Modification of the effects of blood on amino acid metabolism by intravenous isoleucine

Asynchronous supply of indispensable amino acids reduces protein deposition in milk-fed calves

A balanced supply of indispensable amino acids (AA) is required for efficient protein synthesis. Different absorption kinetics (e.g., free vs. protein-bound AA) may, however, create asynchrony in postabsorptive availability of individual AA, thereby reducing the efficiency of protein deposition. We studied the effects of AA asynchrony on protein metabolism in growing, milk-fed calves. In 2 experiments, each with a change-over design including 8 calves, a milk replacer deficient in Lys and Thr was used. In Expt. 1, L-Lys and L-Thr were parenterally supplemented, either in synchrony (SYN), asynchrony (ASYN), or partial asynchrony (PART) with dietary AA. In Expt. 2, l-Lys and l-Thr were orally supplemented, either in SYN or ASYN with dietary AA. In Expt. 1, digested protein was used less efficiently for growth for ASYN (31.0%) than for SYN (37.7%), with PART being intermediate (36.0%). Indicator AA oxidation tended (P = 0.06) to be higher for ASYN. In Expt. 2, the efficiency of protein utilization was lower for ASYN (34.9%) than for SYN (46.6%). Calves spared AA from oxidation when the limiting AA were provided in excess after a short period (<24 h) of deprivation. Restoring AA balance by parenteral supplementation resulted in a 19% lower efficiency of digestible protein utilization than by oral supplementation, likely caused by splanchnic oxidation of imbalanced AA in excess to Thr. In conclusion, asynchronous availability of individual indispensable AA reduces the efficiency by which digested protein is retained in milk-fed calves. Furthermore, an AA imbalance in the splanchnic tissues may result in disproportionate AA oxidation.

Protein synthesis is severely diminished following a simulated upper GI bleed in patients with cirrhosis

BACKGROUND/AIMS

An upper gastrointestinal (GI) bleed in cirrhotic patients has been hypothesised to induce catabolism due to the absence of the essential branched chain amino acid (BCAA) isoleucine and an abundance of the BCAA leucine in haemoglobin. We tested whether an upper GI bleed produces hypoisoleucinemia via BCAA antagonism and impairs protein synthesis.

METHODS

Isoleucine turnover and oxidation was studied in 5 metabolically stable patients with cirrhosis during a 4-h period of intragastric saline infusion followed by a 4-h period in which an upper GI bleed was simulated by an amino acid solution mimicking haemoglobin.

RESULTS

The simulated upper GI bleed induced hypoisoleucinemia (26% of initial values) and an increase in leucine (400%) and valine (350%) concentrations. Isoleucine flux and isoleucine oxidation decreased to a third of initial values following a simulated bleed, but the fraction of isoleucine flux used for oxidation did not change. Consequently, the non-oxidative portion of isoleucine flux, representing protein synthesis, decreased similarly.

CONCLUSIONS

The present study shows that a simulated upper GI bleed induces hypoisoleucinemia and decreases protein synthesis markedly. The fact that the percentage of isoleucine flux that was oxidized was not influenced by the hypoisoleucinemic state can only be explained by BCAA antagonism.

Aromatic amino acid metabolism during liver failure

Liver failure is associated with hepatic encephalopathy (HE). An imbalance in plasma levels of aromatic amino acids (AAA) phenylalanine, tyrosine, and tryptophan and branched chain amino acids (BCAA) and their BCAA/AAA ratio has been suggested to play a causal role in HE by enhanced brain AAA uptake and subsequently disturbed neurotransmission. Until recently, data on this subject and the role of the liver and splanchnic bed were scarce, particularly in humans, due to inaccessibility of portal and hepatic veins. Here, we discuss, against a background of relevant literature, data obtained in patients undergoing liver resection or with a transjugular intrahepatic portasystemic stent shunt (TIPSS), where these veins are accessible. The BCAA/AAA ratio remained unchanged after major liver resection, but plasma AAA levels were inversely correlated (P < 0.001) with residual liver volume, in keeping with the observed hepatic AAA uptake. In patients with stable cirrhosis and a TIPSS, the plasma BCAA/AAA ratio was lower than in controls (1.19 +/- 0.09 vs. controls: 3.63 +/- 0.34). Gastrointestinal bleeding in cirrhotics with a TIPSS induced disturbances in BCAA levels and the BCAA/AAA ratio and induced catabolism, which could partly be corrected by isoleucine administration. AAA may be important in the pathogenesis of HE, but it is unlikely that they are the sole factors. HE most likely is a syndrome with multifactorial pathogenesis, where hyperammonemia, AAA/BCAA imbalances, inflammation, brain edema, and neurotransmitter changes interact. Novel therapies to normalize AAA levels in patients with liver failure (such as the molecular adsorbent recirculating system dialysis device) should probably be combined with supplementation of e.g. isoleucine and enhancing ammonia excretion by the kidneys.

Isoleucine infusion during "simulated" upper gastrointestinal bleeding improves liver and muscle protein synthesis in cirrhotic patients

Upper gastrointestinal (GI) bleeding in cirrhotic patients has a high incidence of mortality and morbidity. Postbleeding catabolism has been hypothesized to be partly due to the low biological value of hemoglobin, which lacks the essential amino acid isoleucine. The aims were to study the metabolic consequences of a "simulated" upper GI bleed in patients with cirrhosis of the liver and the effects of intravenous infusion of isoleucine. Portal drained viscera, liver, muscle, and kidney protein kinetics were quantified using a multicatheterization technique during routine portography. Sixteen overnight-fasted, metabolically stable patients who received an intragastric infusion of an amino acid solution mimicking hemoglobin every 4 hours were randomized to saline or isoleucine infusion and received a mixture of stable isotopes (L-[ring-2H5]phenylalanine, L-[ring-2H4]tyrosine, and L-[ring-2H2]tyrosine) to determine organ protein kinetics. This simulated bleed resulted in hypoisoleucinemia that was attenuated by isoleucine infusion. Isoleucine infusion during the bleed resulted in a positive net balance of phenylalanine across liver and muscle, whereas renal and portal drained viscera protein kinetics were unaffected. In the control group, no significant effect was shown.

CONCLUSION

The present study investigated hepatic and portal drained viscera protein metabolism selectively in humans. The data show that hepatic and muscle protein synthesis is stimulated by improving the amino acid composition of the upper GI bleed by simultaneous intravenous isoleucine administration.

Systemic and regional hemodynamics in pigs with acute liver failure and the effect of albumin dialysis

OBJECTIVE

Acute liver failure (ALF) is haemodynamically characterized by a hyperdynamic circulation. The aims of this study were to investigate the systemic and regional haemodynamics in ALF, to measure changes in nitric oxide metabolites (NOx) and to evaluate whether these haemodynamic disturbances could be attenuated with albumin dialysis.

MATERIAL AND METHODS

Norwegian Landrace pigs (23-30 kg) were randomly allocated to groups as controls (sham-operation, n = 8), ALF (hepatic devascularization, n = 8) and ALF + albumin dialysis (n = 8). Albumin dialysis was started 2 h after ALF induction and continued for 4 h. Systemic and regional haemodynamics were monitored. Creatinine clearance, nitrite/nitrate and catecholamines were measured. A repeated measures ANOVA was used to analyse the data.

RESULTS

In the ALF group, the cardiac index increased (PGT < 0.0001), while mean arterial pressure (PG = 0.02) and systemic vascular resistance decreased (PGT < 0.0001). Renal resistance (PG = 0.04) and hind-leg resistance (PGT = 0.003) decreased in ALF. There was no difference in jejunal blood flow between the groups. ALF pigs developed renal dysfunction with increased serum creatinine (PGT = 0.002) and decreased creatinine clearance (P = 0.02). Catecholamines were significantly higher in ALF, but NOx levels were not different. Albumin dialysis did not attenuate these haemodynamic or renal disturbances.

CONCLUSIONS

The haemodynamic disturbances during the early phase of ALF are characterized by progressive systemic vasodilatation with no associated changes in metabolites of NO. Renal vascular resistance decreased and renal dysfunction developed independently of changes in renal blood flow. After 4 h of albumin dialysis there was no attenuation of the haemodynamic or renal disturbances.

Adequate range for sulfur-containing amino acids and biomarkers for their excess: lessons from enteral and parenteral nutrition

The adequacy range of dietary requirements of specific amino acids in disease states is difficult to determine. In health, several techniques are available allowing rather precise quantification of requirements based on growth of the organism, rises in plasma concentration, or increases in the oxidation of marker amino acids during incremental administration of the amino acid under study. Requirements may not be similar in disease with regard to protein synthesis or with regard to specific functions such as scavenging of reactive oxygen species by compounds including glutathione. Requirements for this purpose can be assessed only when such a function can be measured and related to clinical outcome. There is apparent consensus concerning normal sulfur amino acid (SAA) requirements. WHO recommendations amount to 13 mg/kg per 24 h in healthy adults. This amount is roughly doubled in artificial nutrition regimens. In disease or after trauma, requirements may be altered for methionine, cysteine, and taurine. Although in specific cases of congenital enzyme deficiency, prematurity, or diminished liver function, hypermethionemia or hyperhomocysteinemia may occur, SAA supplementation can be considered safe in amounts exceeding 2-3 times the minimal recommended daily intake. Apart from some very specific indications (e.g., acetaminophen poisoning), the usefulness of SAA supplementation is not yet established. There is a growing body of data pointing out the potential importance of oxidative stress and resulting changes in redox state in numerous diseases including sepsis, chronic inflammation, cancer, AIDS/HIV, and aging. These observations warrant continued attention for the potential role of SAA supplementation. In particular, N-acetylcysteine remains promising for these conditions.

Casein and Soy Protein Meals Differentially Affect Whole-Body and Splanchnic Protein Metabolism in Healthy Humans

Dietary protein quality is considered to be dependent on the degree and velocity with which protein is digested, absorbed as amino acids, and retained in the gut as newly synthesized protein. Metabolic animal studies suggest that the quality of soy protein is inferior to that of casein protein, but confirmatory studies in humans are lacking. The study objective was to assess the quality of casein and soy protein by comparing their metabolic effects in healthy human subjects. Whole-body protein kinetics, splanchnic leucine extraction, and urea production rates were measured in the postabsorptive state and during 8-h enteral intakes of isonitrogenous [0.42 g protein/(kg body weight . 8 h)] protein-based test meals, which contained either casein (CAPM; n = 12) or soy protein (SOPM; n = 10) in 2 separate groups. Stable isotope techniques were used to study metabolic effects. With enteral food intake, protein metabolism changed from net protein breakdown to net protein synthesis. Net protein synthesis was greater in the CAPM group than in the SOPM group [52 +/- 14 and 17 +/- 14 nmol/(kg fat-free mass (FFM) . min), respectively; P < 0.02]. Urea synthesis rates decreased during consumption of both enteral meals, but the decrease tended to be greater in the subjects that consumed CAPM (P = 0.07). Absolute splanchnic extraction of leucine was higher in the subjects that consumed CAPM [306 +/- 31 nmol/(kg FFM . min)] vs. those that consumed SOPM [235 +/- 29 nmol/(kg FFM . min); P < 0.01]. In conclusion, a significantly larger portion of soy protein is degraded to urea, whereas casein protein likely contributes to splanchnic utilization (probably protein synthesis) to a greater extent. The biological value of soy protein must be considered inferior to that of casein protein in humans.

Amino Acid Adequacy in Pathophysiological States

Amino acid utilization and, therefore, demand differ between the healthy state and various disease states. In the healthy state most circulating amino acids are derived from dietary proteins that are stored and broken down in the gut and released gradually into the portal circulation, and from continuous turnover of body protein. In disease states, the amino acid composition of amino acids derived from periferal protein breakdown and released in the circulation, is different, for example because a substantial part of the branched-chain amino acids is broken down to yield glutamine and alanine, which are released in the circulation. It appears to be advantageous to mimic this continuous autoinfusion in patients, dependent of parenteral of enteral tube feeding. In disease, different endpoints should be used to assess the adequacy of the administered amino acid mix. Maintenance of a positive nitrogen balance and growth is less important than support of wound healing and immune function. Several amino acids such as glutamine, cysteine, and taurine are shown or suggested to be conditionally essential in disease, and to form substrate in the stressed patient for anabolic processes in liver, immune system, and injured sites. Amino acid toxicity is rare, and protein restriction for patients with renal or liver failure is obsolete because this only aggravated malnutrition. A true example of protein toxicity consists of gastrointestinal hemorrhage that precipitates hepatic encephalopathy in liver insufficiency, most likely because hemoglobin is an unbalanced protein because it lacks the essential amino acid isoleucine.

Renal metabolism of amino acids: its role in interorgan amino acid exchange

The kidneys play a role in the synthesis and interorgan exchange of several amino acids. The quantitative importance of renal amino acid metabolism in the body is not, however, clear. We review here the role of the kidney in the interorgan exchange of amino acids, with emphasis on quantitative aspects. We reviewed relevant literature by using a computerized literature search (PubMed) and checking relevant references from the identified articles. Our own data are discussed in the context of the literature. The kidney takes up glutamine and metabolizes it to ammonia. This process is sensitive to pH and serves to maintain acid-base homeostasis and to excrete nitrogen. In this way, the metabolism of renal glutamine and ammonia is complementary to hepatic urea synthesis. Citrulline, derived from intestinal glutamine breakdown, is converted to arginine by the kidney. Renal phenylalanine uptake is followed by stoichiometric tyrosine release, and glycine uptake is accompanied by serine release. Certain administered oligopeptides (eg, glutamine dipeptides) are converted by the kidneys to their constituent components before they can be used in metabolic processes. The kidneys play an important role in the interorgan exchange of amino acids. Quantitatively, for several important amino acids, the kidneys are as important as the gut in intermediary metabolism. The kidneys may be crucial "mediators" of the beneficial effects of specialized, disease-specific feeding solutions such as those enriched in glutamine dipeptides.

The pathophysiological basis of acute-on-chronic liver failure

The vast majority of patients that are referred to a specialist hepatological centre suffer from acute deterioration of their chronic liver disease. Yet, this entity of acute-on-chronic liver failure remains poorly defined. With the emergence of newer liver support strategies, it has become necessary to define this entity, its pathophysiology and the short and long-term prognosis. This review focuses upon how a precipitant such as an episode of gastrointestinal bleeding or sepsis may start a cascade of events that culminate in end-organ dysfunction and liver failure. We briefly review the pathophysiological basis of the therapeutic modalities that are available. Our current strategy for the management of liver failure involves supportive therapy for the end-organs with the hope that the liver function would recover if sufficient time for such a recovery is allowed. Because liver failure, whether of the acute or acute-on-chronic variety, is potentially reversible, the stage is set for the application of newer liver support strategies to enhance the recovery process.

Induced hyperammonemia alters neuropsychology, brain MR spectroscopy and magnetization transfer in cirrhosis

Hyperammonemia is a universal finding after gastrointestinal hemorrhage in cirrhosis. We administered an oral amino acid solution mimicking the hemoglobin molecule to examine neuropsychological changes, brain glutamine levels, and brain magnetization transfer ratio (MTR). Forty-eight metabolically stable patients with cirrhosis and no evidence of "overt" hepatic encephalopathy (HE) were randomized to receive 75 g of amino acid solution or placebo; measurements were performed before and 4 hours after administration. Neuropsychological tests included the Trails B Test, Digit Symbol Substitution Test, memory subtest of the Randt battery, and reaction time. Plasma was collected for ammonia and amino acid measurements, and brain metabolism was studied using proton magnetic resonance (MR) spectroscopy in the first 16 randomized patients. In 7 other patients, MTR was measured. A significant increase in ammonia levels was observed in the amino acid group (amino acid group, 76 +/- 7.3 to 121 +/- 6.4 micromol/L; placebo, 83 +/- 3.3 to 78 +/- 2.9 micromol/L; P <.001). Neuropsychological function improved significantly in the placebo group, but no significant change in neuropsychological function was observed in the amino acid group. Brain glutamate/glutamine (Glx)/creatine (Cr) ratio increased significantly in the amino acid group. MTR decreased significantly from 30 +/-2.9 to 23 +/- 4 (P <.01) after administration of the amino acid solution. In conclusion, an improvement in neuropsychological test results followed placebo, which was not observed in patients administered the amino acid solution. Induced hyperammonemia resulted in an increase in brain Glx/Cr ratio and a decrease in MTR, which may indicate an increase in brain water as the operative mechanism.

Interorgan ammonia metabolism in liver failure

In the post-absorptive state, ammonia is produced in equal amounts in the small and large bowel. Small intestinal synthesis of ammonia is related to amino acid breakdown, whereas large bowel ammonia production is caused by bacterial breakdown of amino acids and urea. The contribution of the gut to the hyperammonemic state observed during liver failure is mainly due to portacaval shunting and not the result of changes in the metabolism of ammonia in the gut. Patients with liver disease have reduced urea synthesis capacity and reduced peri-venous glutamine synthesis capacity, resulting in reduced capacity to detoxify ammonia in the liver. The kidneys produce ammonia but adapt to liver failure in experimental portacaval shunting by reducing ammonia release into the systemic circulation. The kidneys have the ability to switch from net ammonia production to net ammonia excretion, which is beneficial for the hyperammonemic patient. Data in experimental animals suggest that the kidneys could have a major role in post-feeding and post-haemorrhagic hyperammonemia.During hyperammonemia, muscle takes up ammonia and plays a major role in (temporarily) detoxifying ammonia to glutamine. Net uptake of ammonia by the brain occurs in patients and experimental animals with acute and chronic liver failure. Concomitant release of glutamine has been demonstrated in experimental animals, together with large increases of the cerebral cortex ammonia and glutamine concentrations. In this review we will discuss interorgan trafficking of ammonia during acute and chronic liver failure. Interorgan glutamine metabolism is also briefly discussed, since glutamine synthesis from glutamate and ammonia is an important alternative pathway of ammonia detoxification. The main ammonia producing organs are the intestines and the kidneys, whereas the major ammonia consuming organs are the liver and the muscle.

L-arginine supplementation in pigs decreases liver protein turnover and increases hindquarter protein turnover both during and after endotoxemia

BACKGROUND

Accumulating evidence suggests that L-arginine, under conditions of septicemia, not only enhances immune function but also improves protein metabolism.

OBJECTIVE

Because the effect of L-arginine administration on the protein metabolism of different organs is unknown, the aim of the study was to elucidate the effects of exogenous supplementation of L-arginine during endotoxemia on the in vivo protein metabolism of individual organs and at the whole-body level.

DESIGN

Female pigs were cannulated with catheters in the aorta and the splenic, caval, portal, hepatic, and renal veins, enabling measurements across the hindquarter, portal-drained viscera, liver, and kidneys. Endotoxemia was induced by a 24-h continuous intravenous infusion of endotoxin (3 microg x kg body wt(-1) x h(-1)). At 8 h, an intravenous infusion of L-arginine was started (n = 8). Control pigs (n = 6) received L-alanine. At 24 h, blood was sampled. After cessation of the endotoxin infusion, L-arginine and L-alanine infusions were continued as a supplement in the enterally infused diet. At 48 h, blood samples were obtained during the postendotoxemic and nutritionally supported conditions. Stable isotopes were used to assess protein metabolism and phenylalanine hydroxylation.

RESULTS

Both during and after the endotoxin challenge, L-arginine administration enhanced protein synthesis and degradation across the hindquarter and simultaneously reduced protein synthesis and degradation in the liver at equal rates. Protein turnover across the kidneys and portal-drained viscera remained unaffected. After endotoxemia, L-arginine infusion decreased whole-body protein turnover without affecting the net protein balance.

CONCLUSION

L-Arginine administration affects protein turnover of the muscle area and the liver oppositely.

Interorgan amino acid exchange

This review is concerned with the status of our current research related to the exchange of amino acids across organs. Accumulation of knowledge regarding how amino acid pools are maintained within the body remains a work in progress. In recent years, the use of organ balance measurement techniques in combination with isotopic tracers has much increased our understanding of the role of the kidney and splanchnic organs in amino acid metabolism, and in kidney and liver gluconeogenesis from amino acids. An interorgan cooperation between the kidney and splanchnic organs for leucine-ketoisocaproate metabolism has also been demonstrated.

Enhanced renal vein ammonia efflux after a protein meal in the pig

BACKGROUND/AIMS

The intake of dietary protein has been associated with increased arterial ammonia levels. However, the origin of this rise in ammonia levels is unknown. This study was designed to examine whether this increase is caused by ammonia formed by the gut escaping hepatic clearance, or ammonia formed by the kidney and subsequently released into the circulation.

METHODS

Splanchnic and renal fluxes of ammonia and amino acids were studied in 10 pigs that were fed in a randomized cross-over design with a protein meal (n = 8), a meal with an equimolar amount of free amino acids (n = 8) or an iso-osmolar NaCl solution (n = 6).

RESULTS

After the protein meal, and less pronounced after the amino acid meal, arterial ammonia levels increased from approximately 25 to 75 micromol/l. Arterial pH changes and splanchnic ammonia release were negligible. The renal vein ammonia efflux increased after the protein meal (0.67+/-0.10 to 1.94+/-0.35 micromol/kg bw/min) and to a lesser degree after the amino acid meal (to 1.20+/-0.39 micromol/kg bw/ min). Renal uptake of alanine, and not glutamine, increased stoichiometrically, paralleling the enhanced renal vein ammonia efflux.

CONCLUSIONS

Arterial ammonia increases after a meal in pigs, coinciding with a negligible splanchnic ammonia release, but increased renal vein ammonia efflux. Thus, post-prandial plasma ammonia levels appear to be mainly related to renal ammoniagenesis. Alanine appears to be the main precursor for this renal ammoniagenesis in the pig.

Upper gastrointestinal bleeding: an ammoniagenic and catabolic event due to the total absence of isoleucine in the haemoglobin molecule

Upper gastrointestinal bleeding causes increased urea concentrations in patients with normal liver function and high ammonia concentrations in patients with impaired liver function. This ammoniagenesis may precipitate encephalopathy. The haemoglobin molecule is unique because it lacks the essential amino acid isoleucine and has high amounts of leucine and valine. Upper gastrointestinal bleeding therefore presents the gut with protein of very low biologic value, which may be the stimulus to induce a cascade of events culminating in net catabolism. This may influence the function of rapidly dividing cells and short half-life proteins. We hypothesize that, following a variceal bleed in a cirrhotic patient, the lack of isoleucine in blood protein is the cause of the exaggerated ammoniagenesis and catabolism. We propose that intravenous administration of isoleucine may serve as a simple therapeutic that transforms blood protein in a balanced protein, resulting in only a short-lived rise in ammonia and urea production, and preventing interference with protein synthesis.

Infusion of soy and casein protein meals affects interorgan amino acid metabolism and urea kinetics differently in pigs

For routine evaluation of the quality of dietary protein, amino acid scoring patterns were used. Evaluation of this pattern for soy and casein revealed that these proteins are of almost equal quality. However, in vivo studies showed a large difference. To study the biological effects of meals with casein and soy protein, the contributions of individual amino acids to net protein retention and amino acid kinetics in gut, liver and muscle in healthy pigs were investigated. Isonitrogenous enteral nutrition, infused at a rate of 10 mL. kg body wt-1. h-1 and consisting of maltodextrin (137 g/L) with added casein (53 g/L) or soy protein (68 g/L), was given to conscious, healthy female multicathetized pigs (20-22 kg, n = 12). A primed-constant infusion protocol with L-[ring-2,6-3H]phenylalanine, L-[3,4-3H]valine and [15N-15N]urea was used to measure amino acid and urea kinetics in gut, liver and muscle. Measurements were done postabsorptively and 2-6 h after initiation of the enteral nutrition. During the meal, appearance of amino acids into the portal vein and the uptake by the liver was lower with casein infusion. Muscle uptake did not differ. Gut protein synthesis tended to be lower with soy infusion (P = 0.1). Liver protein synthesis and degradation were higher with casein infusion (P < 0.05), while in muscle, soy infusion stimulated protein turnover (P < 0.05). In comparison to the postabsorptive condition, liver urea production was unchanged after casein infusion, while it was significantly increased after soy infusion. These results suggest that the quality of soy protein is inferior to that of casein protein.

In vivo inter-organ protein metabolism of the splanchnic region and muscle during trauma, cancer and enteral nutrition

The study of protein kinetics has entered a new era by the recognition that whole body protein turnover only poorly reflects the true events occurring in several organs and with regard to the multitude of proteins present in the body. It is also increasingly recognized that the simultaneous synthesis and degradation of proteins is important in regulation and adaptation during several metabolic conditions like starvation, feeding, after trauma, and during exercise. Especially important is the recognition that the kinetics of individual proteins may change in opposite directions, thereby leading to fluxes of alpha-amino-nitrogen that serve to adapt to and survive a changing environment. At present, much emphasis is put upon molecular biological regulation. However, it is important that the metabolic processes that occur in the intact organism are still poorly defined. New technology allows the exploration of these processes, which should therefore prompt the initiation of further research in this area.

Decreased plasma isoleucine concentrations after upper gastrointestinal haemorrhage in humans

BACKGROUND

A decrease in arterial isoleucine values after intragastric blood administration in pigs has been observed. This contrasted with increased values of most other amino acids, ammonia, and urea. After an isonitrogenous control meal in these pigs all amino acids including isoleucine increased, and urea increased to a lesser extent, suggesting a relation between the arterial isoleucine decrease and uraemia after gastrointestinal haemorrhage.

METHODS

To extend these findings to humans, plasma amino acids were determined after gastrointestinal haemorrhage in patients with peptic ulcers (n = 9) or oesophageal varices induced by liver cirrhosis (n = 4) and compared with preoperative patients (n = 106).

RESULTS

After gastrointestinal haemorrhage, isoleucine decreased in all patients by more than 60% and normalised within 48 hours. Most other amino acids increased and also normalised within 48 hours. Uraemia occurred in both groups, hyperammonaemia was seen in patients with liver cirrhosis.

CONCLUSIONS

These results confirm previous findings in animals and healthy volunteers that plasma isoleucine decreases after simulated upper gastrointestinal haemorrhage. This supports the hypothesis that the absence of isoleucine in blood protein causes decreased plasma isoleucine values after gastrointestinal haemorrhage, and may be a contributory factor to uraemia and hyperammonaemia in patients with normal and impaired liver function, respectively. Intravenous isoleucine administration after gastrointestinal haemorrhage could be beneficial and will be the subject of further research.

Intragastric bolus feeding of meals containing elementary, partially hydrolyzed or intact protein causes comparable changes in interorgan substrate flux in the pig

Dietary protein given as pre-digested protein improves the nutritional value of the meal. However, studies measuring absorption kinetics of pre-digested protein or free amino acid mixtures are scarce and suffer from methodological problems. Therefore, the study was designed to study whether differences in absorption kinetics play a role. The kinetics of substrate production or consumption after a rapid gastrically-infused meal was studied across the portal drained viscera, liver and hindquarter in conscious, multicatheterized healthy pigs of 20-22 kg (n = 12). The meal contained carbohydrates and protein (1.44 g/kg body weight) as intact whey protein isolate, moderately-hydrolyzed protein digest or equivalent amino acid mixture (including glutamine and asparagine). For almost all amino acids and glucose, intestinal production, liver and hindquarter uptake were similar. The higher liver urea production (less than 15% of total alpha-amino intake) after the meals with pre-digested protein or free amino acids was related to the marginally higher intestinal glutamine breakdown (not significant) and ammonia production. Our results suggest that in the normal healthy pig, uptake and metabolism of moderately hydrolyzed,free amino acid or intact protein meals with identical composition is not different.

Supplementation of enteral nutrition with butyrate leads to increased portal efflux of amino acids in growing pigs with short bowel syndrome

Previously, short-chain fatty acids (SCFAs) infused into the hindgut or administered intravenously have been shown to stimulate intestinal adaptation after massive small bowel resection. To study the effects of enterally supplemented n-butyrate on food digestion and absorption in growing pigs with short bowel syndrome, the authors examined the portal efflux of glucose and amino acids during a meal. In 12 growing pigs, 75% of the small intestine was resected. Five control (CONT) animals underwent transection and reanastomosis of the small bowel. A splenic vein, the aorta, the portal vein, and the stomach were catheterized. Postoperatively, seven enterectomized (ENT) pigs and the CONT pigs were fed by infusion of a liquid diet, without SCFAs, through the gastrostomy catheter. Five enterectomized animals received the same diet, supplemented with butyrate (ENTB) (0.26 g/kg body weight/d). After 3 weeks, the portal efflux of amino acids and glucose was measured after 2 hours of constant feeding. The portal efflux of glucose expressed per kilogram of body weight in the ENT group was 10% of that in the CONT group, and in the ENTB group it was 42%. No significant difference in portal glucose efflux between the ENT and the ENTB groups was found. The portal efflux of amino acids during a meal in the ENT group in relation to the CONT groups was 34%; in the ENTB group it was 63%. These data suggest that enteral supplementation with SCFAs leads to improvement of intestinal food digestion and absorption during short bowel syndrome, possibly related to improved intestinal adaptation.

Influence of storage conditions on normal plasma amino-acid concentrations

Conflicting information in the literature is given concerning the optimal preparation and storage conditions of plasma samples for amino-acid analysis. To assess the optimal pre-storage treatment, we compared several methods and studied their influence on plasma amino-acid levels of rats and humans, stored at different temperatures. In rat plasma, the frequently reported degradation of glutamine was not measurable at a storage temperature of -70 degrees C. However, storage of native, not deproteinised plasma at this temperature, resulted in a 32% decrease of arginine and a 30% increase in ornithine after 24 weeks. Deproteinisation prohibited this arginine decay. At -20 degrees C, arginine decay was even more pronounced, whereas glutamine decreased by 14% in untreated plasma, by 10% in sulfosalicylic acid deproteinised plasma and by 3% if the deproteinisation was followed by removal of the protein pellet and subsequent neutralisation. To confirm these unexpected results in humans, we repeated this experiment with plasma of 6 volunteers. In contrast to rat plasma, we did not observe any changes in arginine and ornithine concentrations in human plasma stored at -70 degrees C. At -20 degrees C the reduction in glutamine was only 4-5%. These results suggest that interspecies differences in enxymatic activity exist in plasma. Finally, having assessed the optimal treatment and storage conditions (deproteinisation followed by storage at -70 degrees C), samples were obtained from a total of 112 human volunteers, stratified for age and sex, and amino-acids were measured. In the female group, we found a tendency to a gradual increase in most amino-acid concentrations with advancing age, which however only reached significance for histidine, citrulline, alanine and leucine. These observations demonstrate that plasma samples for amino-acid analysis should be deproteinised and stored at -70 degrees C. Also important interspecies differences appear to exist in plasma enzymatic activity. Finally, control samples should be taken from an age and sex matched control group.

Modification of the analysis of amino acids in pig plasma

Determination of amino acids in pig plasma with the classical ninhydrin system is influenced by the excessive amount of protein and lipophilic compounds in the sample, leading to a decline in resolution. This problem was eliminated by using 80 mg of sulphosalicylic acid per ml of plasma, and solid-phase extraction with a C18 cartridge as an additional clean up step. The latter resulted in significantly higher quantities of threonine, asparagine, glutamic acid, glutamine, glycine, alanine, valine and lysine, and lower levels of phenylalanine and tryptophan (P < 0.05). The use of a C18 cartridge had a minor effect on the analytical error.