Dynamic storage of glutamate in rat brain synaptic vesicles.

Storage of [3H]glutamate accumulated by highly purified synaptic vesicles from brain was characterized. [3H]Glutamate was lost with single exponential kinetics with a time constant of minutes after synaptic vesicles were diluted into medium that allowed uptake to continue but that contained unlabeled glutamate in place of [3H]glutamate. This [3H]glutamate efflux occurred at similar rates in media containing 50 and 500 microM glutamate, which suggests that it did not depend on the rate of glutamate transport and was independent of the external and internal glutamate concentrations. All efflux was blocked at 0 degrees C. These results imply that glutamate stored in synaptic vesicles turns over with a half-time of minutes, even during active uptake under physiological conditions.