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.
D000612:4-Aminobutyrate Transaminase / Q000037:antagonists & inhibitors / Q000378:metabolism; D000625:Aminooxyacetic Acid / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D007700:Kinetics; D051381:Rats; D013574:Synaptosomes / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D000625:Aminooxyacetic Acid; D005680:gamma-Aminobutyric Acid; D000612:4-Aminobutyrate Transaminase