Body and intestinal growth of broiler chicks on a commercial starter diet.3. Development and characteristics of tryptophan transport

1. A study was conducted to characterise the development of amino acid tran sport in broiler chicks, using L-tryptophan as a model. The chicks were mai ntained on a broiler starter diet between hatch and 21 d of age. 2. There was a significant reduction in the rate of uptake of 0.04 mM L-try ptophan with age in both the jejunum and ileum. Uptake was enhanced in the presence of 50 mM sodium chloride to different degrees depending on age and intestinal site. At both intestinal sites, uptake capacity increased with age while there was a reduction in uptake efficiency with age. 3. At a concentration of 25 mM, both sodium chloride and potasium chloride increased uptake by ileal brush-border membrane vesicles (BBMV) of 7-d-old chicks but uptake was reduced when potassium chloride was included at a con centration of 50 mM. In the presence of valinomycin, uptake by jejunal BBMV was stimulated by 25 mM sodium chloride. In the presence of both sodium ch loride and potassium chloride and in the absence of valinomycin, uptake was increased by 42.6% but this was reduced to 23.4% when the ionophore was in cluded in the buffer. 4. The Na+-independent uptake of L-tryptophan into jejunal vesicles of 21-d -old chicks was lower in the presence of D-tryptophan than in the presence of 2-aminobicyclo-{2,2,1}-heptane-2-carboxylic acid ( BCH). The inclusion o f BCH in the incubation medium at low concentrations significantly enhanced the uptake of 0.04 mM L-tryptophan by jejunal BBMV. 5. At similar concentration (0.04 mM) to L-tryptophan, lysine, methionine a nd alanine in the presence of Na+ also stimulated L-tryptophan uptake. The uptake of L-tryptophan was reduced at a higher concentration, 25 mM, of the se amino acids. 6. The study revealed a decline in rate of amino acid uptake and an increas e in total uptake capacity with age. Tryptophan uptake was both Na+-indepen dent and dependent, and occurred more in the ileum than in the jejunum. The uptake of L-tryptophan depended on the concentration of other amino acids and other factors in the diet and digesta.