Expression of the recombinant human excitatory amino aid transporters, EAAT1 and EAAT2, in Xenopus laevis oocytes allows electrogenic transport to be studied under voltage clamp conditions. 2. We have investigated the transport of the pharmacological substrate, L-serine-O-sulphate transport by EAAT1 and EAAT2. The EC50 values for L-serine-O-sulphate transport by EAAT2 showed a steep voltage-dependence, increasing from 152+/-11 microM at - 100 mV to 1930+/-160 microM at 0 mV. In contrast to EAAT2, EC50 values for L-serine-O-sulphate transport by EAAT1 were relatively constant over the membrane potential range of - 100 mV to 0 mV. The EC50 values for L-glutamate and D-aspartate transport, by EAAT2, were also relatively constant over this membrane potential range. 3. Chloride ions modulated the voltage-dependent changes in EC50 values for transport by EAAT2. This effect was most apparent for L-serine-O-sulphate transport, and to a lesser extent for L-glutamate and not at all for D-aspartate transport by EAAT2. 4. Extracellular sodium and proton concentrations also modulated the voltage-dependence of L-serine-O-sulphate EC50 values for EAAT2. 5. We speculate that these different properties of L-serine-O-sulphate transport by EAAT2 compared to other substrates may be due to the much stronger acidity of the sulphate group of L-serine-O-sulphate compared to carboxyl groups of L-glutamate or D-aspartate. 6. These results highlight some of the differences in the way different glutamate transporter subtypes transport substrates. This may be used to understand further the transport process and develop subtype selective inhibitors of glutamate transport.
