L-Aspartate Transport into Plasma Membrane Vesicles Derived from Rat Brain Synaptosomes

Abstract: Aspartate uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on a Na⁺ gradient (Na⁺] outside > Na⁺] inside). Active transport of aspartate is strictly dependent upon the presence of sodium and maximal extent of transport is reached when both Na⁺ and Cl⁻ ions are present. The uptake is transport into an osmotically active space and not a binding artifact as indicated by the effect of increasing the medium osmolarity. The uptake of aspartate is stimulated by a membrane potential (negative inside), as demonstrated by the effect of the ionophore carbonyl cyanide m -chlorophenylhydrazone and anions with different permeabilities. The presence of ouabain, an inhibitor of (Na⁺+ K⁺)-ATPase, does not affect aspartate transport. The kinetic analysis shows that aspartate is accumulated by two systems with different affinities, showing K _m and V _max values of similar order to those found in slightly "cruder" preparations. Inhibition of the l -aspartate uptake by d -aspartate and d - and l -glutamate indicates that a common carrier is involved in the process, this being stereospecific for the d - and l -glutamate stereoisomers.