Intravenous Amino Acid Infusion Research Articles

Post-ruminal or intravenous infusions of carbohydrates or amino acids to dairy cows 2. Late lactation

The objectives of this study were to compare the effects of post-ruminal and intravenous infusions of wheat starch or glucose (CHO) or a mixture of amino acids (AA) on milk protein yield, nitrogen (N) utilisation, plasma metabolites and mammary extraction rate of dairy cows in late lactation. Eight cow, ruminally fistulated, was assigned to two 4 × 4 Latin squares during 14-day periods, where the last 7 days were for infusions. Infusions were: (1) starch in the abomasum (SP), (2) glucose in the blood (GB), (3) AA in the abomasum (AP), and (4) AA in the blood (AB). The experiment started 165 ± 4 days (mean ± s.e.) post partum (milk yield 22.5 ± 1.1 kg) Daily amounts of nutrients infused were 257, 283, 233, and 260 g for SP, GB, AP and AB, respectively. The cows were fed a basal diet consisting of a concentrate mixture and grass silage (55:45 on a dry-matter (DM) basis), where total dry-matter intake (DMI) was 13.3 kg/day. Milk production was affected by site of infusion within substrate, whereas infusion substrates within infusion site (CHO or AA) were of minor importance. Responses to intravenous infusions (GB or AB) were similar to those in early lactation, but more pronounced. Compared with SP infusion, GB infusion increased ( P < 0.05) milk yield, energy-corrected milk (ECM), protein and lactose yield by 1.4 and 0.9 kg, 38 and 59 g, respectively. The AB infusion had 1.4 and 1.3 kg, 51, 52 and 50 g higher ( P < 0.05) milk yield, ECM, protein, fat and lactose yields than the AP infusion, respectively. N balance data indicated higher losses of metabolic faecal nitrogen (MFN) by abomasal than by intravenous infusions, but the catabolism of AA was lower than in early lactation indicated by no difference ( P < 0.05) in urinary N excretion between treatments. Intravenous AA infusion increased plasma glucose and insulin above that of intravenous glucose infusion. The treatment effects on plasma insulin concentrations were higher in late than in early lactation, suggesting a higher sensitivity in late lactation even at similar negative energy balance. Compared with the SP infusion, GB infusion showed lower ( P < 0.05) concentrations of essential AA (EAA) and branched-chain AA (BCAA) resulting in a higher AA utilisation because of a higher milk protein production. AP infusion increased ( P < 0.05) plasma non-essential AA concentration compared with AB infusion, but infusion site of AA had no effect ( P>0.05) on plasma EAA or BCAA. It is concluded that it is the nutrient supply and not the lactation stage per se that is important for the response in milk production. Nevertheless, stage of lactation affects the N metabolism and the response in plasma hormone concentrations even when cows are in negative energy balance in both lactation stages.

Post-ruminal or intravenous infusions of carbohydrates or amino acids to dairy cows 1. Early lactation

The objectives of this study were to compare the effects of post-ruminal and intravenous infusions of wheat starch or glucose (CHO) or a mixture of amino acids (AA) on milk protein yield, nitrogen utilisation, plasma metabolites and mammary extraction rate of dairy cows in early lactation. Eight cow, ruminally fistulated, was assigned to two 4 × 4 Latin squares during 14-day periods, where the last 7 days were for infusions. Infusions were: (1) starch in the abomasum (SP), (2) glucose in the blood (GB), (3) AA in the abomasum (AP), and (4) AA in the blood (AB). The experiment started 54 ± 4 days (mean ± s.e.) post partum (milk yield 33.4 ± 1.7 kg). Daily amounts of nutrients infused were 378, 365, 341, and 333 g for SP, GB, AP and AB, respectively. The cows were fed a basal diet consisting of a concentrate mixture and grass silage (55:45 on dry-matter (DM) basis), and DM intake was 17.2 kg/day. Milk production was affected by site of infusion within substrate, whereas infusion substrates within infusion site (CHO or AA) were of minor importance. Compared with SP infusion, GB infusion increased ( P < 0.05) milk protein yield and concentration by 55 g and 1 g/kg. The AB infusion tended to ( P < 0.10) increase milk yield and ECM and increased ( P < 0.05) protein yield and concentration by 1.8 and 2.2 kg, 83 g and 1.1 g/kg compared with AP infusion, respectively. Nitrogen balance data indicated higher losses of metabolic faecal nitrogen (MFN) by abomasal than by intravenous infusions, and an increased ( P < 0.05) catabolism for AP and AB infusions compared with SP and GB infusions. GB infusion did not increase ( P>0.10) plasma glucose or insulin concentrations above that of SP infusion. Compared with the SP infusion, the GB infusion had minor effect on plasma AA. AP infusion increased ( P < 0.05) plasma non-essential AA (NEAA) concentration compared with AB infusion, whereas infusion site of AA had no effect ( P>0.05) on essential AA (EAA) or branched-chain AA (BCAA). Although a higher milk protein synthesis was observed for AB infusion, the mammary extraction rate was not higher ( P>0.05) than for AP infusion. Across infusion site, AP and AB infusions increased plasma concentration of EAA and BCAA, but compared with GB infusion, the mammary extraction rates tended ( P < 0.10) to be lower. It is concluded that abomasal nutrient infusion increases loss of MFN and that the gastrointestinal metabolism influences the nutrients available for milk synthesis. Our conclusion is that when glucose was infused, AA limited a further milk protein synthesis, but when AA was infused, glucose or energy substrate might have been the limiting factor. Our results verify that glucogenic substrates are limiting when cows are in negative energy balance.

Characterization of proteinuria and tubular protein uptake in a new model of oral L-lysine administration in rats

Intravenous infusion of basic amino acids is used experimentally and pharmacologically to prevent renal proximal tubular uptake of filtered proteins. Intravenously injected L-lysine is rapidly cleared from plasma and the effect on tubular protein reabsorption is transient. To obtain a more sustained effect, we developed a model of oral L-lysine administration and characterized this model by analyzing urinary protein excretion and proximal tubule uptake of filtered proteins. Rats placed in metabolic cages were treated with 20 mmol/kg/6 h of L-lysine, glycine, or water. Urines were analyzed for proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and radioimmunoassay. Proximal tubule uptake of proteins and expression of apical membrane receptors were investigated by immunocytochemistry. In vitro uptake and receptor expression were studied using a yolk sac cell line. L-lysine administration produced increased urinary excretion of a large number of proteins while the effect on tubular accumulation of selected proteins was variable. L-lysine treatment induced changes in the localization of two receptors responsible for tubular endocytosis of filtered proteins. In conclusion, oral L-lysine treatment induced proteinuria, in particular albuminuria, as efficiently as previous reports on intravenous infusion. The effect on tubular protein accumulation was variable suggesting differential effects on tubular reabsorption and degradation of filtered proteins. Changes in tubular protein handling were accompanied by changes in the localization of the endocytic receptors, megalin, and cubilin. In vitro experiments supported the in vivo observations. The findings suggest that L-lysine may affect receptor trafficking in addition to possible effects on the direct binding of ligands to the receptors.

Improvement of regional cerebral blood flow after treatment with branched-chain amino acid solutions in patients with cirrhosis

Objective The administration of solutions rich in branched-chain amino acids leads to mental recovery from acute hepatic encephalopathy in patients with liver cirrhosis. However, the mechanism of action of branchedchain amino acids remains unclear. The purpose of this study was to evaluate the effect of intravenous infusion of branched-chain amino acids on brain perfusion in patients with liver cirrhosis. Methods Single photon emission computed tomography scans were performed in 14 patients with liver cirrhosis before and after the administration of branched-chain amino acids in a single-day split-dose protocol. The per cent change in regional brain perfusion was calculated in high frontal, parietal, temporal, occipital lobes and cerebellum. Thereafter, statistical parametric mapping was performed to identify brain regions with abnormal cerebral perfusion. Results Intravenous infusion of solutions enriched with branched-chain amino acids induced a 13-20% increase in regional cerebral blood flow. Cirrhotic patients had regions of significant hypoperfusion, as determined by statistical parametric mapping, in the left superior parietal and posterior cingulate as compared to the control group. This hypoperfusion of parietal and cingulate regions was not detected after treatment with solutions of branchedchain amino acids. Conclusions The results of the present study suggest that administration of solutions enriched with branched-chain amino acids improves cerebral perfusion in patients with cirrhosis.

Regulation of translation initiation by insulin and amino acids in skeletal muscle of neonatal pigs.

Previous studies have shown that intravenous infusion of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates and that insulin and amino acids act independently to produce this effect. The goal of the present study was to delineate the regulatory roles of insulin and amino acids on muscle protein synthesis in neonates by examining translational control mechanisms, specifically the eukaryotic translation initiation factors (eIFs), which enable coupling of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. Insulin secretion was blocked by somatostatin in fasted 7-day-old pigs (n = 8-12/group), insulin was infused to achieve plasma levels of approximately 0, 2, 6, and 30 microU/ml, and amino acids were clamped at fasting or fed levels or, at the high insulin dose, below fasting. Both insulin and amino acids increased the phosphorylation of ribosomal protein S6 kinase (S6K1) and the eIF4E-binding protein (4E-BP1), decreased the binding of 4E-BP1 to eIF4E, increased eIF4E binding to eIF4G, and increased fractional protein synthesis rates but did not affect eIF2B activity. In the absence of insulin, amino acids had no effect on these translation initiation factors but increased the protein synthesis rates. Raising insulin from below fasting to fasting levels generally did not alter translation initiation factor activity but raised protein synthesis rates. The phosphorylation of S6K1 and 4E-BP1 and the amount of 4E-BP1 bound to eIF4E and eIF4E bound to eIF4G were correlated with insulin level, amino acid level, and protein synthesis rate. Thus insulin and amino acids regulate muscle protein synthesis in skeletal muscle of neonates by modulating the availability of eIF4E for 48S ribosomal complex assembly, although other processes also must be involved.

Effect of preoperative amino acid infusion on thermoregulatory response during spinal anaesthesia

Intravenous amino acid infusion during general anaesthesia prevents decreases in core temperature resulting from increased energy expenditure and heat accumulation. We investigated whether such stimulation also occurs during spinal anaesthesia, which blocks sympathetic nervous activity. We examined the effect of i.v. amino acid infusion on changes in core temperature during spinal anaesthesia. Thirty-five patients were divided into two groups: an i.v. amino acid infusion group (4 kJ kg(-1) h(-1) starting 2 h before surgery); and a saline infusion group. Tympanic membrane core temperature, forearm-fingertip temperature gradient (an index of peripheral vasoconstriction) and mean skin temperature were measured for 90 min after the onset of spinal anaesthesia. Changes in mean arterial pressure and heart rate did not differ significantly between the groups during the study period. Mean final core temperature 90 min after induction of spinal anaesthesia was 35.8 (SEM 0.1) degrees C in the saline group and 36.6 (0.1) degrees C in the amino acid group (P<0.05). The increased level of oxygen consumption in the amino acid group compared with the saline group was preserved even after the onset of spinal anaesthesia. The thermal vasoconstriction threshold, defined as the tympanic membrane temperature that triggered a rapid increase in forearm-fingertip temperature gradient, was increased in the amino acid group [36.8 (0.1) degrees C] compared with the saline group [36.5 (0.1) degrees C] (P<0.05). Preoperative infusion of amino acids effectively prevents spinal anaesthesia-induced hypothermia by maintaining a higher metabolic rate and increasing the threshold core temperature for thermal vasoconstriction.

The effect of a maternal infusion of amino acids on umbilical uptake in pregnancies complicated by intrauterine growth restriction

Objective: To establish whether, in human pregnancies complicated by intrauterine growth restriction (IUGR), the maternal intravenous infusion of amino acids can increase fetal amino acid concentrations and umbilical uptake. Study Design: Before elective cesarean delivery, a solution of amino acids was infused into a maternal vein in 8 patients with pregnancies complicated by IUGR (experimental group). At cesarean delivery, maternal, umbilical venous, and arterial blood samples were obtained. Ten comparable IUGR pregnancies were compared with those in the experimental group. Results: In the experimental group, all maternal amino acid concentrations were increased significantly. In the umbilical vein, valine, methionine, isoleucine, leucine, phenylalanine, arginine, serine, glycine, and proline concentrations were elevated. Umbilical venoarterial differences of amino acid per mole of oxygen for leucine, isoleucine, methionine, arginine, glycine, serine, and proline were elevated but not for lysine, histidine, threonine, valine, and phenylalanine. Conclusion: In pregnancies complicated by IUGR, increasing the maternal concentration of amino acids leads to an increased umbilical uptake of some of the amino acids to the fetus. There was no evidence of a change in the uptake of 3 essential amino acids: lysine, histidine, and threonine. (Am J Obstet Gynecol 2002;187:741-6.)

Responses of milk production to the intravenous infusion of amino acids in dairy cows given diets of grass silage and cereal-based supplements.

Three experiments were carried out to examine responses of milk production to the intravenous infusion of amino acids in dairy cows given diets of grass silage and supplements based on barley, with or without added soyabean meal and ranging in crude protein content from 16 to 19% in dry matter. Particular attention was given to histidine, administered alone or in combination with methionine, lysine and tryptophan. Responses of milk protein secretion to infusion of histidine were seen only when the diet contained a supplement of barley alone. When soyabean meal was included, there were no responses of milk production to infusion of any of the infused amino acids. Calculations suggested that, although histidine remained first-limiting when soya was included in the diet, any response to infusion of histidine was blocked by the rapidly emerging deficiency of another amino acid, probably leucine. The results confirm that, for diets based on grass silage and supplements of cereal only, histidine is first-limiting such that increases of milk protein secretion can be obtained in response to infusion of histidine alone. In assessing the practical significance of this finding, it should be remembered that greater responses in the yield of milk protein can probably be obtained by substituting 1 kg of soyabean meal for 1 kg of cereal, which is likely to be an easier and cheaper option.

Effects of insulin and amino acids on leucine metabolism in young and middle-aged humans.

Aging is characterized by loss of muscle mass. In healthy subjects this process is associated with hormone and nutritional changes which take place over many decades. Aim of the study To investigate the effects of insulin and amino acids on amino acid metabolism in middle-aged humans. We evaluated leucine kinetics by means of the intravenous infusion of [1-(14C)]leucine, in 8 young (age 24 +/- 2 yr, BMI 21+/- 2 kg/M2) and in 6 middle-aged (age 53 +/- 4 yr, BMI 26 +/- 1 kg/M2) healthy subjects. Studies were performed under fasting conditions (basal), and after 180 min of euglycemic hyperinsulinemic clamp (study I), or 180 min of euglycemic hyperinsulinemia in combination with an intravenous amino acid infusion (study II). In the basal state endogenous leucine flux (ELF, an index of proteolysis), normalized for IBW, averaged 1.71 +/- 0.12 and 1.66 +/- 0.14 micromol/kg x min in young and middle-aged subjects, respectively. Basal leucine oxidation (0.22 +/- 0.03 vs 0.28 +/- 0.03 micromol/ kg x min, p < 0.05) was lower in middle-aged with respect to young subjects. Non-oxidative leucine disposal (NOLD, an index of protein synthesis: 1.44 +/- 0.11 vs 1.43 +/- 0.11 micromol/kg x min) was similar in young and middle-aged subjects, respectively. In response to insulin (study I) the absolute and percent decline of ELF and LOX were similar in young and middle-aged subjects: ELF declined to 1.05 +/- 0.06 micromol/kg x min (-39 +/- 5 %) and 1.07 +/- 0.14 micromol/kg x min (-36 +/- 4%), in young and middle-aged, respectively (both p < 0.01 vs basal); LOX declined to 0.21 +/- 0.02 micromol/kg min (-35 +/- 3 %), and 0.18 +/- 0.05 micromol/kg x min (-28 +/- 3%, p < 0.05 vs basal) in young and middle-aged individuals respectively (both p < 0.01 vs basal). In contrast, insulin-mediated whole-body glucose uptake was lower in middle-aged subjects (6.6 +/- 1.4 mg/ kg x min) with respect to young individuals (8.1+/-1.7 mg/kg min, p < 0.05). During study II (insulin plus AA) a significant rise in NOLD was obtained in both young (1.72 +/- 0.10 micromol/kg x min, p < 0.01 vs basal) and middle-aged subjects (1.76 +/- 0.25 micromol/kg x min, p < 0.01 vs basal). Similarly, net leucine balance rose significantly in both young (+0.62 +/- 0.13 vs -0.25 +/- 0.02 micromol/kg x min, p < 0.01 vs basal) and middle-aged subjects (+0.37 +/- 0.08 vs -0.22 +/- 0.03 micromol/ kg x min, p < 0.01 vs basal) suggesting that the anabolic response to amino acids is preserved in middle-aged subjects. In middle-aged subjects we observed 1) a moderate decline in basal leucine oxidation; 2) a normal antiproteolytic response to insulin and a reduction in glucose uptake; and 3) a normal anabolic response to AA plus insulin. In conclusion, the data provide evidence for a normal regulation of protein anabolism and an early dissociation between the metabolic effects of insulin on glucose uptake and proteolysis in middle-aged subjects.

Effects of intravenous infusion of amino acids and glucose on the yield and concentration of milk protein in dairy cows.

To test the hypothesis that the availability of glucose or its precursors can influence the response of milk protein concentration to the intravenous infusion of amino acids, five cows were used in a 5 x 5 Latin square design with period lengths of 7 d. The five treatments were the basal diet of grass silage ad lib. plus 5 kg/d of a cereal-based supplement containing feather meal (Basal); Basal plus 4 g/d histidine, 8 g/d methionine and 26 g/d lysine (4H); Basal plus 8 g/d histidine, 8 g/d methionine and 26 g/d lysine (SH); and these two amino acid mixtures together with 600 g/d of gluctose (4HG and 8HG respectively). Earlier experiments with this basal diet had shown that histidine was first-limiting for secretion of milk protein, followed by methionine and lysine. The yield of milk protein was increased progressively with the amount of histidine infused. The efficiency of transfer of histidine into milk protein was 0.42 for the 4H and 4HG and 0.35 for the 8H and 8HG treatments, and the concentration of milk protein was increased over Basal by all infusion treatments. However, milk protein concentrations were higher, and lactose concentrations in the milk were lower, in the absence of added glucose. Concentrations of insulin in blood plasma were not affected by treatment. It is concluded that, with the treatments without added glucose, a shortage of glucose prevented an increase in lactose secretion, and hence limited the increase in milk yield, leading to an increased concentration of protein in the milk.

Continuous Renal Replacement Therapy in the Initial Management of Neonatal Hyperammonemia Due to Urea Cycle Defects

After completing this article, readers should be able to: 1. Describe the metabolic acidosis that may be seen in urea cycle defects. 2. Delineate the most important factor in determining neurologic outcome in urea cycle defects. 3. Explain the factors that affect the blood ammonia concentration in continuous renal replacement therapy (CRRT). 4. Delineate the most difficult issue in pursuing CRRT in neonates. 5. Describe the morbidities associated with CRRT. MP was born at 39 weeks' estimated gestational age by vaginal delivery to a 35-year-old gravida 3 para 2 woman who had an uncomplicated pregnancy. The 3- and 6-year-old female siblings of the patient were healthy. Apgar scores were 8 at 1 minute and 9 at 5 minutes. The newborn was discharged to home after a 2-day stay in the well baby nursery during which time she appeared to be alert and was breastfeeding appropriately. She presented to the emergency department less than 1 day later after her parents noted increasing lethargy and very poor feeding at home. She was admitted to the neonatal intensive care unit (NICU). Results of a complete blood count with differential count and C-reactive protein evaluation were unremarkable, but the patient progressed to worsening lethargy, respiratory failure, and abdominal distension requiring intubation within hours of admission. The anterior fontanelle was soft on admission physical examination. Further testing revealed a base deficit of 6.1 on arterial blood gas, serum ammonia level of 810 mcmol/L (1,134 mcg/dL), serum lactate level of 5.6 mmol/L (5.6 mEq/L), and serum pyruvate level of 0.06 mmol/L. Results of liver function tests were abnormal, and the infant had a prothrombin time of 35.9 seconds, a partial thromboplastin time of 54.3 seconds, and a fibrinogen concentration of 1.75 g/L (175 mg/dL). Genetic and nephrology consultations were requested immediately. Additional metabolic studies were obtained, including measurement of plasma amino …

Variability in the ranking of the three most-limiting amino acids for milk protein production in dairy cows consuming grass silage and a cereal-based supplement containing feather meal

Previous experiments showed that histidine was clearly first-limiting for milk protein production in cows consuming grass silage and a cereal-based supplement containing feather meal. Two experiments were carried out to investigate further the responses of milk production to the intravenous infusion of amino acids in dairy cows consuming this basal diet. In Experiment 1, four cows received the following treatments: (1) the basal diet (Basal); (2) Basal plus jugular infusion of 114 g d−1 of essential amino acids containing 6 g d−1 of histidine (EAA); (3) Basal plus histidine, methionine and lysine in the amounts supplied by EAA (3AA); and (4) Basal plus 6 g d−1 of histidine (His). The EAA and 3AA treatments increased (P < 0.05) the yield of milk protein by 106 g d−1; the corresponding increase (P < 0.05) for the His treatment was 60 g d−1. This result confirmed those of earlier experiments in showing histidine to be clearly first-limiting. In Experiment 2, five cows received the following treatments: (1) basal diet (Basal); (2) Basal plus jugular infusion of (g d−1) histidine, 9.0, methionine, 10.0, lysine, 25.5, and tryptophan, 4.8 (4AA); (3) Basal plus 4AA minus methionine (−Met); (4) Basal plus 4AA minus lysine (−Lys); and (5) Basal plus 4AA minus tryptophan (−Trp). Milk protein yield was increased (P < 0.05) only by the 4AA and −Trp treatments; the increases were 101 and 126 g d−1 respectively. These results indicate that methionine and lysine were also limiting. Taking the results of these experiments, together with those published previously, suggests that, despite all reasonable attempts to maintain a constant diet composition, the ranking of the three most-limiting amino acids varied between experiments. © 2000 Society of Chemical Industry

Stimulation of gallbladder by intravenous infusion of amino acid: a new method to obtain duodenal bile for bile analyses.

Bile analysis is performed on duodenal bile obtained after stimulation of the gallbladder (GB) with cholecystokinin or its analog. Rapid amino acids (AA) infusion contracts the GB. Our objective was to assess qualitatively the adequacy of duodenal bile obtained following AA infusion. We prospectively studied 15 patients each with non ulcer dyspepsia (NUD) and gallstones (GS), and 17 patients with acute idiopathic pancreatitis (AIP). Duodenal bile was obtained after duodenal intubation and stimulation of GB by rapid AA infusion. The GB contractile response to AA infusion was assessed by functional ultrasonography. Bile was analyzed for lipids, nucleation time, cholesterol saturation index (CSI), and biliary crystals. Adequate duodenal bile could be collected in all but one patient with AIP and one with GS. The mean +/- SD percent ejection fraction of GB following AA infusion in patients with NUD, GS and AIP was 53.41 +/- 16.23, 52.06 +/- 16.07, and 43.37 +/- 14.16, respectively. Biliary microscopy showed the presence of cholesterol monohydrate crystals (CMC) in 7 of 16 patients with AIP, 12 of 14 patients with GS, and 1 of 15 patients with NUD. Mean CSIs in patients with NUD, GS, CMC-positive AIP, and CMC-negative AIP were 0.78, 1.97, 1.85, and 1.00, respectively. Nucleation time in patients with NUD and gallstones was 21.1 and 8.42 days. respectively. In conclusion, qualitatively adequate bile can be obtained for chemical and microscopic examination following rapid intravenous infusion of AA to stimulate the GB.

Determination of the first-limiting amino acid for milk production in dairy cows consuming a diet of grass silage and a cereal-based supplement containing feather meal

Lactating dairy cows consuming a diet of grass silage and a cereal-based supplement containing feather meal were given intravenous infusions of amino acids to determine the first-limiting amino acid for milk production, methionine having been shown to be not-limiting in a previous experiment. The three infusion treatments were a mixture of methionine, lysine, histidine and tryptophan (4AA); the mixture without lysine (-Lys); and the mixture without histidine (-His). The 4AA treatment markedly increased the yield of milk protein by about 18% and this response was not diminished by omission of lysine. However, exclusion of histidine produced no response over basal, confirming histidine as the first-limiting amino acid. In a second experiment, lactating cows receiving a similar basal diet were used to examine the effects on milk production of progressively substituting avian blood meal (rich in histidine and poor in methionine) for part of the feather meal. Blood meal was substituted for 0, 0.10, 0.20 and 0.40 of the feather meal in the supplement. The yield of milk protein was increased by about 15% by the first level of inclusion of blood meal, but there was no further response beyond the first level of inclusion. The results of the feeding trial confirm that dietary addition of protein rich in histidine and of low ruminal degradability substantially increased milk production with this basal diet, although it should be noted that the calculated supply of all the essential amino acids were also increased, by varying degrees, by substitution of blood meal. The results of the two experiments are discussed in relation to the likely importance of histidine as a limiting amino acid in dairy cows consuming diets typical of those used in practice. © 1999 Society of Chemical Industry

Umbilical amino acid uptake at increasing maternal amino acid concentrations: Effect of a maternal amino acid infusate

Objective: Our purpose was to establish whether, in normal human pregnancies, the maternal intravenous infusion of amino acids can increase fetal amino acid uptake and amino acid concentrations. Study Design: Twenty-six normal pregnancies were studied at the time of cesarean delivery (38-40 weeks’ gestation). In 10 cases an amino acid formulation (Freamine 8.5% III, Baxter) was infused into a maternal vein before cesarean delivery. Maternal blood samples were obtained during the course of the study. Umbilical venous and arterial samples were obtained from the clamped segment of the cord. There were no differences between the 2 groups for fetal and placental weights and for fetal oxygenation and acid-base balance. Results: Maternal amino acid concentrations increased significantly in the group receiving infusions. Significant increases in umbilical venous concentrations were observed for most amino acids, except for histidine and threonine. The amino acid umbilical arteriovenous differences per mole of oxygen (AA/O 2 ratio) increased significantly for leucine, isoleucine, valine, methionine, phenylalanine, arginine, glycine, serine, alanine, and proline. There were no significant increases for lysine, histidine, and threonine. Conclusion: An increase in maternal concentrations leads to an increase in the delivery of most amino acids to the fetus. (Am J Obstet Gynecol 1999;181:477-83.)

Brain substrate utilisation during acute hypoglycaemia.

Our study was undertaken to examine directly the utilisation of glucose and alternative substrates, in particular amino acids, during hypoglycaemia. Catheters were positioned in the jugular venous bulb and an artery in six healthy subjects in the overnight fasted state. Arterio-venous differences for glucose, amino acids, lactate and pyruvate were measured in the basal state, during hyperinsulinaemic euglycaemia and during hyperinsulinaemic hypoglycaemia. The subjects were studied on two different occasions, once during intravenous infusion of amino acids and once during infusion of saline. In the basal state the fractional extraction of glucose across the brain was 10 +/- 2%, glucose uptake accounted for 106 +/- 5% of the brain's oxidative metabolism. There was a small release of lactate and pyruvate. During hyperinsulinaemia glucose uptake continued to account for the entire fuel requirement of the brain. Hyperaminoacidaemia did not result in net amino acid uptake by the brain. During hypoglycaemia (2.4 +/- 0.2 mmol/l) fractional extraction of glucose by the brain increased (p < 0.01) and glucose uptake accounted for 90 +/- 15% of the brain's oxidative metabolism. Uptake of amino acids, lactate or pyruvate could not be detected. 1) Brain fractional extraction of glucose increases during hypoglycaemia, 2) hyperinsulinaemia does not change fractional extraction of glucose by the brain, 3) augmented availability of amino acids does not result in brain amino acid uptake during euglycaemia or hypoglycaemia and 4) under the present study conditions glucose remains the major substrate for cerebral metabolism during hypoglycaemia; lactate or pyruvate uptake by the brain can not be detected.

Bolus Intravenous Infusion of Amino Acids or Lipids Does Not Stimulate Gallbladder Contraction in Neonates on Total Parenteral Nutrition

Stimulation of gallbladder contraction in patients receiving total parenteral nutrition (TPN) may be beneficial in preventing cholestatic jaundice associated with TPN. Bolus intravenous administration of either amino acids or fat promotes gallbladder contraction in normal adult volunteers after a period of starvation. This phenomenon has not been investigated in patients receiving continuous TPN. The aim of this study was to test the hypothesis that bolus intravenous infusion of amino acids or fat produces gallbladder contraction in neonates receiving TPN. Methods: Twentyeight studies of gallbladder contraction were performed in 18 neonates receiving continuous TPN (median weight, 2.3 kg; range, 1.3 to 7.3; gestational age, 37 weeks; range, 28 to 40; age, 6.5 days; range, 2 to 180). Different types of experiments were performed: (1) bolus intravenous infusion of amino acid or fat for 60 minutes at twice the normal infusion rate; (2) bolus intravenous infusion of amino acid or fat for 15 minutes at four times the normal infusion rate. Gallbladder volume was measured by real‐time ultrasonography by a single investigator. Measurements were made before starting the bolus infusion and every 15 minutes for 1 hour. Results: The volume of the gallbladder before bolus intravenous infusion was not significantly different between experiments (median, 985; range, 603 to 1,802 mm3). These volumes are consistent with an enlarged and atonic gallbladder as previously reported in infants receiving parenteral nutrition. In all experiments there was no significant decrease in gallbladder volume after bolus infusion of either amino acids or fat. Conclusions: Contrary to their effects in adults, bolus infusions of amino acids or fat did not induce gallbladder contraction in neonates on TPN. This may have been because of lack of starvation in the neonates and/or the effect of continuous glucose infusion.

Effects of Insulin and Amino Acids on Glucose and Leucine Metabolism in CAPD Patients

This study investigates the basal and insulin-stimulated glucose metabolism, substrate utilization, and protein turnover in eight patients maintained on continuous ambulatory peritoneal dialysis (CAPD) (mean age 39+/-5 yr, body mass index [BMI] 108+/-6) and 14 control subjects (mean age 33+/-4 yr, BMI 103+/-3). Euglycemic insulin clamp studies (180 min) were performed in combination with continuous indirect calorimetry and 1-14C leucine infusion (study I). Postabsorptive glucose oxidation was higher (1.75+/-0.18 versus 1.42+/-0.14 mg/kg per min) and lipid oxidation was lower (0.43+/-0.09 versus 0.61+/-0.12 mg/kg per min) in CAPD patients than in control subjects (P<0.05 versus control subjects). During the last 60 min of euglycemic hyperinsulinemia, the total rate of glucose metabolism was similar in CAPD and control subjects (6.33+/-0.51 versus 6.54+/-0.62 mg/kg per min). Both insulin-stimulated glucose oxidation (2.53+/-0.27 versus 2.64+/-0.37 mg/kg per min) and glucose storage (3.70+/-0.48 versus 3.90+/-0.58 mg/kg per min) were similar in CAPD and control subjects. Basal leucine flux (an index of endogenous proteolysis) was significantly lower in CAPD patients than in control subjects (1.21+/-0.15 versus 1.65+/-0.07 micromol/kg per min). Leucine oxidation (0.13+/-0.02 versus 0.26+/-0.02 micromol/kg per min) and nonoxidative leucine disposal (an index of protein synthesis) (1.09+/-0.16 versus 1.35+/-0.05 micromol/kg per min) were also reduced in CAPD compared with control subjects (P<0.01 versus control subjects). In response to insulin (study I), endogenous leucine flux decreased to 0.83+/-0.08 and 1.05+/-0.05 micromol/kg per min in CAPD and control subjects, respectively (all P<0.01 versus basal). Leucine oxidation declined to 0.06+/-0.01 and to 0.19+/-0.02 micromol/kg per min in CAPD and control subjects, respectively (P<0.01 versus basal). A second insulin clamp was performed in combination with an intravenous amino acid infusion (study II). During insulin plus amino acid administration, nonoxidative leucine disposal rose to 1.23+/-0.17 and 1.42+/-0.09 micromol/kg per min in CAPD and control subjects, respectively (both P<0.05 versus basal, P = NS versus control subjects), and leucine balance, an index of the net amino acid flux into protein, become positive in both groups (0.30+/-0.05 versus 0.40+/-0.07 micromol/kg per min in CAPD and control subjects, respectively) (both P<0.01 versus basal, P = NS versus control subjects). In summary, in CAPD patients: (1) basal glucose oxidation is increased; (2) basal lipid oxidation is decreased; (3) insulin-mediated glucose oxidation and storage are normal; (4) basal leucine flux is reduced; (5) the antiproteolitic action of insulin is normal; and (6) the anabolic response to insulin plus amino acid administration is normal. Uremic patients maintained on CAPD treatment show a preferential utilization of glucose as postabsorptive energy substrate; however, their anabolic response to substrate administration and the sensitivity to insulin are normal.

Role of tryptophan in the elevated serotonin-turnover in hepatic encephalopathy.

The increase of the brain levels of 5-hydroxyindoleacetic acid (5-HIAA) in hepatic encephalopathy (HE) suggests an increased turnover of serotonin (5-HT). To study the role of tryptophan on the increased brain 5-HT metabolism in HE, we attempted to monitor brain levels of tryptophan in rats with thioacetamide-induced acute liver failure by intravenous infusion of branched-chain amino acids (BCAA). The effect of this treatment on 5-HT synthesis and metabolism was investigated in five brain areas. BCAA-infusions (1 and 2 gm/kg/24 h) increased the ratio BCAA/aromatic amino acids in plasma two- and fourfold, respectively, and lowered both plasma and brain levels of tryptophan. At the higher BCAA-dose all parameters suggesting an altered brain 5-HT metabolism (increased brain levels of 5-HT and 5-HIAA, increased 5-HIAA/5-HT ratio) were almost completely normalized. These results provide further evidence for the role of tryptophan in the elevation of brain 5-HT metabolism and for a potential role of BCAA in the treatment of HE.

Development of a Minimally Invasive Protocol for the Determination of Phenylalanine and Lysine Kinetics in Humans during the Fed State

The primed, continuous intravenous infusion of amino acids labeled with 13C together with measurement of isotopic enrichment in plasma is commonly used to study amino acid metabolism. However, a less invasive, oral infusion that also produces an isotopic steady state in CO2 and urine would be useful, particularly for pediatric studies. We measured the 13C enrichments of expired CO2, plasma and urine free phenylalanine and lysine and estimated flux and oxidation rates in adult humans (n = 12) who received a 4-h oral, primed, equal dose infusion of either L-[1-13C]phenylalanine, L-[1-13C]lysine (D-lysine = 1.6%) or L-[1-13C]lysine (D-lysine ≤ 0.2%). Steady fed state conditions were established by feeding subjects eight hourly meals beginning 4 h before the start of the oral infusion protocol. Isotopic plateau in CO2, plasma and urine was achieved within 120 min of phenylalanine or lysine infusion. At isotopic plateau, the mean ratio of plasma to urine enrichment was 1.0 ± 0.04 (SEM), 0.39 ± 0.03 and 0.97 ± 0.02 for L-[1-13C]phenylalanine, L-[1-13C]lysine (D-lysine = 1.6%) and L-[1-13C]lysine (D-lysine≤ 0.2%), respectively. There was good agreement between isotopic enrichment in plasma and urine for L-[1-13C]phenylalanine and L-[1-13C]lysine (D-lysine ≤ 0.2%). However, use of L-[1-13C]lysine (D-lysine = 1.6%) resulted in significantly higher enrichment in urine, probably due to renal tubular discrimination of D-lysine. Mean flux rates for phenylalanine and lysine were consistent with the literature. Thus, the oral, primed, equal dose infusion produces the isotopic steady-state condition required for amino acid flux and oxidation determination within an 8-h period.