1 for 28 d. Body weight of pigs in the L group was less than those in the N group (P < 0.01). However, fractional body weight was greater for L pigs compared with their N siblings from day 24 to 28 of feeding regardless of formula (P < 0.01). In addition, feed efficiency (P < 0.0001) and efficiently of protein accretion (P < 0.0001) were greater for L than N pigs regardless of supplementation. Pigs fed the B formula had greater plasma leucine, isoleucine, and valine concentrations compared with those fed the C formula (P < 0.05). Longissimus dorsi Sestrin2 protein expression was less for pigs in the L group compared with those in the N group (P < 0.01), but did not result in a corresponding increase in translation initiation signaling. Longissimus dorsi mRNA expression of BCAT2 was less for LB pigs compared with those in the LC group, and was intermediate for NC and NB pigs (P < 0.05). Hepatic mRNA expression of BCKDHA was greater for pigs in the L compared with those in the N groups (P < 0.05). However, plasma branched-chain keto-acid concentration was reduced for C compared with those in the B group (P < 0.05). These data suggest that branched-chain amino acid supplementation does not improve lean tissue accretion of low and normal birth weight pigs, despite a reduction in Sestrin2 expression in skeletal muscle of low birth weight pigs. The modest improvement in fractional growth rate of low birth weight pigs compared with their normal birth weight siblings was likely due to a more efficient dietary protein utilization.

The use of functional amino acids in different categories of pigs - A review

The present review deals with a particularly important topic: the use of functional amino acids in different categories of pigs. It is especially relevant in the context of the current efforts to reduce the use of antibiotics in pig farming and the search for possible alternatives to replace them. The review is based on the definition that functional amino acids (FAAs) are classified as dispensable amino acids, but with additional biological functions, i.e., not only are they used for protein formation, but they are also involved in regulating essential metabolic pathways to improve health, survival, growth, and development. We describe the mechanism of action of individual FAAs and their potential use in pigs, including glutamate, glutamine, arginine, branched-chain amino acids (i.e., leucine, isoleucine, and valine), tryptophan and glycine. The work is divided into three parts. The first part deals with the FAAs and their role in the overall health of sows and their offspring. The second part describes the use of functional amino acids in piglets after weaning. Part three examines the use of functional amino acids in growing and fattening pigs and their impact on meat quality.

Regulation of skeletal muscle protein synthesis in the preterm pig by intermittent leucine pulses during continuous parenteral feeding

BACKGROUND

Extrauterine growth restriction is a common complication of preterm birth. Leucine (Leu) is an agonist for the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) signaling pathway that regulates translation initiation and protein synthesis in skeletal muscle. Previously, we showed that intermittent intravenous pulses of Leu to neonatal pigs born at term receiving continuous enteral nutrition increases muscle protein synthesis and lean mass accretion. Our objective was to determine the impact of intermittent intravenous pulses of Leu on muscle protein anabolism in preterm neonatal pigs administered continuous parenteral nutrition.

METHODS

Following preterm delivery (on day 105 of 115 gestation), pigs were fitted with umbilical artery and jugular vein catheters and provided continuous parenteral nutrition. Four days after birth, pigs were assigned to receive intermittent Leu (1600 µmol kg-1  h-1 ; n = 8) or alanine (1600 µmol kg-1  h-1 ; n = 8) parenteral pulses every 4 h for 28 h. Anabolic signaling and fractional protein synthesis were determined in skeletal muscle.

RESULTS

Leu concentration in the longissimus dorsi and gastrocnemius muscles increased in the leucine (LEU) group compared with the alanine (ALA) group (P < 0.0001). Despite the Leu-induced disruption of the Sestrin2·GATOR2 complex, which inhibits mTORC1 activation, in these muscles (P < 0.01), the abundance of mTOR·RagA and mTOR·RagC was not different. Accordingly, mTORC1-dependent activation of 4EBP1, S6K1, eIF4E·eIF4G, and protein synthesis were not different in any muscle between the LEU and ALA groups.

CONCLUSION

Intermittent pulses of Leu do not enhance muscle protein anabolism in preterm pigs supplied continuous parenteral nutrition.

Chronic Fetal Leucine Infusion Increases Rate of Leucine Oxidation but Not of Protein Synthesis in Late Gestation Fetal Sheep

BACKGROUND

Leucine increases protein synthesis rates in postnatal animals and adults. Whether supplemental leucine has similar effects in the fetus has not been determined.

OBJECTIVE

To determine the effect of a chronic leucine infusion on whole-body leucine oxidation and protein metabolic rates, muscle mass, and regulators of muscle protein synthesis in late gestation fetal sheep.

METHODS

Catheterized fetal sheep at ∼126 d of gestation (term = 147 d) received infusions of saline (CON, n = 11) or leucine (LEU; n = 9) adjusted to increase fetal plasma leucine concentrations by 50%-100% for 9 d. Umbilical substrate net uptake rates and protein metabolic rates were determined using a 1-13C leucine tracer. Myofiber myosin heavy chain (MHC) type and area, expression of amino acid transporters, and abundance of protein synthesis regulators were measured in fetal skeletal muscle. Groups were compared using unpaired t tests.

RESULTS

Plasma leucine concentrations were 75% higher in LEU fetuses compared with CON by the end of the infusion period (P < 0.0001). Umbilical blood flow and uptake rates of most amino acids, lactate, and oxygen were similar between groups. Fetal whole-body leucine oxidation was 90% higher in LEU (P < 0.0005) but protein synthesis and breakdown rates were similar. Fetal and muscle weights and myofiber areas were similar between groups, however, there were fewer MHC type IIa fibers (P < 0.05), greater mRNA expression levels of amino acid transporters (P < 0.01), and a higher abundance of signaling proteins that regulate protein synthesis (P < 0.05) in muscle from LEU fetuses.

CONCLUSIONS

A direct leucine infusion for 9 d in late gestation fetal sheep does not increase protein synthesis rates but results in higher leucine oxidation rates and fewer glycolytic myofibers. Increasing leucine concentrations in the fetus stimulates its own oxidation but also increases amino acid transporter expression and primes protein synthetic pathways in skeletal muscle.

Management and Feeding Strategies in Early Life to Increase Piglet Performance and Welfare around Weaning: A Review

The performance of piglets in nurseries may vary depending on body weight, age at weaning, management, and pathogenic load in the pig facilities. The early events in a pig's life are very important and may have long lasting consequences, since growth lag involves a significant cost to the system due to reduced market weights and increased barn occupancy. The present review evidences that there are several strategies that can be used to improve the performance and welfare of pigs at weaning. A complex set of early management and dietary strategies have been explored in sows and suckling piglets for achieving optimum and efficient growth of piglets after weaning. The management strategies studied to improve development and animal welfare include: (1) improving sow housing during gestation, (2) reducing pain during farrowing, (3) facilitating an early and sufficient colostrum intake, (4) promoting an early social interaction between litters, and (5) providing complementary feed during lactation. Dietary strategies for sows and suckling piglets aim to: (1) enhance fetal growth (arginine, folate, betaine, vitamin B12, carnitine, chromium, and zinc), (2) increase colostrum and milk production (DL-methionine, DL-2-hydroxy-4-methylthiobutanoic acid, arginine, L-carnitine, tryptophan, valine, vitamin E, and phytogenic actives), (3) modulate sows' oxidative and inflammation status (polyunsaturated fatty acids, vitamin E, selenium, phytogenic actives, and spray dried plasma), (4) allow early microbial colonization (probiotics), or (5) supply conditionally essential nutrients (nucleotides, glutamate, glutamine, threonine, and tryptophan).

Chronic Fetal Leucine Infusion Does Not Potentiate Glucose-Stimulated Insulin Secretion or Affect Pancreatic Islet Development in Late-Gestation Growth-Restricted Fetal Sheep

BACKGROUND

Growth-restricted fetuses have attenuated glucose-stimulated insulin secretion (GSIS), smaller pancreatic islets, less pancreatic β-cells, and less pancreatic vascularization compared with normally growing fetuses. Infusion of leucine into normal late-gestation fetal sheep potentiates GSIS, as well as increases pancreatic islet size, the proportion of the pancreas and islet comprising β-cells, and pancreatic and islet vascularity. In addition, leucine stimulates hepatocyte growth factor (HGF ) mRNA expression in islet endothelial cells isolated from normal fetal sheep.

OBJECTIVE

We hypothesized that a 9-d leucine infusion would potentiate GSIS and increase pancreatic islet size, β-cells, and vascularity in intrauterine fetal growth restriction (IUGR) fetal sheep. We also hypothesized that leucine would stimulate HGF mRNA in islet endothelial cells isolated from IUGR fetal sheep.

METHODS

Late-gestation Columbia-Rambouillet IUGR fetal sheep (singleton or twin) underwent surgeries to place vascular sampling and infusion catheters. Fetuses were randomly allocated to receive a 9-d leucine infusion to achieve a 50-100% increase in leucine concentrations or a control saline infusion. GSIS was measured and pancreas tissue was processed for histologic analysis. Pancreatic islet endothelial cells were isolated from IUGR fetal sheep and incubated with supplemental leucine. Data were analyzed by mixed-models ANOVA; Student, Mann-Whitney, or a paired t test; or a test of equality of proportions.

RESULTS

Chronic leucine infusion in IUGR fetuses did not affect GSIS, islet size, the proportion of the pancreas comprising β-cells, or pancreatic or pancreatic islet vascularity. In isolated islet endothelial cells from IUGR fetuses, HGF mRNA expression was not affected by supplemental leucine.

CONCLUSIONS

IUGR fetal sheep islets are not responsive to a 9-d leucine infusion with respect to insulin secretion or any histologic features measured. This is in contrast to the response in normally growing fetuses. These results are important when considering nutritional strategies to prevent the adverse islet and β-cell consequences in IUGR fetuses.

Intermittent leucine pulses during continuous feeding alters novel components involved in skeletal muscle growth of neonatal pigs

When neonatal pigs continuously fed formula are supplemented with leucine pulses, muscle protein synthesis and body weight gain are enhanced. To identify the responsible mechanisms, we combined plasma metabolomic analysis with transcriptome expression of the transcriptome and protein catabolic pathways in skeletal muscle. Piglets (n = 23, 7-day-old) were fed continuously a milk replacement formula via orogastric tube for 21 days with an additional parenteral infusion (800 μmol kg^-1 h^-1) of either leucine (LEU) or alanine (CON) for 1 h every 4 h. Plasma metabolites were measured by liquid chromatography-mass spectrometry. Gene and protein expression analyses of longissimus dorsi muscle were performed by RNA-seq and Western blot, respectively. Compared with CON, LEU pigs had increased plasma levels of leucine-derived metabolites, including 4-methyl-2-oxopentanoate, beta-hydroxyisovalerate, β-hydroxyisovalerylcarnitine, and 3-methylglutaconate (P ≤ 0.05). Leucine pulses downregulated transcripts enriched in the Kyoto Encyclopedia of Genes and Genomes terms "spliceosome," "GAP junction," "endocytosis," "ECM-receptor interaction," and "DNA replication". Significant correlations were identified between metabolites derived from leucine catabolism and muscle genes involved in protein degradation, transcription and translation, and muscle maintenance and development (P ≤ 0.05). Further, leucine pulses decreased protein expression of autophagic markers and serine/threonine kinase 4, involved in muscle atrophy (P ≤ 0.01). In conclusion, results from our studies support the notion that leucine pulses during continuous enteral feeding enhance muscle mass gain in neonatal pigs by increasing protein synthetic activity and downregulating protein catabolic pathways through concerted responses in the transcriptome and metabolome.

Regulation of Muscle Growth in Early Postnatal Life in a Swine Model

Skeletal muscle growth during the early postnatal period is rapid in the pig and dependent on the capacity of muscle to respond to anabolic and catabolic stimuli. Muscle mass is driven by the balance between protein synthesis and degradation. Among these processes, muscle protein synthesis in the piglet is exceptionally sensitive to the feeding-induced postprandial changes in insulin and amino acids, whereas muscle protein degradation is affected only during specific catabolic states. The developmental decline in the response of muscle to feeding is associated with changes in the signaling pathways located upstream and downstream of the mechanistic target of rapamycin protein complex. Additionally, muscle growth is supported by an accretion of nuclei derived from satellite cells. Activated satellite cells undergo proliferation, differentiation, and fusion with adjacent growing muscle fibers. Enhancing early muscle growth through modifying protein synthesis, degradation, and satellite cell activity is key to maximizing performance, productivity, and lifelong pig health.

Short- and long-term effects of leucine and branched-chain amino acid supplementation of a protein- and energy-reduced diet on muscle protein metabolism in neonatal pigs

The objective of this study was to determine if enteral leucine or branched-chain amino acid (BCAA) supplementation increases muscle protein synthesis in neonates who consume less than their protein and energy requirements, and whether this increase is mediated via the upregulation of the mechanistic target of rapamycin complex 1 (mTORC1) pathway or the decrease in muscle protein degradation signaling. Neonatal pigs were fed milk replacement diets containing reduced energy and protein (R), R supplemented with BCAA (RBCAA), R supplemented with leucine (RL), or complete protein and energy (CON) at 4-h intervals for 9 (n = 24) or 21 days (n = 22). On days 9 and 21, post-prandial plasma amino acids and insulin were measured at intervals for 4 h; muscle protein synthesis rate and activation of mTOR-related proteins were determined at 120 min post-feeding in muscle. For all parameters measured, the effects of diet were not different between day 9 or day 21. Compared to CON and R, plasma leucine and BCAA were higher (P ≤ 0.01) in RL- and RBCAA-fed pigs, respectively. Body weight gain, protein synthesis, and activation of S6 kinase (S6K1), 4E-binding protein (4EBP1), and eukaryotic initiation factor 4 complex (eIF4E·eIF4G) were decreased in RBCAA, RL, and R relative to CON (P < 0.01). RBCAA and RL upregulated (P ≤ 0.01) S6K1, 4EBP1, and eIF4E·eIF4G compared to R. In conclusion, when protein and energy are restricted, both leucine and BCAA supplementation increase mTOR activation, but do not enhance skeletal muscle protein synthesis and muscle growth in neonatal pigs.

Role of the gut, melanocortin system and malonyl-CoA in control of feed intake in non-ruminant animals

Regulation of feed intake is under complex control, involving physical, chemical, hormonal and neuronal responses. Understanding the regulation of feed intake in farm animals is key to optimisation of intake to meet production and profitability goals. Fundamental mechanisms regulating feed intake include constraints imposed by the gut, systems monitoring current and long-term energy status to increase or decrease intake, and hedonic, reward-related drives. Feed intake is closely related to the rate of passage of digesta and the capacity of the gastrointestinal tract. Indigestible fibre increases the rate of digesta passage and feed intake until excess distension sends signals of satiety to the brain. The presence of partially digested nutrients and products of microbial fermentation in the distal intestines releases peptides (PYY, OXM, GPL-1, Apo A-IV, amylin) from gut and pancreas to activate the intestinal brake, which slows the rate of passage and reduces feed intake. These peptides also act on orexigenic (NPY, AgRP) and anorexigenic (POMC, CART) peptides of the melanocortin system of the hypothalamus to reduce intake over the long term. Immediate energy status of the animal is monitored through the ratio of AMP : ATP via adenosine monophosphate-activated kinase and mammalian target of rapamycin, whereas the overall animal energy status is monitored by insulin, leptin and ghrelin. These energy-monitoring systems control short- and long-term intakes through the melanocortin system of the hypothalamus, primarily via malonyl-CoA, to alter the relative expression of orexigenic and anorexigenic peptides. Gut and hypothalamic control of feed intake can be over-ridden by hedonic, reward-related centres of the brain, predominantly through the release of dopamine. These hedonic responses can lead to over-consumption and obesity under some circumstances or reduced feed intake under stressful or other negative environmental situations. Knowledge of these mechanisms can be used to identify practical strategies for either increasing or decreasing voluntary intake in pigs.

Intermittent vs. continuous enteral feeding to prevent catabolism in acutely ill adult and pediatric patients

PURPOSE OF REVIEW

In clinical management of acutely ill adults and children, continuous enteral feeding (CEF), being considered the most tolerable approach, in comparison to other temporal patterns of nutrient administration (i.e. intermittent, cyclic and bolus), is the most frequently applied method. However, uncertainties remain about the most efficient approach to counteract protein catabolism.

RECENT FINDINGS

In critically ill adults, protein loss is mainly driven by increased protein breakdown whereas, in pediatric patients, acute illness is mainly characterized by blunted regulation of protein synthesis and stunted growth. Kinetic studies in fed adult volunteers indicate that protein synthesis can be stimulated for a limited period only. However, continuous feeding persistently improves protein balance through a sustained suppression of protein breakdown. This leads to the hypothesis that CEF could be more anticatabolic than intermittent enteral feeding (IEF) in these patients. Differently from adults, experimental models of acute disease in growing animals have consistently indicated that IEF can improve protein anabolism more efficiently than CEF, mainly through protein synthesis stimulation. The scarce number of clinical studies in acutely ill adults or pediatric patients, mostly performed with inadequate methodology, could not define the best approach to maintain protein balance.

SUMMARY

There is a need for pragmatic studies to directly compare the protein anabolic action of CEF and IEF using accurate methodologies, such as stable isotopes of amino acids, in both adult and pediatric patients with acute illness.

Dietary Leucine Supplementation Decreases Whole-Body Protein Turnover before, but Not during, Immune System Stimulation in Pigs

BACKGROUND

Immune system stimulation (ISS) adversely affects protein metabolism and reduces pig productivity. Leu has a regulatory role in skeletal muscle and whole-body protein turnover, which may be affected by ISS.

OBJECTIVE

We sought to determine the effect of supplemental Leu intake on whole-body protein turnover in pigs before and during ISS.

METHODS

Pigs [mean ± SD initial body weight (BW): 10.6 ± 1.1 kg] were surgically fitted with jugular vein catheters and assigned to 1 of 3 treatments: 1.36% standardized ileal-digestible (SID) Leu (CON; n = 13); 2.04% SID Leu (LEU-M; n = 8); and 2.72% SID Leu (LEU-H; n = 7). Pigs were infused continuously with 0.66 ± 0.05 mmol ¹⁵N ⋅ kg BW^-1 ⋅ d^-1 to determine whole-body protein kinetics. The study consisted of a 72-h prechallenge period followed by a 36-h challenge period. At the start of the challenge period, ISS was induced in all LEU-M and LEU-H pigs and half of the CON pigs with LPS (ISS+); the remaining CON pigs were administered saline (ISS-).

RESULTS

Whole-body protein synthesis (309, 273, and 260 ± 14 mmol N ⋅ kg BW^-1 ⋅ d^-1 for CON, LEU-M, and LEU-H pigs, respectively) and protein degradation (233, 203, and 185 ± 14 mmol N ⋅ kg BW^-1 ⋅ d^-1 for CON, LEU-M, and LEU-H pigs, respectively) were reduced with increasing Leu intake during the prechallenge period (P < 0.05). ISS reduced whole-body protein synthesis (203 compared with 169 ± 12 mmol N ⋅ kg BW^-1 ⋅ d^-1 for ISS- and ISS+ pigs fed CON, respectively; P < 0.05) and protein deposition (PD) (64.9 compared with 45.0 ± 2.9 mmol N ⋅ kg BW^-1 ⋅ d^-1 for ISS- and ISS+ pigs fed CON, respectively; P < 0.01), whereas ISS did not affect whole-body protein degradation. Leu intake did not affect whole-body protein synthesis or degradation in ISS+ pigs.

CONCLUSIONS

Our results indicate that supplemental Leu intake improves the efficiency of PD rather than PD directly in healthy pigs but did not affect whole-body protein turnover during ISS.