Abstract
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.
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