Glutamine Synthetase Activity In Muscle Research Articles

Ornithine phenylacetate targets alterations in the expression and activity of glutamine synthase and glutaminase to reduce ammonia levels in bile duct ligated rats

In liver failure, ammonia homeostasis is dependent upon the function of the ammonia metabolising enzymes, glutamine synthetase (GS) and glutaminase (GA) but data about their protein expression and activity are lacking. The aims of this study were to determine the protein expression and activity of GS and GA in individual organs in a rat model of chronic liver disease and to test whether the treatment with the ammonia-lowering agent ornithine phenylacetate (OP) modulates their activities. 49 SD rats were studied 35 days after sham-operation or bile duct ligation (BDL). The BDL group received: L-ornithine (0.6 mg/kg/day), Phenylacetate (0.6 mg/kg/day), OP (0.6 mg/kg/day) or placebo (saline) for 5 days prior to sacrifice. Arterial ammonia, amino acids and liver biochemistry were measured. Expressions of GS and GA were determined by Western-blotting and activities by end-point methods in liver, muscle, gut, kidney, lung, and frontal cortex. In BDL rats, hepatic GS enzyme activity was reduced by more than 80% compared to sham rats. Further, in BDL rats GA activity was reduced in liver but increased in the gut, muscle and frontal cortex compared to sham rats. OP treatment resulted in a reduction in hyperammonemia in BDL rats, associated with increased GS activity in the muscle and reduced gut GA activity. In a rat model of chronic liver failure, hyperammonemia is associated with inadequate compensation by liver and muscle GS activity and increased gut GA activity. OP reduces plasma ammonia by increasing GS in the muscle and reducing GA activity in the gut providing additional insights into its mechanism of its action. GS and GA may serve as important future therapeutic targets for hyperammonemia in liver failure.

Effect of intermittent glutamine supplementation on skeletal muscle is not long-lasting in very old rats.

Muscle is the major site for glutamine synthesis via glutamine synthetase (GS). This enzyme is increased 1.5-2 fold in 25-27-mo rats and may be a consequence of aging-induced stress. This stimulation is similar to the induction observed following a catabolic state such as glucocorticoid treatment (6 to 24 months). Although oral glutamine supply regulates the plasma glutamine level, nothing is known if this supplementation is interrupted before the experiment. Adult (8-mo) and very old (27-mo) female rats were exposed to intermittent glutamine supplementation for 50 % of their age lifetime. Treated rats received glutamine added to their drinking water and control rats water alone but the effect of glutamine supplementation was only studied 15 days after the last supplementation. Glutamine pretreatment discontinued 15 days before the experiment increased plasma glutamine to ~ 0.6 mM, a normal value in very old rats. However, it failed to decrease the up-regulated GS activity in skeletal muscle from very old rats. Our results suggest that long-term treatment with glutamine started before advanced age but discontinued 15 days before rat sacrifice is effective in increasing plasma glutamine to recover basal adult value and in maintaining plasma glutamine in very old rats, but has no long-lasting effect on the GS activity of skeletal muscle with advanced age.

P57 Administration of branched chain amino acids to hyperammonemic cirrhotic animals is associated with worsening of hyperammonemia and increase in brain oedema

IntroductionDespite branched chain amino acids (BCAA) being used for over 30 years, its efficacy in treating hyperammonemia remains controversial. Recent studies suggest that the use of BCAA in cirrhosis is...

The freshwater Amazonian stingray, Potamotrygon motoro, up-regulates glutamine synthetase activity and protein abundance, and accumulates glutamine when exposed to brackish (15‰) water

This study aimed to examine whether the stenohaline freshwater stingray, Potamotrygon motoro, which lacks a functional ornithine-urea cycle, would up-regulate glutamine synthetase (GS) activity and protein abundance, and accumulate glutamine during a progressive transfer from freshwater to brackish (15 per thousand) water with daily feeding. Our results revealed that, similar to other freshwater teleosts, P. motoro performed hyperosmotic regulation, with very low urea concentrations in plasma and tissues, in freshwater. In 15 per thousand water, it was non-ureotelic and non-ureoosmotic, acting mainly as an osmoconformer with its plasma osmolality, [Na+] and [Cl-] comparable to those of the external medium. There were significant increases in the content of several free amino acids (FAAs), including glutamate, glutamine and glycine, in muscle and liver, but not in plasma, indicating that FAAs could contribute in part to cell volume regulation. Furthermore, exposure of P. motoro to 15 per thousand water led to up-regulation of GS activity and protein abundance in both liver and muscle. Thus, our results indicate for the first time that, despite the inability to synthesize urea and the lack of functional carbamoyl phosphate synthetase III (CPS III) which uses glutamine as a substrate, P. motoro retained the capacity to up-regulate the activity and protein expression of GS in response to salinity stress. Potamotrygon motoro was not nitrogen (N) limited when exposed to 15 per thousand water with feeding, and there were no significant changes in the amination and deamination activities of hepatic glutamate dehydrogenase. In contrast, P. motoro became N limited when exposed to 10 per thousand water with fasting and could not survive well in 15 per thousand water without food.

Fasting up-regulates muscle glutamine synthetase while it down-regulates liver glutamine synthetase in male rats during aging

Summary Background & aims Muscle is an important site of Gln synthesis and release in the whole body, while liver can take up circulating Gln even if it has the capacity to both degrade and synthesize Gln. During fasting, skeletal muscle in rats exports large amounts of Gln but also increases Gln production by glutamine synthetase (GS) to maintain its Gln pool, whatever the age. The aim of this study was to determine whether fasting induced GS activity in muscle may have consequences on liver Gln homeostasis during aging (by changing GS and glutaminase (GA) activities). Methods Liver GS and GA activities were assessed in fed and 5 day fasted male Wistar rats during aging (2, 5, 15, 22 and 24 months). To compare potential cooperation between liver and muscle, GS activity was also measured in skeletal muscle. Results After 5 days fasting, both total GS and GA activities in liver were decreased irrespective of age. As expected, muscle GS activity was increased by a factor of 1.5–2 in 2 to 24 month rats. In addition, 24 month fed and fasted rats had the same GS response. In contrast, in the fed state, both age and nutritional state played a role in the response of GA activity. Conclusion Five day fasting induced an adaptation of Gln metabolism in male Wistar rats, in relation to aging. Muscle and liver synergistically played a role in this regulation. The specific role of Gln in satisfying increased requirements in very old rats was confirmed.

Dietary Glutamine Supplementation Increases the Activity of Peritoneal Macrophages and Hemopoiesis in Early-Weaned Mice Inoculated with Mycobacteriumbovis Bacillus Calmette-Guérin

Infants who are breast-fed have been shown to have a lower incidence of certain infectious diseases compared with formula-fed infants. Glutamine is one of the most abundant amino acids found in maternal milk and it is essential for the function of immune system cells such as macrophages. The purpose of this study was to investigate the effect of glutamine supplementation on the function of peritoneal macrophages and on hemopoiesis in early-weaned mice inoculated with Mycobacterium bovis bacillus Calmette-Guérin (BCG). Mice were weaned at 14 d of age and distributed to 2 groups and fed either a glutamine-free diet (n = 16) or a glutamine-supplemented diet (+Gln) (n = 16). Both diets were isonitrogenous (with addition of a mixture of nonessential amino acids) and isocaloric. At d 21, 2 subgroups of mice (n = 16) were intraperitoneally injected with BCG and all mice were killed at d 28. Plasma, muscle and liver glutamine concentrations and muscle glutamine synthetase activity were not affected by diet or inoculation with BCG. The +Gln diet led to increased leukocyte and lymphocyte counts in the peripheral blood (P < 0.05) and granulocyte and lymphocyte counts in the bone marrow and spleen (P < 0.05). The +Gln diet increased spreading and adhesion capacities, hydrogen peroxide, nitric oxide, and tumor necrosis factor-α (TNF α) syntheses and the phagocytic and fungicidal activity of peritoneal macrophages (P < 0.05). The interaction between the +Gln diet and BCG inoculation increased the area under the curve of interleukin (IL)-1 β and TNF α syntheses (P < 0.05). In conclusion, the intake of glutamine increases the function of peritoneal macrophages and hemopoiesis in early-weaned and BCG-inoculated mice. These data have important implications for the design of breast milk substitutes for human infants.

Alterations in glutamine synthetase activity in rat skeletal muscle are associated with advanced age

Objective Glutamine synthetase (GS), a key enzyme in the production of glutamine, is preserved in skeletal muscle during early aging (<24 mo). Because the effects of advanced age on GS are unknown, we investigated the effect of advanced age (>24 mo) on GS activity in skeletal muscle. We hypothesized that advanced age would enhance muscle GS activity. Methods Muscle GS activities were assessed in adult (8 mo), mature adult (15 mo), aged (20–22 mo), advanced age (25–27 mo), or very advanced age (29–32 mo) female Wistar rats. Male Wistar (6–27 mo) were used to investigate the effect of gender on this activity. Results Glutamine synthetase activity remained low and unaltered in rats from 8 to 22 mo of age, as previously demonstrated. In contrast, GS activity was high (∼75% of individual values were higher than the low value mean) in 25-mo to 27-mo-old rats. In very-old-aged rats (29–32 mo), ∼55% of GS activity data points exhibited low values. Changes in GS protein content paralleled those in GS activities. In male rats, GS activity was also high (∼80% of individual values were higher than the mean value of 6-mo to 19-mo-old rats) at the upper limit of life expectancy (27 mo). Conclusion There is enhanced GS activity in old female and male rats suggesting a greater need for glutamine. In some very old rats, low GS activity may be associated with longevity or reflect a limitation in glutamine production due to extremely advanced age per se.

Cellular concentrations of glutamine synthetase in murine organs

Glutamine synthetase (GS) is the only enzyme that can synthesize glutamine, but it also functions to detoxify glutamate and ammonia. Organs with high cellular concentrations of GS appear to function primarily to remove glutamate or ammonia, whereas those with a low cellular concentration appear to primarily produce glutamine. To validate this apparent dichotomy and to clarify its regulation, we determined the GS concentrations in 18 organs of the mouse. There was a >100-fold difference in GS mRNA, protein, and enzyme-activity levels among organs, whereas there was only a 20-fold difference in the GS protein:mRNA ratio, suggesting extensive transcriptional and posttranscriptional regulation. In contrast, only small differences in the GS enzyme activity : protein ratio were found, indicating that posttranslational regulation is of minor importance. The cellular concentration of GS was determined by relating the relative differences in cellular GS concentration, detected using image analysis of immunohistochemically stained tissue sections, to the biochemical data. There was a >1000-fold difference in cellular concentrations of GS between GS-positive cells in different organs, and cellular concentrations were up to 20x higher in subpopulations of cells within organs than in whole organs. GS activity was highest in pericentral hepatocytes (approximately 485 micromol.g(-1).min-(1), followed in descending order by epithelial cells in the epididymal head, Leydig cells in the testicular interstitium, epithelial cells of the uterine tube, acid-producing parietal cells in the stomach, epithelial cells of the S3 segment of the proximal convoluted tubule of the kidney, astrocytes of the central nervous tissue, and adipose tissue. GS activity in muscle amounted to only 0.4 micromol.g(-1).min(-1). Our findings confirmed the postulated dichotomy between cellular concentration and GS function.

Does acute glutamine depletion enhance the response of glutamine synthesis to fasting in muscle in adult and old rats?

In earlier studies, skeletal muscle glutamine synthetase (GS) activity was shown to be enhanced by fasting and glucocorticoids, and inhibited by exogenous glutamine (Gln) supplementation. The current study was designed to determine whether phenylbutyrate (PhiB), a Gln-chelating agent in humans, (1) could trap Gln and produce a decline in plasma Gln in rats, as it does in humans, and (2) if so, whether (Phi)B would further enhance the response of muscle GS activity to fasting in rats. Adult (6-8 months) and aged (20-21 months) rats were fasted for 5 days and received two doses of 0.5 g(Phi)Bby orogastric route at times 0 and 4 h, and were then sacrificed at 5.5 h. Plasma Gln was measured by enzymatic methods, other amino acids were quantified by amino acid analysis. GS activity was measured in soleus (SO) and tibialis anterior (TA) muscles. (Phi)B treatment was associated with: (1) a 20% decline in plasma Gln concentration from 572+/-54 to 424+/-34 micromol/L (P<0.05) and from 476+/-49 to 360+/-80 micromol/L (P<0.05) in fasted adult and old rats, respectively; and (2) a preservation of GS up-regulation by fasting in TA and SO muscles in both adult and aged rats, with TA muscle GS activities of 198+/-65 vs. 203+/-68 ((Phi)B-treated vs. vehicle-treated, NS), and 244+/-81 vs. 274+/-59 (NS) nmol/h/mg protein in adult and aged rats, respectively. These data suggest that: (1) large doses of (Phi)B deplete plasma Gln in fasted rats, regardless of age, (2) Gln depletion induced by Phi)B does not alter GS activity.

Metabolic pathways implicated in the kinetic impairment of muscle glutamine homeostasis in adult and old glucocorticoid-treated rats.

An impairment of muscle glutamine metabolism in response to dexamethasone (DEX) occurs with aging. To better characterize this alteration, we have investigated muscle glutamine release with regard to muscle glutamine production (net protein breakdown, de novo glutamine synthesis) in adult and old glucocorticoid-treated rats. Male Sprague-Dawley rats (3 or 24 mo old) were divided into seven groups: three groups received 1.5 mg/kg of DEX once a day by intraperitoneal injection for 3, 5, or 7 days; three groups were pair fed to the three treated groups, respectively; and one control group of healthy rats was fed ad libitum. Muscle glutamine synthetase activity increased earlier in old rats (day 3) than in adult rats (day 7), whereas an increase in muscle glutamine release occurred later in old rats (day 5) than in adult DEX-treated rats (day 3). Consequently, muscle glutamine concentration decreased later in old rats (day 5) than in adults (day 3). Finally, net muscle protein breakdown increased only in old DEX-treated rats (day 7). In conclusion, the impairment of muscle glutamine metabolism is due to a combination of an increase in glutamine production and a delayed increase in glutamine release.

The role of adrenal hormones in the response of glutamine synthetase to fasting in adult and old rats

Background & Aims: During fasting, skeletal muscle exports increased amounts of glutamine (Gln) while increasing the production of this amino acid by glutamine synthetase (GS) in order to maintain the intramuscular Gln pool. Glucocorticoid hormones are believed to be the principal mediators of GS induction during stress conditions. The aim of this study was to evaluate (1) the effect of fasting on GS activity and expression in skeletal muscle during aging and consequently, (2) the role of glucocorticoids in fasting-induced GS activity. Methods: Male Wistar rats (6-, 22-month old) were fasted for 5 days and both the activity and expression of GS were measured in tibialis anterior muscle. To better demonstrate the role of glucocorticoids in the response of GS to fasting, we suppressed their action by RU38486 administration (a potent glucocorticoid antagonist) and their production by adrenalectomy in fed and fasted rats. Results: An increase in fasting-induced GS activity was observed in skeletal muscles from both adult and aged rats. Adrenalectomy, but surprisingly not RU38486, suppressed the fasting-induced increase in GS activity and expression. Conclusion: The data clearly show that the GS responsiveness to fasting was not modified by aging in skeletal muscle.

Do sex steroids regulate glutamine synthesis with age?

Glutamine synthetase, a key enzyme in the production of glutamine, is known to be induced by glucocorticoids and preserved in skeletal muscle during aging, but the effect of other steroids, such as sex steroids (progesterone, estradiol), is unknown in vivo. The aim of this study was to determine whether progesterone or estradiol plays a role in the regulation of glutamine synthetase (GS) with aging. The effects of glucocorticoids and sex steroids on muscle GS activity and mRNA expression were measured in adult (6-8 mo; n = 7 in each group) and aged (26 mo; n = 10 in each group) female Wistar rats after adrenalectomy (ADX), ovariectomy (OV), or both (ADXOV) and were compared with those in sham-operated (Sham) control rats. In tibialis anterior muscle, ADX noticeably decreased both GS activity and expression irrespective of age (50-60%; P < 0.05), whereas OV had no effect at either age. Progesterone and estradiol replacement had no effect on the recovery of muscle GS response in either ADX or OV rats, regardless of age. In contrast, heart GS activity was decreased by ADX in aged animals only. These results suggest that the reproductive endocrine status of female rats does not affect muscle GS activity either in muscle or in heart, in young or aged animals, and that the heart GS response to steroids may be differently regulated in aged rats.

Dietary glutamine suppresses endogenous glutamine turnover in the rat

Plasma glutamine turnover was determined using 1-14C-labeled glutamine in rats that consumed crystalline amino acid diets containing the equivalent of 16% protein with 25% of the amino acids as glutamine or a control diet containing no glutamine (or glutamate) for 10 days. Glutamine turnover in glutamine-fed animals was 66% of the rate in the control group. Glutamine feeding caused 20% higher levels of arterial plasma glutamine. Arterial-portal differences across the portal-drained viscera showed net glutamine uptake in control animals but no net uptake or release in the glutamine-fed group. Skeletal muscle glutamine synthetase activity was similar in both groups. The results indicate that long-term consumption of relatively large amounts of dietary glutamine decreases the turnover of plasma glutamine and thus reduces the need for endogenous glutamine synthesis.

Changes in the activity levels of glutamine synthetase, glutaminase and glycogen synthetase in rats subjected to hypoxic stress.

Exposure to high altitude causes loss of body mass and alterations in metabolic processes, especially carbohydrate and protein metabolism. The present study was conducted to elucidate the role of glutamine synthetase, glutaminase and glycogen synthetase under conditions of chronic intermittent hypoxia. Four groups, each consisting of 12 male albino rats (Wistar strain), were exposed to a simulated altitude of 7620 m in a hypobaric chamber for 6 h per day for 1, 7, 14 and 21 days, respectively. Blood haemoglobin, blood glucose, protein levels in the liver, muscle and plasma, glycogen content, and glutaminase, glutamine synthetase and glycogen synthetase activities in liver and muscle were determined in all groups of exposed and in a group of unexposed animals. Food intake and changes in body mass were also monitored. There was a significant reduction in body mass (28-30%) in hypoxia-exposed groups as compared to controls, with a corresponding decrease in food intake. There was rise in blood haemoglobin and plasma protein in response to acclimatization. Over a three-fold increase in liver glycogen content was observed following 1 day of hypoxic exposure (4.76 +/- 0.78 mg.g-1 wet tissue in normal unexposed rats; 15.82 +/- 2.30 mg.g-1 wet tissue in rats exposed to hypoxia for 1 day). This returned to normal in later stages of exposure. However, there was no change in glycogen synthetase activity except for a decrease in the 21-days hypoxia-exposed group. There was a slight increase in muscle glycogen content in the 1-day exposed group which declined significantly by 56.5, 50.6 and 42% following 7, 14, and 21 days of exposure, respectively. Muscle glycogen synthetase activity was also decreased following 21 days of exposure. There was an increase in glutaminase activity in the liver and muscle in the 7-, 14- and 21-day exposed groups. Glutamine synthetase activity was higher in the liver in 7- and 14-day exposed groups; this returned to normal following 21 days of exposure. Glutamine synthetase activity in muscle was significantly higher in the 14-day exposed group (4.32 mumol gamma-glutamyl hydroxamate formed.g protein-1.min-1) in comparison to normal (1.53 mumol gamma-glutamyl hydroxamate formed.g protein-1.min-1); this parameter had decreased by 40% following 21 days of exposure. These results suggest that since no dramatic changes in the levels of protein were observed in the muscle and liver, there is an alteration in glutaminase and glutamine synthetase activity in order to maintain nitrogen metabolism in the initial phase of hypoxic exposure.

Measurement of glutamine synthetase activity in rat muscle by a colorimetric assay

Glutamine synthetase catalyses the formation of l-Gln from l-Glu and NH 4 +. This enzyme also exerts a glutamyl-transferase activity that produces γ-glutamyl-hydroxamate from Gln and hydroxylamine. This γ-glutamyl-transfer reaction can be used to determine glutamine synthetase activity by colorimetric assay. This method has never been applied to rat muscle. The aim of this work was to study and optimize the glutamine synthetase assay conditions in rat muscle. Enzyme activity was linear with time of incubation (30 min at 37°C) and linear with enzyme concentration in the incubation medium. The method was specific. In addition, this assay correlated well with a radiometric assay ( y=0.76 x+340, where x and y are the glutamine synthetase activities measured by radiometry and colorimetry respectively; r=0.94; P=0.05). Finally, no glutamine synthetase activity was found in muscles of rats treated with methionine sulfoximine, an inhibitor of glutamine synthetase, and activity dramatically rose in muscles from rats treated with dexamethasone, an activator of glutamine synthetase (in extensor digitorum longus: 2717±54 nmol/min/g protein in dexamethasone-treated rats versus 1228±114 nmol/min/g protein in control rats, P<0.0001). In conclusion, the method presented here is accurate and reliable for measurement of glutamine synthetase activity in muscles.

Effects of glutamine supplementation during inhibited muscle glutamine synthetase activity on protein turnover

Effects of glutamine supplementation during inhibited muscle glutamine synthetase activity on protein turnover

Influence of Progressive Tumor Growth on Glutamine Metabolism in Skeletal Muscle and Kidney

The effects of progressive malignant growth on glutamine metabolism in skeletal muscle and in kidney were investigated. Fast-growing tumors consume considerable quantities of glutamine and lead to a decrease in circulating glutamine concentrations. Experiments were performed at various stages of tumor growth in rats implanted subcutaneously with the non-metastasizing methylcholanthrene-induced (MCA) fibrosarcoma and in pair-fed non tumor-bearing controls. Tumor growth stimulated a twofold increase in hindquarter (muscle) glutamine release, which was not due to an increase in blood flow, but rather to a doubling in the fractional release rate. Consequently, a progressive decrease in skeletal muscle glutamine concentrations was observed over time. Simultaneously, the activity of glutamine synthetase (GS), the principal enzyme of de novo glutamine biosynthesis, increased more than twofold. This increase in muscle GS activity was accompanied by an increase in GS mRNA but the augmentation in GS expression apparently could not match the increased rate of efflux since muscle depletion developed. In rats with large tumors and severe glutamine depletion, GS activity was not elevated. Glutamine feeding increased muscle glutamine concentrations and glutamine synthetase specific activity. Although tumor growth led to the development of mild systemic acidemia, the classic renal adaptations normally observed, i.e., elevated glutaminase activity and accelerated renal glutamine utilization, were not present in acidotic tumor-bearing rats. Instead, renal GS activity was increased in tumor-bearing animals and ammoniagenesis was enhanced, in spite of a reduction in net renal glutamine uptake. These data suggest that marked alterations in muscle and renal glutamine handling occur in the host with cancer; the enhanced muscle glutamine release in conjunction with no increase in renal consumption is consistent with increased glutamine uptake in other organs, most likely the tumor itself and the liver.

Skeletal muscle glucocorticoid receptor and glutamine synthetase activity in the wasting syndrome in rats treated with 2,3,7,8-tetrachlorodibenzo- p-dioxin

This study demonstrated specific changes in rat skeletal muscles after a single oral dose (100 μg/kg) of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD). The development of the wasting syndrome was characterized by marked body weight loss, as well as atrophy of plantaris and gastrocnemius muscles. Fourteen days after administration of TCDD, gastrocnemius muscle cytosolic glucocorticoid receptor binding, measured at a single saturating concentration of [ 3H]triamcinolone acetonide, was significantly diminished, while plantaris muscle glutamine synthetase activity was strikingly elevated, indicating that specific biochemical alterations occur in skeletal muscle in the wasting syndrome.

28] Glutamine synthetase from muscle

The assay for glutamine synthetase activity in muscle measures the rate of synthesis of radioactive glutamine from radioactive glutamate. In muscle, glutamine synthetase plays a biosynthetic role in maintaining the availability of glutamine as a source of nitrogen and as a respiratory or gluconeogenic substrate when dietary sources of nitrogen or carbon are limited. The sensitivity of the muscle enzymes to inhibition by ADP is probably of physiological significance in the in vivo regulation of glutamine metabolism. These enzymes have a significantly lower apparent Km for MgATP than do the enzymes from brain, liver, and spleen, which are less sensitive to inhibition by ADP. This suggests that an important mechanism of control of glutamine synthesis in muscle may be by the adenylate energy charge or by the ATP/ADP ratio in the cell.

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.