Branched-chain Amino Acids In Muscle Research Articles

Branched chain amino acids activate messenger ribonucleic acid translation regulatory proteins in human skeletal muscle, and glucocorticoids blunt this action.

Branched chain amino acids (BCAA) are particularly effective anabolic agents. Recent in vitro studies suggest that amino acids, particularly leucine, activate a signaling pathway that enhances messenger ribonucleic acid translation and protein synthesis. The physiological relevance of these findings to normal human physiology is uncertain. We examined the effects of BCAA on the phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (eIF4E-BP1) and ribosomal protein S6 kinase (p70(S6K)) in skeletal muscle of seven healthy volunteers. We simultaneously examined whether BCAA affect urinary nitrogen excretion and forearm skeletal muscle protein turnover and whether the catabolic action of glucocorticoids could be mediated in part by inhibition of the action of BCAA on the protein synthetic apparatus. BCAA infusion decreased urinary nitrogen excretion (P < 0.02), whole body phenylalanine flux (P < 0.02), plasma phenylalanine concentration (P < 0.001), and improved forearm phenylalanine balance (P = 0.03). BCAA also increased the phosphorylation of both eIF4E-BP1 (P < 0.02) and p70(S6K) (P < 0.03), consistent with an action to activate the protein synthetic apparatus. Dexamethasone increased plasma phenylalanine concentration (P < 0.001), prevented the BCAA-induced anabolic shift in forearm protein balance, and inhibited their action on the phosphorylation of p70(S6K). We conclude that in human skeletal muscle BCAA act directly as nutrient signals to activate messenger ribonucleic acid translation and potentiate protein synthesis. Glucocorticoids interfere with this action, and that may be part of the mechanism by which they promote net protein catabolism in muscle.

Branched chain amino acid in adenine-induced uremic rats treated with rHuEPO

Background. It is well known that chronic renal failure shows an abnormal amino acid profile characterized by a decrease in branched chain amino acid (BCAA). Methods. To investigate the change in BCAA with uremia and the effect of recombinant human erythropoietin (rHuEPO) on BCAA level, we measured the plasma and muscle levels of BCAA in adenine-induced renal anemia in Wistar rats. Results. A significant correlation was found between plasma and muscle tissue levels of BCAA. Further, not only the plasma level but the muscle level of each BCAA showed an inverse correlation with the serum creatinine value. Both plasma and muscle BCAA levels showed, in addition, significant recovery after rHuEPO treatment. Conclusions. Taking these results into account, we conclude that the decrease in BCAA is a clinicopathological feature essentially associated with uremia (i.e., it is not an epiphenomenon), and that BCAA levels could be recovered by rHuEPO treatment. However, the pathological mechanism by which BCAA level is decreased in the uremic state and is recovered after rHuEPO treatment remains to be elucidated.

Plasma lactate, GH and GH-binding protein levels in exercise following BCAA supplementation in athletes.

Branched chain amino acids (BCAA) stimulate protein synthesis, and growth hormone (GH) is a mediator in this process. A pre-exercise BCAA ingestion increases muscle BCAA uptake and use. Therefore after one month of chronic BCAA treatment (0.2 gkg(-1) of body weight), the effects of a pre-exercise oral supplementation of BCAA (9.64 g) on the plasma lactate (La) were examined in triathletes, before and after 60 min of physical exercise (75% of VO2 max). The plasma levels of GH (pGH) and of growth hormone binding protein (pGHBP) were also studied. The end-exercise La of each athlete was higher than basal. Furthermore, after the chronic BCAA treatment, these end-exercise levels were lower than before this treatment (8.6+/-0.8 mmol L(-1) after vs 12.8+/-1.0 mmol L(-1) before treatment; p < 0.05 [mean +/- std. err.]). The end-exercise pGH of each athlete was higher than basal (p < 0.05). Furthermore, after the chronic treatment, this end-exercise pGH was higher (but not significantly, p = 0.08) than before this treatment (12.2+/-2.0 ng mL(-1) before vs 33.8+/-13.6 ngmL(-1) after treatment). The end-exercise pGHBP was higher than basal (p < 0.05); and after the BCAA chronic treatment, this end-exercise pGHBP was 738+/-85 pmol L(-1) before vs 1691+/-555 pmol L(-1) after. pGH/pGHBP ratio was unchanged in each athlete and between the groups, but a tendency to increase was observed at end-exercise. The lower La at the end of an intense muscular exercise may reflect an improvement of BCAA use, due to the BCAA chronic treatment. The chronic BCAA effects on pGH and pGHBP might suggest an improvement of muscle activity through protein synthesis.

Feeding and arginine deficient diets differentially alter free amino acid concentrations of hindlimb muscle in young rats

The objective of these experiments was to examine short- and long-term (7 d) effects of arginine-deficient diets on free amino acid concentrations in hindlimb muscle of rats. In rats fed the control diet containing arginine (+Arg), muscle alanine and methionine concentrations were higher 1 and 2h after feeding compared to food-deprived rats, whereas branched-chain amino acids, arginine and asparagine concentrations were lower postprandially. In Experiment 1, rats were fed an arginine-deficient (−Arg) diet with glutamate (+Glu) substituted for arginine; alanine (+Ala), ornithine (+Orn) or citrulline (+Cit) were substituted for arginine in Experiment 2. In Experiment 1, arginine concentrations decreased in blood but not in muscle. This contrasts with rats fed −Arg/+Ala or −Arg/+Orn diets which had muscle arginine concentrations less than half the concentrations in controls or in rats fed the −Arg/+Cit diet. Muscle essential amino acids in Experiment 2 did not differ by diet, but muscle branched-chain amino acids were elevated relative to controls in the rats fed −Arg/+Ala or −Arg/+Orn diets; however, rats fed the −Arg/+Cit diet had levels similar to the controls. Also, muscle branched-chain amino acids were correlated with glutamine concentrations in both blood and muscle. The measurements in the post-meal period suggest that muscle amino acid concentrations may more closely reflect dietary amino acid patterns than do blood amino concentrations.

Hyperammonemia-induced depletion of glutamate and branched-chain amino acids in muscle and plasma

Background/Aims: Exogenous hyperammonemia is known to decrease the plasma levels of branched-chain amino acids (BCAA). To investigate whether changes in intracellular amino acid concentrations of muscle are associated with and may, at least in part, mediate this effect, experiments were carried out on a total of 60 male Wistar rats. Methods: Five groups were formed in a randomized manner. Group A: no treatment; groups B1 and B2: 2-hour and 6-hour continuous central-venous infusions, respectively, of sodium salts; groups C1 and C2: 2-hour and 6-hour infusions of ammonium salts. We obtained venous blood samples and muscle biopsies. Plasma ammonia, whole blood glucose, serum insulin, blood pH, and amino acids in plasma as well as in the intracellular water of muscle were measured. Results: As compared with control group A, groups C1 and C2 displayed a 3.3- and a 4-fold increase, respectively, in the plasma ammonium concentration. Regarding insulin, the ammonium-infused rats were similar to group A but not to the sodium-infused B groups, which had significantly lower insulin concentrations. Administering ammonium brought about a decline in BCAA concentrations in plasma after 2 hours and in muscle after 6 hours. The ammonium-induced fall in intracellular BCAA values was preceded by an increase of glutamine as well as by a decrease of glutamate and alanine in both plasma and muscle. Conclusions: It is pointed out that the inter-group differences in serum insulin, although possibly accounting for some of the findings, can by no means explain the entire pattern of amino acid concentrations seen after the ammonium infusions. Instead, our results agree with the hypothesis that hyperammonemia indirectly lowers the plasma levels of BCAA by stimulating glutamine synthesis, thus reducing the intracellular glutamate pool, which is likely to be restored, at least in part, by an intensified BCAA transamination. Clarification is needed as to whether carbon skeletons derived from valine and isoleucine additionally contribute to replenishing the glutamate pool.

Mechanisms of activation of muscle branched-chain alpha-keto acid dehydrogenase during exercise in man.

1. Exercise leads to activation (dephosphorylation) of the branched-chain alpha-keto acid dehydrogenase (BCKADH). Here we investigate the effect of low pre-exercise muscle glycogen content and of branched-chain amino acid (BCAA) ingestion on the activity of BCKADH at rest and after 90 min of one-leg knee-extensor exercise at 65% maximal one-leg power output in five subjects. 2. Pre-exercise BCAA ingestion (308 mg BCAAs (kg body wt)-1) caused an increased muscle BCAA uptake, a higher intramuscular BCAA concentration and activation of BCKADH both at rest (9 +/- 1 versus 25 +/- 5% for the control and BCAA test, respectively) and after exercise (27 +/- 4 versus 54 +/- 7%). 3. At rest the percentage active BCKADH was not different, 6 +/- 2% versus 5 +/- 1%, in the normal and low glycogen content leg (392 +/- 21 and 147 +/- 34 mumol glycosyl units (g dry muscle)-1, respectively). The post-exercise BCKADH activity was higher in the low (46 +/- 2%) than in the normal glycogen content leg (26 +/- 2%). 4. It is concluded that: (1) the mechanism of activation by BCAA ingestion probably involves an increase of the muscle BCAA concentration; (2) BCKADH activation caused by exercise and BCAA ingestion are additive; (3) low pre-exercise muscle glycogen content augments the exercise-induced BCKADH activation without an increase in muscle BCAA concentration; and (4) the mechanism of BCKADH activation via BCAA ingestion and low muscle glycogen content are different.

Prolonged activation of the branched‐chain α‐keto acid dehydrogenase complex in muscle of zymosan treated rats

Whole-body oxidation rates of branched chain amino acids (BCAA) are increased during catabolic diseases. A significant role for muscle in this feature has been suggested and, therefore, activities of the rate limiting enzyme in the degradative pathway of the BCAA in muscle were investigated in a catabolic rat model (intraperitoneal zymosan injection). Both actual and total activities of the branched chain alpha-keto acid dehydrogenase complex zymosan treated rats and compared with values measured in pair fed and ad libitum fed controls. The actual activity and the percentage of the enzyme in the active form were increased 2 and 6 days after the zymosan challenge. Total activity of the BC-complex and the activities of mitochondrial marker enzymes were reduced 2 days after zymosan treatment. We conclude that zymosan treatment leads to (1) a reduction of the mitochondrial content in skeletal muscle and (2) a prolonged activation of the BC-complex in muscle which may explain enhanced oxidation of BCAA during catabolic diseases.

Leucine metabolism during chronic ethanol consumption

Chronic ethanol consumption is known to increase plasma concentrations of branched-chain amino acids (BCAA) in rats and man, but the mechanisms of this effect are not known. Chronic ethanol consumption may increase levels of BCAA by altering protein turnover and/or by affecting the oxidation of BCAA. These possibilities were investigated in rats pair-fed liquid diets containing either 0% or 36% of total calories as ethanol for 21 days. In the fed state, ethanol-treated rats had a plasma ethanol level of 20 ± 5 mmol/L and twofold increases in BCAA concentrations in plasma. There were also significant increases (37% to 63%) in muscle, liver, and jejunal mucosa BCAA concentrations. Chronic ethanol consumption significantly increased whole-body rates ( μ mol 100 g h ) of leucine turnover (73.8 ± 7.5 v 104 ± 5.6, P < .01) and oxidation (12.0 ± 1.7 v 17.7 ± 1.1, P < .05). In contrast, it significantly decreased leucine incorporation (nmol/mg protein/240 min) into both muscle (0.61 ± 0.07 v 0.35 ± 0.05, P < .01) and liver (13.25 ± 1.40 v 6.78 ± 0.98, P < .01) proteins. Incorporation of leucine into the mucosal proteins of jejunum (17.42 ± 1.42 v 15.85 ± 1.90, P = NS) was not significantly altered by ethanol. These results suggest that reduced protein synthesis and/or increased protein breakdown may account for the elevated tissue BCAA concentrations in chronic ethanol consumption. The consequences of these increased tissue concentrations are increases in tissue oxidation and plasma concentrations of BCAA.

Effect of Infused Branched-Chain Amino Acids on Muscle and Whole-Body Amino Acid Metabolism in Man

1. Using the forearm balance method, together with systemic infusions of L-[ring-2,6-3H]phenylalanine and L-[1-14C]leucine, we examined the effects of infused branched-chain amino acids on whole-body and skeletal muscle amino acid kinetics in 10 postabsorptive normal subjects; 10 control subjects received only saline. 2. Infusion of branched-chain amino acids caused a four-fold rise in arterial branched-chain amino acid levels and a two-fold rise in branched-chain keto acids; significant declines were observed in circulating levels of most other amino acids, including phenylalanine, which fell by 34%. Plasma insulin levels were unchanged from basal levels (8 +/- 1 mu-units/ml). 3. Whole-body phenylalanine flux, an index of proteolysis, was significantly suppressed by branched-chain amino acid infusion (P less than 0.002), and forearm phenylalanine production was also inhibited (P less than 0.03). With branched-chain amino acid infusion total leucine flux rose, with marked increments in both oxidative and non-oxidative leucine disposal (P less than 0.001). Proteolysis, as measured by endogenous leucine production, showed a modest 12% decrease, although this was not significant when compared with saline controls. The net forearm balance of leucine and other branched-chain amino acids changed from a basal net output to a marked net uptake (P less than 0.001) during branched-chain amino acid infusion, with significant stimulation of local leucine disposal. Despite the rise in whole-body non-oxidative leucine disposal, and in forearm leucine uptake and disposal, forearm phenylalanine disposal, an index of muscle protein synthesis, was not stimulated by infusion of branched-chain amino acids. 4. The results suggest that in normal man branched-chain amino acid infusion suppresses skeletal muscle proteolysis independently of any rise of plasma insulin. Muscle branched-chain amino acid uptake rose dramatically in the absence of any apparent increase in muscle protein synthesis, as measured by phenylalanine disposal, or in branched-chain keto acid release. Thus, an increase in muscle branched-chain amino acid concentrations and/or local branched-chain amino acid oxidation must account for the increased disposal of branched-chain amino acids.

Glucocorticoid regulation of muscle branched-chain amino acid metabolism

Branched-chain alpha-keto acid dehydrogenase (BCKAD) is a multisubunit complex regulated by phosphorylation and is considered to be rate-limiting for branched-chain amino acid (BCAA) metabolism in skeletal muscle. Glucocorticoids increase net protein degradation in muscle; associated with this increased breakdown of muscle protein is an elevated rate of BCAA oxidation. The effects of glucocorticoids on skeletal muscle BCKAD were investigated in different rat models. BCKAD was activated after glucocorticoid treatment (both acutely, within 2 h, and chronically). The amount of enzyme per muscle cell increased after 5 d of cortisone acetate treatment. Insulin administration partially blocked the acute effects of glucocorticoids on muscle BCKAD. Activation was also observed during metabolic acidosis, insulinopenic diabetes mellitus, and endotoxic shock, three conditions characterized by elevated circulating glucocorticoids, increased BCAA oxidation, and increased net protein breakdown. Activation of BCKAD may account for the increased oxidation of BCAA observed during hypercortisolemia. The sequelae of this accelerated catabolism may include increased glutamine and alanine production for gluconeogenesis and provision of ATP for muscle work.

Activities of Branched-Chain Amino Acid-Degrading Enzymes in Liver from Rats Fed Different Dietary Levels of Protein

The relationships among dietary protein intake, plasma branched-chain amino acid (BCAA) and keto acid (BCKA) concentrations, and liver BCAA-degrading enzyme activities were investigated in rats fed, for 5 h/d for 2, 6 or 9 d, diets containing from 0 to 50% casein. Plasma, liver and muscle BCAA concentrations were proportional to protein intake over the entire range tested; plasma BCKA concentration, however, was proportional only in the range from 0 to 20% casein, after which a plateau was reached. By d 2, liver cytosolic BCAA aminotransferase activity had increased in rats fed 50% casein; by d 9, activity had increased in rats fed 0 or 5% casein as well. Liver mitochondrial BCAA aminotransferase activity was unresponsive to dietary treatment. Basal liver BCKA dehydrogenase activity and the percent active complex were proportional to protein intake on d 2 and 6. On d 2, total BCKA dehydrogenase activity was the same in all groups; by d 6, total activity had increased in rats fed 30 or 50% casein. We conclude that although the adaptive changes in BCAA-degrading enzyme activities are small, they are sufficient to compensate for excessively high or low protein intakes, so that tolerable concentrations of BCAA and BCKA are maintained.

Intra- and extracellular amino acid concentrations in portacaval-shunted rabbits. Role of hyperammonemia and effects of branched-chain amino acid-enriched parenteral nutrition

Intra- and extracellular amino acid concentrations were measured in rabbits in order to elucidate the possible role of hyperammonemia in lowering the postabsorptive plasma levels of branched-chain amino acids (BCAA) and to assess the effects of BCAA-enriched total parenteral nutrition (TPN) on the amino acid pattern of muscle. The pathophysiological part of this paper deals with portacaval anastomosis (PCA) and is aimed at substantiating or rejecting our hypothesis that excessive ammonia-by stimulating glutamine synthesis-reduces the intracellular glutamate pool which is then restored, at least in part, by an intensified BCAA degradation. Regarding infusion therapy, we were mainly interested in whether an amino acid solution adapted to the metabolism in liver cirrhosis causes an accumulation of BCAA in muscle or modifies the intracellular content of glutamate and glutamine. Eighteen rabbits did not undergo surgery and served as controls (group A), while 30 were given a portacaval end-to-side anastomosis (group B). Two weeks after creating the PCA, venous blood samples were taken and muscle biopsies (Bergström's technique) were performed postabsorptively. An 18-h TPN was then started, the regimen administered included dextrose, fat and, in addition, either a conventional (group B1, n = 15) or an adapted amino acid solution (group B2, n = 15). We obtained second blood specimens and muscle biopsies at the end of the infusion period. With the control animals, the same time schedule for blood sampling and muscle biopsies was followed. Fourteen days after the operation, the PCA rabbits displayed a mean plasma ammonia level 5.1 times higher than that measured in the controls (p less than or equal to 0.001). Conventional blood chemistry did not reveal any impairment of liver cell integrity or over-all hepatic function, whereas the nutritional state of the shunted animals worsened, as indicated by body weight and biochemical variables. Since in the PCA rabbits, the total amino acid pools of muscle and plasma were seen to be increased and decreased, respectively, the results concerning the individual amino acids are given in terms of both the absolute and percentage values, the latter more often revealing high levels of statistical significance. PCA induced a marked rise in the intra- and extracellular concentrations of glutamine, while the values of glutamate and alanine showed a decline in muscle and plasma. The extracellular levels of methionine, phenylalanine, and tyrosine were raised, while those of the BCAA were diminished.(ABSTRACT TRUNCATED AT 400 WORDS)

PHARMACOLOGIC REVERSAL Of ABNORMAL GLUCOSE REGULATION, BCAA UTILIZATION AND MUSCLE CATAROLISM IN SEPSIS BY DICHLOROACETATE

Sepsis has been shown to decrease skeletal muscle glucose oxidation by inhibiting the pyruvate dehydrogenase activity (PDHa) and to increase proteolysis and use of branched-chain amino acids (BCAA). The effects of dichloroacetate (DCA), which reverses PDHa inhibition, were studied in skeletal muscle from a septic (S) rat model of intra-abdominal abscess (E. coli + B. fragilis) and compared to control (C) and sterile inflammatory abscess (I) animals. In one set of S, I, and C animals, DCA (1 mmol/kg) was injected intraperitoneally at 0, 30, and 60 min. Septic, but not I, rats had a twofold increase in skeletal muscle lactate concentrations over C, but no changes in pyruvate. After DCA, both lactate and pyruvate were reduced (p less than 0.001) to same level in S, I, and C. Skeletal muscle alanine was increased in S compared to I or C, but after DCA was reduced threefold in C, S, and I (p less than 0.001) suggesting that alanine synthesis may be impaired due to decreased pyruvate availability. Like alanine, skeletal muscle BCAA were increased in S compared to C, but not altered in I. Following DCA, BCAA levels in muscle from S were reduced (p less than 0.001) to values seen in C or I. Muscle phenylalanine content was significantly elevated in S (p less than 0.05) compared to C or I, but was reduced (p less than 0.05) after DCA in S but not in C or I. Decreased muscle phenylalanine associated with lowered BCAA suggests DCA may decrease septic muscle protein catabolism and/or enhance protein synthesis. Coupled with an increased PDHa and reduced lactate levels, this suggests that DCA may reverse the excess muscle catabolism and BCAA dependence of sepsis by increasing glucose and lactate oxidation and may be a useful therapeutic modality.

Branched-chain amino acid metabolism in rat muscle: abnormal regulation in acidosis.

Branched-chain amino acid (BCAA) metabolism is frequently abnormal in pathological conditions accompanied by chronic metabolic acidosis. To study how metabolic acidosis affects BCAA metabolism in muscle, rats were gavage fed a 14% protein diet with or without 4 mmol NH4Cl X 100 g body wt-1 X day-1. Epitrochlearis muscles were incubated with L-[1-14C]-valine and L-[1-14C]leucine, and rates of decarboxylation, net transamination, and incorporation into muscle protein were measured. Plasma and muscle BCAA levels were lower (P less than 0.05) in acidotic rats. Rates of valine and leucine decarboxylation and net transamination were higher (P less than 0.05) in muscles from acidotic rats; these differences were associated with a 79% increase in the total activity of branched-chain alpha-keto acid dehydrogenase and a 146% increase in the activated form of the enzyme. We conclude that acidosis affects the regulation of BCAA metabolism by enhancing flux through the transaminase and by directly stimulating oxidative catabolism through activation of branched-chain alpha-keto acid dehydrogenase.

Branched-chain amino acid metabolism and alanine formation in rat muscles in vitro. Mitochondrial-cytosolic interrelationships

Muscle branched-chain amino acid metabolism is coupled to alanine formation via branched-chain amino acid aminotransferase and alanine aminotransferase, but the subcellular distributions of these and other associated enzymes are uncertain. Recovery of branched-chain aminotransferase in the cytosol fraction after differential centrifugation was shown to be accompanied by leakage of mitochondrial-matrix marker enzymes. By using a differential fractional extraction procedure, most of the branched-chain aminotransferase activity in rat muscle was located in the mitochondrial compartment, whereas alanine aminotransferase was predominantly in the cytosolic compartment. Phosphoenolpyruvate carboxykinase, like aspartate aminotransferase, was approximately equally distributed between these subcellular compartments. This arrangement necessitates a transfer of branched-chain amino nitrogen and carbon from the mitochondria to the cytosol for alanine synthesis de novo to occur. In incubations of hemidiaphragms from 48 h-starved rats with 3mM-valine or 3mM-glutamate, the stimulation of alanine release was inhibited by 69% by 1 mM-aminomethoxybut-3-enoate, a selective inhibitor of aspartate aminotransferase. Leucine-stimulated alanine release was unaffected. These data implicate aspartate aminotransferase in the transfer of amino acid carbon and nitrogen from the mitochondria to the cytosol, and suggest that oxaloacetate, via phosphoenolpyruvate carboxykinase, can serve as an intermediate on the route of pyruvate formation for muscle alanine synthesis.

The effect of clofibric acid on muscle branched-chain amino acid metabolism and alanine release in vitro

The effect of clofibric acid on muscle branched-chain amino acid metabolism and alanine release <i>in vitro</i>

Ammonia uptake by skeletal muscle in the hyperammonaemic rat.

A two-stage surgical occlusion of the portal vein was employed to produce hyperammonaemia in the rat. The procedure resulted in a significant rise of arterial blood ammonia level from 70 . 5 +/- 6 . 5 mumol/l (mean +/- SEM, n = 10) to 214 . 0 +/- 37 . 7 mumol/l and in a rise of venous blood ammonia from 65 . 0 +/- 9 . 4 mumol/l to 122 . 2 +/- 7 . 4 mumol/l during the first day following the complete vein occlusion. A marked increase of the arteriovenous difference of ammonia concentration from virtually zero in sham-operated controls to 72 +/- 9 (n = 8) mumol/l in rats 1 day after the surgical manipulation suggested uptake of ammonia by skeletal muscle. Rat muscle glutamine synthetase activity increased from 0 . 46 +/- 0 . 06 u/mg (n = 7) in controls to 2 . 7 +/- 0 . 3 u/mg (n = 7) on the fourth day following portal vein ligation, and muscle branched chain amino acids aminotransferase increased from 0 . 2 +/- 0 . 05 u/mg in controls to 0 . 96 +/- 0 . 1 u/mg (n = 7) during the first day of ligation. Glutamine dehydrogenase and aspartate aminotransferase activities were not affected by the surgical procedure. These observations suggest that ammonia trapping in skeletal muscle is coupled to glutamine formation via amination of glutamic acid. This conclusion was further supported by the finding that ammonia uptake correlated (r = 0 . 92) with enhanced release of glutamine from muscle and that treatment with methionine sulfoximine, a potent inhibitor of glutamine synthetase, changed the arteriovenous difference of glutamine from -0 . 92 +/- 0 . 01 mmol/l in ligated animals (net release) to +0 . 12 +/- 0 . 01 mmol/l (net uptake) in ligated and inhibitor-treated animals. Similarly, the inhibitor also abolished the arterio-venous difference of ammonia. Thus, the animal model of hyperammonaemia and the muscle enzyme assays reveal that skeletal muscle is involved in the regulation of blood ammonia level by conversion of ammonia, via glutamic acid, to glutamine.

The Branched-Chain Amino Acid Antagonism in Chicks

The effects of dietary supplements of branched-chain amino acids on growth, food consumption and metabolism in chicks were investigated. When an adequate diet contained 1.20, 1.60, 2.25, 3.75, or 5.00% leucine, increasing leucine content caused reduced food consumption and weight gains, coupled with impaired efficiency of food utilization. When the diet deficient in branched-chain amino acids contained 0.98, 1.46, 2.25, 3.75, or 5.00% leucine, increasing leucine resulted in increased food consumption and reduced efficiency of food utilization when levels of leucine up to 3.75% were fed. Excess leucine depressed plasma concentrations of isoleucine and valine. Excesses of isoleucine or valine caused smaller depressions of concentrations of the other two branched-chain amino acids. All these effects were seen during the first 8 days of experiment, after which they diminished or disappeared. Muscle branched-chain amino acid aminotransferase (BCAT) (L-leucine:2-oxoglutarate aminotransferase, EC 2.6.1.6) activity was increased in chicks fed excess leucine but not in those fed excess isoleucine or valine. Hepatic alpha-ketoisocaproic dehydrogenase (KADH) (2-oxoisocaproate:lipoate oxidoreductase, EC 1.2.4.3) activity and muscle polyribosomal aggregation were unaffected by diet. When chicks were fed diets containing either 0.98 or 2.25% leucine, production of 14CO2 from [1-14C]isoleucine and [1-14C]valine was increased in chicks fed the higher level of leucine. The increase was small in both cases, representing approximately 2% of consumed isoleucine and valine. Increased production of 14CO2 was observed within 12 hours of feeding excess leucine; however, BCAT increased only after 2 to 4 days. No differences were seen in excreted 14C or in the relative distribution of 14C along the small intestine. We conclude that the chick is able to adapt in part to excesses of dietary leucine and that the branched-chain amino acid antagonism may involve increased catabolism of the limiting branched-chain amino acids.