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 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.

A High‐Affinity, Na+‐Dependent Uptake System for γ‐Hydroxybutyrate in Membrane Vesicles Prepared from Rat Brain

“…A computer analysis of the data using the mathematical treatment of Feldman (1972) (two sitedone ligand) indicates that the high-affinity system has a K,, of 34.3 p M and a V, , , , of 135 pmollmidmg protein. The value for the high-affinity constant is of the same order as those found for other neurotransmitters in the membrane vesicles, such as glycine, aspartate, glutamate, and GABA Marvizon et al, 1981;Kanner, 1978;Kanner and Sharon, 1978;Roskoski et al, 1981).…”

NMDA‐receptor antagonist requirements in conantokin‐G

“…The NMDA-receptor antagonist activity of each peptide was measured with the [ Q H]MK-801 binding assay. This NMDA receptor speci¢c open channel blocker has been used widely as a radioligand in binding assays to monitor the e¡ects of various modulators of the NMDA receptor [23]. Conantokins have been shown to be capable of inhibiting the spermine-enhanced [ Q H]MK-801 binding to rat brain membranes in a dose-dependent manner [8,16].…”

Studies on the Osmotic Disruption and Resealing of Synaptosomes

“…The production of resealed synaptosomes in which the internal milieu can be controlled and manipulated is a worthy aim, and its accomplishment in the erythrocyte has yielded much useful information (see, for example, Whittam and Ager, 1964;Garrahan and Glynn, 1967). However, this aim does not seem so easily realisable in the synaptosome preparation, even though several reports (Kanner and Sharon, 1978a,b;Aragon et al, 1981;Marviz6n et al, 1981; have appeared in which hypoosmotically ruptured synaptosome preparations have been used for transport studies. The conditions we have used encompass those described by Kanner (1978a,b); and, although it is clear that disruption occurs, its extent is highly variable at the lower range of osmotic strength, and a small but significant (about 10%) residue of undisrupted.particles always remains (Fig.…”