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

Effects of shunt embolization on hepatic encephalopathy recurrence in patients with major portosystemic shunts: A systematic review and meta‑analysis

This study addresses the effects of shunt embolization on the recurrence of hepatic encephalopathy (HE) in patients with major portosystemic shunts. MEDLINE via PubMed, Google Scholar, and Scopus was searched to find the relevant full-text articles published from inception until August 2024. The primary outcome was the degree of HE or mental state change determined by the West-Heaven classification system. Dichotomous data were compared using odds ratios (OR). 95% confidence (CI) intervals were provided for each outcome in the report. The random-effects model was used to analyze the data. Trim and fill, Egger's regression and funnel plot were employed to evaluate publication bias in this body of literature. A total of 7 articles and 254 patients were included in the present meta-analysis. It was found that shunt embolization significantly reduced the recurrence of HE in patients with portosystemic shunts due to liver cirrhosis. Overall analysis showed that the pooled OR was 0.253 and the overall heterogeneity of the data was substantial (95% CI: 0.117-0.550, I 2=60.52% and P=0.001). The funnel plot was reasonably symmetrical and no study was trimmed to either side of the mean. Begg's (P=0.229) and Egger's tests (P=0.273) showed no significant risk of publication bias. Quality assessment showed that the majority of the included studies were of low quality. In conclusion, the present meta-analysis indicates that shunt embolization after portosystemic shunt significantly reduces the recurrence of HE in patients with liver cirrhosis. However, the findings should be interpreted with caution due to the low quality and low number of the included studies. Future research should prioritize higher-quality trials to validate these results and explore long-term outcomes.

TIPS and hepatic encephalopathy in patients with cirrhosis

Despite a better understanding in its prognosis and pathogenesis, hepatic encephalopathy (HE) remains one of the major complications of Transjugular Intrahepatic Portosystemic Shunt (TIPS) with a prevalence ranging from 35 to 50%. Its epidemiology differs according to the indication for TIPS (salvage/rescue TIPS, preemptive (pTIPS) or elective TIPS). In salvage/rescue TIPS, the prognosis is linked to that of bleeding, and HE should not be a contraindication to TIPS, especially as bleeding is a common precipitating factor of HE. In pTIPS, i.e. TIPS performed within the 72 h after stabilization of acute variceal bleeding in high-risk patients, the risk rebleeding and HE is reduced, when compared to endoscopic and drugs treatment. As a consequence, the Baveno VII recommendations state that HE at admission should not be considered as a contraindication to pTIPS placement. In elective situations, such as refractory (intractable ascites (intolerance to diuretics) or resistant ascites (i.e. despite optimal diuretic treatment (spironolactone 400 mg/d and Furosemide 160 mg/d combined with low-salt treatment (< 5.2 g/day) or recurrent ascites (the need for at least 3 paracenteses per year) and secondary prophylaxis of variceal bleeding, it is recommended to systematically look for risk factors for HE, and chronic or refractory HE remain not recommended to TIPS in most centers. Chronic HE involves persistent neurological symptoms with fluctuating acute episodes. Recurrent HE refers to repeated episodes occurring within 6 months, while refractory HE is resistant to standard treatments, often requiring more aggressive management (Vilstrup et al. 2014). A careful selection of patients is mandatory before elective TIPS decision. Risk factors must be identified and corrected if possible before any TIPS decision is made. Management of HE after TIPS is based on identification of precipitating factors, curative treatment with lactulose as first-line therapy and rifaximin as second-line therapy, and nutritional management. In elective TIPS, prophylactic administration of rifaximin is recommended in order to decrease the risk of further HE development in selected patients (not in everyone, at least according to Baveno VII). Liver transplantation (LT) should be discussed with a multidisciplinary team as an alternative option to TIPS in case of high-risk of post-TIPS HE, and in case of refractory HE after TIPS.

Impact of nutritional status on the outcome of transjugular intrahepatic portosystemic shunt in patients with cirrhosis: a systematic review

OBJECTIVES

Malnutrition and sarcopenia have been reported to adversely affect the outcome of patients with cirrhosis of the liver. There is an emerging body of evidence suggesting malnutrition and sarcopenia increase the risk of hepatic encephalopathy (HE) and mortality after transjugular intrahepatic portosystemic shunt (TIPS). The current systematic review aims to determine whether the body of evidence supports an association between nutritional status and post-TIPS outcomes in patients with cirrhosis.

METHODS

Electronic databases of PubMed, Embase, and Scopus were searched from inception to June 3, 2023, for studies analysing the effect of nutritional status on post-TIPS outcomes in patients with cirrhosis.

RESULTS

A total of 22 studies were included in the systemic review. Assessment of sarcopenia was done by skeletal muscle index (SMI) at the L3 level, transversal psoas muscle thickness, psoas muscle density, malnutrition as per ICD, relative sarcopenia with excess adiposity, lipid profile, controlling nutritional status score, body composition analysis, hospital frailty risk score, and visceral and subcutaneous fat area index. Ten out of 12 studies in this systematic review showed a significant association with the incidence of post-TIPS HE. Thirteen out of 14 studies reported that the presence of malnutrition was associated with increased odds of mortality following TIPS. One study reported sarcopenia as an independent predictor of liver failure, and another study reported that Pre-TIPS SMI was an independent predictor of substantial improvement in post-TIPS SMI.

CONCLUSIONS

The current systematic review shows that the presence of pre-TIPS malnutrition or sarcopenia is an independent predictor of adverse outcomes after TIPS. Incorporating these parameters into present prediction models can provide additional prognostic information.

ADVANCES IN KNOWLEDGE

Nutritional assessment should be part of the evaluation of patients planned for TIPS for prediction of adverse events after the procedure.

A novel score predicts mortality after transjugular intrahepatic portosystemic shunt: MOTS - Modified TIPS Score

BACKGROUND AND AIMS

The high risk for severe shunting-related post-interventional complications demands a stringent selection of candidates for transjugular intrahepatic portosystemic shunt (TIPS). We aimed to develop a simple and reliable tool to accurately predict early post-TIPS mortality.

METHODS

144 cases of TIPS implantation were retrospectively analysed. Using univariate and multivariate Cox regression analysis of factors predicting mortality within 90 days after TIPS, a score integrating urea, international normalized ratio (INR) and bilirubin was developed. The Modified TIPS-Score (MOTS) ranges from 0 to 3 points: INR >1.6, urea >71 mg/dl and bilirubin >2.2 mg/dl account for one point each. Additionally, MOTS was tested in an external validation cohort (n = 187) and its performance was compared to existing models.

RESULTS

Modified TIPS-Score achieved a significant prognostic discrimination reflected by 90-day mortality of 8% in patients with MOTS 0-1 and 60% in patients with MOTS 2-3 (p < .001). Predictive performance (area under the curve) of MOTS was accurate (c = 0.845 [0.73-0.96], p < .001), also in patients with renal insufficiency (c = 0.830 [0.64-1.00], p = .02) and in patients with refractory ascites (c = 0.949 [0.88-1.00], p < .001), which are subgroups with particular room for improvement of post-TIPS mortality prediction. The results were reproducible in the validation cohort.

CONCLUSIONS

Modified TIPS-Score is a novel, practicable tool to predict post-TIPS mortality, that can significantly simplify clinical decision making. Its practical applicability should be further investigated.

Review article: post-TIPSS hepatic encephalopathy-current knowledge and future perspectives

BACKGROUND

In light of the global rise in the burden of chronic liver diseases and liver cirrhosis, the number of patients suffering from decompensation events is expected to increase. Transjugular intrahepatic portosystemic shunts (TIPSS) provide effective long-term symptom control and may prolong transplant-free survival in portal hypertension-driven recurrent ascites and variceal bleeding. New-onset or recurrent hepatic encephalopathy (HE) after TIPSS insertion (post-TIPSS HE) represents the most severe post-interventional complication.

AIMS

To provide insight into the epidemiology and risk factors for post-TIPSS HE and scrutinize the current state of the art in treatment and drug therapy options.

METHODS

We conducted a literature search on post-TIPSS HE in patients with liver cirrhosis.

RESULTS

Post-TIPSS HE occurs in up to 54.5% of cases and particularly early recurrent HE is associated with a dismal prognosis. In recent years, several risk factors for the development of post-TIPSS HE have been identified. These include not only parameters reflecting liver function (model for end-stage liver disease score/Child-Pugh score) as well as cognitive dysfunction caused by minimal HE but also extrahepatic factors such as sarcopenia and common medications such as proton pump inhibitors. In addition, new data on the benefit of rifaximin and of smaller stent grafts emerged and may improve the prevention of post-TIPSS HE.

CONCLUSIONS

Careful selection of TIPSS candidates is of utmost importance to reduce the risk of post-TIPSS HE. In this narrative review, we provide a concise overview of the current epidemiology and risk factors of the treatment options for post-TIPSS HE.

Mitochondrial Metabolism behind Region-Specific Resistance to Ischemia-Reperfusion Injury in Gerbil Hippocampus. Role of PKCβII and Phosphate-Activated Glutaminase

Ischemic episodes are a leading cause of death worldwide with limited therapeutic interventions. The current study explored mitochondrial phosphate-activated glutaminase (GLS1) activity modulation by PKCβII through GC-MS untargeted metabolomics approach. Mitochondria were used to elucidate the endogenous resistance of hippocampal CA2-4 and dentate gyrus (DG) to transient ischemia and reperfusion in a model of ischemic episode in gerbils. In the present investigation, male gerbils were subjected to bilateral carotids occlusion for 5 min followed by reperfusion (IR). Gerbils were randomly divided into three groups as vehicle-treated sham control, vehicle-treated IR and PKCβII specific inhibitor peptide βIIV5-3-treated IR. Vehicle or βIIV5-3 (3 mg/kg, i.v.) were administered at the moment of reperfusion. The gerbils hippocampal tissue were isolated at various time of reperfusion and cell lysates or mitochondria were isolated from CA1 and CA2-4,DG hippocampal regions. Recombinant proteins PKCβII and GLS1 were used in in vitro phosphorylation reaction and organotypic hippocampal cultures (OHC) transiently exposed to NMDA (25 μM) to evaluate the inhibition of GLS1 on neuronal viability. PKCβII co-precipitates with GAC (GLS1 isoform) in CA2-4,DG mitochondria and phosphorylates GLS1 in vitro. Cell death was dose dependently increased when GLS1 was inhibited by BPTA while inhibition of mitochondrial pyruvate carrier (MPC) attenuated cell death in NMDA-challenged OHC. Fumarate and malate were increased after IR 1h in CA2-4,DG and this was reversed by βIIV5-3 what correlated with GLS1 activity increases and earlier showed elevation of neuronal death (Krupska et al., 2017). The present study illustrates that CA2-4,DG resistance to ischemic episode at least partially rely on glutamine and glutamate utilization in mitochondria as a source of carbon to tricarboxylic acid cycle. This phenomenon depends on modulation of GLS1 activity by PKCβII and remodeling of MPC: all these do not occur in ischemia-vulnerable CA1.

TNFα increases STAT3-mediated expression of glutaminase isoform KGA in cultured rat astrocytes

Glutamate related excitotoxicity and excess of cerebral levels of tumor necrosis factor alpha (TNFα) are interrelated and well documented abnormalities noticed in many central nervous system diseases. Contribution of kidney type glutaminase (KGA) and shorter alternative splicing form (GAC) to glutamine degradation in astrocytes has been recently a matter of dispute and extensive study but the regulation of the GLS isoforms by inflammatory factors is still not well known. Here we show that treatment of cultured rat cortical astrocytes with pathophysiologically relevant (50 ng/ml) concentration of TNFα specifically increases the expression of KGA but not GAC and increases activity of GLS. No changes in the expression of either of two GLS isoforms were observed following treatment with other tested cytokines IL-1β and IL-6. The TNFα mediated KGA expression was associated with increased phosphorylation of signal transducer and activator of transcription 3 (STAT3). Stimulatory effect of TNF-α on KGA expression was reduced by selective inhibition of (STAT3) but not by inhibition of STAT1 nor nuclear transcription factor kappa. Additionally, the role of miRNA in TNFα-induced expression of KGA in astrocytes was excluded, since the expression of miR-23a/b and miR-200c, potential regulators of KGA expression, was unchanged. This study documents increased KGA expression in the astrocytes under inflammatory stimulation, identifying TNFα as a cytokine mediating this response, and demonstrates the specific and selective involvement of STAT3.

Portacaval shunting causes differential mitochondrial superoxide production in brain regions

The portacaval shunting (PCS) prevents portal hypertension and recurrent bleeding of esophageal varices. On the other hand, it can induce chronic hyperammonemia and is considered to be the best model of mild hepatic encephalopathy (HE). Pathogenic mechanisms of HE and dysfunction of the brain in hyperammonemia are not fully elucidated, but it was originally suggested that the pathogenetic defect causes destruction of antioxidant defense which leads to an increase in the production of reactive oxygen species (ROS) and the occurrence of oxidative stress. In order to gain insight into the pathogenic mechanisms of HE in the brain tissue, we investigated the effects of PCS in rats on free radicals production and activity levels of antioxidant and prooxidant enzymes in mitochondria isolated from different brain areas. We found that O₂^·- production, activities of Mn-superoxide dismutase (Mn-SOD), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione transferase (GT), nitric oxide synthase (NOS), and levels of carbonylated proteins differed between the four brain regions both in the amount and response to PCS. In PCS rats, Mn-SOD activity in the cerebellum was significantly decreased, and remained unchanged in the neocortex, hippocampus and striatum compared with that in sham-operated animals. Among the four brain regions in control rats, the levels of the carbonyl groups in mitochondrial proteins were maximal in the cerebellum. 4 weeks after PCS, the content of carbonylated proteins were higher only in mitochondria of this brain region. Under control conditions, O₂^·- production by submitochondrial particles in the cerebellum was significantly higher than in other brain regions, but was significantly increased in each brain region from PCS animals. Indeed, the production of O₂^·- by submitochondrial particles correlated with mitochondrial ammonia levels in the four brain regions of control and PCS-animals. These findings are the first to suggest that in vivo levels of ammonia in the brain directly affect the rate of mitochondrial O₂^·- production.

Current diagnosis and management of post-transjugular intrahepatic portosystemic shunt refractory hepatic encephalopathy

Transjugular intrahepatic portosystemic shunt has evolved into an important option for management of complications of portal hypertension. The use of polytetrafluoroethylene covered stents enhances shunt patency. Hepatic encephalopathy (HE) remains a significant problem after TIPS placement. The approach to management of patients with refractory hepatic encephalopathy typically requires collaboration between different specialties. Patient selection for TIPS requires careful evaluation of risk factors for HE. TIPS procedure-related technical factors like stent size, attention to portosystemic pressure gradient reduction and use of adjunctive variceal embolization maybe important. Conservative medical therapy in combination with endovascular therapies often results in resolution or substantial reduction of symptoms. Liver transplantation is, however, the ultimate treatment.

Acute liver failure in rats activates glutamine-glutamate cycle but declines antioxidant enzymes to induce oxidative stress in cerebral cortex and cerebellum

Background and Purpose

Liver dysfunction led hyperammonemia (HA) causes a nervous system disorder; hepatic encephalopathy (HE). In the brain, ammonia induced glutamate-excitotoxicity and oxidative stress are considered to play important roles in the pathogenesis of HE. The brain ammonia metabolism and antioxidant enzymes constitute the main components of this mechanism; however, need to be defined in a suitable animal model. This study was aimed to examine this aspect in the rats with acute liver failure (ALF).

Methods

ALF in the rats was induced by intraperitoneal administration of 300 mg thioacetamide/Kg. b.w up to 2 days. Glutamine synthetase (GS) and glutaminase (GA), the two brain ammonia metabolizing enzymes vis a vis ammonia and glutamate levels and profiles of all the antioxidant enzymes vis a vis oxidative stress markers were measured in the cerebral cortex and cerebellum of the control and the ALF rats.

Results

The ALF rats showed significantly increased levels of ammonia in the blood (HA) but little changes in the cortex and cerebellum. This was consistent with the activation of the GS-GA cycle and static levels of glutamate in these brain regions. However, significantly increased levels of lipid peroxidation and protein carbonyl contents were consistent with the reduced levels of all the antioxidant enzymes in both the brain regions of these ALF rats.

Conclusion

ALF activates the GS-GA cycle to metabolize excess ammonia and thereby, maintains static levels of ammonia and glutamate in the cerebral cortex and cerebellum. Moreover, ALF induces oxidative stress by reducing the levels of all the antioxidant enzymes which is likely to play important role, independent of glutamate levels, in the pathogenesis of acute HE.

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

BACKGROUND & AIMS

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Covert hepatic encephalopathy: not as minimal as you might think

Hepatic encephalopathy (HE) is a serious neuropsychiatric and neurocognitive complication of acute and chronic liver disease. Symptoms are often overt (confusion, disorientation, ataxia, or coma) but can also be subtle (difficulty with cognitive abilities such as executive decision-making and psychomotor speed). There is consensus that HE is characterized as a spectrum of neuropsychiatric symptoms in the absence of brain disease, ranging from overt HE (OHE) to minimal HE (MHE). The West Haven Criteria are most often used to grade HE, with scores ranging from 0-4 (4 being coma). However, it is a challenge to diagnose patients with MHE or grade 1 HE; it might be practical to combine these entities and name them covert HE for clinical use. The severity of HE is associated with the stage of liver disease. Although the pathologic mechanisms of HE are not well understood, they are believed to involve increased levels of ammonia and inflammation, which lead to low-grade cerebral edema. A diagnosis of MHE requires dedicated psychometric tests and neurophysiological techniques rather than a simple clinical assessment. Although these tests can be difficult to perform in practice, they are cost effective and important; the disorder affects patients' quality of life, socioeconomic status, and driving ability and increases their risk for falls and the development of OHE. Patients with MHE are first managed by excluding other causes of neurocognitive dysfunction. Therapy with gut-specific agents might be effective. We review management strategies and important areas of research for MHE and covert HE.

Hepatic encephalopathy after transjugular intrahepatic portosystemic shunt

Transjugular intrahepatic portosystemic shunt (TIPS) has been used for more than 20 years to treat some of the complications of portal hypertension. When TIPS was initially proposed, it was claimed that the optimal calibration of the shunt could allow an adequate reduction of portal hypertension, avoiding, at the same time, the occurrence of hepatic encephalopathy (HE), a neurologic syndrome. However, several clinical observations have shown that HE occurred rather frequently after TIPS, and HE has become an important issue to be taken into consideration in TIPS candidates and a problem to be faced after the procedure.

Brain biomolecules oxidation in portacaval-shunted rats

BACKGROUND

Oxidative stress induced by a high ammonia concentration has been suggested to be implicated in the pathophysiology of hepatic encephalopathy (HE). Therefore, oxidative damage of brain biomolecules could contribute towards explaining the neurological and motor alterations observed in HE.

METHODS

Portacaval-shunted (PCS) rats (n = 5) were used as an animal model of chronic HE. Plasma and brain ammonia were measured by the l-glutamate dehydrogenase method. Reactive oxygen species was measured by the dichlorodihydrofluorescein diacetate method. Lipid peroxidation was measured as thiobarbituric acid-reactive substances (TBARS) by a colorimetric method; malondialdehyde (MDA) and 4-hydroxy-2-noneal (HNE) were measured by HPLC and an immunological method respectively. Protein oxidation (carbonylation) was measured as total carbonyl after labelling with 2,4-dinitrophenyl hydrazine (DNPH) using a spectrophotometric method. Individual protein oxidation was studied, after labelling with DNPH and its separation by one-dimensional (1D) electrophoresis, by an immunological method.

RESULTS

Ammonia-induced oxidative stress in PCS rats was associated with increased MDA and HNE, together with increased protein oxidation, evidenced by total carbonyl quantification and by the analysis of individual protein bands separated by 1D electrophoresis. However, lipid peroxidation measured as TBARS did not show differences.

CONCLUSION

Our data show an increased evidence of oxidative stress in PCS rat brain; moreover, PCS rat brain proteins are oxidized (carbonylated), some proteins being more sensitive to oxidation than others. These data also show that at least six specific brain proteins in PCS rats are highly sensitive to carbonylation. Identification of these proteins may be crucial for a better understanding of HE pathophysiology.

Current concepts in the pathophysiology and management of hepatic encephalopathy

Hepatic encephalopathy (HE) represents a broad continuum of neuropsychological dysfunction in patients with acute or chronic liver disease and/or portosystemic shunting of blood flow. The pathophysiology of this disease is quite complex, as it involves overproduction and reduced metabolism of various neurotoxins, particularly ammonia. Recent hypotheses implicate low-grade cerebral edema as a final common pathway for the pathophysiology of HE. Management of this condition is multifaceted and requires several steps: elimination of precipitating factors; removal of toxins, both by reducing them at their source and by augmenting scavenging pathways; modulation of resident fecal flora; proper nutritional support; and downregulation of systemic and gut-derived inflammation.

Role of astrocytes in brain function and disease

Astrocytes assume multiple roles in maintaining an optimally suited milieu for neuronal function. Select astrocytic functions include the maintenance of redox potential, the production of trophic factors, the regulation of neurotransmitter and ion concentrations, and the removal of toxins and debris from the cerebrospinal fluid (CSF). Impairments in these and other functions, as well as physiological reactions of astrocytes to injury, can trigger or exacerbate neuronal dysfunction. This review addresses select metabolic interactions between neurons and astrocytes and emphasizes the role of astrocytes in mediating and amplifying the progression of several neurodegenerative disorders, such as Parkinson's disease (PD), hepatic encephalopathy (HE), hyperammonemia (HA), Alzheimer's disease (AD), and ischemia.

Hepatic encephalopathy therapy: An overview

Type-C hepatic encephalopathy (HE) is a severe complication of cirrhosis, which seriously affects quality of life and is strongly related to patient survival. Treatment based on a classical pharmacological approach that is aimed at reducing the production of gut-derived toxins, such as ammonia, is still under debate. Currently, results obtained from clinical trials do not support any specific treatment for HE and our competence in testing old and new treatment modalities by randomized controlled trials with appropriate clinically relevant end-points urgently needs to be improved. On the other hand, patients who are at risk for HE are now identifiable, based on studies on the natural history of the disease. Today, very few studies that are specifically aimed at establishing whether HE may be prevented are available or in progress. Recent studies have looked at non absorbable disaccharides or antibiotics and other treatment modalities, such as the modulation of intestinal flora. In the treatment of severe stage HE, artificial liver supports have been tested with initial positive results but more studies are needed.

Energy metabolism in brain cells: effects of elevated ammonia concentrations

Both neurons and astrocytes have high rates of glucose utilization and oxidative metabolism. Fully 20% of glucose consumption is used for astrocytic production of glutamate and glutamine, which during intense glutamatergic activity leads to an increase in glutamate content, but at steady state is compensated for by an equally intense oxidation of glutamate. The amounts of ammonia used for glutamine synthesis and liberated during glutamine hydrolysis are large, compared to the additional demand for glutamine synthesis in hyperammonemic animals and patients with hepatic encephalopathy. Nevertheless, elevated ammonia concentrations lead to an increased astrocytic glutamine production and an elevated content of glutamine combined with a decrease in glutamate content, probably mainly in a cytosolic pool needed for normal activity of the malate-asparate shuttle (MAS); another compartment generated by glutamine hydrolysis is increased. As a result of reduced MAS activity the pyruvate/lactate ratio is decreased in astrocytes but not in neurons and decarboxylation of pyruvate to form acetyl coenzyme A is reduced. Elevated ammonia concentrations also inhibit decarboxylation of alpha-ketoglutarate in the TCA cycle. This effect occurs in both neurons and astrocytes, is unrelated to MAS activity and seen after chronic treatment with ammonia even in the absence of elevated ammonia concentrations.