Activation of pyruvate dehydrogenase during metabolism of ammonium ions in hemoglobin-free perfused rat liver

Pivotal role of glutamine synthetase in ammonia detoxification

Glutamine synthetase (GS) catalyzes condensation of ammonia with glutamate to glutamine. Glutamine serves, with alanine, as a major nontoxic interorgan ammonia carrier. Elimination of hepatic GS expression in mice causes only mild hyperammonemia and hypoglutaminemia but a pronounced decrease in the whole-body muscle-to-fat ratio with increased myostatin expression in muscle. Using GS-knockout/liver and control mice and stepwise increments of enterally infused ammonia, we show that ∼35% of this ammonia is detoxified by hepatic GS and ∼35% by urea-cycle enzymes, while ∼30% is not cleared by the liver, independent of portal ammonia concentrations ≤2 mmol/L. Using both genetic (GS-knockout/liver and GS-knockout/muscle) and pharmacological (methionine sulfoximine and dexamethasone) approaches to modulate GS activity, we further show that detoxification of stepwise increments of intravenously (jugular vein) infused ammonia is almost totally dependent on GS activity. Maximal ammonia-detoxifying capacity through either the enteral or the intravenous route is ∼160 μmol/hour in control mice. Using stable isotopes, we show that disposal of glutamine-bound ammonia to urea (through mitochondrial glutaminase and carbamoylphosphate synthetase) depends on the rate of glutamine synthesis and increases from ∼7% in methionine sulfoximine-treated mice to ∼500% in dexamethasone-treated mice (control mice, 100%), without difference in total urea synthesis.

CONCLUSIONS

Hepatic GS contributes to both enteral and systemic ammonia detoxification. Glutamine synthesis in the periphery (including that in pericentral hepatocytes) and glutamine catabolism in (periportal) hepatocytes represents the high-affinity ammonia-detoxifying system of the body. The dependence of glutamine-bound ammonia disposal to urea on the rate of glutamine synthesis suggests that enhancing peripheral glutamine synthesis is a promising strategy to treat hyperammonemia. Because total urea synthesis does not depend on glutamine synthesis, we hypothesize that glutamate dehydrogenase complements mitochondrial ammonia production. (Hepatology 2017;65:281-293).

Selenium homeostasis and antioxidant selenoproteins in brain: implications for disorders in the central nervous system

The essential trace element selenium, as selenocysteine, is incorporated into antioxidant selenoproteins such as glutathione peroxidases (GPx), thioredoxin reductases (TrxR) and selenoprotein P (Sepp1). Although comparatively low in selenium content, the brain exhibits high priority for selenium supply and retention under conditions of dietary selenium deficiency. Liver-derived Sepp1 is the major transport protein in plasma to supply the brain with selenium, serving as a "survival factor" for neurons in culture. Sepp1 expression has also been detected within the brain. Presumably, astrocytes secrete Sepp1, which is subsequently taken up by neurons via the apolipoprotein E receptor 2 (ApoER2). Knock-out of Sepp1 or ApoER2 as well as neuron-specific ablation of selenoprotein biosynthesis results in neurological dysfunction in mice. Astrocytes, generally less vulnerable to oxidative stress than neurons, are capable of up-regulating the expression of antioxidant selenoproteins upon brain injury. Occurrence of neurological disorders has been reported occasionally in patients with inadequate nutritional selenium supply or a mutation in the gene encoding selenocysteine synthase, one of the enzymes involved in selenoprotein biosynthesis. In three large trials carried out among elderly persons, a low selenium status was associated with faster decline in cognitive functions and poor performance in tests assessing coordination and motor speed. Future research is required to better understand the role of selenium and selenoproteins in brain diseases including hepatic encephalopathy.

Critical role of specific ions for ligand-induced changes regulating pyruvate dehydrogenase kinase isoform 2

In the complete absence of K+ and phosphate (Pi), pyruvate dehydrogenase kinase isoform 2 (PDHK2) was catalytically very active but with an elevated Km for ATP, and this activity is insensitive to effector regulation. We find that K+ or 5-fold lower levels of NH4+ markedly enhanced quenching of Trp383 fluorescence of PDHK2 by ADP and ATP. K+ binding caused an approximately 40-fold decrease in the equilibrium dissociation constants (Kd) for ATP from approximately 120 to 3.0 microM and an approximately 25-fold decrease in Kd for ADP from approximately 950 to 38 microM. Linked reductions in Kd of PDHK2 for K+ were from approximately 30 to approximately 0.75 mM with ATP bound and from approximately 40 to approximately 1.7 mM with ADP bound. Without K+, there was little effect of ADP on pyruvate binding, but with 100 mM K+ and 100 microM ADP, the L0.5 of PDHK2 for pyruvate was reduced by approximately 14 fold. In the absence of K+, Pi had small effects on ligand binding. With 100 mM K+, 20 mM Pi modestly enhanced binding of ADP and hindered pyruvate binding but markedly enhanced the binding of pyruvate with ADP; the L0.5 for pyruvate was specifically decreased approximately 125-fold with 100 microM ADP. Pi effects were minimal when NH4+ replaced K+. We have quantified coupled binding of K+ with ATP and ADP and elucidated how linked K+ and Pi binding are required for the potent inhibition of PDHK2 by ADP and pyruvate.

Rat liver responsiveness to gluconeogenic substrates during insulin-induced hypoglycemia

Hepatic responsiveness to gluconeogenic substrates during insulin-induced hypoglycemia was investigated. For this purpose, livers were perfused with a saturating concentration of 2 mM glycerol, 5 mM L-alanine or 5 mM L-glutamine as gluconeogenic substrates. All experiments were performed 1 h after an ip injection of saline (CN group) or 1 IU/kg of insulin (IN group). The IN group showed higher (P<0.05) hepatic glucose production from glycerol, L-alanine and L-glutamine and higher (P<0.05) production of L-lactate, pyruvate and urea from L-alanine and L-glutamine. In addition, ip injection of 100 mg/kg glycerol, L-alanine and L-glutamine promoted glucose recovery. The results indicate that the hepatic capacity to produce glucose from gluconeogenic precursors was increased during insulin-induced hypoglycemia.

Effect of a meal feeding schedule on hepatic glycogen synthesis and gluconeogenesis in rats

We investigated the effect of a meal feeding schedule (MFS) on food intake, hepatic glycogen synthesis, hepatic capacity to produce glucose and glycemia in rats. The MFS comprised free access to food for a 2-hour period daily at a fixed mealtime (8.00-10.00 a.m.) for 13 days. The control group was composed of rats with free access to food from day 1 to 12, which were then starved for 22 h, refed with a single meal at 8.00-10.00 a.m. and starved again for another 22 h. All experiments were performed at the meal time (i.e. 8.00 a.m.). The MFS group exhibited increased food intake and higher glycogen synthase activity. Since gluconeogenesis from L-glutamine or L-alanine was not affected by MFS, we conclude that the increased food intake and higher glycogen synthase activity contributed to the better glucose maintenance showed by MFS rats at the fixed meal time.

Hepatic glutamine transport and metabolism

Although the liver was long known to play a major role in the uptake, synthesis, and disposition of glutamine, metabolite balance studies across the whole liver yielded apparently contradictory findings suggesting that little or no net turnover of glutamine occurred in this organ. Efforts to understand the unique regulatory properties of hepatic glutaminase culminated in the conceptual reformulation of the pathway for glutamine synthesis and turnover, especially as regards the role of sub-acinar distribution of glutamine synthetase and glutaminase. This chapter describes these processes as well as the role of glutamine in hepatocellular hydration, a process that is the consequence of cumulative, osmotically active uptake of glutamine into cells. This topic is also examined in terms of the effects of cell swelling on the selective stimulation or inhibition of other far-ranging cellular processes. The pathophysiology of the intercellular glutamine cycle in cirrhosis is also considered.

Modulation of phosphoenolpyruvate carboxykinase mRNA levels by the hepatocellular hydration state

Exposure of isolated perfused rat livers to hypo-osmotic (225 mosmol/l) perfusion media for 3 h led to a decrease of about 60% in mRNA levels for phosphoenolpyruvate carboxy-kinase (PEPCK) compared with normo-osmotic (305 mosmol/l) perfusions. Conversely, PEPCK mRNA levels increased about 3-fold during hyperosmotic (385 mosmol/l) perfusions. The anisotonicity effects were not explained by changes in the intracellular cyclic AMP (cAMP) concentration or by changes of the extracellular Na+ or Cl- activity. Similar effects of aniso-osmolarity on PEPCK mRNA levels were found in cultured rat hepatoma H4IIE.C3 cells, the experimental system used for further characterization of the effect. Whereas during the first hour of anisotonic exposure no effects on PEPCK mRNA levels were detectable, near-maximal aniso-osmolarity effects were observed within the next 2-3 h. PEPCK mRNA levels increased sigmoidally with the osmolarity of the medium, and the anisotonicity effects were most pronounced upon modulation of osmolarity between 250 and 350 mosmol/l. The aniso-osmolarity effects on PEPCK mRNA were not affected in presence of Gö 6850, protein kinase C inhibitor. cAMP increased the PEPCK mRNA levels about 2.3-fold in normo-osmotic media, whereas insulin lowered the PEPCK mRNA levels to about 8%. The effects of cAMP and insulin were also observed during hypo-osmotic and hyperosmotic exposure, respectively, but the anisotonicity effects were not abolished in presence of the hormones. The data suggest that hepatocellular hydration affects hepatic carbohydrate metabolism also over a longer term by modulating PEPCK mRNA levels. This is apparently unrelated to protein kinase C or alterations of cAMP levels. The data strengthen the view that cellular hydration is an important determinant for cell metabolic function by extending its regulatory role in carbohydrate metabolism to the level of mRNA.

Reciprocal changes in gluconeogenesis and ureagenesis induced by fatty acid oxidation

Fatty acids produced a stimulation of gluconeogenesis and either inhibition or no effect on ureagenesis in livers perfused with gluconeogenic substrates and having NH4Cl plus ornithine as the nitrogen source. This finding indicates that stimulation of flux through pyruvate carboxylase is not sufficient to enhance urea production from ammonia. The metabolic action of fatty acids showed the following characteristics: (1) it was concentration-dependent, showing saturation-type kinetics similar to those described for fatty acid oxidation; (2) the stimulatory action on gluconeogenesis was constant and independent of NH4Cl concentration, whereas the inhibition of ureagenesis was variable and dependent on NH4Cl concentration and the degree of reduction of the gluconeogenic substrate; and (3) fatty acids produced apparent reciprocal changes in the state of reduction of the cytosolic and mitochondrial NAD systems. Fatty acid oxidation exerted its effect mainly, if not exclusively, by preventing the gluconeogenic substrate-induced stimulation of ureagenesis. Fatty acids also inhibited ureagenesis without stimulating gluconeogenesis (lactate < 1 mmol/L), ruling out a limiting energy availability as the cause of the inhibition. One or both of the following two mechanisms seem to account for the fatty acid-induced inhibition of ureagenesis from NH4Cl. First, a decreased uptake of ornithine, and second, decreased flux through pyruvate dehydrogenase and probably other NAD(P)-linked mitochondrial dehydrogenases. The correlation found between the ability of fatty acids to inhibit ureagenesis and the state of activation of pyruvate dehydrogenase supports the latter point.

Interrelationships between ureogenesis and gluconeogenesis in perfused rat liver

Stimulation of ureogenesis by ornithine and/or NH4Cl inhibited gluconeogenesis from lactate but not from equimolar concentrations of pyruvate in perfused rat liver. Neither a shortage of energy nor a decrease in alpha-ketoglutarate availability seems to be responsible for this inhibition. With lactate as substrate the extracellular concentration of pyruvate attained was approximately equal to 0.15 mM that assuming reflects its cytosolic concentration it would be limiting for its mitochondrial transport. Stimulation of ureogenesis from NH4Cl enhances flux through pyruvate dehydrogenase. Furthermore, activation of pyruvate dehydrogenase by dichloroacetate led to stimulation of ureogenesis and inhibition of glucose production. Conversely, inhibition of pyruvate dehydrogenase flux by fatty acid enhanced glucose production and inhibited ureogenesis. Thus, ornithine and/or NH4Cl seem to inhibit lactate to glucose flux by shifting the mitochondrial partitioning of pyruvate from carboxylation towards decarboxylation with the result of a decreased oxaloacetate formation. Gluconeogenic substrates enhanced the hepatic uptake of ornithine. However, no correlation seems to exist between the uptake of ornithine, ornithine-induced stimulation of ureogenesis and total rates of urea production. Ornithine produced a concentration-dependent acidification of the hepatic outflow perfusate, suggesting that it may be transported in exchange for H+.

Real-time study of the urea cycle using 15N n.m.r. in the isolated perfused rat liver

1. Isolated rat liver was perfused with 10 mM-15NH4Cl, 5 mM-lactate and 1 mM-ornithine, or with 3 mM-[15N]alanine and 1 mM-ornithine, in haemoglobin-free medium. The liver was physiologically stable for over 3 h and synthesized urea at the rate of 1.15 mumol.min-1.g of liver-1 (15NH4(+)-perfused) or 0.41 mumol.min-1.g-1 ([15N]alanine-perfused). 2. The perfused liver was continuously monitored by 15N n.m.r. spectroscopy at 20.27 MHz for 15N. Well-resolved 15N resonances of precursors and intermediates of the urea cycle, present at tissue concentrations of 0.2-3.0 mumol/g, were observed from the intact liver in 5-40 min of acquisition. Key metabolites in liver extract and the final perfusion medium were analysed by n.m.r. and by biochemical assays to determine fractional 15N enrichment and the total 15N recovery. 3. In 15NH4(+)-perfused liver (n = 6), 15N incorporation into glutamate and alanine (1.0-1.3 mumol/g), as well as progressive formation of [15N2]urea, was observed during the first 2 h of perfusion. In the second and third hour, hepatic concentrations of [omega-15N]citrulline and [omega,omega'-15N]argininosuccinate increased to n.m.r.-detectable levels (0.3-0.9 mumol/g). The [15N]aspartate pool was large in the absence of added ornithine, but on its addition was rapidly incorporated into argininosuccinate (n = 3). 4. In [15N]alanine-perfused liver, major metabolites were [15N]glutamate, [gamma-15N]glutamine and [15N]urea. Urea-cycle intermediates were undetectable. 5. The results suggest that, in intact liver provided with excess ammonia, low concentrations of cytosolic argininosuccinate synthetase and argininosuccinate lyase limited the rate of metabolite flux in the urea cycle. By contrast, in alanine-perfused liver at a physiological rate of urea synthesis, mitochondrial carbamoylphosphate synthetase was rate-limiting. 6. The potential utility of 15N n.m.r. for study of metabolite channelling through urea-cycle enzymes in intact liver is discussed.

Control of hepatic proteolysis by amino acids. The role of cell volume

1. Proteolysis in isolated perfused rat liver was monitored as [3H]leucine release into effluent perfusate after in vivo labeling by intraperitoneal injection of [3H]leucine about 16 h prior to the perfusion experiment. Exposure of the livers to hypotonic perfusion media (175-295 mOsmol.l-1) increased liver mass due to cell swelling and inhibited [3H]leucine release. The extent of inhibition of [3H]leucine release was linearly related to the liver-mass increase, regardless of whether livers from fed or 24-h-starved rats were studied. 2. Infusion of glycine (0.5-3 mmol.l-1) or glutamine (0.5-3 mmol.l-1) during normotonic perfusions (305 mOsmol.l-1) led to a concentration-dependent increase of liver mass and inhibition of [3H]leucine release. The inhibition of [3H]leucine release was again strongly dependent upon the increase of liver mass, regardless of whether cell swelling was induced by glutamine or glycine in normotonic perfusions, by exposure of the liver to hypotonic media or whether amino-acid-induced cell swelling was modified by the nutritional state. The effects of glutamine and glycine on [3H]leucine release were additive to the same extent as that found when the liver-mass increase was observed. 3. Alanine, serine and proline inhibited [3H]leucine release in parallel to the extent of amino-acid-induced liver-mass increase; however, the inhibition of [3H]leucine release was about twice that found when comparable degrees of cell swelling were induced either by hypotonic exposure or by addition of glutamine or glycine. The relationship between alanine-induced liver-mass increase and the inhibition of [3H]leucine release was also maintained in presence of aminooxyacetate (0.2 mmol.l-1). 4. Infusion of an amino acid mixture, roughly mimicking the concentrations found in portal venous blood, to livers from 24-h-starved or fed rats inhibited [3H]leucine release by 56.0 +/- 2.4% (n = 6) or 31.1 +/- 2.3% (n = 3), respectively, and increased liver mass by 5.0 +/- 0.1% (n = 6) or 2.2 +/- 0.3% (n = 3), respectively. Regardless of the nutritional state, there was a close relationship between the amino-acid-mixture-induced (and also phenylalanine-induced) increase of liver mass and the degree of inhibition of [3H]leucine release; however, the inhibition of [3H]leucine release was about fourfold higher than that found when comparable degrees of cell swelling were induced by hypotonic exposure.(ABSTRACT TRUNCATED AT 400 WORDS)

Hepatocyte heterogeneity in uptake and metabolism of malate and related dicarboxylates in perfused rat liver

1. In isolated perfused rat liver a near-maximal net malate uptake of about 120 nmol g-1 min-1 was observed at influent malate concentrations above 100 mumol l-1 and a half-maximal uptake at about 50 mumol l-1 in influent. 14CO2 production from added [U-14C]malate paralleled hepatic net malate uptake, however, 14CO2 production exceeded net malate uptake by 20-25%. This was observed in antegrade as well as in retrograde perfusions and regardless of whether NH4Cl was added to the influent perfusate. Stimulation of glutamine synthesis by NH4Cl only slightly affected net malate uptake and 14CO2 production, but resulted in a marked stimulation of [14C]glutamine release from the liver. 2. Because [U-14C]malate uptake by the liver (reflecting the influent/effluent concentration difference of labeled malate) could at least in part involve a malate/malate exchange mechanism, net malate uptake (as determined from the influent/effluent concentration difference of enzymatically assayable malate) may underestimate hepatic [U-14C]malate uptake. On the other hand, during metabolic steady states 14CO2 production from added [U-14C]malate can be considered as an upper limit estimate of [U-14C]malate uptake by the liver. Assuming that 14CO2 production equals [U-14C]malate uptake by the liver, extrapolation studies suggest that during maximal rates of NH4Cl-stimulated glutamine synthesis 80-110% of the [U-14]malate taken up by the liver was used for glutamine synthesis. This was true for retrograde and antegrade perfusions. Similar data, i.e. a 100-130% incorporation regardless of the direction of perfusion, were obtained when [U-14C]malate uptake was assumed to equal net malate uptake by the liver. 3. Substitution of Na+ in the perfusion fluid by choline abolished net malate uptake by the liver and inhibited 14CO2 production from [U-14C]malate by more than 90%. 4. 2-Oxoglutarate inhibited [14C]malate uptake and [1-14C]oxoglutarate uptake by the liver was inhibited by malate, fumarate, succinate and oxaloacetate, but not by aspartate and glutamate. Inhibition of [1-14C]oxoglutarate uptake and of 14CO2 production from added labeled 2-oxoglutarate by malate and fumarate seemed largely competitive. Malate, fumarate and succinate not only inhibited [1-14C]oxoglutarate uptake, but also stimulated the release of unlabeled 2-oxoglutarate from the liver. 5. The data are consistent with a predominant uptake of vascular malate by perivenous glutamine synthetase containing hepatocytes when glutamine synthesis is stimulated to Vmax values by NH4Cl. Malate and other citric acid cycle dicarboxylates, but not aspartate and glutamate, may compete with 2-oxoglutarate for uptake into perivenous glutamine synthesizing hepatocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of hepatic nitrogen metabolism and glutathione release by cell volume regulatory mechanisms

1. Urea synthesis was studied in isolated perfused rat liver during cell volume regulatory ion fluxes following exposure of the liver to anisotonic perfusion media. Lowering of the osmolarity in influent perfusate from 305 mOsm/l to 225 mOsm/l (by decreasing influent [NaCl] by 40 mmol/l) led to an inhibition of urea synthesis from NH4Cl (0.5 mmol/l) by about 60% and a decrease of hepatic oxygen uptake by 0.43 +/- 0.03 mumol g-1 min-1 [from 3.09 +/- 0.13 mumol g-1 min-1 to 2.66 +/- 0.12 mumol g-1 min-1 (n = 9)]. The effects on urea synthesis and oxygen uptake were observed throughout hypotonic exposure (225 mOsm/l). They persisted although volume regulatory K+ efflux from the liver was complete within 8 min and were fully reversible upon reexposure to normotonic perfusion media (305 mOsm/l). A 42% inhibition of urea synthesis from NH4Cl (0.5 mmol/l) during hypotonicity was also observed when the perfusion medium was supplemented with glucose (5 mmol/l). Urea synthesis was inhibited by only 10-20% in livers from fed rats, and was even stimulated in those from starved rats when an amino acid mixture (twice the physiological concentration) plus NH4Cl (0.2 mmol/l) was infused. 2. The inhibition of urea synthesis from NH4Cl (0.5 mmol/l) during hypotonicity was accompanied by a threefold increase of citrulline tissue levels, a 50-70% decrease of the tissue contents of glutamate, aspartate, citrate and malate, whereas 2-oxoglutarate, ATP and ornithine tissue levels, and the [3H]inulin extracellular space remained almost unaltered. Further, hypotonic exposure stimulated hepatic glutathione (GSH) release with a time course roughly paralleling volume regulatory K+ efflux. NH4Cl stimulated lactate release from the liver during hypotonic but not during normotonic perfusion. In the absence of NH4Cl, hypotonicity did not significantly affect the lactate/pyruvate ratio in effluent perfusate. With NH4Cl (0.5 mmol/l) present, the lactate/pyruvate ratio increased from 4.3 to 8.2 in hypotonicity, whereas simultaneously the 3-hydroxybutyrate/acetoacetate ratio slightly, but significantly decreased. 3. Addition of lactate (2.1 mmol/l) and pyruvate (0.3 mmol/l) to influent perfusate did not affect urea synthesis in normotonic perfusions, but completely prevented the inhibition of urea synthesis from NH4Cl (0.5 mmol/l) induced by hypotonicity. Restoration of urea production in hypotonic perfusions by addition of lactate and pyruvate was largely abolished in the presence of 2-cyanocinnamate (0.5 mmol/l). Addition of 3-hydroxybutyrate (0.5 mmol/l), but not of acetoacetate (0.5 mmol/l) largely reversed the hypotonicity-induced inhibition of urea synthesis from NH4Cl.(ABSTRACT TRUNCATED AT 400 WORDS)

Liver glutamine metabolism

A fundamental conceptional change in the field of hepatic glutamine metabolism is derived from an understanding of the unique regulatory properties of hepatic glutaminase, the occurrence of glutamine cycling, and the discovery of marked hepatocyte heterogeneities in nitrogen metabolism, with metabolic interactions between differently localized subacinar hepatocyte populations. This change provided new insight into the role of the liver in maintaining ammonia and bicarbonate homeostasis under physiologic and pathologic conditions. Glutamine synthetase is present only in a specialized cell population at the hepatic venous outflow of the liver acinus; these cells act as scavengers for ammonia and probably also for various signal molecules ("perivenous scavenger cell hypothesis"). The function of mitochondrial glutaminase is that of a pH- and hormone-modulated ammonia amplification system that controls carbamoylphosphate synthesis and urea cycle flux in periportal hepatocytes. Not only is hepatic glutamine metabolism essential for maintenance of bicarbonate and ammonia homeostasis, but glutamine itself can act in the liver as a signal modulating hepatic metabolism. This article summarizes some major aspects of hepatic glutamine metabolism, based on previous reviews.

Interactions between glutamine metabolism and cell-volume regulation in perfused rat liver

1. In the presence of near-physiological glutamine concentrations, exposure of perfused rat liver to hypotonic perfusion media switched glutamine balance across the liver from net release to net uptake. This was due to both stimulation of flux through glutaminase and inhibition of flux through glutamine synthetase. Conversely, during exposure to hypertonic media, net glutamine release from the liver increased due to inhibition of glutaminase flux and slight stimulation of flux through glutamine synthetase. The effect of perfusate osmolarity on glutaminase flux was observed at an NH4Cl concentration (0.5 mM) sufficient for near-maximal ammonia stimulation of glutaminase. This indicates the involvement of different mechanisms of glutaminase flux control by extracellular osmolarity changes and ammonia. The effects of anisotonicity on flux through glutamine-metabolizing enzymes were fully reversible. Glutamine (0.6 mM) stimulated urea synthesis from NH4Cl (0.5 mM) during hypotonic and normotonic conditions. 2. Exposure to hypotonic and hypertonic media led, after initial liver-cell swelling and shrinkage, respectively to volume-regulatory K+ fluxes which largely restored the initial liver-cell volume despite the continuing osmotic challenge. Even after completion of cell-volume regulatory K+ fluxes, the effects of perfusate osmolarity on hepatic glutamine metabolism persisted. This indicates that in anisotonicity the liver cell is left in an altered metabolic state, even after completion of volume-regulatory responses. 3. During perfusion with isotonic media, addition of glutamine (3 mM) led to an increase of liver mass by about 4% within 2 min, which was accompanied by a net K+ uptake by the liver. Thereafter, the new steady state of increased liver mass was maintained throughout glutamine infusion. When the liver mass had reached this new steady state, a net release of K+ from the liver of about 3 mumol/g liver was observed during the following 10 min. Withdrawal of glutamine was followed by a slow reuptake of K+ and the liver mass returned to its initial value. Following exposure to glutamine (3 mM), the intracellular glutamine concentration (as calculated from glutamine tissue levels, taking into account the extracellular space determined with the [3H]inulin technique) rose from about 1 mM to 30-35 mM within about 12 min, indicating a 10-12-fold concentrative uptake of glutamine into the liver cells and an osmotic challenge for the hepatocyte. When intracellular glutamine had reached its steady-state concentration, net K+ efflux from the liver was also terminated.(ABSTRACT TRUNCATED AT 400 WORDS)

Benzoate stimulates glutamate release from perfused rat liver

In isolated perfused rat liver, benzoate addition to the influent perfusate led to a dose-dependent, rapid and reversible stimulation of glutamate output from the liver. This was accompanied by a decrease in glutamate and 2-oxoglutarate tissue levels and a net K+ release from the liver; withdrawal of benzoate was followed by re-uptake of K+. Benzoate-induced glutamate efflux from the liver was not dependent on the concentration (0-1 mM) of ammonia (NH3 + NH4+) in the influent perfusate, but was significantly increased after inhibition of glutamine synthetase by methionine sulphoximine or during the metabolism of added glutamine (5 mM). Maximal rates of benzoate-stimulated glutamate efflux were 0.8-0.9 mumol/min per g, and the effect of benzoate was half-maximal (K0.5) at 0.8 mM. Similar Vmax. values of glutamate efflux were obtained with 4-methyl-2-oxopentanoate, ketomethionine (4-methylthio-2-oxobutyrate) and phenylpyruvate; their respective K0.5 values were 1.2 mM, 3.0 mM and 3.8 mM. Benzoate decreased hepatic net ammonia uptake and synthesis of both urea and glutamine from added NH4Cl. Accordingly, the benzoate-induced shift of detoxication from urea and glutamine synthesis to glutamate formation and release was accompanied by a decreased hepatic ammonia uptake. The data show that benzoate exerts profound effects on hepatic glutamate and ammonia metabolism, providing a new insight into benzoate action in the treatment of hyperammonaemic syndromes.

Functional hepatocyte heterogeneity. Vascular 2-oxoglutarate is almost exclusively taken up by perivenous, glutamine-synthetase-containing hepatocytes

1. In isolated perfused rat liver maximal rates of 2-[1-14C]oxoglutarate uptake were about 0.4 mumol.g-1 .min-1; half-maximal rates of 2-[14C]oxoglutarate uptake were observed with influent concentrations of about 100 microM. 2-[14C]Oxoglutarate uptake by the liver was not affected by the direction of perfusion, but was decreased by about 80-90% when Na+ in the perfusion fluid was substituted by choline+, suggesting a Na+-dependence of hepatic 2-oxoglutarate uptake. In the absence of added ammonia, [14C]oxoglutarate uptake by the liver was about twice the net oxoglutarate uptake, indicating a simultaneous release of unlabeled oxoglutarate from perfused rat liver. 2. 14C-Labeled metabolites derived from [1-14C]oxoglutarate and recovered in the effluent perfusate were 14CO2 and 14C-labeled glutamate and glutamine; they accounted for 85-100% of the radiolabel taken up by the liver. 14CO2 was the major product (more than 70%) from [1-14C]oxoglutarate taken up the liver, provided glutamine synthesis was either inhibited by methionine sulfoximine or the endogenous rate of glutamine production was below 40 nmol.g-1.min-1. 3. Stimulation of glutamine synthesis by ammonia did not affect [14C]oxoglutarate uptake by the liver, but considerably increased net hepatic oxoglutarate uptake, indicating a decreased release of unlabeled oxoglutarate from the liver. Stepwise stimulation of hepatic glutamine synthesis led to a gradual decrease of 14CO2 production and radiolabel was recovered increasingly as [14C]glutamine in the effluent. At high rates of glutamine formation (i.e. about 0.6 mumol.g-1.min-1), about 60% of the [1-14C]oxoglutarate taken up by the liver was recovered in the effluent as [14C]glutamine. 14CO2 and [14C]glutamine production from added [1-14C]oxoglutarate were dependent on the rate of hepatic glutamine synthesis but not on the direction of perfusion. Extrapolation of 14C incorporation into glutamine to maximal rates of hepatic glutamine synthesis yielded an about 100% utilization of the [14C]oxoglutarate taken up by the liver for glutamine synthesis. This was again true for both the antegrade and the retrograde perfusion directions. On the other hand, addition of ammonia did not affect 14CO2 production from labeled oxoglutarate, when glutamine synthetase was inhibited by methionine sulfoximine. 4. The data suggest that vascular oxoglutarate is almost exclusively taken up by the small perivenous hepatocyte population containing glutamine synthetase, i.e. a cell population comprising only 6-7% of all hepatocytes. Thus, the findings demonstrate the existence of a, to date, uniquely zonally distributed oxoglutarate transport system which is probably Na+-dependent in the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of thromboxane release by extracellular UTP and ATP from perfused rat liver. Role of icosanoids in mediating the nucleotide responses

1. In isolated perfused rat liver, infusion of UTP (20 microM) led to a transient, about sevenfold stimulation of thromboxane release (determined as thromboxane B2), which did not parallel the time course of the UTP-induced stimulation of glucose release. An increased thromboxane release was also observed after infusion of ATP (20 microM). Although the maximal increase of portal pressure following ATP was much smaller than with UTP (4.2 vs 11.5 cm H2O), the peak thromboxane release was similar with both nucleotides. 2. Indomethacin (10 microM) inhibited the UTP-induced stimulation of thromboxane release and decreased the UTP-induced maximal increase of glucose output and of portal pressure by about 30%. The thromboxane A2 receptor antagonist BM 13.177 (20 microM) completely blocked the pressure and glucose response to the thromboxane A2 analogue U-46619 (200 nM) and decreased the ATP- and UTP-induced stimulation of glucose output by about 25%, whereas the maximal increase of portal pressure was inhibited by about 50% and 30%, respectively. BM 13.177 and indomethacin inhibited the initial nucleotide-induced overshoot of portal pressure increase, but had no effect on the steady-state pressure increase which is obtained about 5 min after addition of ATP or UTP. 3. The leukotriene D4/E4 receptor antagonist LY 171883 (50 microM) inhibited not only the glucose and pressure response of perfused rat liver to leukotriene D4, but also to leukotriene C4 by about 90%. This suggests that leukotriene D4 (not C4) is the active metabolite in perfused liver and the effects of leukotriene C4 are probably due to its rapid conversion to leukotriene D4. LY 171883 also inhibited the response to the thromboxane A2 analogue U-46619 by 75-80%, whereas the response of perfused liver to leukotriene C4 was not affected by the thromboxane receptor antagonist BM 13.177 (20 microM). The glucose and pressure responses of the liver to extracellular UTP were inhibited by LY 171883 and by BM 13.177 by about 30%. This suggests that the inhibitory action of LY 171883 was due to a thromboxane receptor antagonistic side-effect and that peptide leukotrienes do not play a major role in mediating the UTP response. 4. In isolated rat hepatocytes extracellular UTP (20 microM), ATP (20 microM), cyclic AMP (50 microM) and prostaglandin F2 alpha (3 microM) increased glycogen phosphorylase a activity by more than 100%.(ABSTRACT TRUNCATED AT 400 WORDS)

Liver carbonic anhydrase and urea synthesis. The effect of diuretics

In isolated perfused rat liver, urea synthesis is rapid and reversibly inhibited not only by the well-known carbonic anhydrase inhibitors acetazolamide, methazolamide and ethoxzolamide, but also by diuretics, like xipamide, mefruside, chlortalidone, and chlorothiazide. Furosemide was without effect. Similar to findings with isolated perfused rat liver, acetazolamide inhibits urea synthesis from ammonium ions in normal and cirrhotic human liver slices. Inhibition of urea synthesis by xipamide and acetazolamide is accompanied by a 70% decrease of the cellular citrulline content and the tissue levels of 2-oxoglutarate and citrate, suggesting a block of urea synthesis at a step prior to citrulline formation. At a constant extracellular pH (7.4), inhibition of urea synthesis by xipamide, mefruside and acetazolamide was overcome by increasing the extracellular concentrations of HCO3- and CO2 to above twice the normal values. This shows that inhibition of urea synthesis by these diuretics is not due to an unspecific inhibition of one of the urea cycle enzymes but is due to an inhibition of mitochondrial carbonic anhydrase and therefore due to an impaired HCO3- provision for mitochondrial carbamoylphosphate synthesis. It is concluded that the activity of mitochondrial carbonic anhydrase is required for urea synthesis also in human liver and that several diuretics impair urea synthesis by inhibition of mitochondrial carbonic anhydrase. The pathophysiological significance of these data is discussed with respect to the development of diuretics-induced hyperammonemia and alkalosis in liver disease.

The effect of urea synthesis on extracellular pH in isolated perfused rat liver

In a non-recirculating system of isolated liver perfusion, stimulation of urea synthesis by NH4Cl is followed by a decrease of effluent pH by up to 0.2 pH unit. This effect is not observed when urea synthesis is inhibited by amino-oxyacetate or norvaline. When the urea formed by the liver is immediately hydrolysed with urease before the effluent perfusate reaches the pH electrode, the urea-synthesis-induced acidification is no longer observed. This indicates that accompanying alterations in hepatic metabolism after stimulation of urea synthesis, such as increased energy provision and consumption, are not responsible for the extracellular acidification, but that the effect is due to the formation of urea itself. The acidification of the extracellular space after stimulation of urea synthesis by NH4Cl is quantitatively explained by the consumption of 2 mol of HCO3-/mol of urea formed: 1 mol being incorporated into urea, the other being protonated to yield CO2 and H2O. The data match the theoretically predicted HCO3- consumption during ureogenesis and underline the role of hepatic urea synthesis for disposal of HCO3- by converting it into the excretable products CO2 and urea.

Utilization of nitrogen from 15NH4Cl and [15N]alanine for urea synthesis in hepatocytes from fed and starved rats

Utilization of N from 15NH4Cl and [15N]alanine for urea synthesis in hepatocytes isolated from fed and 24 hr starved rats was investigated. In hepatocytes isolated from fed rats, 54 and 65% of the added [15N]ammonia was utilized for urea synthesis in the presence of 0.5 and 2.0 mM NH4Cl, respectively. This utilization of [15N]ammonia in hepatocytes from starved rats was 2-fold lower. The amount of urea synthetized from endogenous sources was, in the presence of 0.5 and 2.0 mM NH4Cl, about 44 and 60% higher than in the control conditions (without NH4Cl). The considerable amount of added ammonia (30-44%) was utilized in processes other than urea synthesis. Alanine markedly diminished the utilization of 15N from NH4Cl in hepatocytes from both fed and starved rats. In these conditions (NH4Cl present), alanine significantly increased the urea formation in hepatocytes from starved rats and failed to affect the urea production in hepatocytes from fed rats. On the basis of 15N determination, it was concluded that both NH4Cl and alanine caused an increase in the utilization of nitrogen from endogenous sources in rat hepatocytes. This conclusion is in contrast with the results based only on the changes in ammonia and urea concentrations.

Regulation of hepatic ammonia metabolism: the intercellular glutamine cycle

In the liver acinus, urea synthesis and glutaminase activity are predominantly localized in the periportal area, whereas glutamine synthetase activity is perivenous. Because ammonium ions at low concentrations are effectively removed by glutamine synthetase, but not by urea synthesis, the two pathways of ammonia detoxication in the liver acinus represent the sequence of a low-affinity, but high-capacity system (ureogenesis) and a perivenous high-affinity system (glutamine synthesis). In agreement with these findings, obtained in experiments with the metabolically and structurally intact perfused rat liver, perivenous glutamine synthesis was almost completely inhibited after induction of perivenous liver cell necrosis by carbon tetrachloride, whereas periportal urea synthesis was not affected. The structural and functional organization of hepatic ammonium and glutamine metabolism and the metabolic interactions of different subacinar hepatocyte populations provide a new understanding of hepatic nitrogen metabolism under physiological and pathological conditions. Periportal glutaminase and perivenous glutamine synthetase are simultaneously active, resulting in an intercellular (as opposed to intracellular) glutamine cycle, being under complex metabolic and hormonal control. The intercellular glutamine cycle provides an effective means for almost complete conversion of portal ammonium ions into urea without accompanying net glutamine formation. This is achieved by additional substrate feeding into the urea cycle by the glutaminase reaction, both pathways being localized in the periportal compartment, and the perivenous resynthesis of glutamine from ammonium ions which escaped periportal urea synthesis. This complete conversion of portal ammonium ions into urea by means of glutamine cycling represents the situation of a well-balanced pH homeostasis. Because urea synthesis, in contrast to glutamine synthesis, is a major pathway for removal of bicarbonate, the switching of hepatic ammonium detoxication from urea synthesis to glutamine synthesis in acidosis points to an important role of the liver in maintaining pH homeostasis. The acid-base-induced changes of the route of hepatic ammonium detoxication and therefore bicarbonate removal are performed by the regulatory properties of the enzymes of the intercellular glutamine cycle.

Role of plasma membrane transport in hepatic glutamine metabolism

In livers of fed rats and in perfused livers supplied with a physiological portal glutamine concentration of 0.6 mM, the mitochondrial and cytosolic glutamine concentrations are 20 mM and 7 mM, respectively, thus, the mitochondrial/cytosolic glutamine concentration gradient is 2-3. Uptake and release of glutamine by periportal and perivenous hepatocytes occurs predominantly by an Na+-dependent transport system (so-called system 'N'). Histidine in near-physiological concentrations inhibits both glutamine uptake by periportal hepatocytes and its release by perivenous hepatocytes. This is not due to an inhibition of glutamine-metabolizing enzymes by histidine or its metabolites. With physiological portal glutamine concentrations (0.6 mM), stimulation of glutaminase flux or of glutamine transaminase flux is followed by a decrease of hepatic glutamine levels to about 80% or 30%, respectively, glutamine levels are further decreased to 50% or 20% in the presence of histidine. When glutamine is synthesized endogenously (no glutamine added), the histidine-induced inhibition of glutamine release is paralleled by a 210% increase of the hepatic tissue level of glutamine. In experiments with and without methionine sulfoximine and in the absence of added glutamine, the glutamine content in the small perivenous hepatocyte population containing glutamine synthetase is estimated to be about 3.5 mumol/g wet weight and that in the periportal hepatocytes as low as 0.1 mumol/g wet weight. In contrast to the prevailing view, it is concluded that glutamine transport across the plasma membrane of hepatocytes is a potential regulatory site in glutamine degradation and synthesis, especially under the influence of effectors like histidine.

Hepatic urea synthesis and pH regulation. Role of CO2, HCO3-, pH and the activity of carbonic anhydrase

In isolated perfused rat liver, urea synthesis from ammonium ions was dependent on extracellular HCO3- and CO2 concentrations when the HCO3-/CO2 ratio in the influent perfusate was constant (pH 7.4). Urea synthesis was half-maximal at HCO3- = 4 mM, CO2 = 0.19 mM and was maximal at HCO3- and CO2 concentrations above 20 mM and 0.96 mM, respectively. At physiological HCO3- (25 mM) and CO2 (1.2 mM) concentrations in the influent perfusate, acetazolamide, the inhibitor of carbonic anhydrase, inhibited urea synthesis from ammonium ions (1 mM) by 50-60% and led to a 70% decrease in citrulline tissue levels. Acetazolamide concentrations required for maximal inhibition of urea synthesis were 0.01-0.1 mM. At subphysiological HCO3- and CO2 concentrations, inhibition of urea synthesis by acetazolamide was increased up to 90%. Inhibition of urea synthesis by acetazolamide was fully overcome in the presence of unphysiologically high HCO3- and CO2 concentrations, indicating that the inhibitory effect of acetazolamide is due to an inhibition of carbonic-anhydrase-catalyzed HCO3- supply for carbamoyl-phosphate synthetase, which can be bypassed when the uncatalyzed intramitochondrial HCO3- formation from portal CO2 is stimulated in the presence of high portal CO2 concentrations. With respect to HCO3- supply of mitochondrial carbamoyl-phosphate synthetase, urea synthesis can be separated into a carbonic-anhydrase-dependent (sensitive to acetazolamide at 0.5 mM) and a carbonic-anhydrase-independent (insensitive to acetazolamide) portion. Carbonic-anhydrase-independent urea synthesis linearly increased with the portal 'total CO2 addition' (which was experimentally determined to be CO2 addition plus 0.036 HCO3- addition) and was independent of the perfusate pH. At a constant 'total CO2 addition', carbonic-anhydrase-dependent urea synthesis was strongly affected by perfusate pH and increased about threefold when the perfusate pH was raised from 6.9 to 7.8. It is concluded that the pH dependent regulation of urea synthesis is predominantly due to mitochondrial carbonic anhydrase-catalyzed HCO3- supply for carbamoyl phosphate synthesis, whereas there is no control of urea synthesis by pH at the level of the five enzymes of the urea cycle. Because HCO3- provision for carbamoyl phosphate synthetase increases with increasing portal CO2 concentrations even in the absence of carbonic anhydrase activity, susceptibility of ureogenesis to pH decreases with increasing portal CO2 concentrations. This may explain the different response of urea synthesis to chronic metabolic and chronic respiratory acidosis in vivo.

Effects of ATP and adenosine addition on activity of oxoglutarate dehydrogenase and the concentration of cytoplasmic free Ca2+ in rat hepatocytes

Addition of ATP (100 microM) to hepatocytes from starved rats incubated with 5 mM [1-14C]glutamine caused a stimulation of glucose formation; the magnitude of the concomitant increases in 14CO2 production and glutamine consumption indicate that flux from glutamine to glucose was increased. ATP also caused a simultaneous decrease in the cell content of oxoglutarate; together with the increased flux this is consistent with an activation of oxoglutarate dehydrogenase. In corroboration of this, a stimulation by ATP of gluconeogenesis and a decrease in oxoglutarate was also observed with 5 mM proline as substrate. ATP caused an increase in hepatocyte cytoplasmic free Ca2+ concentration, [Ca2+]c, as indicated by the increase in the fluorescence of cytoplasmically trapped quin2, from a resting value of about 0.2 microM to greater than 1 microM. The mechanism of oxoglutarate dehydrogenase activation may be via an increase in mitochondrial Ca2+ content as a consequence of the increase in [Ca2+]c. The effects of 100 microM adenosine were also investigated. An increase in flux from glutamine to glucose was observed together with a decrease in the cell oxoglutarate, thus indicating that adenosine addition to hepatocytes could also activate oxoglutarate dehydrogenase. The activation by adenosine was less than that produced by ATP. Adenosine caused a small apparent increase in [Ca2+]c to 0.3-0.4 microM; it remains to be established if this effect, which is small relative to that of ATP, is sufficient to elicit the activation of oxoglutarate dehydrogenase: alternative mechanisms may exist.

Reciprocal effects of energy utilization on palmitate oxidation and esterification in hepatocytes of fed rats

The effects of the energy-dependent process of urea synthesis from NH4Cl on the partition of [1-14C]palmitate between oxidation and esterification were examined in hepatocytes of fed rats. A high rate of urea formation from NH4Cl resulted in stimulation of total palmitate oxidation by 25 and 15% at 0.2 and 1 mM fatty acid, respectively. The stimulation of palmitate oxidation was reciprocally correlated with diminished palmitate incorporation into lipids, mainly triacylglycerols. This relationship was almost stoichiometric. NH4Cl increased the palmitate oxidation/esterification ratio from 0.72 to 1.13 and from 0.94 to 1.36 in the presence of 0.2 mM and 1 mM palmitate, respectively. The transaminase inhibitor, aminooxyacetate, strongly inhibited urea synthesis from NH4Cl, had little effect on the low beta-hydroxybutyrate/acetoacetate ratio in the presence of NH4Cl, completely reversed the changes in palmitate metabolism caused by NH4Cl and did not affect palmitate metabolism in the absence of NH4Cl. Therefore, the increased utilization of energy for urea synthesis was the causative factor by which NH4Cl stimulated total palmitate oxidation and led in consequence to its decreased esterification into lipids. Accordingly, these observations indicate that in liver cells the rate of ATP utilization is one of the determinants of triacylglycerol synthesis.

Influence of insulin and glucose on pyruvate catabolism in perfused rat hindlimbs

The effects of insulin and glucose on the oxidative decarboxylation of pyruvate in isolated rat hindlimbs was studied in non-recirculating perfusion with [1-14C]pyruvate. Insulin increased the calculated pyruvate decarboxylation rate in a concentration-dependent manner. At supramaximal insulin concentrations, the calculated pyruvate decarboxylation rate was increased by about 40% in perfusions with 0.15-1.5 mM-pyruvate. Glucose up to 20 mM had no effect. In the presence of insulin and low physiological pyruvate concentrations (0.15 mM), glucose increased the calculated pyruvate oxidation. This effect was abolished by high concentrations of pyruvate (1 mM). The data provide evidence that in resting perfused rat skeletal muscle insulin primarily increased the activity of the pyruvate dehydrogenase complex. The effect of glucose was due to increased intracellular pyruvate supply.

Regulation of hepatic glutamate metabolism. Role of 2-oxoacids in glutamate release from isolated perfused rat liver

In isolated perfused rat liver, addition of the oxoanalogues of leucine, isoleucine, methionine and phenylalanine is followed by a rapid and reversible stimulation of glutamate release. This is not observed with the corresponding amino acids or 2-oxoisovalerate, 2-oxoglutarate or oxaloacetate. The increased glutamate release by the liver is accompanied by a decrease in the tissue contents of 2-oxoglutarate and glutamate by about 25% and 50%, respectively. During the metabolism of glutamine, i.e. conditions with elevated tissue glutamate concentrations, 2-oxoacid-induced glutamate release is stimulated. In the presence of glutamine (5 mM), 2-oxoisocaproate, 2-oxo-4-methylvalerate and 2-oxo-4-methylthiobutyrate were found to be most effective and glutamate release by the liver increased linearly from about 80 nmol g-1 min-1 to 600 nmol g-1 min-1 at increasing 2-oxoacid concentrations up to 1 mM. When glutamate tissue levels were decreased by phenylephrine, stimulation of glutamate release by 2-oxoisocaproate was markedly diminished. 2-Oxoacid-stimulated glutamate release is independent of oxoacid metabolism, indicating that the effect is probably not explained by a 2-oxoacid/glutamate exchange across the liver plasma membrane. 2-Oxoacid-induced glutamate export predominantly occurs in a sodium-independent way. At low concentrations of 2-oxoisocaproate (below 0.2 mM), the increased glutamate release was accompanied by a slight inhibition of 14CO2 production from added [14C]glutamate, indicating a simultaneous glutamate uptake and release also under these conditions. Stimulation of glutamate release by 2-oxoisocaproate is followed by a decreased rate of urea and glutamine synthesis from portal ammonia, as a consequence of an increased glutamate release.

Effect of phenylephrine on glutamate and glutamine metabolism in isolated perfused rat liver

Addition of phenylephrine to isolated perfused rat liver is followed by an increased 14CO2 production from [1-14C]glutamate, [1-14C]glutamine, [U-14C]proline and [3-14C]pyruvate, but by a decreased 14CO2 production from [1-14C]pyruvate. Simultaneously, there is a considerable decrease in tissue content of 2-oxoglutarate, glutamate and citrate. Stimulation of 14CO2 production from [1-14C]glutamate is also observed in the presence of amino-oxyacetate, suggesting a stimulation of glutamate dehydrogenase and 2-oxoglutarate dehydrogenase fluxes by phenylephrine. Inhibition of pyruvate dehydrogenase flux by phenylephrine is due to an increased 2-oxoglutarate dehydroxygenase flux. Phenylephrine stimulates glutaminase flux and inhibits glutamine synthetase flux to a similar extent, resulting in an increased hepatic glutamine uptake. Whereas the effects of NH4+ ions and phenylephrine on glutaminase flux were additive, activation of glutaminase by glucagon was considerably diminished in the presence of phenylephrine. The reported effects are largely overcome by prazosin, indicating the involvement of alpha-adrenergic receptors in the action of phenylephrine. It is concluded that stimulation of gluconeogenesis from various amino acids by phenylephrine is due to an increased flux through glutamate dehydrogenase and the citric acid cycle.

The elucidation of the effect of ammonium chloride on pyruvate distribution and pyruvate dehydrogenase interconversion in isolated rat hepatocytes

The distribution of pyruvate between cell compartments measured in isolated hepatocytes in the presence of lactate was in agreement with delta pH across plasma and mitochondrial membranes. In isolated liver mitochondria NH4Cl decreased the transmembrane potential (delta psi) by about 14 mV, whereas no change of delta pH was observed. In the presence of lactate or alanine NH4Cl increased the mitochondrial pyruvate concentration presumably due to the inhibition of the flux through pyruvate carboxylase. In the presence of lactate or alanine changes in the amount of the active form of pyruvate dehydrogenase (PDHa) were correlated with the mitochondrial pyruvate concentration, NH4Cl increased the amount of PDHa by lowering the mitochondrial ATP/ADP and NADH/NAD+ ratios.

Hepatocyte heterogeneity in glutamine and ammonia metabolism and the role of an intercellular glutamine cycle during ureogenesis in perfused rat liver

1. The metabolism of glutamine and ammonia was studied in isolated perfused rat liver in relation to its dependence on the direction of perfusion by comparing the physiological antegrade (portal to caval vein) to the retrograde direction (caval to portal vein). 2. Added ammonium ions are mainly converted to urea in antegrade and to glutamine in retrograde perfusions. In the absence of added ammonia, endogenously arising ammonium ions are converted to glutamine in antegrade, but are washed out in retrograde perfusions. When glutamine synthetase is inhibited by methionine sulfoximine, direction of perfusion has no effect on urea synthesis from added or endogenous ammonia. 3. 14CO2 production from [1-14C]glutamine is higher in antegrade than in retrograde perfusions as a consequence of label dilution during retrograde perfusions. 4. The results are explained by substrate and enzyme activity gradients along the liver lobule under conditions of limiting ammonia supply for glutamine and urea synthesis, and they are consistent with a perivenous localization of glutamine synthetase and a predominantly periportal localization of glutaminase and urea synthesis. Further, the data indicate a predominantly periportal localization of endogenous ammonia production. The results provide a basis for an intercellular (as opposed to intracellular) glutamine cycling and its role under different metabolic conditions.

Control of rat-liver glutaminase by ammonia and pH

Regulation by ammonia of phosphate-dependent glutaminase in isolated rat-liver mitochondria was studied at pH values near the cytosolic pH of 7.0. 1. Glutaminase activity, both in the absence and presence of bicarbonate, was completely dependent on the presence of ammonia. 2. Glutaminase activity, both in the absence and presence of bicarbonate, was strongly depressed by decreasing the pH of the incubation medium from 7.0 to 6.8 when the ammonia concentration was below 0.5 mM. 3. Bicarbonate stimulated glutaminase activity only in the presence of low concentrations of ammonia. 4. The data indicate that the reported inhibition of glutamine degradation in the perfused liver at low pH [e.g. Häussinger et al. (1980) Hoppe-Seyler's Z. Physiol. Chem. 361, 995-1001] is due to a decreased affinity of glutaminase for ammonia.

Decreased flux through pyruvate dehydrogenase during calcium ion movements induced by vasopressin, alpha-adrenergic agonists and the ionophore A23187 in perfused rat liver

Vasopressin or alpha-adrenergic agents such as phenylephrine or adrenaline, but not glucagon, elicited an initial decrease in flux through pyruvate dehydrogenase assayed by 14CO2 production from [1-14C]pyruvate in perfused rat liver. This rapid decrease in 14CO2 production was maximal within 1-2 min of exposure, concomitant with a rise in effluent pyruvate concentration: a subsequent return towards initial values in both parameters was completed well before 5 min. This time course was superposed with Ca2+ efflux from perfused liver, maximal (at 116 nmol/min per g wet wt. of liver) at 1-2 min of exposure. The percentage of the active (dephospho) form of pyruvate dehydrogenase was not decreased at 2 min of exposure. The effect on flux through pyruvate dehydrogenase by phenylephrine was abolished by prazosine, phentolamine or phenoxybenzamine. Ionophore A23187 also caused a depression in 14CO2 production from [1-14C]pyruvate and a rise in effluent pyruvate concentration, but this effect was stable for longer times, and it was delayed when Ca2+ was omitted from the perfusion medium. Responses of phenylephrine and A23187 were not additive. The results demonstrate that under the experimental conditions employed in intact perfused liver, the mitochondrial multienzyme system of pyruvate dehydrogenase is sensitive to vasopressin, alpha-adrenergic agents and A23187. The similar time course in Ca2+ efflux may be indicative of the involvement of Ca2+ in mediating this effect.

Ammonium ions enhance the flow through the pyruvate dehydrogenase in Ehrlich ascites tumor cells

The flow through pyruvate dehydrogenase was assayed in glycolysing cells by the evolution of 14CO2 from [1-14C] pyruvate. Parallel incubations were carried out in high bicarbonate buffer (25 mM) and in bicarbonate-free buffer. The activation of the complex by NH+4 was only observed in high bicarbonate buffer, because the dilution of labelled CO2 in the presence of an excess of bicarbonate enables the quantitative determination of labelled CO2 evolved from pyruvate in the decarboxylase step. In the bicarbonate-free buffer the activation of the complex was not observed, because the 14CO2 evolved from pyruvate was consumed by biosynthetic processes inside the cell. On the contrary in isolated hepatocytes the NH+4 activation of the pyruvate dehydrogenase was observed in both buffers. In Ehrlich ascites cells, in common with other mammalian tissues, pyruvate dehydrogenase activity was found to be inversely correlated to the intramitochondrial ATP/ADP ratio.

Effects of ammonia on liver lysosomal functionality in Salmo gairdneri Rich

Salmo gairdneri specimens were exposed for 4-48 hours to three different concentrations of un-ionized ammonia (UIA). An increase in lysosomal sensitivity to osmotic shock and in total proteolytic activity occurs in the liver. The amount of this increase depends upon the exposure time or upon environmental and tissue ammonia levels. On the other hand ammonia does not affect lysosome hydrolases activity in vitro.

Peroxisomal fatty acid oxidation as detected by H2O2 production in intact perfused rat liver

1. H2O2 formation associated with the metabolism of added fatty acids was quantitatively determined in isolated haemoglobin-free perfused rat liver (non-recirculating system) by two different methods. 2. Organ spectrophotometry of catalase Compound I [Sies & Chance (1970) FEBS Lett. 11, 172-176] was used to detect H2O2 formation (a) by steady-state titration with added hydrogen donor, methanol or (b) by comparison of fatty-acid responses with those of the calibration compound, urate. 3. In the use of the peroxidatic reaction of catalase, [14C]methanol was added as hydrogen donor at an optimal concentration of 1 mM in the presence of 0.2 mM-L-methionine, and 14CO2 production rates were determined. 4. Results obtained by the different methods were similar. 5. The yield of H2O2 formation, expressed as the rate of H2O2 formation in relation to the rate of fatty-acid supply, was less than 1.0 in all cases, indicating that, regardless of chain length, less than one acetyl unit was formed per mol of added fatty acid by the peroxisomal system. In particular, the standard substrate used with isolated peroxisomal preparations (C16:0 fatty acid) gave low yield (close to zero). Long-chain monounsaturated fatty acids exhibit a relatively high yield of H2O2 formation. 6. The hypolipidaemic agent bezafibrate led to slightly increased yields for most of the acids tested, but the yield with oleate was decreased to one-half the original yield. 7. It is concluded that in the intact isolated perfused rat liver the assayable capacity for peroxisomal beta-oxidation is used to only a minor degree. However, the observed rates of H2O2 production with fatty acids can account for a considerable share of the endogenous H2O2 production found in the intact animal.

The stimulation of glutamine hydrolysis in isolated rat liver mitochondria by Mg2+ depletion and hypo-osmotic incubation conditions

1. In respiring rat liver mitochondria EDTA stimulates glutaminase activity measured in the presence of phosphate and HCO3- ions. The stimulation can be reversed by the addition of low concentrations of MgCl2. EGTA does not stimulate glutamine hydrolysis. 2. Glutaminase activity assayed in disrupted mitochondria is not significantly affected by EDTA or MgCl2. 3. The addition of EDTA results in a decrease in the concentration of phosphate required for half-maximal glutaminase activity. 4. Depletion of mitochondrial Mg2+ by the addition of the ionophore A23187 also stimulates glutamine hydrolysis in both the presence and the absence of EDTA. The effect of the ionophore can be abolished by the addition of MgCl2. 5. Hypo-osmotic incubation conditions increase the rate of mitochondrial glutamine hydrolysis. The effect of hypo-osmoticity on glutaminase is much less when EDTA is present. 6. It is suggested that glutaminase is partially and indirectly inhibited by endogenous mitochondrial Mg2+ and that the inner membrane may play a role in the regulation of glutaminase activity.

Effects of low ammonia levels on NAD(P)H levels and glutamate secretion during calcium-dependent depolarization of CNS slices

Simultaneously with an evoked release of endogenous glutamate, redox changes in NAD(P)H levels occur as a response to electrical or chemical stimulation of the isolated CNS tissue. While electrical-field stimulation induces a transient increase in tissue NAD(P)H, KCl stimulation produces a decrease in NAD(P)H. One possible interpretation for this difference is that elevated KCl induced glutamate release from larger cell populations, including glia, while electrical stimulation might have a more neuro-specific action. In the present report, it is shown that the electrically or biochemically evoked release of endogenous glutamate is strongly inhibited by ammonium ions at 3--5 mM in the hippocampus and frontal cortex. At the same time, ammonium ions inverted the NAD(P)H response of the tissue during electrical stimulation, making both electrical and KCl depolarization induce a decrease in NAD(P)H. There was no effect of ammonium ions per se on the NAD(P)H levels, and to obtain the effects on both NAD(P)H and glutamate release, the tissue had to be exposed for 40--60 minutes to ammonium ions. The results are interpreted to indicate that ammonium ions influence regulatory control mechanisms in cell populations in the tissue slice that secretes glutamate in response to depolarization.