Uptake and metabolism of malate in neurons and astrocytes in primary cultures

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Abstract
Uptake and oxidative metabolism of [14C]malate as well as its incorporation into aspartate, glutamate, glutamine, and GABA were studied in cultured cerebral cortical neurons (GABAergic), cerebellar granule neurons (glutamatergic), and cerebral cortical astrocytes. All cell types exhibited high affinity uptake of malate (Km 10–85 μM) with slightly higher Vmax values in neurons (0.1–0.2 nmol × min−1 × mg−1) than in astrocytes (0.06 nmol × min−1 × mg−1). Malate was oxidatively metabolized in all three cell types with nominal rates of 14CO2 production of 2–15 pmol × min−1 × mg−1. The oxidation of malate was only slightly inhibited by 5 mM aminooxyacetic acid (AOAA). In granule cell preparations [14C]malate was incorporated into aspartate and glutamate and, to a much less extent, into glutamine. This incorporation was blocked by 5 mM AOAA. Astrocytes exhibited slightly higher incorporation rates into aspartate and glutamate, but in these cells glutamine was labelled to a considerable extent. AOAA (5 mM) inhibited the incorporation by 60–70%. In cultures of cerebral cortical neurons, very low levels of radioactivity derived from [14C]malate were found in aspartate and glutamate, and GABA was not labelled at all. Glutamine had the same specific activity as glutamate, indicating that the low rates of incorporation of radioactivity into amino acids in this preparation is likely to exclusively represent metabolism of malate in the small population of astrocytes (5% of total cell number), contaminating the neuronal cultures. The findings suggest that exogenous malate to a quantitatively limited extent may serve as a precursor for transmitter glutamate in glutamaterigc neurons. Astrocytes are able to metabolize malate to glutamate and related amino acids, but oxidize little malate to CO2. © Wiley‐Liss, Inc.