The transport of dipeptides by the small intestine

During steady-state transfer across the vascularly perfused anuran small intestine, the intracellular concentration of L-leucine never exceeded its concentration in the lumen. However, during periods of stopped vascular flow accumulation did occur and this was dependent upon the presence of sodium 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 sodium was considerably lower than from the free amino acid. In contrast, the dipeptide L-carnosine (beta-alanyl-L-histidine) was not accumulated by the tissue in the presence or absence of luminal sodium. Also, the uptake of this poorly hydrolyzed peptide by tissue rings was unaffected either by sodium substitution or by the presence of L-leucylglycine, glycyl-L-leucine, or glycyl-L-proline. Only free L-leucine was found to inhibit the peptide's uptake significantly. It was concluded that there are two routes of uptake for amino acids from dipeptides. One is shared with free amino acids and is sodium dependent and concentrative. The other "peptide route" is nonconcentrative and not influenced by luminal sodium. Carnosine appears to be taken up by a second, apparently sodium-independent route.