FUNCTIONAL-PROPERTIES AND SUBSTRATE-SPECIFICITY OF THE CLONED L-GLUTAMATE L-ASPARTATE TRANSPORTER GLAST-1 FROM RAT-BRAIN EXPRESSED IN XENOPUS OOCYTES

The rat brain L-glutamate/L-aspartate transporter GLAST-1 is a member of a family of Na+-dependent high-affinity L-glutamate transporters pr oposed to be involved in the termination and modulation of excitatory neurotransmitter signals. Application of electrophysiological and radi otracer techniques on Xenopus oocytes expressing cloned GLAST-1 reveal ed that the apparent K-m value of the transporter for L-glutamate and Na+ ions did not depend on voltage while the maximal transport rate in creased with more negative potentials, indicative of a low-field acces s channel. The apparent K-m value of the transporter for L-glutamate d epends on the Na+ concentration, suggesting that substrate and ions ar e transported by GLAST-1 in a simultaneous manner. All of the L-glutam ate uptake blockers tested either were substrates or did not affect th e current induced by L-glutamate. The changes in the amplitude of the current induced by simultaneous application of two substrates can be i nterpreted by a competition for one binding site.