TRANSPORT OF <scp>l</scp>‐TRYPTOPHAN INTO SLICES OF RAT CEREBRAL CORTEX¹

Abstract

Abstract —Slices of rat cerebral cortex, when incubated aerobically at 37°C in Krebs‐Ringer‐phosphate solution (pH 7.0) containing 10 m m glucose and 1.0 m m l ‐tryptophan [1‐ 14 C], accumulated tryptophan. Within the first 15 min of incubation the ratio of the concentration of the amino acid in the tissue to that in the medium reached 3.5:1. Uptake of tryptophan was linear for the first 30 min and attained a maximum concentration ratio (tissue:medium) of 6.5:1 within 60 min. The transport mechanism became saturated at 1.0‐3.0 m m tryptophan. Entry of the amino acid into the cortical cells was thereafter directly proportional to its initial concentration in the medium. The tissue: medium ratio at 15 min decreased significantly under the following experimental conditions: (1) lowering the incubation temperature to 0°C; (2) incubating under N 2 ; (3) omitting glucose; (4) decreasing the Na + concentration below 50 m m ; (5) removing K + from the medium; or (6) adding 1.0 m m NaCN or 0.1 m m protoveratrine B to the medium. These results provided evidence that the accumulation of tryptophan against its concentration gradient was an active process. The effects of a number of amino acids on the uptake of tryptophan were studied: of these, l ‐phenylalanine, dl ‐ p ‐chlorophenylalanine, l ‐tyrosine, l ‐DOPA, the branched chain aliphatic amino acids ( l ‐leucine, l ‐isoleucine, l ‐valine) and l ‐glutamic acid were found to be the most potent inhibitors of tryptophan transport. Several tryptophan metabolites were tested; only l ‐kynurenine inhibited the uptake of tryptophan.