Citrulline is an effective arginine precursor in enterally fed neonatal piglets

Evaluating the probiotic effects of spraying lactiplantibacillus plantarum P-8 in neonatal piglets

BACKGROUND

Gut microbes play an important role in the growth and health of neonatal piglets. Probiotics can promote the healthy growth of neonatal piglets by regulating their gut microbes. The study investigated the effects of spraying Lactiplantibacillus plantarum P-8 (L. plantarum P-8) fermentation broth on the growth performance and gut microbes of neonatal piglets.

RESULTS

The animals were randomly divided into probiotics groups (109 neonatal piglets) and control groups (113 neonatal piglets). The probiotics group was sprayed with L. plantarum P-8 fermented liquid from 3 day before the expected date of the sow to the 7-day-old of piglets, while the control group was sprayed with equal dose of PBS. Average daily gain (ADG), immune and antioxidant status and metagenome sequencing were used to assess the changes in growth performance and gut microbiota of neonatal piglets. The results showed that L. plantarum P-8 treatment significantly improved the average daily gain (P < 0.05) of neonatal piglets. L. plantarum P-8 increased the activities of CAT and SOD but reduced the levels of IL-2 and IL-6, effectively regulating the antioxidant capacity and immunity in neonatal piglets. L. plantarum P-8 adjusted the overall structure of gut microflora improving gut homeostasis to a certain extent, and significantly increased the relative abundance of gut beneficial bacteria such as L. mucosae and L. plantarum.

CONCLUSION

Spraying L. plantarum P-8 can be a feasible and effective probiotic intervention not only improving the growth of neonatal piglets, regulating the antioxidant capacity and immunity of neonatal piglets, but also improving the gut homeostasis to a certain extent.

Citrulline, Biomarker of Enterocyte Functional Mass and Dietary Supplement. Metabolism, Transport, and Current Evidence for Clinical Use

L-Citrulline is a non-essential but still important amino acid that is released from enterocytes. Because plasma levels are reduced in case of impaired intestinal function, it has become a biomarker to monitor intestinal integrity. Moreover, oxidative stress induces protein citrullination, and antibodies against anti-citrullinated proteins are useful to monitor rheumatoid diseases. Citrullinated histones, however, may even predict a worse outcome in cancer patients. Supplementation of citrulline is better tolerated compared to arginine and might be useful to slightly improve muscle strength or protein balance. The following article shall provide an overview of L-citrulline properties and functions, as well as the current evidence for its use as a biomarker or as a therapeutic supplement.

Hydrolysable Tannin Supplementation Alters Digestibility and Utilization of Dietary Protein, Lipid, and Carbohydrate in Grass Carp (Ctenopharyngodon idellus)

Tannin, an antinutritional component of plant proteins was fed to grass carp (Ctenopharyngodon idellus, 8. 18 ± 0.81 g) for 8 weeks to investigate their tolerance levels. Semi-purified diets (T0, T1, T2, and T3) with varying levels of hydrolysable tannin (0, 0.75, 1.25, and 1.75% respectively) were used. No significant difference was obtained in weight gain, while feed conversion ratio of T0 was significantly lower than T2. Muscle protein content of T0 and T3 were significantly higher than T2, while lipid content of T0 was significantly higher than other groups. Muscle and hepatic glycogen in T0 were significantly lower than other groups. Muscle saturated fatty acids in T3 were significantly higher than T0, and lowest in T1 and T2, while the poly-unsaturated fatty acids in T1 and T2 were higher than T0 and lowest in T3. Significant increases were obtained in trypsin and amylase activities as tannin levels increased, the lipase activity of T0 and T1 was significantly higher than T2 and T3. Activities of hepatic alanine aminotransferase and aspartate aminotransferase decreased with increasing tannin level. The total protein in serum of T2 was significantly higher than T0 and T1 and lowest in T3, whereas globulin of T2 was significantly higher than T0 and T3 and lowest in T1, while albumin of T1 was significantly higher than other groups. Urea nitrogen of T0 was significantly higher than other groups, triglyceride and total cholesterol significantly increased with tannin level and decreased in T3, significant decreases were obtained in low-density lipoprotein cholesterol and high-density lipoprotein cholesterol in T3. mRNA expression of intestinal TOR was significantly upregulated as dietary tannin increased. In hepatopancreas, the expression of glucokinase in T1 was significantly higher than T2, and lowest in T0 and T3, pyruvate kinase in T2 was significantly higher than T0 and T1 and lowest T3. The expression of lipoprotein lipase upregulated as tannin level and downregulated in T3, and fatty acid synthase downregulated as tannin level. In conclusion, grass carp could tolerate 1.75% dietary tannin without influencing growth. However, 1.25% tannin impaired digestion and metabolism of protein, decreased the deposition of lipid and promoted utilization of carbohydrate.

Dietary mulberry leaf powder affects growth performance, carcass traits and meat quality in finishing pigs

This study was conducted to evaluate the effect of mulberry leaves as an alternative source of protein on growth performance, carcass traits and meat quality in finishing pigs. A total of 180 Xiangcun Black pigs were randomly assigned to five treatment groups with six pens of six pigs per pen. The pigs were provided with a basal diet or a diet contained 3%, 6%, 9% or 12% of mulberry leaf powder during a 50-day experiment period. The results showed that dietary mulberry leaf powder had no negative effect on growth performance in Xiangcun Black pigs, except in the 12% mulberry group, where final body weight and average daily gain decreased (p < .05) and feed to gain ratio of the pigs increased (p < .05). Dietary mulberry inclusion decreased (quadratic, p < .05) the back fat thickness, fibre mean cross-sectional area (CSA) in the longissimus dorsi (LD) muscle and mRNA expression levels of myosin heavy chain (MyHC) IIb in LD and biceps femoris (BF) muscles, while increased (linear or quadratic, p < .05) the plasma concentration of albumin, levels of crude protein (CP), inosine monophosphate (IMP) and several amino acids in muscle tissues. When compared with the other groups, the 9% mulberry diet increased (p < .05) loin-eye area and contents of CP and IMP in muscles, while decreased (p < .05) plasma activity of cholinesterase and concentrations of uric acid and urea. The 6% mulberry diet had the lowest fibre mean CSA and shear force and increased total fibre number of the LD muscle, when compared with the other groups. These results suggest that including mulberry in the diet at <12% is an effective feed crop to improve meat quality and the chemical composition of muscle without negatively affecting growth performance.

BOARD-INVITED REVIEW: Arginine nutrition and metabolism in growing, gestating, and lactating swine

Arginine (Arg) has traditionally not been considered as a deficient nutrient in diets for gestating or lactating swine due to the assumption that these animals can synthesize sufficient amounts of Arg to meet their physiological needs. The lack of full knowledge about Arg nutrition has contributed to suboptimal efficiency of pork production. Over the past 25 yr, there has been growing interest in Arg metabolism in the pig, which is an agriculturally important species and a useful model for studying human biology. Arginine is a highly abundant amino acid in tissues of pigs, a major amino acid in allantoic fluid, and a key regulator of gene expression, cell signaling, and antioxidative reactions. Emerging evidence suggests that dietary supplementation with 0.5% to 1% Arg maintains gut health and prevents intestinal dysfunction in weanling piglets, while enhancing their growth performance and survival. Also, the inclusion of 1% Arg in diets is required to maximize skeletal muscle accretion and feed efficiency in growing pigs, whereas dietary supplementation with 1% Arg reduces muscle loss in endotoxin-challenged pigs. Furthermore, supplementing 0.83% Arg to corn- and soybean meal-based diets promotes embryonic/fetal survival in swine and milk production by lactating sows. Thus, an adequate amount of dietary Arg as a quantitatively major nutrient is necessary to support maximum growth, lactation, and reproduction performance of swine. These results also have important implications for improving the nutrition and health of humans and other animals.

Effects of L-citrulline supplementation on heat stress physiology, lactation performance and subsequent reproductive performance of sows in summer

Lactating sows are susceptible to heat stress (HS). Part of the thermoregulatory response to HS is to increase peripheral blood flow, which is mediated in part by the vasodilator, nitric oxide (NO). Therefore, the aim of this experiment was to determine the effect of supplementation of L-citrulline, a NO precursor, on symptoms of HS, lactation performance and subsequent reproductive performance of sows in summer. A total of 221 summer farrowing mixed parity sows were fed either a control diet or supplemented with 1% L-citrulline upon entry to the farrowing house (6 ± 1.8 days for mean ± standard deviation [SD] before farrowing) until weaning (26 ± 1.5 days). The average daily minimum and maximum temperature in the farrowing house was 21.0 ± 1.88 and 29.2 ± 3.82°C (mean ± SD). Rectal temperature, respiration rate, and plasma and urinary nitrite and nitrate (NOx) of sows were measured on the 19th day post-farrowing. Supplemental L-citrulline in the diet did not affect the number of piglets born alive, feed intake of sows, body weight or backfat thickness of sows at weaning, or litter weight gain. L-citrulline tended to reduce piglet pre-weaning mortality rate from 18.6% to 15.6% (p = 0.058). L-citrulline reduced the respiration rate of sows compared to the control diet at 17:00 hr (Time × Diet, p < 0.001); however, rectal temperature was not affected. L-citrulline tended to increase urinary NOx concentrations (127 vs. 224 µM, p = 0.057) but not plasma NOx concentrations. L-citrulline did not affect farrowing rate or number of piglets born alive in the subsequent parity. In conclusion, L-citrulline supplementation reduced respiration rate of lactating sows and reduced piglet pre-weaning mortality rate in summer. Whether the effects were due to a NO-dependent mechanism requires further validation.

The intestinal-renal axis for arginine synthesis is present and functional in the neonatal pig

The intestinal-renal axis for endogenous arginine synthesis is an interorgan process in which citrulline produced in the small intestine is utilized by the kidney for arginine synthesis. The function of this axis in neonates has been questioned because during this period the enzymes needed for arginine synthesis argininosuccinate synthase (ASS1) and lyase (ASL) are present in the gut. However, evidence of high plasma citrulline concentrations in neonates suggests otherwise. We quantified in vivo citrulline production in premature (10 days preterm), neonatal (7 days old), and young pigs (35 days old) using citrulline tracers. Neonatal pigs had higher fluxes (69 µmol·kg^-1·h^-1, P < 0.001) than premature and young pigs (43 and 45 µmol·kg^-1·h^-1, respectively). Plasma citrulline concentration was also greater in neonatal pigs than in the other age groups. We also determined the site of synthesis and utilization of citrulline in neonatal and young pigs by measuring organ balances across the gut and the kidney. Citrulline was released from the gut and utilized by the kidney in both neonatal and young pigs. The abundance and localization of the enzymes involved in the synthesis and utilization were determined in intestinal and kidney tissue. Despite the presence of ASS1 and ASL in the neonatal small intestine, the lack of colocalization with the enzymes that produce citrulline results in the release of citrulline by the PDV and its utilization by the kidney to produce arginine. In conclusion, the intestinal-renal axis for arginine synthesis is present in the neonatal pig.

Effects of dietary protein/energy ratio on growth performance, carcass trait, meat quality, and plasma metabolites in pigs of different genotypes

BACKGROUND

The protein/energy ratio is important for the production performance and utilization of available feed resources by animals. Increased protein consumption by mammals leads to elevated feed costs and increased nitrogen release into the environment. This study aimed to evaluate the effects of dietary protein/energy ratio on the growth performance, carcass traits, meat quality, and plasma metabolites of pigs of different genotypes.

METHODS

Bama mini-pigs and Landrace pigs were randomly assigned to two dietary treatment groups (Chinese conventional diet with low protein/energy ratio or National Research Council diet with high protein/energy ratio; n = 24 per treatment) in a 2 × 2 factorial arrangement. Blood and muscle samples were collected at the end of the nursery, growing, and finishing phases.

RESULTS

We observed significant interactions (P < 0.05) between breed and diet for total fat percentage, intramuscular fat (IMF) content, protein content in biceps femoris (BF) muscle, and plasma urea nitrogen (UN) concentration in the nursery phase; for average daily gain (ADG), average daily feed intake (ADFI), dry matter, IMF content in psoas major (PM) muscle, and plasma total protein and albumin concentrations in the growing phase; and for drip loss and plasma UN concentration in the finishing phase. Breed influenced (P < 0.05) growth performance, carcass traits, and meat quality, but not plasma metabolites. Throughout the trial, Landrace pigs showed significantly higher (P < 0.05) ADG, ADFI, dressing percentage, lean mass rate, and loin-eye area than did Bama mini-pigs, but significantly lower (P < 0.05) feed/gain ratio, fat percentage, backfat thickness, and IMF content. Dietary protein/energy ratio influenced the pH value, chemical composition of BF and PM muscles, and plasma activities of glutamic-pyruvic transaminase and gamma-glutamyl transpeptidase, and plasma concentration of UN.

CONCLUSIONS

Compared with Landrace pigs, Bama mini-pigs showed slower growth and lower carcass performance, but had better meat quality. Moreover, unlike Landrace pigs, the dietary protein/energy ratio did not affect the growth performance of Bama mini-pigs. These results suggest that, in swine production, low dietary protein/energy ratio may be useful for reducing feed costs and minimizing the adverse effects of ammonia release into the environment.

Dietary arginine requirements for growth are dependent on the rate of citrulline production in mice

BACKGROUND

In many species, including humans, arginine is considered a semiessential amino acid because under certain conditions endogenous synthesis cannot meet its demand. The requirements of arginine for growth in mice are ill defined and seem to vary depending on the genetic background of the mice.

OBJECTIVE

The objective of this study was to determine the metabolic and molecular basis for the requirement of arginine in 2 mouse strains.

METHODS

Institute of Cancer Research (ICR) and C57BL/6 (BL6) male mice were fed arginine-free or arginine-sufficient diets (Expt. 1) or 1 of 7 diets with increasing arginine concentration (from 0- to 8-g/kg diet, Expt. 2) between day 24 and 42 of life to determine the arginine requirements for growth. Citrulline production and "de novo" arginine synthesis were measured with use of stable isotopes, and arginine requirements were determined by breakpoint analysis and enzyme expression by reverse transcriptase-polymerase chain reaction.

RESULTS

In Expt. 1, ICR mice grew at the same rate regardless of the arginine concentration of the diet (mean ± SE: 0.66 ± 0.04 g/d, P = 0.80), but BL6 mice had a reduced growth rate when fed the arginine-free diet (0.25 ± 0.02 g/d, P < 0.001) compared to the 8-g arginine/kg diet (0.46 ± 0.03 g/d). ICR mice showed at least a 2-fold greater expression (P < 0.001) of ornithine transcarbamylase (OTC) than BL6 mice, which translated into a greater rate of citrulline (25%) and arginine synthesis (49%, P < 0.002). In Expt. 2, breakpoint analysis showed that the requirement for growth of BL6 mice was met with 2.32 ± 0.39 g arginine/kg diet; for ICR mice, however, no breakpoint was found.

CONCLUSION

Our data indicate that a reduced expression of OTC in BL6 mice translates into a reduced production of citrulline and arginine compared with ICR mice, which results in a dietary arginine requirement for growth in BL6 mice, but not in ICR mice.

Nitrogen in dietary glutamate is utilized exclusively for the synthesis of amino acids in the rat intestine

Although previous studies have shown that virtually the entire carbon skeleton of dietary glutamate (glutamate-C) is metabolized in the gut for energy production and amino acid synthesis, little is known regarding the fate of dietary glutamate nitrogen (glutamate-N). In this study, we hypothesized that dietary glutamate-N is an effective nitrogen source for amino acid synthesis and investigated the fate of dietary glutamate-N using [(15)N]glutamate. Fischer male rats were given hourly meals containing [U-(13)C]- or [(15)N]glutamate. The concentration and isotopic enrichment of several amino acids were measured after 0-9 h of feeding, and the net release of each amino acid into the portal vein was calculated. Most of the dietary glutamate-C was metabolized into CO(2), lactate, or alanine (56, 13, and 12% of the dietary input, respectively) in the portal drained viscera (PDV). Most of the glutamate-N was utilized for the synthesis of other amino acids such as alanine and citrulline (75 and 3% of dietary input, respectively) in the PDV, and only minor amounts were released into the portal vein in the form of ammonia and glutamate (2 and 3% of the dietary input, respectively). Substantial incorporation of (15)N into systemic amino acids such as alanine, glutamine, and proline, amino acids of the urea cycle, and branched-chain amino acids was also evident. These results provide quantitative evidence that dietary glutamate-N distributes extensively to amino acids synthesized in the PDV and, consequently, to circulating amino acids.

De novo synthesis is the main source of ornithine for citrulline production in neonatal pigs

Citrulline is an amino acid synthesized in the gut and utilized for the synthesis of the conditionally essential amino acid arginine. Recently, the origin of the ornithine utilized for citrulline synthesis has become a matter of discussion. Multiple physiological factors may have contributed to the differences found among different researchers; one of these is the developmental stage of the subjects studied. To test the hypothesis that during the neonatal period de novo synthesis is the main source of ornithine for citrulline synthesis, neonatal piglets were infused intravenously or intragastrically with [U-(13)C(6)]arginine, [U-(13)C(5)]glutamine, or [U-(13)C(5)]proline during the fasted and fed periods. [ureido-(15)N]citrulline and [(2)H(2)]ornithine were infused intravenously for the entire infusion protocol. During fasting, plasma proline (13%) and ornithine (19%) were the main precursors for citrulline synthesis, whereas plasma arginine (62%) was the main precursor for plasma ornithine. During feeding, enteral (27%) and plasma (12%) proline were the main precursors for the ornithine utilized in the synthesis of citrulline, together with plasma ornithine (27%). Enteral proline and glutamine were utilized directly by the gut to produce ornithine utilized for citrulline synthesis. Arginine was not utilized by the gut, which is consistent with the lack of arginase activity in the neonate. Arginine, however, was the main source (47%) of plasma ornithine and in this way contributed to citrulline synthesis. In conclusion, during the neonatal period, the de novo pathway is the predominant source for the ornithine utilized in the synthesis of citrulline, and proline is the preferred precursor.

Enteral arginine does not increase superior mesenteric arterial blood flow but induces mucosal growth in neonatal pigs

Arginine is an essential amino acid in neonates synthesized by gut epithelial cells and a precursor for NO that regulates vasodilatation and blood flow. Arginine supplementation has been shown to improve intestinal integrity in ischemia-reperfusion models and low plasma levels are associated with necrotizing enterocolitis. We hypothesized that enteral arginine is a specific stimulus for neonatal intestinal blood flow and mucosal growth under conditions of total parenteral nutrition (TPN) or partial enteral nutrition (PEN). We first tested the dose dependence and specificity of acute (3 h) enteral arginine infusion on superior mesenteric artery (SMA) blood flow in pigs fed TPN or PEN. We then determined whether chronic (4 d) arginine supplementation of PEN increases mucosal growth and if this was affected by treatment with the NO synthase inhibitor, N(G)-nitro-l-arginine methyl ester (L-NAME). Acute enteral arginine infusion increased plasma arginine dose dependently in both TPN and PEN groups, but the plasma response was markedly higher (100-250%) in the PEN group than in the TPN group at the 2 highest arginine doses. Baseline SMA blood flow was 90% higher in the PEN (2.37 ± 0.32 L⋅kg(-1)⋅h(-1)) pigs than in the TPN pigs (1.23 ± 0.17 L⋅kg(-1)⋅h(-1)), but was not affected by acute infusion individually of arginine, citrulline, or other major gut fuels. Chronic dietary arginine supplementation in PEN pigs induced mucosal growth in the intestine, but this effect was not prevented by treatment with L-NAME. Intestinal crypt cell proliferation, protein synthesis, and phosphorylation of mammalian target of rapamycin and p70S6 kinase were not affected by dietary arginine. We conclude that partial enteral feeding, but not acute enteral arginine, increases SMA blood flow in the neonatal pig. Furthermore, supplementing arginine in partial enteral feeding modestly increases intestinal mucosal growth and was NO independent.

Arginine is synthesized from proline, not glutamate, in enterally fed human preterm neonates

In neonatal mammals, arginine is synthesized in the enterocyte, with either proline or glutamate as the dietary precursor. We have shown several times in piglets that proline is the only precursor to arginine, although in vitro evidence supports glutamate in this role. Because of this uncertainty, we performed a multitracer stable isotope study to determine whether proline, glutamate, or both are dietary precursors for arginine in enterally fed human neonates. Labeled arginine (M + 2), proline (M + 1), and glutamate (M + 3) were given enterally to 15 stable, growing preterm infants (GA at birth 30-35 wk) at 1-3 wk postnatal age. Enrichment in urine of the tracer amino acids and the M + 1 and M + 3 isotopomers of arginine were measured by LC-tandem mass spectrometry to determine the contribution of proline and glutamate to arginine synthesis. Plateau enrichments of arginine and proline tracers were measurable in urine. Urinary glutamate enrichment was not detected. Conversion of proline to arginine was detected. However, the M + 3 isotopomer of arginine, which would have been synthesized from glutamate, was not detected. We conclude that, in contrast to the current consensus in the literature based on in vitro studies, proline is the major contributor to arginine synthesis in human preterm infants.

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.

Effect of intestinal resections on arginine metabolism: practical implications for nutrition support

PURPOSE OF REVIEW

The present review relates recent developments in the understanding of arginine and citrulline metabolism and complementation after intestinal resection.

RECENT FINDINGS

Arginine metabolism is disturbed after significant intestinal resection, with reduced fluxes and circulating and tissue concentrations. There is also a reduction in citrulline production, a major source of endogenous arginine by enterocyte metabolism. There is evidence to suggest that arginine or citrulline supplementation may be important in this situation.

SUMMARY

In experimental intestinal resection, arginine availability decreases as intestinal citrulline synthesis decreases. In this setting, there is debate over the efficiency of arginine supplementation on intestinal adaptation, perhaps due to different doses used. In contrast, citrulline, a precursor for arginine synthesis, whether provided enterally or parenterally, is more efficient at 1 g/kg/day than complementation with arginine (at the same dose) in sustaining arginine pools. In addition, citrulline is more effective than arginine in maintaining nitrogen homeostasis. Clinical studies are vital in order to establish the value of citrulline supplementation in short bowel patients.

The human neonatal small intestine has the potential for arginine synthesis; developmental changes in the expression of arginine-synthesizing and -catabolizing enzymes

Background

Milk contains too little arginine for normal growth, but its precursors proline and glutamine are abundant; the small intestine of rodents and piglets produces arginine from proline during the suckling period; and parenterally fed premature human neonates frequently suffer from hypoargininemia. These findings raise the question whether the neonatal human small intestine also expresses the enzymes that enable the synthesis of arginine from proline and/or glutamine. Carbamoylphosphate synthetase (CPS), ornithine aminotransferase (OAT), argininosuccinate synthetase (ASS), arginase-1 (ARG1), arginase-2 (ARG2), and nitric-oxide synthase (NOS) were visualized by semiquantitative immunohistochemistry in 89 small-intestinal specimens.

Results

Between 23 weeks of gestation and 3 years after birth, CPS- and ASS-protein content in enterocytes was high and then declined to reach adult levels at 5 years. OAT levels declined more gradually, whereas ARG-1 was not expressed. ARG-2 expression increased neonatally to adult levels. Neurons in the enteric plexus strongly expressed ASS, OAT, NOS1 and ARG2, while varicose nerve fibers in the circular layer of the muscularis propria stained for ASS and NOS1 only. The endothelium of small arterioles expressed ASS and NOS3, while their smooth-muscle layer expressed OAT and ARG2.

Conclusion

The human small intestine acquires the potential to produce arginine well before fetuses become viable outside the uterus. The perinatal human intestine therefore resembles that of rodents and pigs. Enteral ASS behaves as a typical suckling enzyme because its expression all but disappears in the putative weaning period of human infants.

Comparative aspects of tissue glutamine and proline metabolism

The cellular metabolism of glutamine and proline are closely interrelated, because they can be interconverted with glutamate and ornithine via the mitochondrial pathway involving pyrroline-5-carboxylate (P5C). In adults, glutamine and proline are converted via P5C to citrulline in the gut, then citrulline is converted to arginine in the kidney. In neonates, arginine is a semiindispensable amino acid and is synthesized from proline completely in the gut; because of low P5C synthase activity, glutamine is not an important precursor for neonatal arginine synthesis. Thus, splanchnic metabolism of glutamine and proline is important, because both amino acids serve as key precursors for arginine synthesis with some developmental differences. Studies investigating splanchnic extraction demonstrate that about two-thirds of dietary glutamine and almost all dietary glutamate are extracted on first pass and the vast majority is oxidized in the gut. This capacity to extract glutamine and glutamate appears to be very large, so diets high in glutamine or glutamate probably have little impact on circulating concentrations and consequent potential toxicity. In contrast, it appears that very little proline is extracted by the gut and liver, at least in the neonate, which may result in hyperprolinemia and potential toxicity. Therefore, the upper limits of safe dietary intake for glutamine and proline, and other amino acids, appear to be substantially different depending on the extent of first-pass splanchnic extraction and irreversible catabolism.

Dose-ranging effects of citrulline administration on plasma amino acids and hormonal patterns in healthy subjects: the Citrudose pharmacokinetic study

Previous experimental studies have highlighted that citrulline (CIT) could be a promising pharmaconutrient. However, its pharmacokinetic characteristics and tolerance to loading have not been studied to date. The objective was to characterise the plasma kinetics of CIT in a multiple-dosing study design and to assess the effect of CIT intake on the concentrations of other plasma amino acids (AA). The effects of CIT loading on anabolic hormones were also determined. Eight fasting healthy males underwent four separate oral loading tests (2, 5, 10 or 15 g CIT) in random order. Blood was drawn ten times over an 8 h period for measurement of plasma AA, insulin and growth hormone (Gh). Urine samples were collected before CIT administration and over the next 24 h. None of the subjects experienced side effects whatever the CIT dose. Concerning AA, only CIT, ornithine (ORN) and arginine (ARG) plasma concentrations were affected (maximum concentration 146 (sem 8) to 303 (sem 11) micromol/l (ARG) and 81 (sem 4) to 179 (sem 10) micromol/l (ORN); time to reach maximum concentration 1.17 (sem 0.26) to 2.29 (sem 0.20) h (ARG) and 1.38 (sem 0.25) to 1.79 (sem 0.11) h (ORN) according to CIT dose). Even at high doses, urinary excretion of CIT remained low ( < 5 %). Plasma insulin and Gh were not affected by CIT administration. Short-term CIT administration is safe and well-tolerated. CIT is a potent precursor of ARG. However, at the highest doses, CIT accumulated in plasma while plasma ARG levels increased less than expected. This may be due to saturation of the renal conversion of CIT into ARG.

A multitracer stable isotope quantification of the effects of arginine intake on whole body arginine metabolism in neonatal piglets

We have previously shown that deficient arginine intake increased the rate of endogenous arginine synthesis from proline. In this paper, we report in vivo quantification of the effects of arginine intake on total endogenous arginine synthesis, on the rates of conversion between arginine, citrulline, ornithine, and proline, and on nitric oxide synthesis. Male piglets, with gastric catheters for diet and isotope infusion and femoral vein catheters for blood sampling, received a complete diet for 2 days and then either a generous (+Arg; 1.80 g x kg(-1) x day(-1); n = 5) or deficient (-Arg; 0.20 g.kg(-1).day(-1); n = 5) arginine diet for 5 days. On day 7, piglets received a primed, constant infusion of [guanido-(15)N(2)]arginine, [ureido-(13)C;5,5-(2)H(2)]citrulline, [U-(13)C(5)]ornithine, and [(15)N;U-(13)C(5)]proline in an integrated study of the metabolism of arginine and its precursors. Arginine synthesis (micromol x kg(-1) x h(-1)) from both proline (+Arg: 42, -Arg: 74, pooled SE: 5) and citrulline (+Arg: 67, -Arg: 120; pooled SE: 15) were higher in piglets receiving the -Arg diet (P < 0.05); and for both diets proline accounted for approximately 60% of total endogenous arginine synthesis. The conversion of proline to citrulline (+Arg: 39, -Arg: 67, pooled SE: 6) was similar to the proline-to-arginine conversion, confirming that citrulline formation limits arginine synthesis from proline in piglets. Nitric oxide synthesis (micromol x kg(-1) x h(-1)), measured by the rate conversion of [guanido-(15)N(2)]arginine to [ureido-(15)N]citrulline, was greater in piglets receiving the +Arg diet (105) than in those receiving the -Arg diet (46, pooled SE: 10; P < 0.05). This multi-isotope method successfully allowed many aspects of arginine metabolism to be quantified simultaneously in vivo.

Citrulline and the gut

PURPOSE OF REVIEW

Citrulline, a nonprotein amino acid, is an important source of endogenous arginine. The gut is the main source of citrulline in humans. Hence, citrulline is a potential biomarker of short bowel function. Conversely, citrulline uptake by the gut is important for an oral supply of this amino acid as an alternative to arginine. This review discusses these two aspects of citrulline, as well as the recent developments in the understanding of its metabolism.

RECENT FINDINGS

Citrullinemia is such an efficient marker when the active mass of the bowel is affected that it can be used as a prognostic marker for parenteral nutrition weaning (if citrullinemia is >20 micromol/l) and as a factor for deciding between parenteral and enteral nutrition (as long as the pathology is considered). Citrullinemia should be used with care as a marker either of the intestinal absorption or following small bowel transplantation.

SUMMARY

Citrulline is easily taken up by the gut, with a broad set of transporters that can remove it from the lumen in the enterocytes. This is confirmed by pharmacokinetic studies and the efficacy is so great that oral complementation with citrulline seems more efficient than complementation with arginine to provide arginine.

Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism

AIMS

Oral L-arginine supplementation has been used in several studies to improve endothelium-dependent, nitric oxide (NO)-mediated vasodilation. L-Arginine treatment is hampered by extensive presystemic elimination due to intestinal arginase activity. In contrast, L-citrulline is readily absorbed and at least in part converted to L-arginine. The aim of our study was to assess this metabolic conversion and its subsequent pharmacodynamic effects.

METHODS

In a double-blind, randomized, placebo-controlled cross-over study, 20 healthy volunteers received six different dosing regimes of placebo, citrulline, and arginine. Pharmacokinetic parameters (C(max), T(max), C(min), AUC) were calculated after 1 week of oral supplementation. The ratio of plasma L-arginine over asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase (arginine/ADMA ratio), urinary cyclic guanosine monophosphate (cGMP) and nitrate excretion rates, and flow-mediated vasodilation (FMD) was measured to assess pharmacodynamic effects.

RESULTS

L-Citrulline dose-dependently increased AUC and C(max) of plasma L-arginine concentration more effectively than L-arginine (P < 0.01). The highest dose of citrulline (3 g bid) increased the C(min) of plasma L-arginine and improved the L-arginine/ADMA ratio from 186 +/- 8 (baseline) to 278 +/- 14 [P < 0.01, 95% confidence interval (CI) 66, 121]. Moreover, urinary nitrate and cGMP were increased from 92 +/- 10 to 125 +/- 15 micromol mmol(-1) creatinine (P = 0.01, 95% CI 8, 58) and from 38 +/- 3.3 to 50 +/- 6.7 nmol mmol(-1) creatinine (P = 0.04, 95% CI 0.4, 24), respectively. No treatment improved FMD over baseline. However, pooled analysis of all FMD data revealed a correlation between the increase of arginine/ADMA ratio and improvement of FMD.

CONCLUSION

Our data show for the first time that oral L-citrulline supplementation raises plasma L-arginine concentration and augments NO-dependent signalling in a dose-dependent manner.

Nutritional consequences of interspecies differences in arginine and lysine metabolism

Differences in lysine and arginine requirements among various species such as omnivores (humans, pigs, rats, dogs), carnivores (cats), herbivores (rabbits, horses), ruminants (cattle), poultry, and fish, are covered in detail in this article. Although lysine is classified as an indispensable amino acid across species, the classification of arginine as either an indispensable or dispensable amino acid is more ambiguous because of differences among species in rates of de novo arginine synthesis. Because lysine is most often the limiting amino acid in the diet, its requirement has been extensively studied. By use of the ideal protein concept, the requirements of the other indispensable amino acids can be extrapolated from the lysine requirement. The successful use of this concept in pigs is compared with potential application of the ideal protein concept in humans. The current dietary arginine requirement varies widely among species, with ruminants, rabbits, and rats having relatively low requirements and carnivores, fish, and poultry having high requirements. Interspecies differences in metabolic arginine utilization and reasons for different rates of de novo arginine synthesis are reviewed in detail, as these are the primary determinants of the dietary arginine requirement. There is presently no dietary requirement for humans of any age, although this needs to be reassessed, particularly in neonates. A thorough understanding of the factors contributing to the lysine and arginine requirements in different species will be useful in our understanding of human amino acid requirements.

Parenterally fed neonatal piglets have a low rate of endogenous arginine synthesis from circulating proline

Parenterally fed neonatal piglets cannot synthesize sufficient arginine to maintain arginine status, presumably due to the intestinal atrophy that occurs with parenteral feeding. Parenteral feeding-induced atrophy can be reduced by the infusion of glucagon-like peptide 2 (GLP-2). GLP-2 infusion was hypothesized to increase the rate of endogenous arginine synthesis from proline, the major arginine precursor, in parenterally fed piglets receiving an arginine-deficient diet. Male piglets, fitted with jugular vein catheters for diet and isotope infusion, and femoral vein catheters for blood sampling (d 0), were allocated to a continuous infusion of either GLP-2 (n = 5; 10 nmol x kg(-1) x d(-1)) or saline (n = 5) for 7 d. Piglets received 2 d of a complete diet, followed by 5 d of an arginine-deficient [0.60 g x kg(-1) x d(-1)] diet. Piglets received primed, constant infusions of [guanido-(14)C]arginine to measure arginine flux (d 6) and [U-(14)C]proline (d 7) to measure proline conversion to arginine. Plasma arginine concentrations and arginine fluxes indicated a similar whole-body arginine status. Piglets receiving GLP-2 showed improvements in intestinal variables, including mucosal mass (P < 0.01) and villus height (P < 0.001), and a greater rate of arginine synthesis (micromol x kg(-1) x h(-1)) from proline (11.6 vs. 6.3) (P = 0.03). Mucosal mass (R(2) = 0.71; P = 0.002) and villus height were correlated (R(2) = 0.66; P = 0.004) with arginine synthesis. This study was the first to quantitate arginine synthesis in parenterally fed neonates and showed that although GLP-2 infusion increased arginine synthesis in a manner directly related to mucosal mass, this increased arginine synthesis was insufficient to improve whole-body arginine status in piglets receiving a low arginine diet.

Oral N-carbamylglutamate supplementation increases protein synthesis in skeletal muscle of piglets

This study investigated the potential mechanisms by which oral supplementation of N-carbamylglutamate (NCG), an analogue of endogenous N-acetylglutamate (an activator of arginine synthesis) increases growth rate in sow-reared piglets. Two piglets of equal body weight (BW) and of the same gender from each lactating sow were allotted to receive oral administration of 0 (control) or 50 mg of NCG/kg BW every 12 h for 7 d. Piglets (n=32; BW=3 kg) were studied in the food-deprived or fed state following the 7 d of treatment. Overnight food-deprived piglets were given NCG or water (control) at time 0 and 60 min. Piglets studied in the fed state were gavage-fed sow's milk with their respective NCG treatment at 0 and 60 min. At 60 min, the piglets were administered a flooding dose of [3H]phenylalanine and killed at 90 min to measure tissue protein synthesis. Piglets treated with NCG gained 28% more weight than control pigs (P<0.001) over the 7-d period. Fed pigs had greater rates of protein synthesis in longissimus dorsi and gastrocnemius muscles and duodenum compared with food-deprived pigs (P<0.001). Absolute protein synthesis rates in longissimus dorsi (P=0.050) and gastrocnemius (P=0.068) muscles were 30 and 21% greater, respectively, in NCG-treated compared with control pigs. Piglets supplemented with NCG also had greater plasma concentrations of arginine and somatotropin than control pigs (P<0.001). The results suggest that oral NCG supplementation increases plasma arginine and somatotropin levels, leading to an increase in growth rate and muscle protein synthesis in nursing piglets.

Coadministration of ornithine and alpha-ketoglutarate is no more effective than ornithine alone as an arginine precursor in piglets enterally fed an arginine-deficient diet

Simultaneous administration of alpha-ketoglutarate and ornithine, in a 1:2 molar ratio, may improve the effectiveness of ornithine as an arginine precursor in neonatal piglets by shifting ornithine metabolism away from oxidation and toward the synthesis of arginine and other metabolically important compounds. To study this proposed mechanism, enterally fed piglets were allocated to receive 1 of 4 diets for 5 d: an arginine-deficient [1.2 mmol/(kg . d) arginine] diet (basal), or the basal diet supplemented with either alpha-ketoglutarate [4.6 mmol/(kg x d)] (+alpha-KG), ornithine [9.2 mmol/(kg x d)] (+Orn), or both ornithine and alpha-ketoglutarate (+alpha-KG/+Orn, molar ratio 1:2). Primed, constant infusions of [1-(14)C]ornithine given both intragastrically and intraportally were used to measure ornithine kinetics and determine the role of first-pass intestinal metabolism in ornithine metabolism. Whole body arginine and glutamate kinetics were measured using a primed, constant intragastric infusion of [guanido-(14)C]arginine and [3,4-(3)H]glutamate. The diets did not affect plasma arginine or ammonia concentrations, arginine flux, or arginine synthesis from ornithine. Therefore, arginine synthesis was not increased by the simultaneous infusion of ornithine and alpha-ketoglutarate. Piglets that received dietary ornithine had a 2-fold greater rate of proline synthesis from ornithine (P < 0.05) and oxidized a greater (P < 0.05) portion of the infused ornithine than piglets in the basal and +alpha-KG groups. Overall, ornithine addition to an arginine deficient diet had a greater effect on ornithine and arginine metabolism than the addition of alpha-ketoglutarate. First-pass intestinal metabolism was critical for ornithine synthesis and conversion to other metabolites but not for ornithine oxidation.