SOME DYNAMIC ASPECTS OF BRAIN METABOLISM IN RATS GIVEN A PORTOCAVAL ANASTOMOSIS

Limited capacity for ammonia removal by brain in chronic liver failure: potential role of nitric oxide

Chronic liver failure leads to hyperammonemia and consequently increased brain ammonia concentrations, resulting in hepatic encephalopathy. When the liver fails to regulate ammonia concentrations, the brain, devoid of a urea cycle, relies solely on the amidation of glutamate to glutamine through glutamine synthetase, to efficiently clear ammonia. Surprisingly, under hyperammonemic conditions, the brain is not capable of increasing its capacity to remove ammonia, which even decreases in some regions of the brain. This non-induction of glutamine synthetase in astrocytes could result from possible limiting substrates or cofactors for the enzyme, or an indirect effect of ammonia on glutamine synthetase expression. In addition, there is evidence that nitration of the enzyme resulting from exposure to nitric oxide could also be implicated. The present review summarizes these possible factors involved in limiting the increase in capacity of glutamine synthetase in brain, in chronic liver failure.

Neurobiology of ammonia

Hyperammonemia resulting from inherited urea cycle enzyme deficiencies or liver failure results in severe central nervous system dysfunction including brain edema, convulsions and coma. Neuropathologic evaluation in these disorders reveals characteristic alterations of astrocyte morphology ranging from cell swelling (acute hyperammonemia) to Alzheimer Type II astrocytosis (chronic hyperammonemia). Having no effective urea cycle, brain relies on glutamine synthesis for the removal of excess ammonia and the enzyme responsible, glutamine synthetase, has a predominantly astrocytic localization. Accumulation of ammonia in brain results in a redistribution of cerebral blood flow and metabolism from cortical to sub-cortical structures. In addition to changes in astrocyte morphology, increased brain ammonia concentrations result in alterations in expression of key astrocyte proteins including glial fibrillary acidic protein, glutamate and glycine transporters and "peripheral-type" (mitochondrial) benzodiazepine receptors. Such changes result in alterations of astrocytic volume and increased extracellular concentrations of excitatory and inhibitory substances. In addition, the ammonium ion has direct effects on excitatory-inhibitory transmission via distinct mechanisms involving cellular chloride extrusion and postsynaptic receptor function. Acute ammonia exposure leads to activation of NMDA receptors and their signal transduction pathways. Chronic hyperammonemia also results in increased concentrations of neuroactive L-tryptophan metabolites including serotonin and quinolinic acid. Therapy in hyperammonemic syndromes continues to rely on ammonia-lowering strategies via peripheral mechanisms (reduction of ammonia production in the gastrointestinal tract, increased ammonia removal by muscle).

Local uptake of (14)C-labeled acetate and butyrate in rat brain in vivo during spreading cortical depression

Spreading depression severely disrupts ion homeostasis, causes sensory neglect and motor impairment, and is associated with stroke and migraine. Glucose utilization (CMR(glc)) and lactate production rise during spreading depression, but the metabolic changes in different brain cell types are unknown. Uptake of (14)C-labeled compounds known to be preferentially metabolized by the glial tricarboxylic acid cycle was, therefore, examined during unilateral KCl-induced spreading cortical depression in conscious, normoxic rats. [(14)C]Metabolites derived from [(14)C]butyrate in K+ -treated tissue rose 21% compared to that of untreated contralateral control cortex, whereas incorporation of H(14)CO(3) into metabolites in K+ -treated tissue was reduced to 86% of control. Autoradiographic analysis showed that laminar labeling of cerebral cortex by both (14)C-labeled acetate and butyrate was elevated heterogeneously throughout cortex by an average of 23%; the increase was greatest (approximately 40%) in tissue adjacent to the K+ application site. Local uptake of acetate, butyrate, and deoxyglucose showed similar patterns, and monocarboxylic acid uptake was highest in the structures in which apparent loss of labeled metabolites of [6-(14)C]glucose was greatest. Enhancement of net uptake of acetate and butyrate in cerebral cortex during spreading depression is tentatively ascribed to increased astrocyte metabolism.

Changes in the brain monoamine metabolism in acute liver failure produced by ischemia-reperfusion injury in rats

OBJECTIVE

To investigate the relationship between behavioral alterations and changes in monoaminergic systems provoked by ischemia-reperfusion liver injury in rats.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University animal laboratory.

SUBJECTS

Male Wistar rats.

INTERVENTIONS

Acute liver failure was induced by occlusion of the left portal vein and the hepatic artery for 90 mins. Twenty animals were subjected to the behavioral examination. The brain water content was measured in 12 animals. Forty-two animals were used for the evaluation of brain monoamine turnover. Half of animals in each experiment were subjected to the ischemic operation.

MEASUREMENTS AND MAIN RESULTS

A step-through passive avoidance test was used for the behavioral evaluation 48 hrs after the ischemic operation. Then, plasma concentrations of amino acids were determined. The brain water content was measured with the dry weight method. The brain monoamine turnover was evaluated by the depletion of norepinephrine and dopamine induced by alpha-methyl-p-tyrosine, or the accumulation of 5-hydroxyindoleacetic acid induced by probenecid. In the plasma analysis performed 48 hrs after the operation, marked damage was found in animals subjected to liver ischemia. Injured rats demonstrated impairment in the passive avoidance test. The plasma concentrations of branch-chain amino acids were decreased, and the plasma concentrations of aromatic amino acids were increased. However, the brain water content was not changed by liver ischemia. The turnover of both norepinephrine in the cerebral cortex and dopamine in the striatum was decreased. The turnover of 5-hydroxytryptamine in the cerebral cortex was increased markedly.

CONCLUSION

In liver injury caused by liver ischemia, the excitatory neurotransmission by norepinephrine and dopamine is depressed and the inhibitory neurotransmission mediated by 5-hydroxytryptamine is facilitated, especially in the telencephalon.

Long-term changes in glial fibrillary acidic protein and glutamine synthetase immunoreactivities in the supraoptic nucleus of portacaval shunted rats

The present study was undertaken to ascertain whether, and to what extent, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) expressions in the supraoptic nucleus (SON) could be modulated after one month and six months of portacaval shunting (PCS) in rats. GFAP and GS immunoreactivities were significantly higher in PCS rats than in control rats at one and six months. The increased GFAP and GS immunoreactivities observed in the SON astrocytes were directly related to the duration of PCS. In PCS rats, the number and length of both GFAP and GS immunopositive astroglial processes increased not only in the hypothalamic nucleus but in the perinuclear zone, where glutamatergic pathways have been described, whereas GFAP and GS expressions decreased in the ventral glial lamina. Since GS is one of the glutamate metabolizing enzymes and the SON is one of the areas of glutamatergic activity, our results show that astrocytes respond differentially to glutamate toxicity. This suggests that overexpression of GFAP and GS immunoreactivities could be associated with glutamatergic neurotransmission disorders.

Brain glucose levels in portacaval-shunted rats with chronic, moderate hyperammonemia: implications for determination of local cerebral glucose utilization

Rates of glucose utilization (lCMRglc) in many structures of the brain of fed, portacaval-shunted rats, when assayed with the [14C]deoxyglucose (DG) method in our laboratory, were previously found to be unchanged (30 of 36 structures) or depressed (6 structures) during the first 4 weeks after shunting, but to rise progressively to higher than normal values in 25 of 36 structures from 4-12 weeks. In contrast, lCMRglc, when assayed with the [14C]glucose method in another laboratory, was depressed in most structures of brains of 4-8-week shunted rats that had relatively high brain ammonia levels. There was a possibility that the increases in lCMRglc obtained with the [14C]DG method may have been artifactual, due, in part, to a change in brain glucose content which could alter the value of the lumped constant of the DG method. Brain glucose levels of shunted rats were, therefore, assayed by both direct chemical measurement in freeze-blown samples and by determination of steady-state brain:plasma distribution ratios for [14C]methylglucose; the methylglucose distribution ratio varies as a function of plasma and tissue glucose contents. Within a week after shunting, ammonia levels in blood and brain rose to 0.25-0.30 mM and 0.35-0.70 mumol/g, respectively, and mean plasma glucose levels fell from 9-10 mM to 7.4-8.5 mM, and then remained nearly constant. Brains of fed-shunted rats had normal glycogen levels and stable but moderately reduced glucose contents between 1 and 12 weeks (i.e., 1.9-2.2 mumol/g). [14C]Methylglucose distribution ratios were essentially the same as those in controls in 22 brain structures at 2 and 8 weeks after shunting. Because brain glucose levels remained stable from 1 to 12 weeks after shunting, there is no evidence to support the hypothesis that the value of the lumped constant would have changed and caused an artifactual rise in lCMRglc.

Region-selective reductions in activities of glutamine synthetase in rat brain following portacaval anastomosis

Portacaval anastomosis in the rat results in liver atrophy, sustained hyperammonemia and mild encephalopathy. Previous studies have demonstrated region-selective alterations of glutamine and other ammonia-related amino acids in brain following portacaval anastomosis. Ammonia removal by brain relies on glutamine synthesis and the enzyme responsible, glutamine synthetase, has an almost exclusively astrocytic localization. Glutamine synthetase activities were measured using a radioenzymatic assay in homogenates of seven brain regions of rats four weeks after end-to-side portacaval anastomosis. Enzyme activities were significantly reduced in hippocampus (by 25%, p < 0.01), in cerebellum (by 29%, p < 0.01) and in cerebral cortex (by 14%, p < 0.05). Enzyme activities in other brain regions were within normal limits. Region-selective reductions of glutamine synthetase following portacaval anastomosis could result in disruption of neuron-glial metabolic interactions and in a deficit of glutamatergic synaptic regulation. Similar mechanisms could be implicated in the pathogenesis of hepatic encephalopathy accompanying chronic liver disease in humans.

Effect of portacaval anastomosis on glutamine synthetase activities in liver, brain, and skeletal muscle

Glutamine synthetase is responsible for the ATP-dependent amidation of glutamate to glutamine. In liver the enzyme is highly localized in perivenous hepatocytes; in brain the enzyme is localized in astrocytes. Portacaval anastomosis resulted in liver atrophy, hyperammonemia, and up to 90% loss of glutamine synthetase activity in liver homogenates. This effect, which appears to be irreversible, probably reflects the selective loss of perivenous hepatocytes following portacaval anastomosis. Glutamine synthetase activities in brain were unaffected by portacaval anastomosis of up to 12 weeks' duration. Enzyme activities in homogenates of skeletal muscle, on the other hand, were significantly increased at one and four weeks after shunt surgery. These effects were not the result of decreased food intake in shunted animals. These findings suggest fundamentally different regulatory mechanisms for glutamine synthetase in these tissues. Skeletal muscle may thus provide an important alternative site for ammonia detoxification after portal-systemic shunting.

Aspartate aminotransferase, malate dehydrogenase, and pyruvate carboxylase activities in rat cerebral synaptic and nonsynaptic mitochondria: effects of in vitro treatment with ammonia, hyperammonemia and hepatic encephalopathy

The effects of in vitro treatment with ammonium chloride, hepatic encephalopathy (HE) due to thioacetamide (TAA) induced liver failure and chronic hyperammonemia produced by i.p. administration of ammonium acetate on the activity of the two malate-aspartate shuttle enzymes: aspartate aminotransferase (AAT), malate dehydrogenase (MDH), and on the pyruvate carboxylase (PC) activity were examined in synaptic and nonsynaptic mitochondria from rat brain. With regard to the shuttle enzymes the response to ammonium ions in vitro (3mM NH4Cl) was observed in nonsynaptic mitochondria only, and was manifested by a 27% decrease of AAT activity and a 16% decrease in MDH activity. By contrast, both in vivo conditions primarily affected the synaptic mitochondrial enzymes: TAA-induced HE produced a 26% decrease of synaptic mitochondrial AAT and a 50% decrease of synaptic mitochondrial MDH. Hyperammonemia inhibited synaptic mitochondrial AAT by 30% and synaptic mitochondrial MDH by 45%. HE produced no effect at all in nonsynaptic mitochondria while hyperammonemia produced a 30% increase in the AAT activity, but no changes in MDH. All the experimental conditions affected the nonsynaptic mitochondria PC: ammonium chloride in vitro produced a 20% decrease, TAA-induced HE--a 30% decrease, whereas hyperammonemia inhibited the enzyme by 53%. The PC activity in synaptic mitochondria was very low (about 2% of that measured in nonsynaptic mitochondria), which is consistent with the primarily astrocytic localization of the enzyme.

Cerebrospinal fluid amino acids in relation to neurological status in experimental portal-systemic encephalopathy

Using an indwelling cisterna magna catheter technique, serial CSF samples were analyzed for amino acid content in rats at various stages of portal-systemic encephalopathy resulting from ammonium acetate administration following portacaval anastomosis. Anastomosis alone resulted in increased CSF concentrations of glutamine, tyrosine, phenylalanine, glutamate and alanine. GABA levels, on the other hand were not significantly changed. Onset of severe neurological symptoms following ammonium acetate administration resulted in selectively increased CSF alanine. Other amino acids were not further increased at severe stages of encephalopathy. Increased CSF alanine probably results from increased glutamine transamination in the brains of portacaval shunted rats.

Ammonia metabolism in normal and portacaval-shunted rats

Ammonia is generated from a large number of metabolically important reactions. Despite its central importance in whole body nitrogen homeostasis excess ammonia is neurotoxic and its concentration must be kept low. Ammonia generated in most extrahepatic tissues is detoxified by incorporation into glutamine (amide). This glutamine may be used in a number of biosynthetic reactions (e.g. in pyrimidine synthesis). Alternatively, as a means of maintaining nitrogen balance, glutamine may be released to the blood. Resting skeletal muscle is particularly important 1) as a "sink" for removal of blood ammonia, and 2) as a major source of circulating glutamine. However, during vigorous exercise skeletal muscle may become a net contributor of ammonia to the blood. A few tissues and cell types (e.g. lymphocytes, macrophages, enterocytes, colonocytes, thymocytes, fibroblasts, bone) and tumors exhibit marked rates of glutamine utilization. In the kidney, glutamine is an important source of urinary ammonia. Ammonia generated from 1) the breakdown of nitrogenous substances in the gut, and 2) from the use of glutamine as a metabolic fuel in the small intestine, is taken up by the liver wherein it is detoxified by conversion to urea and to a lesser extent, glutamine. Some portal vein glutamine acts as a source of urea nitrogen. Ultimately, however, most excess ammonia nitrogen is detoxified indirectly (via glutamine (blood)----glutamine (small intestine)----ammonia (portal vein) or directly in the liver as urea. Portal-systemic shunting of blood, as occurs in chronic cirrhosis of the liver or following the surgical construction of a portacaval shunt results in portal blood bypassing the normal ammonia detoxification machinery of the liver. Under this condition blood ammonia levels rise markedly, increasing the burden on extrahepatic tissues, such as skeletal muscle, brain, and kidney, in maintaining ammonia homeostasis. The most commonly employed animal model of human liver disease is the rat in which an end-to-side portacaval shunt (PCS) has been surgically constructed. Brain glutamine synthetase activity is not increased in PCS rats and in some areas of the brain there may even be a decrease in activity. The brain glutamine synthetase appears to be working at near maximal capacity. Thus, the PCS rats exhibit profound neurological dysfunction when administered ammonium salts in amounts easily tolerated by normal animals. Because of the limited capacity of brain to remove excess ammonia, a rational approach to the treatment of patients with liver disease should include a regimen directed toward lowering the associated hyperammonemia.

Regional differences in the capacity for ammonia removal by brain following portocaval anastomosis

Portocaval anastomosis (PCA) in the rat leads, within 4 weeks, to severe liver atrophy, sustained hyperammonemia, and increased brain ammonia. Because brain is not equipped with an effective urea cycle, removal of ammonia involves glutamine synthesis and PCA results in significantly increased brain glutamine. Glutamine synthetase activities, however, are decreased by 15% in cerebral cortex and are unchanged in brainstem of shunted rats. Administration of ammonium acetate to rats following PCA results in severe encephalopathy (loss of righting reflex and, ultimately, coma). Glutamine concentrations in brainstem of comatose rats are increased a further two-fold, whereas those of cerebral cortex are unchanged. Consequently, ammonia levels in cerebral cortex reach disproportionately high levels (of the order of 5 mM). These findings suggest a limitation in the capacity of cerebral cortex to remove additional blood-borne ammonia by glutamine formation following PCA. Such mechanisms may explain the hypersensitivity of rats with PCA and of patients with portal-systemic shunting to small increases of blood ammonia. Disproportionately high levels of brain ammonia in certain regions, such as cerebral cortex, may then result in alterations of inhibitory neurotransmission and, ultimately, loss of cellular (astrocytic) integrity.

Serotonin metabolism in the central nervous system following sepsis or portacaval shunt in the rat

Similar neurological disturbances and metabolic alterations have been observed in liver insufficiency and in bacterial sepsis. In both liver failure and sepsis an altered neurotransmitter profile in the central nervous system (CNS) has been implicated in the pathogenesis of encephalopathic symptoms. It has been suggested that equivalent disturbances in brain neurotransmitters, especially serotonin, play a role in the encephalopathy accompanying sepsis and liver failure. The objective of this study was to compare the CNS serotonin metabolism in rats with an end-to-side portacaval shunt (PCS) with that found in rats with 12 or 24 hr of intraabdominal sepsis. The metabolism of CNS serotonin was estimated after inhibition of two enzymes acting in the 5-hydroxyindole synthetic pathway (decarboxylase and monoamine oxidase). The 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined in different regions of the CNS, thereby permitting evaluation of the synthetic activity of the serotonin neurotransmitter system. As previously reported, a marked increase in CNS serotonin synthetic rate was noted following PCS. In contrast, and in contradistinction to several recent reports, no major changes in the CNS serotonin synthesis rate were present following 12 or 24 hr of sepsis. CNS levels of the serotonin metabolite 5-HIAA were elevated in both sepsis and PCS rats. These data indicate that sepsis and liver failure have different effects upon serotonin metabolism in the CNS and suggest that differing pathogenetic mechanisms may underlie the encephalopathy clinically associated with these conditions.

Cerebral aminoacids in portal-systemic encephalopathy: lack of evidence for altered gamma-aminobutyric acid (GABA) function

Construction of an end-to-side portocaval anastomosis in the rat resulted, 4 weeks later, in sustained hyperammonemia and two- to threefold increases in brain ammonia. Measurement of cerebral amino acids using a sensitive double-isotope dansyl microtechnique revealed substantial increases in the glutamine content of cerebral cortex and brain stem. Glutamate levels were found to be concomitantly reduced in both brain regions compared to those of sham-operated controls. The gamma-aminobutyric acid (GABA) content of cerebral cortex and brain stem was unaffected by portocaval shunting, as were activities of the GABA nerve-terminal marker enzyme glutamic acid decarboxylase (GAD). These findings suggest that impaired GABA function may not play a major role in the pathogenesis of hepatic encephalopathy associated with portocaval shunts. Preliminary evidence suggests that decreased cerebral glutamate may reflect its loss from the releasable (neurotransmitter) pool.

Regional blood-brain barrier transport of glucose after portacaval anastomosis

The regional influx of glucose across the blood-brain barrier was measured in rats 5 to 6 weeks after a portacaval anastomosis or sham operation. D-[14C]Glucose was infused intravenously for 15 sec while arterial blood was sampled continuously for measurement of plasma radioactivity and glucose concentration. Brain tissue radioactivity was measured by quantitative autoradiography. Glucose influx and plasma clearance (permeability times surface area; PS) were calculated from the net disintegrations per minute per gram in brain, the plasma radioactivity integral, and the plasma glucose concentration. In shunted rats influx was decreased by about 22% (in the brain as a whole) compared to that in controls. This decrease was almost entirely due to the decrease in plasma glucose concentrations (27%). The PS, normalized to take plasma concentrations into account, showed a slight decrease in most of the brain except the telencephalon. For the brain as a whole this decrease amounted to 11%. The regional PS and glucose utilization are known to be coupled and the relationship between these was the same in sham-operated and shunted rats. The decrease in PS observed in shunted rats was commensurate with their lower rates of glucose use; thus, the transport process of glucose from plasma to brain appeared to be unaffected by portacaval shunting.

Portacaval shunt in the rat: selective alterations in behavior and brain serotonin

Portacaval shunted (PCS) rats and sham-operated controls were investigated for spontaneous activity, exploration, somatosensory reactivity, swim latencies in a water maze, motor coordination, and passive avoidance 2 to 3 weeks after operation. The rats were subsequently decapitated and indole metabolism was investigated in different brain regions. The results showed that shunted rats were impaired in both open field tests (spontaneous activity and exploration) and in somatosensory reactivity (latency to respond, maximal response and integrated response). Results from motor coordination tasks and learning and memory tests (water maze and passive avoidance) did not demonstrate differences between the groups. There was an increased brain indolamine metabolism in PCS compared to sham-operated rats. No correlation between the behavioral impairment and the altered indolamine metabolism could be demonstrated with multiple correlation analysis.

Spontaneous motor activity increases after portacaval anastomosis in rats

Spontaneous motor activity was monitored in rats at various times, up to 14 weeks, after portacaval anastomosis (PCA) or sham operation. Total 24 hour activity scores rose significantly after PCA and remained significantly higher than those of sham operated rats for twelve weeks. After PCA, activity during the twelve hour lighted period was greatly increased, whereas dark period activity was unchanged. Increased activity in the light period was found to be well correlated with the degree of elevation in brain concentrations of tryptophan, tyrosine, phenylalanine and glutamine. These results are consistent with previously reported EEG studies and suggest that motor-activity measurement may facilitate the correlation of biochemical changes with behaviour in rats with portal systemic shunting.

Metabolism of acetate to amino acids in brains of rats after complete hepatectomy

The concentration of glutamine increases in the brain after hepatectomy. In the present studies the conversion of intravenously given [14C]acetate to [14C]glutamate and [14C]glutamine was studied in control rats and in rats at 6 h after complete hepatectomy. The incorporation of label into glutamate was only slightly inhibited, but the further incorporation into glutamine was greatly inhibited, after hepatectomy. These data, and previous data using [14C]glucose as precursor, indicate that synthesis of glutamine in brain is inhibited after hepatectomy, and suggest that its concentration must increase because degradation is inhibited to an even greater extent.

Decreased startle reactivity in the end-to-side portacaval shunted rat

A rat with an end-to-side portacaval anastomosis (PCA) is an extensively used experimental animal for characterization of biochemical alterations following diversion of portal blood flow from the liver; but few behavioral abnormalities have been detected. Biochemical changes observed in rats after PCA include increased plasma and brain levels of the serotonin precursor tryptophan, with increased brain serotonin and its metabolite 5-hydroxyindoleacetic acid. Accumulating evidence indicates that serotonin may be involved in the modulation of the startle response in the rat. In this study, the magnitude of startle responses to both tactile and auditory stimuli were shown to be abnormally decreased in chronic PCA rats. The simplicity of this animal model and the reproducibility of the effect suggest that evaluation of startle responses in rats with PCA may enable assessments of the efficacy of putative therapeutic strategies for the treatment of some of the dysfunctions associated with hepatic encephalopathy.

Rapid steady-state analysis of blood-brain glucose transfer in rat

A new kinetic analysis of blood-brain glucose transport is described, based on a steady-state model that takes account of cerebral blood flow, mean capillary glucose concentration, and cerebral metabolic rate. The maximal rate (Tmax) and half-saturation constant (Km) of glucose transport from blood to brain were determined in rats by measuring the rate of blood-to-brain glucose transfer at different blood glucose concentrations. Each determination lasted 20 seconds. For whole-brain, Tmax and Km averaged 258 +/- 33 (S.E.) mumol L(100 g)-1 min-1 and 5.9 +/- 1.6 (S.E.) mmol 1-1, respectively. The regional variations were insignificant. The new approach permits kinetic parameters to be measured locally in brain in rapidly changing functional states.

Morphometric studies of rat glial cell ultrastructure after urease-induced hyperammonaemia

The ultrastructure of astrocytes and oligodendrocytes was investigated in hyperammonaemic rats injected daily with urease for 4 days. Glial cells were randomly photographed and magnified x28 000. Cell and nuclear sizes were estimated by planimetry and mitochondrial size and density were measured by image analysis. After 4 days of hyperammonaemia the astrocyte cytoplasmic area was increased by 46%. Mitochondrial area was increased by 20%, but after correction for cytoplasmic oedema the number and size of mitochondria were not significantly increased. The nuclear and cytoplasmic areas of oligodendrocytes were unchanged. The mitochondria of oligodendrocytes were small in the hyperammonaemic group and so was their percentage area to cytoplasmic area, but their numbers were unchanged. It was concluded that hyperammonaemia induces astrocyte oedema and increases the astrocyte mitochondrial content. These findings support the assumption that the astrocytes are the active cells in the brain metabolism of ammonia. The decrease in oligodendrocyte mitochondrial content could be considered a point against an active function of oligodendrocyte mitochondria in ammonia metabolism in hyperammonaemia.

Brain metabolism and specific transport at the blood-brain barrier after portocaval anastomosis in the rat

The rapid metabolism of [1-14C]butyrate and [2-14C]pyruvate in the brain was studied after intracarotid injection in control rats and rats given an end-to-side portocaval anastomosis (PCA). At 10 or 50 sec after injection there was a 40-50% lowering of the total amount of radioactivity in the brain of operated rats for both compounds. The percentage distribution of label in various metabolic fractions, e.g. amino acids, was unaltered by PCA. The results from the metabolic studies focused on an impaired transport of short chain monocarboxylic acids between blood and brain in rats with a PCA. The brain uptake index (BUI) of acetate, butyrate, pyruvate and glucose was determined by rapid intracarotid injection of the 14C test compound plus 3H2O as a reference marker. At 3 weeks after PCA, transport of all 3 monocarboxylic acids into brain was reduced by 40-50%. The reduction in [2-14C]pyruvate uptake was absent at one week, but thereafter, up to 50 weeks after operation, was consistently present. Eadie-Hofstee analysis of influx at varying substrate concentrations indicated a reduction in Vmax values of pyruvate and butyrate uptake without changes in Km. The calculated influx rate of glucose in operated rats was reduced in direct proportion to the lowering of plasma glucose. Portocaval anastomosis in the rat induces selective changes on substrates that are transported across the blood-brain barrier via a facilitated transport process.

Cerebral amino acid levels and uptake in rats after portocaval anastomosis: I. Regional studies in vitro

Uptake of various amino acids was studied in slices from brain regions of rats four weeks after portocaval anastomosis. No differences of the inulin compartment were observed between control and experimental animals. After 60-minutes incubation, uptake showed an overall pattern of diminution. This was more evident for some amino acids: valine, methionine, and lysine exhibited a lowering of about 30%, which was fairly uniform in the four tested regions; others showed a regional decrease-- alanine in pons-medulla, phenylalanine in cerebellum, histidine and GABA in mesodiencephalon. This decrease did not seen to be related to transport classes. The restricted entry of amino acids into brain cells in portocaval encephalopathy is somewhat difficult to explain; a decreased rate of protein synthesis may be of some importance, but other factors, such as a "carrier" impairment, effects on release and on amino acid metabolism, may also be involved.

Capillary size and density in the cerebral cortex of rats with a porto-caval anastomosis

Morphometric investigation by automatic image analysis of alkaline phosphatase positive brain capillaries was performed in control rats and in rats with a 30 days old porto-caval anastomosis. Rats with a porto-caval anastomosis showed an increased capillary diameter Di and as a consequence of this a decreased capillary surface-to-volume ratio Si/Vi. The free capillary distance was increased and the capillary length per unit brain volume LVi was decreased in the shunted rats compared to the controls. There was no difference in the percentage of the projected capillary area AAi% between the two groups. It is suggested that the dilatation of the capillaries in rats with a porto-caval anastomosis is caused by the altered haemodynamic properteries with a low mean blood pressure although the cardiac output is increased and that the alteration in free capillary distance and length per unit brain volume LVi is caused by dispersion by cortical edema induced by the porto-caval anastomosis.

Glial changes in pigs with porto-caval anastomosis and temporary or total hepatic artery clamping

The number and size of astrocyte, oligodendrocyte, and neurone nuclie were determined in cortex and corpus striatum (putamen) of pigs with porto-caval anastomosis (PCA) and total or temporary clamping of the hepatic artery. Animals with PCA and total clamping became comatose and died on average 18 1/2 hours postoperatively. Their brains showed slight oedema but no changes in the glial and neurone nuclear numbers. Animals with PCA and temporary clamping of the hepatic artery (30-60 min.) all survived without clinical symptoms of encephalopathy. However, the brains showed vacuolization of the deeper cortical layers, diffuse occurrence of Alzheimer type II astrocytes, and the counts revealed an increased number of astrocytic nuclei. The total number of glial cells decreased due to loss of oligodendrocytes, but the number of neurones was unchanged. The findings indicate that the neuropathological changes after PCA and temporary hepatic artery clamping are concerned primarily with changes of the glial cells.

Glial and nerve cell changes in rats with porto-caval anastomosis

Nuclear size and density were determined in brain regions with different glial--neurone composition in rats up to 35 weeks after porto-caval anastomosis. In the white matter, i.e. corpus callosum, both the total cell count and the percentage of astrocytes and oligodendrocytes were unchanged. In the corpus striatum, where the glial/neurone ratio is about 1, the number of nuclei registered as astrocytes increased, and after 35 weeks astrocytes comprised 29% of glial cells (compared with 15% in controls). However, the number of oligodendrogial nuclei decreased simultaneously, leaving the total glial number unchanged. In the animals with longest experimental period there was a 15% loss of neurones. In a region with higher glial/neurone ratio, i.e. the Purkinje cell layer, the neurones showed a similar reduction, whereas the number of Bergmann astrocyte nuclei increased less than striatal astrocytes. A small group of animals with pronounced signs of encephalopathy had a higher loss of neurones and, furthermore, the glial number in corpus striatum and callosum was reduced, due to loss of oligodendrocytes. Despite the use of perfusion fixation, the size of astrocyte nuclei increased, this was reversible, as only slight changes were seen after 35 weeks. A possible explanation of the increase in astrocyte nuclear count and decrease in oligodendroglial count could be that nuclei normally considered to be oligodendroglial are transformed into nuclei with morphological characteristics of astrocytes.

Glial cell reactions in rats with hyperammoniemia induced by urease or porto-caval anastomosis

The density and size of astrocyte, oligodendrocyte and neurone nuclei were determined in the corpus striatum of rats with urease-induced hyperammoniemia (plasma NH4+-concentration about 800 micronmol/1). No changes in the number of neurone nuclei were found. After 4 days the density of astrocyte nuclei increased from 13 to 23 per cent of the glial nuclei. However, the number of oligodendroxyte nuclei decreased correspondingly and the total glial nuclear number remained constant. The number of astrocyte nuclei was normalized 1 week after the 4-day period of hyperammoniemia. Moreover, intracerebral injections of colchicine at different times of the experimental period revealed no mitoses, indicating that no astrocyte divisions took place in pure hyperammoniemia. The astrocyte nuclei were of normal size in the urease animals in contrast to the animals with porto-caval anastomosis (PCA) which showed enlarged astrocyte nuclei. Thus, hyperammoniemia caused a reversible transformation of glial nuclei, but no real proliferation. A comparison of the glial reactions 4 days after a brain lesion showed the same frequency of astrocyte mitoses in control and urease animlas but a higher incidence of mitoses in the PCA animals. The number of Alzheimer type I astrocytes was the same in control and PCA animals, whereas no such cells were found in the urease animals, indicating that this form of hyperammoniemia did not lead to arrest of astrocyte metaphases with subsequent formation of Alzheimer type 1 cells.

Number of Purkinje cells and Bergmann astrocytes in rats with CCl4--induced liver disease

The nuclei of Purkinje cells and Bergmann astrocytes were counted on sagittal sections from cerebellum and the length of stratum gangliosum was measured in rats with CCl4-induced liver disease, using an electronic image analyzer. After 8 weeks of CCl4-administration a reduction was found in the number of Purkinje cells, many of which showed homogenizating changes. Ten weeks after termination of the administration period the number of Purkinje cells was reduced by 12 percent. The number of Bergmann astrocytes remained significantly increased after 8 weeks of CCl4-administration (max. 20 per cent). The changes of Purkinje cell and Bergmann astrocyte density developed during the period of severe liver necrosis, whereas only minor changes were found in the ensuing period of liver "cirrhosis". In the perfusion fixed specimens, the Bergmann astrocyte nuclei increased in volume up to 65 per cent and immersion fixed brains showed typical Alzheimer type II nuclear changes. The impact of the increased plasma ammonia concentration on the astrocytes is discussed.

On glutaminase activity in mammalian synaptosomes

A large proportion (30%) of tissue glutaminase activity was found localized in synaptosome fractions as well as purified mitochondrial fractions, where it is also enriched on a protein basis (2-fold). Sulphate was more effective (2-8-fold) than phosphate or chloride in activating the enzyme at concentrations above 10-12 mM, but equivalent to phosphate at lower concentrations (1-3 mM). At CSF levels (0.5 mM), glutamine is readily used by synaptosomes as a substrate in the presence of 10 mM glucose. It constitutes at least 50% to the carbon of aspartate, glutamate and GABA. Pool sizes of these amino acids are maintained in the presence of glutamine and glucose at CSF levels, but not by glucose alone. Ammonium ion at 1 mM substantially (30%) inhibits glutaminase utilisation by synaptosomes. It is suggested that CSF glutamine is a major substrate for the nerve ending in situ.