Efflux of L-glutamate by synaptic plasma membrane vesicles isolated from rat brain

Synaptic plasma membrane vesicles isolated from rat brain were loaded with L-glutamate either passively, by using a freeze-thaw technique, or by active transport. Subsequently the ion dependency of glutamate efflux from these vesicles was studied. With each of the types of loading similar results were obtained. Efflux requires the simultaneous presence of internal sodium ions and external potassium ions. The process is also stimulated by chloride ions, but either internal or external chloride ions cause stimulation. Addition of unlabeled L-glutamate stimulates efflux about 2-fold. It is concluded that efflux of L-glutamate is in many aspects symmetrical with its influx [Kanner, B. I., & Sharon, I. (1978) Biochemistry 17, 3949--3954]. It appears that in order for L-glutamate to interact with the transporter, sodium has to be present on the same side as L-glutamate whereas potassium has to be simultaneously present on the opposite site. The simplest way to account for these and the previous data is to postulate that the L-glutamate transporter catalyzes sodium and L-glutamate cotransport, while it simultaneously catalyzes antiport of potassium.