Factors affecting the movement of amino acids and small peptides across the vascularly perfused Anuran small intestine.

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Abstract
In the vascularly perfused frog small intestine, the characteristics of the washout into the vascular bed of L-leucine and glycine were investigated following loading either as free amino acids or as the dipeptide glycyl-L-leucine. In both cases, the washout of the 2 amino acids was biexponential and could be ascribed a fast rate constant K1 and a slow rate constant K2. The K1 for glycine was always smaller than that for L-leucine. The K1 and K2 for L-leucine had a Q10 of 1.71 and 2.66, respectively, while steady-state transfer of the amino acid exhibited a Q10 of 2.45. Vascular perfusion rate affected the K1 but not the K2 for L-leucine. Na+ replacement in the lumen did not affect K1 and K2 for either amino acid from the mixture or the peptide, but did reduce the pool size of both when taken up as free amino acids. Peptide uptake, as measured by pool size, was not affected by luminal Na+ replacement. If Na+ in the lumen was replaced with Li+, all substitutions of Na in the vascular bed inhibited the exit of L-leucine. Na pumping across the basolateral membranes of the epithelial cells to keep the intercellular spaces patent is an important factor in the movement of amino acids from the enterocytes to the blood stream.