Relationships between ethylenediamine and GABA transport systems in rat brain slices

[ 14C]EDA was accumulated by slices of adult rat cerebral cortex, although the tissue:medium ratios achieved were very much lower than those for GABA. EDA uptake was temperature dependent and appeared to take place by both sodium dependent and sodium independent mechanisms. Kinetic analysis of the uptake revealed a major low affinity component with an apparent K m of 1.11 ± 0.05 mM and a V max of 9.8 ± 0.2 μmol/hg wet wt, with a second site of K m about 20 μM but a 50 fold lower V max. Inhibition studies indicate that EDA may be transported in part by the ‘small basic’ amino acid transport system and in part by polyamine systems shown to be present in CNS tissue. High levels of displaceable binding of radioactive EDA to glass-fibre filters were observed; studies using [ 14C]EDA may be complicated by binding to tissue macromolecules. Potassium stimulated, calcium dependent release of radioactivity from brain slices labelled with [ 14C]EDA in the presence of sodium ions was observed. Extracellular EDA stimulated the release of [ 3H]GABA and [ 3H]beta-alanine from preloaded slices, although GABA and beta-alanine did not stimulate [ 14C]EDA release. It appears that extracellular EDA can counterexchange with intracellular GABA or beta-alanine, but that EDA which is accumulated by the tissue may then be bound or move to pools not directly accessible to these amino acids. Ouabain released radioactivity from slices labelled by [ 14C]EDA in the presence of sodium but not from slices labelled in the absence of sodium. These results suggests that EDA is not acting simply as a substrate for GABA transport sites.