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Hepatic encephalopathy and nitric oxide

2001; Elsevier BV; Volume: 34; Issue: 4 Linguagem: Inglês

10.1016/s0168-8278(01)00051-4

1600-0641

Freimut Schliess, Dieter Häussinger,

Liver Disease Diagnosis and Treatment

Hepatic encephalopathy (HE) represents a major neuropsychiatric complication associated with fulminant (e.g. viral or toxic hepatitis) or chronic (e.g. cirrhosis) liver failure. Symptoms are highly variable and range from mild personality changes to deep coma, but are, at each level of severity, potentially reversible (for review see [1Hazell A.S Butterworth R.F Hepatic encephalopathy: an update of pathophysiologic mechanisms.Proc Soc Exp Biol Med. 1999; 222: 99-112Crossref PubMed Scopus (245) Google Scholar, 2Butterworth R.F Complications of cirrhosis III. Hepatic encephalopathy.J Hepatol. 2000; 32: 171-180Abstract Full Text PDF PubMed Google Scholar, 3Häussinger D Kircheis G Fischer R Schliess F vom Dahl S Hepatic encephalopathy in chronic liver disease: a clinical manifestation of astrocyte swelling and low grade cerebral oedema.J Hepatol. 2000; 32: 1035-1038Abstract Full Text Full Text PDF PubMed Scopus (373) Google Scholar, 4Albrecht J Jones E.A Hepatic encephalopathy: molecular mechanisms underlying the clinical syndrome.J Neurol Sci. 1999; 170: 138-146Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar]). The pathomechanism(s) underlying the development of HE are still incompletely understood. However, a central role of ammonia due to impaired clearance of this gut-derived toxin from the blood by the injured liver is beyond doubt [[5]Norenberg M.D Astrocytic-ammonia interactions in hepatic encephalopathy.Semin Liver Dis. 1996; 16: 245-253Crossref PubMed Scopus (158) Google Scholar]. In positron emission tomography studies on patients with chronic liver failure an increased cerebral metabolic rate for ammonia as well as an increased permeability of the blood brain barrier for ammonia was found [[6]Lockwood A.H Yap E.W Rhoades H.M Wong W.H Altered cerebral blood flow and glucose metabolism in patients with liver disease and minimal encephalopathy.J Cereb Blood Flow Metab. 1991; 11: 331-336Crossref PubMed Scopus (136) Google Scholar]. Cerebral detoxification of ammonia is performed by the astrocytes which represent the only cellular compartment in brain expressing the glutamine synthetase [[7]Martinez H.A Bell K.P Norenberg M.D Glutamine synthetase: glial localization in brain.Science. 1977; 195: 1356-1358Crossref PubMed Scopus (978) Google Scholar]. Hyperammonaemia induces astrocyte swelling due to osmotic water influx following intracellular glutamine accumulation. Astrocyte swelling in the sense of a cytotoxic brain oedema was identified in absence of a clinically overt increase of intracranial pressure already in the subclinical stage of HE in chronic liver disease [8Häussinger D Laubenberger J vom Dahl S Ernst T Bayer S Langer M et al.Proton magnetic resonance spectroscopy studies on human brain myo-inositol in hypo-osmolarity and hepatic encephalopathy.Gastroenterology. 1994; 107: 1475-1480Abstract Full Text PDF PubMed Google Scholar, 9Morgan M.Y Cerebral magnetic resonance imaging in patients with chronic liver disease.Metab Brain Dis. 1998; 13: 273-290Crossref PubMed Scopus (42) Google Scholar]. In the case of fulminant hepatic failure (FHF) astrocyte swelling contributes to the development of severe brain oedema associated with a rise in intracranial pressure. Indeed brain oedema and herniation is a major cause of death in FHF. Inhibition of the glutamine synthetase, which decreases ammonia-induced astrocyte swelling, ameliorates brain oedema and improves the survival rate in animal models of FHF [10Warren K.S Schencker S Effect of an inhibitor of glutamine synthesis (methionine sulfoximine) on ammonia toxicity and metabolism.J Lab Clin Med. 1964; 64: 442-449PubMed Google Scholar, 11Blei A.T Olafsson S Therrien G Butterworth R.F Ammonia-induced brain oedema and intracranial hypertension in rats after portacaval anastomosis.Hepatology. 1994; 19: 1437-1444Crossref PubMed Scopus (206) Google Scholar]. A more recent line of evidence suggested cerebral oedema in FHF to arise as a consequence of cerebral vasodilatation. Mild hypothermia, which reduces cerebral blood flow rather than glutamine accumulation in the brain, ameliorated ammonia-induced brain oedema [[12]Cordoba J Crespin J Gottstein J Blei A.T Mild hypothermia modifies ammonia-induced brain oedema in rats after portacaval anastomosis.Gastroenterology. 1999; 116: 686-693Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar]. As recently reviewed by Andres T. Blei and Fin Stolze Larsen in the Journal of Hepatology [[13]Blei A.T Larsen F.S Pathophysiology of cerebral oedema in fulminant hepatic failure.J Hepatol. 1999; 31: 771-776Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar], the ‘glutamine hypothesis’ and a contribution of vasodilatation to ammonia-induced brain oedema are not mutually exclusive but may in fact be related to each other. These authors proposed an ‘unified theory’ of brain oedema which considers astrocyte swelling to represent a critical step for the development of cerebral circulatory changes in response to FHF. One consequence of astrocyte swelling is the elevation of extracellular glutamate levels resulting from both, glutamate release and impaired glutamate clearance from the extracellular space by the swollen astrocytes [[14]Albrecht J Roles of neuroactive amino acids in ammonia neurotoxicity.J Neurosci Res. 1998; 51: 133-138Crossref PubMed Scopus (63) Google Scholar]. Elevation of extracellular glutamate levels may cause overstimulation of the neuronally expressed N-methyl-d-aspartate (NMDA) receptors. NMDA receptor activation triggers an increase in intracellular free calcium, which after binding to calmodulin activates the neuronal nitric oxide synthase (nNOS). nNOS-catalyzed synthesis of nitric oxide (NO) from l-arginine in turn leads to the formation of cyclic guanosine monophosphate (cGMP) by activation of the soluble guanylate cyclase, which finally may produce vasodilation in brain [[13]Blei A.T Larsen F.S Pathophysiology of cerebral oedema in fulminant hepatic failure.J Hepatol. 1999; 31: 771-776Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar]. This hypothesis is strengthened by the finding, that systemic administration of NMDA in rats induces an increase in CBF, which was sensitive to NOS inhibitors [[15]Pelligrino D.A Gay III, R.L Baughman V.L Wang Q NO synthase inhibition modulates NMDA-induced changes in cerebral blood flow and EEG activity.Am J Physiol. 1996; 271: H990-H995PubMed Google Scholar]. In addition, death following acute ammonia intoxication of mice is prevented by NMDA receptor antagonists but not by inhibitors of kainate/α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors [[16]Hermenegildo C Marcaida G Montoliu C Grisolia S Minana M.D Felipo V NMDA receptor antagonists prevent acute ammonia toxicity in mice.Neurochem Res. 1996; 21: 1237-1244Crossref PubMed Scopus (127) Google Scholar]. Moreover, high doses of ammonia increase extracellular cGMP levels in rat brain by a mechanism sensitive to NMDA receptor antagonism and a good correlation exists between the increase in cGMP and the severity of the neurological symptoms elicited by different doses of ammonia [[17]Hermenegildo C Monfort P Felipo V Activation of N-methyl-d-aspartate receptors in rat brain in vivo following acute ammonia intoxication: characterization by in vivo brain microdialysis.Hepatology. 2000; 31: 709-715Crossref PubMed Scopus (232) Google Scholar]. In line with this the protection of rats from acute ammonia toxicity by priming the animals with chronic moderate hyperammonaemia correlates with a downregulation of the NMDA receptor-mediated cGMP formation [18Felipo V Hermenegildo C Montoliu C Llansola M Minana M.D Neurotoxicity of ammonia and glutamate: molecular mechanisms and prevention.Neurotoxicology. 1998; 19: 675-681PubMed Google Scholar, 19Monfort P Montoliu C Hermenegildo C Munoz M Felipo V Differential effects of acute and chronic hyperammonemia on signal transduction pathways associated to NMDA receptors.Neurochem Int. 2000; 37: 249-253Crossref PubMed Scopus (21) Google Scholar]. Thus, current evidence suggests that NMDA receptor activation in fact plays a major role in mediating toxicity of high doses of ammonia. However, the mode of the ammonia-induced NMDA-receptor activation and the downstream signalling events are not completely clarified. A role of glutamate in NMDA receptor activation following acute ammonia intoxication of rats was recently questioned due to the finding, that glutamate receptor-stimulated signalling occurs before an increase in extracellular glutamate [[17]Hermenegildo C Monfort P Felipo V Activation of N-methyl-d-aspartate receptors in rat brain in vivo following acute ammonia intoxication: characterization by in vivo brain microdialysis.Hepatology. 2000; 31: 709-715Crossref PubMed Scopus (232) Google Scholar]. In this study ammonia-induced glutamate release into the extracellular space was even blocked by an inhibitor of NMDA receptors, suggesting glutamate release to be a consequence of NMDA receptor activation. Other uncertainty within the ‘unified theory’ concerns the role of NO and glutamine synthesis in mediating ammonia-induced injury at least in acutely ammonia-intoxicated mice [[16]Hermenegildo C Marcaida G Montoliu C Grisolia S Minana M.D Felipo V NMDA receptor antagonists prevent acute ammonia toxicity in mice.Neurochem Res. 1996; 21: 1237-1244Crossref PubMed Scopus (127) Google Scholar]. Inhibitors of NOS only partly reduce mortality and inhibition of glutamine synthetase is without effect on mortality in this model. Recent work by A.T. Blei's group demonstrated, that inhibition of glutamine synthetase significantly diminishes the ammonia-induced increase of both, cerebrospinal fluid (CSF), glutamine concentration and CBF in rats after portocaval anastomosis [[20]Master S Gottstein J Blei A.T Cerebral blood flow and the development of ammonia-induced brain oedema in rats after portacaval anastomosis.Hepatology. 1999; 30: 876-880Crossref PubMed Scopus (147) Google Scholar]. In the paper presented by Fin Stolze Larsen, Jeanne Gottstein and Andres T. Blei in this issue of the Journal of Hepatology [[21]Larsen F.S Gottstein J Blei A.T Cerebral hyperemia and nitric oxide synthase in rats with ammonia-induced brain edema.J Hepatol. 2001; 34: 548-554Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar] the pathogenesis of brain oedema and cerebral hyperperfusion in FHF is further addressed with a focus on the role of glutamine formation, brain water increase and NOS activation. As in [[20]Master S Gottstein J Blei A.T Cerebral blood flow and the development of ammonia-induced brain oedema in rats after portacaval anastomosis.Hepatology. 1999; 30: 876-880Crossref PubMed Scopus (147) Google Scholar] the authors used portocaval anastomized rats (PCA rats), which were challenged with an ammonium acetate infusion 24 h after PCA construction. PCA rats represent a well established animal model to study ammonia-induced brain oedema under conditions of impaired liver function [[22]Blei A.T Omary R Butterworth R.F Animal models of hepatic encephalopathies.in: Boulton A Baker G Butterworth R.F Animal models of neurological disease II. The Humana Press Inc, 1992: 183-222Crossref Google Scholar]. In this study for the first time the chronological sequence of alterations in glutamine synthesis, brain water, intracranial pressure and CBF was investigated. It is demonstrated that ammonia-induced elevation of brain water content precedes the increase in intracranial pressure and CBF, respectively. Most important, a significant correlation between the increase in brain water and the rise in CBF is reported, supporting the hypothesis, that cerebral accumulation of water (most probably due to astrocyte swelling) triggers the development of cerebral hyperaemia. Interestingly, glutamine concentration in the cerebrospinal fluid (CSF) rose very slowly and was significantly elevated only in the animal group showing a threefold increase in intracranial pressure. Clearly CSF glutamine must not necessarily reflect the glutamine concentration inside the cells and therefore a poor correlation between CSF glutamine and brain water may not argue against a causal relationship between glutamine production and brain swelling. On the other hand, as suggested earlier [[11]Blei A.T Olafsson S Therrien G Butterworth R.F Ammonia-induced brain oedema and intracranial hypertension in rats after portacaval anastomosis.Hepatology. 1994; 19: 1437-1444Crossref PubMed Scopus (206) Google Scholar], the ammonia-induced increase in brain water in PCA rats may not solely result from glial glutamine accumulation, although inhibition of glutamine synthesis ameliorated brain oedema and prevented raise in intracranial pressure. It should be taken into account that glial swelling most likely is a consequence from the synergistic action of multiple factors with intracellular glutamine accumulation being one of them. Thus, hyponatraemia, which is frequently associated with acute and chronic liver failure, lowers glutamine accumulation but exacerbates brain oedema in response to ammonia in the PCA rat model [[23]Cordoba J Gottstein J Blei A.T Chronic hyponatremia exacerbates ammonia-induced brain oedema in rats after portacaval anastomosis.J Hepatol. 1998; 29: 589-594Abstract Full Text PDF PubMed Scopus (89) Google Scholar]. In addition, acute hyperammonaemic states are associated with upregulation of the peripheral-type benzodiazepinreceptor (PBR), which is functionally associated with neurosteroid synthesis [[1]Hazell A.S Butterworth R.F Hepatic encephalopathy: an update of pathophysiologic mechanisms.Proc Soc Exp Biol Med. 1999; 222: 99-112Crossref PubMed Scopus (245) Google Scholar]. It was shown, that PBR agonists as well as neurosteroids markedly enhance ammonia-induced swelling in cultured astrocytes [[24]Bender A.S Norenberg M.D Effect of benzodiazepines and neurosteroids on ammonia-induced swelling in cultured astrocytes.J Neurosci Res. 1998; 54: 673-680Crossref PubMed Scopus (87) Google Scholar]. The second major finding of the study presented by Larsen et al. in this issue [[21]Larsen F.S Gottstein J Blei A.T Cerebral hyperemia and nitric oxide synthase in rats with ammonia-induced brain edema.J Hepatol. 2001; 34: 548-554Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar] concerns the role of NO in mediating ammonia-induced brain oedema. Neither systemic application of the non-specific NOS inhibitor NΩ-nitro-l-arginine (l-NNA), nor of 1-(2-trifluoromethylphenyl)imidazole (TRIM), a recently developed inhibitor specifically blocking nNOS [[25]Towler P.K Bennett G.S Moore P.K Brain S.D Neurogenic oedema and vasodilatation: effect of a selective neuronal NO inhibitor.NeuroReport. 1998; 9: 1513-1518Crossref PubMed Scopus (21) Google Scholar], was effective to ameliorate ammonia-induced increase in intracranial pressure and CBF, respectively. This argues at first glance against a role of NO in mediating the ammonia effects on cerebral blood flow and intracranial pressure. Although NOS activity in brain was not measured in the present study, TRIM was used at a concentration effectively inhibiting nNOS in other studies [[25]Towler P.K Bennett G.S Moore P.K Brain S.D Neurogenic oedema and vasodilatation: effect of a selective neuronal NO inhibitor.NeuroReport. 1998; 9: 1513-1518Crossref PubMed Scopus (21) Google Scholar], rendering it unlikely that nNOS-derived NO plays a major role in mediating the ammonia effects on CBF in PCA rats. The action of l-NNA, which was chronically administrated during the five days prior to installation of the PCA, was indirectly confirmed by the elevated arterial pressure found in presence of the inhibitor. However, as demonstrated recently [[15]Pelligrino D.A Gay III, R.L Baughman V.L Wang Q NO synthase inhibition modulates NMDA-induced changes in cerebral blood flow and EEG activity.Am J Physiol. 1996; 271: H990-H995PubMed Google Scholar], chronic l-NNA administration did not influence the NMDA-induced increase in rat cortical CBF, whereas acute pretreatment of the rats with l-NNA effectively blocked this effect. This points to the possibility that chronic l-NNA administration recruits additional signalling mechanism which compensate for deficiency in NOS activity. If so, ammonia could activate alternative signalling routes after chronic l-NNA treatment, which possibly play a minor role in absence of the inhibitor or after its application acutely before ammonia intoxication. Therefore, the data presented do not exclude the possibility of ammonia-induced NO synthesis by eNOS and iNOS expressed in neurons or non-neuronal cells. That this possibility must be taken into account is suggested by cell culture studies. Ammonia stimulates arginine uptake in cultured astrocytes [[26]Hazell A.S Norenberg M.D Ammonia and manganese increase arginine uptake in cultured astrocytes.Neurochem Res. 1998; 23: 869-873Crossref PubMed Scopus (41) Google Scholar] and high ammonia concentrations induce the expression of argininosuccinate synthetase and the argininosuccinate lyase, which catalyze the regeneration of arginine from citrulline (another product of the NO-generating reaction sequence) and thereby increase substrate availability for the NOS [[27]Braissant O Honegger P Loup M Iwase K Takiguchi M Bachmann C Hyperammonemia: regulation of argininosuccinate synthetase and argininosuccinate lyase genes in aggregating cell cultures of fetal rat brain.Neurosci Lett. 1999; 266: 89-92Crossref PubMed Scopus (38) Google Scholar]. Measurement of NOS activity in brain and representation of NOS activity by NADPH diaphorase histochemistry in brain slices may help to get deeper insight into the role of NO in the generation of acute HE by ammonia intoxication and the effectivity of inhibitor action in these the PCA rat model. Nevertheless, potential additional / alternative players such as potassium and the arachidonic acid-derived metabolites need to be considered in the future. Arachidonic acid in cultured astrocytes induces swelling and impairs volume regulation in response to hypoosmotic swelling [28Sanchezolea R Moralesmulia M Moran J Pasantesmorales H Inhibition by polyunsaturated fatty acids of cell volume regulation and osmolyte fluxes in astrocytes.Am J Physiol. 1995; 38: C96-C102Google Scholar, 29Staub F Winkler A Peters J Kempski O Baethmann A Mechanisms of glial swelling by arachidonic acid.Acta Neurochir Suppl Wien. 1994; 60: 20-23PubMed Google Scholar]. In HE ammonia-induced arachidonic acid release could increase glial swelling thereby amplifying ammonia toxicity. Indometacine, which inhibits the cyclooxygenase COX-2, was reported to decrease the intracranial pressure in patients with FHF by decreasing cerebral blood volume, suggesting a participation of prostaglandin E2 vasorelaxation [[30]Clemmesen J.O Hansen B.A Larsen F.S Indomethacin normalizes intracranial pressure in acute liver failure: a 23-year old woman treated with indomethacin.Hepatology. 1997; 25: 1423-1426Crossref Scopus (81) Google Scholar]. However, this does not exclude a role of NO because COX-2 can be activated by NO [[31]Salvemini D Misko T.P Masferrer J.L Seibert K Currie M.G Needleman P Nitric oxide activates cyclooxygenase enzymes.Proc Natl Acad Sci USA. 1993; 90: 7240-7244Crossref PubMed Scopus (1378) Google Scholar] and iNOS-synthesized NO was shown to mediate prostaglandin E2 production in brain after cerebral ischemia [[32]Nogawa S Forster C Zhang F Nagayama M Ross M.E Iadecola C Interaction between inducible nitric oxide synthase and cyclooxygenase-2 after cerebral ischemia.Proc Natl Acad Sci USA. 1998; 95: 10966-10971Crossref PubMed Scopus (280) Google Scholar]. Prostacyclin PGI2 is another candidate to mediate ammonia-induced vasorelaxation, supported by the neuronal localization of the PGI2 synthase [[33]Mehl M Bidmon H.J Hilbig H Zilles K Dringen R Ullrich V Prostacyclin synthase is localized in rat, bovine and human neuronal brain cells.Neurosci Lett. 2001; 271: 187-190Crossref Scopus (21) Google Scholar]. PGI2 formation requires cyclooxygenase activity and is inhibited by NO due to nitration of a critical tyrosine residue [[34]Zou M Klein T Pasquet J.P Ullrich V Interleukin 1β decreases prostacyclin synthase activity in rat mesangial cells via endogenous peroxynitrite formation.Biochem J. 1998; 336: 507-512Crossref PubMed Scopus (64) Google Scholar]. Last but not least, a reduction in the synthesis of vasoconstrictors in response to ammonia should not be ignored. The paper of Larsen et al. [[21]Larsen F.S Gottstein J Blei A.T Cerebral hyperemia and nitric oxide synthase in rats with ammonia-induced brain edema.J Hepatol. 2001; 34: 548-554Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar] impressively supports the view that astrocyte swelling represents an integrative trigger within the complex signalling network generating the increase in CBF and probably the neurological symptoms in HE. Further work on animal and cell culture models is required to unravel the molecular mechanism of ammonia toxicity and especially to finally define the role of NO in pathogenesis of acute and chronic HE.