Purpose: The purpose of this study was to characterize the transport of uracil, a pyrimidine nucleobase, in Caco-2 and LLC-PK, 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 Km and Vmax 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 Km value of 3.67 +/- 0.40 microM, a Vmax of 11.31 +/- 0.91 pmol/cm2/min, and a Na+:uracil coupling stoichiometry of 1.28 +/- 0.20. The second system was Na+ independent and satuable with a low affinity (Km, 50.37 +/- 9.61 microM) and Vmax (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.
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