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J. Biol. Chem., Vol. 268, Issue 20, 14594-14596, Jul, 1993

Electrogenic L-glutamate uptake in Xenopus laevis oocytes expressing a cloned rat brain L-glutamate/L-aspartate transporter (GLAST-1)

U Klockner, T Storck, M Conradt and W Stoffel
Institute of Biochemistry, Medical Faculty of the University of Cologne, Germany.

The transport of L-glutamate into Xenopus laevis oocytes expressing the cloned L-glutamate/L-aspartate transporter (GLAST-1) from rat brain was studied using the voltage clamp technique. At a holding potential of - 90 mV, a bath application of 100 microM L-glutamate induced an inward current (IGLAST) with an amplitude ranging from -5 to -30 nA. IGLAST did not require extracellular Ca2+, Mg2+, or Cl-, was larger at negative potentials, and did not reverse up to +80 mV. The current was dependent on external L-glutamate and Na+ with half-maximal amplitudes at 11 microM L-glutamate and 41 mM Na+. IGLAST saturated at 100 microM L-glutamate and 80 mM Na+. The Hill coefficient for Na+ and L-glutamate was 3.3 and 1.3, respectively, suggesting that 3 Na+ accompany the transport of 1 L-glutamate molecule. At low [Na+]o, IGLAST was enhanced by reducing [K+]o, an indication for the countertransport of K+. Reducing external pH from 7.4 to 6.0 did not change the amplitude of IGLAST. This argues against a glutamate/proton cotransport. The results provide evidence for GLAST-1 carrying out a high affinity, sodium- dependent L-glutamate transport with a proposed stoichiometry of 3 Na+, 1 L-glutamate-/1 K+.
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