The metabolism of glutamate and aspartate in rat cerebral cortical slices

The fate of [14C]glutamate and [14C]aspartate in rat cerebral cortical slices was followed quantitatively by means of the previously described automatic scanning technique of paper radio-chromatograms, in the presence and absence of non-radiocative glucose, and in the case of [14C]glutamate, also of the following non-radioactive tricarboxylic cycle acids: pyruvate, citrate, α-oxoglutarate, malate, oxalo-acetate and succinate. Both amino-acids were readily oxidized to CO2, glutamate increasing, aspartate not affecting the rate of endogenous oxygen uptake of the tissue. In addition the following metabolites accumulated: (a) From glutamate: aspartate, glutamine and γ-aminobutyrate, in order of decreasmg amounts. (b) From aspartate: glutamate, malate and citrate, in order of decreasmg amounts, with traces of glutamine, γ aminobutyrate and lactate. The presence of non-radioactive glucose as co-substrate caused the following effects. 1. On glutamate-.The total production of 14CO2aswell as the oxygen uptake increased. This indicated that glucose oxidation had not replaced glutamate oxidation, but actually caused its stimulation. The conversion into glutamine was increased by a factor of 3, an effect similar to that caused by K+ ions. The conversion into aspartate was decreased by a factor of 4. The accumulation of intracellular glutamate was much enhanced. 2. On aspartate: both 14CO2 production and oxygen uptake were stimulated, indicating that, as in the case of glutamate, glucose oxidation not only had not replaced the oxidation of amino-acid, but stimulated it. The production of glutamate, glutamine, γ-aminobutyrate and lactate was considerably increased while that of malate and citrate was unaffected. The following effects on glutamate metabolism were noted in the presence of non-radioactive tricarboxylic cycle acids: None of the acids affected 14CO2 production except α-oxoglutarate which caused a decrease of over half. This decrease was shown to be due to a dilution of [14C]glutamate through equilibration with non-radioactive α-oxoglutarate; the latter acquired about half of the radioactivity of the [14C]glutarnate added during the incubation. Pyruvate, like glucose, decreased the conversion to aspartate and increased glutamine formation, the latter, however, to a much lower degree than glucose. It also enhanced the accumulation of intracellular glutamate. Citrate, succinate, malate and oxaloacetate, particularly the latter two co-substrates, repressed the formation of [14C]glutamine, but had no other effect on the pattern of distiibution of the metabolites formed from glutamate and on the permeability of this amino-acid into the cell.