KINETICS OF A HUMAN GLUTAMATE TRANSPORTER

Currents mediated by a glutamate transporter cloned from human motor c ortex were measured in Xenopus oocytes. In the absence of glutamate, v oltage jumps induced Na+-dependent capacitive currents that were block ed by kainate, a competitive transport antagonist. The pre-steady-stat e currents can be described by an ordered binding model in which a vol tage-dependent Na+ binding is followed by a voltage-independent kainat e binding. At -80 mV, two charges are translocated per molecule of glu tamate, with a cycling time of approximately 70 ms, which is significa ntly slower than the predicted time course of synaptically released gl utamate. The results suggest that glutamate diffusion and binding to t ransporters, rather than uptake, are likely to dominate the synaptic c oncentration decay kinetics.