A comparison of inhibition of steady state, new transport, and exchange fluxes of amino acids in brain slices.

Abstract

1. The steady state, net transport, and exchange flux at steady state of four amino acids (aminoisobutyric acid, leucine, lysine and d-glutamic acid) whose transport is primarily mediated by different transport systems in brain slices were compared. The effects of various metabolic inhibitors (1 mM cyanide, 10 μM ouabain) and of sub-optimal incubation conditions (sodium- and glucose-free mediums and incubations of 0°) were studied. 2. Steady-state amino acid levels were reduced to a “new” lower level by the above experimental conditions. The changes in steady state appear to be due primarily to changes in net transport. The maintenance of steady state required the expenditure of metabolic energy. 3. The inhibition of influx varied with the particular amino acid studied. This is consistent with differences in sensitivity to the experimental conditions of the transport systems involved. 4. The exit of amino acids was increased by the experimental conditions. The increased exit is not likely to be explained by an inhibition of re-uptake. 5. Exchange influx at steady state compared to influx from identical medium concentrations was significantly greater for lysine. The inhibition of exchange influx by the various experimental conditions was not always identical to that of influx. Exchange influx but not net transport of aminoisobutyric acid and lysine was sodium independent. 6. Exchange efflux was greater than exit for all four amino acids. The ratio of exchange efflux and exchange influx approximated a 1:1 exchange ratio. The inhibition of exchange efflux was similar to the inhibition of exchange influx; inhibition of re-uptake could not explain all the observed data. 7. Whether the differences between exchange flux and net transport were due to exchange diffusion representing different transport systems or to other factors such as variation of properties of the carriers or membrane changes induced by the experimental conditions can not be answered at this time.