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PubMed1984

Regulation of glutamine-repressible gene products by the GLN3 function in Saccharomyces cerevisiae.

Mutants of the yeast Saccharomyces cerevisiae have been isolated which fail to derepress glutamine synthetase upon glutamine limitation. The mutations define a single nuclear gene, GLN3, which is located on chromosome 5 near HOM3 and HIS1 and is unlinked to the structural gene for glutamine synthetase, GLN1. The three gln3 mutations are recessive, and one is amber suppressible, indicating that the GLN3 product is a positive regulator of glutamine synthetase expression. Four polypeptides, in addition to the glutamine synthetase subunit are synthesized at elevated rates when GLN3+ cultures are shifted from glutamine to glutamate media as determined by pulse-labeling and one- and two-dimensional gel electrophoresis. The response of all four proteins is blocked by gln3 mutations. In addition, the elevated NAD-dependent glutamate dehydrogenase activity normally found in glutamate-grown cells is not found in gln3 mutants. Glutamine limitation of gln1 structural mutants has the opposite effect, causing elevated levels of NAD-dependent glutamate dehydrogenase even in the presence of ammonia. We suggest that there is a regulatory circuit that responds to glutamine availability through the GLN3 product.

Ammonia↗
PubMed1996

Corneal organ culture: effects of serum and a stabilised form of L-glutamine.

AIMS: Organ culture medium for corneas contains labile components, such as L-glutamine, whose loss could be a limiting factor to the length of storage. The medium is also supplemented with fetal bovine serum (FBS), which can vary significantly between different batches. The aim of this study was to establish the need for FBS during corneal organ culture, and to determine whether substitution of L-glutamine by the stable dipeptide L-analyl-L-glutamine was beneficial. METHODS: Porcine corneoscleral discs were suspended in 80 ml of organ culture medium (HEPES buffered Eagle's MEM with Earle's salts, 26 mmol/l NaHCO3, penicillin, streptomycin, and amphotericin B) and kept at 34 degrees C. The medium contained either 2 mmol/l L-glutamine or 2 mmol/l L-analyl-L-glutamine, and was either serum free or contained 2% FBS. At weekly intervals, five corneas from each group were stained with trypan blue and alizarin red S, and the surface area and shape of 100 endothelial cells were determined for each cornea. RESULTS: No differences were observed between corneas in organ culture medium with L-glutamine or L-analyl-L-glutamine. In serum free medium, endothelial cell density remained constant for the first week, but then declined rapidly over the next 2 weeks. With 2% FBS, there was no loss of endothelial cells for the first 2 weeks, but cell density had halved by the fourth week of organ culture. CONCLUSION: The presence of 2% FBS extended the period of endothelial stability, but no advantage was gained from the stabilised form of L-glutamine. The overall loss of endothelial cells was much greater than would be expected for human corneas.

Animals↗
PubMed1999

The effects of glutamine on intestinal epithelial cell proliferation in parenterally fed rats.

BACKGROUND: Several papers have indicated that glutamine is a preferred fuel for the enterocyte and that it can increase intestinal epithelial cell proliferation. AIMS: To investigate the effects of glutamine on intestinal epithelial cell proliferation in the parenterally fed rat. METHODS: Five groups of six rats were fed parenterally; a group of orally fed rats was also studied. Crypt cell proliferation was studied after six days using native mitoses in microdissected crypts and bromodeoxyuridine labelling. RESULTS: No effect of treatment was seen on intestinal weight; however, the weights of the small intestine, caecum, and colon were all significantly heavier in the orally fed group than in the total parenteral nutrition groups (p<0.001). There was no effect of any of the glutamine treatments on mitotic activity in the small intestine. In the colon there was a small increase in native mitoses with glutamine (p=0.03). There was also an indication of increased proliferative activity in the first fifth of the small intestine and colon with glutamine. Little effect of glutamine on bromodeoxyuridine labelling in either site was observed, but there was a small but significant reduction in growth fraction of the colon of the glutamine treated group. The labelling distribution curve from sections and the mitotic distribution curve obtained from crypt squashes showed a good correlation. CONCLUSION: Glutamine has a small, but significant effect on mitotic activity but only in the colon. Modest effects on the distribution of labelled cells were also seen.

Animals↗
PubMed2000

Effect of L-ornithine-L-aspartate on patients with and without TIPS undergoing glutamine challenge: a double blind, placebo controlled trial.

BACKGROUND AND AIM: An oral glutamine load in cirrhotic patients awaiting liver transplantation was shown to cause a rise in blood ammonia and psychometric abnormalities which were reversed by hepatic transplantation. L-Ornithine-L-aspartate (LOLA) has been shown to reduce ammonia and improve psychometric function in patients with hepatic encephalopathy. The aim of the present study was to assess the effect of LOLA in healthy patients with cirrhosis and no evidence of clinical encephalopathy after challenging the central nervous system by administration of oral glutamine. PATIENTS AND METHODS: Eight cirrhotics (Child's B or C) without transjugular intrahepatic portosystemic shunts (TIPS) and seven with TIPS underwent two oral glutamine (20 g) challenges, receiving LOLA (5 g intravenously) on one occasion and placebo on the other in random order. Psychometric tests, including choice reaction time (CRT) and number connection test, were performed before and after glutamine, together with electroencephalography and blood ammonia. RESULTS: Mean basal ammonia was 27 (SEM 5) micromol/l in non-TIPS and 76 (10) micromol/l in TIPS patients (p<0.05). Basal CRT 2 was 0.643 (0.033) s in non-TIPS and 0.825 (0.076) s in TIPS patients (p<0.02). In non-TIPS patients, ammonia increased to 36 (10) micromol/l when LOLA was administered and to 62 (13) micromol/l with placebo (p<0.02). There was no alteration in psychometric function in non-TIPS patients after glutamine when LOLA was given but when placebo was given, glutamine caused prolongation of CRT (p=0.02). Glutamine did not affect psychometric function in TIPS patients with or without LOLA. CONCLUSION: This study showed that LOLA ameliorated the deleterious psychometric effects of glutamine in Child's grade B and C patients with cirrhosis without TIPS and supports its use in clinical practice in hepatic encephalopathy.

Ammonia↗
PubMed2003

Glutamine deprivation facilitates tumour necrosis factor induced bacterial translocation in Caco-2 cells by depletion of enterocyte fuel substrate.

BACKGROUND AND AIMS: Factors that induce luminal bacteria to cross the intestinal epithelium following injury remain poorly defined. The aim of this study was to investigate the interaction between glutamine metabolism, energy supply, and inflammatory mediators in determining the translocation of non-pathogenic bacteria across cultured enterocytes. METHODS: The effect of tumour necrosis factor alpha (TNF-alpha) on translocation of Escherichia coli C25 across Caco-2 epithelial monolayers was studied in the presence of products and inhibitors of glutamine metabolism. Simultaneous measurements of transepithelial electrical resistance (TEER) and flux of lucifer yellow were used to assess effects on the paracellular pathway. Lactate dehydrogenase release was used to monitor enterocyte integrity. Imaging of monolayers in these experimental conditions was undertaken with transmission electron microscopy. RESULTS: Exposure to basolateral TNF-alpha (20 ng/ml) for six hours induced translocation of E coli across Caco-2 but only if accompanied by simultaneous glutamine depletion (p<0.01). Translocation was inhibited by addition of glutamine for two hours (p<0.01) but not by an isonitrogenous mixture of non-glutamine containing amino acids. Inhibition of glutamine conversion to alpha-ketoglutarate, but not blockade of glutathione or polyamine synthesis, also induced translocation in the presence of TNF-alpha. Manipulations that induced bacterial translocation were associated with a marked reduction in enterocyte ATP levels. No effect of these treatments on paracellular permeability or lactate dehydrogenase release was observed. Conditions in which translocation occurred were associated with the presence of bacteria within enterocyte vacuoles but not the paracellular space. CONCLUSIONS: In inflammatory conditions, the availability of glutamine as an enterocyte fuel substrate is essential for the preservation of a functional barrier to microorganisms. In conditions of acute glutamine depletion, cytokine mediated bacterial translocation appears to be primarily a transcellular process.

Adenosine Triphosphate↗
PubMed2003

Glutamine delays spontaneous apoptosis in neutrophils.

Nuclear, mitochondrial, and plasma membrane events associated with apoptosis were investigated in rat neutrophils cultivated for 3, 24, and 48 h in the absence or presence of glutamine (0.5, 1.0, and 2.0 mM). Condensation of chromatin was reduced after 24 or 48 h of culture in the presence of glutamine compared with its absence as assessed by Hoechst 33342 staining. The level of Escherichia coli phagocytosis in the presence of glutamine was markedly increased compared with the level achieved by cells cultured in the absence of glutamine. Annexin V binding to externalized phosphatidylserine was reduced in the presence of glutamine. Sensitive fluorochrome rhodamine 123, as determined by fluorescence-activated cell sorting and confocal microscopy, was used to monitor loss of the mitochondrial transmembrane potential. In the absence of glutamine, neutrophils exhibited a marked reduction in the uptake of rhodamine 123. In the presence of 1.0 or 2.0 mM glutamine, the uptake of rhodamine was 20 or 38% higher, respectively. Similar effect was found in human neutrophils by measuring DNA fragmentation and mitochondrial transmembrane potential. Therefore, glutamine protects from events associated with triggering and executing apoptosis in both rat and human neutrophils.

Animals↗
PubMed1998

Glutamine transport by the blood-brain barrier: a possible mechanism for nitrogen removal.

Glutamine and glutamate transport activities were measured in isolated luminal and abluminal plasma membrane vesicles derived from bovine brain endothelial cells. Facilitative systems for glutamine and glutamate were almost exclusively located in luminal-enriched membranes. The facilitative glutamine carrier was neither sensitive to 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid inhibition nor did it participate in accelerated amino acid exchange; it therefore appeared to be distinct from the neutral amino acid transport system L1. Two Na-dependent glutamine transporters were found in abluminal-enriched membranes: systems A and N. System N accounted for approximately 80% of Na-dependent glutamine transport at 100 microM. Abluminal-enriched membranes showed Na-dependent glutamate transport activity. The presence of 1) Na-dependent carriers capable of pumping glutamine and glutamate from brain into endothelial cells, 2) glutaminase within endothelial cells to hydrolyze glutamine to glutamate and ammonia, and 3) facilitative carriers for glutamine and glutamate at the luminal membrane may provide a mechanism for removing nitrogen and nitrogen-rich amino acids from brain.

Animals↗
PubMed2005

Effect of intravenous amino acids on glutamine and protein kinetics in low-birth-weight preterm infants during the immediate neonatal period.

Glutamine may be a conditionally essential amino acid in low-birth-weight (LBW) preterm neonates. Exogenously administered amino acids, by providing anaplerotic carbon into the tricarboxylic acid cycle, could result in greater cataplerotic efflux and glutamine de novo synthesis. The effect of dose and duration of amino acid infusion on glutamine and nitrogen (N) kinetics was examined in LBW infants in the period immediately after birth. Preterm neonates (<32 weeks gestation, birth weights 809-1,755 g) were randomized to initially receive either 480 or 960 micromol x kg(-1) x h(-1) of an intravenous amino acid solution for 19-24 hours, followed by a higher or lower amino acid load for either 5 h or 24 h. Glutamine de novo synthesis, leucine N, phenylalanine, and urea kinetics were determined using stable isotopic tracers. An increase in amino acid infusion from 480 to 960 micromol x kg(-1) x h(-1) for 5 h resulted in decreased glutamine de novo synthesis in every neonate (384.4 +/- 38.0 to 368.9 +/- 38.2 micromol x kg(-1) x h(-1), P < 0.01) and a lower whole body rate of proteolysis (P < 0.001) and urea synthesis (P < 0.001). However, when the increased amino acid infusion was extended for 24 h, glutamine de novo synthesis increased (369.7 +/- 92.6 to 483.4 +/- 97.5 micromol x kg(-1) x h(-1), P < 0.001), whole body rate of proteolysis did not change, and urea production increased. Decreasing the amino acid load resulted in a decrease in glutamine rate of appearance (R(a)) and leucine N R(a), but had no effect on phenylalanine R(a). Acutely stressed LBW infants responded to an increase in amino acid load by transiently suppressing whole body rate of glutamine synthesis, proteolysis, and oxidation of protein. The mechanisms of this transient effect on whole body protein/nitrogen metabolism remain unknown.

Amino Acids↗
PubMed1980

Mithcondrial glutamine permeability and renal ammonia production in metabolic acidosis.

These experiments were undertaken to determine the correspondence between acidosis-induced in situ motochondrial glutamine uptake and the process by which glutamine moves across the mitochondrial membrane. Feeding rats 1.5% NH4Cl for 2 wk accelerated the in situ uptake rate from 0.12 +/- 0.08 to 1.89 +/- 0.28 mu mol/min or some 16-fold. To determine glutamine uptake independent of its metabolic conversion, D-glutamine was employed. In isolated mitochondria from non-acidotic rat kidneys, D-glutamine diffused into 71 +/- 10% of the mitochondria water volume; in acidotic mitochondria the diffusion volume increased to 127 +/- 16%. The reflection coefficients (sigma) for a series of amides, including glutamine, were determined by gravimetrically following volume decrements in increasing concentrations of solute; D-glutamine's sigma fell from 1.05 +/- 0.08 to 0.50 +/- 0.06 in acidotic rat kidney mitchondria, The permeability coefficients corresponding to the measured sigma were 10(-7) cm/s and 10(-5) cm/s in nonacidosis and acidosis, respectively. When viewed in situ proximal tubule mitochondria undergo dramatic alterations during chronic acidosis. These involved an enlargement in the mitochondria particularly in the basal region of the cell and a reduction in number. Furthermore, numerous autophagic vacuoles, containing mitochondria, appear in the basal region. The findings are consistent availability that becomes activated during acidosis as a consequence of mitochondrial glutamine permeability resulting in increased ammoniagenesis and accelerated organelle turnover.

Acidosis↗
PubMed1991

Glutamine and glucose metabolism in rat splenocytes and mesenteric lymph node lymphocytes.

The metabolism of glutamine (2 mM) and glucose (5 mM) was studied in splenocytes and mesenteric lymph node lymphocytes of Wistar-Furth rats to assess their relative importance as energy substrates. The major products from glutamine were ammonia, glutamate, aspartate, and CO2, whereas those from glucose were lactate, pyruvate, and CO2 in cells from both lymphoid organs. The individual rates of glutamine and glucose metabolism were decreased in the presence of both substrates, compared with the rates when present separately. The rates of glucose and some (but not all) aspects of glutamine metabolism were higher (P less than 0.01) in splenocytes than in mesenteric lymphocytes. In cells from both lymphoid organs, glutamine and glucose could potentially contribute almost equal amounts of ATP in the presence of both substrates. Glutamine and glucose individually were able to provide sufficient amounts of ATP to maintain its concentrations in the cells throughout a 2-h incubation period at the same levels as with both substrates present. We also found that splenocyte concentration (3.3-100 x 10(6) cells/ml) in the incubations is an important determinant of rates of metabolite formation from glutamine when expressed per 10(6) cells. We conclude that glucose is not the only quantitatively significant energy substrate or even the major one for lymphocytes, because glutamine at near-physiological concentration can be readily utilized by these cells.

Adenosine Triphosphate↗
PubMed1993

Glucocorticoid regulation of splanchnic glutamine, alanine, glutamate, ammonia, and glutathione fluxes.

Interorgan glutamine and associated metabolite fluxes were measured across the gut and liver to delineate splanchnic bed fluxes secondary to enhanced arterial loads mobilized in the periphery by glucocorticoid. Experiments were performed on adrenalectomized rats since adrenalectomy doubled the hepatic glucocorticoid receptor population compared with intact animals. Under these conditions, triamcinolone supplement (40 micrograms.day-1.100 g body wt-1) enhanced the combined net glutamine uptake by gut and liver eightfold, whereas combined gut and liver unidirectional breakdown and synthesis fluxes both increased (3.4- and 7.4-fold, respectively). Triamcinolone supplement also altered the pattern of metabolite released; gut released predominantly ammonium and some alanine, whereas the liver removed more alanine along with glutamine and released more urea, glutamate, and glutathione. Mechanistically, enhanced cellular glutamine uptake could be attributed to a three- to fourfold acceleration of glutamine transport associated with a rise in intracellular glutamine content. However, uptake by isolated membrane vesicles revealed only a small (27%) increase in System N activity, whereas extraction and reconstitution of the transporter into proteoliposomes failed to demonstrate increased transporter activity. Similarly, activity of phosphate-dependent glutaminase and glutamate dehydrogenase increased in crude homogenates (2-fold), but the former disappears in completely disrupted preparations. Furthermore, whereas messenger RNA and assayable enzymic activity for glutamate dehydrogenase clearly increased with glucocorticoid, glutaminase message was less significantly increased. Thus glucocorticoid appears directly capable of accelerating hepatic glutamine extraction primarily by modulating transporter activity that is closely coupled to glutamine utilization.

Adrenalectomy↗
PubMed1995

Glutamine-glutamate exchange between placenta and fetal liver.

The hypothesis that glutamine shuttles nitrogen between placenta and fetal liver via interconversion with glutamate was explored by infusing L-[1,2-13C2]glutamine in six fetal sheep chronically catheterized for sampling of the umbilical and hepatic circulations. Fetal plasma glutamine disposal rate was 19.9 +/- 1.3 mumol.min-1.kg fetus-1. Entry of glutamine from the placenta accounted for approximately 60% of the total glutamine entry rate in fetal plasma. Glutamine was taken up by fetal liver, and 45.3 +/- 7.9% of the glutamine taken up was released as glutamate. The fetal liver released large quantities of glutamate, as evidenced by a sixfold increase in plasma glutamate concentration in the blood flowing through the left hepatic lobe and a hepatic glutamate output-to-O2 uptake molar ratio of 0.149 +/- 0.013. In conjunction with a previous study of fetal glutamate metabolism, these data demonstrate that glutamine entering the fetal circulation is converted to glutamate by the fetal liver at a rate of approximately 3-4 mumol.min-1.kg fetus-1.

Animals↗
PubMed1999

The metabolic consequences of critical illness: acute effects on glutamine and protein metabolism.

Net protein loss and large decreases in plasma glutamine concentration are characteristics of critical illness. We have used [2-15N]glutamine and [1-13C]leucine to investigate whole body glutamine and leucine kinetics in a group of critically ill patients and matched healthy controls. Glutamine appearance rate (Ra,Gln) was similar in both groups. However, in the patients, the proportion of Ra,Gln arising from protein breakdown was higher than in the control group (43 +/- 3 vs. 32 +/- 2%, P < 0.05). Glutamine metabolic clearance rate (MCR) was 92 +/- 8% higher (P < 0.001), whereas plasma glutamine concentration was 38 +/- 5% lower (P < 0.001) than in the control group. Leucine appearance rate (whole body proteolysis) and nonoxidative leucine disposal (whole body protein synthesis) were 59 +/- 14 and 49 +/- 15% higher in the patients (P < 0.001). Leucine oxidation and MCR were increased in the patients by 104 +/- 37 and 129 +/- 39%, respectively (P < 0.05). These results demonstrate that critical illness is associated with a major increase in protein turnover. The acute decrease in plasma glutamine concentration and the unaltered plasma Ra,Gln suggest that the increase in proteolysis is insufficient to meet increased demand for glutamine in this severe catabolic state.

Adult↗
PubMed1999

Effect of intravenous glutamine on duodenal mucosa protein synthesis in healthy growing dogs.

To determine whether glutamine acutely stimulates protein synthesis in the duodenal mucosa, five healthy growing dogs underwent endoscopic biopsies of duodenal mucosa at the end of three 4-h primed, continuous intravenous infusions of L-[1-13C]leucine on three separate days, while receiving intravenous infusion of 1) saline, 2) L-glutamine (800 micromol. kg-1. h-1), and 3) isonitrogenous amounts of glycine. The three infusions were performed after 24 h of fasting, a week apart from each other and in a randomized order. Glutamine infusion induced a doubling in plasma glutamine level, and glycine caused a >10-fold rise in plasma glycine level. During intravenous infusions of [13C]leucine, the plasma leucine labeling attained a plateau value between 3.22 and 3.68 mole % excess (MPE) and [13C]ketoisocaproate ([13C]KIC) of 2.91-2. 84 MPE; there were no significant differences between glutamine, glycine, and saline infusion days. Plasma leucine appearance rate was 354 +/- 33 (SE), 414 +/- 28, and 351 +/- 35 micromol. kg-1. h-1 (not significant) during glycine, saline, and glutamine infusion, respectively. The fractional synthetic rate (FSR) of duodenal mucosa protein was calculated from the rise in protein-bound [13C]leucine enrichment in the biopsy sample, divided by time and with either plasma [13C]KIC or tissue free [13C]leucine as precursor pool enrichment. Regardless of the precursor pool used in calculations, duodenal protein FSR failed to rise significantly during glutamine infusion (65 +/- 11%/day) compared either with saline (84 +/- 18%/day) or glycine infusion days (80 +/- 15%/day). We conclude that 1) plasma [13C]KIC and tissue free [13C]leucine can be used interchangeably as precursor pools to calculate gut protein FSR; and 2) short intravenous infusion of glutamine does not acutely stimulate duodenal protein synthesis in well-nourished, growing dogs.

Aging↗
PubMed2000

Role of glucose in the regulation of glutamine metabolism in health and in type 1 insulin-dependent diabetes.

To determine the effect of glucose availability on glutamine metabolism, glutamine kinetics were assessed under conditions of hyperglycemia resulting from 1) intravenous infusion of 7.5% dextrose in healthy adults and 2) insulin deficiency in young adults with insulin-dependent diabetes mellitus (IDDM). Eight healthy adults and five young adults with IDDM were studied in the postabsorptive state by use of a primed continuous infusion of D-[U-(14)C]glucose, L-[5,5,5-(2)H(3)]leucine, and L-[3, 4-(13)C]glutamine. Whether resulting from insulin deficiency or dextrose infusion, the rise in plasma glucose was associated with increased glucose turnover (23.5 +/- 0.7 vs. 12.9 +/- 0.3 micromol. kg(-1). min(-1), P < 0.01 and 20.9 +/- 2.5 vs. 12.8 +/- 0.4 micromol. kg(-1). min(-1), P = 0.03, in health and IDDM, respectively). In both cases, high blood glucose failed to alter glutamine appearance rate (R(a)) into plasma [298 +/- 9 vs. 312 +/- 14 micromol. kg(-1). h(-1), not significant (NS) and 309 +/- 23 vs 296 +/- 26 micromol. kg(-1). h(-1), NS, in health and IDDM, respectively] and the estimated fraction of glutamine R(a) arising from de novo synthesis (210 +/- 7 vs. 217 +/- 10 micromol. kg(-1). h(-1), NS and 210 +/- 16 vs. 207 +/- 21 micromol. kg(-1). h(-1), NS, in health and IDDM, respectively). When compared with the euglycemic day, the apparent contribution of glucose to glutamine carbon skeleton increased when high plasma glucose resulted from intravenous dextrose infusion in healthy volunteers (10 +/- 0.8 vs. 4.8 +/- 0.3%, P < 0.01) but failed to do so when hyperglycemia resulted from insulin deficiency in IDDM. We conclude that 1) the contribution of glucose to the estimated rate of glutamine de novo synthesis does not increase when elevation of plasma glucose results from insulin deficiency, and 2) the transfer of carbon from glucose to glutamine may depend on insulin availability.

Adult↗
PubMed1990

Development of L-glutamine-stimulated electroneutral sodium absorption in piglet jejunum.

Glutamine is the primary metabolic fuel of the small intestine. To determine the effects of glutamine on intestinal electrolyte transport, piglet (3 days to 3 wk old) jejunum was bathed in Ussing chambers in a buffer containing 10 mM serosal glucose, and the effects of different concentrations of mucosal L-glutamine and D-glucose on short-circuit current and transmucosal Na+ and Cl- transport were measured. Resting jejunum secreted Na+ and Cl- in an electrogenic manner. In contrast to mucosal D-glucose (30 mM), which promoted electrogenic Na+ absorption (1.8 mueq.cm-2.h-1), mucosal L-glutamine (30 mM) stimulated both Na+ (2.7 mueq.cm-2.h-1) and Cl- (2.2 mueq.cm-2.h-1) absorption. This NaCl-absorptive jejunal response depended on the presence of both Na+ and Cl-, did not appear until animals were greater than 7 days of age, and was not observed with glucose, phenylalanine, or mannitol. Serosal, as well as mucosal, glutamine (30 mM) promoted electroneutral NaCl absorption. A small electrogenic Na(+)-absorptive response to L-glutamine was also observed. The effect of L-glutamine on jejunal NaCl transport resembles that of other metabolic fuels on colonic transport; its mechanism remains to be determined. We conclude that glutamine promotes electroneutral salt absorption in the small intestine.

Aging↗
PubMed1991

Absorption and metabolic effects of enterally administered glutamine in humans.

To assess absorption and metabolic effects of enterally delivered glutamine, a total of 10 healthy subjects received perfusions of natural L-glutamine at graded infusion rates (ranging from 0 to 126 mmol/h; n = 2-8 subjects at each rate) along with a nonabsorbable marker (polyethylene glycol) through a double-lumen nasojejunal tube. Perfusions were administered after an overnight fast during three consecutive 1- or 2.5-h periods and in a randomized order. In eight subjects, continuous intravenous infusion of D-[6,6-2H2]glucose, L-[1-13C]leucine, and L-[15N]alanine was simultaneously performed. Glutamine was nearly quantitatively absorbed over the 30-cm study segment; estimated Km and Vmax of glutamine absorption were 2.48 and 2.32 mmol/min over the 30-cm study segment. Enteral glutamine administration induced 1) a dose-dependent increase in plasma glutamine level; 2) a rise in the plasma level and appearance rate (Ra) of alanine (from 191 +/- 42 to 213 +/- 51 mumols.kg-1.h-1, P less than 0.05, for 0 and 46.8-mmol/h glutamine infusion rates, respectively) and in plasma levels of glutamate, citrulline, aspartate, and urea; 3) a decline in plasma free fatty acid and glycerol levels; and 4) no change in leucine or glucose Ra. We conclude that glutamine is efficiently absorbed by human jejunum in vivo and may directly inhibit lipolysis, whereas it neither affects proteolysis nor glucose production in healthy postabsorptive humans.

Adult↗
PubMed1995

Oxidation of glucose carbon entering the TCA cycle is reduced by glutamine in small intestine epithelial cells.

The influence of glutamine on glucose oxidation was assessed in epithelial cells isolated from the mucosa of the proximal, mid-, and distal small intestine of young, fed, male rats. Glucose oxidation declined along the length of the small intestine, with values from the mid- and distal segments representing approximately 55% and 40%, respectively, of the value from the proximal segment. A gradient along the small intestine was noted also in the influence of glutamine on glucose oxidation: glutamine suppressed glucose oxidation approximately 60% in the proximal small intestine, 39% in the mid-intestine, and 31% in the distal small intestine. Glutamine suppressed the oxidation of glucose carbon that entered the tricarboxylic acid (TCA) cycle; this was determined using CO2 ratios derived from acetate and glucose isotopes. In cells from the proximal segment, the probability that carbon entering the cycle would complete one full turn was reduced by glutamine from 0.77 to 0.28. The entry of glucose-derived pyruvate into the TCA cycle did not appear to be influenced by the presence of glutamine, however. Glutamine had no influence on the proportion of glucose metabolism that occurred via the pentose phosphate pathway (which averaged 5% or less), but reduced flux of carbon through pyruvate carboxylase relative to flux through pyruvate dehydrogenase from 40% to 9% in cells from the proximal segment. These data suggest that, in the presence of glutamine, the fate of pyruvate carbon (derived from glucose or elsewhere) entering the TCA cycle is altered from that of oxidation to anaplerosis and subsequent efflux of TCA cycle intermediates into newly synthesized compounds.

Animals↗