Adenosine influences the high-affinity uptake of transmitter glutamate and aspartate under conditions of hepatic encephalopathy

Proteomic and metabolomic analysis of marine medaka (Oryzias melastigma) after acute ammonia exposure

Ammonia is both a highly toxic environmental pollutant and the major nitrogenous waste produced by ammoniotelic teleosts. Although the acute toxic effects of ammonia have been widely studied in fish, the biochemical mechanisms of its toxicity have not been understood comprehensively. In this study, we performed comparative proteomic and metabolomic analysis between ammonia-challenged (1.2 and 2.6 mmol L^-1 NH₄Cl for 96 h) and control groups of marine medaka (Oryzias melastigma) to identify changes of the metabolite and protein profiles in response to ammonia stress. The metabolic responses included changes of multiple amino acids, carbohydrates (glucose and glycogen), energy metabolism products (ATP and creatinine), and other metabolites (choline and phosphocholine) after ammonia exposure, indicating that ammonia mainly caused disturbance in energy metabolism and amino acids metabolism. The two-dimensional electrophoresis-based proteomic study identified 23 altered proteins, which were involved in nervous system, locomotor system, cytoskeleton assembly, immune stress, oxidative stress, and signal transduction of apoptosis. These results suggested that ammonia not only induced oxidative stress, immune stress, cell injury and apoptosis but also affected the motor ability and central nervous system in marine medaka. It is the first time that metabolomic and proteomic approaches were integrated to elucidate ammonia toxicity in marine fishes. This study is of great value in better understanding the mechanisms of ammonia toxicity in marine fishes and in practical aspects of aquaculture.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Purinergic signaling in liver disease

Adenosine triphosphate (ATP) is essential for the myriad of metabolic processes upon which life is based and is known widely as the universal energy currency unit of intracellular biologic reactions. ATP, adenosine diphosphate, adenosine, as well as other purines and pyrimidines also serve as ubiquitous extracellular mediators which function through the activation of specific receptors (viz. P2 receptors for nucleotides and purinergic P1 receptors for adenosine). Extracellular nucleotides are rapidly converted to nucleosides, such as adenosine, by highly regulated plasma membrane ectonucleotidases that modulate many of the normal biological and metabolic processes in the liver - such as gluconeogenesis and insulin signaling. Under inflammatory conditions, as with ischemia reperfusion, sepsis or metabolic stress, ATP and other nucleotides can also act as 'damage-associated molecular patterns' causing inflammasome activation in innate immune cells and endothelium resulting in tissue damage. The phosphohydrolysis of ATP by ectonucleotidases, such as those of the CD39/ENTPD family, results in the generation of immune suppressive adenosine, which in turn markedly limits inflammatory processes. Experimental studies by others and our group have implicated purinergic signaling in experimental models of hepatic ischemia reperfusion and inflammation, transplant rejection, hepatic regeneration, steatohepatitis, fibrosis and cancer, amongst others. Expression of ectonucleotidases on sinusoidal endothelial, stellate or immune cells allows for homeostatic integration and linking of the control of vascular inflammatory and immune cell reactions in the liver. CD39 expression also identifies hepatic myeloid dendritic cells and efficiently distinguishes T-regulatory-type cells from other resting or activated T cells. Our evolving data strongly indicate that CD39 serves as a key 'molecular switch' and is an integral component of the suppressive machinery of myeloid, dendritic and T cells. Increased understanding of mechanisms of extracellular ATP scavenging and specifically conversion to nucleosides by ectonucleotidases of the CD39 family have also led to novel insights into the exquisite balance of nucleotide P2-receptor and adenosinergic P1-receptor signaling in inflammatory and hepatic diseases. Further, CD39 and other ectonucleotidases exhibit genetic polymorphisms in humans which alter levels of expression/function and are associated with predisposition to inflammatory and immune diseases, diabetes and vascular calcification, amongst other problems. Development of therapeutic strategies targeting purinergic signaling and ectonucleotidases offers promise for the management of disordered inflammation and aberrant immune reactivity.

Sensitivity of ventilation and brain metabolism to ammonia exposure in rainbow trout, Oncorhynchus mykiss

Ammonia has been documented as a respiratory gas that stimulates ventilation, and is sensed by peripheral neuroepithelial cells (NECs) in the gills in ammoniotelic rainbow trout. However, the hyperventilatory response is abolished in trout chronically exposed (1+ months) to high environmental ammonia [HEA; 250 μmol l(-1) (NH4)2SO4]. This study investigates whether the brain is involved in the acute sensitivity of ventilation to ammonia, and whether changes in brain metabolism are related to the loss of hyperventilatory responses in trout chronically exposed to HEA ('HEA trout'). Hyperventilation (via increased ventilatory amplitude rather than rate) and increased total ammonia concentration ([TAmm]) in brain tissue were induced in parallel by acute HEA exposure in control trout in a concentration-series experiment [500, 750 and 1000 μmol l(-1) (NH4)2SO4], but these inductions were abolished in HEA trout. Ventilation was correlated more closely to [TAmm] in brain rather than to [TAmm] in plasma or cerebrospinal fluid. The close correlation of hyperventilation and increased brain [TAmm] also occurred in control trout acutely exposed to HEA in a time-series analysis [500 μmol l(-1) (NH4)2SO4; 15, 30, 45 and 60 min], as well as in a methionine sulfoxamine (MSOX) pre-injection experiment [to inhibit glutamine synthetase (GSase)]. These correlations consistently suggest that brain [TAmm] is involved in the hyperventilatory responses to ammonia in trout. The MSOX treatments, together with measurements of GSase activity, TAmm, glutamine and glutamate concentrations in brain tissue, were conducted in both the control and HEA trout. These experiments revealed that GSase plays an important role in transferring ammonia to glutamate to make glutamine in trout brain, thereby attenuating the elevation of brain [TAmm] following HEA exposure, and that glutamate concentration is reduced in HEA trout. The mRNAs for the ammonia channel proteins Rhbg, Rhcg1 and Rhcg2 were expressed in trout brain, and the expression of Rhbg and Rhcg2 increased in HEA trout, potentially as a mechanism to facilitate the efflux of ammonia. In summary, the brain appears to be involved in the sensitivity of ventilation to ammonia, and brain ammonia levels are regulated metabolically in trout.

Neurotransmitter receptor alterations in hepatic encephalopathy: a review

Hepatic encephalopathy (HE), a complex neuropsychiatric syndrome with symptoms ranging from subtle neuropsychiatric and motor disturbances to deep coma and death, is thought to be a clinical manifestation of a low-grade cerebral oedema associated with an altered neuron-astrocyte crosstalk and exacerbated by hyperammonemia and oxidative stress. These events are tightly coupled with alterations in neurotransmission, either in a causal or a causative manner, resulting in a net increase of inhibitory neurotransmission. Therefore, research focussed mainly on the potential role of γ-aminobutyric acid-(GABA) or glutamate-mediated neurotransmission in the pathophysiology of HE, though roles for other neurotransmitters (e.g. serotonin, dopamine, adenosine and histamine) or for neurosteroids or endogenous benzodiazepines have also been suggested. Therefore, we here review HE-related alterations in neurotransmission, focussing on changes in the levels of classical neurotransmitters and the neuromodulator adenosine, variations in the activity and/or concentrations of key enzymes involved in their metabolism, as well as in the densities of their receptors.

Pathological roles of purinergic signaling in the liver

Purinergic signaling has been postulated as a mechanism of cellular signaling since the early 1970s. Cellular responses triggered by extracellular nucleotides and nucleosides occur by defined adenosine (P1) and ATP (P2) receptors, respectively, and play a prominent role in many aspects of health and disease, including those involving the liver. In normal physiology, extracellular nucleotides modulate many of the normal biologic and hepatic metabolic processes such as gluconeogenesis and insulin responsiveness. Further, in multiple disease states, ATP and certain nucleotides serve as danger signals and are involved in heightened purinergic receptor activation in a myriad of pathologic processes. Recently, others and we have shown the regulation of purinergic signaling by ectonucleotidases to play an important role in the acute vascular pathobiology of liver inflammation, regeneration, and immunity, as in ischemia reperfusion and transplantation. Increased understanding into mechanisms of extracellular ATP metabolism by such ecto enzymes has also led to novel insights into the exquisite balance of nucleotide P2-receptor and adenosinergic P1-receptor signaling in those chronic hepatic diseases characterized by steatosis, fibrosis, and malignancy. This review will explore the developing role of purinergic signaling in the pathophysiology of liver disease and comment on potential future clinical applications.

Dogmas and controversies in the handling of nitrogenous wastes: ammonia tolerance in the oriental weatherloach Misgurnus anguillicaudatus

The oriental weatherloach Misgurnus anguillicaudatus is an extremely ammonia-tolerant fish. Many ammonia-protection mechanisms have been reported in this fish. Six strategies used by this fish to deal with the problem of excess ammonia are described. The fish can (1) reduce ammonia production through reduction in protein and/or amino acid catabolism; (2) reduce ammonia production and obtain energy through partial amino acid catabolism leading to alanine formation; (3) detoxify ammonia to glutamine; (4) tolerate very high ammonia levels in its tissues; (5) get rid of ammonia as NH(3) gas and, probably, (6) possesses background K(+) channels that are impermeable to NH(4)(+). The effects of extracellular ammonia on the contraction performance of the heart from this fish were found to be the same as in rainbow trout, an ammonia-sensitive fish. It suggests that the hearts of most, if not all, fish species are protected against ammonia. MK-801, an NMDA receptor blocker, was found to have a protective effect against ammonia intoxication in the oriental weatherloach, which suggests that the NMDA receptor, as in mammals, is involved in ammonia toxicity.

The swamp eel Monopterus albus reduces endogenous ammonia production and detoxifies ammonia to glutamine during 144 h of aerial exposure

The swamp eel Monopterus albus inhabits muddy ponds, swamps, canals and rice fields, where it can burrow within the moist earth during the dry summer season, thus surviving for long periods without water. This study aimed to elucidate the strategies adopted by M. albus to defend against endogenous ammonia toxicity when kept out of water for 144 h (6 days). Like any other fish, M. albus has difficulties in excreting ammonia during aerial exposure. In fact, the rates of ammonia and urea excretions decreased significantly in specimens throughout the 144 h of aerial exposure. At 144 h, the ammonia and urea excretion rates decreased to 20% and 25%, respectively, of the corresponding control values. Consequently, ammonia accumulated to high levels in the tissues and plasma of the experimental specimens. Apparently, M. albus has developed relatively higher ammonia tolerance at the cellular and subcellular levels compared with many other teleost fish. Since the urea concentration in the tissues of specimens exposed to air remained low, urea synthesis was apparently not adopted as a strategy to detoxify endogenous ammonia during 144 h of aerial exposure. Instead, ammonia produced through amino acid catabolism was detoxified to glutamine, leading to the accumulation of glutamine in the body during the first 72 h of aerial exposure. Complementing the increased glutamine formation was a significant increase in glutamine synthetase activity in the liver of specimens exposed to air for 144 h. Formation of glutamine is energetically expensive. It is probably because M. albus remained relatively inactive on land that the reduction in energy demand for locomotory activity facilitated its exploitation of glutamine formation to detoxify endogenous ammonia. There was a slight decrease in the glutamine level in the body of the experimental animals between 72 h and 144 h of aerial exposure, which indicates that glutamine might not be the end product of nitrogen metabolism. In addition, these results suggest that suppression of endogenous ammonia production, possibly through reductions in proteolysis and amino acid catabolism, acts as the major strategy to avoid ammonia intoxication in specimens exposed to air for >/=72 h. It is concluded that glutamine formation and reduction in ammonia production together served as effective strategies to avoid the excessive accumulation of ammonia in the body of M. albus during 144 h of aerial exposure. However, these strategies might not be adequate to sustain the survival of M. albus in the mud for longer periods during drought because ammonia and glutamine concentrations had already built up to high levels in the body of specimens exposed to air for 144 h.

Ammonia toxicity in fish

Ammonia is present in the aquatic environment due to agricultural run-off and decomposition of biological waste. Ammonia is toxic to all vertebrates causing convulsions, coma and death, probably because elevated NH4+ displaces K+ and depolarizes neurons, causing activation of NMDA type glutamate receptor, which leads to an influx of excessive Ca2+ and subsequent cell death in the central nervous system. Present ammonia criteria for aquatic systems are based on toxicity tests carried out on, starved, resting, non-stressed fish. This is doubly inappropriate. During exhaustive exercise and stress, fish increase ammonia production and are more sensitive to external ammonia. Present criteria do not protect swimming fish. Fish have strategies to protect them from the ammonia pulse following feeding, and this also protects them from increases in external ammonia, as a result starved fish are more sensitive to external ammonia than fed fish. There are a number of fish species that can tolerate high environmental ammonia. Glutamine formation is an important ammonia detoxification strategy in the brain of fish, especially after feeding. Detoxification of ammonia to urea has also been observed in elasmobranches and some teleosts. Reduction in the rate of proteolysis and the rate of amino acid catabolism, which results in a decrease in ammonia production, may be another strategy to reduce ammonia toxicity. The weather loach volatilizes NH3, and the mudskipper, P. schlosseri, utilizes yet another unique strategy, it actively pumps NH4+ out of the body.

Synaptosomal and brain slice cerebrocortical [3H]L-glutamate uptake in a rat model of chronic hepatic encephalopathy

Cerebrocortical [3H]L-glutamate uptake was examined using brain slices and synaptosomes obtained from rats with portal vein and bile duct ligation. In addition, the effect of in vitro addition of 5 mM ammonia on glutamate uptake parameters was determined. There was no significant difference in brain slice or synaptosomal glutamate uptake in rats with portal vein and bile duct ligation compared to control rats. In vitro addition of ammonia had no effect on uptake kinetics in either brain slices or synaptosomes. These results suggest that glutamate uptake kinetics are not perturbed in this animal model of chronic hepatic encephalopathy.

L-glutamate and gamma-aminobutyric acid uptake in synaptosomes from the cerebral cortex of dogs with congenital chronic hepatic encephalopathy

High affinity [3H]gamma-aminobutyric acid (GABA) and [3H]L-glutamate uptake were determined in synaptosomes prepared from the cerebral cortex of dogs with congenital hepatic encephalopathy and control dogs. The Km value for GABA uptake was increased by 35% but there was a concomitant 34% increase in Vmax suggesting that GABA uptake capacity was not changed in HE dogs. In contrast, mean Vmax for glutamate uptake in HE dogs was 85% greater than mean Vmax in control dogs; mean Km was increased by 25% in HE dogs. Therefore, overall synaptosomal high affinity glutamate uptake capacity was increased in HE dogs compared to controls.

Ammonia stimulates the release of taurine from cultured astrocytes

The effect of ammonia on the release of the neuroactive amino acids taurine (TAU), gamma-aminobutyric acid (GABA) and D-aspartate (D-ASP), an analog of L-glutamate (L-GLU), from cultured rat cortical astrocytes was studied. NH4Cl (1 and 5 mM) induced the release of TAU. TAU release was reduced when Na+ was removed, and was almost completely abolished when Cl- was omitted. In contrast, TAU basal release was enhanced upon removal of Na+ or Cl-. Ammonia inhibited the release of GABA and D-ASP. Ammonia-induced release of astroglial TAU may modify the neuronal excitability accompanying hyperammonemic conditions.

Molecular mechanism of acute ammonia toxicity and of its prevention by L-carnitine

In summary, we propose that acute ammonia intoxication leads to increased extracellular concentration of glutamate in brain and results in activation of the NMDA receptor. Activation of this receptor mediates ATP depletion and ammonia toxicity since blocking the NMDA receptor with MK-801 prevents both phenomena. Ammonia-induced metabolic alterations (in glycogen, glucose, pyruvate, lactate, glutamine, glutamate, etc) are not prevented by MK-801 and, therefore, it seems that they do not play a direct role in ammonia-induced ATP depletion nor in the molecular mechanism of acute ammonia toxicity. The above results suggest that ammonia-induced ATP depletion is due to activation of Na+/K(+)-ATPase, which, in turn, is a consequence of decreased phosphorylation by protein kinase C. This can be due to decreased activity of PKC or to increased activity of a protein phosphatase. We also show that L-carnitine prevents glutamate toxicity in primary neuronal cultures. The results shown indicate that carnitine increases the affinity of glutamate for the quisqualate type (including metabotropic) of glutamate receptors. Also, blocking the metabotropic receptor with AP-3 prevents the protective effect of L-carnitine, indicating that activation of this receptor mediates the protective effect of carnitine. We suggest that the protective effect of carnitine against acute ammonia toxicity in animals is due to the protection against glutamate neurotoxicity according to the above mechanisms.