Postnatal development of carrier-mediated absorption of a foreign amino acid from the rat lung.

To investigate the postnatal development of the carrier-mediated transport process by which certain amino acids are absorbed from the air space of the lung into the blood-stream, a solution of the 14C-labeled nonmetabolized amino acid cycloleucine was administered into the lungs of anesthetized rats of various ages and the rate of absorption of the compound measured. Carrier transport was barely detectable in animals 3-6 days of age; it increased markedly by th age of 12 days, and it reached nearly mature levels by 18 days of age. The rate of development of this carrier process was considerably slower than that previously reported for the organic anion carrier system of the rat lung.