The use of 3H-gamma-aminobutyric acid for transport studies with isolated nerve-terminals from rat brain.

Isolated synaptosomes were used to study the problem of net accumulation of neurotransmitters. The time-course and the kinetics of exogenous and endogenous GABA transport were studied by liquid-scintillation counting and HPLC-amino acid analysis respectively. Different pools of GABA were suggested by a 6-fold difference in tissue-to-medium-ratio of endogenous vs. exogenous GABA. Net accumulation, exchange and net efflux of GABA was found to be a function of the GABA concentration in the incubation medium. The Kms for net accumulation and for 3H-GABA accumulation were 2.68 +/- 1.16 and 6.19 +/- 1.26 microM respectively, whereas the Vmaxs were 5.9 +/- 4.9 and 134 +/- 13 pmol/mg w.w. min respectively. This means that the transport studies which use exogenous substances (e.g. 3H-GABA) considerably overestimate the transport by overlooking the magnitude of the counter transport.