Biomedical subjects

E Kosenko

Publications and source records attributed to E Kosenko.

7 recordsLinked to original sources

PubMed1996

Glutamate induces a calcineurin-mediated dephosphorylation of Na+,K(+)-ATPase that results in its activation in cerebellar neurons in culture.

In primary cultures of cerebellar neurons glutamate neurotoxicity is mainly mediated by activation of the NMDA receptor, which allows the entry of Ca2+ and Na+ into the neuron. To maintain Na+ homeostasis, the excess Na+ entering through the ion channel should be removed by Na+,K(+)-ATPase. It is shown that incubation of primary cultured cerebellar neurons with glutamate resulted in activation of the Na+,K(+)-ATPase. The effect was rapid, peaking between 5 and 15 min (85% activation), and was maintained for at least 2 h. Glutamate-induced activation of Na+,K(+)-ATPase was dose dependent: It was appreciable (37%) at 0.1 microM and peaked (85%) at 100 microM. The increase in Na+,K(+)-ATPase activity by glutamate was prevented by MK-801, indicating that it is mediated by activation of the NMDA receptor. Activation of the ATPase was reversed by phorbol 12-myristate 13-acetate, an activator of protein kinase C, indicating that activation of Na+,K(+)-ATPase is due to decreased phosphorylation by protein kinase C. W-7 or cyclosporin, both inhibitors of calcineurin, prevented the activation of Na+,K(+)-ATPase by glutamate. These results suggest that activation of NMDA receptors leads to activation of calcineurin, which dephosphorylates an amino acid residue of the Na+,K(+)-ATPase that was previously phosphorylated by protein kinase C. This dephosphorylation leads to activation of Na+,K(+)-ATPase.

Animals↗

PubMed1995

Nitroarginine, an inhibitor of nitric oxide synthetase, attenuates ammonia toxicity and ammonia-induced alterations in brain metabolism.

We have proposed that acute ammonia toxicity is mediated by activation of the N-methyl-D-aspartate type of glutamate receptors. MK-801, a selective antagonist of these receptors, prevents death of animals induced by acute ammonia intoxication as well as ammonia-induced depletion of ATP. It seems therefore that, following activation of the N-methyl-D-aspartate receptors, the subsequent events in ammonia toxicity should be similar to those involved in glutamate neurotoxicity. As it has been shown that inhibitors of nitric oxide synthetase such as nitroarginine prevent glutamate toxicity, we have tested whether nitroarginine prevents ammonia toxicity and ammonia-induced alterations in brain energy and ammonia metabolites. It is shown that nitroarginine prevents partially (approximately 50%), but significantly death of mice induced by acute ammonia intoxication. Nitroarginine also prevents partially ammonia-induced depletion of brain ATP. It also prevents completely the rise in glucose and pyruvate and partially that in lactate. Injection of nitroarginine alone, in the absence of ammonia, induces a remarkable accumulation of glutamine and a decrease in glutamate. The results reported indicate that nitroarginine attenuates acute ammonia toxicity and ammonia-induced alterations in brain energy metabolites. The effects of MK-801 and of nitroarginine are different, suggesting that ammonia can induce nitric oxide synthetase by mechanisms other than activation of N-methyl-D-aspartate receptors.

Amino Acid Oxidoreductases↗

PubMed1994

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

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

Ammonia↗

PubMed1994

High ammonia levels decrease brain acetylcholinesterase activity both in vivo and in vitro.

We have tested the effect of ammonium injection on the activity of acetylcholinesterase in rat brain. Fifteen minutes after ip injection of 7 mmol/kg of ammonium acetate, the activity of acetylcholinesterase in brain was reduced significantly. The inhibitory effect varied in a wide range, with a maximum decrease of 60%, and was proportional to the concentration of ammonia reached in the brain. It is also shown that ammonium salts added in vitro to the assay mixture inhibit acetylcholinesterase in brain homogenates competitively. The Ki values for inhibition of the enzyme in vitro were 7.2 and 8.5 mM for ammonium acetate and ammonium chloride, respectively, when acetylcholinesterase was assayed in rat brain homogenates, and 7.6 and 8.3 mM when assayed in mice brain homogenates. These results suggest that at least part of the neurologic effects of ammonia could be mediated by an increase of acetylcholine as a consequence of the inhibition of acetylcholinesterase.

Acetates↗

PubMed1994

Brain ATP depletion induced by acute ammonia intoxication in rats is mediated by activation of the NMDA receptor and Na+,K(+)-ATPase.

Injection of large doses of ammonia into rats leads to depletion of brain ATP. However, the molecular mechanism leading to ATP depletion is not clear. The aim of the present work was to assess whether ammonium-induced depletion of ATP is mediated by activation of the NMDA receptor. It is shown that injection of MK-801, an antagonist of the NMDA receptor, prevented ammonia-induced ATP depletion but did not prevent changes in glutamine, glutamate, glycogen, glucose, and ketone bodies. Ammonia injection increased Na+,K(+)-ATPase activity by 76%. This increase was also prevented by previous injection of MK-801. The molecular mechanism leading to activation of the ATPase was further studied. Na+,K(+)-ATPase activity in samples from ammonia-injected rats was normalized by "in vitro" incubation with phorbol 12-myristate 13-acetate, an activator of protein kinase C. The results obtained suggest that ammonia-induced ATP depletion is mediated by activation of the NMDA receptor, which results in decreased protein kinase C-mediated phosphorylation of Na+,K(+)-ATPase and, therefore, increased activity of the ATPase and increased consumption of ATP.

Acetates↗

PubMed1993

Chronic hyperammonemia prevents changes in brain energy and ammonia metabolites induced by acute ammonium intoxication.

Acute ammonia toxicity has been attributed to the depletion of energy metabolite intermediates. Ingestion of an ammonium containing diet produces hyperammonemia and protects rats against acute ammonium intoxication. We have tested the effect of chronic hyperammonemia on the brain contents of energy and ammonia metabolite intermediates and on the effect on these contents of acute ammonia intoxication (i.p. injection of 7 mmol/kg of ammonium acetate). Chronic hyperammonemia was induced in rats by feeding them a diet containing 20% ammonium acetate. Control rat were fed the same diet without addition of ammonium acetate. It is shown that chronic hyperammonemia did not affect the content of most metabolites, the only remarkable changes are the increases of the contents of ammonia (46%), glutamine (81%), acetoacetate (31%) and of the mitochondrial NAD+/NADH ratio (32%) as well as the marked decrease of beta-hydroxybutyrate (by 86%). Chronic hyperammonemia prevents most changes in metabolites induced by acute ammonium intoxication (i.p. injection of 7 mmol/kg of ammonium acetate). In control rats it was a marked breakdown of glycogen and increased contents of glucose, lactate and pyruvate, with decreased cytosolic NAD+/NADH ratio and beta-hydroxybutyrate and ATP contents. These changes were nearly completely prevented in hyperammonemic rats. In controls, ammonia increased 12.8-fold while glutamate and aspartate decreased by approximately 40% and glutamine and alanine raised by 37% and 93%, respectively; in hyperammonemic rats ammonia increased 6.9-fold while glutamate, glutamine and alanine were not significantly affected. Also the mitochondrial NAD+/NADH ratio raised by 18-fold in controls and by 6-fold in hyperammonemic rats. These results indicate that chronic hyperammonemia markedly prevents the alterations of the contents of energy and ammonia metabolites induced by acute ammonium intoxication.

Ammonia↗

PubMed1991

Ammonium ingestion prevents depletion of hepatic energy metabolites induced by acute ammonium intoxication.

Ingestion of an ammonium containing diet produces hyperammonemia and protects rats against acute ammonium intoxication. Acute ammonium toxicity has been attributed to the depletion of energy metabolite intermediates. We show here that hyperammonemia affords considerable protection against depletion of hepatic energy metabolites evoked by ammonium acetate injection. In control rats there were marked decreases in the content of acetoacetate, beta-hydroxybutyrate, ATP, 2-oxoglutarate, lactate, and pyruvate while phosphoenolpyruvate increased markedly. In hyperammonemic rats beta-hydroxybutyrate, ATP, 2-oxoglutarate, and lactate were not significantly affected while pyruvate increased markedly and phosphoenolpyruvate slightly. These results suggest that in controls the activity of pyruvate kinase is inhibited after ammonium injection while in hyperammonemic rats it is not inhibited. The content of alanine (an inhibitor of pyruvate kinase) reached 2.8 mumol/g in controls and 1.6 mumol/g in hyperammonemic rats, 15 min after ammonium injection. This could explain the different effects of ammonium injection on control and hyperammonemic rats.

Acetates↗