ALPHA-KETOISOCAPROATE ALTERS THE PRODUCTION OF BOTH LACTATE AND ASPARTATE FROM [U-C-13]GLUTAMATE IN ASTROCYTES - A C-13 NMR-STUDY

The present study determined the metabolic fate of [U-C-13]glutamate i n primary cultures of cerebral cortical astrocytes from rat brain and also in cultures incubated in the presence of 1 or 5 mM alpha-ketoisoc aproate (alpha-KIC). When astrocytes were incubated with 0.2 mM [U-C-1 3]glutamate, 64.1% of the C-13 metabolized was converted to glutamine, and the remainder was metabolized via the tricarboxylic acid (TCA) cy cle. The formation of [1,2,3-C-13(3)]glutamate demonstrated metabolism of the labeled glutamate via the TCA cycle. In control astrocytes, 8. 0% of the [C-13]glutamate metabolized was incorporated into intracellu lar aspartate, and 17.2% was incorporated into lactate that was releas ed into the medium. In contrast, there was no detectable incorporation of [C-13]glutamate into aspartate in astrocytes incubated in the pres ence of alpha-KIC. In addition, the intracellular aspartate concentrat ion was decreased 50% in these cells. However, there was increased inc orporation of [C-13]glutamate into the 1,2,3-C-13(3)-isotopomer of lac tate in cells incubated in the presence of alpha-KIC versus controls, with formation of lactate accounting for 34.8% of the glutamate metabo lized in astrocytes incubated in the presence of alpha-KIC. Altogether more of the [C-13] glutamate was metabolized via the TCA cycle, and l ess was converted to glutamine in astrocytes incubated in the presence of alpha-KIC than in control cells. Overall, the results demonstrate that the presence of alpha-KIC profoundly influences the metabolic dis position of glutamate by astrocytes and leads to altered concentration s of other metabolites, including aspartate, lactate, and leucine. The decrease in formation of aspartate from glutamate and in total concen tration of aspartate may impair the activity of the malate-aspartate s huttle and the ability of astrocytes to transfer reducing equivalents into the mitochondria and thus compromise overall energy metabolism in astrocytes.