Phenylalanine transport in rabbit small intestine.

1. The proposal that rabbit small intestine possesses a separate, sodium‐dependent carrier of phenylalanine has been examined by measurements of the unidirectional influx of amino acids across the brush‐border membrane of the intact epithelium of the rabbit small intestine. 2. We demonstrate that, like alanine, glycine and leucine, phenylalanine shares sodium‐dependent as well as sodium‐independent transport with lysine. 3. Using the distal ileum we applied the A (phenylalanine)‐B (leucine)‐C (alanine) test on the sodium‐dependent, lysine‐resistant transport of phenylalanine. For phenylalanine, K1/2 (concentration required for half‐maximal transport) was 3.1 +/‐ 0.2 mM (n = 7) and Ki (inhibitor constant) against leucine transport was 3.1 +/‐ 0.2 mM (n = 4). For leucine, K1/2 was 1.1 +/‐ 0.1 mM (n = 4) and Ki against transport of phenylalanine was 1.1 +/‐ 0.1 mM (n = 4). For alanine, K1/2 was 12.6 +/‐ 1.1 mM (n = 3), Ki against phenylalanine was 13.1 +/‐ 1.8 mM (n = 4) and Ki against leucine was 11.0 +/‐ 0.4 mM (n = 4). 4. Using the jejunum we applied the A (phenylalanine)‐B (alanine)‐C (methionine) test on the lysine‐resistant, sodium‐dependent transport of phenylalanine. For phenylalanine, K1/2 was 4.7 +/‐ 0.2 mM (n = 7) and Ki against alanine was 4.8 +/‐ 0.2 mM (n = 4). For alanine, K1/2 was 15.6 +/‐ 0.8 mM (n = 7) and Ki against phenylalanine was 18.1 +/‐ 0.9 mM (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)