OXIDATION OF AMINO ACIDS BY ISOLATED RAT DIAPHRAGM AND THE INFLUENCE OF INSULIN

Branched-chain amino acids in health and disease: metabolism, alterations in blood plasma, and as supplements

Branched-chain amino acids (BCAAs; valine, leucine, and isoleucine) are essential amino acids with protein anabolic properties, which have been studied in a number of muscle wasting disorders for more than 50 years. However, until today, there is no consensus regarding their therapeutic effectiveness.

In the article is demonstrated that the crucial roles in BCAA metabolism play: (i) skeletal muscle as the initial site of BCAA catabolism accompanied with the release of alanine and glutamine to the blood; (ii) activity of branched-chain keto acid dehydrogenase (BCKD); and (iii) amination of branched-chain keto acids (BCKAs) to BCAAs. Enhanced consumption of BCAA for ammonia detoxification to glutamine in muscles is the cause of decreased BCAA levels in liver cirrhosis and urea cycle disorders. Increased BCKD activity is responsible for enhanced oxidation of BCAA in chronic renal failure, trauma, burn, sepsis, cancer, phenylbutyrate-treated subjects, and during exercise. Decreased BCKD activity is the main cause of increased BCAA levels and BCKAs in maple syrup urine disease, and plays a role in increased BCAA levels in diabetes type 2 and obesity. Increased BCAA concentrations during brief starvation and type 1 diabetes are explained by amination of BCKAs in visceral tissues and decreased uptake of BCAA by muscles.

The studies indicate beneficial effects of BCAAs and BCKAs in therapy of chronic renal failure. New therapeutic strategies should be developed to enhance effectiveness and avoid adverse effects of BCAA on ammonia production in subjects with liver cirrhosis and urea cycle disorders. Further studies are needed to elucidate the effects of BCAA supplementation in burn, trauma, sepsis, cancer and exercise. Whether increased BCAA levels only markers are or also contribute to insulin resistance should be known before the decision is taken regarding their suitability in obese subjects and patients with type 2 diabetes.

It is concluded that alterations in BCAA metabolism have been found common in a number of disease states and careful studies are needed to elucidate their therapeutic effectiveness in most indications.

Interactions in the Metabolism of Glutamate and the Branched-Chain Amino Acids and Ketoacids in the CNS

Glutamatergic neurotransmission entails a tonic loss of glutamate from nerve endings into the synapse. Replacement of neuronal glutamate is essential in order to avoid depletion of the internal pool. In brain this occurs primarily via the glutamate-glutamine cycle, which invokes astrocytic synthesis of glutamine and hydrolysis of this amino acid via neuronal phosphate-dependent glutaminase. This cycle maintains constancy of internal pools, but it does not provide a mechanism for inevitable losses of glutamate N from brain. Import of glutamine or glutamate from blood does not occur to any appreciable extent. However, the branched-chain amino acids (BCAA) cross the blood-brain barrier swiftly. The brain possesses abundant branched-chain amino acid transaminase activity which replenishes brain glutamate and also generates branched-chain ketoacids. It seems probable that the branched-chain amino acids and ketoacids participate in a "glutamate-BCAA cycle" which involves shuttling of branched-chain amino acids and ketoacids between astrocytes and neurons. This mechanism not only supports the synthesis of glutamate, it also may constitute a mechanism by which high (and potentially toxic) concentrations of glutamate can be avoided by the re-amination of branched-chain ketoacids.

Role of plasma membrane transporters in muscle metabolism

Muscle plays a major role in metabolism. Thus it is a major glucose-utilizing tissue in the absorptive state, and changes in muscle insulin-stimulated glucose uptake alter whole-body glucose disposal. In some conditions, muscle preferentially uses lipid substrates, such as fatty acids or ketone bodies. Furthermore, muscle is the main reservoir of amino acids and protein. The activity of many different plasma membrane transporters, such as glucose carriers and transporters of carnitine, creatine and amino acids, play a crucial role in muscle metabolism by catalysing the influx or the efflux of substrates across the cell surface. In some cases, the membrane transport process is subjected to intense regulatory control and may become a potential pharmacological target, as is the case with the glucose transporter GLUT4. The goal of this review is the molecular characterization of muscle membrane transporter proteins, as well as the analysis of their possible regulatory role.

Phosphate-dependent glutaminase of rat skeletal muscle. Some properties and possible role in glutamine metabolism

A relatively high activity (26.7 nmol/min per mg mitochondrial protein) of phosphate-dependent glutaminase (EC 3.5.1.2; L-glutamine amidohydrolase) was found in rat skeletal muscle (mixed type from hindlegs) mitochondria incubated in 200 mM potassium phosphate (pH 8.2); the activity was lower in rat heart and diaphragm mitochondria. Phosphate-dependent glutaminase was also found in human skeletal muscle mitochondria, but the activity was about 3-5 times lower than in rat skeletal muscle. Multiplying the specific activity of mitochondrial glutaminase by the amount of mitochondrial protein present in 1 g of rat skeletal muscle the maximum glutaminase activity was found to be 0.352 mumol/min per g wet tissue. The rat skeletal muscle enzyme appears to be similar in many respects to phosphate-dependent glutaminase of the kidney (e.g., S0.5 for glutamine, K0.5 for phosphate, the pH activity profile, inhibition by glutamate). These properties make the skeletal muscle enzyme very similar to the 'kidney type' glutaminase isoenzyme of rat tissues. A significant difference between rat kidney and skeletal muscle enzymes is their adaptive response during acidosis. While the kidney enzyme increases during acidosis, the skeletal muscle glutaminase activity does not. A possible role of glutaminase in the glutamine metabolism in rat skeletal muscle is discussed.

Glutamine metabolism in skeletal muscles from the broiler chick (Gallus domesticus) and the laboratory rat (Rattus norvegicus)

Oxidative decarboxylation of L-[1-14C]glutamine was studied in isolated chick and rat skeletal muscles incubated in the presence of glucose, insulin and plasma concentrations of amino acids. (1) The rate of oxidative decarboxylation of L-[1-14C]glutamine was high, and exceeded that of L-[1-14C]leucine in all muscles. (2) The rate of oxidative decarboxylation of L-[1-14C]glutamine increased with increasing intracellular concentrations of glutamine. (3) The activities of glutamine aminotransferases K and L were more than 10-fold greater in rat than in chick skeletal muscles. (4) Mitochondrial phosphate-activated glutaminase activity was approx. 10-fold greater in chick than in rat skeletal muscles and increased with increasing glutamine concentrations. (5) An inhibitor of glutaminase, 6-diazo-5-oxo-L-norleucine, inhibited the rate of glutamine decarboxylation in chick, but not in rat, skeletal muscle. These findings suggest that glutamine degradation in skeletal muscle may be substantial and may make an important contribution to the regulation of intramuscular glutamine concentrations. A species difference in the pathways and the subcellular location for the conversion of glutamine into 2-oxoglutarate in rat and chick skeletal muscles is implied by the relative activities of glutamine-degrading enzymes.

Possible involvement of alanine and pyruvate in the regulation of glucose transport in heart muscle cells

In isolated rat heart muscle cells, addition of L-alanine (1.5 mmol/l) or of L-valine (3 mmol/l) resulted in either a ca 1.5- or 1.3-fold increase in glucose transport, resp. half-maximal stimulation was observed in the presence of L-alanine, but not of L-valine, within a physiological plasmatic range of concentrations. D-Alanine (1.5 mmol/l) was ineffective and the stimulating effect of L-alanine could be prevented by an excess of L-serine (15-30 mmol/l). L-Alanine produced an increase in 3-O-methyl-D-glucose transport Vmax (from 44.6 to 81.5 pmol.s-1.mg protein-1) without affecting the Km (12.2 in control vs 12.8 mmol/l in alanine-treated cells). Pyruvate (1.5 mmol/l) inhibited glucose transport by 20% and prevented the stimulating action of L-alanine (1.5 mmol/l). These results suggest that the effect of L-alanine in cardiac myocytes occurs through the interaction with an intracellular site and that both alanine and pyruvate may play a role in the regulation of glucose transport in these cells.

Anticatabolic properties of branched chain amino-acids in post-operative patients. A prospective study

The effect of infusion of branched chain amino-acids (BCAA) on post-operative protein metabolism was analysed in 19 elective surgical patients treated for the first 5 post-operative days with a nutritional regimen of 30 kcal kg(-1) day(-1) and 2 g of amino-acids kg(-1) day(-1). The patients were divided into three groups whose only difference was the amount of BCAA delivered. Our results showed that an increased BCAA input improved nitrogen balance and reduced protein catabolism as estimated by the excretion of 3-methyl-histidine. Since nitrogen retention was maximal during the first 3 post-operative days and the reduction in 3-methylhistidine excretion was observed only on post-operative days 4 and 5, a dual action of BCAA on improving protein synthesis and reducing catabolism is postulated, even though the reduction in catabolism seems to be the main action. This dual action may reflect the unique role of BCAA, which is both 'nutritional' (as they constitute 40% of total amino-acid daily requirements of the healthy subject) and 'pharmacological (as they reduce protein catabolism and improve synthesis in muscle and liver with a dose-dependent effect). Of the three BCAA, isoleucine and leucine seemed to have an 'anticatabolic' effect, whereas an analysis of literature data showed that valine probably has none.

In-vitro stimulation of the rat epitrochlearis muscle. I. Contractile activity per se affects myofibrillar protein degradation and amino acid metabolism

The influence of contractile activity on protein degradation and amino acid metabolism in skeletal muscle was investigated by utilizing an in-vitro electrical stimulation model with the rat epitrochlearis muscle preparation. Graded decreases in contraction force and in the muscle content of ATP and PCr, and increases in lactate were recorded with different rates of stimulation (1 h) and with both isometric twitches and tetanic contractions. 3-Methylhistidine and phenylalanine were chosen as indicators of myofibrillar and total protein degradation, respectively. The release of 3-methylhistidine was significantly stimulated by contractile activity, but a significant increase in the total amount of this amino acid (released amount + tissue content) occurred only at the most intense contraction rates. The release rate, tissue content and total amount of phenylalanine were not influenced by the contractions. Glutamate formation was generally inhibited, but its release was increased. Alanine synthesis was increased in moderately and intensely stimulated muscles. Glutamine and glycine were not influenced by the contractions, however. Inhibition of protein synthesis did not significantly influence protein degradation or amino acid release. The data suggest that in the absence of anabolic factors in the medium, myofibrillar protein degradation is increased in heavily activated muscle. This takes place without total protein breakdown being affected.

Adaptation of a perfused rat hemidiaphragm preparation to the study of intermediary metabolism

Glucose catabolism of a vascular perfused rat hemidiaphragm was determined at rest and during stimulation of the phrenic nerve with trains of either 5 (T5) or 15 (T15) pulses (20 msec intervals) per second. Tissues were perfused and bathed in HEPES-buffered medium containing 11 mM D-[U-14C, 5-3H]glucose, equilibrated with 100% O2. Resting glucose catabolism via the Emden-Meyerhof pathway was indicated by a 3H2O production rate per hemidiaphragm of 1.45 +/- 0.07 mumol/h, of which 47% was recovered as [14C]lactate with the remainder assumed to be metabolised by mitochondria. During the first 30 min of T5 and T15 stimulation, peak isometric tension declined from an initial value of 105 +/- 8 g by 54% and 79%, respectively. The resulting peak tensions of 48 and 22 g remained constant for the next 60 min. These tensions were associated with linear rates of 3H2O production of 2.93 +/- 0.41 and 2.84 +/- 0.25 mumol/h. Stimulation by T5 and T15 increased mitochondrial metabolism of glucose by 64% and 95%, respectively, with no significant alterations in lactate formation from either exogenous or endogenous sources. The results suggest that the initial decline in tension is due to fatigue of the fast anaerobic myofibers; whereas, the sustained force beyond 30 min is attributable to the mitochondrial-rich slow myofibers.

Effects of diabetes and starvation on skeletal muscle branched-chain alpha-keto acid dehydrogenase activity

The activation state of branched-chain alpha-keto acid dehydrogenase (BCDH) was studied in rat hindlimb muscles during starvation and insulinopenic diabetes, conditions in which circulating branched-chain amino acids (BCAA) are increased and their oxidation is accelerated. Muscle BCDH is predominantly inactive (phosphorylated) in postabsorptive rats but is activated by increased circulating leucine. Diabetes (streptozotocin-induced and spontaneous BB/W) increased circulating BCAA four- to fivefold and BCDH activity approximately threefold. Insulin treatment caused near normalization of circulating BCAA without correcting BCDH activity. Adrenalectomy of diabetics decreased (without normalizing) circulating BCAA and BCDH activation. Starvation caused mild, progressive increases in circulating BCAA and significant activation of BCDH only after 4 days. Leucine infusion activated BCDH in muscle but the activation by leucine was markedly blunted by diabetes. In isolated perfused hindlimbs (control and diabetic) insulin did not affect BCDH significantly; perfusion with leucine activated BCDH, and this response appeared blunted in diabetics. Activation of muscle BCDH may contribute to increased BCAA catabolism in diabetes; the blunted activation response to hyperleucinemia may spare BCAA and contribute to their persistent elevation in plasma.

Effect of pyruvate, octanoate and glucose on leucine degradation in skeletal muscle from fed and fasted chicks

1. Pyruvate at 5 mM decreased the rate of leucine oxidative decarboxylation and increased the rate of 2-oxoisocaproate production in extensor digitorum communis (EDC) muscles from both fed and 24-hr fasted chicks. Pyruvate at 5 mM increased the net rate of leucine transamination in EDC muscle from fed chicks and had no effect in EDC muscles from 24-hr fasted chicks. 2. Octanoate at 0.2 and 1 mM markedly increased the rates of net leucine transamination, leucine oxidative decarboxylation and oxidation of decarboxylated leucine carbons 2-6 in EDC muscles from fed chicks, but had no effect on these parameters of leucine degradation in muscles from 24-hr fasted chicks. 3. Glucose at 5 and 12 mM decreased the rates of leucine oxidative decarboxylation and oxidation of decarboxylated leucine carbons 2-6, and increased the net rate of 2-oxoisocaproate production as compared to control (no glucose) group in muscles from fed chicks. Glucose had no effect on these parameters of leucine degradation in muscles from 24-hr fasted chicks.

Amino acid metabolism by perfused rat hindquarter. Effects of insulin, leucine and 2-chloro-4-methylvalerate

Hindquarters from starved rats were perfused without substrates but in the presence of an O2- and CO2-carrying perfluorocarbon emulsion to evaluate principally the metabolism of individual endogenous and protein-derived amino acids by this muscle preparation. This experimental model was shown, by a battery of metabolite measurements, to maintain cellular homoeostasis for at least 2h. The net appearance of most amino acids closely approximated their frequency of occurrence in muscle proteins, showing that they are not significantly metabolized. Exceptions were the branched-chain amino acids, methionine and those amino acids that are interconvertible with intermediates of the citrate cycle and pyruvate through coupled transaminations. The evidence indicates that only valine, isoleucine, aspartate and probably methionine can be catabolized by skeletal muscle to provide carbon precursors for glutamate/glutamine and alanine that are formed de novo by protein-catabolic muscle. The protein-sparing effects of insulin and leucine were confirmed. Although each decreased proteolysis and the net appearance of free amino acids, they were generally without effect on the ratios of amino acids formed. 2-Chloro-4-methylvalerate selectively stimulated the removal rate for the branched-chain amino acids, confirming the idea that the branched-chain oxo acid dehydrogenase normally limits the rate of their oxidation by muscle. It is also concluded that, since alanine was not formed in excess of that found in muscle proteins when no glucose was added as substrate, the excess of alanine (carbon) released from muscles in other studies is derived to a large extent, but not exclusively, from preformed carbohydrate.

Effects of jejunoileal bypass on food intake, amino acid levels, and indoleamine metabolism in rats

Adult rats were subjected to either a 90 to 95 percent jejunoileal bypass or a sham operation and were sacrificed 35 days after surgery. Rats with jejunoileal bypass lost 33 percent of their original weight, whereas the sham operated rats gained 14 percent. Food intake per 100 g body weight was significantly increased between postoperative days 14 and 35 in the jejunoileal bypass rats. Levels of tryptophan were significantly reduced in the cortex, hypothalamus, striatum, hippocampus, mesencephalon, diencephalon, pons-oblongata, and cerebellum, whereas serotonin concentrations were lowered in the diencephalon, pons-medulla, and cerebellum in jejunoileal rats compared with control rats. Levels of 5-HIAA were reduced in the hypothalamus, cortex, mesencephalon, and diencephalon. In the plasma of bypassed rats, concentrations of valine, leucine, isoleucine, tryptophan, methionine, threonine, and tyrosine were significantly lower than in the control rats. In the cerebral cortex, levels of phenylalanine, tyrosine, histidine, and glutamine were increased. The results suggest involvement of indoleamine metabolism in disrupted eating after jejunoileal bypass. The elevated brain levels of glutamine, phenylalanine, tyrosine, and histidine resemble similar changes seen after portosystemic shunting in rats.

Regulatory effects of fatty acids on decarboxylation of leucine and 4-methyl-2-oxopentanoate in the perfused rat heart

The regulatory effects of fatty acids on the oxidative decarboxylation of leucine and 4-methyl-2-oxopentanoate were investigated in the isolated rat heart. Infusion of the long-chain fatty acid palmitate resulted in both an inactivation of the branched-chain 2-oxo acid dehydrogenase and an inhibition of the measured metabolic flux through this enzyme complex. Pyruvate addition also caused both an inactivation and an inhibition of the flux through the complex. On the other hand, the medium-chain fatty acid octanoate caused an activation of and a stimulation of flux through the branched-chain 2-oxo acid dehydrogenase when the perfusion conditions before octanoate addition maintained the enzyme complex in its inactive state. When the enzyme complex was activated before octanoate infusion, this fatty acid caused a significant inhibition of the flux through the branched-chain 2-oxo acid dehydrogenase reaction. Inclusion of glucose in the perfusion medium prevented the octanoate-mediated activation of the branched-chain 2-oxo acid dehydrogenase.

Effect of branched-chain amino acids and insulin on postinjury protein catabolism in growing animals

Muscle proteolysis continues to occur in hypercatabolic states despite the administration of carbohydrates and proteins. Recent clinical and experimental studies have demonstrated that, under catabolic conditions, treatment with either branched-chain amino acids (BCAA) or insulin may decrease negative nitrogen balance. However, the use of BCAA-enriched solutions to inhibit muscle proteolysis has never been tested in growing animals. A study was therefore undertaken to assess the effectiveness of such solutions, with or without insulin, as compared to a more balanced amino acid solution, in preventing or diminishing postinjury protein catabolism in growing animals. Fifteen-day-old rabbits, exposed to standard moderate trauma in the form of crushing the muscle mass of one rear thigh, received one of two amino acid formulations--a balanced amino acid solution (18.8% BCAA) or a 35% BCAA-enriched solution--for 96 hr. Insulin was given to subgroups of both series. The results indicate that: (1) nitrogen balance in nontraumatized animals is clearly superior when balanced amino acids are administered; (2) BCAA-enriched solutions may decrease postinjury muscle protein catabolism; (3) after trauma, insulin also has a nitrogen-conserving effect, which is demonstrated when it is combined both with BCAA-enriched (35%) and balanced amino acid (18.8%) solutions. However, a better nitrogen balance is achieved when insulin is associated with the balanced amino acid solution.

Infusion of branched chain-enriched amino acid solutions in sepsis

The goal of nutritional support in sepsis is, as in other conditions, to prevent the use of endogenous protein as an energy substrate and, ideally, to promote the synthesis of proteins specifically required in responding to the particular insult or stress at hand. This entails provision of an utilizable fuel, in sufficient quantity, that does not inhibit the use of endogenous nonprotein sources; preservation of the existing protein mass by minimizing skeletal muscle and visceral proteolysis; provision of amino acids in sufficient quantity and in the appropriate proportions such that protein synthesis is optimized. Specifically, this includes the synthesis of those proteins required to maintain hyperdynamic function of the essential organs as well as the hepatic and leukocytic synthesis of proteins required in immunologic defense. This study has assessed one aspect of this goal during the administration of nutrient solutions differing primarily in branched chain amino acid content. We conclude that leucine is fundamental among the branched chain amino acids for reducing skeletal muscle proteolysis. Solutions designed for sepsis or stress should, therefore, contain adequate amounts of this amino acid.

Interaction of various metabolites and agents with branched-chain 2-oxo acid oxidation in rat and human muscle in vitro

The interaction of various metabolites and agents with the 14CO2 production from 0.1 mM [1-14C]-labelled 2-oxoisocaproate (KIC) and 2-oxoisovalerate (KIV) was studied in rat and human heart and skeletal muscle preparations. Glucose and carnitine had no effect in any of the studied systems; palmitate gave a small increase of KIC oxidation only in soleus muscle. With rat hemidiaphragms a considerable decrease was found in the presence of high concentrations of a competitive branched-chain 2-oxo acid and of pyruvate, and in the presence of ketone bodies. A considerable increase was found in the presence of the branched-chain 2-oxo acid dehydrogenase kinase inhibitor 2-chloroisocaproate and the transminase inhibitor amino-oxyacetate. 2-Oxoglutarate increased and clofibric acid decreased only KIC oxidation. Divergent effects were given by intermediates of the degradation route of KIC and KIV and by monocarboxylate translocator inhibitors. The observed interactions are discussed and related to regulatory mechanisms which are known to affect the branched-chain 2-oxo acid dehydrogenase complex.

Nitrogen sparing effects and mechanisms of branched-chain amino acids in the injured rat

A series of experiments in a rat injury model were designed to elucidate the role and mechanisms of branched-chain amino acids in the post-injury catabolism. Our results suggest that: 1. Nutritional support can maintain nitrogen equilibrium in the early post-operative state. 2. Branched chain amino acids exert a nitrogen sparing effect and thus prevent or minimise post-operative catabolism. 3. Increasing the amount of infused branched chain amino acids results in nitrogen retention. 4. A balanced amino-acid mixture containing 45 per cent branched chain amino acids seems to be optimal for nutritional support in the post-injury state.

Use of aromatic amino acids as monitors of protein turnover

Phenylalanine and tyrosine were metabolized by the perfused rat heart via a mitochondrial aminotransferase. When L-[alanyl-2,3-3H]phenylalanine and L-[alanyl-2,3-3H]tyrosine were used, release of 3H2O was progressive over 2 h of perfusion. Metabolism of L-[U-14C]phenylalanine to 14CO2 or production of 3H2O from L-[ring-2,6-3H]phenylalanine or L-[ring-2,6-3H]tyrosine was not detected. Although 3H2O production from L-[alanyl-2,3-3H]phenylalanine was rapid, net production of phenylpyruvate or other metabolites of phenylalanine was negligible. As a result, use of aromatic amino acids as monitors of protein turnover in heart muscle was validated. Production of 3H2O from L-[alanyl-2,3-3H]phenylalanine was catalyzed by a mitochondrial enzyme, which is thought to be aspartate aminotransferase (EC 2.6.1.1). The rate of 3H2O production by both intact and detergent-treated mitochondria exceeded that of phenylpyruvate by a factor of 10 and occurred in the absence of alpha-ketoglutarate. These data provide an explanation for the production of 3H2O from L-[alanyl-2,3-3H]phenylalanine by perfused rat heart without the concomitant production of [3H]phenylpyruvate.

Effects of lactation on L-leucine metabolism in the rat. Studies in vivo and in vitro

1. The turnover rate of L-[1-14C]leucine was increased by 35% in lactating rats compared with virgin rats. Starvation or removal of pups (24 h) returned the value to that of the virgin rat. 2. Incorporation of L-[U-14C]leucine into lipid and protein of mammary glands of lactating rats in vivo increased 7-fold and 6-fold respectively compared with glands of virgin rats. Lactation caused no change in the incorporation of L-[U-14C]leucine into hepatic lipid and protein. 3. The production of 14CO2 from L[l-14C]leucine (in the presence of glucose) was similar in isolated acini from glands of fed (chow) and starved lactating rats. Feeding with a 'cafeteria' diet caused a slight decrease, and removal of pups a large decrease, in the oxidative decarboxylation of leucine. 4. Oxidation of L-[2-14C]leucine to 14CO2 was increased about 3-fold in acini from starved lactating rats or lactating rats fed on a 'cafeteria' diet compared with rats fed on a chow diet. Insulin decreased the formation of 14CO2 in all three situations. 5. Incorporation of L-[U-14C]- and [2-14C]-leucine into lipid was decreased in acini from starved lactating rats and lactating rats fed on a 'cafeteria' diet. Insulin tended to increase the conversion of [2-14C]leucine into lipid, but this was significant only in the case of the acini from 'cafeteria'-fed rats. 6. Experiments with (-)-hydroxycitrate indicate that the major route for conversion of leucine carbon into lipid in acini is via citrate translocation from the mitochondria. 7. The physiological implications of these findings are discussed.

[U-14C]glucose, -alanine, and -leucine oxidation in rats at rest and two intensities of running

The oxidations of injected [U-14C]glucose, [U-14C]alanine, and [U-14C]leucine were investigated in laboratory rats during rest or 2 h of easy and hard treadmill running. After [U-14C]glucose injection, the rate and magnitude of 14CO2 evolution were relatively low at rest and increased as a linear function of metabolic rate (VO2). Evolution of 14CO2 after [U-14C]alanine injection was faster and larger during exercise than rest. The peak of alanine decarboxylation occurred before glucose and, therefore, did not reflect conversion of alanine to glucose prior to decarboxylation. The rate and magnitude of 14CO2 evolution after [U-14C]leucine injection were proportional to metabolic rate, but less than after glucose or alanine injection. During exercise, levels of alanine and leucine in muscle and blood were unchanged or elevated compared to rest. During exercise, alanine levels were unchanged or increased in liver. Liver leucine levels were depressed when exercise began, but increased toward control values during exercise. The metabolism of selected amino acids is joined to carbon flow sustaining exercise.

Effect of stress and sampling site on metabolite concentration in rat plasma

The effect of mild stress on various plasma metabolites in the rat has been studied. Mild stress resulted in significant decreases in liver size and glycogen content, as well as in an increase of blood glucose. In addition, plasma lactate, insulin, glycerol and urea, as well as a number of amino acids were altered by stress. These data indicate that minimal stress can have major effects upon the composition of blood, and suggest the need for strict precautions on the handling of animals during blood sampling. The site of blood extraction--tail tip vs. neck--was also found to have a significant effect on plasma lactate, glucose and urea concentrations. In stressed animals the differences between tail- and neck blood composition were increased.

[Semiessential and nonessential amino acids in parenteral feeding]

With reference to a critical study of the relevant literature and to results from their own investigations, the authors emphasize the importance of the semi-essential and non-essential amino acids arginine, histidine, tyrosine, cystine and glutamic acid for the completion of essential amino-acid mixtures destined for parenteral feeding. MADDEN'S assumption that intravenously applied glutamic acid is intolerable was not supported by the authors' experiments in dogs. As evidenced by the increase of blood urea, the glutamic acid-containing amino-acid mixture was well utilizable. When glutamic acid is present, proline and alanine are obviously superfluous in amino-acid mixtures for parenteral feeding, since they are easily formed from glutamic acid by intermediary metabolism.

Plasma amino acids in children and adolescents on hemodialysis

Fasting plasma amino acid concentrations were measured in 16 children on regular hemodialysis for renal failure. Reductions compared to normal were found in valine, leucine, isoleucine, lysine, histidine, tyrosine, and serine; and increases were found in glycine, citruline, proline, and 1- and 3-methylhistidine. Acute reductions in amino acid concentrations occurred in response to i.v. glucose, similar to those reported in normal adults, but plasma alanine, which was raised only in those with poor glucose tolerance, fell to normal and did not vary in those with normal glucose tolerance. No correlations were found with growth, but the plasma glycine concentration was highest in those patients with poorest energy intakes. Plasma alanine concentrations correlated with raised triglyceride concentrations. It is suggested that many of the abnormalities are due to the excessive utilization of protein for energy because of impaired availability of conventional energy sources in uremia.

Protein synthesis in central nervous tissue: studies on retina in vitro

Rabbit retinas were maintained in vitro in medium that resembled CSF but with leucine varied from 2 to 1000 microM. Both leucine and threonine were isotopically labelled. When leucine in the medium was 100-1000 microM, leucine was incorporated into protein at 2.03 +/- 0.04 (S.E.M.) mumol/g dry wt./h, a turnover per h of 0.55% of the leucine in retinal protein. Incorporation was constant for at least 7 h. It was reduced 34% when the other amino acids were omitted from the medium and 24% when they were increased 15 fold above physiological levels. When medium leucine was reduced to 2 microM with other amino acids constant, 14C-leucine incorporation fell 70% without significant change in 3H-threonine incorporation, indicating a fall in intracellular specific activity of leucine. The intracellular/extracellular concentration ratio of labelled leucine was 4:1 with medium leucine 23 microM. It fell markedly when medium leucine was reduced to 2 microM or increased to 1000 microM. The concentration ratio of labelled threonine was 15:1 with medium leucine at physiological levels but fell to 6:1 when medium leucine was increased to 1000 microM. Decarboxylation removed 1.5% of free intracellular leucine per min and, at physiological concentrations, was 7.7% the rate of protein incorporation. The ratio of protein synthesis/breakdown, estimated from changes in leucine and 7 other essential amino acids in the medium, was nearly unity. The potential of this preparation for study of CNS protein metabolism is discussed.

Metabolism of branched-chain amino acids in altered nutrition

Plasma concentrations of the branched-chain amino acids (leucine, isoleucine, and valine) are more prominently affected than the concentrations of other amino acids by changes in dietary-caloric, protein, fat, and carbohydrate-intake in man. For example, within a day of starvation or protein deprivation, there are increases or decreases, respectively, in concentrations of these amino acids in the plasma of healthy human volunteers. The cellular mechanisms of these changes have been investigated in rats, since the changes in the plasma branched-chain amino acid concentrations in response to the previously stated dietary alterations are similar to those found in man. Among the tissues studied (liver, skeletal muscle, heart, kidney, and intestine) only liver and the skeletal muscle exhibit changes in branched-chain amino acid concentrations in response to dietary alteation. Changes in plasma concentrations appear to reflect more intimately those of the muscle than theliver. After 8 days of starvation, there is a 25% decrease in the muscle protein, but after 8 days of protein deprivation, there is no significant change in the muscle mass. Increases in concentrations of branched-chain amino acids in the muscle are much smaller than the amounts of these amino acids lost as protein constituents form the muscle during fasting. Changes in tissue transport, transamination, oxidation, or metabolic conversions of branched-chain amino acids in tissues. It is concluded that increased muscle protein breakdown, which provides substrates for enhanced gluconeogenesis in the liver and enhanced branched-chain amino acid oxidation in the muscle, is the major mechanism of hyperbranched-chain aminoacdemia in starvation. On the other hand, the principal factors in the development of hypobranched-chain aminoacidemia during protein deprivation are absence of exogenous amino acids as well as curtailed muscle protein breakdown.