Glutamine as a pathogenic factor in hepatic encephalopathy

Hepatic and extrahepatic metabolic modulation in hbv-related decompensated cirrhosis and acute-on-chronic liver failure

Acute-on-chronic liver failure (ACLF) and decompensated cirrhosis (DC) are life-threatening syndromes that can develop at the end-stage of chronic hepatitis B virus (HBV) infection. Both ACLF and DC are complicated by hepatic and extrahepatic pathogeneses. To better understand the compartment-specific metabolic modulations related to their pathogenesis, HBV-DC, HBV-ACLF patients, and controls (30 each) were analyzed by metabolomics using portal (Port), hepatic vein (Hep), and peripheral (Peri) serum. Compartment ratios of metabolites (RatioHep/Port, RatioPeri/Hep, and RatioPort/Peri) were calculated. The liver tissues (10 per group) were analyzed using transcriptomics and metabolomics. An additional 75 patients with ACLF, 20 with DC, and 20 with liver cirrhosis (LC) were used to confirm oxlipid dysregulation. Both multi-omics datasets suggest suppressed energy, amino acid, and pyrimidine metabolism in the ACLF/DC liver. The serum metabolomic variations were contributed primarily by disease rather than sampling compartments, as both HBV-ACLF and HBV-DC patients demonstrated abnormal profiles of amino acids and peptides, indoles, purines, steroids, and benzimidazoles. In ACLF/DC patients, impaired hepatic metabolism resulted in a highly correlated hepatic and portal vein serum metabolome and release of inflammatory lipids and heme metabolites from the liver. HBV-ACLF showed higher RatioPeri/Hep of extrahepatic inflammatory oxlipids, while HBV-DC patients showed higher RatioPort/Peri of gut microbial metabolites. An inflammatory oxlipid outburst was confirmed in the early stages of HBV-ACLF. The inflammatory effects of the selected oxlipids were confirmed in monocytes. These findings support a synergy between liver-specific mechanisms and systemic inflammation in ACLF/DC development, and that pro-inflammatory oxlipids are metabolic signatures of early HBV-ACLF.

Pattern Clustering of Symmetric Regional Cerebral Edema on Brain MRI in Patients with Hepatic Encephalopathy

Purpose

Metabolic abnormalities in hepatic encephalopathy (HE) cause brain edema or demyelinating disease, resulting in symmetric regional cerebral edema (SRCE) on MRI. This study aimed to investigate the usefulness of the clustering analysis of SRCE in predicting the development of brain failure.

Materials and Methods

MR findings and clinical data of 98 consecutive patients with HE were retrospectively analyzed. The correlation between the 12 regions of SRCE was calculated using the phi (Φ) coefficient, and the pattern was classified using hierarchical clustering using the φ2 distance measure and Ward's method. The classified patterns of SRCE were correlated with clinical parameters such as the model for end-stage liver disease (MELD) score and HE grade.

Results

Significant associations were found between 22 pairs of regions of interest, including the red nucleus and corpus callosum (Φ = 0.81, p < 0.001), crus cerebri and red nucleus (Φ = 0.72, p < 0.001), and red nucleus and dentate nucleus (Φ = 0.66, p < 0.001). After hierarchical clustering, 24 cases were classified into Group I, 35 into Group II, and 39 into Group III. Group III had a higher MELD score (p = 0.04) and HE grade (p = 0.002) than Group I.

Conclusion

Our study demonstrates that the SRCE patterns can be useful in predicting hepatic preservation and the occurrence of cerebral failure in HE.

Recomendaciones de manejo de la hiperamonemia en neonatos

La hiperamonemia se define como el aumento de las concentraciones de amonio en el plasma, de forma aguda o crónica. Frecuentemente, se presenta en diversos tipos de errores innatos del metabolismo, enfermedades que deben diagnosticarse y manejarse de manera inmediata y adecuada, debido a que el retraso en su manejo genera secuelas neurológicas graves y permanentes, así como desenlaces fatales. El objetivo del artículo es aportar herramientas al clínico para la sospecha, el abordaje diagnóstico y el manejo del recién nacido con hiperamonemia primaria, teniendo en cuenta la correlación entre fisiopatología, etiología, aproximación clínica y de laboratorio, así como recomendaciones de manejo farmacológico y no farmacológico.

Transporters at the Interface between Cytosolic and Mitochondrial Amino Acid Metabolism

Mitochondria are central organelles that coordinate a vast array of metabolic and biologic functions important for cellular health. Amino acids are intricately linked to the bioenergetic, biosynthetic, and homeostatic function of the mitochondrion and require specific transporters to facilitate their import, export, and exchange across the inner mitochondrial membrane. Here we review key cellular metabolic outputs of eukaryotic mitochondrial amino acid metabolism and discuss both known and unknown transporters involved. Furthermore, we discuss how utilization of compartmentalized amino acid metabolism functions in disease and physiological contexts. We examine how improved methods to study mitochondrial metabolism, define organelle metabolite composition, and visualize cellular gradients allow for a more comprehensive understanding of how transporters facilitate compartmentalized metabolism.

Uraemic Encephalopathy in a Persian Cat with Chronic Kidney Disease

Uraemic encephalopathy (UE) is rarely associated with acute kidney injury or chronic kidney disease in domestic animals, and we now report the first case in a cat. The animal presented with hypothermia, apathy, lethargy, depression, severe dehydration, uraemic breath, elevated serum urea nitrogen and creatine concentrations, and eventual seizures and coma prior to death. Gross necropsy findings included severe bilateral renal scarring, ulcerative stomatitis and glossitis, and uraemic gastropathy. Microscopic lesions of diffuse interstitial fibrosis, multifocal mineralization and lymphoplasmacytic interstitial nephritis were seen in the kidneys. There was symmetrical, bilateral spongy vacuolation of the white matter of the basal nuclei and cerebellum and Alzheimer type II astrocytes in the cerebral cortex and hippocampus. Glial fibrillary acid protein immunolabelling was absent or faint in astrocytes of the cerebral grey matter. UE should be included in the differential diagnosis in animals with chronic kidney disease and neurological signs.

Umbelliferone prevents oxidative stress, inflammation and hematological alterations, and modulates glutamate-nitric oxide-cGMP signaling in hyperammonemic rats

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication that occurs as a result of liver failure. Umbelliferone (UMB; 7-hydroxycoumarin) is a natural product with proven hepatoprotective activity; however, nothing has yet been reported on its protective effect against hyperammonemia, the main culprit behind the symptoms of HE. Here, we evaluated the effect of UMB against ammonium chloride (NH₄Cl)-induced hyperammonemia, oxidative stress, inflammation and hematological alterations in rats. We demonstrated the modulatory role of UMB on the glutamate-nitric oxide (NO)-cGMP pathways in the cerebrum of rats. Rats received intraperitoneal injections of NH₄Cl (3 times/week) for 8 weeks and concomitantly received 50 mg/kg UMB. NH₄Cl-induced rats showed significantly elevated blood ammonia and liver function markers. Lipid peroxidation and NO were increased in the liver and cerebrum of rats while the antioxidant defenses were declined. UMB significantly reduced blood ammonia, liver function markers, lipid peroxidation and NO, and enhanced the antioxidant defenses in NH₄Cl-induced rats. UMB significantly prevented anemia, leukocytosis, thrombocytopenia and prolongation of PT and aPTT. Hyperammonemic rats showed elevated levels of cerebral TNF-α, IL-1β and glutamine as well as increased activity and expression of Na⁺/K⁺-ATPase, effects that were significantly reversed by UMB. In addition, UMB down-regulated nitric oxide synthase and soluble guanylate cyclase in the cerebrum of hyperammonemic rats. In conclusion, this study provides evidence that UMB protects against hyperammonemia via attenuation of oxidative stress and inflammation. UMB prevents hyperammonemia associated hematological alterations and therefore represents a promising protective agent against the deleterious effects of excess ammonia.

Commiphora molmol Modulates Glutamate-Nitric Oxide-cGMP and Nrf2/ARE/HO-1 Pathways and Attenuates Oxidative Stress and Hematological Alterations in Hyperammonemic Rats

Hyperammonemia is a serious complication of liver disease and may lead to encephalopathy and death. This study investigated the effects of Commiphora molmol resin on oxidative stress, inflammation, and hematological alterations in ammonium chloride- (NH₄Cl-) induced hyperammonemic rats, with an emphasis on the glutamate-NO-cGMP and Nrf2/ARE/HO-1 signaling pathways. Rats received NH₄Cl and C. molmol for 8 weeks. NH₄Cl-induced rats showed significant increase in blood ammonia, liver function markers, and tumor necrosis factor-alpha (TNF-α). Concurrent supplementation of C. molmol significantly decreased circulating ammonia, liver function markers, and TNF-α in hyperammonemic rats. C. molmol suppressed lipid peroxidation and nitric oxide and enhanced the antioxidant defenses in the liver, kidney, and cerebrum of hyperammonemic rats. C. molmol significantly upregulated Nrf2 and HO-1 and decreased glutamine and nitric oxide synthase, soluble guanylate cyclase, and Na⁺/K⁺-ATPase expression in the cerebrum of NH₄Cl-induced hyperammonemic rats. Hyperammonemia was also associated with hematological and coagulation system alterations. These alterations were reversed by C. molmol. Our findings demonstrated that C. molmol attenuates ammonia-induced liver injury, oxidative stress, inflammation, and hematological alterations. This study points to the modulatory effect of C. molmol on glutamate-NO-cGMP and Nrf2/ARE/HO-1 pathways in hyperammonemia. Therefore, C. molmol might be a promising protective agent against hyperammonemia.

In vivo N-15 MRS study of glutamate metabolism in the rat brain

In vivo ¹⁵N MRS has made a unique contribution to kinetic studies of the individual pathways that control glutamate flux in the rat brain. This review covers the following topics: (1) the advantages and limitations of in vivo ¹⁵N MRS and its indirect detection through coupled ¹H; (2) kinetic methods; (3) major findings from our and other laboratories in the areas: (a) the uptake of the neurotransmitter glutamate from the extracellular fluid into glia; (b) the metabolism of glutamate to glutamine; (c) glutamine transport to the extracellular fluid; (d) hydrolysis of neuronal glutamine to glutamate; and (e) contribution of transamination from leucine to replenish the glutamate nitrogen. In vivo glutamine synthetase activities measured at several levels of hyperammonemia showed that this enzyme becomes saturated at blood ammonia concentration >0.9 μmol/g, and causes the elevation of brain ammonia. Implications of the results for the cause of hyperammonemic encephalopathy are discussed. Leucine provides >25% of glutamate nitrogen. An intriguing possibility that supplementing leucine may restore cognitive function after brain injury is discussed. Finally, some characteristics of ¹⁵N MRS that may facilitate the future application of this technique to the study of the human brain at 4 or 7 T are described.

Glutamine synthetase desensitizes differentiated adipocytes to proinflammatory stimuli by raising intracellular glutamine levels

The role of glutamine synthetase (GS) during adipocyte differentiation is unclear. Here, we assess the impact of GS on the adipocytic response to a proinflammatory challenge at different differentiation stages. GS expression at the late stages of differentiation desensitized mature adipocytes to bacterial lipopolysaccharide (LPS) by increasing intracellular glutamine levels. Furthermore, LPS-activated mature adipocytes were unable to produce inflammatory mediators; LPS sensitivity was rescued following GS inhibition and the associated drop in intracellular glutamine levels. The ability of adipocytes to differentially respond to LPS during differentiation negatively correlates to GS expression and intracellular glutamine levels. Hence, modulation of intracellular glutamine levels by GS expression represents an endogenous mechanism through which mature adipocytes control the inflammatory response.

Simultaneous recovery of dual pathways for ammonia metabolism do not improve further detoxification of ammonia in HepG2 cells

BACKGROUND

Key enzyme deficiency in the dual-pathway of ammonia metabolism leads to low detoxification capacity of HepG2 cells. Previously, we established a HepG2/AFhGS cell line with overexpression of human glutamine synthetase (hGS) in pathway 1 and a HepG2/(hArgI+hOTC)4 cell line with overexpression of human arginase I (hArgI) and human ornithine transcarbamylase (hOTC) in pathway 2. The present study aimed to investigate whether simultaneous recovery of the two pathways contributes to the further improvement of ammonia detoxification in HepG2 cells.

METHODS

We adopted a recombinant retrovirus carrying the hGS gene to infect HepG2/(hArgI+hOTC)4 cells and selected a new recombinant HepG2 cell line. The capacities of ammonia tolerance and detoxification in cells were detected by biochemical methods. Cell cycle PCR chip was used to assess the changes of gene expression.

RESULTS

Introducing hGS into HepG2/(hArgI+hOTC)4 cells did not lead to hGS overexpression, but inhibited hArgI expression. The levels of synthetic glutamine and urea in HepG2/(hArgI+hOTC+AFhGS)1 cells were significantly lower than those in HepG2/(hArgI+hOTC)4 cells when cultured in the medium with 10 and 15 mmol/L glutamate (Glu) and with 60 and 180 mmol/L NH4Cl, respectively. In addition, the comparison of different cell growth showed that HepG2/AFhGS cells significantly lagged behind the other cells by the 5th and 7th day, indicating that introduction of hGS impedes HepG2 cell proliferation. Analysis of the mechanism suggested that the decreased expression of BCL2 played an important role.

CONCLUSIONS

This study demonstrated that the recovery of two ammonia metabolic pathways in HepG2 cells is not helpful in increasing ammonia metabolism. The reinforcement of the pathway of urea metabolism is more important and valuable in improving the ammonia metabolism capacity in HepG2 cells.

Can Lactobacillus acidophilus improve minimal hepatic encephalopathy? A neurometabolite study using magnetic resonance spectroscopy

BACKGROUND AND STUDY AIMS

Minimal hepatic encephalopathy (MHE) is diagnosed when hepatic patients perform worse on psychometric tests compared to healthy controls. This study aimed to evaluate probiotics as alternative therapy in MHE.

PATIENTS AND METHODS

This is an open-label randomised controlled trial, performed in the Department of Tropical Medicine and Infectious Diseases, Tanta University Hospitals, from March 2010 to January 2012. A total of 90 patients with MHE were allocated by simple randomisation to three parallel equal groups. Group A received lactulose, group B a probiotic (Lactobacillus acidophilus) and group C served as the control. After informed consent, patients were tested for gut micrecology, fasting blood ammonia, liver functions and magnetic resonance spectroscopy (MRS) examination to study brain metabolites, mainly choline (Cho), myo-inositol (mI), glutamine+glutamate (Glx) and creatinin (Cre). Patients who developed overt encephalopathy were excluded from analysis. The whole battery of investigations was repeated in the same order after 4weeks.

RESULTS

The probiotic was better tolerated than lactulose. The relative risk reduction (RRR) of developing overt encephalopathy was 60% in the case of lactulose and 80% in the case of probiotic, with a number needed to treat (NNT) of 2.4 and 2.3, respectively. The differential but not total microecology count was significantly shifted towards saccharolytic rather than proteolytic bacteria. The mI/Cre and (Cho+mI)/Glx ratios were significantly increased and the Glx/Cre ratio was significantly reduced after 1month-follow-up in the probiotic group compared to the lactulose group and in both treatment groups compared to the control group.

CONCLUSION

Both probiotic and lactulose therapy can improve blood ammonia and psychometric tests in MHE and reduce the risk of developing overt encephalopathy. MRS showed more improvement in the levels of brain neurometabolites in the probiotic group.

Stable overexpression of arginase I and ornithine transcarbamylase in HepG2 cells improves its ammonia detoxification

HepG2 is an immortalized human hepatoma cell line that has been used for research into bioartificial liver systems. However, a low level of ammonia detoxification is its biggest drawback. In this work, a recombinant HepG2 cell line with stable overexpression of human arginase I (hArgI) and human ornithine transcarbamylase (hOTC), HepG2/(hArgI + hOTC)4, was developed using a eukaryotic dual gene expression vector pBudCE4.1. (1) The hArgI and hOTC enzymatic activity in HepG2/(hArgI + hOTC)4 cells were higher than in the control cells. (2) The ammonia tolerance capacity of HepG2/(hArgI + hOTC)4 cells was three times that of HepG2 cells and 37.5% of that of primary human hepatocytes in cultivation. In the experiment of ammonia detoxification, HepG2/(hArgI + hOTC)4 cells produced 3.1 times more urea (at 180 mM NH(4) Cl) and 3.1 times more glutamine (at 120 mM NH(4) Cl and 15 mM glutamate) than HepG2 cells, reaching 63.1% and 36.0% that of primary human hepatocytes, respectively. (3) The hArgI and hOTC overexpression did not influence the growth of HepG2 cells and also promoted the expression of other ammonia detoxification associated proteins including glutamine synthetase (GS), arginase II (ArgII), arginosuccinate synthase (ASS) and arginosuccinate lyase (ASL) in HepG2 cells. This work illustrates that the modification reported here made significant progress in the improvement of HepG2 cell function and the HepG2/(hArgI + hOTC)4 cells will provide a better selection for the application of bioartificial liver system.

Arginase I deficiency: severe infantile presentation with hyperammonemia: more common than reported?

Enzyme defects of the urea cycle typically present with significant hyperammonemia and its associated toxicity, in the first few months of life. However, arginase I (ARG1) deficiency, a rare autosomal recessive disorder, has classically been the exception. ARG1 deficiency usually presents later in life with spasticity, seizures, failure to thrive and developmental regression. Neonatal and early infantile presentation of ARG1 deficiency with severe hyperammonemia remains rare and only six such cases have been described. We report a severely affected infant with ARG1 deficiency who presented at 6 weeks of age with lethargy, poor feeding and severe encephalopathy caused by hyperammonemia. The clinical and biochemical features of the proband and six other previously reported cases with neonatal or infantile-onset presentation of ARG1 deficiency with hyperammonemia are reviewed. In addition, the clinical spectrum of seven previously unpublished patients with later onset ARG1 deficiency, who also experienced recurrent hyperammonemia, is presented. Several biochemical abnormalities have been postulated to play a role in the pathogenesis of the neurological changes in ARG1 deficiency including hyperargininemia, elevated guanidino compounds and elevated glutamine levels, as well as the hyperammonemia. The index case demonstrated many of these. The cases reviewed here suggest a genotype/phenotype correlation and advocate for the addition of arginine as a primary target in newborn screening programs.

Continuous veno-venous single-pass albumin hemodiafiltration in children with acute liver failure

OBJECTIVE

To investigate the applicability, efficacy, and safety of single-pass albumin dialysis in children.

DESIGN

Retrospective data review of uncontrolled clinical data.

SETTING

University-based pediatric intensive care unit collaborating with a local center for liver transplantation.

PATIENTS

Nine children, aged 2 to 15 yrs, who were treated with single-pass albumin dialysis for acute liver failure of various origins under a compassionate-use protocol between 2000 and 2006. All patients met high-urgency liver transplantation criteria.

INTERVENTIONS

Single-pass albumin dialysis was performed as rescue therapy for children with acute liver failure.

MEASUREMENTS AND MAIN RESULTS

The decrease in hepatic encephalopathy (grades 1-4) and the serum levels of bilirubin, bile acids, and ammonium were measured to assess the efficacy of detoxification. As a measure of liver synthesis function, thromboplastin time and fibrinogen were analyzed. The safety of the procedure was assessed by documenting adverse effects on mean arterial blood pressure, platelet count, and clinical course. Seven out of nine patients were bridged successfully to either native organ recovery (n = 1) or liver transplantation (n = 6), one of them twice. Six out of nine patients undergoing single-pass albumin dialysis (ten treatments) survived. In six patients, hepatic encephalopathy could be reduced at least by one degree. Ammonium, bilirubin, and bile acid levels decreased in all patients. One patient had an allergic reaction to albumin.

CONCLUSIONS

In childhood acute liver failure, treatment with single-pass albumin dialysis was generally well tolerated and seems to be effective in detoxification and in improving blood pressure, thus stabilizing the critical condition of children before liver transplantation and facilitating bridging to liver transplantation. It may be beneficial in avoiding severe neurologic sequelae after acute liver failure and thereby improve survival. Single-pass albumin dialysis is an inexpensive albumin-based detoxification system that is easy to set up and requires little training. Whether and to what extent single-pass albumin dialysis can support children with acute liver failure until native liver recovery remains unclear.

Nitric oxide, ammonia, and CRP levels in cirrhotic patients with hepatic encephalopathy: is there a connection?

GOALS

Comparison of nitric oxide (NO) levels in cirrhotic patients with and without hepatic encephalopathy (HE), evaluation of possible correlation between HE and other clinical or laboratory characteristics, and estimation of utilization of NO levels in clinical practice.

BACKGROUND

HE is a neuropsychiatric complication of cirrhosis. The exact pathogenetic mechanisms underlying the presence of HE are not known. However, dysfunction of the NO pathway and ammonia detoxification are thought to play a major role.

STUDY

Sixty-seven cirrhotic patients, 36 (53.7%) without HE, and 31 (46.3%) with HE were included in the study. Eighteen healthy individuals were used as control group. Clinical and laboratory data, including ammonia and stable end products of NO using Griess reaction, were collected.

RESULTS

NOx levels were statistically significantly higher in cirrhotic patients (225.5 μmol/L) than in control group [(67.94 μmol/L) (P=0.000)]. NOx levels were, also, statistically significantly higher in patients with HE compared with patients without HE (324.67 μmol/L vs. 141.96 μmol/L, P=0.000). Significant correlation between the presence of HE and NOx, ammonia, C-reactive protein, albumin, Model for End-Stage Liver Disease score, and Child-Pugh classification revealed. NOx levels also correlated with severity of HE. NOx and ammonia are independent factors predicting HE according to regression analysis. Diagnostic accuracy for the diagnosis of HE using a combination of NOx and ammonia was superior compared with standalone NOx or ammonia utilization.

CONCLUSIONS

NOx levels are correlated with the presence and severity of HE. NOx levels determination, in addition to ammonia levels, could contribute in diagnosis of HE.

Two-dimensional MR spectroscopy of minimal hepatic encephalopathy and neuropsychological correlates in vivo

PURPOSE

To evaluate regional cerebral metabolic and structural changes in patients with minimal hepatic encephalopathy (MHE) using two-dimensional (2D) MR spectroscopy (MRS) and T( (1) )-weighted MRI, to correlate the observed MR changes with neuropsychological (NP) test scores, and to compare the diagnostic accuracy of MRI, 2D MRS, and NP tests in discriminating between patients and healthy subjects.

MATERIALS AND METHODS

Thirty-three MHE patients and 30 healthy controls were investigated. The 2D localized correlated spectroscopy (L-COSY) was performed in the frontal and occipital brain on a 1.5 Tesla (T) MR scanner. The NP test battery included 15 tests, grouped into 6 cognitive domains. Globus pallidus signal intensities were calculated from T(1)-weighted images.

RESULTS

The 2D MRS showed significant differences in ratios of the following metabolite(s) peaks with respect to creatine (Cr): decreased myo-inositol (mI), choline (Ch), mICh, and increased (glutamate plus glutamine) (Glx) in patients compared with healthy subjects in both occipital and frontal lobes. Frontal lobe taurine also showed a decline in patients. The NP test results revealed declines in cognitive speed, motor function, executive function, and global cognitive status. Significant correlations were found between the altered metabolites and NP tests. Alteration in the mICh/Cr ratio was noted as a powerful discriminant between healthy subjects and the patients.

CONCLUSION

The study demonstrates that relative metabolite levels determined by 2D MRS, in particular mICh/Cr, provide the best diagnostic prediction for MHE. The results suggest that depletions of myo-inositol, choline and taurine with respect to creatine correlate with measures of neuropsychological impairment.

Brain edema in acute liver failure: inhibition by L-histidine

Brain edema and the associated increase in intracranial pressure are potentially lethal complications of acute liver failure (ALF). Astrocyte swelling (cytotoxic edema) represents a significant component of the brain edema in ALF, and elevated blood and brain ammonia levels have been strongly implicated in its formation. We earlier showed in cultured astrocytes that oxidative stress (OS) and the mitochondrial permeability transition (mPT) play major roles in the mechanism of ammonia-induced astrocyte swelling. Glutamine, a byproduct of ammonia metabolism, has also been shown to induce OS, the mPT, and astrocyte swelling. Such effects of glutamine were suggested to be mediated by its hydrolysis in mitochondria, potentially yielding high levels of ammonia in this organelle and leading to OS and the mPT. L-histidine, an inhibitor of mitochondrial glutamine transport, was recently shown to mitigate OS, mPT, and cell swelling in cultured astrocytes treated with ammonia. The present study examined whether L-histidine similarly abolishes OS, the mPT, and brain edema in a rat model of ALF. Treatment of rats with thioacetamide caused a significant degree of brain edema, which was associated with induction of OS and the mPT. These changes were completely abolished by L-histidine, supporting a key role of mitochondrial glutamine transport and hydrolysis in the mechanism of the brain edema associated with ALF.

Effect of elevated ammonia on tissue nitrogen metabolites in the ureotelic gulf toadfish (Opsanus beta) and the ammoniotelic midshipman (Porichthys notatus)

We investigated possible biochemical pathways explaining extreme ammonia tolerance by the gulf toadfish and specifically tested the prediction that the gulf toadfish Opsanus beta is more tolerant than the plainfin midshipman Porichthys notatus, two confamilial species, because it reverses brain glutamine accumulation during high ammonia exposure. This prediction stems from previous studies demonstrating that gulf toadfish produce urea from glutamine, a pathway not present in the ammoniotelic midshipman. Our results show that at the same water NH(3) concentration, ammonia increases more from control levels in brains of midshipman than toadfish. After 48 h of exposure to 50% of their respective LC(50) (96 h) value for ammonia, toadfish are able to return the ammonia-induced increase in brain glutamine back to control values, reducing brain glutamine by 2,500 micromol kg(-1). However, in midshipman, brain glutamine remains significantly elevated from control throughout the experiment. Toadfish exposed to 33% of their LC(50) (96 h) showed an initial increase in whole body urea, which is then reduced at a rate of 104 micromol kg fish(-1) h(-1) and could be directly excreted into the water. We discuss how the special handling of glutamine in toadfish may explain in part their great tolerance to ammonia.

Metabolic fate of isoleucine in a rat model of hepatic encephalopathy and in cultured neural cells exposed to ammonia

Hepatic encephalopathy is a severe neuropathological condition arising secondary to liver failure. The pathogenesis is not well understood; however, hyperammonemia is considered to be one causative factor. Hyperammonemia has been suggested to inhibit tricarboxylic acid (TCA) cycle activity, thus affecting energy metabolism. Furthermore, it has been suggested that catabolism of the branched-chain amino acid isoleucine may help curb the effect of hyperammonemia by by-passing the TCA cycle block as well as providing the carbon skeleton for glutamate and glutamine synthesis thus fixating ammonia. Here we present novel results describing [U-(13)C]isoleucine metabolism in muscle and brain analyzed by mass spectrometry in bile duct ligated rats, a model of chronic hepatic encephalopathy, and discuss them in relation to previously published results from neural cell cultures. The metabolism of [U-(13)C]isoleucine in muscle tissue was about five times higher than that in the brain which, in turn, was lower than in corresponding cell cultures. However, synthesis of glutamate and glutamine was supported by catabolism of isoleucine. In rat brain, differential labeling patterns in glutamate and glutamine suggest that isoleucine may primarily be metabolized in the astrocytic compartment which is in accordance with previous findings in neural cell cultures. Lastly, in rat brain the labeling patterns of glutamate, aspartate and GABA do not suggest any significant inhibition by ammonia of TCA cycle activity which corresponds well to findings in neural cell cultures. Branched-chain amino acids including isoleucine are used for treating hepatic encephalopathy and the present findings shed light on the possible mechanism involved. The low turn-over of isoleucine in rat brain suggests that this amino acid does not serve the role of providing metabolites pertinent to TCA cycle function and hence energy formation as well as the necessary carbon skeleton for subsequent ammonia fixation in hyperammonemia. The higher metabolism of isoleucine in muscle could, however, contribute to ammonia fixation and thus likely be of value in the treatment of hepatic encephalopathy.

Amino acids in CSF and plasma in hyperammonaemic coma due to arginase1 deficiency

We report the CSF and plasma amino acid concentrations and their ratios in a male patient with arginase1 deficiency with an unusual early presentation at 34 days of age. He developed hyperammonaemic coma (ammonia >400 μmol/L; normal <90 μmol/L) on postnatal day 35. CSF and plasma concentrations were assayed by ion-exchange chromatography on day 36. Arginine was increased both in plasma (971 μmol/L; controls (mean ± 2SD) 50 ± 42) and in CSF (157 μmol/L; controls 19 ± 8.6), resulting in a normal CSF/plasma ratio of 0.16 (controls 0.41 ± 0.26). Interestingly, glutamine was disproportionately high in CSF (3114 μmol/L; controls 470 ± 236) but normal in plasma (420 μmol/L; controls 627 ± 246); the ratio exceeded unity (7.4; controls 0.76 ± 0.31). The CSF/plasma ratios of most neutral amino acids were elevated but not those of the imino- and of the dibasic amino acids lysine and ornithine. The mechanism leading to the increase of most neutral amino acids in brain is not known.

CONCLUSION

A normal glutamine in plasma does not exclude an increased concentration in CSF; it could be useful to ascertain by MRS that a high CSF glutamine concentration truly reflects a high concentration in brain tissue for better understanding its pathogenesis.

Role of Magnetic Resonance in Understanding the Pathogenesis of Hepatic Encephalopathy

A spectrum of neuropsychiatric abnormalities caused by portosystemic venous shunting occurs in hepatic encephalopathy (HE) patients with or without liver dysfunction. It is not completely clear how the astrocyte swelling leads to glial-neuronal dysfunction, and how the symptoms are manifested in HE. A major goal of this work is to review the current status of information available from the existing magnetic resonance (MR) modalities including MR imaging (MRI) and MR Spectroscopy (MRS) as well as other modalities in the understanding the pathogenesis of HE. First, we discuss briefly neuron-histopathology, neurotoxins, neuropsychological and neurophysiological tests. A short review on the progress with single-photon emission computed tomography (SPECT) and positron emission tomography (PET) is then presented. In the remaining part of the manuscript, the following topics pertinent to understanding the pathogenesis of HE are discussed: MRI, diffusion tensor imaging (DTI), one-dimensional MRS based single- and multi-voxel based spectroscopic imaging techniques and two-dimensional MRS.

The anaplerotic flux and ammonia detoxification in hepatic encephalopathy

Metabolic alterations in the brain underly many of the mechanisms leading to acute and chronic Hepatic Encephalopathy (HE). Controversy exists about the role of glutamine accumulation as a causal factor in HE. Glutamine formation contributes to detoxify ammonia, whereby anaplerotic mechanisms in the astrocytes have to be sufficient to replenish Krebs cycle intermediates. The application of ex vivo high-resolution nuclear magnetic resonance (NMR) spectroscopy permits direct measurements of metabolites and different metabolic pathways. Ex vivo (13)C-NMR studies in experimental animal models of acute and chronic HE have provided new insights. In an experimental rat model of ALF, (13)C isotopomer analysis of glucose metabolism showed that alterations of glucose flux through astrocytic pyruvate carboxylase might be linked to the pathogenesis of ALF as a limited anaplerotic flux in the brain, but not in the muscle, correlates with the development of brain edema. Moreover, (13)C-NMR data from a rat model of mild HE demonstrated relative differences in the pathway of glucose through pyruvate carboxylase in thalamus compared to frontal cortex, which might explain the vulnerability of this brain region compared to thalamus. These findings further support that glutamine accumulation might be not the primary cause of neurological symptoms in HE, and show that anaplerotic mechanisms could be essential for ammonia detoxification in HE.

NMDA receptors in hyperammonemia and hepatic encephalopathy

The NMDA type of glutamate receptors modulates learning and memory. Excessive activation of NMDA receptors leads to neuronal degeneration and death. Hyperammonemia and liver failure alter the function of NMDA receptors and of some associated signal transduction pathways. The alterations are different in acute and chronic hyperammonemia and liver failure. Acute intoxication with large doses of ammonia (and probably acute liver failure) leads to excessive NMDA receptors activation, which is responsible for ammonia-induced death. In contrast, chronic hyperammonemia induces adaptive responses resulting in impairment of signal transduction associated to NMDA receptors. The function of the glutamate-nitric oxide-cGMP pathway is impaired in brain in vivo in animal models of chronic liver failure or hyperammonemia and in homogenates from brains of patients died in hepatic encephalopathy. The impairment of this pathway leads to reduced cGMP and contributes to impaired cognitive function in hepatic encephalopathy. Learning ability is reduced in animal models of chronic liver failure and hyperammonemia and is restored by pharmacological manipulation of brain cGMP by administering phosphodiesterase inhibitors (zaprinast or sildenafil) or cGMP itself. NMDA receptors are therefore involved both in death induced by acute ammonia toxicity (and likely by acute liver failure) and in cognitive impairment in hepatic encephalopathy.

A metabonomic investigation on the biochemical perturbation in liver failure patients caused by hepatitis B virus

A metabonomic study was performed to investigate the biochemical perturbation of the serum samples from liver failure patients induced by hepatitis B virus (HBV; n=24) and control normal subjects (n=23). The serum metabonome was detected using gas chromatography-mass spectrometry (GC-MS) technique integrated with a commercial mass spectral library for the peak identification. After peak deconvolution, identification, and matching, the acquired GC-MS data were normalized and processed by principal component analysis (PCA) and canonical discriminant analysis (CDA). Specific changes in the metabolic composition of serum samples from patients including amino acids (AAs) and glucose were shown in GC-MS total ion current (TIC) chromatograms. The distinctive biochemical difference between the healthy subjects and liver failure patients was displayed by the pattern recognition methods. We also found that the liver failure patients with different degree of severity categorized as MELD (model for end-stage of liver diseases) could be clearly classified by the corresponding metabonomic data. In comparison, the current routine clinical indices cannot characterize the global phenotyping of liver failure. The result demonstrated that the GC-MS technique is an alternative tool for the characterization of the metabolic perturbation and the metabonomic study promises to provide an integrative criterion to evaluate the severity and the prognosis of liver diseases.

Role of the nurse in managing patients with hepatic cerebral oedema

Acute liver failure occurs abruptly often without pre-existing liver disease being present. Management comprises support of the patient and treatment of symptoms until recovery or transplantation can occur. Cerebral oedema is one of the recognized complications of acute liver failure with a mortality rate of 30-50% when it occurs. Effective supportive nursing management includes use of sedative agents, positioning and thermoregulation if the patient is to recover or be optimized for transplantation.

Hyperammonemia causes protein oxidation and enhanced proteasomal activity in response to mitochondria-mediated oxidative stress in rat primary astrocytes

Hyperammonemia, as a consequence of severe liver failure, is strongly associated with the neurological syndrome hepatic encephalopathy (HE) whereby excessive ammonia is metabolized by astrocytes, followed by cell and brain swelling in vivo. In the present study we were able to show that ammonia treatment of primary astrocytes in vitro is followed by cell swelling and a loss of cell viability at higher ammonia concentrations. Lower ammonia concentrations are accompanied by mitochondria-derived oxidative stress, as demonstrated by using inhibitors of mitochondrial glutaminase I, 143B-rho (0) cells and isolated mitochondria. The oxidative stress generated by mitochondria is accompanied by protein oxidation. In further studies we could show, that an activation of the proteasomal system takes place during ammonia exposure and protects cells. The proteasome acitvation can be blocked by antioxidants or by inhibitors of enzymes of glutamine metabolism. We conclude that oxidative stress-mediated proteasomal activation is important for survival of astroglial cells.

Acute liver failure in children

OBJECTIVE

To review the incidence, etiologies, pathophysiology, and treatment of acute liver failure (ALF) in children. Emphasis will be placed on the initial management of the multiple organ system involvement of ALF.

METHOD

MEDLINE search from 1970 to March 2005 was performed. Search headings were as follows: acute liver failure, fulminant liver failure, pediatric liver failure, hepatic encephalopathy, and liver transplantation. Studies written in English were selected. Pediatric studies were emphasized. Adult studies were referenced if there were no pediatric studies available in regard to a specific aspect of liver failure.

CONCLUSIONS

Pediatric acute liver failure is a rare but life-threatening disease. The common etiologies differ for given age groups. Management includes treating specific causes and supporting multiple organ system failure. Commonly associated disorders that require initial recognition and treatment include energy production deficiencies (hypoglycemia), coagulation abnormalities, immune system dysfunctions, encephalopathy, and cerebral edema. Criteria used to determine the need for liver transplant are reviewed as well as the difficulties associated with predicting which patients will meet these criteria and how rapidly liver transplant will become the only option. Finally, experimental procedures that may provide additional time for the liver to recover are briefly reported.

Nuclear magnetic resonance studies of energy metabolism and glutamine shunt in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy (HE) in both acute and chronic liver failure is more likely a reversible functional disease rather than an irreversible pathological lesion of brain cells. Metabolic alterations underlie many of the mechanisms leading to HE. This paper summarizes in vivo and ex vivo (1)H-, (13)C-, and (15)N-nuclear magnetic resonance (NMR) spectroscopy data on patients and experimental models of HE. In vivo NMR spectroscopy provides a unique opportunity to study metabolic changes noninvasively in the brain in vivo, and to quantify various metabolites in localized brain areas, and ex vivo NMR permits the high-resolution measurement of metabolites and the identification of different metabolic pathways. In vivo and ex vivo (1)H-NMR investigations consistently reveal severalfold increases in brain glutamine and concomitant decreases in myo-inositol, an important osmolyte in astrocytes. An osmotic disturbance in these cells has long been suggested to be responsible for astrocyte swelling and brain edema. However, ex vivo (13)C-NMR studies have challenged the convention that glutamine accumulation is the major cause of brain edema in acute HE. They rather indicate a limited anaplerotic flux and capacity of astrocytes to detoxify ammonia by glutamine synthesis and emphasize distortions of energy and neurotransmitter metabolism. However, recent (15)N-NMR investigations have demonstrated that glutamine fluxes between neurons and astrocytes are affected by ammonia. Further NMR studies may provide novel insights into the relationship between brain edema and/or astrocyte pathology and changes in inter- and intracellular glutamine homeostasis, which may secondarily alter brain energy metabolism.

A marriage of old and new: chemostats and microarrays identify a new model system for ammonium toxicity

Toxicity is related to an organism's ability to rid itself of the offending molecules. This primer provides insights into how this can be monitored by highlighting the case of ammonium toxicity.

Ornithine transcarbamylase and arginase I deficiency are responsible for diminished urea cycle function in the human hepatoblastoma cell line HepG2

A possible cell source for a bio-artificial liver is the human hepatblastoma-derived cell line HepG2 as it confers many hepatocyte functions, however, the urea cycle is not maintained resulting in the lack of ammonia detoxification via this cycle. We investigated urea cycle activity in HepG2 cells at both a molecular and biochemical level to determine the causes for the lack of urea cycle expression, and subsequently addressed reinstatement of the cycle by gene transfer. Metabolic labelling studies showed that urea production from 15N-ammonium chloride was not detectable in HepG2 conditioned medium, nor could 14C-labelled urea cycle intermediates be detected. Gene expression data from HepG2 cells revealed that although expression of three urea cycle genes Carbamoyl Phosphate Synthase I, Arginosuccinate Synthetase and Arginosuccinate Lyase was evident, Ornithine Transcarbamylase and Arginase I expression were completely absent. These results were confirmed by Western blot for arginase I, where no protein was detected. Radiolabelled enzyme assays showed that Ornithine Transcarbamylase functional activity was missing but that Carbamoyl Phosphate Synthase I, Arginosuccinate Synthetase and Arginosuccinate Lyase were functionally expressed at levels comparable to cultured primary human hepatocytes. To restore the urea cycle, HepG2 cells were transfected with full length Ornithine Transcarbamylase and Arginase I cDNA constructs under a CMV promoter. Co-transfected HepG2 cells displayed complete urea cycle activity, producing both labelled urea and urea cycle intermediates. This strategy could provide a cell source capable of urea synthesis, and hence ammonia detoxificatory function, which would be useful in a bio-artificial liver.

Phosphate-activated glutaminase activity is enhanced in brain, intestine and kidneys of rats following portacaval anastomosis

AIM: To assess whether portacaval anastomosis (PCA) in rats affects the protein expression and/or activity of glutaminase in kidneys, intestines and in three brain areas of cortex, basal ganglia and cerebellum and to explain the neurological alterations found in hepatic encephalopathy (HE).

METHODS: Sixteen male Wistar rats weighing 250-350 g were grouped into sham-operation control (n = 8) or portacaval shunt (n = 8). Twenty-eight days after the procedure, the animals were sacrificed. The duodenum, kidney and brain were removed, homogenised and mitochondria were isolated. Ammonia was measured in brain and blood. Phosphate-activated glutaminase (PAG) activity was determined by measuring ammonia production following incubation for one hour at 37 °C with O-phthalaldehyde (OPA) and specific activity expressed in units per gram of protein (µkat/g of protein). Protein expression was measured by immunoblotting.

RESULTS: Duodenal and kidney PAG activities together with protein content were significantly higher in PCA group than in control or sham-operated rats (duodenum PAG activity was 976.95±268.87 µkat/g of protein in PCA rats vs 429.19±126.92 µkat/g of protein in sham-operated rats; kidneys PAG activity was 1259.18 ± 228.79 µkat/g protein in PCA rats vs 669.67± 400.8 µkat/g of protein in controls, P < 0.05; duodenal protein content: 173% in PCA vs sham-operated rats; in kidneys the content of protein was 152% in PCA vs sham-operated rats). PAG activity and protein expression in PCA rats were higher in cortex and basal ganglia than those in sham-operated rats (cortex: 6646.6 ± 1870.4 µkat/g of protein vs 3573.8 ± 2037.4 µkat/g of protein in control rats, P < 0.01; basal ganglia, PAG activity was 3657.3 ± 1469.6 μkat/g of protein in PCA rats vs 2271.2 ± 384 μkat/g of protein in sham operated rats, P < 0.05; In the cerebellum, the PAG activity was 2471.6 ± 701.4 μkat/g of protein vs 1452.9 ± 567.8 μkat/g of protein in the PCA and sham rats, respectively, P < 0.05; content of protein: cerebral cortex: 162% ± 40% vs 100% ± 26%, P < 0.009; and basal ganglia: 140% ± 39% vs 100% ± 14%, P < 0.05; but not in cerebellum: 100% ± 25% vs 100% ± 16%, P = ns).

CONCLUSION: Increased PAG activity in kidney and duodenum could contribute significantly to the hyperammonaemia in PCA rats, animal model of encephalopathy. PAG is increased in non-synaptic mitochondria from the cortex and basal ganglia and could be implicated in the pathogenesis of hepatic encephalopathy. Therefore, PAG could be a possible target for the treatment of HE or liver dysfunction.

Role of phosphate-activated glutaminase in the pathogenesis of hepatic encephalopathy

Disturbed body nitrogen homeostasis due to impaired hepatic urea synthesis leads to an alteration in inter-organ ammonia trafficking, resulting in hyperammonemia. Glutamine (Gln) synthase is the alternative pathway for ammonia detoxification. Gln taken up by several organs is split by the intramitochondrial phosphate-activated enzyme glutaminase (PAG) into glutamate (Glu) and ammonia. In cirrhotic patients with portosystemic intrahepatic shunt, the main source of systemic hyperammonemia is the small intestine, and ammonia derives mainly from Gln deamidation. Recently, PAG has been found increased in cirrhotics showing minimal hepatic encephalopathy and, therefore, could be implicated in the production of systemic hyperammonemia in these patients. Intestinal PAG activity correlates with psychometric test and magnetic resonance spectroscopy findings. Moreover, nitric oxide and tumor necrosis factor seem to be the major factors regulating intestinal ammonia production in cirrhotics. In the brain, PAG localized into the astrocytes is responsible for ammonia and free-radical production. The blockade of PAG, using 6-oxo-5-norleucine, avoids the toxic effects of Gln accumulation in the brain. These data support an important role for intestinal and brain glutaminase in the pathogenesis of hepatic encephalopathy and could be a new target for future therapies.

Endogenous neuro-protectants in ammonia toxicity in the central nervous system: facts and hypotheses

The paper overviews experimental evidence suggestive of the engagement of three endogenous metabolites: taurine, kynurenic acid, and glutathione (GSH) in the protection of central nervous system (CNS) cells against ammonia toxicity. Intrastriatal administration of taurine via microdialysis probe attenuates ammonia-induced accumulation of extracellular cyclic guanosine monophosphate (cGMP) resulting from over-activation of the N-methyl-D: -aspartate/nitric oxide (NMDA/NO) pathway, and this effect involves agonistic effect of taurine on the GABA-A and glycine receptors. Taurine also counteracts generation of free radicals, increased release of dopamine, and its metabolism to dihydroxyphenylacetic acid (DOPAC). Taurine reduces ammonia-induced increase of cell volume (edema) in cerebrocortical slices by a mechanism involving GABA-A receptors. Massive release of radiolabeled or endogenous taurine from CNS tissues by ammonia in vivo and in vitro is thought to promote its neuroprotective action, by making the amino acid available for interaction with cell membranes and/or by driving excess water out of the CNS cells (astrocytes) that underwent ammonia-induced swelling. Ammonia in vivo and in vitro affects in variable ways the synthesis of kynurenic acid (KYNA). Since KYNA is an endogenous NMDA receptor antagonist with a high affinity towards its glycine site, changes in its content may counter over-activation or depression of glutaminergic transmission observed at the different stages of hyperammonemia. GSH is a major antioxidant in the CNS whose synthesis is partly compartmented between neurons and astrocytes: astrocytic GSH is a source of precursors for the synthesis of neuronal GSH. Ammonia in vitro stimulates GSH synthesis in cultured astrocytes, which may compensate for increased GSH consumption (decreased GSH/GSSG ratio) in neurons.

Cytotoxic edema is responsible for raised intracranial pressure in fulminant hepatic failure: in vivo demonstration using diffusion-weighted MRI in human subjects

It is not clear whether cerebral edema in fulminant hepatic failure is predominantly vasogenic or cytotoxic, though cytotoxic edema due to astrocyte swelling is more likely. Diffusion-weighted magnetic resonance imaging can differentiate vasogenic from cytotoxic edema. We performed diffusion-weighted imaging in patients with fulminant hepatic failure to clarify the issue by measuring apparent diffusion coefficient, which quantifies movement of water molecule across cell membrane. Seven patients with fulminant hepatic failure underwent conventional and diffusion-weighted magnetic resonance imaging. Apparent diffusion coefficient was measured in four cortical areas and 12 deep white and gray matter regions in both cerebral hemispheres. Thirteen healthy subjects served as controls. The apparent diffusion coefficient values in patients and controls were compared using Wilcoxon signed rank test. Two patients who survived underwent repeat imaging using same protocol. Patients with FHF had significantly lower apparent diffusion coefficient in all cortical and deep white and gray matter regions of interest compared to controls (p < 0.001), suggesting cytotoxic cell swelling. In two survivors with repeat imaging, one showed complete resolution while the changes persisted in the other, suggesting ischemic injury. Cerebral edema in fulminant hepatic failure is predominantly due to cytotoxic edema.

Ammonia affects brain nitrogen metabolism but not hydration status in the Gulf toadfish (Opsanus beta)

Laboratory rodents made hyperammonemic by infusing ammonia into the blood show symptoms of brain cell swelling and increased intracranial pressure. These symptoms could be caused in part by an increase in brain glutamine formed when brain glutamine synthetase (GS) naturally detoxifies ammonia to glutamine. Previous studies on the Gulf toadfish (Opsanus beta) demonstrated that it is resistant to high ammonia exposure (HAE) (96 h LC(50)=10mM) despite an increase in brain glutamine. This study attempts to resolve whether the resistance of O. beta is mediated by special handling of brain water in the face of changing glutamine concentrations. Methionine sulfoximine (MSO), an inhibitor of GS, was used to pharmacologically manipulate glutamine concentrations, and magnetic resonance imaging (MRI) was used to assess the status of brain water. Ammonia or MSO treatment did not substantially affect blood acid-base parameters. Exposure to 3.5mM ammonium chloride in seawater for 16 or 40 h resulted in a parallel increase in brain ammonia (3-fold) and glutamine (2-fold) and a decrease in brain glutamate (1.3-fold). Pre-treatment with MSO prevented ammonia-induced changes in glutamine and glutamate concentrations. HAE also induced an increase in plasma osmolality (by 7%) which was probably due to a disturbance of osmoregulatory processes but which did not result in broader whole body dehydration as indicated by muscle water analysis. The increase in brain glutamine was not associated with any changes in brain water in toadfish exposed to 3.5 mM ammonia for up to 40 h or even at 10, 20 and 30 mM ammonia consecutively and for one hour in each concentration. The lack of brain water accumulation implies that ammonia toxicity in toadfish appears to be via pathways other than cerebral swelling. Furthermore, toadfish pre-treated with MSO did not survive a normally sub-lethal exposure to 3.5 mM ammonia for 40 h. The enhancement of ammonia toxicity by MSO suggests that GS function is critical to ammonia tolerance in this species.

Chronic and acute ammonia toxicity in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti: brain ammonia and glutamine contents, and effects of methionine sulfoximine and MK801

The objective of this study was to elucidate if chronic and acute ammonia intoxication in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti, were associated with high levels of ammonia and/or glutamine in their brains, and if acute ammonia intoxication could be prevented by the administration of methionine sulfoximine [MSO; an inhibitor of glutamine synthetase (GS)] or MK801 [an antagonist of N-methyl D-aspartate type glutamate (NMDA) receptors]. For P. schlosseri and B. boddaerti exposed to sublethal concentrations (100 and 8 mmol l(-1) NH4Cl, respectively, at pH 7.0) of environmental ammonia for 4 days, brain ammonia contents increased drastically during the first 24 h, and they reached 18 and 14.5 micromol g(-1), respectively, at hour 96. Simultaneously, there were increases in brain glutamine contents, but brain glutamate contents were unchanged. Because glutamine accumulated to exceptionally high levels in brains of P. schlosseri (29.8 micromol g(-1)) and B. boddaerti (12.1 micromol g(-1)) without causing death, it can be concluded that these two mudskippers could ameliorate those problems associated with glutamine synthesis and accumulation as observed in patients suffering from hyperammonemia. P. schlosseri and B. boddaerti could tolerate high doses of ammonium acetate (CH3COONH4) injected into their peritoneal cavities, with 24 h LC50 of 15.6 and 12.3 micromol g(-1) fish, respectively. After the injection with a sublethal dose of CH3COONH4 (8 micromol g(-1) fish), there were significant increases in ammonia (5.11 and 8.36 micromol g(-1), respectively) and glutamine (4.22 and 3.54 micromol g(-1), respectively) levels in their brains at hour 0.5, but these levels returned to normal at hour 24. By contrast, for P. schlosseri and B. boddaerti that succumbed within 15-50 min to a dose of CH3COONH4 (15 and 12 micromol g(-1) fish, respectively) close to the LC50 values, the ammonia contents in the brains reached much higher levels (12.8 and 14.9 micromol g(-1), respectively), while the glutamine level remained relatively low (3.93 and 2.67 micromol g(-1), respectively). Thus, glutamine synthesis and accumulation in the brain was not the major cause of death in these two mudskippers confronted with acute ammonia toxicity. Indeed, MSO, at a dosage (100 microg g(-1) fish) protective for rats, did not protect B. boddaerti against acute ammonia toxicity, although it was an inhibitor of GS activities from the brains of both mudskippers. In the case of P. schlosseri, MSO only prolonged the time to death but did not reduce the mortality rate (100%). In addition, MK801 (2 microg g(-1) fish) had no protective effect on P. schlosseri and B. boddaerti injected with a lethal dose of CH3COONH4, indicating that activation of NMDA receptors was not the major cause of death during acute ammonia intoxication. Thus, it can be concluded that there are major differences in mechanisms of chronic and acute ammonia toxicity between brains of these two mudskippers and mammalian brains.

Differential response of glutamine in cultured neurons and astrocytes

Glutamine, a byproduct of ammonia detoxification, is found elevated in brain in hepatic encephalopathy (HE) and other hyperammonemic disorders. Such elevation has been implicated in some of the deleterious effects of ammonia on the central nervous system (CNS). Recent studies have shown that glutamine results in the induction of the mitochondrial permeability transition (MPT) in cultured astrocytes. We examined whether glutamine shows similar effects in cultured neurons. Both cultured astrocytes and neurons were exposed to glutamine (6.5 mM) for 24 hr and the MPT was assessed by changes in cyclosporin A (CsA)-sensitive inner mitochondrial membrane potential (DeltaPsi(m)) using the potentiometric dye tetramethylrhodamine ethyl ester (TMRE). Glutamine significantly dissipated the DeltaPsi(m) in astrocytes as demonstrated by a decrease in mitochondrial TMRE fluorescence, a process that was blocked by CsA. On the other hand, treatment of cultured neurons with glutamine had no effect on the DeltaPsi(m). Dissipation of the DeltaPsi(m) in astrocytes by glutamine was blocked by treatment with 6-diazo-5-oxo-L-norleucine (DON; 100 microM), suggesting that glutamine hydrolysis and the subsequent generation of ammonia, which has been shown previously to induce the MPT, might be involved in MPT induction by glutamine. These data indicate that astrocytes but not neurons are vulnerable to the toxic effects of glutamine. The selective induction of oxidative stress and the MPT by glutamine in astrocytes may partially explain the deleterious affects of glutamine on the CNS in the setting of hyperammonemia, as well as account for the predominant involvement of astrocytes in the pathogenesis of HE and other hyperammonemic conditions.

Oxidative stress in the pathogenesis of hepatic encephalopathy

The pathogenesis of hepatic encephalopathy (HE) remains elusive. While it is clear that ammonia is the likely toxin and that astrocytes are the main target of its neurotoxicity, precisely how ammonia brings about cellular injury is poorly understood. Studies over the past decade have invoked the concept of oxidative stress as a pathogenetic mechanism for ammonia neurotoxicity. This review sets out the arguments in support of this concept based on evidence derived from human observations, animal studies, and cell culture investigations. The consequences and potential therapeutic implications of oxidative stress in HE are also discussed.

Selective alterations of brain osmolytes in acute liver failure: protective effect of mild hypothermia

The principal cause of mortality in patients with acute liver failure (ALF) is brain herniation resulting from intracranial hypertension caused by a progressive increase of brain water. In the present study, ex vivo high-resolution 1H-NMR spectroscopy was used to investigate the effects of ALF, with or without superimposed hypothermia, on brain organic osmolyte concentrations in relation to the severity of encephalopathy and brain edema in rats with ALF due to hepatic devascularization. In normothermic ALF rats, glutamine concentrations in frontal cortex increased more than fourfold at precoma stages, i.e. prior to the onset of severe encephalopathy, but showed no further increase at coma stages. In parallel with glutamine accumulation, the brain organic osmolytes myo-inositol and taurine were significantly decreased in frontal cortex to 63% and 67% of control values, respectively, at precoma stages (p<0.01), and to 58% and 67%, respectively, at coma stages of encephalopathy (p<0.01). Hypothermia, which prevented brain edema and encephalopathy in ALF rats, significantly attenuated the depletion of myo-inositol and taurine. Brain glutamine concentrations, on the other hand, did not respond to hypothermia. These findings demonstrate that experimental ALF results in selective changes in brain organic osmolytes as a function of the degree of encephalopathy which are associated with brain edema, and provides a further rationale for the continued use of hypothermia in the management of this condition.

Effects of hypothermia on brain glucose metabolism in acute liver failure: a H/C-nuclear magnetic resonance study

BACKGROUND & AIMS

Mild hypothermia has a protective effect on brain edema and encephalopathy in both experimental and human acute liver failure. The goals of the present study were to examine the effects of mild hypothermia (35 degrees C) on brain metabolic pathways using combined (1)H and (13)C-Nuclear Magnetic Resonance (NMR) spectroscopy, a technique which allows the study not only of metabolite concentrations but also their de novo synthesis via cell-specific pathways in the brain.

METHODS

(1)H and (13)C NMR spectroscopy using [1-(13)C] glucose was performed on extracts of frontal cortex obtained from groups of rats with acute liver failure induced by hepatic devascularization whose body temperature was maintained either at 37 degrees C (normothermic) or 35 degrees C (hypothermic), and appropriate sham-operated controls.

RESULTS

At coma stages of encephalopathy in the normothermic acute liver failure animals, glutamine concentrations in frontal cortex increased 3.5-fold compared to sham-operated controls (P < 0.001). Comparable increases of brain glutamine were observed in hypothermic animals despite the absence of severe encephalopathy (coma). Brain glutamate and aspartate concentrations were respectively decreased to 60.9% +/- 7.7% and 42.2% +/- 5.9% (P < 0.01) in normothermic animals with acute liver failure compared to control and were restored to normal values by mild hypothermia. Concentrations of lactate and alanine in frontal cortex were increased to 169.2% +/- 15.6% and 267.3% +/- 34.0% (P < 0.01) respectively in normothermic rats compared to controls. Furthermore, de novo synthesis of lactate and alanine increased to 446.5% +/- 48.7% and 707.9% +/- 65.7% (P < 0.001), of control respectively, resulting in increased fractional (13)C-enrichments in these cytosolic metabolites. Again, these changes of lactate and alanine concentrations were prevented by mild hypothermia.

CONCLUSIONS

Mild hypothermia (35 degrees C) prevents the encephalopathy and brain edema resulting from hepatic devascularization, selectively normalizes lactate and alanine synthesis from glucose, and prevents the impairment of oxidative metabolism associated with this model of ALF, but has no significant effect on brain glutamine. These findings suggest that a deficit in brain glucose metabolism rather than glutamine accumulation is the major cause of the cerebral complications of acute liver failure.

Glutamine synthetase activity and glutamine content in brain: modulation by NMDA receptors and nitric oxide

Acute intoxication with large doses of ammonia leads to rapid death. The main mechanism for ammonia elimination in brain is its reaction with glutamate to form glutamine. This reaction is catalyzed by glutamine synthetase and consumes ATP. In the course of studies on the molecular mechanism of acute ammonia toxicity, we have found that glutamine synthetase activity and glutamine content in brain are modulated by NMDA receptors and nitric oxide. The main findings can be summarized as follows. Blocking NMDA receptors prevents ammonia-induced depletion of brain ATP and death of rats but not the increase in brain glutamine, indicating that ammonia toxicity is not due to increased activity of glutamine synthetase or formation of glutamine but to excessive activation of NMDA receptors. Blocking NMDA receptors in vivo increases glutamine synthetase activity and glutamine content in brain, indicating that tonic activation of NMDA receptors maintains a tonic inhibition of glutamine synthetase. Blocking NMDA receptors in vivo increases the activity of glutamine synthetase assayed in vitro, indicating that increased activity is due to a covalent modification of the enzyme. Nitric oxide inhibits glutamine synthetase, indicating that the covalent modification that inhibits glutamine synthetase is a nitrosylation or a nitration.Inhibition of nitric oxide synthase increases the activity of glutamine synthetase, indicating that the covalent modification is reversible and it must be an enzyme that denitrosylate or denitrate glutamine synthetase.NMDA mediated activation of nitric oxide synthase is responsible only for part of the tonic inhibition of glutamine synthetase. Other sources of nitric oxide are also contributing to this tonic inhibition. Glutamine synthetase is not working at maximum rate in brain and its activity may be increased pharmacologically by manipulating NMDA receptors or nitric oxide content. This may be useful, for example, to increase ammonia detoxification in brain in hyperammonemic situations.

Excitotoxic mechanism of cell swelling in rat cerebral cortical slices treated acutely with ammonia

Swelling of CNS cells due to endogenous ammonia is a major cause of cerebral oedema in hyperammonaemic encephalopathies. In the present study, incubation in the presence of 5mM ammonium acetate ("ammonia") decreased steady-state distribution of [14C]inulin within incubated rat cerebrocortical minislices, indicating cell swelling. NMDA receptor antagonists, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cycloheptene-5,10-imine maleate,10 microM) and DL-AP-5 (DL-2-amino-5-phosphonovaleric acid, 250 microM), a nitric oxide synthase inhibitor, L-nitroarginine (L-NNA, 500 microM), and an antioxidant, taurine (Tau, 10 mM), markedly attenuated the cell volume-increasing effect of ammonia. The effect of Tau (10mM) was abolished by the GABA(A) receptor antagonist bicuculline (100 microM), but was unaffected by the Tau transport inhibitor guanidynoethyl-sulfonate (GES, 500 microM). Ammonia increased the slice content of Gln, an amino acid whose excess accumulation has been implicated in hyperammonemic oedema. However, treatments that reduced the cell volume did not affect Gln content. These results indicate that ammonia-induced cell swelling is in a large degree mediated by overactivation of NMDA receptors and the ensuing generation of NO and free radicals.

Glutamine and acute illness

This is a literature review over a time period of the past 2 years concerning glutamine in clinical nutrition. Emphasis is put upon studies of glutamine in clinical settings, but a brief overview of the large range of literature over the role of glutamine in various experimental settings is also included. The most interesting concept for the past 2 years is the suggestion to use plasma glutamine concentration at admission to the intensive care unit as a prognostic marker and as a possible indicator for indication of glutamine supplementation.