Nucleoside Transport in Cultured Llc-Pk1 Epithelia

The transport of nucleosides by LLC-PK1 cells, a continuous epithelial cell line derived from pig kidney, was characterised. Uridine influx was saturable (apparent K(m) approximately 34-mu-M at 22-degrees-C) and inhibited by > 95% by nitrobenzylthioinosine (NBMPR), dilazep and a variety of purine and pyrimidine nucleosides. In contrast to other cultured animal cells, the NBMPR-sensitive nucleoside transporter in LLC-PK1 cells exhibited both a high affinity for cytidine (apparent K(i) approximately 65-mu-M for influx) and differential 'mobility' of the carrier (the kinetic parameters of equilibrium exchange of formycin B are greater than those for formycin B influx). An additional minor component of sodium-dependent uridine influx in LLC-PK, cells became detectable when the NBMPR-sensitive nucleoside transporter was blocked by the presence of 10-mu-M NBMPR. This active transport system was inhibited by adenosine, inosine and guanosine but thymidine and cytidine were without effect, inhibition properties identical to the N1 sodium-dependent nucleoside carrier in bovine renal outer cortical brush-border membrane vesicles (Williams and Jarvis (1991) Biochem. J. 274, 27-33). Late proximal tubule brush-border membrane vesicles of porcine kidney were shown to have a much reduced Na+-dependent uridine uptake activity compared to early proximal tubule porcine brush-border membrane vesicles. These results, together with the recent suggestion of thc late proximal tubular origin of LLC-PK1 cells, suggest that in vivo nucleoside transport across thc late proximal tubule cell may proceed mainly via a facilitated-diffusion process.