B. Barbour et al., CHANGES IN NAD(P)H FLUORESCENCE AND MEMBRANE CURRENT PRODUCED BY GLUTAMATE UPTAKE INTO SALAMANDER MULLER CELLS, Journal of physiology, 466, 1993, pp. 573-597
1. Glutamate uptake into isolated, whole-cell patch-clamped glial cell
s was studied by monitoring the increase of cell fluorescence generate
d as glutamate and NAD(P) were converted into alpha-ketoglutarate and
NAD(P)H by glutamate dehydrogenase. The current generated by the gluta
mate uptake carrier was recorded simultaneously. 2. L-Glutamate evoked
an increase of cell fluorescence and an inward uptake current. L- and
D-aspartate generated an uptake current but no fluorescence response,
consistent with the amino acid specificity of glutamate dehydrogenase
. 3. In the absence of external sodium the glutamate-evoked fluorescen
ce response and uptake current were abolished, showing that there is n
o sodium-independent glutamate uptake across the cell membrane. 4. Var
ying the glutamate concentration altered both the fluorescence respons
e and the uptake current. The fluorescence response saturated at a low
er glutamate concentration than the uptake current, and depended in a
Michaelis-Menten fashion on the uptake current. 5. The fluorescence re
sponse and the uptake current were reduced by membrane depolarization,
and also by removal of intracellular potassium. 6. The dependence of
the fluorescence response on uptake current when membrane potential wa
s altered or intracellular potassium was removed was the same as that
seen when the external glutamate concentration was altered. 7. These f
luorescence studies show that glutamate uptake is inhibited by depolar
ization and by removal of intracellular potassium, consistent with the
conclusion of earlier work in which uptake was monitored solely as a
membrane current. The data are consistent with high-affinity electroge
nic sodium- and potassium-dependent glutamate uptake with fixed stoich
iometry being the only significant influx route for glutamate. Other p
ossible interpretations of the data are also discussed.