PROPERTIES OF THE UPTAKE AND RELEASE OF GLUTAMIC ACID BY SYNAPTOSOMES FROM RAT CEREBRAL CORTEX

Abstract

Abstract– (1) The uptake and release of glutamic acid by guinea‐pig cerebral cortex slices and rat synaptosomal fractions were studied, comparing the naturally occurring l‐ and non‐natural d‐isomers. Negligible metabolism of d‐glutamic acid was observed in the slices. (2) Whereas in the cerebral slices the accumulation of glutamic acid was almost the same for the two isomers, d‐glutamic acid was accumulated into the synaptosomal fraction at a markedly lower rate than was the L‐isomer. (3) The uptake systems for d‐isomer into the slices and synaptosomal fraction were found to be of single component, in contrast with the two component systems, high and low affinity components, for the uptake of l‐glutamic acid. The apparent Km values for the uptake of d‐glutamic acid into the slices and synaptosomal fraction were comparable with those reported for the low affinity components for l‐isomer. The uptake systems for d‐glutamic acid were dependent on the presence of Na+ ions in the medium, like those for l‐glutamic acid and GABA. (4) The evoked release of radioactive preloaded d‐glutamic acid was observed both from the slices and synaptosomal fraction following stimulation by high K+ ions in the medium. From these observations, it is evident that the evoked release of an amino acid by depolarization in vitro is not necessarily accompanied by a high affinity uptake process. (5) The uptake of l‐glutamic acid, expecially into the synaptosomal fraction, was highly resistant to ouabain. On the other hand, the uptake rate of d‐glutamic acid and GABA into the synaptosomal fraction was inhibited by varying concentrations of ouabain in accordance with the inhibition for brain Na‐K ATPase. (6) The uptake of l‐glutamic acid into subfractions of the P2 fraction was studied in relation to the distribution of the ‘synaptosomal marker enzymes’. An attempt to correlate the activities of enzymes of glutamic acid metabolism with the uptake of l‐glutamic acid into the synaptosomal fraction from various parts of brain was unsuccessful. The high affinity uptake of l‐glutamic acid was found to be very active in the synaptosomal fraction from any part of brain examined.