Excitatory amino acid-stimulated uptake of 22Na+ in primary astrocyte cultures.
In this study we have found that L-glutamic acid, as well as being taken up by a Na+-dependent mechanism, will stimulate the uptake of 22Na+ by primary astrocyte cultures from rat brain in the presence of ouabain. By simultaneously measuring the uptake of 22Na+ and L-3H-glutamate a stoichiometry of 2-3 Na+ per glutamate was measured, implying electrogenic uptake. Increasing the medium K+ concentration to depolarize the cells inhibited L-3H-glutamate uptake, while calculations of the energetics of the observed L-3H-glutamate accumulation also supported an electrogenic mechanism of at least 2 Na+:1 glutamate. In contrast, kinetic analysis of the Na+ dependence of L-3H-glutamate uptake indicated a stoichiometry of Na+ to glutamate of 1:1, but further analysis showed that the stoichiometry cannot be resolved by purely kinetic studies. Studies with glutamate analogs, however, showed that kainic acid was a very effective stimulant of 22Na+ uptake, but 3H-kainic acid showed no Na+ -dependent uptake. Furthermore, while L-3H-glutamate uptake was very sensitive to lowered temperatures, glutamate-stimulated 22Na+ uptake was relatively insensitive. These results indicate that glutamate-stimulated uptake of 22Na+ in primary astrocytes cultures cannot be explained solely by cotransport of Na+ with glutamate, and they suggest that direct kainic acid-type receptor induced stimulation of Na+ uptake also occurs. Since both receptor and uptake effects involve transport of Na+, accurate measurements of the Na+ :glutamate stoichiometry for uptake can only be done using completely specific inhibitors of these 2 systems.
Kimelberg
H K
HK
Department of Biochemistry, Albany Medical College, Albany, New York 12208.
Pang
S
S
Treble
D H
DH
D000596:Amino Acids / Q000494:pharmacology* / Q000502:physiology; D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D055598:Chemical Phenomena; D002621:Chemistry; D005971:Glutamates / Q000378:metabolism / Q000494:pharmacology; D018698:Glutamic Acid; D007700:Kinetics; D008433:Mathematics; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D012979:Sodium Radioisotopes; D013696:Temperature; D014316:Tritium
D000596:Amino Acids; D005971:Glutamates; D012979:Sodium Radioisotopes; D014316:Tritium; D018698:Glutamic Acid; D012964:Sodium; D011188:Potassium