Characterization of the transport of uracil across Caco-2 and LLC-PK1 cell monolayers

Purpose. The purpose of this study was to characterize the transport of uracil, a pyrimidine nucleobase, in Caco-2 and LLC-PK1 cells. Methods. Caco-2 and LLC-PK1 cells were grown to confluency on a permeable polycarbonate membrane insert to permit transport and uptake experiments after the loading of uracil on either the apical or basolateral side. Results. The vectorial transport of uracil in both directions was saturable with comparable K-m and V-max in Caco-2 cell monolayers, probably because of a Na+-independent transport system located on the basolateral membrane. In LLC-PK1 cell monolayers, two distinct transport systems, namely a Na+-dependent and a Na+-independent, were functional in the apical to basolateral (A-B) transport of uracil. The first system was saturable with a K-m value of 3.67 +/- 0.40 muM, a V-max of 11.31 +/- 0.91 pmol/cm(2)/min, and a Na+:uracil coupling stoichiometry of 1.28 +/- 0.20. The second system was Na+ independent and satuable with a low affinity (K-m, 50.37 +/- 9.61 muM) and V-max (16.01 +/- 4.48 pmol/cm2/min). The two transport systems appeared to be located on the apical membrane. Conclusion. The mechanism of uracil transport differs depending on cell lines; a Na+-independent system on the basolateral membrane in Caco-2 cells and both Na+-dependent and Na+-independent systems on the apical membrane in LLC-PK1 cells seem to be responsible for the difference.