Endogenous expression of the renal high-affinity H+-peptide cotransporter in LLC-PK1 cells

The reabsorption of filtered di- and tripeptides as well as certain peptide mimetics from the tubular lumen into renal epithelial cells is mediated by an H+-coupled high-affinity transport process. Here we demonstrate for the first time H+-coupled uptake of dipeptides into the renal proximal tubule cell line LLC-PK1. Transport was assessed 1) by uptake studies using the radiolabeled dipeptide D-[3H]Phe-L-Ala, 2) by cellular accumulation of the fluorescent dipeptide D-Ala-Lys-AMCA, and 3) by measurement of intracellular pH (pHi) changes as a consequence of H+-coupled dipeptide transport. Uptake of D-Phe-L-Ala increased linearly over 11 days postconfluency and showed all the characteristics of the kidney cortex high-affinity peptide transporter, e.g., a pH optimum for transport of D-Phe-L-Ala of 6.0, an apparent Km value for influx of 25.8 +/- 3. 6 microM, and affinities of differently charged dipeptides or the beta-lactam antibiotic cefadroxil to the binding site in the range of 20-80 microM. pHi measurements established the peptide transporter to induce pronounced intracellular acidification in LLC-PK1 cells and confirm its postulated role as a cellular acid loader.