Effects of perfusate leucine concentration on the metabolism of valine by the isolated rat hindquarter

Myocardial oxidative metabolism and protein synthesis during mechanical circulatory support by extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) provides essential mechanical circulatory support necessary for survival in infants and children with acute cardiac decompensation. However, ECMO also causes metabolic disturbances, which contribute to total body wasting and protein loss. Cardiac stunning can also occur, which prevents ECMO weaning, and contributes to high mortality. The heart may specifically undergo metabolic impairments, which influence functional recovery. We tested the hypothesis that ECMO alters oxidative metabolism and protein synthesis. We focused on the amino acid leucine and integration with myocardial protein synthesis. We used a translational immature swine model in which we assessed in heart 1) the fractional contribution of leucine (FcLeucine) and pyruvate to mitochondrial acetyl-CoA formation by nuclear magnetic resonance and 2) global protein fractional synthesis (FSR) by gas chromatography-mass spectrometry. Immature mixed breed Yorkshire male piglets (n = 22) were divided into four groups based on loading status (8 h of normal circulation or ECMO) and intracoronary infusion [(13)C(6),(15)N]-L-leucine (3.7 mM) alone or with [2-(13)C]-pyruvate (7.4 mM). ECMO decreased pulse pressure and correspondingly lowered myocardial oxygen consumption (∼40%, n = 5), indicating decreased overall mitochondrial oxidative metabolism. However, FcLeucine was maintained and myocardial protein FSR was marginally increased. Pyruvate addition decreased tissue leucine enrichment, FcLeucine, and Fc for endogenous substrates as well as protein FSR. The heart under ECMO shows reduced oxidative metabolism of substrates, including amino acids, while maintaining 1) metabolic flexibility indicated by ability to respond to pyruvate and 2) a normal or increased capacity for global protein synthesis.

Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans

Leucine has been proposed as an in vivo regulator of protein metabolism, although the evidence for this in humans remains inconclusive. To test this hypothesis, we infused either L-leucine (154 +/- 1 mumol.kg-1 x h-1) or saline intravenously in six healthy men in two separate studies. L-Leucine infusion increased plasma concentrations of leucine and alpha-ketoisocaproate from 112 +/- 6 and 38 +/- 3 mumol/l to 480 +/- 27 (P < 0.001) and 94 +/- 13 mumol/l (P < 0.001), respectively, without any significant change in circulating insulin or C peptide levels. Leucine infusion decreased plasma concentrations of several amino acids and decreased whole body valine flux and valine oxidation (using L-[1-13C]valine as a tracer) and phenylalanine flux (using [2H5]-phenylalanine as a tracer). According to arteriovenous differences across the leg, the net balance of phenylalanine, valine, and lysine shifted toward greater retention during leucine infusion, whereas alanine balance did not change. Valine release and phenylalanine release from the leg (estimated from the dilution of respective tracers) decreased, indicating inhibition of protein degradation by leucine infusion. We conclude that leucine decreases protein degradation in humans and that this decreased protein degradation during leucine infusion contributes to the decrease in plasma essential amino acids. This study suggests a potential role for leucine as a regulator of protein metabolism in humans.

Metabolic effect of short-term total parenteral nutrition highly enriched with leucine or valine in rats recovering from severe trauma

The metabolic impact of infusing a large amount of leucine (Leu) or valine (Val) was examined with regard to the corrective effect of total parenteral nutrition (TPN). Rats recovering from severe sepsis received either Leu- or Val-enriched TPN solution for 30 hours. The in vivo behavior of the amino acids administered was explored by a pulse injection of 14C-labeled Leu or Val. The recovery of 14CO2 from Leu increased by 64% in the septic rats of Leu-TPN group (41% of dose; p less than .01), as compared with control rats receiving the same TPN solution, whereas no significant rise in the 14CO2 recovery from Val occurred in the septic rats given Val-TPN (45% of dose) in comparison with the corresponding controls. The enhancement of Leu catabolism to CO2 in the Leu-TPN group was compatible with the alterations of urinary nitrogen excretion, plasma Leu level, and metabolite contents of liver and muscle. The only difference in metabolite levels observed between the two TPN groups was in hepatic total adenine nucleotides. Plasma amino acid levels were largely unaffected by infusion of these TPN solutions highly enriched with branched-chain amino acids (45%), except for an approximately threefold elevation of the Val level in Val-TPN rats. Thus, when administered in a large quantity during such short-term TPN, Leu can exert its metabolic effect without causing an imbalance in plasma amino acids under severe catabolic conditions.

Transamination and oxidative decarboxylation of L-isoleucine, L-alloisoleucine and related 2-oxo acids in perfused rat hind limb muscle

Metabolism of L-isoleucine, L-alloisoleucine and corresponding 2-oxo acids in rat hind limb muscle was comparatively studied under steady-state perfusion conditions. At 0.5 mM L-[1-14C]isoleucine, apparent transamination and 2-oxo acid decarboxylation rates amounted to about 17 and 4 nmol/min per g of muscle, respectively. With L-allo[1-14C]isoleucine, the corresponding rates were about 5- and 10-fold lower, respectively. After addition of dichloroacetate (1-5 mM), the portion of (S)- and (R)-methyl-2-oxopentanoate undergoing further oxidative decarboxylation within the tissue was similarly increased by over 40%. In perfusions with 0.5 mM (R,S)-3-methyl-2-oxopentanoate and tracer doses of 1-14C-labeled (S)- or (R)-enantiomer, the 14CO2 production was comparable (about 0.5 nmol/min per g of muscle). Dichloroacetate caused a several-fold increase in 14CO2 release from either enantiomer, apparent 2-oxo acid transamination rates remaining unaffected. Indications for a racemization of 2-oxo acid were not obtained in the experiments. The results are discussed with respect to the appearance/disappearance of L-alloisoleucine in vivo and to the fact that (R)-3-methyl-2-oxopentanoate, but not L-alloisoleucine, can support growth of rats on a diet deficient in L-isoleucine.

Effects of branched-chain amino acids on protein turnover

Amino acid availability rapidly regulates protein synthesis and degradation. Increasing amino acid concentrations above the levels found in post-absorptive plasma stimulates protein synthesis in a dose-dependent manner at the level of mRNA translation-initiation and inhibits protein degradation by inhibiting lysosomal autophagy. The anabolic effects of insulin on protein synthesis and protein degradation are exerted at the same sites (i.e., peptide chain initiation and lysosomal stabilization) allowing for a rapid synergistic response when both amino acids and insulin increase after a protein-containing meal. In perfused liver preparations, protein anabolic effects are exerted by a group of amino acids acting in concert. The BCAA are among the amino acids required for stimulation of hepatic protein synthesis, but there is no evidence that BCAA or leucine alone are effective. Leucine alone is an important inhibitor of hepatic protein degradation, but maximal inhibition requires in addition several other regulatory amino acids. In heart and skeletal muscle in vitro, increasing the concentration of the three BCAA or of leucine alone reproduces the effects of increasing the supply of all amino acids in stimulating protein synthesis and inhibiting protein degradation. Skeletal muscle is the largest repository of metabolically active protein and a major contributor to total body nitrogen balance. Supplying energy alone (i.e., carbohydrate and lipids) cannot prevent negative nitrogen balance (net protein catabolism) in animals or humans; only provision of amino acids allows the attainment of nitrogen balance. In rats and in humans nourished parenterally, provision of balanced amino acid solutions or of only the three BCAA cause similar improvements in nitrogen balance for several days. There is some evidence that infusions of leucine alone can stimulate muscle protein synthesis in vivo; the effect may be transitory and was not observed by all investigators; provisions of excess leucine alone does not seem to affect total body or muscle protein degradation in vivo. In postabsorptive rats, in vivo, infusion of the three BCAA together stimulates muscle protein synthesis as much as the infusion of a complete amino acid mixture or of a mixture of essential amino acids; the in vivo effect requires coinfusion of glucose or of small (physiological) doses of insulin, suggesting synergism between insulin and amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucocorticoid-mediated activation of muscle branched-chain alpha-keto acid dehydrogenase in vivo

Muscle branched-chain alpha-keto acid dehydrogenase, the rate-limiting enzyme for branched-chain amino acid oxidation in skeletal muscle, was measured after treatment of rats with glucocorticoids. Cortisone treatment (10 mg X 100 g body wt-1 X day-1 for 2-5 days) resulted in an approximate doubling of the percentage of active enzyme. To further characterize this effect, the enzyme complex was measured 4 h after the intraperitoneal injection of 6 alpha-methylprednisolone, a water-soluble glucocorticoid with rapid onset effects. The percentage of active enzyme increased linearly as the dose of methylprednisolone was increased from 0.125 to 12.5 mg/100 g body wt, while total enzyme activity was unchanged. Administration of insulin with glucose had no significant effect on the activity of the enzyme. However, treatment of rats with insulin and glucose after methylprednisolone administration partially blocked branched-chain alpha-keto acid dehydrogenase activation. The activity of the enzyme complex was correlated with the concentration of leucine in plasma and muscle. Activation of skeletal muscle branched-chain alpha-keto acid dehydrogenase by increased glucocorticoids may play a role in the acceleration of branched-chain amino acid oxidation observed during severe stress.

Branched-chain amino acid interactions in skeletal muscle: isoleucine and L-alloisoleucine

Parenteral administration of a mixture of branched-chain amino acid (BCAA) solutions is known to alter plasma levels of the BCAA (ILE, LEU, VAL), their corresponding alpha-ketoacids (KMV, KIC, KIV) and L-alloisoleucine (ALLO), a stereoisomer of ILE. Although variously formulated mixtures of BCAA are administered, the metabolic implications of individual BCAA interactions have been only partially elucidated. Using the incubated, isolated, and intact rat epitrochlearis muscle, we measured the effect of graded increases (0.05, 0.10, 0.5, and 1.0 mM) in media concentrations of a single BCAA or ALLO on (1) the rate of decarboxylation of the other BCAA, and (2) the release of branched chain ketoacids from muscle. A graded increase of media ILE concentration to 1.0 mM changed the decarboxylation of LEU by -28%, and the release of KIC by +23%, but VAL decarboxylation increased by +25%, and the release of KIV declined by -56%. A graded increase of media LEU to 1.0 mM increased ILE decarboxylation by +146%, and its corresponding ketoacid (KMV) by +61%. However, VAL decarboxylation changed by only +25% and KIV release declined by -65%. A graded increase in media VAL to 1.0 mM accelerated ILE decarboxylation by +37%, but KMV release was unchanged. Similarly, LEU decarboxylation fell by -26%, and the release of KIC by only +6%. ALLO 0.025 mM increased ILE release by +55% but had an inconsistent effect on ILE decarboxylation and did not alter protein synthesis or degradation (estimated by phenylalanine incorporation and tyrosine release, respectively). Increasing ILE did not affect ALLO release.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimation of tissue protein synthesis in sheep during sustained elevation of plasma leucine concentration by intravenous infusion

1. The fractional rate of protein synthesis (FSR) was determined in skeletal muscle, liver, rumen and cardiac muscle of wether sheep by continuous intravenous infusion of L-[4,5-3H]leucine accompanied by infusion of 0, 7.6, 15.2 or 22.8 mmol leucine/h (three sheep per treatment). FSR was calculated assuming plasma (ksp) or intracellular (ksi) leucine-specific radioactivity (SRA) was representative of the leucine precursor pool SRA for protein synthesis. 2. Plasma leucine concentration (plateau) was linearly related to leucine infusion rate, 22.8 mmol/h evoking a tenfold increase in plasma concentration. 3. Difference between plasma leucine SRA and intracellular leucine SRA in all tissues diminished as plasma leucine concentration increased. 4. There were significant differences between ksi and ksp estimates for liver and rumen in control sheep. 5. As leucine infusion rate increased, differences between ksi and ksp diminished in all tissues. With increasing leucine infusion, in liver ksi decreased and ksp was increased, in rumen ksi decreased and ksp was stable, while in cardiac and skeletal muscle ksi and ksp both increased. 6. At a leucine infusion rate of 22.8 mmol/h, mean ksp and ksi respectively were: 11 (SE 2), 13 (SE 1); liver 19 (SE 2), 21 (SE 2); cardiac muscle 3.6 (SE 0.4), 3.8 (SE 0.3); skeletal muscle 4.1 (SE 0.2), 4.5 (SE 0.5) and did not differ significantly in any tissue.