Uptake of γ-aminobutyric acid and l-glutamic acid by synaptosomes from postmortem human cerebral cortex: multiple sites, sodium dependence and effect of tissue preparation

Abstract

The uptake of γ-aminobutyric acid (GABA) and l-glutamic acid by synaptosomes prepared from frozen postmortem human brain was shown to be affected via distinct high and low affinity sites. At approximately 17 h postmortem delay, the kinetic parameters for GABA uptake were: high affinity site, K m 7.1 ± 2.5 μM, V max 18.7 ± 4.8nmol·min −1 per 100 mg protein; low affinity site, K m 2 ± 1mM, V max 425 ± 250nmol·min −1 per 100 mg protein (means±S.E.M., n = 13). Kinetic parameters for l-glutamate uptake were: high affinity site, K m 7.5 ± 1.0 μM, V max 85 ± 8nmol·min −1 per 100 mg protein; low affinity site, K m 1.8 ± 1.2mM, V max 780 ± 175nmol·min −1 per 100 mg protein ( n = 11). A detailed kinetic analysis of high affinity GABA uptake was performed over a range of sodium ion concentrations. The results were consistent with a coupling ratio of one Na + ion to one GABA molecule; a similar results was found with rat brain synaptosomes. However, rat and human synaptosomes differed in the degree to which the substrate affinity of the high affinity GABA uptake site varied with decreasing Na + ion concentration. High affinity GABA uptake was markedly affected by the method used to freeze and divide the tissue, but did not vary greatly in different cortical regions. There was some decline of high affinity GABA uptake activity with postmortem delay, apparently due to a loss of sites rather than a change in site affinity.