Effects of Ammonia and β‐Methylene‐dl‐Aspartate on the Oxidation of Glucose and Pyruvate by Neurons and Astrocytes in Primary Culture

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Abstract
Both ammonia and β-methylene-dl-aspartate (β-MA), an irreversible inhibitor of aspartate aminotransferase activity and thus of the malate-aspartate shuttle, were found previously to decrease oxidative metabolism in cerebral cortex slices. In the present work, the possibility that ammonia and β-MA affect energy metabolism by a common mechanism (i.e., via inhibition of the malate-aspartate shuttle) was investigated using primary cultures of neurons and astrocytes. Incubation of astrocytes for 30 min with 5 mMβ-MA resulted in a decreased production of 14CO2 from [U-14Clglucose, but did not affect 14CO2 production from [2–14C] pyruvate. Conversely, incubation of astrocytes with 3 mM ammonium chloride resulted in decreased 14CO2 production from [2–14C] pyruvate, but 14CO2 production from [U-14C] glucose was not significantly affected. Ammonium chloride had no significant effect on 14CO2 production from either [U-14C] glucose or [2–14]pyruvate by neurons. However, incubation of neurons with β-MA or β-MA plus ammonium chloride resulted in a 45% decrease of 14CO2 production from both [U-14C] glucose and [2–14C] pyruvate. A 2-h incubation of astrocytes with β-MA resulted in no change in ATP levels, but a 35% decrease in phosphocreatine. Similar treatment of neurons resulted in >50% decrease in ATP, but had little effect on phosphocreatine. β-MA also caused a decrease in glutamate and aspartate content of neurons, but not of astrocytes. The different metabolic responses of neurons and astrocytes towards β-MA were probably not due to a differential inhibition of aspartate aminotransferase which was inhibited by ∼45% in astrocytes and by ∼55% in neurons.