Neutral amino acid transport into rat skeletal muscle: competition, adaptive regulation, and effects of insulin

Portal glucose delivery stimulates muscle but not liver protein metabolism

Portal vein glucose delivery (the portal glucose signal) stimulates glucose uptake and glycogen storage by the liver, whereas portal amino acid (AA) delivery (the portal AA signal) induces an increase in protein synthesis by the liver. During a meal, both signals coexist and may interact. In this study, we compared the protein synthesis rates in the liver and muscle in response to portal or peripheral glucose infusion during intraportal infusion of a complete AA mixture. Dogs were surgically prepared with hepatic sampling catheters and flow probes. After a 42-h fast, they underwent a 3-h hyperinsulinemic (4× basal) hyperglucagonemic (3× basal) hyperglycemic (≈160 mg/dl) hyperaminoacidemic (hepatic load 1.5× basal; delivered intraportally) clamp (postprandial conditions). Glucose was infused either via a peripheral (PeG; n = 7) or the portal vein (PoG; n = 8). Protein synthesis was assessed with a primed, continuous [(14)C]leucine infusion. Net hepatic glucose uptake was stimulated by portal glucose infusion (+1 mg·kg(-1)·min(-1), P < 0.05) as expected, but hepatic fractional AA extraction and hepatic protein synthesis did not differ between groups. There was a lower arterial AA concentration in the PoG group (-19%, P < 0.05) and a significant stimulation (+30%) of muscle protein synthesis associated with increased expression of LAT1 and ASCT2 AA transporters and p70S6 phosphorylation. Concomitant portal glucose and AA delivery enhances skeletal muscle protein synthesis compared with peripheral glucose and portal AA delivery. These data suggest that enteral nutrition support may have an advantage over parenteral nutrition in stimulating muscle protein synthesis.

Amino acid availability affects amino acid flux and protein metabolism in the porcine mammary gland

A kinetic model was used to examine transmembrane flux kinetics of lysine, methionine and valine across the porcine mammary gland (MG) under dietary amino acid (AA) limiting, adequate and excess conditions. Lactating sows (3 per treatment) were offered three diets: lysine-deficient [LD, 4.9 lysine and 9.9 valine (g/kg diet)], adequate (Control, 9.7 and 10.2) and valine-excess (VE, 9.8 and 13.4). On d 18 of lactation, 2-(15)N-lysine, 5-methyl-(2)H(3)-methionine and 1-(13)C-valine were infused into a jugular vein for 20.5 h. Milk and arterial and mammary venous blood samples were collected at 2- and 1-h intervals, respectively. Compared with Control, milk yield and litter growth rate decreased (P < 0.05) in sows fed the LD diet. Model estimates of mammary protein synthesis (PS), breakdown (PB) and net PS decreased (P < 0.05) in sows fed the LD diet. Net uptake of lysine decreased (P < 0.05) in sows fed the LD diet as a result of decreases in inward and outward transport of lysine. Inward transport of methionine tended to be reduced (P < 0.10) in sows fed the LD diet, resulting in a decrease in net methionine uptake. In sows fed the VE diet, PB was reduced (P < 0.05) and PS unchanged compared with Control. Outward transport of valine and net lysine uptake were reduced (P < 0.05), but net valine uptake was unchanged in sows fed the VE diet compared with Control. In conclusion, the kinetic model provided estimates of PS that were similar to empirical measurements of milk protein output and mammary protein accretion. Transport of lysine and methionine by the porcine MG is closely linked to regulation of mammary PS. Lysine availability has little effect on the transmembrane flux of valine.

Reduction of large neutral amino acid levels in plasma and brain of hyperleucinemic rats

Neurological dysfunction is common in patients with maple syrup urine disease (MSUD). However, the mechanisms underlying the neuropathology of this disorder are poorly known. In the present study we investigated the effect of acute hyperleucinemia on plasma and brain concentrations of amino acids. Fifteen-day-old rats were injected subcutaneously with 6 micromol L-leucine per gram body weight. Controls received saline in the same volumes. The animals were sacrificed 30--120 min after injection, blood was collected and their brain rapidly removed and homogenized. The amino acid concentrations were determined by HPLC using orthophtaldialdehyde for derivatization and fluorescence for detection. The results showed significant reductions of the large neutral amino acids (LNAA) L-phenylalanine, L-tyrosine, L-isoleucine, L-valine and L-methionine, as well as L-alanine, L-serine and L-histidine in plasma and of L-phenylalanine, L-isoleucine, L-valine and L-methionine in brain, as compared to controls. In vitro experiments using brain slices to study the influence of leucine on amino acid transport and protein synthesis were also carried out. L-Leucine strongly inhibited [14C]-L-phenylalanine transport into brain, as well as the incorporation of the [14C]-amino acid mixture, [14C]-L-phenylalanine and [14C]-L-lysine into the brain proteins. Although additional studies are necessary to evaluate the importance of these effects for MSUD, considering previous findings of reduced levels of LNAA in plasma and CSF of MSUD patients during crises, it may be speculated that a decrease of essential amino acids in brain may lead to reduction of protein and neurotransmiter synthesis in this disorder.

Localization and expression of BCAT during pregnancy and lactation in the rat mammary gland

During lactation, branched-chain aminotransferase (BCAT) gene expression increases in the mammary gland. To determine the cell type and whether this induction is present only during lactation, female rats were randomly assigned to one of three experimental groups: pregnancy, lactation, or postweaning. Mammary gland BCAT activity during the first days of pregnancy was similar to that of virgin rats, increasing significantly from day 16 to the last day of pregnancy. Maximal BCAT activity occurred on day 12 of lactation. During postweaning, BCAT activity decreased rapidly to values close to those observed in virgin rats. Analyses by Western and Northern blot revealed that changes in enzyme activity were accompanied by parallel changes in the amount of enzyme and its mRNA. Immunohistochemical studies of the mammary gland showed a progressive increase in mitochondrial BCAT (mBCAT)-specific staining of the epithelial acinar cells during lactation, reaching high levels by day 12. Immunoreactivity decreased rapidly after weaning. There was a significant correlation between total BCAT activity and milk production. These results indicate that the pattern of mBCAT gene expression follows lactogenesis stages I and II and is restricted to the milk-producing epithelial acinar cells. Furthermore, BCAT activity is associated with milk production in the mammary gland during lactation.

Reduction of large neutral amino acid concentrations in plasma and CSF of patients with maple syrup urine disease during crises

Neurological dysfunction is common in patients with maple syrup urine disease (MSUD). However, the mechanisms underlying the neuropathology of this disorder are poorly understood. We determined the concentrations of all amino acids in plasma of patients with MSUD during crises (with severe CNS symptoms) and after recovery in the hope of detecting possible alterations of these levels during metabolic decompensation. Blood samples obtained from 11 children with MSUD aged 1 month to 7 years and from 10 age-matched controls (5 months to 6 years) with no evidence of metabolic disease were examined for their amino acid content by high-performance liquid chromatography. We observed that leucine, isoleucine and valine concentrations were respectively 30, 9 and 3 times higher than normal values, whereas the concentrations of the large neutral amino acids (LNAA) phenylalanine, tyrosine, tryptophan and methionine were significantly lower during metabolic decompensation as compared to the controls. In addition, concentrations of leucine, but not of valine or isoleucine, were inversely related to the LNAA concentrations in plasma. The concentrations of these amino acids in plasma returned to normal values when patients were clinically well. CSF amino acid concentrations also showed decreased amounts of LNAA and increased concentrations of branched-chain amino acids. It is possible that the decrease in plasma concentrations of LNAA may lead to a deficit of these essential amino acids in the brain as well as of their products such as proteins and neurotransmitters, a fact that might be related to the neurological dysfunction of MSUD.

Hepatic histidase gene expression responds to protein rehabilitation in undernourished growing rats

We studied the effect of nutritional rehabilitation with a 6, 18 or 50% casein diet in undernourished rats on histidase (Hal) expression. Undernutrition was induced by feeding rats a 0.5% casein diet for 5 wk. Over this period, growth, serum total proteins and insulin-like growth factor-I (IGF-I) levels were significantly lower than those of rats that freely consumed an 18% casein diet. During this period, undernutrition also significantly reduced Hal activity and Hal-mRNA concentration. Nutritional rehabilitation for 21 d with a 6% casein diet did not change any of these variables. Nutritional rehabilitation with an 18 or 50% casein diet for 1 d initiated the restoration of Hal activity and mRNA concentration. After 10 d of consuming 18 or 50% casein diets, Hal activity was 5- and 14-fold, and mRNA concentration was 8.5- and 23-fold higher, respectively, than in the protein-undernourished group (PU). During this period, body weight, total serum proteins and IGF-I levels were also significantly (P < 0.05) higher than those of the PU group. At the end of 21 d of rehabilitation with an 18 or 50% casein diet, Hal activity was 14- and 31-fold higher and Hal mRNA concentration was 10- and 24-fold higher, respectively, than in the PU group. In conclusion, our data showed that rehabilitation of undernourished rats with a 6% casein diet was not sufficient to re-establish growth indicators, Hal activity or gene expression, and that nutritional rehabilitation with an 18 or 50% casein diet effectively re-established body weight , biochemical variables and the capacity of histidase gene expression to eliminate the excess of protein.

Dietary protein level regulates expression of the mitochondrial branched-chain aminotransferase in rats

The first step in the degradation of branched-chain amino acids (BCAA) is transamination catalyzed by the branched-chain aminotransferase (BCAT), which is located in extrahepatic tissues. Studies of the effect of dietary protein on BCAT activity have given contradictory results. Therefore, we established the levels of BCAT activity and mitochondrial BCAT (BCATm) mRNA expression in different organs and tissues of rats. We then determined the effect of different levels of dietary protein in well-nourished rats, the effect of feeding a 0.5% casein diet for 5 wk (protein-malnourished rats) and nutritional rehabilitation of these rats with different levels of dietary protein on BCAT activity and BCATm mRNA expression in selected tissues. Finally, the response of tissue BCAT activity and BCATm mRNA levels in rats fed a 10% casein diet and injected with glucagon (4 d) or hydrocortisone (7 d) was determined. The highest concentration of BCATm mRNA was found in stomach, followed by kidney, heart, muscle, brain, skin and lung. Low levels were found in intestine, and no BCATm mRNA was detectable in liver. Although BCAT activity was significantly higher in muscle, kidney and brain from rats adapted to consume a 50% casein diet for 7 h/d for 10 d than in rats fed 6, 18 or 35% casein diets, only muscle had significantly higher levels of BCATm mRNA. In protein-malnourished rats, BCAT activity and BCATm mRNA expression in kidney, muscle and heart were not different from those of rats with free access to an 18% casein diet. Nutritional rehabilitation of the protein-malnourished rats with 50% casein for 21 d significantly increased the BCAT activity and BCATm mRNA expression in muscle. Neither hydrocortisone nor glucagon injection affected BCAT activity or BCATm mRNA concentrations in rat kidney, muscle or heart. We conclude that the nutritional regulation of BCATm is extrahepatic, tissue specific and may involve transcriptional and post-translational mechanisms.

Maintaining muscle protein anabolism after a metabolic stress: role of dextrose vs. amino acid availability

The effect of chronic hypocaloric parenteral infusions of amino acids (AA) vs. dextrose (D) on protein homeostasis after a generalized metabolic stress was examined. Multicatheterized mongrel dogs were metabolically challenged by a 4-day fast and then administered a 4-day intravenous infusion of saline (S, n = 8), D (n = 8), or isocaloric AA (n = 7). Although nitrogen balance (g.kg.1.day-1) was similarly negative with S (-0.37 +/- 0.05), D (-0.28 +/- 0.03), and AA (-0.37 +/- 0.04) during the fasting period, it was less negative (P < or = 0.05) with AA (-0.06 +/- 0.04) than with D (-0.20 +/- 0.03) or S (-0.23 +/- 0.04) during nutrient infusion. AA resulted in net hindlimb uptake and D in net hindlimb release of essential AA (570 +/- 261 vs. -248 +/- 59 nmol.kg-1.min-1). Whereas S and D infusions led to net hindlimb muscle protein loss (-37 +/- 24 and -89 +/- 33 micrograms.kg-1.min-1, respectively, P < or = 0.05 vs. AA), parenteral AA resulted in net deposition (169 +/- 62 micrograms.kg-1.min-1) as measured using L-[ring-2H5]phenylalanine. Thus hypocaloric parenteral D infusion after a metabolic stress does not favor nitrogen conservation, because net whole body nitrogen loss, skeletal muscle protein catabolism, and hindlimb AA release were not blunted compared with S infusion. Conversely, hypocaloric AA infusion preserves whole body and muscle protein stores.

Characterization of system L and system y+ amino acid transport activity in cultured vascular smooth muscle cells

The uptake of L-leucine and L-lysine into vascular smooth muscle cells cultured from the aortas of rats has been investigated. Both amino acids are taken up by saturable systems that are independent of the presence of a Na+ gradient and can be stimulated in trans by neutral bulky amino acids for leucine and cationic amino acids for lysine. Leucine uptake is inhibited competitively in cis by several neutral amino acids, whereas lysine uptake is inhibited strongly by other cationic amino acids but also significantly by neutral amino acids such as leucine. The leucine inhibition is noncompetitive. Cells preloaded with leucine and lysine could also export these amino acids and the rate of efflux was stimulated by the presence of appropriate amino acids in trans. These data are all consistent with leucine being transported largely if not entirely by System L and lysine by the System y+ transporter.

Competition for transport of amino acids into rat heart: effect of competitors on protein synthesis and degradation

Transport of the neutral amino acids, 2-(methylamino)isobutyrate (MeAIB) and Phe, was examined in isolated rat hearts perfused by the Langendorff method. Hearts were perfused by recirculating for various time periods buffer containing [14C]-MeAIB or [14C]-Phe plus desired additions. Uptake of MeAIB was linear for approximately 30 minutes; Phe uptake was linear for a maximum of 5 minutes, and reached a steady state after 15 minutes. Km and Vmax for MeAIB were 1.1 +/- 0.03 mmol/L and 37.7 +/- 0.4 pmol/microL intracellular fluid (ICF)/min; values for Phe were 1.8 +/- 0.02 mmol/L and 364 +/- 5 pmol/microL ICF/minute. Uptake of MeAIB (0.2 mmol/L) was reduced 95% in the presence of Ser (10 mmol/L), and less severely by large neutral amino acids ([LNAA], 10 mmol/L) such as Phe and Leu (by 46% and 54%, respectively). Uptake of Phe (0.2 mmol/L) was reduced by LNAA such as Val, Leu, and Ile (by 51%, 78%, and 81%, respectively), or by commercial preparations used in parenteral nutrition, eg, Travasol or Travasol plus extra branched-chain amino acids (BCAA) (Branchamin); Ser had little effect (8% reduction). Insulin in the perfusion medium increased the fractional rate of protein synthesis. Individual BCAA at physiological concentrations (0.2 mmol/L) did not alter the rate of protein synthesis. Branchamin or Travasol plus Branchamin also had no effect on the rate of protein synthesis in heart, but did depress the rate of degradation. These studies suggest that amino acid transport into heart may be affected by normal levels of plasma amino acids, whereas protein synthesis is not.

Effects of prior exercise on the action of insulin-like growth factor I in skeletal muscle

Prior exercise increases insulin sensitivity for glucose and system A neutral amino acid transport activities in skeletal muscle. Insulin-like growth factor I (IGF-I) also activates these transport processes in resting muscle. It is not known, however, whether prior exercise increases IGF-I action in muscle. Therefore we determined the effect of a single exhausting bout of swim exercise on IGF-I-stimulated glucose transport activity [assessed by 2-deoxy-D-glucose (2-DG) uptake] and system A activity [assessed by alpha-(methylamino)isobutyric acid (MeAIB) uptake] in the isolated rat epitrochlearis muscle. When measured 3.5 h after exercise, the responses to a submaximal concentration (0.2 nM), but not a maximal concentration (13.3 nM), of insulin for activation of 2-DG uptake and MeAIB uptake were enhanced. In contrast, prior exercise increased markedly both the submaximal (5 nM) and maximal (20 nM) responses to IGF-I for activation of 2-DG uptake, whereas only the submaximal response to IGF-I (3 nM) for MeAIB uptake was enhanced after exercise. We conclude that 1) prior exercise significantly enhances the response to a submaximal concentration of IGF-I for activation of the glucose transport and system A neutral amino acid transport systems in skeletal muscle and 2) the enhanced maximal response for IGF-I action after exercise is restricted to the signaling pathway for activation of the glucose transport system.

Harry M. Vars Research Award. A new model to determine in vivo the relationship between amino acid transmembrane transport and protein kinetics in muscle

The bidirectional transmembrane transport rates of leucine (Leu), valine (Val), phenylalanine (Phe), lysine (Lys), and alanine (Ala) were measured in vivo in the hindlimb muscle of five dogs and related to the rates of protein synthesis and degradation. The compartmental model was based on the systemic continuous infusion of stable isotopic tracers of the amino acids, and the measurement of the enrichment and concentration in the arterial and femoral vein plasma and the intracellular free water in muscle (obtained by biopsy). The transport rate from plasma to tissue (in micromoles per minute) was: Leu, 18.1 +/- 1.8; Val, 26.9 +/- 3.5; Phe, 10.5 +/- 1.6 Lys; 12.2 +/- 1.8; and Ala, 10.7 +/- 3.4. The transport rate from tissue to plasma (in micromoles per minute) was: Leu, 25.5 +/- 2.5; Val, 32.4 +/- 2.8; Phe, 17.0 +/- 2.8; Lys, 24.9 +/- 3.4; Ala, 34.4 +/- 9.0. When the transmembrane transport rate was normalized per unit of amino acid concentration in the source pool, we found that the transport of Leu, Val, and Phe was significantly faster (p less than .05) than the transport of Lys and Ala. The calculated rates of incorporation into hindlimb muscle protein of Phe and Lys (in micromoles per minute) were 4.2 +/- 1.3 and 19.4 +/- 5.3, respectively, and the rates of intracellular appearance from breakdown were 10.7 +/- 1.9 and 32.1 +/- 6.6, respectively. We concluded, therefore, that (1) the transmembrane amino acid transport rate can be measured in vivo in muscle with a relatively noninvasive technique, (2) in the dog hindlimb the equilibration between tissue and plasma free amino acid pool is different for each amino acid depending on the kinetics of the transmembrane transport systems, and (3) the transport rates of amino acids and their rate of appearance from protein breakdown are roughly comparable, suggesting that variations in transport rates could play a role in controlling the rate of protein synthesis.