The Effect of Epinephrine, Glucagon, and the Nutritional State on the Oxidation of Branched Chain Amino Acids and Pyruvate by Isolated Hearts and Diaphragms of the Rat

Why Are Branched-Chain Amino Acids Increased in Starvation and Diabetes?

Branched-chain amino acids (BCAAs; valine, leucine, and isoleucine) are increased in starvation and diabetes mellitus. However, the pathogenesis has not been explained. It has been shown that BCAA catabolism occurs mostly in muscles due to high activity of BCAA aminotransferase, which converts BCAA and α-ketoglutarate (α-KG) to branched-chain keto acids (BCKAs) and glutamate. The loss of α-KG from the citric cycle (cataplerosis) is attenuated by glutamate conversion to α-KG in alanine aminotransferase and aspartate aminotransferase reactions, in which glycolysis is the main source of amino group acceptors, pyruvate and oxaloacetate. Irreversible oxidation of BCKA by BCKA dehydrogenase is sensitive to BCKA supply, and ratios of NADH to NAD⁺ and acyl-CoA to CoA-SH. It is hypothesized that decreased glycolysis and increased fatty acid oxidation, characteristic features of starvation and diabetes, cause in muscles alterations resulting in increased BCAA levels. The main alterations include (i) impaired BCAA transamination due to decreased supply of amino groups acceptors (α-KG, pyruvate, and oxaloacetate) and (ii) inhibitory influence of NADH and acyl-CoAs produced in fatty acid oxidation on citric cycle and BCKA dehydrogenase. The studies supporting the hypothesis and pros and cons of elevated BCAA concentrations are discussed in the article.

Metabolic Coordination of Physiological and Pathological Cardiac Remodeling

Metabolic pathways integrate to support tissue homeostasis and to prompt changes in cell phenotype. In particular, the heart consumes relatively large amounts of substrate not only to regenerate ATP for contraction but also to sustain biosynthetic reactions for replacement of cellular building blocks. Metabolic pathways also control intracellular redox state, and metabolic intermediates and end products provide signals that prompt changes in enzymatic activity and gene expression. Mounting evidence suggests that the changes in cardiac metabolism that occur during development, exercise, and pregnancy as well as with pathological stress (eg, myocardial infarction, pressure overload) are causative in cardiac remodeling. Metabolism-mediated changes in gene expression, metabolite signaling, and the channeling of glucose-derived carbon toward anabolic pathways seem critical for physiological growth of the heart, and metabolic inefficiency and loss of coordinated anabolic activity are emerging as proximal causes of pathological remodeling. This review integrates knowledge of different forms of cardiac remodeling to develop general models of how relationships between catabolic and anabolic glucose metabolism may fortify cardiac health or promote (mal)adaptive myocardial remodeling. Adoption of conceptual frameworks based in relational biology may enable further understanding of how metabolism regulates cardiac structure and function.

Impact of glucagon-like peptide-1 on myocardial glucose metabolism revisited

The gut hormone glucagon-like peptide-1 (GLP-1) is an insulinotropic incretin with significant cardiovascular impact. Two classes of medication, GLP-1 analogues and DPP-4 inhibitors, have been developed that circumvent the rapid degradation of GLP-1 by the enzyme dipeptidyl peptidase-4 (DPP-4), both enhance the incretin effect and were developed for the treatment of type 2 diabetes. Several mechanisms suggesting that DPP-4 inhibitors, GLP-1, and analogues could have a protective effect on the cardiovascular risk profile have been forwarded; e.g., reductions of blood glucose, body weight, blood pressure, improvement in left ventricular ejection fraction, myocardial perfusion, atherosclerosis development, and endothelial function. Despite this, the reasons for a decreased risk of developing cardiovascular disease and reduced post-ischaemia damage are still poorly understood. The potentially beneficial effect of GLP-1 stimulation may rely on, among others, improved myocardial glucose metabolism. This review focuses on the dogma that GLP-1 receptor stimulation may provide beneficial cardiovascular effects, possibly due to enhanced myocardial energetic efficiency, by increasing myocardial glucose uptake. The published literature was systematically reviewed and the applied models evaluated since the outcomes of varying studies differ substantially. Reports on the effect of GLP-1R stimulation on myocardial metabolism are conflicting and should be evaluated carefully. There is limited and conflicting information on the impact of these agents in real life patients and while clinical outcome studies investigating the cardiovascular effects of GLP-1 based therapies have been initiated, the first two studies, both on DPP-4 inhibitors, designed specifically to evaluate cardiac safety reported largely neutral outcomes.

Action and therapeutic potential of oxyntomodulin

Oxyntomodulin (OXM) is a peptide hormone released from the gut in post-prandial state that activates both the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR) resulting in superior body weight lowering to selective GLP1R agonists. OXM reduces food intake and increases energy expenditure in humans. While activation of the GCGR increases glucose production posing a hyperglycemic risk, the simultaneous activation of the GLP1R counteracts this effect. Acute OXM infusion improves glucose tolerance in T2DM patients making dual agonists of the GCGR and GLP1R new promising treatments for diabetes and obesity with the potential for weight loss and glucose lowering superior to that of GLP1R agonists.

Unraveling oxyntomodulin, GLP1's enigmatic brother

Oxyntomodulin (OXM) is a peptide secreted from the L cells of the gut following nutrient ingestion. OXM is a dual agonist of the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR) combining the effects of GLP1 and glucagon to act as a potentially more effective treatment for obesity than GLP1R agonists. Injections of OXM in humans cause a significant reduction in weight and appetite, as well as an increase in energy expenditure. Activation of GCGR is classically associated with an elevation in glucose levels, which would be deleterious in patients with T2DM, but the antidiabetic properties of GLP1R agonism would be expected to counteract this effect. Indeed, OXM administration improved glucose tolerance in diet-induced obese mice. Thus, dual agonists of the GCGR and GLP1R represent a new therapeutic approach for diabetes and obesity with the potential for enhanced weight loss and improvement in glycemic control beyond those of GLP1R agonists.

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.

Glucagon and a glucagon-GLP-1 dual-agonist increases cardiac performance with different metabolic effects in insulin-resistant hearts

BACKGROUND AND PURPOSE

The prevalence of heart disease continues to rise, particularly in subjects with insulin resistance (IR), and improved therapies for these patients is an important challenge. In this study we evaluated cardiac function and energy metabolism in IR JCR:LA-cp rat hearts before and after treatment with an inotropic compound (glucagon), a glucagon-like peptide-1 (GLP-1) receptor agonist (ZP131) or a glucagon-GLP-1 dual-agonist (ZP2495).

EXPERIMENTAL APPROACH

Hearts from IR and lean JCR:LA rats were isolated and perfused in the working heart mode for measurement of cardiac function and metabolism before and after addition of vehicle, glucagon, ZP131 or ZP2495. Subsequently, cardiac levels of nucleotides and short-chain CoA esters were measured by HPLC.

KEY RESULTS

Hearts from IR rats showed decreased rates of glycolysis and glucose oxidation, plus increased palmitate oxidation rates, although cardiac function and energy state (measured by ATP/AMP ratios) was normal compared with control rats. Glucagon increased glucose oxidation and glycolytic rates in control and IR hearts, but the increase was not enough to avoid AMP and ADP accumulation in IR hearts. ZP131 had no significant metabolic or functional effects in either IR or control hearts. In contrast, ZP2495 increased glucose oxidation and glycolytic rates in IR hearts to a similar extent to that of glucagon but with no concomitant accumulation of AMP or ADP.

CONCLUSION AND IMPLICATIONS

Whereas glucagon compromised the energetic state of IR hearts, glucagon-GLP-1 dual-agonist ZP2495 appeared to preserve it. Therefore, a glucagon-GLP-1 dual-agonist may be beneficial compared with glucagon alone in the treatment of severe heart failure or cardiogenic shock in subjects with IR.

Regulation of leucine catabolism by metabolic fuels in mammary epithelial cells

Lactation is associated with elevated catabolism of branched-chain amino acids (BCAA) in mammary glands to produce glutamate, glutamine, alanine, aspartate, and asparagine. This study determined effects of metabolic fuels on the catabolism of leucine (a representative BCAA) in bovine mammary epithelial cells. Cells were incubated at 37 °C for 2 h in Krebs buffer containing 0.5 mM L-leucine and either L-[1-(14)C]leucine or L-[U-(14)C]leucine. The medium also contained 0-5 mM D-glucose, 0-2 mM L-glutamine, 0-4 mM DL-β-hydroxybutyrate, or 0-2 mM oleic acid. Rates of leucine decarboxylation were 60 % lower, but rates of α-ketoisocaproate production were 34 % higher, in the presence of 2 mM glucose than in its absence. All variables of leucine catabolism did not differ between 2 and 5 mM glucose or between 0 and 4 mM DL-β-hydroxybutyrate. Compared with 0-0.25 mM glutamine, 0.5 and 2 mM L-glutamine reduced leucine transport, transamination, and decarboxylation. In contrast, increasing the concentration of oleic acid from 0 to 2 mM dose-dependently stimulated leucine transamination, decarboxylation, and oxidation of carbons 2-6. Oleic acid also enhanced the abundance of cytosolic BCAA transaminase, while reducing the phosphorylated level (inactive state) of the E1α subunit of the mitochondrial branched-chain α-ketoacid dehydrogenase complex. Thus, hypoglycemia or ketosis in early lactation does not likely affect BCAA metabolism in mammary epithelial cells. Increasing circulating levels of BCAA and oleic acid may have great potential to increase the syntheses of glutamate, glutamine, aspartate, alanine, and asparagine by lactating mammary glands, thereby leading to enhanced production of milk for suckling neonates.

Plasma concentration of taurine is higher in malnourished than control children: differences between kwashiorkor and marasmus

Plasma free amino acids were determined in the plasma of severely malnourished children under two years of age. A total of thirty-one patients and eleven controls were evaluated: seventeen cases of kwashiorkor, eight cases of marasmus, and six cases of marasmic-kwashiorkor. Fasting plasma samples were taken in the morning on the day of admission. Fasting plasma samples were also taken from nine patients at discharge after two months in the hospital where they received a balanced diet as treatment. A partial reversal of the signs of malnutrition was observed at discharge. In the whole group of patients ad admission, lower concentrations of tyrosine, methionine, tryptophan, and leucine and higher concentrations of aspartate, glutamate, and taurine were observed compared to controls. Taurine continued to be elevated in the malnourished group at the time of discharge. Marasmic children, as compared to controls, had high aspartate and low tryptophan levels, but taurine levels were not significantly different from controls. Kwashiorkor patients had low tyrosine, methionine, tryptophan, and lysine, and significantly higher taurine plasma levels. The elevated concentration of taurine might be the result of a redistribution of this amino acid to provide specific tissues with the required amount for development.

Hyperglucagonemia and the immediate fate of dietary leucine: a kinetic study in humans

The possible role of glucagon in determining the fate of dietary absorbed amino acids within the splanchnic bed was investigated in five healthy male volunteers. A kinetic study was performed involving a continuous 240-minute infusion of L-[5,5,5-2H3]leucine and D-[6,6-2H2]glucose by vein, while L-[1-13C]leucine was infused by a feeding tube into the duodenum (intragut [i.g.]) along with a constant intravenous (i.v.) infusion of somatotropin release-inhibitory factor (SRIF) combined with insulin, growth hormone, and glucagon. In random order, glucagon was infused at a rate of 0.4 ng x kg(-1) x min(-1) in one experiment and 1.2 ng x kg(-1) x min(-1) in the other experiment, while insulin and growth hormone were kept at constant serum levels, respectively, 37+/-13 pmol x L(-1) and 5+/-0.2 microg x L(-1). The diet was provided as an L-amino acid solution including 60 micromol x kg(-1) x h(-1) leucine without fat and carbohydrate. During the higher rate of glucagon infusion, there was an increase in plasma glucagon and glucose concentrations, glucose flux, and net dietary leucine release into the periphery from the splanchnic bed. Splanchnic removal and uptake of leucine were decreased with increased glucagon infusion. There were no statistical differences in the plasma leucine level and i.v. and i.g. leucine fluxes at the two glucagon levels, although leucine metabolic clearance increased (0.74 v 0.85 L x kg(-1) x h(-1), P=.08) in the case of glucagon excess. Plasma glucose increased with glucagon excess and was negatively correlated (P < .05) with the plasma leucine level (r=-.348) and i.v. (r=-.459) or i.g. (r=-.359) leucine fluxes. The negative correlation between plasma glucagon and leucine levels was also significant (r=-.684). No significant correlation was found between dietary leucine splanchnic removal and glucose, glucagon, or leucine plasma concentrations. We conclude that glucagon in excess has only a small quantitative effect on the overall handling of dietary leucine, and hypothesize that more leucine is exported to the peripheral tissues under these hormonal conditions.

Effect of acute acidosis and alkalosis on leucine kinetics in man

The effects of acute pH changes on whole body leucine kinetics (1-13C-leucine infusion technique) were determined in normal subjects. Plasma insulin, glucagon, and growth hormone concentrations were kept constant by somatostatin and replacement infusions of the three hormones. When acidosis was produced by ingestion of NH4Cl (4 mmol kg-1 p.os; n = 8) arterialized pH decreased within 3 h from 7.39 +/- 0.01 to 7.31 +/- 0.01 (P less than 0.001) and leucine plasma appearance increased by 0.13 +/- 0.04 mumol kg-1 min-1 (P less than 0.02); in contrast, when alkalosis was produced by intravenous infusion of 4 mmol kg-1 NaHCO3 (n = 7, pH 7.47 +/- 0.01), leucine plasma appearance decreased by -0.09 +/- 0.04 mumol kg-1 min-1 (P less than 0.01 vs. acidosis). Whole body leucine flux also increased during acidosis compared to alkalosis (P less than 0.05), suggesting an increase in whole body protein breakdown during acidosis. Apparent leucine oxidation increased during acidosis compared to alkalosis (P = 0.05). Net forearm leucine exchange remained unaffected by acute pH changes. Plasma FFA concentrations decreased during acidosis by -107 +/- 67 mumol l-1 (P less than 0.05) and plasma glucose increased by 1.90 +/- 0.25 mmol l-1 (P less than 0.02); in contrast, alkalosis resulted in an increase in plasma FFA by 83 +/- 40 mumol l-1 (P less than 0.02; P less than 0.01 vs. acidosis), suggesting an increase in lipolysis; plasma glucose decreased compared to acidosis (P less than 0.01). The data demonstrate that acute metabolic acidosis and alkalosis, as they occur in clinical conditions, influence protein breakdown, and in the opposite direction, lipolysis.

Influence of fatty acids on ammonia and amino acid flux from active human muscle

This study examined the dynamics of ammonia and amino acid exchange of human muscle during prolonged steady-state one-legged exercise at 80% of knee extensor maximal work capacity. Subjects (n = 10) performed leg extensor exercise for 1 h (control series), rested for 40 min while an infusion of Intralipid and heparin was begun, and then exercised the contralateral leg with the identical protocol [free fatty acid (FFA) series]. In the control series, ammonia efflux rose progressively, and 4.4 +/- 0.6 mmol were released in 1 h compared with 2.4 +/- 0.5 mmol (P less than 0.05) in the FFA series. The exercise was associated with large effluxes of total amino acids from the active muscle over the hour (12.8 +/- 4.3 and 10.3 +/- 3.3 mmol for control and FFA, respectively). Glutamine and alanine accounted for 47 and 64% of the efflux for the control and FFA series, respectively, while comparable values for essential amino acids were 24 and 20%. The latter implies that a net muscle protein catabolism was occurring during the exercise. The FFA treatment was associated not only with a reduced muscle ammonia release but also with a decreased (P less than 0.05) arterial concentration of nine amino acids (alanine, methionine, lysine, hydroxyproline, serine, glycine, proline, asparagine, and ornithine). Interpretation is limited due to the treatment order effect, but these data are compatible with the hypothesis that plasma clearance was affected by FFA.

The effect of ketone bodies on nitrogen metabolism in skeletal muscle

1. The ketone bodies, D-beta-hydroxybutyrate and acetoacetate, inhibit glycolysis thereby reducing pyruvate availability which leads to a marked inhibition of branched-chain amino acid metabolism and alanine synthesis in skeletal muscles from fasted mammalian and avian species. 2. The rate of glutamine release from skeletal muscles from fasted birds is increased at the expense of alanine in the presence of elevated concentrations of ketone bodies because of an increase in the availability of glutamate for glutamine synthesis. 3. Ketone bodies inhibit both protein synthesis and protein degradation in skeletal muscles from fasted mammalian and avian species in vitro. The mechanisms involved remain unknown. 4. Inhibition of amino acid metabolism and protein turnover in skeletal muscle by ketone bodies may be an important survival mechanism during adaptation to catabolic states such as prolonged fasting.

Interaction of cortisol and epinephrine in the regulation of leucine kinetics in man

To assess the interaction of the two major stress hormones epinephrine and cortisol in the regulation of leucine kinetics in man, epinephrine (50 ng/kg/min) was infused either alone or in combination with cortisol (2 micrograms/kg/min) into two groups of 6 postabsorptive normal male subjects during 180 min. Plasma leucine concentrations decreased by 28% (p less than 0.05) from baseline during epinephrine treatment (plasma levels 515 pg/ml); this was due to a decrease of leucine appearance (determined by 1-13C-leucine infusions) by 23% (p less than 0.025); leucine oxidation decreased by 29% (p less than 0.05). However, when plasma cortisol concentrations were elevated to supraphysiological levels (16.3 mumol/l) during epinephrine administration, the decreases of leucine plasma concentrations, appearance and oxidation were abolished. Plasma glucose and FFA concentrations were similarly elevated during both kinds of treatment. Since leucine appearance represents a measurement of total body protein breakdown and leucine disappearance into non-oxidative pathways reflects protein synthesis, the data indicate that plasma epinephrine concentrations during severe stress exert a protein anabolic effect in man which may counteract catabolic properties of elevated plasma cortisol.

Ketone bodies inhibit leucine degradation in chick skeletal muscle

1. DL-beta-hydroxybutyrate (4 mM) increased the net rate of leucine transamination and the net rate of 2-oxoisocaproate (KIC) production in extensor digitorum communis muscles from fed chicks. 2. DL-beta-hydroxybutyrate at 1 and 4 mM inhibited leucine oxidative decarboxylation in muscles from fed chicks. 3. Acetoacetate at 1 and 4 mM inhibited leucine oxidative decarboxylation and total leucine oxidation, but increased net KIC production in muscles from fed chicks. 4. Both DL-beta-hydroxybutyrate and acetoacetate at 1 and 4 mM inhibited the net rate of leucine transamination and the rates of leucine oxidative decarboxylation and total leucine oxidation in muscles from 24-hr fasted chicks.

Alanine and inter-organ relationships in branched-chain amino and 2-oxo acid metabolism. Review

Branched-chain amino acid metabolism in skeletal muscle promotes the production of alanine, an important precursor in hepatic gluconeogenesis. There is controversy concerning the origin of the carbon skeleton of alanine produced in muscle, specifically whether it is derived from carbohydrate via glycolysis (the glucose-alanine cycle) or from amino acid precursors (viz. glutamate, valine, isoleucine, methionine, aspartate, asparagine) via a pathway involving phosphoenolpyruvate (PEP) carboxykinase and pyruvate kinase, or NADP-malate dehydrogenase (malic enzyme). The relevant literature is reviewed and it is concluded that neogenic flux from amino acids is unlikely to be of major quantitative importance for provision of the carbon skeleton of alanine either in vitro or in vivo. Evidence is presented that branched-chain amino acid oxidation in muscle is incomplete and that the branched-chain 2-oxo acids and the products of their partial oxidation (including glutamine) are released. The role of these metabolites is discussed in the context of fuel homeostasis in starvation.

Catabolism of branched-chain amino acids by diaphragm muscles of fasted and diabetic rats

In vitro catabolism of branched-chain amino acids, leucine and valine, was investigated using diaphragm muscles from normal, streptozotocin-diabetic and overnight fasted rats. Oxidation and transamination of [1-14C] branched-chain amino acids were both stimulated to a similar extent by diabetes or fasting, when diaphragms were incubated with glucose. Transamination of leucine and valine was increased when diaphragms were incubated with pyruvate; stimulation of transamination was greatest in diaphragms from diabetic rats. Leucine and valine oxidation by control diaphragms was inhibited by pyruvate while it was unchanged or slightly stimulated in diaphragms from fasted or diabetic rats. Thus diaphragms from diabetic rats oxidized two to threefold more branched-chain amino acids than controls when they were incubated with pyruvate. The specific radioactivity of extracellular alpha-ketoisocaproate (KIC; the product of leucine transamination) produced by diaphragms incubated with [14C]leucine was similar for all groups (fed, fasted, or diabetic) in the presence or absence of pyruvate. Oxidation of [1-14C]KIC by diaphragms from fasted or diabetic rats, incubated with glucose, was the same or less than KIC oxidation by control diaphragms. Incubation with pyruvate inhibited KIC oxidation by control diaphragms to a significantly greater degree than that by diaphragms from diabetic or fasted rats. These data suggest the following Flux through branched-chain amino acid transaminase is limited by the availability of amino group acceptors in diaphragms from normal and overnight fasted rats, and to a greater extent in diaphragms from diabetic rats. Flux through the transaminase may be a major determinant of accelerated branched-chain amino acid oxidation by diaphragms in fasting and diabetes. In diaphragms of fasted and diabetic rats, flux through the branched-chain alpha-ketoacid dehydrogenase complex is resistant to inhibition by pyruvate, which is normally observed in controls.

Increased leucine flux in short-term fasted human subjects: evidence for increased proteolysis

Plasma leucine concentration increases with short-term fasting in normal humans. In previous studies using an 18-h constant infusion of [2H3]leucine, a 15% decrease in the rate of appearance (Ra) of leucine was observed between 15 and 30 h of fasting. However, incorporation of labeled leucine into and subsequent release from body protein could result in an apparent decrease in leucine Ra. The present studies were undertaken to determine the rate of leucine N and carbon flux in short-term fasted human subjects in using an experimental design that would minimize potential recycling of label. Six normal subjects were infused with [15N]- and [2H3]leucine for two 4-h periods (0600 to 1000 h and 2000 to 2400 h) during a 30-h fast. Between the 15th and 30th h of fasting, plasma leucine concentration (102 +/- 10 to 169 +/- 18 microM, P less than 0.01) and leucine C (1.46 +/- 0.05 to 1.64 +/- 0.10 mumol X kg-1 X min-1, P less than 0.05) and leucine N (1.93 +/- 0.14 to 2.49 +/- 0.26 mumol X kg-1 X min-1, P less than 0.05) flux increased, whereas the metabolic clearance rates of leucine C and N decreased (P less than 0.01 and less than 0.05, respectively). Therefore, when isotope recycling is minimized with short periods of isotope infusion, leucine flux increases between 15 and 30 h of fasting. Because the only source of leucine in the postabsorptive periods is body protein, we conclude that the rate of whole-body proteolysis is increased in short-term fasting in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of epinephrine infusion on leucine and alanine kinetics in humans

Infusion of epinephrine in humans increases glucose production and decreases plasma concentrations of some essential amino acids such as leucine, while not affecting the plasma concentration of the potential gluconeogenic amino acid alanine. To determine whether epinephrine alters alanine and leucine metabolism, rates of appearance (Ra) and disappearance (Rd) of glucose, alanine, and leucine were determined in postabsorptive volunteers using [3H]glucose, [2H3]alanine, [15N]leucine, and [2H3]leucine during a 180-min infusion of epinephrine (50 ng X kg-1 X min-1). Plasma glucose (90 +/- 1 to 142 +/- 5 mg/dl) and insulin (10 +/- 1 to 16 +/- 2 micrograms/ml) increased (P less than 0.05), whereas plasma alanine concentrations did not change and plasma leucine concentrations increased (127 +/- 5 to 72 +/- 3 microM). Glucose Ra increased transiently and returned to basal values by 120 min. In contrast, alanine Ra and Rd increased identically and progressively from 5.7 +/- 0.5 to 14.5 +/- 1.9 mumol X kg-1 X min-1 by 180 min. Although leucine nitrogen Ra increased transiently and returned to basal values, leucine carbon Ra and Rd decreased (P less than 0.05) during the infusion of epinephrine. The calculated rate and percent of leucine nitrogen going to alanine increased, whereas the percent of alanine nitrogen derived from leucine remained constant. The increase in alanine Ra was entirely attributable to increased de novo synthesis because proteolysis, as estimated by leucine carbon flux, decreased.

Ketone body synthesis from leucine by adipose tissue from different sites in the rat

Leucine is catabolized to ketone bodies in adipose tissue, but the contribution of this output to overall ketone metabolism is not known. The intent of the present study was to determine the capacity of different adipose tissues to synthesize ketone bodies from leucine. The amino acid was readily converted into acetoacetate in epididymal, perirenal, and omental fat tissues. In rats fed ad libitum, the rate of acetoacetate synthesis in omental fat (about 2 mumol g tissue-1h-1) was at least 8 times higher than in epididymal or perirenal fat. In omental fat, the rates of acetoacetate formation from alpha-ketoisocaproic acid were 47-55% lower than from leucine at all concentrations examined. There was no significant synthesis of beta-hydroxybutyrate from leucine or alpha-ketoisocaproic acid. After oxidative decarboxylation, a greater proportion (about three-fourths) of leucine in omental fat was metabolized to acetoacetate than to CO2 production through the Krebs cycle. Although addition of glucose, pyruvate, or carnitine did not affect the production of acetoacetate, fasting for 24 h stimulated acetoacetate synthesis from leucine and alpha-ketoisocaproic acid in omental fat. The high rate of leucine conversion to acetoacetate in omental fat was related to high activities of leucine aminotransferase and branched-chain alpha-keto acid dehydrogenase. Moreover, protein content and cytochrome c oxidase activity of omental mitochondria were, respectively, 13 and 12 times higher than in epididymal mitochondria. In contrast, fat content of epididymal adipose tissue was 21 times that of omental adipose tissue. Epididymal depot consisted of 2.0% protein and 75.8% fat, whereas omental depot contains 17.2% protein and 3.6% fat, resembling that of liver and muscle. The results suggest that the high ketogenic capacity of omental fat stems in part from an augmented mitochondrial mass and high activity of branched-chain alpha-keto acid dehydrogenase.

Influence of nutritional status on exertion-induced forearm amino acid metabolism in normal man

Normal volunteers were evaluated in the postabsorptive state, following 10 days of protein-calorie starvation, and during intravenous feeding ( ivf ) to determine the impact of nutritional status upon exertion-induced muscle amino acid metabolism. An isolated forearm model allowed an evaluation of recovery of metabolism following 1 min of submaximal isotonic exercise. Forearm blood flow returned to near basal levels within 15 min after exertion during postabsorptive and ivf conditions, but remained greater than or equal to 150% of basal at 1 hr after exercise during starvation. At 30 and 60 min after exercise, forearm plasma flux of total and essential amino acids were unchanged from basal in the postabsorptive state. However, the pattern of essential amino acid flux demonstrated a relative reduction in isoleucine and leucine efflux compared with basal, and this pattern persisted throughout 1 hr of recovery. During starvation, a significant (P less than 0.05) increase in total and essential amino acid efflux was observed throughout the recovery period. Starvation was also associated with significant increases in alanine and lysine efflux during recovery. Intravenous feeding was associated with a significant (P less than 0.05) uptake of essential amino acids with respect to basal levels at 30 min after exercise. At 60 min, there was a shift to total amino acid efflux but no change from basal flux for essential amino acids. During ivf , the pattern of essential amino acid uptake returned to basal within 1 hr after exertion.

Changes in ovine fetal hindlimb amino acid metabolism during maternal fasting

The flux of various substrates across the ovine fetal and maternal hindlimbs was measured in the fed state and after 5 days of maternal fasting. Whole blood concentrations of glucose, oxygen, ammonia, and six amino acids (glutamate, glutamine, alanine, valine, isoleucine, and leucine) were determined in the fetal and maternal femoral artery and distal inferior vena cava in 15 chronic animal preparations. During fasting the fetal arterial glucose concentration fell by 40% (from 0.828 to 0.494 mM), and the arteriovenous concentration difference decreased by 30% (from 0.148 to 0.099 mM). Similar changes were noted in maternal blood. Fetal oxygen concentrations remained similar between the fed and fasted state, and the fetal arteriovenous oxygen concentration difference increased slightly from 0.861 to 1.02 mM. The glucose oxygen quotient decreased in the fetus from 1.20 to 0.621. In addition, significant changes occurred in the net balance of several amino acids during the fasted state. Both alanine and glutamine, which demonstrated a positive uptake by the fetal hindlimb during the fed state, showed a substantial efflux from the fetal hindlimb during the fasting period. The fetal arteriovenous concentration difference of the branched-chain amino acids (leucine, isoleucine, and valine) increased significantly during fasting. These observations are consistent with the hypothesis that the ovine fetus adapts to a diminished supply of glucose from the mother by enhanced amino acid catabolism and, possibly, proteolysis with subsequent release of gluconeogenic precursors in the form of alanine and glutamine.

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.

Direct anabolic effects of thyroid hormone on isolated mouse heart

The direct effects of L-and D-triiodothyronine (T3) on cardiac protein metabolism were investigated using fetal mouse hearts in organ culture. This model allowed the production of "thyrotoxicosis" in isolated hearts in vitro in the absence of the usual systemic metabolic and hemodynamic effects of thyroid hormones. Hearts were studied during the first 24 h of T3 exposure in culture, before changes in beating rate due to T3 occurred. Phenylalanine release was decreased by 26 +/- 2.3% (P less than 0.001) by the optimal concentrations of T3 (10(-7) to 10(-6) M). Changes were similar in the presence or absence of insulin. D-T3 was also anabolic, decreasing phenylalanine release by 24 +/- 2.5% (P less than 0.001) at concentrations of 10(-6) to 10(-5) M. The L-isomer increased protein synthesis by 23 +/- 6.8% (P less than 0.05) and decreased protein degradation, as measured by phenylalanine release in the presence of cycloheximide, by 5 +/- 1.6% (P less than 0.01). The D-isomer also increased protein synthesis but had no measurable effect on protein degradation. We conclude that thyroid hormones can exert direct anabolic effects on heart in the absence of systemic hemodynamic and metabolic changes. These effects are mediated primarily through an acceleration of the rate of protein synthesis; in the case of L-T3, a small inhibition of proteolysis may also occur.

Branched-chain ketoacid dehydrogenase activity and growth of normal and mutant human fibroblasts: the effect of branched-chain amino acid concentration in culture medium

We investigated changes in cell growth and branched chain ketoacid dehydrogenase (BCKD) activity by varying the concentrations of branched-chain amino acids (BCAAs) in culture medium of diploid fibroblasts from humans with normal BCKD and with impaired enzyme function. For logarithmic growth the two cell populations required similar minimal concentrations (0.05 mM) for each of leucine, isoleucine, and valine tested together. At confluency (saturation density) mutant cells grew less well to the extent of 30 to 40% in the highest concentrations of BCAAs that could be tested, 20.8 mM. BCKD activity was not changed by growth of normal or mutant cells in the absence of BCAAs. This enzyme activity was increased in normal but not mutant cells by growth in 20.8 mM BCAAs. These studies suggest the following: (1) BCKD mutant fibroblasts in culture slow their growth rate in response to high concentrations of BCAAs; (2) the growth disadvantage for mutant cells in high concentrations of BCAAs may be useful to select for enzyme normal hybrids derived when cells with two different mutations affecting BCKD are fused; (3) the increase of BCKD activity in normal but not mutant cells grown in high concentrations of BCAAs can distinguish these phenotypes more precisely in humans; and (4) the mechanism of BCKD stimulation in normal cells grown in high concentrations of BCAAs remains to be explained but can be pursued further with the cell culture conditions described.

Response to trauma of protein, amino acid, and carbohydrate metabolism in injured and uninjured rat skeletal muscles

Soft tissue injury to one hindlimb produced trauma in rats without affecting their food intake or weight gain. Histologic examination showed damage to the soleus and gastrocnemius muscles but not to the extensor digitorum longus muscle. The protein content of the injured soleus muscle was lower than that of the contralateral soleus at one day after injury, and was reflected in vitro by a faster rate of protein degradation. The injured soleus also showed greater rates of protein synthesis, glucose uptake, glycolysis, oxidation of glucose, pyruvate, and leucine, and de novo synthesis of alanine. During three days after the injury, urinary nitrogen excretion increased progressively and was paralleled by a faster rate of protein degradation in uninjured muscles incubated with glucose, insulin, and amino acids. In these muscles, the inhibition of protein degradation by insulin diminished, while its stimulation of protein synthesis was unaffected. This insensitivity of proteolysis to insulin in trauma can explain the increased rate of this process. The oxidation of glucose and pyruvate were lower in the diaphragms of traumatized than of normal rats incubated with leucine, while glycolysis and uptake of 2-deoxyglucose did not differ. The degradation of leucine and isoleucine was greater in the diaphragms of traumatized animals and was associated with a faster de novo synthesis of alanine. For the uninjured soleus muscles of the traumatized rats, the slower rates of oxidation of glucose, glycolysis, and uptake of 2-deoxyglucose in the presence of insulin showed an insensitivity of glucose metabolism to this hormone. In contrast, no differences were seen in these various metabolic processes between the extensor digitorum longus muscles of traumatized and normal rats. These data suggest that the response of skeletal muscles to trauma may depend on their physiologic and biochemical characteristics.

Alterations in protein, carbohydrate, and fat metabolism in injured and septic patients

The physiological and biochemical responses of the body following major injury or severe infection are characterized by hypermetabolism, accelerated gluconeogenesis, and the mobilization of substrates from the carcass to be utilized by visceral organs. These responses in the febrile patient are supported by an elevated cardiac output which insures perfusion of vital organs and provides additional bloodflow to the area of inflammation and/or injury. Because of the accelerated substrate flux and increased catabolism, weight loss and negative nitrogen balance are profound. Cumulative losses rapidly approach near-lethal limits if adequate nutritional support is not instituted. Nutritional maintenance will support the febrile response, maintain body nitrogen and acute phase protein synthesis, and assure an ongoing energy supply. Reparative tissue appears to preferentially utilize substrate, but if malnutrition occurs, wound healing and tissue repair may be limited. The effects of nutritional support on immunological function in these critically ill patients are only now being dissected. Clearly, immunosuppression is related to the initial insult and abnormalities in host defense mechanisms occur almost immediately in patients well nourished before illness. In addition, nutrient depletion and erosion of body mass are also associated with immunological dysfunction so that these two factors combine to impair the patient's resistance to subsequent infection. Present therapy should maintain balance of essential nutrients while complications associated with specialized techniques of nutrient support are minimized. Control of the patient's environment, minimizing pain and discomfort, preventing "bad rest" effect through exercise are all techniques of supportive care, but responses are ablated with resolution of the infection or wound closure, and every effort should be directed toward this goal.

Effect of exercise on rates of oxidation, turnover, and plasma clearance of leucine in human subjects

We have previously hypothesized that increased muscle oxidation of leucine in starvation is an adaptive response to fuel deficiency in this tissue. To investigate this hypothesis further, we have measured the rates of oxidation, turnover, and plasma clearance of [1-14C]leucine in six obese subjects at rest and during 2 h of mild leg exercise. This experimental design was based on the fact that exercise has the greatest impact on energy expenditure in muscle, the principal site for leucine oxidation. Exercise produced a fourfold increase in oxygen consumption. The rate of alpha-decarboxylation of leucine was increased twofold by leg exercise, whereas there were modest decreases (13%) in the rates of turnover and plasma clearance of this amino acid. The plasma concentrations of lactate and alanine increased twofold during exercise, but plasma concentrations of leucine and other amino acids, glucose, beta-hydroxybutyrate, and insulin remained unaltered. Our data suggest that during exercise oxidation of leucine as an energy source increases, whereas the utilization of this amino acid as a substrate for protein synthesis decreases.

A role for leucine in regulation of protein turnover in working rat hearts

Effect of leucine on protein turnover was examined in perfused hearts provided with 1 (0.2 mM) or 5 times (1 mM) plasma levels of leucine and normal plasma levels of other amino acids. When hearts were perfused as Langendorff or working preparations with buffer that contained 15 mM glucose, protein degradation was 2-3 times faster than protein synthesis. As a result, the heart was in marked negative nitrogen balance. Addition of 1 mM leucine significantly improved the nitrogen balance (24-33%) by stimulating protein synthesis in Langendorff preparations (25%) and inhibiting protein degradation in both preparations (14-29%). The stimulatory effect of leucine on protein synthesis was associated with a reduction in levels of ribosomal subunits. In hearts supplied physiological levels of glucose, lactate, beta-hydroxybutyrate, insulin, and glucagon, protein synthesis was more nearly equal to protein degradation. Provision of 1 mM leucine stimulated protein synthesis only in Langendorff preparations (32%) but did not have a significant effect on protein degradation in either preparation. Although leucine did not have a significant effect on either protein synthesis or degradation in working hearts, nitrogen balance became positive with addition of 1 mM leucine. These results suggest that leucine may exert an effect on myocardial nitrogen balance in vivo under conditions that elevate plasma leucine concentrations.

Effects of diabetes on oxidative decarboxylation of branched-chain keto acids

Oxidative decarboxylation is the first irreversible step in the degradation of leucine. The effect of streptozotocin diabetes on this reaction was studied in cell-free rat liver preparations, using [1-14C]alpha-ketoisocaproate as substrate. Diabetes increased the branched-chain ketoacid dehydrogenase (BCKD) activity (per g liver or per mg protein) of homogenates, but the ratios of homogenate BCKD activity to other mitochondrial markers remained unchanged. A cytosolic branched-chain ketoacid decarboxylase activity (15-22% of homogenate activity), which did not require NAD, CoA, or NADP, was also increased in diabetics. Insulin treatment of diabetics normalized enzyme activity in all fractions. The apparent Km of BCKD in homogenates was 43-45 microM; diabetes increased the apparent Vmax from 165 nmol x min-1 x g tissue-1 to 260 nmol x min-1 x g-1. In contrast, the Km for cytosolic alpha-ketoisocaproate decarboxylation was 270 microM in controls, and diabetes resulted in both a lower Km (210 microM) and a higher Vmax. Adrenalectomy did not affect activity in homogenates from controls, but partially reversed the diabetes-associated increase. Glucagon pretreatment of controls did not affect activity. In summary, distinct mitochondrial and cytosolic enzymes decarboxylate alpha-ketoisocaproate in liver. The increased hepatic capacity of diabetic rats to degrade the carbon skeleton of leucine is attributed mainly to a relative increase in mitochondrial mass.

Rhythmic variations of valine and leucine decarboxylation in rat diaphragm

Daily rhythmic variations of valine and leucine decarboxylation in the rat diaphragm were measured. Both valine and leucine decarboxylation increased during the hours of darkness and decreased during hours of light. Hypophysectomy eliminated the daily variation of decarboxylation. When food is available ad lib. to normal rats, the time of most active feeding coincides with the hours of darkness. Therefore, the period of darkness and maximum feeding coincides with the maximum oxidation of these two essential amino acids, valine and leucine, by diaphragm, and an active pituitary appears to be necessary to maintain this relationship. This model can be used to study interrelationships to behavioral, neurohumoral, and metabolic rhythms.

Amino acid degradation and effect of leucine on pyruvate oxidation in rat atrial muscle

Both left and right atria from fasted rats produced significant amounts of 14CO2 during incubation with U-14C-labeled leucine, isoleucine, valine, alanine, glutamate, glutamine, aspartate, asparagine, proline, threonine, or lysine. This pattern of amino acid metabolism resembles that of skeletal muscle. Production of 14CO2 from [1-14C]leucine was 2.5-fold greater in atria from fasted than from fed rats and was due to greater alpha-ketoisocaproic dehydrogenase activity in the tissue from fasted animals. At normal plasma concentrations, leucine reduced the oxidation of glucose and lactate in atria from fasted but not from fed rats by inhibiting pyruvate oxidation and without altering the rate of glycolysis. Leucine also reduced glucose oxidation when added in the presence of ketone bodies or other amino acids and stimulated the release of lactate into the medium. Although the leucine skeleton can be completely oxidized to CO2 and thus can serve as an alternative fuel in fasting in place of glucose, oxidation of leucine (like glucose or lactate oxidation) accounts only for a very small fraction of the total oxygen consumption of the resting atria.

Plasma branched-chain amino acids in cold- and heat-acclimatised rats

The concentrations of plasma branched-chain amino acids, valine, isoleucine and leucine, were significantly elevated in cold-acclimatised rats, while these values were significantly reduced in heat-acclimatised rats, in both 2-week and 4-week temperature acclimatisation.

Biochemical changes in a 100 km run: free amino acids, urea, and creatinine

Free amino acids, urea, and creatinine were analyzed in venous blood and urine of 11 trained (28--81 years old) male subjects before, immediately after, and 1 day after a 100 km running competition. The urinary excretion per minute of all amino acids was lowered after the contest. The renal clearance of creatinine was reduced from 116 to 60 ml/min and the clearance of most amino acids was reduced to a similar extent. However, for the amino acids with a resting clearance under 1 ml/min (x), a high relative clearance ratio (y in % of x) was seen post-exercise: y = -92.3 (log10 x) +23.1, r = -0.83, showing that their high reabsorption capacity had been impaired. Serum concentrations of most free amino acids, including the branched-chain amino acids and alanine, were reduced to 35--85% of the pre-race values. The sulfur amino acids were elevated either at the end of (cystine, to 180%) or 24 h after (methionine, to 155%) the race. Urea production increased by 44% while creatinine production tended to decrease. The production of 3-methylhistidine remained unchanged. These findings are compatible with a stimulation of gluconegenesis at the expense of the amino acid pool without induction of muscle protein catabolism.

Leucine as an in vitro precursor to lipids in rat sciatic nerve

The in vitro incorporation of lucine, isoleucine and pyruvate into lipids was compared and the possibility that leucine might serve as an in situ precursor to the corresponding iso fatty acids in the rat sciatic nerve was studied. The relative incorporation of 14C from leucine into lipids vs. nonlipids was 20%, and the incorporation of label into total lipids from leucine was one-halp that from pyruvate. The incorporation of label from leucine and pyruvate into sterols was nearly equivalent, but the incorporation of label into all other lipid classes from leucine was less than that from pyruvate, and the incorporation of label from isoleucine into lipids was much less in all cases. No detectable label from leucine was incorporated into brached chain fatty acids. It is concluded that leucine may be a substantial in vitro precursor to all major lipids in peripheral nerve, espeically sterols. The possibility and significance of a leucine catabolic pathway in the cytosol in relation to availability of 3-hydroxy-3-methylglutaryl CoA for sterol biosynthesis is discussed.