Role of quantitative myocardial blood flow and 13N-ammonia washout for viability assessment in ischemic cardiomyopathy.

OBJECTIVE: Positron emission tomography (PET) integrating assessment of perfusion with 13N-ammonia (NH3) and viability with 18F-fluorodeoxyglucose (FDG) has high accuracy to identify viable, hibernating myocardium. We tested whether quantification of myocardial blood flow (MBF) and washout (k2) can predict myocardial viability using FDG as standard of reference. METHODS: In 180 consecutive patients with ischemic cardiomyopathy, myocardium was categorized on a segment-level into normal, ischemic, hibernating, and scar. From dynamic images, stress MBF, rest MBF, and k2 were derived and myocardial flow reserve (MFR) and volume of distribution (VD) were calculated. RESULTS: Across myocardial tissues, all parameters differed significantly. The area under the curve (AUC) was 0.564 (95% CI 0.527-0.601), 0.635 (0.599-0.671), 0.553 (0.516-0.591), 0.520 (0.482-0.559), and 0.560 (0.522-0.597) for stress MBF, rest MBF, MFR, k2, and VD. The generalized linear mixed model correctly classified 81% of scar as viable, hibernating myocardium. If the threshold of rest MBF to predict viability was set to 0.45 mL·min-1·g-1, sensitivity and specificity were 96% and 12%, respectively. CONCLUSION: Quantitative NH3 PET parameters have low to moderate diagnostic performance to predict viability in ischemic cardiomyopathy. However, if rest MBF falls below 0.45 mL·min-1·g-1, viability testing by FDG-PET may be safely deferred.

Impact of left bundle branch block on myocardial perfusion and metabolism: A positron emission tomography study.

BACKGROUND: Better understanding of pathophysiological changes, induced by left bundle branch block (LBBB), may improve patient selection for cardiac resynchronization therapy (CRT). Therefore, we assessed the effect of LBBB on regional glucose metabolism, 13N-NH3-derived absolute and semiquantitative myocardial blood flow (MBF), and their relation in non-ischemic CRT candidates. METHODS: Twenty-five consecutive non-ischemic patients with LBBB underwent 18F-FDG and resting dynamic 13N-NH3 PET/CT prior to CRT implantation. Regional 18F-FDG uptake, absolute MBF, and late 13N-NH3 uptake were analyzed and corresponding septal-to-lateral wall ratios (SLR) were calculated. Segmental analysis was performed to evaluate "reverse mismatch," "mismatch," and "match" patterns, based on late 13N-NH3/18F-FDG uptake ratios. RESULTS: A significantly lower 18F-FDG uptake was observed in the septum compared to the lateral wall (SLR 0.53 ± 0.17). A similar pattern was observed for MBF (SLR 0.68 ± 0.18), whereas late 13N-NH3 uptake showed a homogeneous distribution (SLR 0.96 ± 0.13). 13N-NH3/18F-FDG "mismatch" and "reverse mismatch" segments were predominantly present in the lateral (52%) and septal wall (61%), respectively. CONCLUSIONS: Non-ischemic CRT candidates with LBBB demonstrate lower glucose uptake and absolute MBF in the septum compared to the lateral wall. However, late static 13N-NH3 uptake showed a homogenous distribution, reflecting a composite measure of altered regional MBF and metabolism, induced by LBBB.

Arginase plays an important role in ammonia detoxification of yellow catfish Pelteobagrus fulvidraco.

A two-stage study was carried out to test the mechanism of arginase in ammonia detoxification of yellow catfish. At stage 1, fish was injected lethal half concentration ammonium acetate and 0.9% sodium chloride respectively every 12 h in six replicates for 72 h. The result found that no significant different in serum ammonia contents of fish in ammonium acetate group at hours 12, 24, 36, 48, 60 and 72. At stage 2, ammonium acetate group was split in two, one continued to injected with ammonium acetate (NH3 group) and the other with ammonium acetate and valine (an inhibitor of arginase; Val group); Sodium chloride group also was split in two, one continued to injected with sodium chloride (NaCl group) and the other with sodium chloride and valine (NaCl + Val group). The experiment continued for 12 h. Serum ammonia and liver arginine contents of fish in Val group were higher than those of fish in NH3 group; Compared with NaCl group, arginase activity and ARG 1 expression in liver of fish in Val group were lower; Fish in NaCl and NaCl + Val groups had the lowest serum superoxide dismutase activities, malondialdehyde, tumor necrosis factor-α, interleukin 1 and 8 contents, TNF-α, IL-1 and IL-8 expressions than fish in NH3 and Val groups, and had the higher lysozyme activities, complement 3 and 4 contents. This study indicates that ammonia poisoning would lead to oxidative damage, immunosuppression and inflammation in yellow catfish; Arginase may be an important target of ammonia toxicity in yellow catfish; Exogenous arginine supplementation might alleviate the symptoms of ammonia poisoning in yellow catfish.

Identify glioma recurrence and treatment effects with triple-tracer PET/CT.

BACKGROUND: Differential diagnosis of tumour recurrence (TuR) from treatment effects (TrE), mostly induced by radiotherapy and chemotherapy, is still difficult by using conventional computed tomography (CT) or magnetic resonance (MR) imaging. We have investigated the diagnostic performance of PET/CT with 3 tracers, 13N-NH3, 18F-FDOPA, and 18F-FDG, to identify TuR and TrE in glioma patients following treatment. METHODS: Forty-three patients with MR-suspected recurrent glioma were included. The maximum and mean standardized uptake values (SUVmax and SUVmean) of the lesion and the lesion-to-normal grey-matter cortex uptake (L/G) ratio were obtained from each tracer PET/CT. TuR or TrE was determined by histopathology or clinical MR follow-up for at least 6 months. RESULTS: In this cohort, 34 patients were confirmed to have TuR, and 9 patients met the diagnostic standard of TrE. The SUVmax and SUVmean of 13N-NH3 and 18F-FDOPA PET/CT at TuR lesions were significantly higher compared with normal brain tissue (13N-NH3 0.696 ± 0.558, 0.625 ± 0.507 vs 0.486 ± 0.413; 18F-FDOPA 0.455 ± 0.518, 0.415 ± 0.477 vs 0.194 ± 0.203; both P < 0.01), but there was no significant difference in 18F-FDG (6.918 ± 3.190, 6.016 ± 2.807 vs 6.356 ± 3.104, P = 0.290 and 0.493). L/G ratios of 13N-NH3 and 18F-FDOPA were significantly higher in TuR than in TrE group (13N-NH3, 1.573 ± 0.099 vs 1.025 ± 0.128, P = 0.008; 18F-FDOPA, 2.729 ± 0.131 vs 1.514 ± 0.141, P < 0.001). The sensitivity, specificity and AUC (area under the curve) by ROC (receiver operating characteristic) analysis were 57.7%, 100% and 0.803, for 13N-NH3; 84.6%, 100% and 0.938, for 18F-FDOPA; and 80.8%, 100%, and 0.952, for the combination, respectively. CONCLUSION: Our results suggest that although multiple tracer PET/CT may improve differential diagnosis efficacy, for glioma TuR from TrE, 18F-FDOPA PET-CT is the most reliable. The combination of 18F-FDOPA and 13N-NH3 does not increase the diagnostic efficiency, while 18F-FDG is not worthy for differential diagnosis of glioma TuR and TrE.

Differentiation of suprasellar meningiomas from non-functioning pituitary macroadenomas by 18F-FDG and 13N-Ammonia PET/CT.

BACKGROUND: Differentiation of suprasellar meningiomas (SSMs) from non-functioning pituitary macroadenomas (NFPMAs) is useful for clinical management. We investigated the utility of 13N-ammonia combined with 18F-FDG positron emission tomography (PET)/computed tomography (CT) in distinguishing SSMs from NFPMAs retrospectively. METHODS: Fourteen NFPMA patients and eleven SSM patients with histopathologic diagnosis were included in this study. Every patient underwent both 18F-FDG and 13N-ammonia PET/CT scans. The tumor to gray matter (T/G) ratios were calculated for the evaluation of tumor uptake. RESULTS: The uptake of 18F-FDG was higher in NFPMAs than SSMs, whereas the uptake of 13N-ammonia was obviously lower in NFPMAs than SSMs. The differences of 18F-FDG and 13N-ammonia uptake between the two groups were significant respectively (0.92[0.46] vs 0.59[0.29], P < 0.05, 18F-FDG; 1.58 ± 0.56 vs 2.80 ± 1.45, P < 0.05, 13N-ammonia). Tumor classification demonstrated a high overall accuracy of 96.0% for differential diagnosis. When the two traces were combined, only 1 SSM was misclassified into the NFPMA group. CONCLUSION: SSMs and NFPMAs have different metabolic characteristics on 18F-FDG and 13N-ammonia PET images. The combination of these two tracers can effectively distinguish SSMs from NFPMAs.

Short-term results of continuous venovenous haemodiafiltration versus peritoneal dialysis in 40 neonates with inborn errors of metabolism.

Several recent studies have reported that toxic metabolites accumulated in the body as a product of inborn errors of metabolism (IEM) are eliminated more rapidly with continuous venovenous hemodiafiltration (CVVHDF) than with peritoneal dialysis (PD). However, there is still uncertainty about the impacts of dialysis modalities on the short-term outcome. Here, it was aimed to investigate the effects of dialysis modalities on the short-term outcome. This retrospective study included 40 newborn infants who underwent PD (29 patients) or CVVHDF (11 patients) due to inborn errors of metabolism at a tertiary centre, between June 2013 and March 2018. The outcomes and the potential effects of the dialysis modality were evaluated. Of 40 patients, 21 were urea cycle defect, 14 were organic academia, and 5 were maple syrup urine disease. The median 50% reduction time of toxic metabolites were shorter in patients treated with CVVHDF (p < 0.05). Catheter blockage was the most common complication observed in PD group (24.1%), whereas in CVVHDF group hypotension and filter blockage were more common. There was no significant difference in mortality between dialysis groups (38% vs. 45.4%, p > 0.05). In patients with hyperammonaemia, duration of plasma ammonia > 200 µg/dL was the most important factor influencing mortality (OR 1.05, CI 1.01-1.09, p = 0.007).Conclusion: This study showed that CVVHDF is more efficient than PD to rapidly eliminate toxic metabolites caused by IEM in newborn infants, but not in improving survival. What is Known: •Toxic metabolites are eliminated more rapidly with CVVHDF than with PD. •Higher complication rates were reported with rigid peritoneal catheters in PD and catheter blockage in CVVHDF. What is New: •Prolonged duration of plasma ammonia levels above a safe limit (200 µg/dL) was associated with increased mortality. •Lower catheter-related complication rates may have been associated with the use of Tenckhoff catheters in PD and the use of right internal jugular vein in CVVHDF.

Identifying sediment-associated toxicity in rivers affected by multiple pollutants from the contaminant bioavailability.

In this study, we estimated the toxicity risks from river sediments that were affected by multiple pollutants in the Haihe River Basin. We used a range of methods to determine the concentrations, bioavailability, and toxicity of a range of metals and contaminants in sediments and sediment porewater and then assessed the ecological risks and toxicity using various multivariate statistical approaches. We found that more than 70% of the samples were toxic. The concentrations of non-ionic ammonia (0.168-9.295 mg L-1) were generally high in the sediment porewater, while the concentrations of bioavailable chromium (Cr) and polycyclic aromatic hydrocarbons (PAHs) were also high in the porewater samples from NW01 and NW02, respectively. We used the toxic unit (TU) approach, based on the bioavailable pollutant concentrations, to determine the toxicity of PAHs, heavy metals, and non-ionic ammonia in river sediments and sediment porewater. We found that non-ionic ammonia was the main source of toxicity for Daphnia magna, and that Cr and zinc were toxic for Pseudokirchneriella subcapitata and Chironomus dilutus. By combining various indexes, we identified the main contributors to the toxicity in sediments collected from rivers affected by multiple pollutants.

Effects of the ammonium loading rate on nitrite-oxidizing activity during nitrification at a high dose of inorganic carbon.

In this study, the effects of the ammonium loading rate (ALR) and inorganic carbon loading rate (ILR) on the nitrification performance and composition of a nitrifying bacterial community were investigated in a moving bed biofilm reactor, using poly(vinyl alcohol) (PVA) sponge cubes as a supporting carrier. Between the two ALRs of 0.36 and 2.16 kg-N m-1 d-1, stable partial nitritation was achieved at the higher ALR. Inorganic carbon was dosed at high levels: 33.1, 22.0, 16.4, 11.0, and 5.4 times the theoretical amount. Nonetheless, nitrification efficiency was not affected by the ILR at the two ALRs. Quantitative PCR analysis of ammonia- and nitrite-oxidizing bacteria revealed that ALR is an important determinant of partial nitritation by accumulating ammonia-oxidizing bacteria in the nitrification system. In comparison, two nitrite-oxidizing bacterial genera (Nitrobacter and Nitrospira) showed almost the same relative abundance at various ALRs and ILRs. Terminal restriction fragment length polymorphism targeting the gene of ammonia monooxygenase subunit A revealed that Nitrosomonas europaea dominated under all conditions.

Airlift bioreactor system for simultaneous removal of hydrogen sulfide and ammonia from synthetic and actual waste gases.

The effectiveness of an airlift reactor system in simultaneously removing hydrogen sulfide (H2S) and ammonia (NH3) from synthetic and actual waste gases was investigated. The effects of various parameters, including the ratio of inoculum dilution, the gas concentration, the gas retention time, catalyst addition, the bubble size, and light intensity, on H2S and NH3 removal were investigated. The results revealed that optimal gas removal could be achieved by employing an activated inoculum, using a small bubble stone, applying reinforced fluorescent light, adding Fe2O3 catalysts, and applying a gas retention time of 20 s. The shock loading did not substantially affect the removal efficiency of the airlift bioreactor. Moreover, more than 98.5% of H2S and 99.6% of NH3 were removed in treating actual waste gases. Fifteen bands or species were observed in a profile from denaturing gradient gel electrophoresis during waste gas treatment. Phylogenetic analysis revealed the phylum Proteobacteria to be predominant. Six bacterial strains were consistently present during the entire operating period; however, only Rhodobacter capsulatus, Rhodopseudomonas palustris, and Arthrobacter oxydans were relatively abundant in the system. The photosynthetic bacteria R. capsulatus and R. palustris were responsible for H2S oxidation, especially when the reinforced fluorescent light was used. The heterotrophic nitrifier A. oxydans was responsible for NH3 oxidation. To our knowledge, this is the first report on simultaneous H2S and NH3 removal using an airlift bioreactor system. It clearly demonstrates the effectiveness of the system in treating actual waste gases containing H2S and NH3.

Flexible ammonia handling strategies using both cutaneous and branchial epithelia in the highly ammonia-tolerant Pacific hagfish.

Hagfish consume carrion, potentially exposing them to hypoxia, hypercapnia, and high environmental ammonia (HEA). We investigated branchial and cutaneous ammonia handling strategies by which Pacific hagfish (Eptatretus stoutii) tolerate and recover from high ammonia loading. Hagfish were exposed to HEA (20 mmol/l) for 48 h to elevate plasma total ammonia (TAmm) levels before placement into divided chambers for a 4-h recovery period in ammonia-free seawater where ammonia excretion (JAmm) was measured independently in the anterior and posterior compartments. Localized HEA exposures were also conducted by subjecting hagfish to HEA in either the anterior or posterior compartments. During recovery, HEA-exposed animals increased JAmm in both compartments, with the posterior compartment comprising ~20% of the total JAmm compared with ~11% in non-HEA-exposed fish. Plasma TAmm increased substantially when whole hagfish and the posterior regions were exposed to HEA. Alternatively, plasma TAmm did not elevate after anterior localized HEA exposure. JAmm was concentration dependent (0.05-5 mmol/l) across excised skin patches at up to eightfold greater rates than in skin sections that were excised from HEA-exposed hagfish. Skin excised from more posterior regions displayed greater JAmm than those from more anterior regions. Immunohistochemistry with hagfish-specific anti-rhesus glycoprotein type c (α-hRhcg; ammonia transporter) antibody was characterized by staining on the basal aspect of hagfish epidermis while Western blotting demonstrated greater expression of Rhcg in more posterior skin sections. We conclude that cutaneous Rhcg proteins are involved in cutaneous ammonia excretion by Pacific hagfish and that this mechanism could be particularly important during feeding.

Changes of ammonia, urea contents and transaminase activity in the body during aerial exposure and ammonia loading in Chinese loach Paramisgurnus dabryanus.

The Paramisgurnus dabryanus was exposed to 30 mmol L-1 NH4Cl solution and air to assessing the change of body ammonia and urea contents and the activities of alanine aminotransferase (ALT) and aspartate transaminase (AST). After 48 h of ammonia exposure, ammonia concentration in the plasma, brain, liver and muscle were 3.3-fold, 5.6-fold, 3.5-fold and 4.2-fold, respectively, those of the control values. Plasma, brain, liver and muscle ammonia concentrations increased to 2.2-fold, 3.3-fold, 2.5-fold and 2.9-fold, respectively, those of control values in response to 48 h of aerial exposure. Within the given treatment (ammonia or aerial exposure), there was no change in plasma, brain and liver urea concentrations between exposure durations. The plasma ALT activity was significantly affected by exposure time during aerial exposure, while the liver ALT activity was not affected by ammonia or aerial exposure. Exposure to NH4Cl or air had no effect on either plasma or liver AST activity. Our results suggested that P. dabryanus could accumulate quite high level of internal ammonia because of the high ammonia tolerance in its cells and tissues, and NH3 volatilization would be a possible ammonia detoxification strategy in P. dabryanus. Urea synthesis was not an effective mechanism to deal with environmental or internal ammonia problem. The significant increase of ALT activity in plasma during aerial exposure, indicating that alanine synthesis through certain amino acid catabolism may be subsistent in P. dabryanus.

Ammonia stress under high environmental ammonia induces Hsp70 and Hsp90 in the mud eel, Monopterus cuchia.

The obligatory air-breathing mud eel (Monopterus cuchia) is frequently being challenged with high environmental ammonia (HEA) exposure in its natural habitats. The present study investigated the possible induction of heat shock protein 70 and 90 (hsp70, hsc70, hsp90α and hsp90ß) genes and more expression of Hsp70 and Hsp90 proteins under ammonia stress in different tissues of the mud eel after exposure to HEA (50 mM NH4Cl) for 14 days. HEA resulted in significant accumulation of toxic ammonia in different body tissues and plasma, which was accompanied with the stimulation of oxidative stress in the mud eel as evidenced by more accumulation of malondialdehyde (MDA) and hydrogen peroxide (H2O2) during exposure to HEA. Further, hyper-ammonia stress led to significant increase in the levels of mRNA transcripts for inducible hsp70 and hsp90α genes and also their translated proteins in different tissues probably as a consequence of induction of hsp70 and hsp90α genes in the mud eel. However, hyper-ammonia stress was neither associated with any significant alterations in the levels of mRNA transcripts for constitutive hsc70 and hsp90ß genes nor their translated proteins in any of the tissues studied. More abundance of Hsp70 and Hsp90α proteins might be one of the strategies adopted by the mud eel to defend itself from the ammonia-induced cellular damages under ammonia stress. Further, this is the first report of ammonia-induced induction of hsp70 and hsp90α genes under hyper-ammonia stress in any freshwater air-breathing teleost.

De Novo Glutamine Synthesis: Importance for the Proliferation of Glioma Cells and Potentials for Its Detection With 13N-Ammonia.

PURPOSE: The aim of this study was to investigate the role of de novo glutamine (Gln) synthesis in the proliferation of C6 glioma cells and its detection with (13)N-ammonia. METHODS: Chronic Gln-deprived C6 glioma (0.06C6) cells were established. The proliferation rates of C6 and 0.06C6 cells were measured under the conditions of Gln deprivation along with or without the addition of ammonia or glutamine synthetase (GS) inhibitor. (13)N-ammonia uptake was assessed in C6 cells by gamma counting and in rats with C6 and 0.06C6 xenografts by micro-positron emission tomography (PET) scanning. The expression of GS in C6 cells and xenografts was assessed by Western blotting and immunohistochemistry, respectively. RESULTS: The Gln-deprived C6 cells showed decreased proliferation ability but had a significant increase in GS expression. Furthermore, we found that low concentration of ammonia was sufficient to maintain the proliferation of Gln-deprived C6 cells, and (13)N-ammonia uptake in C6 cells showed Gln-dependent decrease, whereas inhibition of GS markedly reduced the proliferation of C6 cells as well as the uptake of (13)N-ammoina. Additionally, microPET/computed tomography exhibited that subcutaneous 0.06C6 xenografts had higher (13)N-ammonia uptake and GS expression in contrast to C6 xenografts. CONCLUSION: De novo Gln synthesis through ammonia-glutamate reaction plays an important role in the proliferation of C6 cells. (13)N-ammonia can be a potential metabolic PET tracer for Gln-dependent tumors.

The reproducibility of time-of-flight PET and conventional PET for the quantification of myocardial blood flow and coronary flow reserve with (13)N-ammonia.

BACKGROUND: This study aimed to validate the reproducibility of quantitative analysis using time-of-flight (TOF) and conventional PET with (13)N-ammonia ((13)N-NH3). METHODS AND RESULTS: Phantom images were reconstructed with and without TOF, and recovery coefficients (RCs) and the percent contrast of each sphere over the percent background variability were assessed. In the clinical study, 21 subjects underwent dynamic (13)N-NH3 PET scanning under stress and rest conditions. The dynamic acquisition images and intra- and inter-observer reproducibility of myocardial blood flow (MBF) and coronary flow reserve (CFR) were compared between reconstructions (with and without TOF). In the phantom study, RCs and the percent contrast of each sphere over the percent background variability was improved with TOF. In the clinical study, the noise of blood pool and myocardial images with TOF was less than that without TOF. Territorial and global intra- and inter-observer reproducibility of MBF and CFR values was excellent. Although segmental intra- and inter-observer reproducibility was excellent, there were larger variations in apex and the segment near the right ventricle (RV) without TOF. These variations became inconspicuous with TOF. CONCLUSION: Visual image quality, RCs, and percent contrast over percent background variability with TOF were better than that without TOF. Excellent correlations and good agreements in quantitative values were observed. TOF improved the variation of segmental values.

Validation of pixel-wise parametric mapping of myocardial blood flow with ¹³NH3 PET in patients with hypertrophic cardiomyopathy.

PURPOSE: Transmural abnormalities in myocardial blood flow (MBF) are important causes of ischaemia in patients with left ventricular (LV) hypertrophy. The study aimed to test whether pixel-wise parametric mapping of (13)NH3 MBF can reveal transmural abnormalities in patients with hypertrophic cardiomyopathy (HCM). METHODS: We submitted 11 HCM patients and 9 age-matched controls with physiological LV hypertrophy to rest and stress (dipyridamole) (13)NH3 PET. We measured MBF using a compartmental model, and obtained rest and stress parametric maps. Pixel MBF values were reorganized to obtain subendocardial and subepicardial MBF of LV segments. RESULTS: MBF at rest was higher in the subendocardial than in the subepicardial layer: 0.78 ± 0.19 vs. 0.60 ± 0.18 mL/min/g in HCM patients; 0.92 ± 0.24 vs. 0.75 ± 0.24 mL/min/g in controls (both p < 0.0001). Transmural perfusion gradient (TPG = subendocardial MBF/subepicardial MBF) at rest was similar: 1.35 ± 0.31 in HCM patients; 1.28 ± 0.27 in controls (NS). During stress, controls maintained higher subendocardial MBF: 2.44 ± 0.54 vs. 1.96 ± 0.67 mL/min/g tissue (p < 0.0001), with a TPG of 1.33 ± 0.35 (NS vs. rest). In HCM patients, the difference between subendocardial and subepicardial MBF was reduced (1.46 ± 0.48 vs. 1.36 ± 0.48 mL/min/g tissue, p < 0.01) and TPG decreased to 1.11 ± 0.34 (p < 0.0001 vs. rest and vs. controls). In HCM patients 8 of 176 segments had subendocardial MBF less than -2 × SD of the mean, versus none of 144 segments in controls (p < 0.01). CONCLUSION: Pixel-wise parametric mapping of (13)NH3 MBF enables the identification of transmural abnormalities in patients with HCM.

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.

In-vitro methods for testing dermal absorption and penetration of toxic gases.

This technical note provides details of an experimental technique for in-vitro skin studies with atmospheric chemical challenge. There appear to be major evidence gaps in relation to dermal exposure of gases. We describe a modification of standard OECD protocols for an atmospheric delivery system which can be used to understand interaction of toxic gases and the skin. The system can be used to examine the mechanisms by which skin uptake occurs. Auxiliary components which allow for parameter variation such as temperature and relative humidity are also described. Methodology presented in this technical note uses examples of gas challenges (ammonia, chlorine) to illustrate its application to gases of differing physicochemical properties. This adapted protocol can be applied in the context of HAZMAT scenarios involving atmospheric toxic chemical release and dermal absorption potential under variable exposure conditions.

Competition between roots and microorganisms for nitrogen: mechanisms and ecological relevance.

Demand of all living organisms on the same nutrients forms the basis for interspecific competition between plants and microorganisms in soils. This competition is especially strong in the rhizosphere. To evaluate competitive and mutualistic interactions between plants and microorganisms and to analyse ecological consequences of these interactions, we analysed 424 data pairs from 41 (15)N-labelling studies that investigated (15)N redistribution between roots and microorganisms. Calculated Michaelis-Menten kinetics based on K(m) (Michaelis constant) and V(max) (maximum uptake capacity) values from 77 studies on the uptake of nitrate, ammonia, and amino acids by roots and microorganisms clearly showed that, shortly after nitrogen (N) mobilization from soil organic matter and litter, microorganisms take up most N. Lower K(m) values of microorganisms suggest that they are especially efficient at low N concentrations, but can also acquire more N at higher N concentrations (V(max)) compared with roots. Because of the unidirectional flow of nutrients from soil to roots, plants are the winners for N acquisition in the long run. Therefore, despite strong competition between roots and microorganisms for N, a temporal niche differentiation reflecting their generation times leads to mutualistic relationships in the rhizosphere. This temporal niche differentiation is highly relevant ecologically because it: protects ecosystems from N losses by leaching during periods of slow or no root uptake; continuously provides roots with available N according to plant demand; and contributes to the evolutionary development of mutualistic interactions between roots and microorganisms.

Integrating cellular metabolism into a multiscale whole-body model.

Cellular metabolism continuously processes an enormous range of external compounds into endogenous metabolites and is as such a key element in human physiology. The multifaceted physiological role of the metabolic network fulfilling the catalytic conversions can only be fully understood from a whole-body perspective where the causal interplay of the metabolic states of individual cells, the surrounding tissue and the whole organism are simultaneously considered. We here present an approach relying on dynamic flux balance analysis that allows the integration of metabolic networks at the cellular scale into standardized physiologically-based pharmacokinetic models at the whole-body level. To evaluate our approach we integrated a genome-scale network reconstruction of a human hepatocyte into the liver tissue of a physiologically-based pharmacokinetic model of a human adult. The resulting multiscale model was used to investigate hyperuricemia therapy, ammonia detoxification and paracetamol-induced toxication at a systems level. The specific models simultaneously integrate multiple layers of biological organization and offer mechanistic insights into pathology and medication. The approach presented may in future support a mechanistic understanding in diagnostics and drug development.