Electroneutral Na+/dicarboxylic amino acid cotransport in rat intestinal brush border membrane vesicles.

ABSTRACT

L-Glutamate and L-aspartate transport into osmotically active intestinal brush border membrane vesicles is specifically increased by Na+ gradient (extravesicular greater than intravesicular) which in addition energizes the transient accumulation (overshoot) of the two amino acids against their concentration gradients. The "overshoot" is observed at minimal external Na+ concentration of 100 mM for L-glutamate and 60 mM for L-aspartate; saturation with respect to [Na+] was observed at a concentration near 100 mM for both amino acids. Increasing amino acid concentration, saturation of the uptake rate was observed for L-glutamate and L-aspartate in the concentration range between 1 and 2 mM. Experiments showing mutual inhibition and transtimulation of the two amino acids indicate that the same Na+ -dependent transport system is shared by the two acidic amino acids. The imposition of diffusion potentials across the membrane vesicles artificially induced by addition of valinomycin in the presence of a K+ gradient supports the conclusion that the cotransport Na+/dicarboxylic amino acid in rat brush border membrane vesicles is electroneutral.