The transport of dipeptides by the small intestine

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Abstract
During steady-state transfer across the vascularly perfused anuran small intestine, the intracellular concentration of L-leucine never exceeded its concentration in the lumen. During periods of stopped vascular flow, accumulation did occur and this was dependent upon the presence of Na in the luminal solution. Similar observations were made for leucine from the dipeptides L-leucylglycine and glycyl-L-leucine, although the intracellular accumulation of leucine from these peptides in the presence of Na was considerably lower than from the free amino acid. The dipeptide L-carnosine (.beta.-alanyl-L-histidine) was not accumulated by the tissue in the presence or absence of luminal Na. The uptake of this poorly hydrolyzed peptide by tissue rings was unaffected by Na substitution or by the presence of L-leucylglycine, glycyl-L-leucine or glycyl-L-proline. Only free L-leucine inhibited the peptide''s uptake significantly. There are evidently routes of uptake of amino acids from dipeptides. One is shared with free amino acids and is Na-dependent and concentrative. The other peptide route is nonconcentrative and not influenced by luminal Na. Carnosine is taken up by a 2nd, apparently Na-independent route.