Does ammonia really disrupt brain oxygen homeostasis?

We found the paper “Abnormal brain oxygen homeostasis in an animal model of liver disease” by Hadjihambi et al.[1]Hadjihambi A. Cudalbu C. Pierzchala K. Simicic D. Donnelly C. Konstantinou C. et al.Abnormal brain oxygen homeostasis in an animal model of liver disease.J Hep Rep. 2022 Aug; 4100509Google Scholar interesting but do have some comments to and reservations about the authors’ interpretation of the results presented in the paper, particularly concerning their conclusion that ammonia causes a metabolically important limitation of brain oxygen availability. The main conclusion reached by the authors is that the bile duct ligated (BDL) animals suffered from abnormal brain oxygen homeostasis. However, when looking at the measured tissue pO2 values in their Figs 2 and 3, the majority of the BDL rats had baseline values well above the range of 6.8 and 8.8 mmHg, which has been shown to be a critical in vivo pO2 level in the brain tissue of BDL rats.[2]Rolett E.L. Azzawi A. Liu K.J. Yongbi M.N. Swartz H.M. Dunn J.F. Critical oxygen tension in rat brain: a combined 31P-NMR and EPR oximetry study.Am J Physiol Regul Integr Comp Physiol. 2000; 279: R9-R16Crossref PubMed Google Scholar Moreover, there are no data in the paper correlating blood ammonia levels with brain tissue oxygen levels or neurological symptoms. In our opinion, the data accordingly do not unambiguously demonstrate that the decreased tissue pO2 should have any metabolic effects or reflect significant tissue hypoxia or ischaemia related to hepatic encephalopathy (HE). In support of this, the authors found markedly decreased brain lactate concentrations on 1H-MRS in the BDL rats when compared to control rats.[1]Hadjihambi A. Cudalbu C. Pierzchala K. Simicic D. Donnelly C. Konstantinou C. et al.Abnormal brain oxygen homeostasis in an animal model of liver disease.J Hep Rep. 2022 Aug; 4100509Google Scholar In case of significant hypoxia, one would expect the anaerobic metabolism – and thus the concentration of lactate – to increase. As recently summarized by us, we addressed the potential direct effects of ammonia on cerebral oxygen metabolism in a series of experimental studies ranging from brain cell cultures to animal models including the BDL model, and clinical studies with healthy controls and patients with cirrhosis without, during, and after an episode of HE.[3]Sørensen M. Walls A.B. Dam G. Bak L.K. Andersen J.V. Ott P. et al.Low cerebral energy metabolism in hepatic encephalopathy reflects low neuronal energy demand. Role of ammonia-induced increased GABAergic tone.Anal Biocem. 2022 Oct 1; 654114766PubMed Google Scholar First, we did not find any evidence that ammonia inhibits the tricarboxylic acid cycle, i.e. there was no evidence of direct inhibition of cerebral energy metabolism.[3]Sørensen M. Walls A.B. Dam G. Bak L.K. Andersen J.V. Ott P. et al.Low cerebral energy metabolism in hepatic encephalopathy reflects low neuronal energy demand. Role of ammonia-induced increased GABAergic tone.Anal Biocem. 2022 Oct 1; 654114766PubMed Google Scholar,[4]Iversen P. Mouridsen K. Hansen M.B. Jensen S.B. Sørensen M. Bak L.K. et al.Oxidative metabolism of astrocytes is not reduced in hepatic encephalopathy: a PET study with [11C]acetate in humans.Front Neurosci. 2014; 8: 353Crossref PubMed Scopus (15) Google Scholar Second, though cerebral blood flow (CBF) decreased during HE, we found no evidence of decreased brain oxygen supply being a cause of HE, rather it seemed to be a result of a decreased metabolic demand causing a decreased cerebral metabolic rate of oxygen.[5]Gjedde A. Keiding S. Vilstrup H. Iversen P. No oxygen delivery limitation in hepatic encephalopathy.Metab Brain Dis. 2010; 25: 57-63Crossref PubMed Scopus (16) Google Scholar This standpoint is also supported by a recent paper showing decreased brain glucose uptake in BDL rats in a combined PET and 1H-MRS study[7]Mosso J. Yin T. Poitry-Yamate C. Simicic D. Lepore M. McLin V.A. et al.PET CMRglc mapping and 1H-MRS show altered glucose uptake and neurometabolic profiles in BDL rats.Anal Biochem. 2022; 647114606Crossref PubMed Scopus (6) Google Scholar from members of the same group as the Hadjihambi paper. The experimental effects of hypercapnia and manipulation of systemic blood pressure by administration of acetazolamide and phenylephrine in the Hadjihambi paper suggest, as stated by the authors,[1]Hadjihambi A. Cudalbu C. Pierzchala K. Simicic D. Donnelly C. Konstantinou C. et al.Abnormal brain oxygen homeostasis in an animal model of liver disease.J Hep Rep. 2022 Aug; 4100509Google Scholar that the cerebrovascular activity and capacity for blood vessels to dilate are intact. They take this to reflect increased vascular tone but, as no CBF measurements are given, it remains unclear whether the experimental interventions reflect significant changes in blood flow. Furthermore, the values of brain oxygen tension in BDL rats after the ammonia-lowering treatment with ornithine + phenylacetate were few and overlapped with those without ornithine + phenylacetate. As the effect is uncertain, even if ammonia-lowering treatment increases brain oxygen tension in BDL animals, this does not show that ammonia has a direct effect on vascular tone in the brain, as proposed by the authors. Overall, we believe that the results presented by Hadjihambi et al.[1]Hadjihambi A. Cudalbu C. Pierzchala K. Simicic D. Donnelly C. Konstantinou C. et al.Abnormal brain oxygen homeostasis in an animal model of liver disease.J Hep Rep. 2022 Aug; 4100509Google Scholar are in support of our conclusions, namely that hyperammonaemia has an indirect inhibitory effect on brain metabolism due to an inhibitory effect on brain activity and that the decreased brain oxygen consumption and decreased CBF reflects a true decrease in metabolic demand, not abnormal brain oxygen homeostasis. This conclusion is also reached in a 1H and 31P-MRS study in BDL rats,[6]Rackayova V. Braissant O. McLin V.A. Berset C. Lanz B. Cudalbu C. 1H and 31P magnetic resonance spectroscopy in a rat model of chronic hepatic encephalopathy: in vivo longitudinal measurements of brain energy metabolism.Metab Brain Dis. 2016; 31: 1303-1314Crossref PubMed Scopus (33) Google Scholar i.e. the same model used in the recent study by Hadjihambi et al. We compliment the authors for their comprehensive and complex experiments, but we do believe that their conclusions are not fully supported by their data. In light of our experimental and clinical studies, we find that the results may as well indicate that low cerebral oxygen consumption in the presence of high ammonia is due to low brain energy requirements and not compromised brain oxygen availability. That said, we would like to emphasize that we do not question the pathogenetic importance of ammonia in HE. The authors received no financial support to produce this manuscript. The manuscript was written by Michael Sørensen and Hendrik Vilstrup, who both reviewed the final version critically. The authors have nothing to disclose. Please refer to the accompanying ICMJE disclosure forms for further details. The following are the supplementary data to this article: Download .pdf (.73 MB) Help with pdf files Multimedia component 1 Abnormal brain oxygen homeostasis in an animal model of liver diseaseJHEP ReportsVol. 4Issue 8PreviewIncreased plasma ammonia concentration and consequent disruption of brain energy metabolism could underpin the pathogenesis of hepatic encephalopathy (HE). Brain energy homeostasis relies on effective maintenance of brain oxygenation, and dysregulation impairs neuronal function leading to cognitive impairment. We hypothesised that HE is associated with reduced brain oxygenation and we explored the potential role of ammonia as an underlying pathophysiological factor. Full-Text PDF Open AccessReply to: “Does ammonia really disrupt brain oxygen homeostasis?”JHEP ReportsVol. 5Issue 5PreviewWe are grateful for Drs. Sørensen and Vilstrup’s interest1 and thoughtful consideration of our recent study2 and are encouraged that they agree with our substantive conclusions. We welcome this opportunity to reply to their concerns regarding our specific interpretation that decreased tissue pO2 could contribute to the metabolic disruption seen in hepatic encephalopathy (HE). Full-Text PDF Open Access