The acute effect of ammonium acetate on levels of amino acids in the intact and decorticated rat neostriatum

Preservation of cerebral blood flow responses to hypoxia and arterial pressure alterations in hyperammonemic rats

Acute hyperammonemia causes cerebral edema, elevated intracranial pressure and loss of cerebral blood flow (CBF) responsivity to CO2. Inhibition of glutamine synthetase prevents these abnormalities. If the loss of CO2 responsivity is secondary to the mechanical effects of edema, one would anticipate loss of responsivity to other physiological stimuli, such as hypoxia and changes in mean arterial blood pressure (MABP). To test this possibility, pentobarbital-anesthetized rats were subjected to either hypoxic hypoxia (PaO2 approximately 30 mm Hg), hemorrhagic hypotension (MABP approximately 70 and 50 mm Hg), or phenylephrine-induced hypertension (MABP approximately 125 and 145 mm Hg). CBF was measured with radiolabeled microspheres. Experimental groups received intravenous ammonium acetate (approximately 50 mumol min-1 kg-1) for 6 h to increase plasma ammonia to 500-600 microM. Control groups received sodium acetate plus HCl to prevent metabolic alkalosis. The increase in CBF during 10 min of hypoxia after 6 h of ammonium acetate infusion (84 +/- 19 to 259 +/- 52 ml min-1 100 g-1) was similar to that after sodium acetate infusion (105 +/- 20 to 265 +/- 76 ml min-1 100 g-1). Cortical glutamine concentration was elevated equivalently in hyperammonemic rats subjected to normoxia only or to 10 min of hypoxia. With severe hypotension, CBF was unchanged in both the ammonium (80 +/- 20 to 76 +/- 24 ml min-1 100 g-1) and the sodium (80 +/- 14 to 73 +/- 16 ml min-1 100 g-1) acetate groups. With moderate hypertension, CBF was unchanged. With the most severe hypertension, significant increases in CBF occurred in both groups, but there was no difference between groups. We conclude that hypoxic and autoregulatory responses are intact during acute hyperammonemia. The previously observed loss of CO2 responsivity is not the result of a generalized vasoparalysis to all physiological stimuli.

Flow injection determination of glutamate in human serum and rat brain samples with immobilized glutamate oxidase and glutamate dehydrogenase reactors

Two methods are proposed for the determination of regional concentrations of glutamate in the rat brain as well as in human serum. Glutamate oxidase was immobilized on non-porous glass beads and glutamate dehydrogenase was immobilized on glass derivatives. These supports were employed for the construction of Single Bead String Reactors and Packed Bed Reactors, respectively, which in turn were linked to Flow Injection Analysis systems with either photometric or fluorometric detection. Analytical working curves are linear in the range 1-200 mumol/l for packed bed reactors and 10-500 mmol/l for single bead string reactors. The samples were pretreated depending on their origin and the applied measuring system. Optimal dilution factors were established for the two techniques. Optimal dilution ratios were established and the influence of several added substances was investigated. Recovery and method comparison studies including high performance liquid chromatography verified the accuracy of the proposed methods. Results from within-day and between-day measurements gave relative standard deviations of 4.7 and 5.9% for serum samples and 2.5 and 4.0% for brain samples, respectively.

Ammonia added in vitro, but not moderate hyperammonemia in vivo, stimulates glutamate uptake and H(+)-ATPase activity in synaptic vesicles of the rat brain

The uptake of radiolabelled neurotransmitters: glutamate (GLU), GABA, and dopamine (DA) and the activity of the vacuolar type H(+)-pumping ATPase (H(+)-ATPase), were measured in crude synaptic vesicles treated in vitro with a neurotoxic (3 mM) dose of NH4+ (acetate or chloride), or isolated from rats with a moderate increase of brain ammonia (to approximately 0.6 mM) induced by i.p. administration of ammonium acetate (HA rats) or a hepatotoxin-thioacetamide (HE rats). In vitro treatment with ammonium salts increased the sodium-independent, chloride-dependent uptake of GLU but did not stimulate the uptake of GABA or DA. The in vitro treatment also stimulated the H(+)-ATPase activity. Since H(+)-ATPase generates the electrochemical gradient driving synaptic vesicular neurotransmitter transport, its stimulation by ammonia may have facilitated GLU uptake. However the GLU specificity of the effect must be related to other factors differentially affecting GLU uptake and the uptake of other neurotransmitters. Enhanced GLU accumulation in the synaptic vesicles may contribute to the increase of synaptic GLU exocytosis previously reported to accompany acute increases of brain ammonia to toxic levels. However, GLU uptake and H(+)-ATPase activity, but also the uptake of GABA and DA, were unchanged in synaptic vesicles prepared from rats with HA or HE. This indicates that changes in GLU and/or GABA release reported for moderate hyperammonemic conditions must be elicited by factors unrelated to the synaptic vesicular transport of the amino acids.

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.

Decreased glucose utilization in discrete brain regions of rat in thioacetamide-induced hepatic encephalopathy as measured with [3H]-deoxyglucose

To evaluate the possible contribution of bioenergetic failure in the particular brain regions to the pathomechanism of hepatic encephalopathy (HE), local cerebral metabolic rate for glucose (LCMRglue) was evaluated from [3H]-deoxyglucose uptake in frontal, visual and auditory cortex, striatum, cerebellum and medulla oblongata of rats with acute HE induced with a hepatotoxin--thioacetamide (TAA). HE caused a decrease of LCMRglue in all the regions studied. The strongest decrease (about 65%) was noted in hippocampus and cerebral cortex--the two regions rich in glutamatergic neurons. The results indicate a possible link between decreased energy metabolism and impaired excitatory, glutamatergic neurotransmission--the two factors whose contribution to HE has so far been implicated separately.

Activation of arginine metabolism to glutamate in rat brain synaptosomes in thioacetamide-induced hepatic encephalopathy: an adaptative response?

Crude (P2) synaptosomes derived from rats with acute hepatic encephalopathy (HE) induced with thioacetamide showed a slightly increased uptake of radiolabeled arginine (ARG) and a 2.5-fold enhanced conversion of newly taken-up ARG to both glutamate (GLU) and gamma-aminobutyric acid (GABA) as compared with control synaptosomes. Pulse treatment of the preloaded synaptosomes with a high potassium medium decreased their radioactive GLU and GABA content without affecting the content of the precursor ARG. This result, which was identical with control or HE preparations, appears to indicate that ARG contributes at least, in part, to the synthesis of neurotransmitter GLU or GABA. As measured in purified synaptosomal preparations, HE increased by about 50% the activities of arginase and ornithine-delta-aminotransferase--the two enzymes of the ARG to GLU shunt. It is postulated that increased conversion of ARG to GLU may compensate for excessive utilization of the latter amino acid as an ammonia trap during HE and, as such, may be considered as an adaptative response of the synaptic compartment to this pathological condition.

Penicillin-induced convulsions have preferential effects on transmitter glutamate pools in rat neostriatum

Convulsant doses of penicillin and elevated ambient pressure of 41 bar enhance the excitability of neurons. Their effects have been studied in neostriatal tissue with methods allowing differentiation between transmitter and metabolic glutamate pools. Levels of glutamate (Glu), glutamine (Gln), aspartate (Asp); gamma-aminobutyric acid and taurine were measured in the intact and decorticated neostriatum and parieto-occipital cortex of rats with a unilateral frontal cortex ablation. Intravenous infusion of penicillin at 1 bar decreased the neostriatal Glu content in the intact but not in the decorticated hemisphere. Pressure of 41 bar significantly decreased the level of Asp in the decorticated side only. Infusion of penicillin at 41 bar reduced the levels of Glu by 20.1% and Gln by 21.0% in the intact neostriatum only, whereas it decreased the Asp level in both sides as compared to control. The cortical Glu content was decreased only after infusion of penicillin at 41 bar. The results suggest that intravenous penicillin has a more pronounced effect on transmitter than on metabolic Glu pools in rat brain.

Effects of acute hyperammonemia on cerebral amino acid metabolism and pHi in vivo, measured by 1H and 31P nuclear magnetic resonance

The effects of an acute intravenous infusion of ammonium acetate on rat cerebral glutamate and glutamine concentrations, energy metabolism, and intracellular pH were measured in vivo with 1H and 31P nuclear magnetic resonance (NMR). The level of blood ammonia maintained by the infusion protocol used in this study (approximately 500 microM, arterial blood) did not cause significant changes in arterial PCO2, PO2, or pH. Cerebral glutamate levels fell to at least 80% of the preinfusion value, whereas glutamine concentrations increased 170% relative to the preinfusion controls. The fall in brain glutamate concentrations followed a time course similar to that of the rise of brain glutamine. There were no detectable changes in the content of phosphocreatine (PCr) or nucleoside triphosphates (NTP), within the brain regions contributing to the sensitive volume of the surface coil, during the ammonia infusion. Intracellular pH, estimated from the chemical shift of the inorganic phosphate resonance relative to the resonance of PCr in the 31P spectrum, was also unchanged during the period of hyperammonemia. 1H spectra, specifically edited to allow quantitation of the brain lactate content, indicated that lactate rose steadily during the ammonia infusion. Detectable increases in brain lactate levels were observed approximately 10 min after the start of the ammonia infusion and by 50 min of infusion had more than doubled. Spectra acquired from rats that received a control infusion of sodium acetate were not different from the spectra acquired prior to the infusion of either ammonium or sodium acetate.(ABSTRACT TRUNCATED AT 250 WORDS)