α‐Ketoglutarate and Malate Uptake and Metabolism by Synaptosomes: Further Evidence for an Astrocyte‐to‐Neuron Metabolic Shuttle

Publisher Website
Google Scholar
Abstract
This study was undertaken to provide further evidence relevant to the hypothesis that astrocytes supply one or more citric acid cycle intermediates to syn‐aptic terminals, thereby serving an anaplerotic function necessitated by the synthesis and release of amino acid neurotransmitters. In our experiments, two populations of synaptosomes obtained from the brain of rats were separated from myelin and mitochondria by using Percoll to generate continuous density gradients. Both synapto‐somal populations readily accumulated 14C‐labelled a‐ke‐toglutarate and L‐malate by high‐affinity transport systems. Hofstee plots of uptake velocity as a function of substrate concentration were highly nonlinear, indicating that uptake was mediated by two or more carriers, or was subject to negative cooperativity. At least one carrier was selective for a‐ketoglutarate and another for malate, whereas a third carrier appeared to be present which transported both substrates. At low concentrations (1 μM), γ‐ketoglutarate transport was almost totally Na + ‐dependent, whereas malate uptake exhibited little Na+‐dependency. The transport of a‐ketoglutarate was associated with a net influx, and therefore was not due to a homoexchange process. γ‐Ketoglutarate and malate were metabolized rapidly to glutamate and aspartate, respectively, by both synaptosomal preparations; however, in all cases, label accumulated in γ‐aminobutyric acid rather slowly. The incorporation of label into glutamine from γ‐ketoglutarate was much greater in the high‐density synaptosomes than in low‐density synaptosomes, an indication that the former contained a higher proportion of astrogliasomes. The results of our study are consistent with the concept that certain types of synaptic terminals are dependent upon astrocytes for a supply of a‐ketoglutarate and/or malate in order to maintain their pools of citrate cycle intermediates and replenish their neuro‐transmitter pools.