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Sørensen M, Walls AB, Dam G, Bak LK, Andersen JV, Ott P, Vilstrup H, Schousboe A. Low cerebral energy metabolism in hepatic encephalopathy reflects low neuronal energy demand. Role of ammonia-induced increased GABAergic tone. Anal Biochem 2022; 654:114766. [PMID: 35654134 DOI: 10.1016/j.ab.2022.114766] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2021] [Revised: 05/11/2022] [Accepted: 05/26/2022] [Indexed: 11/01/2022]
Abstract
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.
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Affiliation(s)
- Michael Sørensen
- Department of Hepatology & Gastroenterology, Aarhus University Hospital, Denmark; Department of Internal Medicine, Viborg Regional Hospital, Denmark.
| | - Anne Byriel Walls
- Department of Drug Design & Pharmacology, University of Copenhagen, Denmark
| | - Gitte Dam
- Department of Hepatology & Gastroenterology, Aarhus University Hospital, Denmark
| | - Lasse Kristoffer Bak
- Department of Drug Design & Pharmacology, University of Copenhagen, Denmark; Department of Clinical Biochemistry, Copenhagen University Hospital, Rigshospitalet, Denmark
| | | | - Peter Ott
- Department of Hepatology & Gastroenterology, Aarhus University Hospital, Denmark
| | - Hendrik Vilstrup
- Department of Hepatology & Gastroenterology, Aarhus University Hospital, Denmark
| | - Arne Schousboe
- Department of Drug Design & Pharmacology, University of Copenhagen, Denmark
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Nammas W, Maaniitty T, Knuuti J, Saraste A. Cardiac perfusion by positron emission tomography. Clin Physiol Funct Imaging 2021; 41:385-400. [PMID: 33969615 DOI: 10.1111/cpf.12708] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2021] [Accepted: 05/05/2021] [Indexed: 01/16/2023]
Abstract
Myocardial perfusion imaging (MPI) with positron emission tomography (PET) is an established tool for evaluation of obstructive coronary artery disease (CAD). The contemporary 3-dimensional scanner technology and the state-of-the-art MPI radionuclide tracers and pharmacological stress agents, as well as the cutting-edge image reconstruction techniques and data analysis software, have all enabled accurate, reliable and reproducible quantification of absolute myocardial blood flow (MBF), and henceforth calculation of myocardial flow reserve (MFR) in several clinical scenarios. In patients with suspected coronary artery disease, both absolute stress MBF and MFR can identify myocardial territories subtended by epicardial coronary arteries with haemodynamically significant stenosis, as defined by invasive coronary fractional flow reserve measurement. In particular, absolute stress MBF and MFR offered incremental prognostic information for predicting adverse cardiac outcome, and hence for better patient risk stratification, over those provided by traditional clinical risk predictors. This article reviews the available evidence to support the translation of the current techniques and technologies into a useful decision-making tool in real-world clinical practice.
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Affiliation(s)
- Wail Nammas
- Heart Center, Turku University Hospital, Turku, Finland
| | - Teemu Maaniitty
- PET Centre, Turku University Hospital and University of Turku, Turku, Finland
| | - Juhani Knuuti
- PET Centre, Turku University Hospital and University of Turku, Turku, Finland
| | - Antti Saraste
- Heart Center, Turku University Hospital, Turku, Finland.,PET Centre, Turku University Hospital and University of Turku, Turku, Finland
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3
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Egerton A, Dunn JT, Singh N, Yu Z, O'Doherty J, Koychev I, Webb J, Claridge S, Turkheimer FE, Marsden PK, Hammers A, Gee A. Evaluation of [ 13N]ammonia positron emission tomography as a potential method for quantifying glutamine synthetase activity in the human brain. EJNMMI Res 2020; 10:146. [PMID: 33270177 PMCID: PMC7714883 DOI: 10.1186/s13550-020-00731-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2020] [Accepted: 11/21/2020] [Indexed: 11/22/2022] Open
Abstract
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.
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Affiliation(s)
- Alice Egerton
- Department of Psychosis Studies, Institute of Psychiatry, Psychology and Neuroscience, King's College London, 16 De Crespigny Park, London, SE5 8AF, UK.
| | - Joel T Dunn
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
- King's College London & Guy's and St. Thomas' PET Centre, London, SE1 7EH, UK
| | - Nisha Singh
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
- King's College London & Guy's and St. Thomas' PET Centre, London, SE1 7EH, UK
- Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, SE5 7AF, UK
- Department of Psychiatry, Warneford Hospital, University of Oxford, Oxford, OX3 7JX, UK
| | - Zilin Yu
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
| | - Jim O'Doherty
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
- King's College London & Guy's and St. Thomas' PET Centre, London, SE1 7EH, UK
- Clinical Imaging Research Centre, National University of Singapore, Singapore, 117599, Singapore
| | - Ivan Koychev
- Department of Psychosis Studies, Institute of Psychiatry, Psychology and Neuroscience, King's College London, 16 De Crespigny Park, London, SE5 8AF, UK
- Department of Psychiatry, Warneford Hospital, University of Oxford, Oxford, OX3 7JX, UK
| | - Jessica Webb
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
| | - Simon Claridge
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
| | - Federico E Turkheimer
- Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, SE5 7AF, UK
| | - Paul K Marsden
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
- King's College London & Guy's and St. Thomas' PET Centre, London, SE1 7EH, UK
| | - Alexander Hammers
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
- King's College London & Guy's and St. Thomas' PET Centre, London, SE1 7EH, UK
| | - Antony Gee
- School of Biomedical Engineering and Imaging Sciences, St Thomas' Hospital, King's College London, London, SE1 7EH, UK
- King's College London & Guy's and St. Thomas' PET Centre, London, SE1 7EH, UK
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4
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Murthy VL, Bateman TM, Beanlands RS, Berman DS, Borges-Neto S, Chareonthaitawee P, Cerqueira MD, deKemp RA, DePuey EG, Dilsizian V, Dorbala S, Ficaro EP, Garcia EV, Gewirtz H, Heller GV, Lewin HC, Malhotra S, Mann A, Ruddy TD, Schindler TH, Schwartz RG, Slomka PJ, Soman P, Di Carli MF, Einstein A, Russell R, Corbett JR. Clinical Quantification of Myocardial Blood Flow Using PET: Joint Position Paper of the SNMMI Cardiovascular Council and the ASNC. J Nucl Cardiol 2018; 25:269-297. [PMID: 29243073 DOI: 10.1007/s12350-017-1110-x] [Citation(s) in RCA: 133] [Impact Index Per Article: 22.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Affiliation(s)
- Venkatesh L Murthy
- Frankel Cardiovascular Center, Division of Cardiovascular Medicine, Department of Internal Medicine, University of Michigan, Ann Arbor, MI, USA.
| | | | - Rob S Beanlands
- National Cardiac PET Centre, Division of Cardiology, University of Ottawa Heart Institute, Ottawa, ON, Canada
| | - Daniel S Berman
- Departments of Imaging and Medicine, Cedars-Sinai Medical Center, Los Angeles, CA, USA
| | - Salvador Borges-Neto
- Division of Nuclear Medicine, Department of Radiology, and Division of Cardiology, Department of Medicine, Duke University School of Medicine, Duke University Health System, Durham, NC, USA
| | | | | | - Robert A deKemp
- National Cardiac PET Centre, Division of Cardiology, University of Ottawa Heart Institute, Ottawa, ON, Canada
| | - E Gordon DePuey
- Division of Nuclear Medicine, Department of Radiology, Mt. Sinai St. Luke's and Mt. Sinai West Hospitals, Icahn School of Medicine at Mt. Sinai, New York, NY, USA
| | - Vasken Dilsizian
- Department of Diagnostic Radiology and Nuclear Medicine, University of Maryland School of Medicine, Baltimore, MD, USA
| | - Sharmila Dorbala
- Cardiovascular Imaging Program, Brigham and Women's Hospital, Boston, MA, USA
| | - Edward P Ficaro
- Division of Nuclear Medicine, University of Michigan, Ann Arbor, MI, USA
| | - Ernest V Garcia
- Department of Radiology and Imaging Sciences, Emory University, Atlanta, GA, USA
| | - Henry Gewirtz
- Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA
| | - Gary V Heller
- Gagnon Cardiovascular Institute, Morristown Medical Center, Morristown, NJ, USA
| | | | - Saurabh Malhotra
- Division of Cardiovascular Medicine, Jacobs School of Medicine and Biomedical Sciences, University at Buffalo, Buffalo, NY, USA
| | | | - Terrence D Ruddy
- National Cardiac PET Centre, Division of Cardiology, University of Ottawa Heart Institute, Ottawa, ON, Canada
| | - Thomas H Schindler
- Division of Nuclear Medicine, Department of Radiology, Johns Hopkins School of Medicine, Baltimore, MD, USA
| | - Ronald G Schwartz
- Cardiology Division, Department of Medicine, and Nuclear Medicine Division, Department of Imaging Sciences, University of Rochester Medical Center, Rochester, NY, USA
| | - Piotr J Slomka
- Departments of Imaging and Medicine, Cedars-Sinai Medical Center, Los Angeles, CA, USA
| | - Prem Soman
- Division of Cardiology, Heart and Vascular Institute, University of Pittsburgh Medical Center, Pittsburgh, PA, USA
| | - Marcelo F Di Carli
- Cardiovascular Imaging Program, Brigham and Women's Hospital, Boston, MA, USA
| | - Andrew Einstein
- Division of Cardiology, Department of Medicine, and Department of Radiology, Columbia University Medical Center and New York-Presbyterian Hospital, New York, NY, USA
| | - Raymond Russell
- Warren Alpert Medical School, Brown University, Providence, RI, USA
| | - James R Corbett
- Frankel Cardiovascular Center, Division of Cardiovascular Medicine, Department of Internal Medicine, and Division of Nuclear Medicine, Department of Radiology, University of Michigan, Ann Arbor, MI, USA
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5
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Murthy VL, Bateman TM, Beanlands RS, Berman DS, Borges-Neto S, Chareonthaitawee P, Cerqueira MD, deKemp RA, DePuey EG, Dilsizian V, Dorbala S, Ficaro EP, Garcia EV, Gewirtz H, Heller GV, Lewin HC, Malhotra S, Mann A, Ruddy TD, Schindler TH, Schwartz RG, Slomka PJ, Soman P, Di Carli MF. Clinical Quantification of Myocardial Blood Flow Using PET: Joint Position Paper of the SNMMI Cardiovascular Council and the ASNC. J Nucl Med 2017; 59:273-293. [PMID: 29242396 DOI: 10.2967/jnumed.117.201368] [Citation(s) in RCA: 147] [Impact Index Per Article: 21.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2017] [Accepted: 09/11/2017] [Indexed: 12/30/2022] Open
Affiliation(s)
- Venkatesh L Murthy
- Frankel Cardiovascular Center, Division of Cardiovascular Medicine, Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan
| | | | - Rob S Beanlands
- National Cardiac PET Centre, Division of Cardiology, University of Ottawa Heart Institute, Ottawa, Ontario, Canada
| | - Daniel S Berman
- Departments of Imaging and Medicine, Cedars-Sinai Medical Center, Los Angeles, California
| | - Salvador Borges-Neto
- Division of Nuclear Medicine, Department of Radiology, and Division of Cardiology, Department of Medicine, Duke University School of Medicine, Duke University Health System, Durham, North Carolina
| | | | | | - Robert A deKemp
- National Cardiac PET Centre, Division of Cardiology, University of Ottawa Heart Institute, Ottawa, Ontario, Canada
| | - E Gordon DePuey
- Division of Nuclear Medicine, Department of Radiology, Mt. Sinai St. Luke's and Mt. Sinai West Hospitals, Icahn School of Medicine at Mt. Sinai, New York, New York
| | - Vasken Dilsizian
- Department of Diagnostic Radiology and Nuclear Medicine, University of Maryland School of Medicine, Baltimore, Maryland
| | - Sharmila Dorbala
- Cardiovascular Imaging Program, Brigham and Women's Hospital, Boston, Massachusetts
| | - Edward P Ficaro
- Division of Nuclear Medicine, University of Michigan, Ann Arbor, Michigan
| | - Ernest V Garcia
- Department of Radiology and Imaging Sciences, Emory University, Atlanta, Georgia
| | - Henry Gewirtz
- Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts
| | - Gary V Heller
- Gagnon Cardiovascular Institute, Morristown Medical Center, Morristown, NJ, USA
| | | | - Saurabh Malhotra
- Division of Cardiovascular Medicine, Jacobs School of Medicine and Biomedical Sciences, University at Buffalo, Buffalo, New York
| | - April Mann
- Hartford Hospital, Hartford, Connecticut
| | - Terrence D Ruddy
- National Cardiac PET Centre, Division of Cardiology, University of Ottawa Heart Institute, Ottawa, Ontario, Canada
| | - Thomas H Schindler
- Division of Nuclear Medicine, Department of Radiology, Johns Hopkins School of Medicine, Baltimore, Maryland
| | - Ronald G Schwartz
- Cardiology Division, Department of Medicine, and Nuclear Medicine Division, Department of Imaging Sciences, University of Rochester Medical Center, Rochester, New York; and
| | - Piotr J Slomka
- Departments of Imaging and Medicine, Cedars-Sinai Medical Center, Los Angeles, California
| | - Prem Soman
- Division of Cardiology, Heart and Vascular Institute, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania
| | - Marcelo F Di Carli
- Cardiovascular Imaging Program, Brigham and Women's Hospital, Boston, Massachusetts
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6
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Shi X, Zhang X, Yi C, Liu Y, He Q. [¹³N]Ammonia positron emission tomographic/computed tomographic imaging targeting glutamine synthetase expression in prostate cancer. Mol Imaging 2015; 13. [PMID: 25431095 DOI: 10.2310/7290.2014.00048] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The purpose of this study was to investigate the expression of glutamine synthetase (GS) in prostate cancer (PCa) and the utility of [¹³N]ammonia positron emission tomography/computed tomography (PET/CT) in the imaging of PCa. The uptake ratio of [¹³N]ammonia and the expression of GS in PC3 and DU145 cells was measured. Thirty-four patients with suspected PCa underwent [¹³N]ammonia PET/CT imaging, and immunohistochemistry staining of GS was performed. The uptake of [¹³N]ammonia in PC3 and DU145 cells elevated along with the decrease in glutamine in medium. The expression of GS messenger ribonucleic acid and protein also increased when glutamine was deprived. In biopsy samples, the GS expression scores were significantly higher in PCa tissue than in benign tissues (p < .001), and there was a positive correlation between the maximum GS expression scores and Gleason scores (Spearman r = .52). In 34 patients, [¹³N]ammonia uptake in PCa segments was significantly higher than that in benign segments (p ≤ .01), and there was a weak correlation between GS expression scores and the uptake of [¹³N]ammonia (Spearman r = .47). The expression of GS in PCa cells upregulated along with the deprivation of glutamine. GS is the main reason for the uptake of [¹³N]ammonia, and [¹³N]ammonia is a useful tracer for PCa imaging.
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7
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Edula RGR, Pyrsopoulos NT. New Methods of Testing and Brain Imaging in Hepatic Encephalopathy: A Review. Clin Liver Dis 2015. [PMID: 26195200 DOI: 10.1016/j.cld.2015.04.001] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The diagnosis of hepatic encephalopathy is predominantly clinical, and the tests available assist in the diagnosis only by excluding other causes. Covert hepatic encephalopathy, which is defined as abnormal performance on psychometric tests when standard neurologic examination is completely normal, has gained widespread attention in recent years due to its effect on quality of life. This review focuses on the tests available to aid in the diagnosis of this significant complication of liver disease, and discusses the complex pathophysiologic mechanisms identified through new imaging techniques and their significance toward development of new therapeutic targets for this condition.
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Affiliation(s)
- Raja G R Edula
- Division of Gastroenterology & Hepatology, Rutgers New Jersey Medical School, 185 South Orange Avenue, MSB H 538, Newark, NJ 07103, USA
| | - Nikolaos T Pyrsopoulos
- Division of Gastroenterology & Hepatology, Rutgers New Jersey Medical School, 185 South Orange Avenue, MSB H 538, Newark, NJ 07103, USA.
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8
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Aldridge DR, Tranah EJ, Shawcross DL. Pathogenesis of hepatic encephalopathy: role of ammonia and systemic inflammation. J Clin Exp Hepatol 2015; 5:S7-S20. [PMID: 26041962 PMCID: PMC4442852 DOI: 10.1016/j.jceh.2014.06.004] [Citation(s) in RCA: 192] [Impact Index Per Article: 21.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/09/2013] [Accepted: 06/05/2014] [Indexed: 12/12/2022] Open
Abstract
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.
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Key Words
- ATP, adenosine triphosphate
- AoCLF, acute-on-chronic liver failure
- BBB, blood–brain barrier
- CBF, cerebral blood flow
- CNS, central nervous system
- GS, glutamine synthetase
- HE, hepatic encephalopathy
- ICH, intracranial hypertension
- MHE, minimal hepatic encephalopathy
- MPT, mitochondrial permeability transition
- PAG, phosphate-activated glutaminase
- PTP, permeability transition pore
- TLR, toll-like receptor
- ammonia
- hepatic encephalopathy
- iNOS, inducible nitric oxide synthase
- infection
- inflammation
- systemic inflammatory response syndrome
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Affiliation(s)
| | | | - Debbie L. Shawcross
- Institute of Liver Studies, King's College London School of Medicine at King's College Hospital, King's College Hospital, Denmark Hill, London SE5 9RS, United Kingdom
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9
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Ott P, Vilstrup H. Cerebral effects of ammonia in liver disease: current hypotheses. Metab Brain Dis 2014; 29:901-11. [PMID: 24488230 DOI: 10.1007/s11011-014-9494-7] [Citation(s) in RCA: 74] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/24/2013] [Accepted: 01/21/2014] [Indexed: 12/15/2022]
Abstract
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.
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Affiliation(s)
- Peter Ott
- Department of Hepatology and Gastroenterology, Aarhus University Hospital, 8000C, Aarhus, Denmark,
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10
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Abstract
Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.
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Affiliation(s)
- Valerie Walker
- Department of Clinical Biochemistry, University Hospital Southampton NHS Foundation Trust, Southampton, United Kingdom.
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11
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Effect of glutamine synthetase inhibition on brain and interorgan ammonia metabolism in bile duct ligated rats. J Cereb Blood Flow Metab 2014; 34:460-6. [PMID: 24346692 PMCID: PMC3948122 DOI: 10.1038/jcbfm.2013.218] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/13/2013] [Revised: 10/28/2013] [Accepted: 11/04/2013] [Indexed: 01/03/2023]
Abstract
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.
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12
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Jacobson O, Abourbeh G, Tsvirkun D, Mishani E. Rat imaging and in vivo stability studies using [11C]-dimethyl-diphenyl ammonium, a candidate agent for PET-myocardial perfusion imaging. Nucl Med Biol 2013; 40:967-73. [PMID: 23999238 DOI: 10.1016/j.nucmedbio.2013.07.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2013] [Revised: 07/07/2013] [Accepted: 07/09/2013] [Indexed: 10/26/2022]
Abstract
BACKGROUND PET myocardial perfusion imaging (MPI) holds several advantages over SPECT for diagnosing coronary artery disease. The short half-lives of prevailing PET-MPI agents hamper wider clinical application of PET in nuclear cardiology; prompting the development of novel PET-MPI agents. We have previously reported on the potential of radiolabeled ammonium salts, and particularly on that of [(11)C]dimethyl-diphenyl-ammonium ([(11)C]DMDPA), for cardiac PET imaging. This study was designed to improve the radiosynthesis and increase the yield of [(11)C]DMDPA, characterize more meticulously the kinetics of radioactivity distribution after its injection via micro-PET/CT studies, and further explore its potential for PET-MPI. METHODS The radiosynthetic procedure of [(11)C]DMDPA was improved with respect to the previously reported one. The kinetics of radioactivity distribution following injection of [(11)C]DMDPA were investigated in juvenile and young adult male SD rats using microPET/CT, and compared to those of [(13)N]NH3. Furthermore, the metabolic fate of [(11)C]DMDPA in vivo was examined after its injection into rats. RESULTS Following a radiosynthesis time of 25-27 min, 11.9 ± 1.1 GBq of [(11)C]DMDPA was obtained, with a 43.7% ± 4.3% radiochemical yield (n = 7). Time activity curves calculated after administration of [(11)C]DMDPA indicated rapid, high and sustained radioactivity uptake in hearts of both juvenile and young adult rats, having a two-fold higher cardiac radioactivity uptake compared to [(13)N]NH3. Accordingly, at all time points after injection to both juvenile and young adult rats, image quality of the left ventricle was higher with [(11)C]DMDPA compared to [(13)N]NH3. In vivo stability studies of [(11)C]DMDPA indicate that no radioactive metabolites could be detected in plasma, liver and urine samples of rats up to 20 min after injection, suggesting that [(11)C]DMDPA is metabolically stable in vivo. CONCLUSIONS This study further illustrates that [(11)C]DMDPA holds, at least in part, essential qualities required from a PET-MPI probe. Owing to the improved radiosynthetic procedure reported herein, [(11)C]DMDPA can be produced in sufficient amounts for clinical use.
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Affiliation(s)
- Orit Jacobson
- Cyclotron-Radiochemistry-MicroPET Unit, Department of Medical Biophysics and Nuclear Medicine, Hadassah Hebrew University Hospital, Jerusalem 91120, Israel
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13
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The combination of 13N-ammonia and 18F-FDG in predicting primary central nervous system lymphomas in immunocompetent patients. Clin Nucl Med 2013; 38:98-102. [PMID: 23334122 DOI: 10.1097/rlu.0b013e318279b6cc] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
OBJECTIVE Accurate identification of primary central nervous system lymphoma (PCNSL) and its differentiation from other brain tumors remain difficult but are essential for treatment. In this study, we investigated whether (13)N-ammonia combined with (18)F-FDG could distinguish PCNSL from solid gliomas effectively. METHODS Ten consecutive patients with final diagnosis of PCNSL (5 female and 5 male patients; mean [SD] age, 59.10 [12.47] years; range, 43-74 years) and another fifteen consecutive patients with solid glioma lesions (5 female and 10 male patients; mean [SD] age, 46.73 [19.61] years; range, 14-72 years) were included in this study. PET/CT imaging was performed for all of them with both (18)F-FDG and (13)N-ammonia as tracers. Tumor-to-gray matter (T/G) ratios were calculated for the evaluation of tumor uptake. Both Student t test and discriminant analysis were recruited to assess the differential efficacy of these 2 tracers. RESULTS The T/G ratios of (18)F-FDG in PCNSL lesions were higher than in solid gliomas (3.26 [1.18] vs 1.56 [0.41], P < 0.001), whereas the T/G ratios of (13)N-ammonia in PCNSL lesions were lower than in solid gliomas significantly (1.38 [0.20] vs 2.11 [0.69], P < 0.001). All the lesions of PCNSL displayed higher T/G ratios of (18)F-FDG than (13)N-ammonia, whereas 14 (77.8%) of 18 glioma lesions showed contrary results. Tumor classification by means of canonical discriminant analysis yielded an overall accuracy of 96.9%, and only one glioma lesion was misclassified into the PCNSL group. CONCLUSIONS PCNSLs and solid gliomas have different metabolic profiles on N-ammonia and F-FDG imaging. The combination of these 2 tracers can distinguish these 2 clinical entities effectively and make an accurate prediction of PCNSL.
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14
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Brain metabolism in patients with hepatic encephalopathy studied by PET and MR. Arch Biochem Biophys 2013; 536:131-42. [PMID: 23726863 DOI: 10.1016/j.abb.2013.05.006] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2013] [Revised: 05/07/2013] [Accepted: 05/13/2013] [Indexed: 01/15/2023]
Abstract
We review PET- and MR studies on hepatic encephalopathy (HE) metabolism in human subjects from the point of views of methods, methodological assumptions and use in studies of cirrhotic patients with clinically overt HE, cirrhotic patients with minimal HE, cirrhotic patients with no history of HE and healthy subjects. Key results are: (1) Cerebral oxygen uptake and blood flow are reduced to 2/3 in cirrhotic patients with clinically overt HE but not in cirrhotic patients with minimal HE or no HE compared to healthy subjects. (2) Cerebral ammonia metabolism is enhanced due to increased blood ammonia in cirrhotic patients but the kinetics of cerebral ammonia uptake and metabolism is not affected by hyperammonemia. (3) Recent advantages in MR demonstrate low-grade cerebral oedema not only in astrocytes but also in the white matter in cirrhotic patients with HE.
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15
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Dam G, Keiding S, Munk OL, Ott P, Vilstrup H, Bak LK, Waagepetersen HS, Schousboe A, Sørensen M. Hepatic encephalopathy is associated with decreased cerebral oxygen metabolism and blood flow, not increased ammonia uptake. Hepatology 2013; 57:258-65. [PMID: 22886493 DOI: 10.1002/hep.25995] [Citation(s) in RCA: 58] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Revised: 12/15/2011] [Accepted: 07/11/2012] [Indexed: 12/19/2022]
Abstract
UNLABELLED Studies have shown decreased cerebral oxygen metabolism (CMRO(2)) and blood flow (CBF) in patients with cirrhosis with hepatic encephalopathy (HE). It remains unclear, however, whether these disturbances are associated with HE or with cirrhosis itself and how they may relate to arterial blood ammonia concentration and cerebral metabolic rate of blood ammonia (CMRA). We addressed these questions in a paired study design by investigating patients with cirrhosis during and after recovery from an acute episode of HE type C. CMRO(2), CBF, and CMRA were measured by dynamic positron emission tomography (PET)/computed tomography (CT). Ten patients with cirrhosis were studied during an acute episode of HE; nine were reexamined after recovery. Nine patients with cirrhosis with no history of HE served as controls. Mean CMRO(2) increased from 0.73 μmol oxygen/mL brain tissue/min during HE to 0.91 μmol oxygen/mL brain tissue/min after recovery (paired t test; P < 0.05). Mean CBF increased from 0.28 mL blood/mL brain tissue/min during HE to 0.38 mL blood/mL brain tissue/min after recovery (P < 0.05). After recovery from HE, CMRO(2) and CBF were not significantly different from values in the control patients. Arterial blood ammonia concentration decreased 20% after recovery (P < 0.05) and CMRA was unchanged (P > 0.30); both values were higher than in the control patients (both P < 0.05). CONCLUSION The low values of CMRO(2) and CBF observed during HE increased after recovery from HE and were thus associated with HE rather than the liver disease as such. The changes in CMRO(2) and CBF could not be linked to blood ammonia concentration or CMRA.
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Affiliation(s)
- Gitte Dam
- PET Centre & Department of Nuclear Medicine, Aarhus University Hospital, Aarhus, Denmark.
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16
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The role of glutamine synthetase and glutamate dehydrogenase in cerebral ammonia homeostasis. Neurochem Res 2012; 37:2439-55. [PMID: 22618691 DOI: 10.1007/s11064-012-0803-4] [Citation(s) in RCA: 69] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2012] [Revised: 04/24/2012] [Accepted: 05/07/2012] [Indexed: 02/07/2023]
Abstract
In the brain, glutamine synthetase (GS), which is located predominantly in astrocytes, is largely responsible for the removal of both blood-derived and metabolically generated ammonia. Thus, studies with [(13)N]ammonia have shown that about 25 % of blood-derived ammonia is removed in a single pass through the rat brain and that this ammonia is incorporated primarily into glutamine (amide) in astrocytes. Major pathways for cerebral ammonia generation include the glutaminase reaction and the glutamate dehydrogenase (GDH) reaction. The equilibrium position of the GDH-catalyzed reaction in vitro favors reductive amination of α-ketoglutarate at pH 7.4. Nevertheless, only a small amount of label derived from [(13)N]ammonia in rat brain is incorporated into glutamate and the α-amine of glutamine in vivo. Most likely the cerebral GDH reaction is drawn normally in the direction of glutamate oxidation (ammonia production) by rapid removal of ammonia as glutamine. Linkage of glutamate/α-ketoglutarate-utilizing aminotransferases with the GDH reaction channels excess amino acid nitrogen toward ammonia for glutamine synthesis. At high ammonia levels and/or when GS is inhibited the GDH reaction coupled with glutamate/α-ketoglutarate-linked aminotransferases may, however, promote the flow of ammonia nitrogen toward synthesis of amino acids. Preliminary evidence suggests an important role for the purine nucleotide cycle (PNC) as an additional source of ammonia in neurons (Net reaction: L-Aspartate + GTP + H(2)O → Fumarate + GDP + P(i) + NH(3)) and in the beat cycle of ependyma cilia. The link of the PNC to aminotransferases and GDH/GS and its role in cerebral nitrogen metabolism under both normal and pathological (e.g. hyperammonemic encephalopathy) conditions should be a productive area for future research.
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17
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Cornelius JF, Langen KJ, Stoffels G, Hänggi D, Sabel M, Steiger HJ. Positron Emission Tomography Imaging of Meningioma in Clinical Practice. Neurosurgery 2011; 70:1033-41; discussion 1042. [DOI: 10.1227/neu.0b013e31823bcd87] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Abstract
Meningiomas represent about 20% of intracranial tumors and are the most frequent nonglial primary brain tumors. Diagnosis is based on computed tomography (CT) and magnetic resonance imaging (MRI). Mainstays of therapy are surgery and radiotherapy. Adjuvant chemotherapy is tested in clinical trials of phase II. Patients are followed clinically by imaging. However, classical imaging modalities such as CT and MRI have limitations. Hence, we need supplementary imaging tools. Molecular imaging modalities, especially positron emission tomography (PET), represent promising new instruments that are able to characterize specific metabolic features. So far, these modalities have only been part of limited study protocols, and their impact on clinical routine management is still under investigation. It may be expected that their extended use will provide new aspects about meningioma imaging and biology.
In the present article, we summarize PET imaging for meningiomas based on a thorough review of the literature. We discuss and illustrate the potential role of PET imaging in the clinical management of meningiomas. Finally, we indicate current limitations and outline directions for future research.
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Affiliation(s)
- Jan Frederick Cornelius
- Neurochirurgische Klinik, Universitätsklinikum Düsseldorf, Heinrich-Heine-Universität, Düsseldorf, Germany
| | - Karl Josef Langen
- Institut für Medizin, Forschungszentrum Jülich, Heinrich-Heine-Universität, Jülich, Germany
| | - Gabriele Stoffels
- Institut für Medizin, Forschungszentrum Jülich, Heinrich-Heine-Universität, Jülich, Germany
| | - Daniel Hänggi
- Neurochirurgische Klinik, Universitätsklinikum Düsseldorf, Heinrich-Heine-Universität, Düsseldorf, Germany
| | - Michael Sabel
- Neurochirurgische Klinik, Universitätsklinikum Düsseldorf, Heinrich-Heine-Universität, Düsseldorf, Germany
| | - Hans Jakob Steiger
- Neurochirurgische Klinik, Universitätsklinikum Düsseldorf, Heinrich-Heine-Universität, Düsseldorf, Germany
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18
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Dam G, Keiding S, Munk OL, Ott P, Buhl M, Vilstrup H, Bak LK, Waagepetersen HS, Schousboe A, Møller N, Sørensen M. Branched-chain amino acids increase arterial blood ammonia in spite of enhanced intrinsic muscle ammonia metabolism in patients with cirrhosis and healthy subjects. Am J Physiol Gastrointest Liver Physiol 2011; 301:G269-77. [PMID: 21636533 DOI: 10.1152/ajpgi.00062.2011] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Branched-chain amino acids (BCAA) are used in attempts to reduce blood ammonia in patients with cirrhosis and intermittent hepatic encephalopathy based on the hypothesis that BCAA stimulate muscle ammonia detoxification. We studied the effects of an oral dose of BCAA on the skeletal muscle metabolism of ammonia and amino acids in 14 patients with cirrhosis and in 7 healthy subjects by combining [(13)N]ammonia positron emission tomography (PET) of the thigh muscle with measurements of blood flow and arteriovenous (A-V) concentrations of ammonia and amino acids. PET was used to measure the metabolism of blood-supplied ammonia and the A-V measurements were used to measure the total ammonia metabolism across the thigh muscle. After intake of BCAA, blood ammonia increased more than 30% in both groups of subjects (both P < 0.05). Muscle clearance of blood-supplied ammonia (PET) was unaffected (P = 0.75), but the metabolic removal rate (PET) increased significantly because of increased blood ammonia in both groups (all P < 0.05). The total ammonia clearance across the leg muscle (A-V) increased by more than 50% in both groups, and the flux (A-V) of ammonia increased by more than 45% (all P < 0.05). BCAA intake led to a massive glutamine release from the muscle (cirrhotic patients, P < 0.05; healthy subjects, P = 0.12). In conclusion, BCAA enhanced the intrinsic muscle metabolism of ammonia but not the metabolism of blood-supplied ammonia in both the patients with cirrhosis and in the healthy subjects.
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Affiliation(s)
- Gitte Dam
- PET Centre, Aarhus University Hospital, Denmark.
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19
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Brusilow SW, Koehler RC, Traystman RJ, Cooper AJL. Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy. Neurotherapeutics 2010; 7:452-70. [PMID: 20880508 PMCID: PMC2975543 DOI: 10.1016/j.nurt.2010.05.015] [Citation(s) in RCA: 153] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2010] [Revised: 05/17/2010] [Accepted: 05/19/2010] [Indexed: 12/21/2022] Open
Abstract
Many theories have been advanced to explain the encephalopathy associated with chronic liver disease and with the less common acute form. A major factor contributing to hepatic encephalopathy is hyperammonemia resulting from portacaval shunting and/or liver damage. However, an increasing number of causes of hyperammonemic encephalopathy have been discovered that present with the same clinical and laboratory features found in acute liver failure, but without liver failure. Here, we critically review the physiology, pathology, and biochemistry of ammonia (i.e., NH3 plus NH4+) and show how these elements interact to constitute a syndrome that clinicians refer to as hyperammonemic encephalopathy (i.e., acute liver failure, fulminant hepatic failure, chronic liver disease). Included will be a brief history of the status of ammonia and the centrality of the astrocyte in brain nitrogen metabolism. Ammonia is normally detoxified in the liver and extrahepatic tissues by conversion to urea and glutamine, respectively. In the brain, glutamine synthesis is largely confined to astrocytes, and it is generally accepted that in hyperammonemia excess glutamine compromises astrocyte morphology and function. Mechanisms postulated to account for this toxicity will be examined with emphasis on the osmotic effects of excess glutamine (the osmotic gliopathy theory). Because hyperammonemia causes osmotic stress and encephalopathy in patients with normal or abnormal liver function alike, the term "hyperammonemic encephalopathy" can be broadly applied to encephalopathy resulting from liver disease and from various other diseases that produce hyperammonemia. Finally, the possibility that a brain glutamine synthetase inhibitor may be of therapeutic benefit, especially in the acute form of liver disease, is discussed.
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Affiliation(s)
- Saul W. Brusilow
- grid.21107.350000000121719311Department of Pediatrics, The Johns Hopkins University School of Medicine, 21287 Baltimore, MD
| | - Raymond C. Koehler
- grid.21107.350000000121719311Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins University School of Medicine, 21287 Baltimore, MD
| | - Richard J. Traystman
- Office of the Vice Chancellor for Research, Department of Pharmacology, University of Colorado Denver, 80045 Aurora, CO
| | - Arthur J. L. Cooper
- grid.260917.b000000010728151XDepartment of Biochemistry and Molecular Biology, New York Medical College, 10595 Valhalla, NY
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20
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Blood-brain barrier permeability for ammonia in patients with different grades of liver fibrosis is not different from healthy controls. J Cereb Blood Flow Metab 2010; 30:1384-93. [PMID: 20216550 PMCID: PMC2949228 DOI: 10.1038/jcbfm.2010.22] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Increased blood-brain barrier (BBB) permeability for ammonia is considered to be an integral part of the pathophysiology of hepatic encephalopathy (HE) in patients with liver cirrhosis. Increased glutamate-/glutamine-signal intensity in magnetic resonance spectroscopic studies of the brain in cirrhotic patients was explained as a consequence of increased cerebral ammonia uptake. As similar spectroscopic alterations are present in patients with liver fibrosis, we hypothesized that BBB permeability for ammonia is already increased in liver fibrosis, and thereby contributing to the development of HE. To test this hypothesis, cerebral perfusion and ammonia metabolism were examined through positron emission tomography with (15)O-water, respectively, (13)N-ammonia in patients with Ishak grades 2 and 4 fibrosis, cirrhosis, and healthy controls. There were neither global nor regional differences of cerebral blood flow, the rate constant of unidirectional transport of ammonia from blood into brain tissue, the permeability surface area product of the BBB for ammonia, the net metabolic clearance rate constant of ammonia from blood into glutamine in brain, or the metabolic rate of ammonia. The hypothesis that increased permeability of the BBB for ammonia in patients with liver fibrosis contributes to the later development of HE could not be supported by this study.
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