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Agricultural Research Council Institute of Animal Physiology, Babraham, Cambridge CB2 4AT

1. Mucosal amino acid uptake by pig proximal colon, measured independently for fourteen different amino acids each used at a concentration of 1 mM, ranged from 0·6 to 8·6 n-mole. cm^-2. min^-1 in the new-born to 0 to 0·3 n-mole. cm^-2. min^-1 in the 2-day-old animal. Long chain amino acids entered the mucosa of new-born pig proximal colon much more readily than did short chain amino acids.

2. Glycine was used extensively to inhibit the uptake of other neutral amino acids. The degree of maximal inhibition produced depended on the amino acid used. The relative inability of glycine to inhibit the uptake of long chain amino acids suggested that these compounds could cross the brush border on a carrier inaccessible to glycine. The glycine-sensitive uptake remained more or less constant for all amino acids tested (1-2 n-mole.cm^-2.min^-1); the glycine-insensitive uptake varied from 0 to 7 n-mole.cm^-2.min^-1 (glycine and methionine respectively).

3. It is suggested that at least two mechanisms exist for the entry of neutral amino acids into pig proximal colon, one showing specificity for hydrophobic amino acids and the other having broad specificity. The mechanism responsible for the uptake of long chain essential amino acids predominates over the less specific mechanism.

4. These results are discussed in relation to previous work carried out on the rabbit ileum where two similar systems for neutral amino acid entry have been shown to be present. Both tissues transport hydrophobic amino acids on their own specific carrier at approximately the same rate; the ability of the pig colon to transport amino acids on the broad specificity carrier is eight times less than in the rabbit ileum. The possibility is raised that this system is subject to regulation.

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