Changes in amino acid influx with Na flow in incubated slices of mouse brain.

The uptake of amino acids into slices of adult and newborn mouse brain was studied in relation to Na+ flow. (1) The level of Na+ and K+ incubated slices of brain depends on the ionic composition of the incubation medium. The intracellular levels of Na+ in adult tissue are below, in fetal tissue above, Na+ levels in the medium used. Rapid net flow into or out of the tissue can be achieved by transferring slices into media of higher or lower Na+ content. (2) Under conditions of net Na+ inflow, the influx of all amino acids tested increased; under conditions of net Na+ outflow, the influx of all amino acids decreased, as compared to slices in ionic equilibrium. The absolute levels of Na+ in the tissue under the experimental conditions had little effect on amino acid uptake. The stimulatory effect of Na+ inflow and the inhibitory effect of Na+ outflow could be observed at all developmental stages--in adult, newborn, and fetal tissue. (3) We conclude that ion movements influence metabolite transport; these effects are smaller in the absence of fully developed ion pumps. The direction of the net ion flow does not seem to be the main determinant: active accumulation occurred both in adult and in fetal brain, although the tissue-to-medium Na+ gradients were in opposite directions; in addition, amino acid inflow occurred in the presence of Na+ outflow.