Brain metabolism of 13N-ammonia during acute hepatic encephalopathy in cirrhosis measured by positron emission tomography

Production, Validation, and Exposure Dose Measurement of [13N]Ammonia Under Academic Good Manufacturing Practice Environments

Objective: Current good manufacturing practice (cGMP) guidance for positron emission tomography (PET) drugs has been established in Europe and the United States. In Japan, the Pharmaceuticals and Medical Devices Agency (PMDA) approved the use of radiosynthesizers as medical devices for the in-house manufacturing of PET drugs in hospitals and clinics, regardless of the cGMP environment. Without adequate facilities, equipment, and personnel required by cGMP regulations, the quality assurance (QA) and clinical effectiveness of PET drugs largely depend on the radiosynthesizers themselves. To bridge the gap between radiochemistry standardization and site qualification, the Japanese Society of Nuclear Medicine (JSNM) has issued guidance for the in-house manufacturing of small-scale PET drugs under academic GMP (a-GMP) environments. The goals of cGMP and a-GMP are different: cGMP focuses on process optimization, certification, and commercialization, while a-GMP facilitates the small-scale, in-house production of PET drugs for clinical trials and patient-specific standard of care. Among PET isotopes, N-13 has a short half-life (10 min) and must be synthesized on site. [13N]Ammonia ([13N]NH3) is used for myocardial perfusion imaging under the Japan Health Insurance System (JHIS) and was thus selected as a working example for the manufacturing of PET drugs in an a-GMP environment. Methods: A [13N]NH3-radiosynthesizer was installed in a hot cell within an a-GMP-compliant radiopharmacy unit. To comply with a-GMP regulations, the air flow was adjusted through HEPA filters. All cabinets and cells were disinfected to ensure sterility once a month. Standard operating procedures (SOPs) were applied, including analytical methods. Batch records, QA data, and radiation exposure to staff in the synthesis of [13N]NH3 were measured and documented. Results: 2.52 GBq of [13N]NH3 end-of-synthesis (EOS) was obtained in an average of 13.5 min in 15 production runs. The radiochemical purity was more than 99%. Exposure doses were 11 µSv for one production run and 22 µSv for two production runs. The pre-irradiation background dose rate was 0.12 µSv/h. After irradiation, the exposed dosage in the front of the hot cell was 0.15 µSv/h. The leakage dosage measured at the bench was 0.16 µSv/h. The exposure and leakage dosages in the manufacturing of [13N]NH3 were similar to the background level as measured by radiation monitoring systems in an a-GMP environments. All QAs, environmental data, bacteria assays, and particulates met a-GMP compliance standards. Conclusions: In-house a-GMP environments require dedicated radiosynthesizers, documentation for batch records, validation schedules, radiation protection monitoring, air and particulate systems, and accountable personnel. In this study, the in-house manufacturing of [13N]NH3 under a-GMP conditions was successfully demonstrated. These findings support the international harmonization of small-scale PET drug manufacturing in hospitals and clinics for future multi-center clinical trials and the development of a standard of care.

Hepatic encephalopathy in cirrhosis: therapies and developments

Hepatic encephalopathy (HE) is a frequent decompensation in patients with cirrhosis, which significantly affects morbidity and mortality. Ammonia is a major neurotoxin implicated in the pathogenesis, progression, and severity of HE, and various organs including the gut, muscle, kidney, and brain are involved in its metabolism. Therefore, therapeutic management involves reducing ammonia production and increasing its elimination from the blood and the brain. Prevention of HE in patients at high risk of first and recurrent episodes is important for prolonging survival. Various anti-ammonia therapies with synergistic and complementary actions have been attempted for overt HE and for prophylaxis of the first and recurrent episodes of HE. In the current review, we summarize the currently used and under-development pharmacotherapies/procedure(s) for HE in cirrhosis and their mechanism of action. Primary and secondary prophylaxis with monotherapies and combination therapies are also discussed.

Different cortical and subcortical astroglial responsiveness in rats with acute liver failure

Hepatic encephalopathy (HE) is a neuropsychiatric complication of liver failure. Previous studies described astroglia alterations in HE, but regional changes have not been well investigated. This study addresses regional astroglial response by exploring glial fibrillary acidic protein (GFAP) immunoreactivity in cortical structures including somatosensory (S1Tr and S1BF), piriform (Pir), and perirhinal (PRh) cortices, and subcortical regions including corpus callosum (CC), ventromedial thalamus (VMT), mammillothalamic tract (MTT), and dorsomedial hypothalamic nucleus (DHN) in rats with acute liver failure (ALF) sacrificed at coma stage. Our data showed decreased numbers of astrocytes in S1Tr, Pir, and CC in ALF rats. GFAP-immunoreactive cells were increased within other regions including PRh, VMT, MTT, and DHN. Cell morphometric analysis showed significant increase in GFAP-immunoreactive astrocyte processes and cell bodies in cortical and subcortical regions but not in CC and DHN. However, astrocyte perimeters were increased, particularly in S1Tr and VMT. Our study demonstrates regional specificity including (1) regions with astrocyte activation associated with an increase of GFAP-immunostaining and astrocyte cell counts, together with (2) unaltered GFAP components, and (3) regions characterized by presumably inactive astrocyte with a reduced GFAP-immunostaining. These findings may reflect either different regional alterations in HE, or stages of an alteration progressing differently in different regions.

Down the road towards hepatic encephalopathy. The elusive ammonia- what determines the arterial concentration?

Elevated arterial ammonia is associated with several complications of liver disease as it predicts mortality for in-patients and decompensation, hospitalization and death in out-patients with cirrhosis. In this review, our aim was to estimate how the individual organs contribute to arterial ammonia based on published data from human studies. The brain removes ammonia from arterial blood in a concentration-dependent fashion. Ammonia that is released from the gut to portal blood is mainly from metabolism of glutamine in the enterocytes using this as a source of energy. Ammonia produced by bacterial metabolism of urea and proteins only partially reach portal blood and is likely recycled into bacterial proteins. In general, the liver efficiently removes ammonia from arterial or portal blood in proportion to the delivered concentration. As a result,- and in some contrast to conventional wisdom-, the hepato-splanchnic region only contributes marginally to arterial ammonia; even during a simulated upper GI bleed. The only exception is acute liver failure where hepatocyte necrosis allows large quantities of portal ammonia to pass. The kidneys release ammonia from glutamine metabolism into systemic blood. The renal ammonia release increases during a simulated upper GI bleed or hypokalemia where it becomes a major source of elevated arterial ammonia. In the resting state, muscles remove ammonia in a concentration-dependent manner and muscles are the primary ammonia lowering organ in most situations with elevated arterial ammonia. During strenuous exercise, muscles produce large amounts of ammonia into systemic blood. Thus, the complete pattern of ammonia metabolism is very dynamic and illustrates the difficulties in designing ammonia lowering therapies.

Hepatic encephalopathy as a result of ammonia-induced increase in GABAergic tone with secondary reduced brain energy metabolism

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome caused by liver insufficiency and/or portosystemic shunting. HE is mostly episodic and as such reversible. Hyperammonemia clearly plays a key role in the pathophysiology, but the precise detrimental events in the brain leading to HE remain equivocal. Several pathogenic models have been proposed, but few have been linked to clinical studies and observations. Decreased oxygen metabolism is observed in both type A and C HE and in this review, we advocate that this reflects an actual reduced oxygen demand and not a primary cause of HE. As driving force, we propose that the hyperammonemia via astrocytic glutamine synthetase causes an increased γ-aminobutyric acid (GABA) mediated neuro-inhibition which subsequently leads to an overall decreased energy demand of the brain, something that can be enhanced by concomitant neuroinflammation. This also explains the reversibility of the condition.

Correlation of ammonia and blood laboratory parameters with hepatic encephalopathy: A systematic review and meta-analysis

BACKGROUND AND OBJECTIVES

Emerging research suggests that hyperammonemia may enhance the probability of hepatic encephalopathy (HE), a condition associated with elevated levels of circulating ammonia in patients with cirrhosis. However, some studies indicate that blood ammonia levels may not consistently correlate with the severity of HE, highlighting the complex pathophysiology of this condition.

METHODS

A systematic review and meta-analysis through PubMed, Scopus, Embase, Web of Science, and Virtual Health Library were conducted to address this complexity, analyzing and comparing published data on various laboratory parameters, including circulating ammonia, blood creatinine, albumin, sodium, and inflammation markers in cirrhotic patients, both with and without HE.

RESULTS

This comprehensive review, which included 81 studies from five reputable databases until June 2024, revealed a significant increase in circulating ammonia levels in cirrhotic patients with HE, particularly those with overt HE. Notably, significant alterations were observed in the circulating creatinine, albumin, sodium, interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNFα) in HE patients.

CONCLUSIONS

These findings suggest an association between ammonia and HE and underscore the importance of considering other blood parameters such as creatinine, albumin, sodium, and pro-inflammatory cytokines when devising new treatment strategies for HE.

Cerebral Aspects of Portal Hypertension: Hepatic Encephalopathy

Portal hypertension has cerebral consequences via its causes and complications, namely hepatic encephalopathy (HE), a common and devastating brain disturbance caused by liver insufficiency and portosystemic shunting. The pathogenesis involves hyperammonemia and systemic inflammation. Symptoms are disturbed personality and reduced attention. HE is minimal or grades I to IV (coma). Bouts of HE are episodic and often recurrent. Initial treatment is of events that precipitated the episode and exclusion of nonhepatic causes. Specific anti-HE treatment is lactulose. By recurrence, rifaximin is add-on. Anti-HE treatment is efficacious also for prophylaxis, but emergence of HE marks advanced liver disease and a dismal prognosis.

Elevated serum neurofilament light chain levels are associated with hepatic encephalopathy in patients with cirrhosis

Increasing evidences implicate vital role of neuronal damage in the development of hepatic encephalopathy (HE). Neurofilament light chain (NfL) is the main frame component of neurons and is closely related to axonal radial growth and neuronal structural stability. We hypothesized that NfL as a biomarker of axonal injury may contribute to early diagnosis of HE. This study recruited 101 patients with liver cirrhosis, 10 healthy individuals, and 7 patients with Parkinson's disease. Minimal hepatic encephalopathy (MHE) was diagnosed using psychometric hepatic encephalopathy score. Serum NfL levels were measured by the electrochemiluminescence immunoassay. Serum NfL levels in cirrhotic patients with MHE were significantly higher than cirrhotic patients without MHE, and increased accordingly with the aggravation of HE. Serum NfL levels were associated with psychometric hepatic encephalopathy score, Child-Pugh score, model for end-stage liver disease score, and days of hospitalization. Additionally, serum NfL was an independent predictor of MHE (odds ratio of 1.020 (95% CI 1.005-1.034); P = 0.007). The discriminative abilities of serum NfL were high for identifying MHE (AUC of 0.8134 (95% CI 0.7130-0.9219); P ˂ 0.001) and OHE (AUC of 0.8852 (95% CI 0.8117-0.9587); P ˂ 0.001). Elevated serum NfL levels correlated with the presence of MHE and associated with the severity of HE, are expected to be a biomarker in patients with cirrhosis. Our study suggested that neuronal damage may play a critical role in the development of HE.

Mitochondrial Changes in Rat Brain Endothelial Cells Associated with Hepatic Encephalopathy: Relation to the Blood-Brain Barrier Dysfunction

The mechanisms underlying cerebral vascular dysfunction and edema during hepatic encephalopathy (HE) are unclear. Blood-brain barrier (BBB) impairment, resulting from increased vascular permeability, has been reported in acute and chronic HE. Mitochondrial dysfunction is a well-documented result of HE mainly affecting astrocytes, but much less so in the BBB-forming endothelial cells. Here we review literature reports and own experimental data obtained in HE models emphasizing alterations in mitochondrial dynamics and function as a possible contributor to the status of brain endothelial cell mitochondria in HE. Own studies on the expression of the mitochondrial fusion-fission controlling genes rendered HE animal model-dependent effects: increase of mitochondrial fusion controlling genes opa1, mfn1 in cerebral vessels in ammonium acetate-induced hyperammonemia, but a decrease of the two former genes and increase of fis1 in vessels in thioacetamide-induced HE. In endothelial cell line (RBE4) after 24 h ammonia and/or TNFα treatment, conditions mimicking crucial aspects of HE in vivo, we observed altered expression of mitochondrial fission/fusion genes: a decrease of opa1, mfn1, and, increase of the fission related fis1 gene. The effect in vitro was paralleled by the generation of reactive oxygen species, decreased total antioxidant capacity, decreased mitochondrial membrane potential, as well as increased permeability of RBE4 cell monolayer to fluorescein isothiocyanate dextran. Electron microscopy documented enlarged mitochondria in the brain endothelial cells of rats in both in vivo models. Collectively, the here observed alterations of cerebral endothelial mitochondria are indicative of their fission, and decreased potential of endothelial mitochondria are likely to contribute to BBB dysfunction in HE.

Congenital Portosystemic Shunts in Dogs and Cats: Classification, Pathophysiology, Clinical Presentation and Diagnosis

Congenital portosystemic shunts (CPSS) are abnormal vascular communications between the portal and the systemic circulation, bypassing the hepatic parenchyma and resulting in liver hypoplasia and hepatic insufficiency. Such connections develop in utero and persist postnatally. CPSS are among the two most common congenital vascular anomalies of the liver in small animals, along with primary hypoplasia of the portal vein without portal hypertension (PHPV without PH). CPSS can be extrahepatic (ECPSS), most commonly diagnosed in small and toy breed dogs and cats, or intrahepatic (ICPSS), most commonly seen in large breed dogs. Single ECPSS is the most common type encountered in both dogs and cats. Clinical signs of CPSS are non-specific and may wax and wane, while laboratory findings can raise clinical suspicion for CPSS, but they are also not specific. Definitive diagnosis will be established by evaluation of liver function tests, such as determination of fasting plasma ammonia (FA) levels, and pre- and postprandial serum bile acids concentrations, and diagnostic imaging. The purpose of this article is to review the definition, classification, pathogenesis, clinical presentation, and diagnosis of CPSS in dogs and cats, highlighted by the authors' clinical experience.

A glance at transient hyperammonemia of the newborn: Pathophysiology, diagnosis, and treatment: A review

Hyperammonemia is the excessive accumulation of ammonia in the blood, and is usually defined as a plasma level above 100 µmol/L in neonates or above 50 µmol/L in term infants, children, and adolescents. Patients with hyperammonemia usually experience life-threatening neuropsychiatric symptoms, especially newborns. It is routinely caused by inherited metabolic diseases and also by acquired disorders, such as liver failure, portosystemic shunting, gastrointestinal hemorrhage, ureterosigmoidostomy, renal tubular acidosis, hypoxic ischemic encephalopathy, infections with urea-metabolizing organisms, and some drugs. Transient hyperammonemia of the newborn (THAN) is a special type of hyperammonemia acknowledged in the field of metabolic disease as an inwell-defined or well-understood entity, which can be diagnosed only after the exclusion of genetic and acquired causes of hyperammonemia. Although the prognosis for THAN is good, timely identification and treatment are essential. Currently, THAN is underdiagnosed and much less is mentioned for early diagnosis and vigorous treatment. Herein, we present common themes that emerge from the pathogenesis, diagnosis, and management of THAN, based on current evidence. When a newborn presents with sepsis, intracranial hemorrhage, or asphyxia that cannot explain coma and seizures, doctors should always keep this disease in mind.

Hypokalaemia - an active contributor to hepatic encephalopathy?

Patients with cirrhosis are prone to electrolyte disorders, including hypokalaemia. The available evidence suggests that hypokalaemia facilitates hyperammonaemia and thus increases the risk for hepatic encephalopathy (HE). In case studies, plasma potassium decrements were followed by plasma ammonia increments and HE progression, which was reversed by potassium supplementation. The explanation to the hyperammonaemia may be that hypokalaemia both stimulates renal ammonia production and reduces hepatic ammonia elimination by urea synthesis. Further, hypokalaemia eases the entrance of the increased ammonia into the central nervous system because the lower potassium ion concentration favours the competition of NH₄⁺ ions for potassium transporters across the blood brain barrier, and because hypokalaemia-induced metabolic alkalosis increases the amount of gaseous ammonia, which freely passes the barrier. Potassium depletion thus seems to be a mechanistic contributor to HE, supporting the clinical notion of routinely correcting low potassium in patients with cirrhosis.

Low cerebral energy metabolism in hepatic encephalopathy reflects low neuronal energy demand. Role of ammonia-induced increased GABAergic tone

Hepatic encephalopathy (HE) is a frequent and devastating but generally reversible neuropsychiatric complication secondary to chronic and acute liver failure. During HE, brain energy metabolism is markedly reduced and it remains unclear whether this is due to external or internal energy supply limitations, or secondary to depressed neuronal cellular functions - and if so, which mechanisms that are in play. The extent of deteriorated cerebral function correlates to blood ammonia levels but the metabolic link to ammonia is not clear. Early studies suggested that high levels of ammonia inhibited key tricarboxylic acid (TCA) cycle enzymes thus limiting mitochondrial energy production and oxygen consumption; however, later studies by us and others showed that this is not the case in vivo. Here, based on a series of translational studies from our group, we advocate the view that the low cerebral energy metabolism of HE is likely to be caused by neuronal metabolic depression due to an elevated GABAergic tone rather than by restricted energy availability. The increased GABAergic tone seems to be secondary to synthesis of large amounts of glutamine in astrocytes for detoxification of ammonia with the glutamine acting as a precursor for elevated neuronal synthesis of vesicular GABA.

Gut-derived systemic inflammation as a driver of depression in chronic liver disease

Depression and chronic liver disease (CLD) are important causes of disability, morbidity and mortality worldwide and their prevalence continues to rise. The rate of depression in CLD is high compared to that of the general population and is comparable to the increased rates observed in other medical comorbidities and chronic inflammatory conditions. Notably, a comorbid diagnosis of depression has a detrimental effect on outcomes in cirrhosis. Systemic inflammation is pivotal in cirrhosis-associated immune dysfunction - a phenomenon present in advanced CLD (cirrhosis) and implicated in the development of complications, organ failure, disease progression, increased infection rates and poor outcome. The presence of systemic inflammation is also well-documented in a cohort of patients with depression; peripheral cytokine signals can result in neuroinflammation, behavioural change and depressive symptoms via neural mechanisms, cerebral endothelial cell and circumventricular organ signalling, and peripheral immune cell-to-brain signalling. Gut dysbiosis has been observed in both patients with cirrhosis and depression. It leads to intestinal barrier dysfunction resulting in increased bacterial translocation, in turn activating circulating immune cells, leading to cytokine production and systemic inflammation. A perturbed gut-liver-brain axis may therefore explain the high rates of depression in patients with cirrhosis. The underlying mechanisms explaining the critical relationship between depression and cirrhosis remain to be fully elucidated. Several other psychosocial and biological factors are likely to be involved, and therefore the cause is probably multifactorial. However, the role of the dysfunctional gut-liver-brain axis as a driver of gut-derived systemic inflammation requires further exploration and consideration as a target for the treatment of depression in patients with cirrhosis.

Gut Microbes and Hepatic Encephalopathy: From the Old Concepts to New Perspectives

Hepatic encephalopathy (HE) is a severe complication of advanced liver disease and acute liver failure. The clinical spectrum ranges from minor cognitive dysfunctions to lethargy, depressed consciousness, and coma and significantly impact the quality of life, morbidity, and mortality of the patients. It is commonly accepted that the gut milieu is essential for the development of HE; however, despite intensive research efforts, the pathogenesis of HE is still not fully elucidated. As our knowledge of gut microbiota moves from the pioneering era of culture-dependent studies, the connection between microbes, inflammation, and metabolic pathways in the pathogenesis of HE is becoming increasingly clear, providing exciting therapeutic perspectives. This review will critically examine the latest research findings on the role of gut microbes in the pathophysiological pathways underlying HE. Moreover, currently available therapeutic options and novel treatment strategies are discussed.

Hepatic encephalopathy and depression in chronic liver disease: is the common link systemic inflammation?

Hepatic encephalopathy and depression share a number of clinical features, such as cognitive impairment and psychomotor retardation, and are highly prevalent in patients with chronic liver disease. Both conditions signify a poor prognosis, carry an increased mortality and are major determinants of reduced health related quality of life. The pathophysiology of hepatic encephalopathy is complex. Whilst cerebral accumulation of ammonia is well-recognised as being central to the development of hepatic encephalopathy, systemic inflammation, which acts in synergy with hyperammonaemia, is emerging as a key driver in its development. The pro-inflammatory state is also widely documented in depression, and peripheral to brain communication occurs resulting in central inflammation, behavioural changes and depressive symptoms. Gut dysbiosis, with a similar reduction in beneficial bacteria, increase in pathogens and decreased bacterial diversity, has been observed in both hepatic encephalopathy and depression, and it may be that the resultant increased bacterial translocation causes their shared inflammatory pathophysiology. Whilst the literature on a positive association between hepatic encephalopathy and depression in cirrhosis remains to be substantiated, there is evolving evidence that treatment with psychobiotics may be of dual benefit, improving cognition and mood in cirrhosis.

The neurogliovascular unit in hepatic encephalopathy

Hepatic encephalopathy (HE) is a neurological complication of hepatic dysfunction and portosystemic shunting. It is highly prevalent in patients with cirrhosis and is associated with poor outcomes. New insights into the role of peripheral origins in HE have led to the development of innovative treatment strategies like faecal microbiota transplantation. However, this broadening of view has not been applied fully to perturbations in the central nervous system. The old paradigm that HE is the clinical manifestation of ammonia-induced astrocyte dysfunction and its secondary neuronal consequences requires updating. In this review, we will use the holistic concept of the neurogliovascular unit to describe central nervous system disturbances in HE, an approach that has proven instrumental in other neurological disorders. We will describe HE as a global dysfunction of the neurogliovascular unit, where blood flow and nutrient supply to the brain, as well as the function of the blood-brain barrier, are impaired. This leads to an accumulation of neurotoxic substances, chief among them ammonia and inflammatory mediators, causing dysfunction of astrocytes and microglia. Finally, glymphatic dysfunction impairs the clearance of these neurotoxins, further aggravating their effect on the brain. Taking a broader view of central nervous system alterations in liver disease could serve as the basis for further research into the specific brain pathophysiology of HE, as well as the development of therapeutic strategies specifically aimed at counteracting the often irreversible central nervous system damage seen in these patients.

Diagnostics of Inherited Metabolic Diseases in Newborns with the Hyperammonemia Syndrome at the Onset of Disease (Pilot Study)

The aim of the study was to develop a diagnostic model that allows with a high degree of probability predicting the development of inherited metabolic disease (IMD) in newborns with the hyperammonemia syndrome at the onset of disease and determine the adequate management tactics for such patients.

Evaluation of [13N]ammonia positron emission tomography as a potential method for quantifying glutamine synthetase activity in the human brain

PURPOSE

The conversion of synaptic glutamate to glutamine in astrocytes by glutamine synthetase (GS) is critical to maintaining healthy brain activity and may be disrupted in several brain disorders. As the GS catalysed conversion of glutamate to glutamine requires ammonia, we evaluated whether [13N]ammonia positron emission tomography (PET) could reliability quantify GS activity in humans.

METHODS

In this test-retest study, eight healthy volunteers each received two dynamic [13N]ammonia PET scans on the morning and afternoon of the same day. Each [13N]ammonia scan was preceded by a [15O]water PET scan to account for effects of cerebral blood flow (CBF).

RESULTS

Concentrations of radioactive metabolites in arterial blood were available for both sessions in five of the eight subjects. Our results demonstrated that kinetic modelling was unable to reliably distinguish estimates of the kinetic rate constant k3 (related to GS activity) from K1 (related to [13N]ammonia brain uptake), and indicated a non-negligible back-flux of [13N] to blood (k2). Model selection favoured a reversible one-tissue compartmental model, and [13N]ammonia K1 correlated reliably (r2 = 0.72-0.92) with [15O]water CBF.

CONCLUSION

The [13N]ammonia PET method was unable to reliably estimate GS activity in the human brain but may provide an alternative index of CBF.

Hepatic encephalopathy: Novel insights into classification, pathophysiology and therapy

Hepatic encephalopathy (HE) is a frequent and serious complication of both chronic liver disease and acute liver failure. HE manifests as a wide spectrum of neuropsychiatric abnormalities, from subclinical changes (mild cognitive impairment) to marked disorientation, confusion and coma. The clinical and economic burden of HE is considerable, and it contributes greatly to impaired quality of life, morbidity and mortality. This review will critically discuss the latest classification of HE, as well as the pathogenesis and pathophysiological pathways underlying the neurological decline in patients with end-stage liver disease. In addition, management strategies, diagnostic approaches, currently available therapeutic options and novel treatment strategies are discussed.

In vitro inhibition of brain phosphate-activated glutaminase by ammonia and manganese

INTRODUCTION

As a consequence of the loss of liver function in chronic liver disease, increased levels of ammonia, manganese, and glutamine have been observed in the brain of hepatic encephalopathy patients.

OBJECTIVE

In the present study, we explored phosphate activated glutaminase (PAG) activity in mitochondrial enriched fractions under treatment with ammonia and manganese.

METHODS

We dissected out the brain cortex, striatum, and cerebellum of male Wistar rats 250-280 g weight; brain sections were pooled to obtain enriched mitochondrial fractions by differential centrifugation. Aliquots equivalent to 200 μg of protein were incubated with semi-log increasing concentrations of ammonia and/or manganese both as chloride salts (from 0 to 10 000 μM) and glutamine (4 mM) for 30 min. Then, the glutamate produced by the reaction was determined by HPLC coupled with fluorescence detection.

RESULTS AND DISCUSSION

Both manganese and ammonia inhibited PAG in a concentration-dependent manner. Non-linear modeling was used to determine IC50 and IC20 for ammonia (120 μM) and manganese (2 mM). We found that PAG activity under the combination of IC20 of ammonia and manganese was equivalent to the sum of the effects of both substances, being PAG inhibition more pronounced in mitochondrial fractions from cerebellum. The PAG inhibition observed here could potentially explain a pathway for glutamine accumulation, by means of the inhibition of PAG activity as a consequence of increased concentrations of manganese and ammonia in the brain under liver damage conditions.

Chronic Valproic Acid Administration Increases Plasma, Liver, and Brain Ammonia Concentration and Suppresses Glutamine Synthetase Activity

Asymptomatic valproic acid (VPA)-induced hyperammonemia in the absence of liver impairment is fairly common. However, the underlying mechanisms through which VPA causes elevation in plasma ammonia (NH4) remains under investigation. Male Sprague Dawley rats (n = 72) were randomly allocated to receive VPA 400 mg/kg, 200 mg/kg, or vehicle IP daily for either 8, 14, or 28 consecutive days. The behavioral effects of VPA were assessed. Plasma, liver, and prefrontal cortex (PFC), striatum (Str), and cerebellum (Cere) were collected 1 h post last injection and assayed for NH4 concentration and glutamine synthetase (GS) enzyme activity. Chronic VPA treatment caused attenuation of measured behavioral reflexes (p < 0.0001) and increase in plasma NH4 concentration (p < 0.0001). The liver and brain also showed significant increase in tissue NH4 concentrations (p < 0.0001 each) associated with significant reduction in GS activity (p < 0.0001 and p = 0.0003, respectively). Higher tissue NH4 concentrations correlated with reduced GS activity in the liver (r = -0.447, p = 0.0007) but not in the brain (r = -0.058, p = 0.4). Within the brain, even though NH4 concentrations increased in the PFC (p = 0.001), Str (p < 0.0001), and Cere (p = 0.01), GS activity was reduced only in the PFC (p < 0.001) and not in Str (p = 0.2) or Cere (p = 0.1). These results suggest that VPA-induced elevation in plasma NH4 concentration could be related, at least in part, to the suppression of GS activity in liver and brain tissues. However, even though GS is the primary mechanism in brain NH4 clearance, the suppression of brain GS does not seem to be the main factor in explaining the elevation in brain NH4 concentration. Further research is urgently needed to investigate brain NH4 dynamics under chronic VPA treatment and whether VPA clinical efficacy in treating seizure disorders and bipolar mania is impacted by its effect on GS activity or other NH4 metabolizing enzymes.

Involvement of ammonia metabolism in the improvement of endurance performance by tea catechins in mice

Blood ammonia increases during exercise, and it has been suggested that this increase is both a central and peripheral fatigue factor. Although green tea catechins (GTCs) are known to improve exercise endurance by enhancing lipid metabolism in skeletal muscle, little is known about the relationship between ammonia metabolism and the endurance-improving effect of GTCs. Here, we examined how ammonia affects endurance capacity and how GTCs affect ammonia metabolism in vivo in mice and how GTCs affect mouse skeletal muscle and liver in vitro. In mice, blood ammonia concentration was significantly negatively correlated with exercise endurance capacity, and hyperammonaemia was found to decrease whole-body fat expenditure and fatty acid oxidation–related gene expression in skeletal muscle. Repeated ingestion of GTCs combined with regular exercise training improved endurance capacity and the expression of urea cycle–related genes in liver. In C2C12 myotubes, hyperammonaemia suppressed mitochondrial respiration; however, pre-incubation with GTCs rescued this suppression. Together, our results demonstrate that hyperammonaemia decreases both mitochondrial respiration in myotubes and whole-body aerobic metabolism. Thus, GTC-mediated increases in ammonia metabolism in liver and resistance to ammonia-induced suppression of mitochondrial respiration in skeletal muscle may underlie the endurance-improving effect of GTCs.

Chemical exchange saturation transfer imaging in hepatic encephalopathy

Hepatic encephalopathy (HE) is a common complication in liver cirrhosis and associated with an invasion of ammonia into the brain through the blood-brain barrier. Resulting higher ammonia concentrations in the brain are suggested to lead to a dose-dependent gradual increase of HE severity and an associated impairment of brain function. Amide proton transfer-weighted (APT_w) chemical exchange saturation transfer (CEST) imaging has been found to be sensitive to ammonia concentration. The aim of this work was to study APT_w CEST imaging in patients with HE and to investigate the relationship between disease severity, critical flicker frequency (CFF), psychometric test scores, blood ammonia, and APT_w signals in different brain regions.

Whole-brain APT_w CEST images were acquired in 34 participants (14 controls, 20 patients (10 minimal HE, 10 manifest HE)) on a 3 T clinical MRI system accompanied by T₁ mapping and structural images. T₁ normalized magnetization transfer ratio asymmetry analysis was performed around 3 ppm after B₀ and B₁ correction to create APT_w images. All APT_w images were spatially normalized into a cohort space to allow direct comparison. APT_w images in 6 brain regions (cerebellum, occipital cortex, putamen, thalamus, caudate, white matter) were tested for group differences as well as the link to CFF, psychometric test scores, and blood ammonia.

A decrease in APT_w intensities was found in the cerebellum and the occipital cortex of manifest HE patients. In addition, APT_w intensities in the cerebellum correlated positively with several psychometric scores, such as the fine motor performance scores MLS1 for hand steadiness / tremor (r = 0.466; p = .044) and WRT2 for motor reaction time (r = 0.523; p = .022). Moreover, a negative correlation between APT_w intensities and blood ammonia was found for the cerebellum (r = −0.615; p = .007) and the occipital cortex (r = −0.478; p = .045). An increase of APT_w intensities was observed in the putamen of patients with minimal HE and correlated negatively with the CFF (r = −0.423; p = .013).

Our findings demonstrate that HE is associated with regional differential alterations in APT_w signals. These variations are most likely a consequence of hyperammonemia or hepatocerebral degeneration processes, and develop in parallel with disease severity.

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Ammonia is suggested to play a key role in the emergence of HE.

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Increase of ammonia in HE patients might be studied with APT_w CEST.

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HE leads to regionally decreasing APT_w CEST signal.

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APT_w CEST correlates with blood ammonia levels and psychometric test scores.

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APT_w CEST is possibly linked to hyperammonemia or hepatocerebral degeneration.

The role of Branched Chain Amino Acids in the treatment of hepatic Encephalopathy

The relationship between intake of nutrients and Hepatic Encephalopathy (HE) dates back to the historical roots of experimental hepatology. Branched-Chain Amino Acids (BCAA; Isoleucine, leucine and valine) have attracted particular interest and in 1956 Müting described the amino acid pattern in patients with cirrhosis. The abnormal plasma pattern has been characterized by the ratio between BCAA and aromatic amino acids in plasma, the so called 'Fischer´s ratio'. This ratio has been associated with the grade of HE. Under normal conditions, ammonia detoxification predominantly takes place in the liver. When the liver fails, the homeostasis is altered and muscle tissue becomes the main alternative organ for at least temporary detoxification of ammonia. BCAA are believed to support this muscle ammonia detoxification and the ammonia lowering effect of BCAA has been intensely investigated. In this review the effect of BCAA on muscle ammonia metabolism and the protein sparing and anabolic effects of BCAA are discussed. A Cochrane metaanalysis showed that BCAA had beneficial effects on HE with a number needed to treat of 5 patients (RR 0.73, 95% CI 0.61 to 0.88). The combined evidence suggests that although the pathophysiology is poorly understood, there is evidence to support clinical benefits of BCAA. BCAAs enhance muscle mass and exert anabolic effects via stimulation of protein synthesis. The beneficial long-term effects of BCAA on HE could be related to these effects and not only related to Branched-Chain Amino Acid increased ammonia metabolism.

The role of 13N-ammonia in the differential diagnosis of gliomas and brain inflammatory lesions

OBJECTIVE

To investigate the utility of ¹³N-ammonia PET/CT imaging in the differential diagnosis of gliomas and brain inflammations.

METHODS

¹³N-ammonia PET/CT imaging data of 77 patients with gliomas and 34 patients with brain inflammations were retrospectively analyzed. No patients received any treatment before ¹³N-ammonia imaging. All the patients were diagnosed by stereotactic biopsy or clinical follow-up. Visual and semi-quantitative analysis was performed to analyze the results of ¹³N-ammonia imaging. Finally, the uptake ratios of each lesion were calculated and its differences among different groups were tested with one-way ANOVA.

RESULTS

29.4% inflammations, 51.6% low-grade gliomas and 91.3% high-grade gliomas were positive by visual analysis in ¹³N-ammonia imaging. The sensitivity, specificity and accuracy for the diagnosis of gliomas were 75.3%, 55.8% and 67.8%, respectively. As for semi-quantitative analysis, the T/G ratios of inflammatory lesions, low-grade gliomas and high-grade gliomas were 0.88 ± 0.24, 1.04 ± 0.43 and 1.43 ± 0.49, respectively. One-way ANOVA revealed that the T/G ratios of high-grade gliomas were significantly higher than those of low-grade gliomas and inflammations (P < 0.05), but there was no statistical difference between low-grade gliomas and inflammations (P = 0.118). Among the inflammatory lesions, T/G ratios were not statistically different between infectious and demyelinating lesions (P > 0.05). ROC curve analysis showed that the optimal cut-off value of T/G ratio in distinguishing gliomas from inflammations was 1.21 with the AUC 0.78. The sensitivity, specificity, accuracy, PPV and NPV were 52.9%, 94.4%, 65.3%, 95.7% and 45.9%, respectively. ROC curve analysis showed that the optimal cut-off value of T/G ratio in distinguishing high-grade gliomas from low-grade gliomas was 1.06 with the AUC 0.78. The sensitivity, specificity, accuracy, PPV and NPV were 81.5%, 67.7%, 76.5%, 81.5% and 67.7%, respectively. ROC curve analysis showed that the optimal cut-off value of T/G ratio in distinguishing high-grade gliomas from low-grade gliomas and inflammations was 1.19 with the AUC 0.84. The sensitivity, specificity, accuracy, PPV and NPV were 70.4%, 85.1%, 78.5%, 79.2% and 78.1%, respectively.

CONCLUSIONS

¹³N-ammonia imaging is effective in distinguishing high-grade gliomas from low-grade gliomas and inflammations, but its role in the differential diagnosis of low-grade gliomas and brain inflammatory lesions is limited, and the accuracy needs to be improved.

Pathogenesis of Hepatic Encephalopathy in Chronic Liver Disease

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome that occurs during chronic liver disease (CLD). While ammonia and other precipitating factors in liver disease including inflammation, bile acids, oxidative stress, and lactate play a role in the pathogenesis of HE, the exact mechanism that leads to HE is not fully understood. Notably, accumulating evidence points toward a synergic effect rather than independent actions among precipitating factors that contributes to the development and severity of HE in CLD. Hence, this review is aimed to briefly discuss the single and synergic interplay of pathological factors in the progression and severity of HE.

Ammonia-weighted imaging by chemical exchange saturation transfer MRI at 3 T

Hepatic encephalopathy (HE) is triggered by liver cirrhosis and is associated with an increased ammonia level within the brain tissue. The goal of this study was to investigate effects of ammonia on in vitro amide proton transfer (APT)-weighted chemical exchange saturation transfer (CEST) imaging in order to develop an ammonia-sensitive brain imaging method. APT-weighted CEST imaging was performed on phantom solutions including pure ammonia, bovine serum albumin (BSA), and tissue homogenate samples doped with various ammonia concentrations. All CEST data were assessed by magnetization transfer ratio asymmetry. In addition, optical methods were used to determine possible structural changes of the proteins in the BSA phantom. In vivo feasibility measurements were acquired in one healthy participant and two patients suffering from HE, a disease associated with increased brain ammonia levels. The CEST effect of pure ammonia showed a base-catalyzed behavior. At pH values greater than 5.6 no CEST effect was observed. The APT-weighted signal was significantly reduced for ammonia concentrations of 5mM or more at fixed pH values within the different protein phantom solutions. The optical methods revealed no protein aggregation or denaturation for ammonia concentrations less than 5mM. The in vivo measurements showed tissue specific and global reduction of the observed CEST signal in patients with HE, possibly linked to pathologically increased ammonia levels. APT-weighted CEST imaging is sensitive to changes in ammonia concentrations. Thus, it seems useful for the investigation of pathologies with altered tissue ammonia concentrations such as HE. However, the underlying mechanism needs to be explored in more detail in future in vitro and in vivo investigations.

The acute effect of cannabis on plasma, liver and brain ammonia dynamics, a translational study

Recent reports of ammonia released during cannabis smoking raise concerns about putative neurotoxic effects. Cannabis (54mg) was administered in a double-blind, placebo-controlled design to healthy cannabis users (n=15) either orally, or through smoking (6.9%THC cigarette) or inhalation of vaporized cannabis (Volcano®). Serial assay of plasma ammonia concentrations at 0, 2, 4, 6, 8, 10, 15, 30, and 90min from onset of cannabis administration showed significant time (P=0.016), and treatment (P=0.0004) effects with robust differences between placebo and edible at 30 (P=0.002), and 90min (P=0.007) and between placebo and vaporized (P=0.02) and smoking routes (P=0.01) at 90min. Furthermore, plasma ammonia positively correlated with blood THC concentrations (P=0.03). To test the hypothesis that this delayed increase in plasma ammonia originates from the brain we administered THC (3 and 10mg/kg) to mice and measured plasma, liver, and brain ammonia concentrations at 1, 3, 5 and 30min post-injection. Administration of THC to mice did not cause significant change in plasma ammonia concentrations within the first 5min, but significantly reduced striatal glutamine-synthetase (GS) activity (P=0.046) and increased striatal ammonia concentration (P=0.016). Furthermore, plasma THC correlated positively with striatal ammonia concentration (P<0.001) and negatively with striatal GS activity (P=0.030). At 30min, we found marked increase in striatal ammonia (P<0.0001) associated with significant increase in plasma ammonia (P=0.042) concentration. In conclusion, the results of these studies demonstrate that cannabis intake caused time and route-dependent increases in plasma ammonia concentrations in human cannabis users and reduced brain GS activity and increased brain and plasma ammonia concentrations in mice.

Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Glucose, Lactate, β-Hydroxybutyrate, Acetate, GABA, and Succinate as Substrates for Synthesis of Glutamate and GABA in the Glutamine-Glutamate/GABA Cycle

The glutamine-glutamate/GABA cycle is an astrocytic-neuronal pathway transferring precursors for transmitter glutamate and GABA from astrocytes to neurons. In addition, the cycle carries released transmitter back to astrocytes, where a minor fraction (~25 %) is degraded (requiring a similar amount of resynthesis) and the remainder returned to the neurons for reuse. The flux in the cycle is intense, amounting to the same value as neuronal glucose utilization rate or 75-80 % of total cortical glucose consumption. This glucose:glutamate ratio is reduced when high amounts of β-hydroxybutyrate are present, but β-hydroxybutyrate can at most replace 60 % of glucose during awake brain function. The cycle is initiated by α-ketoglutarate production in astrocytes and its conversion via glutamate to glutamine which is released. A crucial reaction in the cycle is metabolism of glutamine after its accumulation in neurons. In glutamatergic neurons all generated glutamate enters the mitochondria and its exit to the cytosol occurs in a process resembling the malate-aspartate shuttle and therefore requiring concomitant pyruvate metabolism. In GABAergic neurons one half enters the mitochondria, whereas the other one half is released directly from the cytosol. A revised concept is proposed for the synthesis and metabolism of vesicular and nonvesicular GABA. It includes the well-established neuronal GABA reuptake, its metabolism, and use for resynthesis of vesicular GABA. In contrast, mitochondrial glutamate is by transamination to α-ketoglutarate and subsequent retransamination to releasable glutamate essential for the transaminations occurring during metabolism of accumulated GABA and subsequent resynthesis of vesicular GABA.

The brain following transjugular intrahepatic portosystemic shunt: the perspective from neuroimaging

Hepatic encephalopathy (HE) is a common complication after implantation of a transjugular intrahepatic portosystemic shunt (TIPS). Neuroimaging offers a variety of techniques for non-invasive evaluation of alterations in metabolism, as well as structural and functional changes of the brain in patients after TIPS implantation. In this article, we review the epidemiology and pathophysiology of post-TIPS HE. The potential of neuroimaging including positron emission tomography and multimodality magnetic resonance imaging to investigate the pathophysiology of post-TIPS HE is presented. We also give a perspective on the role of neuroimaging in this field.

Decreased ¹³N-labeled ammonia uptake in the ipsilateral and contralateral hemispheres following carotid endarterectomy

Carotid artery plaques are a leading cause of ischemic stroke, and carotid endarterectomy (CEA) is one of the major treatment approaches for this disease. Changes in cerebral metabolism following CEA remain unclear. The present study aimed to evaluate the effect of cerebral ammonia metabolism following CEA using ¹³N-labeled ammonia positron emission tomography (PET) in humans. A total of 20 patients were enrolled in the present study, with a mean age of 59.5 years, comprising 16 males and four females. Of these patients, eight underwent right CEA and 12 underwent left CEA. The rate of carotid artery stenosis was between 50–69% in six of the patients, between 70–99% in 11 of the patients and was at 100% (thrombosis) in three of the patients, measured by computerised tomography digital subtraction angiography prior to CEA. ¹³N-labeled ammonia (137 MBq) PET scanning was performed prior and subsequent to CEA surgery for each patient. The first ammonia PET scan was performed 1 day prior to CEA, while the second PET scan was performed 1–4 weeks following CEA. Following injection of ¹³N-labeled ammonia, static PET was acquired for 10 min. The region of interest (ROI), covering the major cerebral hemisphere, was selected and ammonia uptake in the ROI was determined in the ipsilateral and contralateral hemispheres. No hyperperfusion syndrome was observed in the patients subsequent to CEA. No significant change in cerebral hemisphere ammonia uptake was observed between the ipsilateral and contralateral hemispheres prior to (ratio =0.98; P>0.01) or following (ratio =1.09; P>0.01) CEA. Ammonia uptake in the ipsilateral and contralateral hemispheres was significantly reduced to 23.2 and 23.5%, respectively, following CEA. Using ¹³N-labeled ammonia PET to evaluate cerebral ammonia metabolism following CEA in patients with severe carotid artery stenosis, the present study demonstrated that uptake of ammonia in the ipsilateral and contralateral hemispheres was significantly reduced.

Biodistribution and estimation of radiation-absorbed doses in humans for 13N-ammonia PET

OBJECTIVE

To evaluate the biodistribution of radiation-absorbed doses of (13)N-ammonia in healthy people.

METHODS

Five healthy human subjects underwent whole-body PET and CT scans after injection of 555-740 MBq of (13)N-ammonia. Five serial dynamic emission scans in each healthy volunteer were acquired. Regions of interest were drawn on the CT image and transferred to the corresponding transverse PET slice. Estimates of the radiation-absorbed doses were calculated using the medical internal radiation dosimetry method.

RESULTS

The highest concentrations of (13)N-ammonia were found in the heart and liver, followed by pancreas, brain, spleen and stomach. The highest absorbed organ doses were to the heart wall (7.14E-03 ± 3.63E-03 mGy/MBq) and kidneys (6.02E-03 ± 3.53E-03 mGy/MBq). The effective dose (ED) was 6.58E-03 ± 1.23E-03 mSv/MBq.

CONCLUSIONS

With these new estimates for (13)N-ammonia dosimetry, the results for Chinese people were not appreciably different from those of the previous study performed with old devices. As one of the most important myocardial perfusion PET tracers, the whole-body (13)N-ammonia PET appears to be safe for humans, yielding a relatively modest radiation burden that would allow multiple PET studies on the same subject per year.

Recent advances in the treatment of hyperammonemia

Ammonia is a neurotoxic agent that is primarily generated in the intestine and detoxified in the liver. Toxic increases in systemic ammonia levels predominantly result from an inherited or acquired impairment in hepatic detoxification and lead to potentially life-threatening neuropsychiatric symptoms. Inborn deficiencies in ammonia detoxification mainly affect the urea cycle, an endogenous metabolic removal system in the liver. Hepatic encephalopathy, on the other hand, is a hyperammonemia-related complication secondary to acquired liver function impairment. A range of therapeutic options is available to target either ammonia generation and absorption or ammonia removal. Therapies for hepatic encephalopathy decrease intestinal ammonia production and uptake. Treatments for urea cycle disorders eliminate ammoniagenic amino acids through metabolic transformation, preventing ammonia generation. Therapeutic approaches removing ammonia activate the urea cycle or the second essential endogenous ammonia detoxification system, glutamine synthesis. Recent advances in treating hyperammonemia include using synergistic combination treatments, broadening the indication of orphan drugs, and developing novel approaches to regenerate functional liver tissue. This manuscript reviews the various pharmacological treatments of hyperammonemia and focuses on biopharmaceutical and drug delivery issues.

13N-ammonia combined with 18F-FDG could discriminate between necrotic high-grade gliomas and brain abscess

PURPOSE

Accurate prediction of brain abscess is beneficial for timely management. In this study, we investigated the utility of 13N-ammonia and its combination with 18F-FDG in differentiating brain abscess from necrotic high-grade gliomas.

PATIENTS AND METHODS

Thirteen patients with ring-like enhancement high-grade gliomas and 11 patients with brain abscess were recruited in our study. All of them underwent both 18F-FDG and 13N-ammonia PET imaging. Lesion uptake was evaluated by lesion to normal gray matter ratio (L/N). Histopathology diagnosis was obtained for all the patients after PET imaging.

RESULTS

The L/N values of 18F-FDG were not significantly different between brain abscess and necrotic high-grade gliomas (P = 0.35). The uptake of 13N-ammonia in gliomas was higher than that in abscess lesions (L/N: 1.38 ± 0.31 vs 0.84 ± 0.18, P < 0.001). The receiver operating characteristic curve analysis determined the optimal L/N cutoff value (13N-ammonia) of 1.0 with the area under the curve of 0.94 and the overall accuracy of 87.5%. Discriminant analysis demonstrated that the combination of 18F-FDG and 13N-ammonia could distinguish the 2 clinical entities with higher accuracy of 95%, and only 1 necrotic glioma lesion was misclassified into the abscess group.

CONCLUSIONS

13N-ammonia is effective in distinguishing brain abscess from necrotic high-grade gliomas, and its combination with 18F-FDG could further elevate the diagnostic accuracy.

Pathogenesis of hepatic encephalopathy: role of ammonia and systemic inflammation

The syndrome we refer to as Hepatic Encephalopathy (HE) was first characterized by a team of Nobel Prize winning physiologists led by Pavlov and Nencki at the Imperial Institute of Experimental Medicine in Russia in the 1890's. This focused upon the key observation that performing a portocaval shunt, which bypassed nitrogen-rich blood away from the liver, induced elevated blood and brain ammonia concentrations in association with profound neurobehavioral changes. There exists however a spectrum of metabolic encephalopathies attributable to a variety (or even absence) of liver hepatocellular dysfunctions and it is this spectrum rather than a single disease entity that has come to be defined as HE. Differences in the underlying pathophysiology, treatment responses and outcomes can therefore be highly variable between acute and chronic HE. The term also fails to articulate quite how systemic the syndrome of HE can be and how it can be influenced by the gastrointestinal, renal, nervous, or immune systems without any change in background liver function. The pathogenesis of HE therefore encapsulates a complex network of interdependent organ systems which as yet remain poorly characterized. There is nonetheless a growing recognition that there is a complex but influential synergistic relationship between ammonia, inflammation (sterile and non-sterile) and oxidative stress in the pathogenesis HE which develops in an environment of functional immunoparesis in patients with liver dysfunction. Therapeutic strategies are thus moving further away from the traditional specialty of hepatology and more towards novel immune and inflammatory targets which will be discussed in this review.

Cerebral effects of ammonia in liver disease: current hypotheses

Hyperammonemia is necessary for development of the cerebral complications to liver disease including hepatic encephalopathy and cerebral edema but the mechanisms are unclear. Ammonia is taken up by the brain in proportion to its arterial concentration. The flux into the brain is most likely by both diffusion of NH3 and mediated transport of NH4 (+) . Astrocytic detoxification of ammonia involves formation of glutamine at concentrations high enough to produce cellular edema, but compensatory mechanisms reduce this effect. Glutamine can be taken up by astrocytic mitochondria and initiate the mitochondrial permeability transition but the clinical relevance is uncertain. Elevated astrocytic glutamine interferes with neurotransmission. Thus, animal studies show enhanced glutamatergic neurotransmission via the NMDA receptor which may be related to the acute cerebral complications to liver failure, while impairment of the NMDA activated glutamate-NO-cGMP pathway could relate to the behavioural changes seen in hepatic encephalopathy. Elevated glutamine also increases GABA-ergic tone, an effect which is aggravated by mitochondrial production of neurosteroids; this may relate to decreased neurotransmission and precipitation of encephalopathy by GABA targeting drugs. Hyperammonemia may compromise cerebral energy metabolism as elevated cerebral lactate is generally reported. Hypoxia is unlikely since cerebral oxygen:glucose utilisation and lactate:pyruvate ratio are both normal in clinical studies. Ammonia inhibits α-ketoglutaratedehydrogenase in isolated mitochondria, but the clinical relevance is dubious due to the observed normal cerebral oxygen:glucose utilization. Recent studies suggest that ammonia stimulates glycolysis in excess of TCA cycle activity, a hypothesis that may warrant further testing, in being in accordance with the limited clinical observations.

Role of N-13 ammonia PET/CT in diagnosing pancreatic necrosis in patients with acute pancreatitis as compared to contrast enhanced CT--results of a pilot study

BACKGROUND

Contrast enhanced computerized tomography (CECT) is used to determine severity of acute pancreatitis based upon the presence and extent of necrosis. However limitations do exist precluding its applicability in renal failure. Positron emission tomography (PET) imaging for cardiac perfusion shows good uptake of N-13 ammonia ((13)NH3) metabolites in pancreas owing to high perfusion.

AIM

To evaluate the role of (13)NH3 PET/CT in acute pancreatitis and compare it with CECT in diagnosing and quantifying pancreatic necrosis.

MATERIAL AND METHODS

Patients presenting within 1 week of acute pancreatitis were studied. Static PET images were acquired after intravenous injection of 370-740 MBq of (13)NH3. (13)NH3 PET/CT was followed by CECT in the absence of renal impairment. Maximum standard-uptake-value (SUVmax) of pancreas (P) and liver (L) were taken and their ratio (P/L) was estimated to determine perfusion. Areas within pancreas with no tracer uptake were considered necrotic. These patients were managed as per institutional protocol. Patients undergoing (13)NH3 PET/CT for coronary artery disease were used as controls.

RESULTS

29 patients (72% males) were studied of whom 6 had elevated serum creatinine. (13)NH3 PET/CT was done in all patients along with 9 controls while CECT was carried out after PET/CT in 23 patients. Median levels of SUVmax (P/L) in the controls, uninvolved pancreas and necrotic areas were 1.0 (0.86-1.03), 0.66 (0.50-0.92) and 0.12 (0.07-0.21) respectively (p < 0.001). Necrosis estimation was similar in 22/23 patients without renal failure while in one patient only (13)NH3 PET/CT picked up necrosis (<30%). 5/6 patients with renal failure had necrosis on (13)NH(3) PET/CT which was confirmed on surgery or subsequent CECT after improvement of renal failure.

CONCLUSION

This pilot study is the first in literature to diagnose necrosis in patients with acute pancreatitis using (13)NH3 PET/CT. With minimal additional radiation burden, it is possible to estimate the absolute tissue perfusion as well. With no adverse renal side effects, this can be an alternative to CECT in patients with renal failure giving similar information. It has good agreement with CECT with a good interobserver acceptability.

Hyperammonemia due to urea cycle disorders: a potentially fatal condition in the intensive care setting

Disorders of the urea cycle are secondary to a defect in the system that converts ammonia into urea, resulting in accumulation of ammonia and other products. This results in encephalopathy, coma, and death if not recognized and treated rapidly. Late-onset urea cycle disorders may be precipitated by acute disease and can be difficult to recognize because patients are already ill. Diagnosis of urea cycle disorders is based on clinical suspicion and determination of blood ammonia in suspected patients with neurological symptoms in the intensive care setting. Treatment is based on the removal of ammonia by dialysis or hemofiltration, reduction of the catabolic state, abolishment of nitrogen administration, and use of pharmacological nitrogen scavenging agents.

Effect of glutamine synthetase inhibition on brain and interorgan ammonia metabolism in bile duct ligated rats

Ammonia has a key role in the development of hepatic encephalopathy (HE). In the brain, glutamine synthetase (GS) rapidly converts blood-borne ammonia into glutamine which in high concentrations may cause mitochondrial dysfunction and osmolytic brain edema. In astrocyte-neuron cocultures and brains of healthy rats, inhibition of GS by methionine sulfoximine (MSO) reduced glutamine synthesis and increased alanine synthesis. Here, we investigate effects of MSO on brain and interorgan ammonia metabolism in sham and bile duct ligated (BDL) rats. Concentrations of glutamine, glutamate, alanine, and aspartate and incorporation of (15)NH(4)(+) into these amino acids in brain, liver, muscle, kidney, and plasma were similar in sham and BDL rats treated with saline. Methionine sulfoximine reduced glutamine concentrations in liver, kidney, and plasma but not in brain and muscle; MSO reduced incorporation of (15)NH(4)(+) into glutamine in all tissues. It did not affect alanine concentrations in any of the tissues but plasma alanine concentration increased; incorporation of (15)NH(4)(+) into alanine was increased in brain in sham and BDL rats and in kidney in sham rats. It inhibited GS in all tissues examined but only in brain was an increased incorporation of (15)N-ammonia into alanine observed. Liver and kidney were important for metabolizing blood-borne ammonia.

The comparison of 13N-ammonia and 18F-FDG in the evaluation of untreated gliomas

OBJECTIVE

Noninvasive evaluation of glioma is of great help for clinical practice. In this study, we investigated the utility of 13N-ammonia in the evaluation of untreated gliomas and compared the results with that of 18F-FDG.

METHODS

Forty-five consecutive patients with final diagnosis of glioma were included in this study. PET/CT imaging was performed for all of them with both 18F-FDG and 13N-ammonia as tracers. Imaging results were analyzed by tumor-to-gray matter (T/G) ratios. Receiver operating characteristic curve analysis was conducted to determine the optimal T/G cutoff values of each tracer between low-grade and high-grade gliomas.

RESULTS

Forty-eight separate lesions were identified in all (grade II, n = 16; grade III, n = 12; and grade IV, n = 20). Twenty-nine out of 32 high-grade lesions (91%) showed higher uptakes than normal gray matter with N-ammonia in comparison with the result of 21 lesions (66%) with 18F-FDG. The optimal T/G cutoff values for 18F-FDG and 13N-ammonia were 0.64 and 0.86 separately with the area under each curve 0.910 and 0.943. The sensitivity and specificity of predicting high-grade gliomas with optimal cutoff values were 83% and 93% for 18F-FDG and 94% and 94% for 13N-ammonia, respectively.

CONCLUSION

13N-Ammonia is superior to 18F-FDG not only in separating low-grade gliomas from high-grade ones but also in the detection of high-grade gliomas for better tumor to normal gray matter contrast.

Hepatic encephalopathy in alcoholic cirrhosis

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication of cirrhosis in alcoholic patients that is characterized clinically by personality changes, sleep abnormalities, and impaired motor coordination, as well as cognitive dysfunction progressing to stupor and coma. Procedures used for diagnosis and grading of HE include neurologic assessment, electroencephalography, psychometric testing, and use of the critical flicker frequency test. Neuropathologically, HE in cirrhosis is principally a disorder of neuroglia characterized by Alzheimer type II astrocytosis and activation of microglia. However, thalamic and cerebellar neuronal pathologies have been noted as well as lesions to globus pallidus and substantia nigra, leading to a condition known as "parkinsonism in cirrhosis." Multiple mechanisms have been proposed to account for the pathogenesis of HE in cirrhosis, including the neurotoxic actions of ammonia and manganese (normally removed via the hepatobiliary route), impaired brain energy metabolism, central proinflammatory mechanisms, and alterations of both excitatory and inhibitory neurotransmission. Treatment of HE in cirrhosis continues to rely on ammonia-lowering strategies such as lactulose, antibiotics, probiotics and l-ornithine l-aspartate with nutritional management consisting of adequate (but not excessive) dietary protein and vitamin B1 supplements. l-DOPA may improve parkinsonian symptoms. Liver transplantation leads to recovery of central nervous system function in the majority of cases.