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SODIUM‐DEPENDENT EFFLUX AND EXCHANGE OF GABA IN SYNAPTOSOMES
Author(s) -
Simon J. R.,
Martin D. L.,
Kroll M.
Publication year - 1974
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1974.tb10750.x
Subject(s) - efflux , chemistry , sodium , stimulation , incubation , biophysics , biochemistry , biology , endocrinology , organic chemistry
— The influx and efflux of [ 3 H]GABA were investigated in synaptosomes. Two efflux components were detected. The first, termed spontaneous efflux, was not affected by the external sodium chloride concentration. The second, termed GABA‐stimulated efflux, was observed when low levels of GABA were added to the incubation medium and was found to require external sodium chloride. The rate of spontaneous efflux at 0°C was about 37 per cent of the rate at 27°C but both GABA‐stimulated efflux and GABA influx were completely inhibited at 0°C. The stimulation of efflux by external GABA followed simple Michaelis–Menten kinetics with respect to external GABA. The concentration of external GABA required for half‐maximal stimulation was 4·9 ± 1·4 μ m and the V max for efflux was 1·0 ± 0·6 nmol. min ‐1 .mg ‐1 of protein. A similar stimulation of efflux was observed with GABA analogue l ‐2,4‐diamino‐butyric acid which is a competitive inhibitor of influx. The concentration of external l ‐2,4‐diaminobutyric acid required for half‐maximal stimulation of efflux was 51 ± 12 μ m and the V max for efflux was 0·8 ± 0·5 nmol.min ‐1 .mg ‐1 of protein. Since the sodium‐dependency, temperature sensitivity, and kinetic properties of the GABA‐stimulated efflux system were similar to the influx system, GABA‐stimulated efflux was attributed to carrier‐mediated exchange diffusion. Measurement of efflux and influx in the same preparation showed there was a net efflux when total fluxes were considered and that the exchange ratio (influx to GABA‐stimulated efflux) was 0·9 when carrier‐mediated fluxes were considered. The effect of the temperature of the fluid used to rinse synaptosomes collected on filters in influx experiments was investigated. There was no detectable difference in measured values of influx between samples rinsed with cold fluid (0°C) and warm fluid (27°C). The endogenous GABA content of synaptosomes was found to be 20·3 ± 2·5 nmol GABA per mg of protein. From this value, the cytoplasmic concentration of GABA in synaptosomes was estimated to be a maximum of 40 m m . About 5 per cent of total cerebral cortical GABA was found in the synaptosomal fraction.