L-Aspartate transport into plasma membrane vesicles derived from rat brain synaptosomes

Aspartate uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on a Na+ gradient ([Na+] outside greater than [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 Km and Vmax 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.