Enteral nutrient intake level determines intestinal protein synthesis and accretion rates in neonatal pigs

Protein turnover in pigs: A review of interacting factors

Protein turnover defines the balance between two continuous and complex processes of protein metabolism, synthesis and degradation, which determine their deposition in tissues. Although the liver and intestine have been studied extensively for their important roles in protein digestion, absorption and metabolism, the study of protein metabolism has focused mainly on skeletal muscle tissue to understand the basis for its growth. Due to the high adaptability of skeletal muscle, its protein turnover is greatly affected by different internal and external factors, contributing to carcass lean-yield and animal growth. Amino acid (AA) labelling and tracking using isotope tracer methodology, together with the study of myofiber type profiling, signal transduction pathways and gene expression, has allowed the analysis of these mechanisms from different perspectives. Positive stimuli such as increased nutrient availability in the diet (e.g., AA), physical activity, the presence of certain hormones (e.g., testosterone) or a more oxidative myofiber profile in certain muscles or pig genotypes promote increased upregulation of translation and transcription-related genes, activation of mTORC1 signalling mechanisms and increased abundance of satellite cells, allowing for more efficient protein synthesis. However, fasting, animal aging, inactivity and stress, inflammation or sepsis produce the opposite effect. Deepening the understanding of modifying factors and their possible interaction may contribute to the design of optimal strategies to better control tissue growth and nutrient use (i.e., protein and AA), and thus advance the precision feeding strategy.

Deglycosylation Differentially Regulates Weaned Porcine Gut Alkaline Phosphatase Isoform Functionality along the Longitudinal Axis

Gut alkaline phosphatases (AP) dephosphorylate the lipid moiety of endotoxin and other pathogen-associated-molecular patterns members, thus maintaining gut eubiosis and preventing metabolic endotoxemia. Early weaned pigs experience gut dysbiosis, enteric diseases and growth retardation in association with decreased intestinal AP functionality. However, the role of glycosylation in modulation of the weaned porcine gut AP functionality is unclear. Herein three different research approaches were taken to investigate how deglycosylation affected weaned porcine gut AP activity kinetics. In the first approach, weaned porcine jejunal AP isoform (IAP) was fractionated by the fast protein-liquid chromatography and purified IAP fractions were kinetically characterized to be the higher-affinity and lower-capacity glycosylated mature IAP (p < 0.05) in comparison with the lower-affinity and higher-capacity non-glycosylated pre-mature IAP. The second approach enzyme activity kinetic analyses showed that N-deglycosylation of AP by the peptide N-glycosidase-F enzyme reduced (p < 0.05) the IAP maximal activity in the jejunum and ileum and decreased AP affinity (p < 0.05) in the large intestine. In the third approach, the porcine IAP isoform-X1 (IAPX1) gene was overexpressed in the prokaryotic ClearColiBL21 (DE3) cell and the recombinant porcine IAPX1 was associated with reduced (p < 0.05) enzyme affinity and maximal enzyme activity. Therefore, levels of glycosylation can modulate plasticity of weaned porcine gut AP functionality towards maintaining gut microbiome and the whole-body physiological status.

L-Theanine Regulates the Abundance of Amino Acid Transporters in Mice Duodenum and Jejunum via the mTOR Signaling Pathway

The intestine is a key organ for the absorption of amino acids. L-theanine (LTA) is a structural analog of glutamine and a characteristic non-protein amino acid found in tea (Camellia sinensis) that regulates lipid and protein metabolism. The present study explored the role of LTA in intestinal amino acid absorption, protein synthesis, and its mechanisms. Overall, our findings suggest that LTA supplementation not only affects serum alkaline phosphatase (AKP), total protein (TP), and urea nitrogen (BUN) levels, but it also upregulates the mRNA and protein expression of amino acid transporters (EAAT3, EAAT1, 4F2hc, y⁺LAT1, CAT1, ASCT2, and B⁰AT1), and activates the mTOR signaling pathway. The downstream S6 and S6K1 proteins are regulated, and the expression of amino acid transporters is regulated. These findings suggest that LTA increases intestinal AA absorption, promotes protein metabolism, and increases nitrogen utilization by upregulating AAT expression, activating the mTOR signaling pathway, and phosphorylating the mTOR downstream proteins S6 and S6K1.

Intermittent Bolus Feeding Enhances Organ Growth More Than Continuous Feeding in a Neonatal Piglet Model

BACKGROUND

Orogastric tube feeding is frequently prescribed for neonates who cannot ingest food normally. In a piglet model of the neonate, greater skeletal muscle growth is sustained by upregulation of translation initiation signaling when nutrition is delivered by intermittent bolus meals, rather than continuously.

OBJECTIVES

The objective of this study was to determine the long-term effects of feeding frequency on organ growth and the mechanism by which feeding frequency modulates protein anabolism in these organs.

METHODS

Eighteen neonatal pigs were fed by gastrostomy tube the same amount of a sow milk replacer either by continuous infusion (CON) or on an intermittent bolus schedule (INT). After 21 d of feeding, the pigs were killed without interruption of feeding (CON; n = 6) or immediately before (INT-0; n = 6) or 60 min after (INT-60; n = 6) a meal, and fractional protein synthesis rates and activation indexes of signaling pathways that regulate translation initiation were measured in the heart, jejunum, ileum, kidneys, and liver.

RESULTS

Compared with continuous feeding, intermittent feeding stimulated the growth of the liver (+64%), jejunum (+48%), ileum (+40%), heart (+64%), and kidney (+56%). The increases in heart, kidney, jejunum, and ileum masses were proportional to whole body lean weight gain, but liver weight gain was greater in the INT-60 than the CON, and intermediate for the INT-0 group. For the liver and ileum, but not the heart, kidney, and jejunum, INT-60 compared with CON pigs had greater fractional protein synthesis rates (22% and 48%, respectively) and was accompanied by an increase in ribosomal protein S6 kinase 1 and eukaryotic initiation factor 4E binding protein 1 phosphorylation.

CONCLUSIONS

These results suggest that intermittent bolus compared with continuous orogastric feeding enhances organ growth and that in the ileum and liver, intermittent feeding enhances protein synthesis by stimulating translation initiation.

Nutrient Restriction has Limited Short-Term Effects on Gut, Immunity, and Brain Development in Preterm Pigs

BACKGROUND

Extrauterine growth restriction (EUGR) in preterm infants is associated with higher morbidity and impaired neurodevelopment. Early nutrition support may prevent EUGR in preterm infants, but it is not known if this improves organ development and brain function in the short and long term.

OBJECTIVE

Using pigs as models for infants, we hypothesized that diet-induced EUGR impairs gut, immunity, and brain development in preterm neonates during the first weeks after birth.

METHODS

Forty-four preterm caesarean-delivered pigs (Danish Landrace × Large White × Duroc, birth weight 975 ± 235 g, male:female ratio 23:21) from 2 sows were fed increasing volumes [32-180 mL/(kg·d)] of dilute bovine milk (EUGR group) or the same diet fortified with powdered bovine colostrum for 19 d (CONT group, 50-100% higher protein and energy intake than the EUGR group).

RESULTS

The EUGR pigs showed reduced body growth (-39%, P < 0.01), lower plasma albumin, phosphate, and creatine kinase concentrations (-35 to 14%, P < 0.05), increased cortisol and free iron concentrations (+130 to 700%, P < 0.05), and reduced relative weights of the intestine, liver, and spleen (-38 to 19%, all P < 0.05). The effects of EUGR on gut structure, function, microbiota, and systemic immunity were marginal, although EUGR temporarily increased type 1 helper T cell (Th1) activity (e.g. more blood T cells and higher Th1-related cytokine concentrations on day 8) and reduced colon nutrient fermentation (lower SCFA concentration; -45%, P < 0.01). Further, EUGR pigs showed increased relative brain weights (+19%, P < 0.01), however, memory and learning, as tested in a spatial T-maze, were not affected.

CONCLUSION

Most of the measured organ growth, and digestive, immune, and brain functions showed limited effects of diet-induced EUGR in preterm pigs during the first weeks after birth. Likewise, preterm infants may show remarkable physiological adaptation to deficient nutrient supply during the first weeks of life although early life malnutrition may exert negative consequences later.

New generation lipid emulsions increase brain DHA and improve body composition, but not short-term neurodevelopment in parenterally-fed preterm piglets

New generation, multicomponent parenteral lipid emulsions provide key fatty acids for brain growth and development, such as docosahexaenoic acid (DHA) and arachidonic acid (AA), yet the content may be suboptimal for preterm infants. Our aim was to test whether DHA and AA-enriched lipid emulsions would increase activity, growth, and neurodevelopment in preterm piglets and limit brain inflammation. Cesarean-delivered preterm pigs were given three weeks of either enteral preterm infant formula (ENT) or TPN with one of three parenteral lipid emulsions: Intralipid (IL), SMOFlipid (SMOF) or an experimental emulsion (EXP). Activity was continuously monitored and weekly blood sampling and behavioral field testing performed. At termination of the study, whole body and tissue metrics were collected. Neuronal density was assessed in sections of hippocampus (HC), thalamus, and cortex. Frontal cortex (FC) and HC tissue were assayed for fatty acid profiles and expression of genes of neuronal growth and inflammation. After 3 weeks of treatment, brain DHA content in SMOF, EXP and ENT pigs was higher (P < 0.01) in FC but not HC vs. IL pigs. There were no differences in brain weight or neuron density among treatment groups. Inflammatory cytokine TNFα and IL-1β expression in brain regions were increased in IL pigs (P < 0.05) compared to other groups. Overall growth velocity was similar among groups, but IL pigs had higher percent body fat and increased insulin resistance compared to other treatments (P < 0.05). ENT pigs spent more time in higher physical activity levels compared to all TPN groups, but there were no differences in exploratory behavior among groups. We conclude that a soybean oil emulsion increased select brain inflammatory cytokines and multicomponent lipid emulsions enriched with DHA and AA in parenteral lipids results in increased cortical DHA and improved body composition without affecting short term neurodevelopmental outcomes.

Translational Advances in Pediatric Nutrition and Gastroenterology: New Insights from Pig Models

Pigs are increasingly important animals for modeling human pediatric nutrition and gastroenterology and complementing mechanistic studies in rodents. The comparative advantages in size and physiology of the neonatal pig have led to new translational and clinically relevant models of important diseases of the gastrointestinal tract and liver in premature infants. Studies in pigs have established the essential roles of prematurity, microbial colonization, and enteral nutrition in the pathogenesis of necrotizing enterocolitis. Studies in neonatal pigs have demonstrated the intestinal trophic effects of akey gut hormone, glucagon-like peptide 2 (GLP-2), and its role in the intestinal adaptation process and efficacy in the treatment of short bowel syndrome. Further, pigs have been instrumental in elucidating the physiology of parenteral nutrition-associated liver disease and the means by which phytosterols, fibroblast growth factor 19, and a new generation of lipid emulsions may modify disease. The premature pig will continue to be a valuable model in the development of optimal infant diets (donor human milk, colostrum), specific milk bioactives (arginine, growth factors), gut microbiota modifiers (pre-, pro-, and antibiotics), pharmaceutical drugs (GLP-2 analogs, FXR agonists), and novel diagnostic tools (near-infrared spectroscopy) to prevent and treat these pediatric diseases.

Alpha-Lactalbumin Enriched Whey Protein Concentrate to Improve Gut, Immunity and Brain Development in Preterm Pigs

Human milk is rich in nutritional factors, such as alpha-lactalbumin (α-Lac), and important for neonatal development, but nutrient supplementation may be required for optimal growth. Using a pig model, we hypothesized that α-Lac-enriched whey protein concentrate (WPC) supplementation improves neonatal development. Cesarean-delivered preterm pigs were fed either dilute bovine milk (REF) or REF milk supplemented with WPC with normal (STANDARD-ALPHA) or high (HIGH-ALPHA) α-Lac. Clinical, gut, immune and cognitive endpoints (open field, T-maze) were assessed and tissues collected at Day 19. The growth of STANDARD-ALPHA and HIGH-ALPHA were higher than REF (31 vs. 19 g/kg/d). Most organ weights, gut, immunity and brain variables were similar between WPC groups. HIGH-ALPHA had a higher bone mineral content, colon microbial diversity and an abundance of specific bacteria and microbial metabolites, and tended to show a faster food transit time (p = 0.07). Relative to REF, WPC pigs showed higher relative organ weights, blood amino acids, blood neutrophil function, and microbial metabolites, but lower brush-border enzyme activities and plasma cortisol. Cognition outcomes did not differ among the groups. In conclusion, WPC supplementation of milk improved some growth, gut and immunity parameters in preterm pigs. However, increasing the α-Lac content beyond human milk levels had limited effects on the immature gut and developing brain.

Differential action of TGR5 agonists on GLP-2 secretion and promotion of intestinal adaptation in a piglet short bowel model

Enteroendocrine L cells and glucagon-like peptide 2 (GLP-2) secretion are activated in the intestinal adaptation process following bowel resection in patients with short bowel syndrome. We hypothesized that enteral activation of Takeda G protein-coupled receptor 5 (TGR5), expressed in enteroendocrine L cells, could augment endogenous GLP-2 secretion and the intestinal adaptation response. Our aim was to assess the efficacy of different TGR5 agonists to stimulate GLP-2 secretion and intestinal adaptation in a piglet short-bowel model. In study 1, parenterally fed neonatal pigs (n = 6/group) were gavaged with vehicle, olive extract (OE; 10 or 50 mg/kg), or ursolic acid (UA; 10 mg/kg), and plasma GLP-2 was measured for 6 h. In study 2, neonatal pigs (n = 6-8/group) were subjected to transection or 80% mid-small intestine resection and, after 2 days, assigned to treatments for 10 days as follows: 1) transection + vehicle (sham), 2) resection + vehicle (SBS), 3) resection + 30 mg UA (SBS + UA), and 4) resection + 180 mg/kg OE (SBS + OE). We measured plasma GLP-2, intestinal histology, cell proliferation, and gene expression, as well as whole body citrulline-arginine kinetics and bile acid profiles. In study 1, GLP-2 secretion was increased by UA and tended to be increased by OE. In study 2, SBS alone, but not additional treatment with either TGR5 agonist, resulted in increased mucosal thickness and crypt cell proliferation in remnant jejunum and ileum sections. SBS increased biliary and ileal concentration of bile acids and expression of inflammatory and farnesoid X receptor target genes, but these measures were suppressed by UA treatment. In conclusion, UA is an effective oral GLP-2 secretagogue in parenterally fed pigs but is not capable of augmenting GLP-2 secretion or the intestinal adaptation response after massive small bowel resection. NEW & NOTEWORTHY Therapeutic activation of endogenous glucagon-like peptide 2 (GLP-2) secretion is a promising strategy to improve intestinal adaptation in patients with short bowel syndrome. This study in neonatal pigs showed that oral supplementation with a selective Takeda G protein-coupled receptor 5 (TGR5) agonist is an effective approach to increase GLP-2 secretion. The results warrant further study to establish a more potent oral TGR5 agonist that can effectively improve intestinal adaptation in pediatric patients with SBS.

Improved growth performance due to hypertrophied intestinal absorptive epithelial cells by heat-killed Lactobacillus sakei HS-1 in broiler chickens1

This study investigated the effect of heat-killed Lactobacillus sakei HS-1 (HK LS HS-1) on the growth performance and intestinal histology of broilers through 2 feeding trials. In trial 1, 48 male broilers were separated into 3 groups: 1) basal diet (1 to 21 d, including antibiotics; 21 to 49 d, not including antibiotics) supplemented with 0 bacterial cells of HK LS HS-1/g (control); 2) 106 bacterial cells were used; and 3) 108 bacterial cells were used. Trial 2 was carried out in a tropical area of Thailand, where 50 7-d-old male broilers were separated into 2 groups: 1) basal diet (control group); and 2) basal diet supplemented with 107 bacterial cells of HK LS HS-1/g. In trial 1, compared with the control, BW gain (linear; P = 0.019) and G:F (linear; P = 0.032) linearly increase with increasing HK LS HS-1 supplementation. In addition, 3 males died in the control group, while none died in the experimental group. In trial 2, growth performance was not significantly different between the groups. Observation of the gross anatomical visceral organs and intestinal histological parameters showed no difference among the groups. However, the weight of the ileum, total small intestine, gizzard, and ceca decreased, while the height of the ileal villus increased with increasing HK LS HS-1 supplementation (P < 0.05). On the duodenal villus apical surface, protuberated cells, cells without microvilli, recently exfoliated regions on villus tips, and deeper cells at the sites of these recently exfoliated cells were more frequently observed in the experimental groups compared to the control; therefore, the duodenal epithelial cells of the experimental groups were hypertrophied by rapid cell turnover. On the ileal villus apical surface, morphological changes (i.e., cell shedding) were not observed, but protuberated cells were observed; thus, the ileal epithelial cells of the experimental groups were hypertrophied by prolonging the detention period of cells on the villus tip without shedding into the intestinal lumen. The results indicate that HK LS HS-1 improves the growth performance of broilers due to the presences of hypertrophied intestinal absorptive epithelial cells on the villus apical surface, which induce enhanced durability against environmental stress.

Postprandial Amino Acid Kinetics of Milk Protein Mixtures are Affected by Composition, But Not Denaturation, in Neonatal Piglets

BACKGROUND

Multiple studies have indicated that formula-fed infants show a different growth trajectory compared with breastfed infants. The observed growth rates are suggested to be linked to higher postprandial levels of branched chain amino acids (BCAAs) and insulin related to differences in protein quality.

OBJECTIVE

We evaluated the effects of milk protein denaturation and milk protein composition on postprandial plasma and hormone concentrations.

METHODS

Neonatal piglets were bolus-fed randomly, in an incomplete crossover design, 2 of 3 milk protein solutions: native whey protein isolate (NWPI), denatured whey protein isolate (DWPI), or protein base ingredient, comprising whey and casein (PBI). Postprandial plasma amino acids (AAs), insulin, glucagon-like peptide 1, glucose, and paracetamol concentrations were assayed. Plasma responses were fitted with a model of first-order absorption with linear elimination.

RESULTS

DWPI (91% denatured protein) compared with NWPI (91% native protein) showed lower essential amino acids (EAAs) (∼10%) and BCAA (13-19%) concentrations in the first 30-60 min. However, total amino acid (TAA) concentration per time-point and area under the curve (AUC), as well as EAA and BCAA AUC were not different. PBI induced a ∼30% lower postprandial insulin spike than NWPI, yet plasma TAA concentration at several time-points and AUC was higher in PBI than in NWPI. The TAA rate constant for absorption (k a) was twofold higher in PBI than in NWPI. Plasma BCAA levels from 60 to 180 min and AUC were higher in PBI than in NWPI. Plasma EAA concentrations and AUCs in PBI and NWPI were not different.

CONCLUSIONS

Denaturation of WPI had a minimal effect on postprandial plasma AA concentration. The differences between PBI and NWPI were partly explained by the difference in AA composition, but more likely differences in protein digestion and absorption kinetics. We conclude that modifying protein composition, but not denaturation, of milk protein solutions impacts the postprandial amino acid availability in neonatal piglets.

Reduced Plasma Amino Acid Levels During Allogeneic Hematopoietic Stem Cell Transplantation Are Associated with Systemic Inflammation and Treatment-Related Complications

Patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT) are challenged by cytotoxic effects of the conditioning regimen, resulting in tissue damage, systemic inflammation, and increased metabolic demands for amino acids to regenerate damaged tissues, reconstitute hematopoietic cells, and establish antioxidant defenses. To date, few studies have addressed the role of plasma amino acid (PAA) levels during transplantation, and it remains unknown if amino acid deficiency can aggravate treatment-related morbidity. We determined plasma levels of the 23 human amino acids in 80 HSCT recipients (age 1.1 to 55.4 years) before conditioning and on days +7 and +21 post-transplant along with C-reactive protein (CRP) and IL-6 levels on day +7. Significant changes were observed in plasma concentrations of several human amino acids during HSCT. On day +7, numerous amino acids were inversely correlated with both CRP and IL-6, including glutamic acid, serine, alanine, glutamine, arginine, cysteine, glycine, histidine, lysine, tryptophan, threonine, taurine, proline, and methionine (r = -.22 to -.66; all P < .05). Patients who developed sinusoidal obstruction syndrome (SOS) had significantly lower mean total PAA levels compared with patients without SOS (2013 ng/L [95% confidence interval (CI), 1709 to 2318 ng/L] versus 2706 ng/L [95% CI, 2261 to 3150 ng/L]; P = .006), along with lower individual levels of glutamic acid, serine, arginine, glycine, lysine, valine, tryptophan, threonine, and proline on day +7 (all P < .05). Patients with severe acute graft-versus-host disease had a lower mean total PAA level (1922 ng/L [95% CI, 1738 to 2106 ng/L] versus 2649 ng/L [95% CI, 2244 to 3055 ng/L]; P = .014) and lower levels of serine, glutamine, cysteine, glycine, lysine, and threonine on day +7 (all P < .05). These results indicate a relationship between low concentrations of certain amino acids and the risk of treatment-related complications.

Nutrient Fortification of Human Donor Milk Affects Intestinal Function and Protein Metabolism in Preterm Pigs

Background

Nutrient fortification of human milk is often required to secure adequate growth and organ development for very preterm infants. There is concern that formula-based fortifiers (FFs) induce intestinal dysfunction, feeding intolerance, and necrotizing enterocolitis (NEC). Bovine colostrum (BC) may be an alternative nutrient fortifier, considering its high content of protein and milk bioactive factors.

Objective

We investigated whether BC was superior to an FF product based on processed bovine milk and vegetable oil to fortify donor human milk (DHM) for preterm pigs, used as a model for infants.

Methods

Sixty preterm pigs from 4 sows (Danish Landrace × Large White × Duroc, birth weight 944 ± 29 g) received decreasing volumes of parenteral nutrition (96-72 mL ⋅ kg-1 ⋅ d-1) and increasing volumes of enteral nutrition (24-132 mL ⋅ kg-1 ⋅ d-1) for 8 d. Pigs were fed donor porcine milk (DPM) and DHM with or without FF or BC fortification (+4.6 g protein ⋅ kg-1 ⋅ d-1).

Results

DPM-fed pigs showed higher growth (10-fold), protein synthesis (+15-30%), villus heights, lactase and peptidase activities (+30%), and reduced intestinal cytokines (-50%) relative to DHM pigs (all P < 0.05). Fortification increased protein synthesis (+20-30%), but with higher weight gain and lower urea and cortisol concentrations for DHM+BC compared with DHM+FF pigs (2- to 3-fold differences, all P ≤ 0.06). DHM+FF pigs showed more diarrhea and reduced lactase and peptidase activities, hexose uptake, and villus heights relative to DHM+BC or DHM pigs (30-90% differences, P < 0.05). Fortification did not affect NEC incidence but DHM+BC pigs had lower colonic interleukin (IL)-6 and IL-8 concentrations relative to the remaining pigs (-30%, P = 0.06). DHM+FF pigs had higher stomach bacterial load than did DHM, and higher bacterial density along intestinal villi than did DHM and DHM+BC pigs (2- to 3-fold, P < 0.05).

Conclusions

The FF product investigated in this study reduced growth, intestinal function, and protein utilization in DHM-fed preterm pigs, relative to BC as fortifier. The relevance of BC as an alternative nutrient fortifier for preterm infants should be tested.

Husbandry practices and gut health outcomes in weaned piglets: A review

The immediate post-weaning period is one of the most stressful phases in a pig's life, and during this period, piglets are usually exposed to environmental, social and psychological stressors which have direct or indirect effects on gut health and overall growth performance. In this review, the impact of husbandry practices on gut health outcomes and performance of piglets is discussed. Husbandry practices in the swine barn generally include nutrition and management practices, maintenance of hygienic standards and disease prevention protocols, and animal welfare considerations. Poor husbandry practices could result in reduced feed intake, stress and disease conditions, and consequently affect gut health and performance in weaned piglets. Reduced feed intake is a major risk factor for impaired gut structure and function and therefore a key goal is to maximize feed intake in newly weaned piglets. In weaned piglets, crowding stress could reduce pig performance, favor the proliferation of pathogenic bacteria resulting in diarrhea, stimulate immune responses and interfere with beneficial microbial activities in the gut. Sanitation conditions in the swine barn plays an important role for optimal piglet performance, because unclean conditions reduced growth performance, shifted nutrient requirements to support the immune system and negatively affected the gut morphology in weaned piglets. Appropriate biosecurity measures need to be designed to prevent disease entry and spread within a swine operation, which in turn helps to keep all pigs and piglets healthy. Collectively, husbandry practices relating to feeding and nutrition, animal welfare, biosecurity and disease prevention are important determinants of gut health and piglet performance. Thus, it is suggested that adopting high husbandry practices is a critical piece in strategies aimed at raising pigs without the use of in-feed antibiotics.

Feeding an Elemental Dietvsa Milk-Based Formula Does Not Decrease Intestinal Mucosal Growth in Infant Pigs

BACKGROUND

We previously showed that the level of enteral nutrient intake determines the rate of intestinal growth in piglets. Our objective was to determine whether providing enteral nutrition in the form of elemental nutrients (glucose, amino acids, lipid [ED]) rather than cow's milk formula (lactose, protein, lipid [FORM]) reduces small intestinal growth and lactase activity.

METHODS

Three-week-old piglets were fed either ED (n = 7) intragastrically or FORM (n = 6) orally for 6 days.

RESULTS

Intestinal protein and DNA masses, villus height, and crypt depth were not different in ED and FORM pigs. Crypt cell proliferation, measured by in vivo bromodeoxyuridine labeling, was significantly (p < .05) higher (+37%) in ED than in FORM pigs. Rates of mucosal protein synthesis (%/d), measured by in vivo 2H-leucine incorporation, were higher (p < .05) in ED than FORM (147 vs 89) pigs. Circulating concentrations (pmol/L) of the intestinotrophic peptide, glucagon-like peptide-2 (GLP-2), were also higher (p < .05) in ED than in FORM (148 vs 87) pigs. The mean lactase-specific activity (micromol/min/g) in proximal and distal segments was higher (p < .05) in FORM than in ED (124 vs 58) pigs.

CONCLUSIONS

We conclude that intestinal mucosal growth and villus morphology are similar in pigs fed ED and FORM, despite higher cell proliferation and protein synthesis rates and lower lactase activity with ED. This implies that elemental diets may be as trophic as polymeric formulas to simultaneously provide nutrition and a stimulus for intestinal growth during bowel rest.

Effects of weaning on intestinal crypt epithelial cells in piglets

Intestinal epithelial cells in the crypt proliferate in piglets in response to weaning. However, the underlying mechanism has been unclear. We examined 40 piglets from eight litters (five piglets per litter) that were weaned at the age of 14 d, and one piglet from each litter was randomly selected for closer investigation. Based on the distended intestinal sac method, we isolated crypt epithelial cells from the mid-jejunum on Days 0, 1, 3, 5, and 7 post-weaning. Protein expression was analyzed using either isobaric tags for relative and absolute quantification or western blotting. Proteins related to the cell cycle, organization of the cellular macromolecular complex subunit, localization of cellular macromolecules, Golgi vesicle transport, fatty acid metabolism, oxidative phosphorylation, and translational initiation were mainly down-regulated, while those involved in glycolysis, cell cycle arrest, protein catabolism, and cellular amino acid metabolism were up-regulated. The amount of proteins active in the mTOR signaling pathway was generally decreased over time. These results indicate that weaning influences energy metabolism, cellular macromolecule organization and localization, and protein metabolism, thereby affecting the proliferation of intestinal epithelial cells in weaned piglets. Moreover, those cellular processes are possibly controlled by that signaling pathway.

Nutrient-intake-level-dependent regulation of intestinal development in newborn intrauterine growth-restricted piglets via glucagon-like peptide-2

The objective of the present study was to investigate the intestinal development of newborn intrauterine growth-restricted (IUGR) piglets subjected to normal nutrient intake (NNI) or restricted nutrient intake (RNI). Newborn normal birth weight (NBW) and IUGR piglets were allotted to NNI or RNI levels for 4 weeks from day 8 postnatal. IUGR piglets receiving NNI had similar growth performance compared with that of NBW piglets. Small intestine length and villous height were greater in IUGR piglets fed the NNI than that of piglets fed the RNI. Lactase activity was increased in piglets fed the NNI compared with piglets fed the RNI. Absorptive function, represented by active glucose transport by the Ussing chamber method and messenger RNA (mRNA) expressions of two main intestinal glucose transporters, Na+-dependent glucose transporter 1 (SGLT1) and glucose transporter 2 (GLUT2), were greater in IUGR piglets fed the NNI compared with piglets fed the RNI regimen. The apoptotic process, characterized by caspase-3 activity (a sign of activated apoptotic cells) and mRNA expressions of p53 (pro-apoptotic), bcl-2-like protein 4 (Bax) (pro-apoptotic) and B-cell lymphoma-2 (Bcl-2) (anti-apoptotic), were improved in IUGR piglets fed the NNI regimen. To test the hypothesis that improvements in intestinal development of IUGR piglets fed NNI might be mediated through circulating glucagon-like peptide-2 (GLP-2), GLP-2 was injected subcutaneously to IUGR piglets fed the RNI from day 8 to day 15 postnatal. Although the intestinal development of IUGR piglets fed the RNI regimen was suppressed compared with those fed the NNI regimen, an exogenous injection of GLP-2 was able to bring intestinal development to similar levels as NNI-fed IUGR piglets. Collectively, our results demonstrate that IUGR neonates that have NNI levels could improve intestinal function via the regulation of GLP-2.

Ethanolamine enhances the proliferation of intestinal epithelial cells via the mTOR signaling pathway and mitochondrial function

Ethanolamine (Etn), which is the base constituent of phosphatidylethanolamine, a major phospholipid in animal cell membranes, is required for the proliferation of many types of mammalian epithelial cells. However, it is not clear whether the proliferation of intestinal epithelial cells requires Etn. The present study was conducted to examine the effects of Etn on the proliferation of intestinal epithelial cells and to elucidate the underlying mechanisms. The addition of Etn at 100 or 200 μM was found to enhance the proliferation of IPEC-1 cells. The expression of cell cycle-related proteins CDK4, RB3, cyclin A, and PCNA was also enhanced by Etn. Moreover, the expression or phosphorylation levels of the mammalian target of rapamycin (mTOR) signaling pathway protein and the expression of proteins related to mitochondrial function were also affected by Etn in IPEC-1 cells. These results indicate that Etn promotes the proliferation of intestinal epithelial cells by exerting effects on mTOR signaling pathway and mitochondrial function.

Expression of apical Na(+)-L-glutamine co-transport activity, B(0)-system neutral amino acid co-transporter (B(0)AT1) and angiotensin-converting enzyme 2 along the jejunal crypt-villus axis in young pigs fed a liquid formula

Gut apical amino acid (AA) transport activity is high at birth and during suckling, thus being essential to maintain luminal nutrient-dependent mucosal growth through providing AA as essential metabolic fuel, substrates and nutrient stimuli for cellular growth. Because system-B(0) Na(+)-neutral AA co-transporter (B(0)AT1, encoded by the SLC6A19 gene) plays a dominant role for apical uptake of large neutral AA including L-Gln, we hypothesized that high apical Na(+)-Gln co-transport activity, and B(0)AT1 (SLC6A19) in co-expression with angiotensin-converting enzyme 2 (ACE2) were expressed along the entire small intestinal crypt-villus axis in young animals via unique control mechanisms. Kinetics of Na(+)-Gln co-transport activity in the apical membrane vesicles, prepared from epithelial cells sequentially isolated along the jejunal crypt-villus axis from liquid formula-fed young pigs, were measured with the membrane potential being clamped to zero using thiocyanate. Apical maximal Na(+)-Gln co-transport activity was much higher (p < 0.05) in the upper villus cells than in the middle villus (by 29 %) and the crypt (by 30 %) cells, whereas Na(+)-Gln co-transport affinity was lower (p < 0.05) in the upper villus cells than in the middle villus and the crypt cells. The B(0)AT1 (SLC6A19) mRNA abundance was lower (p < 0.05) in the crypt (by 40-47 %) than in the villus cells. There were no significant differences in B(0)AT1 and ACE2 protein abundances on the apical membrane among the upper villus, the middle villus and the crypt cells. Our study suggests that piglet fast growth is associated with very high intestinal apical Na(+)-neutral AA uptake activities via abundantly co-expressing B(0)AT1 and ACE2 proteins in the apical membrane and by transcribing the B(0)AT1 (SLC6A19) gene in the epithelia along the entire crypt-villus axis.

Maintaining physical activity during refeeding improves body composition, intestinal hyperpermeability and behavior in anorectic mice

A role of gut-brain axis emerges in the pathophysiology of anorexia nervosa and maintaining adapted physical activity during refeeding remains discussed. We aimed to assess gastrointestinal protein metabolism and investigate the contribution of physical activity during refeeding in C57BL/6 mice with activity-based anorexia (ABA). ABA mice exhibited lower body weight and food intake with increase of lean mass/fat mass ratio and fat oxidation. Colonic permeability was increased in ABA. Ad libitum food access was then restored and ABA group was divided into two subgroups, with access to running wheel (ABA-PA) or not (ABA-NPA). After refeeding, fat free mass was completely restored only in ABA-PA. Colonic permeability was enhanced in ABA-NPA. Finally, muscle kynurenine conversion into kynurenic acid was lower in ABA-NPA who also exhibited altered behavior. Maintaining physical activity during refeeding may thus limit colonic hyperpermeability and improve behavior in anorectic mice.

Early gradual feeding with bovine colostrum improves gut function and NEC resistance relative to infant formula in preterm pigs

It is unclear when and how to start enteral feeding for preterm infants when mother's milk is not available. We hypothesized that early and slow advancement with either formula or bovine colostrum stimulates gut maturation and prevents necrotizing enterocolitis (NEC) in preterm pigs, used as models for preterm infants. Pigs were given either total parenteral nutrition (TPN, n = 14) or slowly advancing volumes (16-64 ml·kg(-1)·day(-1)) of preterm infant formula (IF, n = 15) or bovine colostrum (BC, n = 13), both given as adjunct to parenteral nutrition. On day 5, both enteral diets increased intestinal mass (27 ± 1 vs. 22 ± 1 g/kg) and glucagon-like peptide 2 release, relative to TPN (P < 0.05). The incidence of mild NEC lesions was higher in IF than BC and TPN pigs (60 vs. 0 and 15%, respectively, P < 0.05). Only the IF pigs showed reduced gastric emptying and gastric inhibitory polypeptide release, and increased tissue proinflammatory cytokine levels (IL-1β and IL-8, P < 0.05) and expression of immune-related genes (AOAH, LBP, CXCL10, TLR2), relative to TPN. The IF pigs also showed reduced intestinal villus-to-crypt ratio, lactose digestion, and some plasma amino acids (Arg, Cit, Gln, Tyr, Val), and higher intestinal permeability, compared with BC pigs (all P < 0.05). Colonic microbiota analyses showed limited differences among groups. Early feeding with formula induces intestinal dysfunction whereas bovine colostrum supports gut maturation when mother's milk is absent during the first week after preterm birth. A diet-dependent feeding guideline may be required for newborn preterm infants.

Provision of Amniotic Fluid During Parenteral Nutrition Increases Weight Gain With Limited Effects on Gut Structure, Function, Immunity, and Microbiology in Newborn Preterm Pigs

BACKGROUND

Small enteral boluses with human milk may reduce the risk of subsequent feeding intolerance and necrotizing enterocolitis in preterm infants receiving parenteral nutrition (PN). We hypothesized that feeding amniotic fluid, the natural enteral diet of the mammalian fetus, will have similar effects and improve growth and gastrointestinal (GI) maturation in preterm neonates receiving PN, prior to the transition to milk feeding.

MATERIALS AND METHODS

Twenty-seven pigs, delivered by cesarean section at ~90% of gestation, were provided with PN and also fed boluses with amniotic fluid (AF; n = 13, 24-72 mL/kg/d) or no oral supplements (nil per os [NPO]; n = 14) until day 5 when blood, tissue, and fecal samples were collected for analyses.

RESULTS

Body weight gain was 2.7-fold higher in AF vs NPO pigs. AF pigs showed slower gastric emptying, reduced meal-induced release of gastric inhibitory peptide and glucagon-like peptide 2, changed gut microbiota, and reduced intestinal permeability. There were no effects on GI weight, percentage mucosa, villus height, plasma citrulline, hexose absorptive capacity, and digestive enzymes. Intestinal interleukin (IL)-1β levels and expression of IL1B and IL8 were increased in AF pigs, while blood biochemistry and amino acid levels were minimally affected.

CONCLUSION

Enteral boluses of AF were well tolerated in the first 5 days of life in preterm pigs receiving PN. Enteral provision of AF before the initiation of milk feeding may stimulate body growth and improve hydration in preterm infants receiving PN. Furthermore, it may improve GI motility and integrity, although most markers of GI maturation remain unchanged.

Glucagon-like peptide-2 induces rapid digestive adaptation following intestinal resection in preterm neonates

Short bowel syndrome (SBS) is a frequent complication after intestinal resection in infants suffering from intestinal disease. We tested whether treatment with the intestinotrophic hormone glucagon-like peptide-2 (GLP-2) increases intestinal volume and function in the period immediately following intestinal resection in preterm pigs. Preterm pigs were fed enterally for 48 h before undergoing resection of 50% of the small intestine and establishment of a jejunostomy. Following resection, pigs were maintained on total parenteral nutrition (TPN) without (SBS, n = 8) or with GLP-2 treatment (3.5 μg/kg body wt per h, SBS+GLP-2, n = 7) and compared with a group of unresected preterm pigs (control, n = 5). After 5 days of TPN, all piglets were fed enterally for 24 h, and a nutrient balance study was performed. Intestinal resection was associated with markedly reduced endogenous GLP-2 levels. GLP-2 increased the relative absorption of wet weight (46 vs. 22%), energy (79 vs. 64%), and all macronutrients (all parameters P < 0.05). These findings were supported by a 200% increase in sucrase and maltase activities, a 50% increase in small intestinal epithelial volume (P < 0.05), as well as increased DNA and protein contents and increased total protein synthesis rate in SBS+GLP-2 vs. SBS pigs (+100%, P < 0.05). Following intestinal resection in preterm pigs, GLP-2 induced structural and functional adaptation, resulting in a higher relative absorption of fluid and macronutrients. GLP-2 treatment may be a promising therapy to enhance intestinal adaptation and improve digestive function in preterm infants with jejunostomy following intestinal resection.

GLP-2 delays but does not prevent the onset of necrotizing enterocolitis in preterm pigs

OBJECTIVES

Necrotizing enterocolitis (NEC) is complex disease thought to occur as a result of an immaturity of the gastrointestinal tract of preterm infants. Intestinal dysfunction induced by total parental nutrition (TPN) may increase the risk for NEC upon introduction of enteral feeding. We hypothesized that the intestinal trophic and anti-inflammatory actions previously ascribed to the gut hormone, glucagon-like peptide-2 (GLP-2), would reduce the incidence of NEC when given in combination with TPN in preterm piglets.

METHODS

Preterm, newborn piglets were nourished by TPN and infused continuously with either human GLP-2 (100 μg · kg⁻¹ · day⁻¹) or control saline for 2 days (n = 12/group). On day 3, TPN was discontinued and pigs were given orogastric formula feeding every 3 hours, and continued GLP-2 or control treatment until the onset of clinical signs of NEC for an additional 96 hours and tissue was collected for molecular and histological endpoints.

RESULTS

GLP-2 treatment delayed the onset of NEC but was unable to prevent a high NEC incidence (~70%) and severity that occurred in both groups. GLP-2-treated pigs had less histological injury and increased proximal intestinal weight and mucosal villus height, but not crypt depth or Ki-67-positive cells. Inflammatory markers of intestinal myeloperoxidase were unchanged and serum amyloid A levels were higher in GLP-2-treated pigs.

CONCLUSIONS

GLP-2 did not prevent NEC and a proinflammatory response despite some reduction in mucosal injury and increased trophic effect.

Supplementing monosodium glutamate to partial enteral nutrition slows gastric emptying in preterm pigs(1-3)

Emerging evidence suggests that free glutamate may play a functional role in modulating gastroduodenal motor function. We hypothesized that supplementing monosodium glutamate (MSG) to partial enteral nutrition stimulates gastric emptying in preterm pigs. Ten-day-old preterm, parenterally fed pigs received partial enteral nutrition (25%) as milk-based formula supplemented with MSG at 0, 1.7, 3.0, and 4.3 times the basal protein-bound glutamate intake (468 mg·kg(-1)·d(-1)) from d 4 to 8 of life (n = 5-8). Whole-body respiratory calorimetry and (13)C-octanoic acid breath tests were performed on d 4, 6, and 8. Body weight gain, stomach and intestinal weights, and arterial plasma glutamate and glutamine concentrations were not different among the MSG groups. Arterial plasma glutamate concentrations were significantly higher at birth than after 8 d of partial enteral nutrition. Also at d 8, the significant portal-arterial concentration difference in plasma glutamate was substantial (∼500 μmol/L) among all treatment groups, suggesting that there was substantial net intestinal glutamate absorption in preterm pigs. MSG supplementation dose-dependently increased gastric emptying time and decreased breath (13)CO2 enrichments, (13)CO2 production, percentage of (13)CO2 recovery/h, and cumulative percentage recovery of (13)C-octanoic acid. Circulating glucagon-like peptide-2 (GLP-2) concentration was significantly increased by MSG but was not associated with an increase in intestinal mucosal growth. In contrast to our hypothesis, our results suggest that adding MSG to partial enteral nutrition slows the gastric emptying rate, which may be associated with an inhibitory effect of increased circulating GLP-2.

Modulation of the gut microbiota with antibiotic treatment suppresses whole body urea production in neonatal pigs

We examined whether changes in the gut microbiota induced by clinically relevant interventions would impact the bioavailability of dietary amino acids in neonates. We tested the hypothesis that modulation of the gut microbiota in neonatal pigs receiving no treatment (control), intravenously administered antibiotics, or probiotics affects whole body nitrogen and amino acid turnover. We quantified whole body urea kinetics, threonine fluxes, and threonine disposal into protein, oxidation, and tissue protein synthesis with stable isotope techniques. Compared with controls, antibiotics reduced the number and diversity of bacterial species in the distal small intestine (SI) and colon. Antibiotics decreased plasma urea concentrations via decreased urea synthesis. Antibiotics elevated threonine plasma concentrations and turnover, as well as whole body protein synthesis and proteolysis. Antibiotics decreased protein synthesis rate in the proximal SI and liver but did not affect the distal SI, colon, or muscle. Probiotics induced a bifidogenic microbiota and decreased plasma urea concentrations but did not affect whole body threonine or protein metabolism. Probiotics decreased protein synthesis in the proximal SI but not in other tissues. In conclusion, modulation of the gut microbiota by antibiotics and probiotics reduced hepatic ureagenesis and intestinal protein synthesis, but neither altered whole body net threonine balance. These findings suggest that changes in amino acid and nitrogen metabolism resulting from antibiotic- or probiotic-induced shifts in the microbiota are localized to the gut and liver and have limited impact on whole body growth and anabolism in neonatal piglets.

Continuous enteral administration can enable normal amino acid absorption in rats with methotrexate-induced gastrointestinal mucositis

It is unknown what feeding strategy to use during chemotherapy-induced gastrointestinal mucositis, which causes weight loss and possibly malabsorption. To study the absorptive capacity of amino acids during mucositis, we determined the plasma availability of enterally administered amino acids (AA), their utilization for protein synthesis, and the preferential side of the intestine for AA uptake in rats with and without methotrexate (MTX)-induced mucositis. Four days after injection with MTX (60 mg/kg) or saline (controls), rats received a primed, continuous dual-isotope infusion (intraduodenal and intravenous) of labeled L-leucine, L-lysine, L-phenylalanine, L-threonine, and L-methionine. We collected blood samples, assessed jejunal histology, and determined labeled AA incorporation in proximal and distal small intestinal mucosa, plasma albumin, liver, and thigh muscle. MTX-induced mucositis was confirmed by histology. The median systemic availability of all AA except for leucine was similar in MTX-treated rats and in controls. However, the individual availability of all AA differed substantially within the group of MTX-treated rats, ranging from severely reduced (<10% of intake) to not different from controls (>40% of intake in 5 of 9 rats). More AA originating from basolateral uptake than those originating from apical uptake were used for intestinal protein synthesis in MTX-treated rats (≥420% more, P < 0.05). We conclude that continuous enteral administration can enable normal AA absorption in rats with MTX-induced mucositis. The intestine prefers basolateral AA uptake to meet its need for AA for protein synthesis during mucositis.

Preventative oral methylthioadenosine is anti-inflammatory and reduces DSS-induced colitis in mice

Methylthioadenosine (MTA) is a precursor of the methionine salvage pathway and has been shown to have anti-inflammatory properties in various models of acute and chronic inflammation. However, the anti-inflammatory properties of MTA in models of intestinal inflammation are not defined. We hypothesized that orally administered MTA would be bioavailable and reduce morbidity associated with experimental colitis. We examined clinical, histological, and molecular markers of disease in mice provided oral MTA before (preventative) or after (therapy) the induction of colitis with 3% dextran sulfate sodium (DSS). We found a reduction in disease activity, weight loss, myeloperoxidase activity, and histological damage in mice given preventative MTA compared with DSS alone. We also found that equivalent supplementation with methionine could not reproduce the anti-inflammatory effects of MTA, and that MTA had no detectable adverse effects in control or DSS mice. Expression microarray analysis of colonic tissue showed several dominant pathways related to inflammatory cytokines/chemokines and extracellular matrix remodeling were upregulation by DSS and suppressed in MTA-supplemented mice. MTA is rapidly absorbed in the gastrointestinal tract and disseminated throughout the body, based on a time course analysis of an oral bolus of MTA. This effect is transient, with MTA levels falling to near baseline within 90 min in most organs. Moreover, MTA did not lead to increased blood or tissue methionine levels, suggesting that its effects are specific. However, MTA provided limited therapeutic benefit when administered after the onset of colitis. Our results show that oral MTA supplementation is a safe and effective strategy to prevent inflammation and tissue injury associated with DSS colitis in mice. Additional studies in chronic inflammatory models are necessary to determine if MTA is a safe and beneficial option for the maintenance of remission in human inflammatory bowel disease.

Glucagon-like peptide-2 (GLP-2) increases net amino acid utilization by the portal-drained viscera of ruminating calves

Glucagon-like peptide-2 (GLP-2) increases small intestinal mass and blood flow in ruminant calves, but its impact on nutrient metabolism across the portal-drained viscera (PDV) and liver is unknown. Eight Holstein calves with catheters in the carotid artery, mesenteric vein, portal vein and hepatic vein were paired by age and randomly assigned to control (0.5% bovine serum albumin in saline; n = 4) or GLP-2 (100 μg/kg BW per day bovine GLP-2 in bovine serum albumin; n = 4). Treatments were administered subcutaneously every 12 h for 10 days. Blood flow was measured on days 0 and 10 and included 3 periods: baseline (saline infusion), treatment (infusion of bovine serum albumin or 3.76 μg/kg BW per h GLP-2) and recovery (saline infusion). Arterial concentrations and net PDV, hepatic and total splanchnic fluxes of glucose, lactate, glutamate, glutamine, β-hydroxybutyrate and urea-N were measured on days 0 and 10. Arterial concentrations and net fluxes of all amino acids and glucose metabolism using continuous intravenous infusion of [U13-C]glucose were measured on day 10 only. A 1-h infusion of GLP-2 increased blood flow in the portal and hepatic veins when administered to calves not previously exposed to exogenous GLP-2, but after a 10-day administration of GLP-2 the blood flow response to the 1-h GLP-2 infusion was substantially attenuated. The 1-h GLP-2 infusion also did not appreciably alter nutrient fluxes on either day 0 or 10. In contrast, long-term GLP-2 administration reduced arterial concentrations and net PDV flux of many essential and non-essential amino acids. Despite the significant alterations in amino acid metabolism, glucose irreversible loss and utilization by PDV and non-PDV tissues were not affected by GLP-2. Fluxes of amino acids across the PDV were generally reduced by GLP-2, potentially by increased small intestinal epithelial growth and thus energy and amino acid requirements of this tissue. Increased PDV extraction of glutamine and alterations in PDV metabolism of arginine, ornithine and citrulline support the concept that GLP-2 influences intestine-specific amino acid metabolism. Alterations in amino acid metabolism but unchanged glucose metabolism suggests that the growth effects induced by GLP-2 in ruminants increase reliance on amino acids preferentially over glucose. Thus, GLP-2 increases PDV utilization of amino acids, but not glucose, concurrent with stimulated growth of the small intestinal epithelium in post-absorptive ruminant calves.

Glucagon-like peptide-2 (GLP-2) increases net amino acid utilization by the portal-drained viscera of ruminating calves

Glucagon-like peptide-2 (GLP-2) increases small intestinal mass and blood flow in ruminant calves, but its impact on nutrient metabolism across the portal-drained viscera (PDV) and liver is unknown. Eight Holstein calves with catheters in the carotid artery, mesenteric vein, portal vein and hepatic vein were paired by age and randomly assigned to control (0.5% bovine serum albumin in saline; n = 4) or GLP-2 (100 μg/kg BW per day bovine GLP-2 in bovine serum albumin; n = 4). Treatments were administered subcutaneously every 12 h for 10 days. Blood flow was measured on days 0 and 10 and included 3 periods: baseline (saline infusion), treatment (infusion of bovine serum albumin or 3.76 μg/kg BW per h GLP-2) and recovery (saline infusion). Arterial concentrations and net PDV, hepatic and total splanchnic fluxes of glucose, lactate, glutamate, glutamine, β-hydroxybutyrate and urea-N were measured on days 0 and 10. Arterial concentrations and net fluxes of all amino acids and glucose metabolism using continuous intravenous infusion of [U13-C]glucose were measured on day 10 only. A 1-h infusion of GLP-2 increased blood flow in the portal and hepatic veins when administered to calves not previously exposed to exogenous GLP-2, but after a 10-day administration of GLP-2 the blood flow response to the 1-h GLP-2 infusion was substantially attenuated. The 1-h GLP-2 infusion also did not appreciably alter nutrient fluxes on either day 0 or 10. In contrast, long-term GLP-2 administration reduced arterial concentrations and net PDV flux of many essential and non-essential amino acids. Despite the significant alterations in amino acid metabolism, glucose irreversible loss and utilization by PDV and non-PDV tissues were not affected by GLP-2. Fluxes of amino acids across the PDV were generally reduced by GLP-2, potentially by increased small intestinal epithelial growth and thus energy and amino acid requirements of this tissue. Increased PDV extraction of glutamine and alterations in PDV metabolism of arginine, ornithine and citrulline support the concept that GLP-2 influences intestine-specific amino acid metabolism. Alterations in amino acid metabolism but unchanged glucose metabolism suggests that the growth effects induced by GLP-2 in ruminants increase reliance on amino acids preferentially over glucose. Thus, GLP-2 increases PDV utilization of amino acids, but not glucose, concurrent with stimulated growth of the small intestinal epithelium in post-absorptive ruminant calves.

B-vitamin deficiency is protective against DSS-induced colitis in mice

Vitamin deficiencies are common in patients with inflammatory bowel disease (IBD). Homocysteine (Hcys) is a thrombogenic amino acid produced from methionine (Met), and its increase in patients with IBD indicates a disruption of Met metabolism; however, the role of Hcys and Met metabolism in IBD is not well understood. We hypothesized that disrupted Met metabolism from a B-vitamin-deficient diet would exacerbate experimental colitis. Mice were fed a B(6)-B(12)-deficient or control diet for 2 wk and then treated with dextran sodium sulfate (DSS) to induce colitis. We monitored disease activity during DSS treatment and collected plasma and tissue for analysis of inflammatory tissue injury and Met metabolites. We also quantified Met cycle activity by measurements of in vivo Met kinetics using [1-(13)C-methyl-(2)H(3)]methionine infusion in similarly treated mice. Unexpectedly, we found that mice given the B-vitamin-deficient diet had improved clinical outcomes, including increased survival, weight maintenance, and reduced disease scores. We also found lower histological disease activity and proinflammatory gene expression (TNF-α and inducible nitric oxide synthase) in the colon in deficient-diet mice. Metabolomic analysis showed evidence that these effects were associated with deficient B(6), as markers of B(12) function were only mildly altered. In vivo methionine kinetics corroborated these results, showing that the deficient diet suppressed transsulfuration but increased remethylation. Our findings suggest that disrupted Met metabolism attributable to B(6) deficiency reduces the inflammatory response and disease activity in DSS-challenged mice. These results warrant further human clinical studies to determine whether B(6) deficiency and elevated Hcys in patients with IBD contribute to disease pathobiology.

Intestinal threonine utilization for protein and mucin synthesis is decreased in formula-fed preterm pigs

Threonine is an essential amino acid necessary for synthesis of intestinal (glyco)proteins such as mucin MUC2 to maintain adequate gut barrier function. In premature infants, reduced barrier function may contribute to the development of necrotizing enterocolitis (NEC). Human milk protects against NEC compared with infant formula. Therefore, we hypothesized that formula feeding decreases the MUC2 synthesis rate concomitant with a decrease in intestinal first-pass threonine utilization, predisposing the preterm neonate to NEC. Preterm pigs were delivered by caesarian section and received enteral feeding with formula (FORM; n = 13) or bovine colostrum (COL; n = 6) for 2 d following 48 h of total parenteral nutrition. Pigs received a dual stable isotope tracer infusion of threonine to determine intestinal threonine kinetics. NEC developed in 38% of the FORM pigs, whereas none of the COL pigs were affected (P = 0.13). Intestinal fractional first-pass threonine utilization was lower in FORM pigs (49 ± 2%) than in COL pigs (60 ± 4%) (P = 0.02). In FORM pigs compared with COL pigs, protein synthesis (369 ± 31 mg·kg(-1)·d(-1) vs. 615 ± 54 mg·kg(-1)·d(-1); P = 0.003) and MUC2 synthesis (121 ± 17%/d vs. 184 ± 15%/d; P = 0.02) were lower in the distal small intestine (SI). Our results suggest that formula feeding compared with colostrum feeding in preterm piglets reduces mucosal growth with a concomitant decrease in first-pass splanchnic threonine utilization, protein synthesis, and MUC2 synthesis in the distal SI. Hence, decreased intestinal threonine metabolism and subsequently impaired gut barrier function may predispose the formula-fed infant to developing NEC.

Intestinal barrier function and absorption in pigs after weaning: a review

Under commercial conditions, weaning of piglets is associated with social, environmental and dietary stress. Consequently, small-intestinal barrier and absorptive functions deteriorate within a short time after weaning. Most studies that have assessed small-intestinal permeability in pigs after weaning used either Ussing chambers or orally administered marker probes. Paracellular barrier function and active absorption decrease when pigs are weaned at 3 weeks of age or earlier. However, when weaned at 4 weeks of age or later, the barrier function is less affected, and active absorption is not affected or is increased. Weaning stress is a critical factor in relation to the compromised paracellular barrier function after weaning. Adequate feed intake levels after weaning prevent the loss of the intestinal barrier function. Transcellular transport of macromolecules and passive transcellular absorption decrease after weaning. This may reflect a natural intestinal maturation process that is enhanced by the weaning process and prevents the pig from an antigen overload. It seems that passive and active absorption after weaning adapt accurately to the new environment when pigs are weaned after 3 weeks of age. However, when weaned at 3 weeks of age or earlier, the decrease in active absorption indicates that pigs are unable to sufficiently adapt to the new environment. To improve weaning strategies, future studies should distinguish whether the effect of feed intake on barrier function can be directed to a lack of a specific nutrient, i.e. energy or protein.

Nutritional modulation of the gut microbiota and immune system in preterm neonates susceptible to necrotizing enterocolitis

The gastrointestinal inflammatory disorder, necrotizing enterocolitis (NEC), is among the most serious diseases for preterm neonates. Nutritional, microbiological and immunological dysfunctions all play a role in disease progression but the relationship among these determinants is not understood. The preterm gut is very sensitive to enteral feeding which may either promote gut adaptation and health, or induce gut dysfunction, bacterial overgrowth and inflammation. Uncontrolled inflammatory reactions may be initiated by maldigestion and impaired mucosal protection, leading to bacterial overgrowth and excessive nutrient fermentation. Tumor necrosis factor alpha, toll-like receptors and heat-shock proteins are identified among the immunological components of the early mucosal dysfunction. It remains difficult, however, to distinguish the early initiators of NEC from the later consequences of the disease pathology. To elucidate the mechanisms and identify clinical interventions, animal models showing spontaneous NEC development after preterm birth coupled with different forms of feeding may help. In this review, we summarize the literature and some recent results from studies on preterm pigs on the nutritional, microbial and immunological interactions during the early feeding-induced mucosal dysfunction and later NEC development. We show that introduction of suboptimal enteral formula diets, coupled with parenteral nutrition, predispose to disease, while advancing amounts of mother's milk from birth (particularly colostrum) protects against disease. Hence, the transition from parenteral to enteral nutrition shortly after birth plays a pivotal role to secure gut growth, digestive maturation and an appropriate response to bacterial colonization in the sensitive gut of preterm neonates.

Chronic parenteral nutrition induces hepatic inflammation, steatosis, and insulin resistance in neonatal pigs

Prematurity and overfeeding in infants are associated with insulin resistance in childhood and may increase the risk of adult disease. Total parenteral nutrition (TPN) is a major source of infant nutritional support and may influence neonatal metabolic function. Our aim was to test the hypothesis that TPN induces increased adiposity and insulin resistance compared with enteral nutrition (EN) in neonatal pigs. Neonatal pigs were either fed enteral formula orally or i.v. administered a TPN mixture for 17 d; macronutrient intake was similar in both groups. During the 17-d period, we measured body composition by dual-energy X-ray absorptiometry scanning; fasting i.v. glucose tolerance tests (IVGTT) and hyperinsulinemic-euglycemic clamps (CLAMP) were performed to quantify insulin resistance. On d 17, tissue was collected after 1-h, low-dose CLAMP for tissue insulin signaling assays. TPN pigs gained less lean and more body fat and developed hepatic steatosis compared with EN pigs. After 7 and 13 d, IVGTT showed evidence of insulin resistance in the TPN compared with the EN group. Fasting plasma glucose and insulin also were higher in TPN pigs. CLAMP showed that insulin sensitivity was markedly lower in TPN pigs than in EN pigs. TPN also reduced the abundance of the insulin receptor, insulin receptor substrate 1, and phosphatidylinositol 3 kinase in skeletal muscle and liver and the proliferation of total pancreatic cells and β-cells. Hepatic proinflammatory genes as well as c-Jun-N-terminal kinase 1 phosphorylation, plasma interleukin 6, and tumor necrosis factor-α were all higher in TPN pigs than in EN pigs. The results demonstrate that chronic TPN induces a hepatic inflammatory response that is associated with significant insulin resistance, hepatic steatosis, and fat deposition compared with EN in neonatal pigs. Further studies are warranted to establish the mechanism of TPN-induced insulin resistance and hepatic metabolic dysfunction and whether there are persistent metabolic consequences of this lifesaving form of infant nutritional support.

Arginine-induced stimulation of protein synthesis and survival in IPEC-J2 cells is mediated by mTOR but not nitric oxide

Arginine is an indispensable amino acid in neonates and is required for growth. Neonatal intestinal epithelial cells (IEC) are capable of arginine transport, catabolism, and synthesis and express nitric oxide (NO) synthase to produce NO from arginine. Our aim was to determine whether arginine directly stimulates IEC growth and protein synthesis and whether this effect is mediated via mammalian target of rapamycin (mTOR) and is NO-dependent. We studied neonatal porcine IEC (IPEC-J2) cultured in serum- and arginine-free medium with increasing arginine concentrations for 4 or 48 h. Our results show that arginine enhances IPEC-J2 cell survival and protein synthesis, with a maximal response at a physiological concentration (0.1-0.5 mM). Addition of arginine increased the activation of mTOR, p70 ribosomal protein S6 (p70 S6) kinase, and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) in a time- and dose-dependent manner. The arginine-induced protein synthesis response was not inhibited by the NO inhibitors nitro-l-arginine methyl ester (l-NAME) and aminoguanidine, despite inducible NO synthase expression in IPEC-J2 cells. Moreover, protein synthesis was not increased or decreased in some cases by addition of an NO donor (S-nitroso-N-acetylpenicillamine), arginine precursors (proline and citrulline) in the absence of arginine, or insulin; S-nitroso-N-acetylpenicillamine suppressed phosphorylation of mTOR, p70 S6 kinase, and 4E-BP1. We found a markedly higher arginase activity in IPEC-J2 cells than in primary pig IEC. Furthermore, mTOR inhibition by rapamycin partially (42%) reduced the arginine-induced protein synthesis response and phosphorylation of mTOR and 4E-BP1. We conclude that arginine-dependent cell survival and protein synthesis signaling in IPEC-J2 cells are mediated by mTOR, but not by NO.

Dietary amino acid levels and feed restriction affect small intestinal development, mortality, and weight gain of male broilers

This study investigated the effect of 2 different dietary amino acid treatments and feed restriction in early life versus a control treatment on development of the small intestine segments (weights), mortality, and broiler performance. Each treatment was applied to 6 cages with Ross 308 male broilers and to 6 cages with Cobb 500 male broilers with 24 birds per cage. A control treatment (100% ideal protein) was compared with a treatment with 30% extra ideal protein, a treatment with daily adjustment of the dietary amino acid level and profile, and a feed restriction treatment. The protein treatments were applied from 0 to 14 d of age. The feed restriction was applied from 4 to 21 d of age. Restriction was 15% from d 4 to 14 of age and diminished with equal daily steps thereafter to 5% at 21 d of age. Birds were weighed and dissected for evaluation of small intestine weights at 6, 9, 14, and 36 d of age. Feed intake restriction reduced leg problems in Ross and Cobb broilers. Extra dietary protein reduced leg problems in Ross broilers only. The present experiment does not show that small intestinal weight development is related to mortality. Thirty percent extra dietary ideal protein increased duodenum weight between 6 and 9 d of age. This was not further increased by the daily optimization of the dietary amino acid level and profile. The increased duodenum weights coincided with an improved BW gain. This indicates that duodenum weight may be important in facilitating BW gain in young broilers. Thus, it may be worthwhile to pay more attention to the relation between nutrition and duodenum weight and duodenum function in further studies.

Almost all enteral aspartate is taken up in first-pass metabolism in enterally fed preterm infants

BACKGROUND & AIMS

The intestine is a major site of amino acid metabolism, especially in neonates. Neonatal animals derive energy needed for metabolic processes from dietary glucose and amino acids. Rats were found to oxidize non-essential amino acids such as aspartate, glutamate and glutamine in the intestine at a high rate. We have previously found that glutamate and glucose are important sources of energy for the splanchnic tissues in fully fed preterm infants. However, no data are available on splanchnic aspartate metabolism in human preterm infants. In the present study we studied whole-body and splanchnic aspartate metabolism and determined the metabolic fate of aspartate.

METHODS

In eight, enterally fed, preterm infants (gestational age 31 weeks (wk)+/-3 SD, range: 26-34wk) splanchnic and whole-body aspartate kinetics were assessed by dual tracer ([U-(13)C]aspartate and [D(3)]aspartate) techniques.

RESULTS

Splanchnic first-pass aspartate uptake was almost complete (77+/-15%). Almost all (80+/-9%) of the (13)C administered as [U-(13)C]aspartate used in first-pass was recovered as CO(2) in expired breath.

CONCLUSION

The splanchnic tissues extract almost all of the dietary aspartate in preterm infants. The majority of the labeled carbon is recovered in expired breath, making it most likely that the sequestered carbon skeleton of aspartate is utilized for energy generation.

Carbohydrate maldigestion induces necrotizing enterocolitis in preterm pigs

Necrotizing enterocolitis (NEC) remains the most severe gastrointestinal disorder in preterm infants. It is associated with the initiation of enteral nutrition and may be related to immature carbohydrate digestive capacity. We tested the hypothesis that a formula containing maltodextrin vs. a formula containing lactose as the principal source of carbohydrate would predispose preterm pigs to a higher NEC incidence. Cesarean-derived preterm pigs were given total parenteral nutrition for 48 h followed by total enteral nutrition with a lactose-based (n = 11) or maltodextrin-based (n = 11) formula for 36 h. A higher incidence (91% vs. 27%) and severity (score of 3.3 vs. 1.8) of NEC were observed in the maltodextrin than in the lactose group. This higher incidence of NEC in the maltodextrin group was associated with significantly lower activities of lactase, maltase, and aminopeptidase; reduced villus height; transiently reduced in vivo aldohexose uptake; and reduced ex vivo aldohexose uptake capacity in the middle region of the small intestine. Bacterial diversity was low for both diets, but alterations in bacterial composition and luminal concentrations of short-chain fatty acids were observed in the maltodextrin group. In a second study, we quantified net portal absorption of aldohexoses (glucose and galactose) during acute jejunal infusion of a maltodextrin- or a lactose-based formula (n = 8) into preterm pigs. We found lower net portal aldohexose absorption (4% vs. 42%) and greater intestinal recovery of undigested carbohydrate (68% vs. 27%) in pigs acutely perfused with the maltodextrin-based formula than those perfused with the lactose-based formula. The higher digestibility of the lactose than the maltodextrin in the formulas can be attributed to a 5- to 20-fold higher hydrolytic activity of tissue-specific lactase than maltases. We conclude that carbohydrate maldigestion is sufficient to increase the incidence and severity of NEC in preterm pigs.

Sulfur amino acid deficiency upregulates intestinal methionine cycle activity and suppresses epithelial growth in neonatal pigs

We recently showed that the developing gut is a significant site of methionine transmethylation to homocysteine and transsulfuration to cysteine. We hypothesized that sulfur amino acid (SAA) deficiency would preferentially reduce mucosal growth and antioxidant function in neonatal pigs. Neonatal pigs were enterally fed a control or an SAA-free diet for 7 days, and then whole body methionine and cysteine kinetics were measured using an intravenous infusion of [1-(13)C;methyl-(2)H(3)]methionine and [(15)N]cysteine. Body weight gain and plasma methionine, cysteine, homocysteine, and taurine and total erythrocyte glutathione concentrations were markedly decreased (-46% to -85%) in SAA-free compared with control pigs. Whole body methionine and cysteine fluxes were reduced, yet methionine utilization for protein synthesis and methionine remethylation were relatively preserved at the expense of methionine transsulfuration, in response to SAA deficiency. Intestinal tissue concentrations of methionine and cysteine were markedly reduced and hepatic levels were maintained in SAA-free compared with control pigs. SAA deficiency increased the activity of methionine metabolic enzymes, i.e., methionine adenosyltransferase, methionine synthase, and cystathionine beta-synthase, and S-adenosylmethionine concentration in the jejunum, whereas methionine synthase activity increased and S-adenosylmethionine level decreased in the liver. Small intestine weight and protein and DNA mass were lower, whereas liver weight and DNA mass were unchanged, in SAA-free compared with control pigs. Dietary SAA deficiency induced small intestinal villus atrophy, lower goblet cell numbers, and Ki-67-positive proliferative crypt cells in association with lower tissue glutathione, especially in the jejunum. We conclude that SAA deficiency upregulates intestinal methionine cycle activity and suppresses epithelial growth in neonatal pigs.

Extensive gut metabolism limits the intestinal absorption of excessive supplemental dietary glutamate loads in infant pigs

Glutamate (Glu) is a major intestinal oxidative fuel, key neurotransmitter, and may be a useful dietary supplement to augment health of the infant gut. We quantified the metabolic fate of various supplemental dietary Glu intakes in young pigs surgically implanted with vascular, intraduodenal (ID), or intragastric (IG) catheters and a portal blood flow probe. Piglets were acutely fed a range of dietary Glu intakes using a basal milk formula (100%) supplemented with varying amounts of monosodium Glu (up to 400%) via ID or IG routes. We quantified the gastrointestinal metabolic fate of dietary Glu using [U-(13)C] Glu tracer. The Glu net absorption in the basal 100% group was low in both ID and IG groups, ranging from 13 to 17% of intake. Enteral Glu supplementation significantly increased the absolute absorption rate and arterial concentration of Glu. In both the ID and IG groups, enteral [(13)C]Glu absorption was limited (<5% tracer input) at the basal Glu intake (100%) but increased nearly 4-fold ( approximately 20% input) in the 300% intake group. A substantial fraction (33-50%) of the enteral [(13)C]Glu input was oxidized by the gut to (13)CO(2) in both the 100 and 300% intake groups. We conclude that extensive gut metabolism limits the absorption of supplemental dietary Glu even at excessive intakes.

The gut takes nearly all: threonine kinetics in infants

BACKGROUND

Threonine is an essential amino acid that is abundantly present in intestinally produced glycoproteins. Animal studies show that intestinal first-pass threonine metabolism is high, particularly during a restricted enteral protein intake.

OBJECTIVE

The objective of the study was to quantify intestinal first-pass threonine metabolism in preterm infants during full enteral feeding and during restricted enteral intake.

DESIGN

Eight preterm infants (x +/- SD birth weight: 1.1 +/- 0.1 kg; gestational age: 29 +/- 2 wk) were studied during 2 periods. During period A, 40% of total intake was administered enterally and 60% was administered parenterally. Total threonine intake was 58 +/- 6 micromol kg(-1) h(-1). During period B, the infants received full enteral feeding, and the total threonine intake was 63 +/- 6 micromol kg(-1) h(-1). Dual stable-isotope tracer techniques were used to assess splanchnic and whole-body threonine kinetics.

RESULTS

The fractional first-pass threonine uptake by the intestine was remarkably high in both periods: 82 +/- 6% during partial enteral feeding and 70 +/- 6% during full enteral feeding. Net threonine retention was not affected by the route of feeding.

CONCLUSION

In preterm infants, the splanchnic tissues extract a very large amount of the dietary threonine intake, which indicates a high obligatory visceral need for threonine, presumably for the purposes of synthesis.

Biochemical and morphological developments are partially impaired in intestinal mucosa from growing pigs fed reduced-protein diets supplemented with crystalline amino acids1

The objective of this study was to determine if a reduction in dietary CP, with partial replacement of the intact protein with crystalline AA (CAA), would alter growth, morphology, and free or peptide-bound AA concentrations of intestinal mucosa in growing pigs. Twenty-four barrows (37.0 +/- 1.5 kg of BW) were fed 1 of 4 diets for 24 d: 16.1% CP with no CAA, or 12.8, 10.1, or 7.8% CP (analyzed values, as-fed) containing CAA. As CP decreased, CAA were gradually increased to meet requirements on a true ileal digestible basis. Pigs were euthanized 2 h postmeal on d 24, and mucosal samples from duodenum, jejunum, and ileum were collected. Reducing dietary CP decreased ADG, G:F, and final weight (linear, P < 0.05). With reduced dietary CP, mucosal protein concentration decreased in the jejunum (quadratic, P < 0.05) and tended to decrease in the ileum (linear, P = 0.062). Reduction of the dietary CP concentration from 16.1 to 7.8% tended to decrease the crypt depth (linear, P < 0.10) and decreased villus width (linear, P < 0.05) in duodenum and jejunum mucosa but did not reduce villus height or villus surface area in any regions of the small intestine. In the duodenum, a reduction in dietary CP increased free Lys, Met, and Thr (linear, P < 0.05) and peptide-bound Lys and Thr (quadratic, P < 0.10). In the jejunum, reducing CP decreased free Cys (linear P < 0.05) and tended to decrease free Asn and His (linear, P < 0.10) and peptide-bound His (quadratic, P = 0.061) and Ile, Leu, and Val (linear, P < 0.10). In the ileum, reducing CP decreased free Asn, Ser, Tyr, Arg, His, Phe (linear, P < 0.05), and Leu (linear, P = 0.054) and peptide-bound Gly and Ser (linear, P < 0.05) and tended to decrease peptide-bound Ile, Leu, Phe, Val (linear, P < 0.10), and Lys (linear P < 0.05). In conclusion, reduced-CP diets supplemented with CAA lead to a reduction in growth performance, associated with biochemical and morphological modifications of the intestinal mucosa.

Effects of a bovine colostrum-supplemented diet on some gut parameters in weaned piglets

The present study investigated the effects of a bovine colostrum-supplemented diet on gut post-weaning adaptation and health in piglets. Thirty-six 21-d-old piglets were allocated to one of the three following dietary treatments: sow-reared (SR), weaned on a control starter diet (WCtrl) or on a starter diet supplemented with bovine colostrum (WCol) until slaughter at 28 d or 35 d of age. Gastric pH and intestinal bacteriological, structural and functional parameters were determined. Compared to WCtrl, the gastric pH was lower (P < 0.05) and the duodenal lactobacilli:coliform ratio was higher (P = 0.05) in WCol piglets. The relative small intestine weight was 18% (P < 0.05) higher in WCol piglets than in SR piglets. Duodenal villous height was lower (P < 0.01) in WCtrl than in SR piglets, whereas the value for WCol piglets was intermediate. The weaning-increased crypt cell proliferation was not affected by bovine colostrum supplementation. The mucosal ribosomal capacity was higher (P < 0.05) in W than in SR piglets. In conclusion, a diet supplemented with colostrum induced, although not always significantly, variations of gut parameters, suggesting that globally, colostrum may limit weaning-induced gut structural and microbial alterations. The observed effects occurred early and were maintained throughout the post-weaning adaptive phase.

Measuring splanchnic amino acid metabolism in vivo using stable isotopic tracers1,2

The splanchnic bed comprises the liver and the portal-drained viscera (PDV). The PDV, which include the stomach, intestines, pancreas, and spleen, represent 4 to 6% of BW, yet they account for 20 to 35% of whole-body protein turnover and energy expenditure. Because the PDV are the first to be exposed to the diet, their nutrient needs are met first. Consequently, the extraction of dietary nutrients, especially AA, by the intestine will have a critical influence on their availability to peripheral tissues and therefore, on whole body requirements. Moreover, the systemic availability of dietary AA is a key determinant of lean body growth rate. A complicating factor in the measurement of intestinal nutrient use is that the intestinal epithelial cells receive nutrients from 2 sources: the diet and the arterial circulation. However, combining measurements of the net portal balance with those of isotopic enrichments from enterally and intravenously administered stable isotope-labeled AA provides an in vivo model that can be used to determine the proportion of AA extracted by the intestine from either source. Using this technique in fed animals demonstrated that the PDV contribute significantly to the use of essential (>60% of threonine) and nonessential (>90% of glutamate) AA provided by the diet. The relative use by the PDV of individual AA from the diet and arterial inputs varies widely, and dietary AA are the preferred fuel over dietary glucose. Stable isotope-labeled AA also enable the determination of the metabolic fate of individual AA. Using this technique, studies have shown that an insufficient protein supply or the mode of feeding affects AA use by the PDV, and consequently, may affect whole-body growth.

Glucagon-like peptide 2 dose-dependently activates intestinal cell survival and proliferation in neonatal piglets

Glucagon-like peptide 2 (GLP-2) is a gut hormone that stimulates mucosal growth in total parenteral nutrition (TPN)-fed piglets; however, the dose-dependent effects on apoptosis, cell proliferation, and protein synthesis are unknown. We studied 38 TPN-fed neonatal piglets infused iv with either saline or GLP-2 at three rates (2.5, 5.0, and 10.0 nmol.kg(-1).d(-1)) for 7 d. Plasma GLP-2 concentrations ranged from 177 +/- 27 to 692 +/- 85 pM in the low- and high-infusion groups, respectively. GLP-2 infusion dose-dependently increased small intestinal weight, DNA and protein content, and villus height; however, stomach protein synthesis was decreased by GLP-2. Intestinal crypt and villus apoptosis decreased and crypt cell number increased linearly with GLP-2 infusion rates, whereas cell proliferation and protein synthesis were stimulated only at the high GLP-2 dose. The intestinal activities of caspase-3 and -6 and active caspase-3 abundance decreased, yet procaspase-3 abundance increased markedly with increasing infusion rate and plasma concentration of GLP-2. The GLP-2-dose-dependent suppression of intestinal apoptosis and caspase-3 activity was associated with increased protein kinase B and glycogen-synthase kinase-3 phosphorylation, yet the expression phosphatidylinositol 3-kinase was unaffected by GLP-2. Intestinal endothelial nitric oxide synthase mRNA and protein expression was increased, but only at the high GLP-2 dose. We conclude that the stimulation of intestinal epithelial survival is concentration dependent at physiological GLP-2 concentrations; however, induction of cell proliferation and protein synthesis is a pharmacological response. Moreover, we show that GLP-2 stimulates intestinal cell survival and proliferation in association with induction of protein kinase B and glycogen-synthase kinase-3 phosphorylation and Bcl-2 expression.