[Proceedings: Glutamine synthetase and glutamine gamma-glutamyl transferase in human, canine, and rat kidneys].
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Acyltransferases↗
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[Glutamine oxalacetate and glutamine pyruvate transaminase activity in different regions of the brains of growing dogs].
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Age Factors↗
Effects of corticosteroid on the transport and metabolism of glutamine in rat skeletal muscle.
Intramuscular glutamine falls with injury and disease in circumstances associated with increases in blood corticosteroids. We have investigated the effects of corticosteroid administration (0.44 mg/kg dexamethasone daily for 8 days, 200 g female rats) on intramuscular glutamine and Na+, muscle glutamine metabolism and sarcolemmal glutamine transport in the perfused hindlimb. After dexamethasone treatment intramuscular glutamine fell by 45% and Na+ rose by 25% (the respective muscle/plasma distribution ratios changed from 8.6 to 4.5 and 0.12 to 0.15); glutamine synthetase and glutaminase activities were unchanged at 475 +/- 75 and 60 +/- 19 nmol/g muscle per min. Glutamine output by the hindlimb of anaesthetized rats was increased from 31 to 85 nmol/g per min. Sarcolemmal glutamine transport was studied by paired-tracer dilution in the perfused hindlimb: the maximal capacity (Vmax) for glutamine transport into muscle (by Na(+)-glutamine symport) fell from 1058 +/- 310 to 395 +/- 110 nmol/g muscle per min after dexamethasone treatment, accompanied by a decrease in the Km (from 8.1 +/- 1.9 to 2.1 +/- 0.4 mM glutamine). At physiological plasma glutamine concentration (0.75 mM) dexamethasone appeared to cause a proportional increase in sarcolemmal glutamine efflux over influx. Addition of dexamethasone (200 nM) to the perfusate of control rat hindlimbs caused acute changes in Vmax and Km of glutamine transport similar to those resulting from 8-day dexamethasone treatment. The reduction in muscle glutamine concentration after dexamethasone treatment may be primarily due to a reduction in the driving force for intramuscular glutamine accumulation, i.e., in the Na+ electrochemical gradient. The prolonged increase in muscle glutamine output after dexamethasone treatment (which occurs despite a reduction in the size of the intramuscular glutamine pool) appears to be due to a combination of (a) accelerated sarcolemmal glutamine efflux and (b) increased intramuscular synthesis of glutamine.
Amino Acids↗
Glutamine: the pivot of our nitrogen economy?
Glutamine serves as a shuttle of useful nontoxic nitrogen, supplying nitrogen from glutamine-producing (eg, muscle) to glutamine-consuming tissues. True production rates of glutamine are difficult to measure, but probably are less than 60 to 100 g/d for a 70-kg man. During catabolic stress increased amounts of glutamine are released from muscle, consisting of protein derived glutamine, newly synthesized glutamine, and glutamine losses from the intramuscular free pool. The large and rapid losses of free muscle glutamine are difficult to restore, presumably as a result of disturbances in the Na+ electrochemical gradient across the cell membrane. Whereas increased amounts of glutamine are released from muscle, glutamine consumption by the immune system (liver, spleen) also is enhanced. Thus, during catabolic stress changes occur in the flow of glutamine between organs. These changes are not necessarily reflected by alterations in the whole-body appearance rate of glutamine. In contrast with the gut, where glutamine is taken up in a concentration dependent manner, the immune system actively takes up glutamine despite decreased plasma concentrations. Supplementation with glutamine influences uptake by both the gut and the immune system, as evidenced by increased mucosal glutamine concentrations and gut glutathione production. There is evidence suggesting that this improves gut barrier function. Although the benefit of glutamine supplementation is most evident from experimental studies, clinical studies on the effect of glutamine do exist and suggest that glutamine supplementation has beneficial effects with regard to patient outcome.
Animals↗
Glutamine and glutamate nitrogen exchangeable pools in cultured fibroblasts: a stable isotope study.
Glutamine's role as an energetic fuel has been extensively studied in the past using 14C- and 3H-labeled tracers in cultured human cells. Yet another prominent role of glutamine, that of a nitrogen shuttle, cannot be approached without an N-tracer. We therefore used 15N-labeled glutamine and glutamate to address the following questions: 1) is it possible to study the exchangeable pools of intracellular free glutamine and glutamate nitrogen with stable isotope methods? and 2) to what extent is intracellular glutamine pool regulated by extracellular glutamine? We observed that: 1) intracellular [15N]-glutamine enrichment reached a plateau at 80% within 20 min of incubation in a buffer containing 0.7 mM pure 15N-glutamine and no glutamate; in contrast, intracellular 15N-glutamate enrichment rose only to 40% after 4 hours of incubation in a buffer containing 0.5 mM pure 15N-glutamate and no glutamine; 2) the cell-free glutamine content was tightly dependent on extracellular glutamine level, while the cell-free glutamate remained steady irrespective of the extracellular glutamate level; 3) the cells took up glutamine and glutamate against a concentration gradient; the rate of glutamine uptake accounted for 90% of the cell glutamine turnover rate; and 4) when cells were confronted with a glutamine-free medium, only one fourth of intracellular glutamine was derived from the exchangeable glutamate. We conclude that: 1) The size and turnover rate of the intracellular pool of free glutamine nitrogen are measureable using stable isotope methodology; 2) glutamine uptake from the extracellular medium accounts for most of glutamine turnover rate in cultured fibroblasts; and 3) intracellular free glutamate is divided up between several pools in cultured human fibroblasts.
Body Fluids↗
Glutamine and cancer.
OBJECTIVE: This overview on glutamine and cancer discusses the importance of glutamine for tumor growth, summarizes the alterations in interorgan glutamine metabolism that develop in the tumor-bearing host, and reviews the potential benefits of glutamine nutrition in the patient with cancer. SUMMARY BACKGROUND DATA: Glutamine is the most abundant amino acid in the blood and tissues. It is essential for tumor growth and marked changes in organ glutamine metabolism are characteristic of the host with cancer. Because host glutamine depletion has adverse effects, it is important to study the regulation of glutamine metabolism in cancer and to evaluate the impact of glutamine nutrition in the tumor-bearing state. METHODS: Data from a variety of investigations on glutamine metabolism and nutrition related to the host with cancer were compiled and summarized. RESULTS: Numerous studies on glutamine metabolism in cancer indicate that many tumors are avid glutamine consumers in vivo and in vitro. As a consequence of progressive tumor growth, host glutamine depletion develops and becomes a hallmark. This glutamine depletion occurs in part because the tumor behaves as a "glutamine trap" but also because of cytokine-mediated alterations in glutamine metabolism in host tissues. Animal and human studies that have investigated the use of glutamine-supplemented nutrition in the host with cancer suggest that pharmacologic doses of dietary glutamine may be beneficial. CONCLUSIONS: Understanding the control of glutamine metabolism in the tumor-bearing host not only improves the knowledge of metabolic regulation in the patient with cancer but also will lead to improved nutritional support regimens targeted to benefit the host.
Antimetabolites, Antineoplastic↗
Role of plasma membrane transport in hepatic glutamine metabolism.
In livers of fed rats and in perfused livers supplied with a physiological portal glutamine concentration of 0.6 mM, the mitochondrial and cytosolic glutamine concentrations are 20 mM and 7 mM, respectively, thus, the mitochondrial/cytosolic glutamine concentration gradient is 2-3. Uptake and release of glutamine by periportal and perivenous hepatocytes occurs predominantly by an Na+-dependent transport system (so-called system 'N'). Histidine in near-physiological concentrations inhibits both glutamine uptake by periportal hepatocytes and its release by perivenous hepatocytes. This is not due to an inhibition of glutamine-metabolizing enzymes by histidine or its metabolites. With physiological portal glutamine concentrations (0.6 mM), stimulation of glutaminase flux or of glutamine transaminase flux is followed by a decrease of hepatic glutamine levels to about 80% or 30%, respectively, glutamine levels are further decreased to 50% or 20% in the presence of histidine. When glutamine is synthesized endogenously (no glutamine added), the histidine-induced inhibition of glutamine release is paralleled by a 210% increase of the hepatic tissue level of glutamine. In experiments with and without methionine sulfoximine and in the absence of added glutamine, the glutamine content in the small perivenous hepatocyte population containing glutamine synthetase is estimated to be about 3.5 mumol/g wet weight and that in the periportal hepatocytes as low as 0.1 mumol/g wet weight. In contrast to the prevailing view, it is concluded that glutamine transport across the plasma membrane of hepatocytes is a potential regulatory site in glutamine degradation and synthesis, especially under the influence of effectors like histidine.
Amino Acids↗
Rate at which glutamine enters TCA cycle influences carbon atom fate in intestinal epithelial cells.
Glutamine carbon entry into the tricarboxylic acid (TCA) cycle was assessed in small intestinal epithelial cells by measuring CO2 production from [1-14C]glutamine, and these data together with [U-14C]glutamine data were used to calculate fractional oxidation rates for glutamine. CO2 production from either [1-14C]glutamine or [U-14C]glutamine showed saturation kinetics, and the concentration needed to achieve the half-maximal rate of CO2 production was 0.7 and 0.4 mmol/l, respectively. Maximal rate for [1-14C]glutamine was twice that for [U-14C]glutamine. Increasing glutamine concentration did not cause proportional increases in glutamine entry into the TCA cycle and glutamine oxidation. Consequently, fractional oxidation of glutamine decreased with increasing glutamine concentration. Fractional oxidation could be predicted from the rate at which glutamine carbon entered the TCA cycle. (Aminooxy)acetic acid, an aminotransferase inhibitor, reduced entry of glutamine into the TCA cycle and increased fractional oxidation of glutamine. Glutamate carbon entered the TCA cycle at about one-half the rate of glutamine-derived glutamate carbon and had a higher fractional oxidation rate when provided at equivalent concentrations to glutamine. These differences in the rate of entry predictably account for the differences in the metabolic fate of glutamine vs. glutamate carbon.
Animals↗
Glutamine and glutathione counteract the inhibitory effects of mediators of sepsis in neonatal hepatocytes.
BACKGROUND/PURPOSE: Surgical neonates are at risk of sepsis-associated liver dysfunction. Hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) are important mediators of sepsis, which impair neonatal hepatic metabolism. Glutamine has been shown to have beneficial effects on hepatocyte metabolism during neonatal sepsis. However, the molecular basis of these effects are unknown. The aim of this study was to test the hypotheses that (1) glutamine and its dipeptides counteract the inhibitory effect of septic mediators on neonatal hepatocyte oxygen consumption and (2) the effects of glutamine are specific and not shared by other amino acids. In addition, we wished to determine the metabolic pathways and mediators involved in the action of glutamine. METHODS: Hepatocytes were isolated from suckling rats, and O(2) consumption measured polarographically. Study A: the ability of 10 mmol/L glutamine to reverse the inhibitory effects of 1.5 mmol/L H(2)O(2) and 300 micromol/L S-Nitroso-N-acetylpenicillamine (SNAP; a nitric oxide donor) on O(2) consumption was examined. Study B: the ability of other amino acids and dipeptides of glutamine to reverse the effects of H(2)O(2) was examined. Study C: various concentrations of glutamine were tested for their ability to reverse the H(2)O(2) inhibition of O(2) consumption. Study D: the mechanism of action of glutamine was examined by incubating hepatocytes with either an inhibitor of entry into the Krebs cycle or an inhibitor of glutathione synthesis. Study E: the ability of glutathione to reverse the inhibitory effects of H(2)O(2) was examined. RESULTS: Study A: glutamine reversed the inhibition of hepatocyte O(2) consumption exerted by either H(2)O(2) or NO. Study B: glutamine dipeptides reversed the inhibition of hepatocyte O(2) consumption by H(2)O(2), but other amino acids did not. Study C: the counteracting effect of glutamine was proportional to the dose administered. Study D: blocking entry of glutamine into the Krebs cycle did not abolish the effects of glutamine, but blocking glutathione synthesis completely abolished the effect of glutamine. Study E: exogenous glutathione reversed the inhibitory effect of H(2)O(2) on hepatocyte O(2) consumption. CONCLUSIONS: This study found that glutamine and its dipeptides are unique in reversing the effects of septic mediators on neonatal rat liver oxidative metabolism. The effectiveness of glutamine appears to be mediated via glutathione synthesis. Addition of glutamine, glutamine dipeptides, or glutathione to total parenteral nutrition (TPN) may be beneficial in preventing liver damage in neonatal sepsis.
Animals↗
Effects of branched-chain L-amino acids, L-phenylalanine, and L-methionine on the transport of L-glutamine in rat brain cortex in vitro. Influence of cations.
Uptake of L-glutamine (2 mM) by rat brain cortex slices against a concentration gradient is markedly inhibited (40%) by branched-chain L-amino acids (1 mM), L-phenylalanine (1 mM), or L-methionine (1 mM); that of L-asparagine (2 mM) is much less affected by these amino acids. Other amino acids investigated have little or no effect on cerebral L-glutamine uptake. The suppressions of L-glutamine uptake by the inhibitory amino acids are apparently blocked by high [K+], which itself has little or no effect on glutamine uptake. This abolition of suppression is partly explained by high [K+] retention of endogenous glutamine; in the absence of Ca2+ such retention disappears. The inhibitory amino acids (1 mM) also enhance the release of endogenous glutamine, exogenous glutamine with which slices have been loaded, or glutamine synthesized in the slices from exogenous glutamate. The enhanced release of endogenous glutamine is diminished by high [K+]. The suppression of glutamine uptake by the branched-chain amino acids is independent of the concentration of glutamine at low concentrations (0.25--0.5 mM), indicating non-competition, but is reduced with high concentration of glutamine. The inhibition by L-phenylalanine is noncompetitive. L-Glutamine (2mM) exerts no inhibition of the cerebral uptakes of the branched-chain L-amino acids or L-phenylalanine (0.25--2 mM). The inhibitory amino acids are as active in suppressing L-glutamine uptake with immature rat brain slices as with adult, although the uptake, against a gradient, of L-glutamine in the infant rat brain is about one-half that in the adult. They are also just as inhibitory on the concentrative uptake of L-glutamine by a crude synaptosomal preparation derived from rat brain cortex. Such a nerve ending preparation takes up L-glutamine (0.25 mM), against a gradient, at about ninefold the rate at which it is taken up by cortex slices (for equal amounts of protein), and the uptake process is markedly suppressed by high [K+] in contrast to the effects of high [K+] with slices. The possible physiological and pathological consequences of the suppression of glutamine uptake are discussed.
Amino Acids↗
Route of administration (enteral or parenteral) affects the contribution of L-glutamine to de novo L-arginine synthesis in mice: a stable-isotope study.
A pathway from enteral L-glutamine as substrate for L-arginine synthesis is suggested by previous studies. L-Glutamine and L-glutamine dipeptides exhibit numerous beneficial effects in experimental and clinical studies. In trauma patients, enteral L-glutamine supply increased plasma L-arginine. The present study was designed to quantify the contribution of L-glutamine to the de novo L-citrulline and L-arginine synthesis in mice when L-glutamine is administered in a high dose of labeled L-glutamine or L-alanyl-L-glutamine by the enteral or parenteral route. For this purpose, male Swiss mice (n = 43) underwent a laparotomy, and catheters were inserted for sampling and infusion. A primed, constant, and continuous infusion of L-alanyl-L-[2-(15)N]glutamine (dipeptide groups) or L-[2-(15)N]glutamine (free L-glutamine groups), simultaneously with L-[ureido-(13)C,(2)H(2)]citrulline and L-[guanidino-(15)N(2),(2)H(2)]arginine, was given (steady-state model). Mice received the L-glutamine tracers intravenously (jugular vein) or enterally (duodenum). Enrichments of metabolites were measured by LC-MS. Arterial L-glutamine concentrations were the highest in the intravenous dipeptide group. L-Glutamine was converted to L-citrulline and L-arginine when L-[2-(15)N]glutamine and L-alanyl-L-[2-(15)N]glutamine were given by enteral or parenteral route. The contribution of L-glutamine to the de novo synthesis of L-citrulline and L-arginine was higher in the enteral groups when compared with the intravenous groups (P < 0.005). Therefore, the route of administration (enteral or parenteral) affects the contribution of L-glutamine, provided as free molecule or dipeptide, to the de novo synthesis of L-arginine in mice.
Animals↗
Regulation of interorganal glutamine flow in metabolic acidosis.
Metabolic acidosis redirects interorgan glutamine flow from hepatic utilization to renal ammoniagenesis at the expense of ureagenesis. The roles of arterial glutamine load and organ glutaminase capacity in the regulation of glutamine balance across the gut, liver, and kidneys were studied in control and chronically acidotic rats. In control rats these organs combined to remove 733 nmol glutamine X min-1 X 100 g-1 in agreement with their respective glutaminase content, gut greater than liver greater than kidneys. In chronic metabolic acidosis renal glutamine extraction alone increased to 1,158 nmol X min-1 X 100 g-1 associated with an increased glutaminase capacity. However, the total glutamine deficit across these organs rose to only 1,043 nmol glutamine consumed X min-1 X 100 g-1 as a consequence of hepatic glutamine uptake reversing to net release. This reversal was not dependent on increased hepatic glutamine synthetase capacity, but rather appears to be dependent on the combined effect of reduced portal venous glutamine load and increased ammonia load. The reduction in portal glutamine load is, in turn, a consequence of renal glutamine extraction and reduced arterial glutamine concentration in metabolic acidosis as well as maintained gut glutamine extraction. Elevating arterial glutamine concentration in metabolic acidosis has no effect on renal uptake, but enhances splanchnic bed extraction with the restoration of ureagenesis. Thus the interorgan flow of glutamine and deposition of N into either urea or ammonia appears to be dependent on arterial glutamine concentration and hence glutamine availability in chronic metabolic acidosis in the rat.
Acid-Base Equilibrium↗
Acid-base regulation of hepatic glutamine metabolism and ureagenesis: study with 15N.
The metabolism of (5-15N)glutamine and (2-15N) glutamine has been studied by isolated hepatocytes obtained from either control, chronically acidotic, or alkalotic rats. The main goal was to elucidate the mechanism(s) by which altered acid-base state affects hepatic ureagenesis from glutamine. Isolated hepatocytes were incubated in Krebs buffer (pH 7.4) supplemented with 0.1 mM ornithine plus either 1 mM (5-15N)glutamine or (2-15N)glutamine. To elucidate the role of glutamine cycling in net ammonia metabolism, a separate series of experiments were performed with 1 mM unlabeled glutamine plus 1 mM (15N)H4Cl. Net glutamine utilization was significantly lower in hepatocytes obtained from chronically acidotic rats compared with control or alkalotic rats. The sum of the rates of 15NH3 and (15N)urea production from (5-15N)glutamine was decreased in acidosis compared with alkalosis. After incubations of 50 min, approximately 75, 65, or 90% of the N in carbamoyl-phosphate was derived from the 5-N of glutamine in control, acidosis, or alkalosis respectively. In experiments with (2-15N)glutamine, the production of singly and doubly labeled (15N)urea as well as (15N)aspartate and (15N)H3 was significantly smaller in acidosis compared with alkalosis. Furthermore, a correlation was observed between production rates of (15N)aspartate and (15N)urea, suggesting that alterations in urea production may depend on aspartate formed from glutamine. However, the production of (15N)alanine was higher in acidosis compared with alkalosis with apparent correlation between the production of (15N)alanine and 2-oxoglutaramate, a product of the glutamine aminotransferase pathway. In addition, the rate of glutamine recycling was significantly higher in acidosis compared with control or alkalosis, indicating that both flux through glutamine aminotransferase and flux through glutamine synthetase were elevated in acidosis compared with alkalosis. These data suggest that decreased formation of aspartate from glutamine may limit ureagenesis in chronic metabolic acidosis. The formation of aspartate may depend on the availability of oxaloacetate rather than diminished flux through transaminase reaction. The enhancement of alanine production and glutamine synthesis may provide an alternate route of N disposal in cases of diminished urea formation.
Acid-Base Equilibrium↗
An update on the role of brain glutamine synthesis and its relation to cell-specific energy metabolism in the hyperammonemic brain: further studies using NMR spectroscopy.
Ammonia is a neurotoxin that is implicated in the pathogenesis of hepatic encephalopathy due to acute and chronic liver failure. However, its relation to neurological damage and brain edema is poorly understood. During the last decades, it has been the prevailing hypothesis that an osmotic disturbance induced by the astrocytic accumulation of glutamine leads to brain edema. However, various findings are at variance with this hypothesis. The present review will discuss: (a) correlation of ammonia with encephalopathy and brain edema in HE; (b) glutamine synthesis and astrocyte swelling; (c) glutamine synthesis and the glutamine-cycle: relation to brain energy metabolism; (d) glutamine synthesis and the glutamate-glutamine cycle and its relation to anaplerotic activity; (e) evidence favouring the "glutamine hypothesis"; (f) evidence contradicting the "glutamine hypothesis"; (g) glutamine synthesis and osmoregulation; (h) glutamine synthesis in chronic liver failure; (i) impaired brain energy metabolism in acute liver failure (ALF) and its relation to astrocytic glutamine synthesis. Taken together, the precise role of glutamine in the development of brain edema in ALF remains unclear. Astrocytic changes due to glutamine accumulation may lead secondarily to effects on brain energy metabolism. However, the relation between impaired energy metabolism and glutamine accumulation has not been well established. It is noteworthy that no single biochemical factor appears to be responsible for the many symptoms of HE. For example, brain glutamine accumulation and low-grade brain edema occur in chronic liver failure (CLF) suggesting common mechanisms are responsible for the neurological dysfunction in CLF and ALF. Recent NMR spectroscopic studies have provided considerably new information in this area. Future NMR studies using the stable isotope 13C may be useful in the study of the dynamics of brain metabolism in patients with ALF so as to better elucidate the precise role of glutamine accumulation and of glutamine-independent components to brain edema in ALF.
Animals↗
Metabolism of glutamine and glutamic acid by isolated perfused kidneys of normal and acidotic rats.
1. When isolated kidneys from fed rats were perfused with glutamine the rate of ammonia release at pH7.4 (110-360mumol/h per g dry wt.) was one to two times that of glutamine removal. Glucose formation from 5mm-glutamine was 16mumol/h per g. If kidneys were perfused with glutamine at pH7.1 (10-13mm-sodium bicarbonate) there was no increase in glutamine removal or in the formation of ammonia or glucose. 2. When isolated kidneys from fed rats were perfused with glutamate at pH7.4, glucose formation was 59mumol/h per g, glutamine formation was 182mumol/h per g and ammonia release was negligible. At pH7.1 glutamine synthesis was inhibited and formation of ammonia and glucose were increased. 3. In perfused kidneys from acidotic rats, which had received 1.5% (w/v) NH(4)Cl to drink for 7-10 days, gluconeogenesis from glutamine was enhanced (101mumol/h per g). Glutamine removal and ammonia formation were also increased, compared with the rates in perfused kidney from normal rats. The extra glutamine consumed was equivalent to the extra glucose formed. 4. When the kidney from the 7-10-day-acidotic rat was perfused with glutamate gluconeogenesis was increased (113mumol/h per g). Synthesis of glutamine was decreased, and ammonia release was approximately equal to the rate of glutamate removal. 5. The time-course of these metabolic alterations was investigated after the rapid induction of acidosis by infusion of 0.25m-HCl into the right side of the heart. The increase in gluconeogenesis from glutamine developed gradually over several hours. When kidneys from 6h-acidotic rats were perfused with glutamate, formation of glucose and glutamine were both rapid. 6. In acidotic rat kidneys perfused with glutamine, tissue concentrations of glutamate and glucose 6-phosphate were increased compared with those in control perfused kidneys from non-acidotic rats. 7. The results are discussed in terms of control of the renal metabolism of glutamine. In particular, it is suggested that in acidotic rats glucose formation is the major fate of the carbon of the extra glutamine utilized by the kidney, and that inhibition of glutamine synthetase could contribute to the increase in intracellular ammonia concentration in the kidney.
Acidosis↗
Glutamate, glutamine, aspartate, asparagine, glucose and ketone-body metabolism in chick intestinal brush-border cells.
1. Suspensions of isolated chick jejunal columnar absorptive (brush-border) cells respired on endogenous substrates at a rate 40% higher than that shown by rat brush-border cells. 2. Added d-glucose (5 or 10mm), l-glutamine (2.5mm) and l-glutamate (2.5mm) were the only individual substrates which stimulated respiration by chick cells; l-aspartate (2.5 or 6.7mm), glutamate (6.7mm), glutamine (6.7mm), l-alanine (1 or 10mm), pyruvate (1 or 2mm), l-lactate (5 or 10mm), butyrate (10mm) and oleate (1mm) did not stimulate chick cell respiration; l-asparagine (6.7mm) inhibited slightly; glucose (5mm) stimulated more than did 10mm-glucose. 3. Acetoacetate (10mm) and d-3-hydroxybutyrate (10mm) were rapidly consumed but, in contrast to rat brush-border cells, did not stimulate respiration. 4. Glucose (10mm) was consumed more slowly than 5mm-glucose; the dominant product of glucose metabolism during vigorous respiration was lactate; the proportion of glucose converted to lactate was greater with 10mm- than with 5mm-glucose. 5. Glutamate and aspartate consumption rates decreased, and alanine and glutamine consumption rates increased when their initial concentrations were raised from 2.5 to 6.7 or 10mm. 6. The metabolic fate of glucose was little affected by concomitant metabolism of any one of aspartate, glutamate or glutamine except for an increased production of alanine; the glucose-stimulated respiration rate was unaffected by concomitant metabolism of these individual amino acids. 7. Chick cells produced very little alanine from aspartate and, in contrast to rat cells, likewise produced very little alanine from glutamate or glutamine; in chick cells alanine appeared to be predominantly a product of transmination of pyruvate derived from glucose metabolism. 8. In chick cells, glutamate and glutamine were formed from aspartate (2.5 or 6.7mm); aspartate and glutamine were formed from glutamate (2.5mm) but only aspartate from 6.7mm-glutamate; glutamate was the dominant product formed from glutamine (6.7mm) but aspartate only was formed from 2.5mm-glutamine. 9. Chick brush-border cells can thus both catabolize and synthesize glutamine; glutamine synthesis is always diminished by concomitant metabolism of glucose, presumably by allosteric inhibition of glutamine synthetase by alanine. 10. Proline was formed from glutamine (2.5mm) but not from glutamine (2.5mm)+glucose (5mm) and not from 2.5mm-glutamate; ornithine was formed from glutamine (2.5mm)+glucose (5.0mm) but not from glutamine alone; serine was formed from glutamine (2.5mm)+glucose (5mm) and from these two substrates plus aspartate (2.5mm). 11. Total intracellular adenine nucleotides (22mumol/g dry wt.) remained unchanged during incubation of chick cells with glucose. 12. Intracellular glutathione (0.7-0.8mm) was depleted by 40% during incubation of respiring chick cells without added substrates for 75min at 37 degrees C; partial restoration of the lost glutathione was achieved by incubating cells with l-glutamate+l-cysteine+glycine.
Adenine Nucleotides↗
Effects in vivo of decreased plasma and intracellular muscle glutamine concentration on whole-body and hindquarter protein kinetics in rats.
Glutamine is considered to be a 'conditionally' essential amino acid. During situations of severe stress like sepsis or after trauma there is a fall in plasma glutamine levels, enhanced glutamine turnover and intracellular muscle glutamine depletion. Under these conditions, decreased intramuscular glutamine concentration correlates with reduced rates of protein synthesis. It has therefore been hypothesized that intracellular muscle glutamine levels have a regulatory role in muscle protein turnover rates. Administration of the glutamine synthetase inhibitor methionine sulphoximine (MSO) was used to decrease glutamine levels in male Wistar rats. Immediately after the MSO treatment (t=0 h), and at t=6 h and t=12 h, rats received intraperitoneal injections (10 ml/100 g body weight) with glutamine (200 mM) to test whether this attenuated the fall in plasma and intracellular muscle glutamine. Control animals received alanine and saline after MSO treatment, while saline was also given to a group of normal rats. At t=18 h rats received a primed constant infusion of L-[2,6-3H]phenylalanine. A three-pool compartment tracer model was used to measure whole-body protein turnover and muscle protein kinetics. Administration of MSO resulted in a 40% decrease in plasma glutamine and a 60% decrease in intracellular muscle glutamine, both of which were successfully attenuated by glutamine infusions. The decreased intracellular muscle glutamine levels had no effect on whole-body protein turnover or muscle protein kinetics. Also, glutamine supplementation did not alter these parameters. Alanine supplementation increased both hindquarter protein synthesis and breakdown but the net balance of phenylalanine remained unchanged. In conclusion, our results show that decreased plasma and muscle glutamine levels have no effect on whole-body protein turnover or muscle protein kinetics. Therefore, it is unlikely that, in vivo, the intracellular muscle concentration of glutamine is a major regulating factor in muscle protein kinetics.
Amino Acids↗
Effect of growth hormone on muscle and liver protein synthesis in septic rats receiving glutamine-enriched parenteral nutrition.
OBJECTIVE: Administration of recombinant human growth hormone (rhGH) to critically ill adults in an attempt to attenuate catabolism was associated with increased morbidity and mortality. Possible explanations included inhibition of glutamine release from skeletal muscle and consequent restriction of splanchnic glutamine supply. In this study, we examined the effects of rhGH on plasma glutamine levels and on muscle and liver glutamine concentrations and protein synthesis rates in sepsis. We investigated the possibility that administration of supplemental glutamine might ameliorate any adverse effects of rhGH. DESIGN: Prospective study in rats rendered septic by cecal ligation and puncture. SETTING: University hospital laboratory. SUBJECTS: A total of 78 male Wistar rats in six groups. INTERVENTIONS: Animals received 6-hr tail vein infusions, commencing 18 hrs after cecal ligation and puncture, of either (a) 0.9% sodium chloride, (b) a standard parenteral nutrition (PN) solution without glutamine, or (c) an isocaloric, isonitrogenous PN solution with glutamine. PN groups received 400 microg rhGH or equivolume 0.9% sodium chloride vehicle in a divided subcutaneous and intravenous dose at PN commencement. Sacrifice was at the end of the infusion period. A further group was unoperated and uninfused and killed at 24 hrs as baseline controls. MEASUREMENTS AND MAIN RESULTS: Glutamine concentrations were measured by fluorometry. Protein synthesis in muscle and liver was measured by a "flooding-dose" technique employing L-[4-H]phenylalanine. Plasma glutamine was increased after cecal ligation and puncture except in the saline and glutamine with rhGH animals. Muscle glutamine was reduced after cecal ligation and puncture and was significantly lower in animals receiving standard PN with rhGH vs. saline alone. Liver glutamine was increased in animals receiving saline and those receiving standard PN with rhGH. PN, with or without glutamine, increased muscle protein synthesis, and the administration of rhGH tended to further increase this effect. Neither PN, glutamine, nor rhGH had an effect on the increased liver protein synthesis characteristic of sepsis. CONCLUSIONS: In sepsis, increased muscle protein synthesis with PN and rhGH administration is not associated with increased muscle glutamine levels. Administration of rhGH does not result in reduced liver glutamine levels or rates of hepatic protein synthesis. PN containing glutamine was no more efficacious than standard PN at increasing muscle protein synthesis.
Animals↗