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 ra diolabeled dipeptide D-[H-3]Phe-L-Ala, 2) by cellular accumulation of the f luorescent dipeptide D-Ala-Lys-AMCA, and 3) by measurement of intracellular pH (pH(i)) changes as a consequence of H+-coupled dipeptide transport. Upt ake of D-Phe-L-Ala increased linearly over II days postconfluency and showe d all the characteristics of the kidney cortex high-affinity peptide transp orter, e.g., a pH optimum for transport of D-Phe-L-Ala of 6.0, an apparent K-m value for influx of 25.8 +/- 3.6 mu M, and affinities of differently ch arged dipeptides or the p-lactam antibiotic cefadroxil to the binding site in the range of 20-80 mu M. pH(i) measurements established the peptide tran sporter to induce pronounced intracellular acidification in LLC-PK1 cells a nd confirm its postulated role as a cellular acid loader.