TRANSPORT OF LEUCINE AND SODIUM IN CENTRAL NERVOUS TISSUE: STUDIES ON RETINA IN VITRO1

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Abstract
Unidirectional leucine fluxes were measured in isolated rabbit retina maintained under steady state conditions in medium resembling CSF but with leucine varied from 2 to 20,000 microM. At physiological leucine concentration (11 microM), 1/2 time for outward transport was 88 s and intracellular fluid was cleared of isotopically labelled leucine at 2.3 ml/g dry wt./min; 1/2 time for inward transport was 16 s and interstitial fluid was cleared at 7.5 ml/g dry wt./min. The rate of leucine influx corresponded quite well with its rate of disappearance from the intracellular fluid, over a wide range of concentrations. Exchange diffusion was demonstrated for transport in both directions. There was competition by other amino acids, but no interaction between Na+ and leucine transport could be demonstrated. Kinetic analysis indicated the presence of more than one transport system for leucine. There was an unexpected fall in the efflux coefficient, with reduction in leucine concentration at the lower end of the concentration range, for which an explanation is proposed. Under control conditions, 1/2 time for efflux of intracellular 24Na+ was about 0.9 min. With intracellular Na+ increased 4 fold, 1/2 time for efflux was slightly reduced. Problems encountered in measuring fluxes in organized tissue are discussed.