Dipeptide Uptake and Transport Characteristics in Rabbit Tracheal Epithelial Cell Layers Cultured at an Air Interface.

Purpose. To determine the functional presence of a H⁺/peptide cotransport process in rabbit tracheal epithelial cell layers cultured at an air-interface and its contribution to transepithelial dipeptide transport. Methods. Rabbit tracheocytes were isolated, plated on Transwells, and cultured at an air-interface. After 5 or 6 days in culture, uptake and transepithelial transport of carnosine were examined. Results. Carnosine uptake by tracheocytes was pH-dependent and was saturable with a Michaelis-Menten constant of 170 μM. Moreover, carnosine uptake was inhibited 94% by Gly-L-Phe, 28% by (β-Ala-Gly, but not at all by Gly-D-Phe or by the amino acids β-Ala and L-His. Unexpectedly, transepithelial carnosine transport at pH 7.4 (i.e., in the absence of a transepithelial pH gradient) was similar in both the apical-to-basolateral ( ab) and basolateral-to-apical ( ba) directions. Lowering the apical fluid pH to 6.5 reduced abtransport 1.6 times without affecting ba transport, consistent with predominantly paracellular diffusion of carnosine under an electrochemical potential gradient. Conclusions. The kinetic behavior of carnosine uptake into cultured tracheal epithelial cell layers is characteristic of a H⁺-coupled dipeptide transport process known to exist in the small intestine and the kidney. Such a process does not appear to be rate-limiting in the transport of carnosine across the tracheal epithelial barrier. [ABSTRACT FROM AUTHOR]