Effects of sodium fluxes and ouabain on brain-slice tryptophan transport

Efflux of preloaded [3H]tryptophan from rat cerebral cortex slices has been monitored into superfusion media that were altered in their sodium content. Total replacement of sodium with choline greatly increased the release of tryptophan. This release could be cancelled by re-introducing sodium into the slices. A brief exposure to ouabain, an efficient inhibitor of Na+, K+-ATPase activity, only slightly increased tryptophan efflux at the concentration of 0.1 mM, whereas at 1.0 mM it produced a similar effect as the sodium-free medium. Accordingly, when the slices were superfused in the presence of ouabain and sodium, the change of medium to sodium-free caused much greater relative enhancement of tryptophan efflux with 0.1 than 1.0 mM ouabain. Tryptophan efflux was modified by changes in sodium fluxes also in slices initially depleted of sodium ions and treated with ouabain. The results suggest that the sodium-free medium and ouabain have a similar mechanism of action in modifying the tryptophan transport, and that the cation gradients across the cell membranes are more crucial for normal amino acid transport than the functional Na+, K+-ATPase.