Role of exchange transport in the absorption of L-tryptophan in the small intestine of chicks.

1. In in vitro experiments with accumulating mucosal preparations (AMP) and everted intestinal sacs, as well as in in vivo experiments with isolated loops of the small intestine the stimulating effect of a number of amino acids on L-tryptophan uptake was investigated. 2. Under "switched off" active transport (anoxia, 2,4-DNF treatment, sodium ion replacement by lithium ions in the mucosal solution) an expressed stimulation of L-tryptophan transport was observed within the mucosa and across the wall of the small intestine in the presence of L-proline, glycine, L-alpha-alanine, L-histidine and L-lysine. 3. Preincubation of AMP in the solutions of glycine, L-alpha-alanine and L-lysine was characterized by a stimulation of L-tryptophan transport, and the increase of its concentration in tissue was accompanied by the exit of an equivalent amount of glycine from it. 4. These observations show the participation of exchange transport in the uptake of L-tryptophan in the small intestine of chicks. 5. The mechanism of exchange transport in chicks starts to function on the 25th day after hatching and its intensity depends on the character of amino acid-modifier participating in the process. 6. Maximum activity of the exchange transport of L-tryptophan is demonstrated in the middle ileum. 7. L-alpha-Alanine stimulates the absorption of L-tryptophan from the isolated intestinal loop proving the existence of an exchange transport mechanism in a living organism. 8. An increased intensity of exchange transport is observed when feeding chicks with diets deficient and enriched in tryptophan.