On the significance of perfusion rate in the study of glutamate release from superfused synaptosomes

The effect of perfusion rate on the apparent release of [(3)H]glutamate from prelabelled and superfused rat cortical synaptosomes was examined. The proportion of tissue [(3)H]glutamate released in response to a 4 ml depolarizing pulse of 15 mM K+ increased almost linearly with perfusion rates from 1 ml min(-1) to 10 ml min(-1). Release did not increase markedly between 10 ml min(-1) and 20 ml min(-1). The basal efflux of [(3)H]glutamate also increased with perfusion rate. The increase in both basal efflux and (K+)-induced release is interpreted as being due to a greater amount of released transmitter avoiding recapture by uptake processes as perfusion rate increases. This is supported by the observation that increasing the potential number of uptake sites in the tissue decreases both the basal and (K+)-evoked release of the transmitter. The significance of this with respect to optimal perfusion rates for studies on the regulation of glutamate release is discussed.