Effects of acute hyperammonemia in vivo on oxidative metabolism in nonsynaptic rat brain mitochondria

The effects of hyperammonemia induced in vivo by injecting rats with ammonium acetate on oxidative phosphorylation, malate-aspartate shuttle, some related enzyme activities and metabolite levels in brain mitochondria were studied ex vivo. Rats were found to be either ammonia-sensitive (showing convulsions) or ammonia-resistant (without convulsions) after intraperitoneal injection of ammonium acetate (7 mmol/kg). Ammonium acetate administration to ammonia-sensitive rats led to inhibition of State 3 rates of brain mitochondria utilizing pyruvate, glutamate, isocitrate, and succinate as substrates and to decreased respiratory control index. In brain mitochondria isolated from ammonia-resistant animals, the ammonia-induced effect on such State 3 rates was not observed. In brain mitochondria from hyperammonemic rats without convulsions, a small increase in the activity of malate dehydrogenase was observed; glutamate dehydrogenase, succinate dehydrogenase, and aspartate aminotransferase were not affected. In brain mitochondria from rats with ammonia-induced convulsions, the activities of malate dehydrogenase and succinate dehydrogenase were reduced significantly. Ammonium acetate injection to rats was associated with a 5-fold increase in the brain mitochondrial ammonium ion content and a decrease (ca. 50%) in brain mitochondrial glutamate and aspartate; brain mitochondrial malate and 2-oxoglutarate levels remained unchanged. The rate of the malate-aspartate shuttle in brain mitochondria of hyperammonemic rats was decreased by 20% as compared to corresponding rate in control rats. We conclude that acute administration of ammonium acetate induces serious disturbances in the electron-transport chain, interferences of the malate-aspartate shuttle, alterations of the levels of shuttle intermediates and inhibition of the activities of malate and succinate dehydrogenases in brain mitochondria.