NUCLEOSIDE TRANSPORT IN BRUSH-BORDER MEMBRANE-VESICLES FROM HUMAN KIDNEY

The goal of this study was to elucidate the mechanisms of nucleoside transport in the brush border membrane of the human kidney. [H-3]Uridine was transported into brush border membrane vesicles (BBMV) from human kidney via Na+-independent and Na+-dependent processes. The Na+-dependent transport was saturable (K(m) = 4.76 +/- 0.39-mu-M; V(max) = 6.42 +/- 0.17 pmol/mg proteins per s) and was trans-stimulated by unlabeled uridine. Structural analogs of uridine (100-mu-M), 2'-deoxyuridine (2-dU) and dideoxyuridine (ddU), significantly inhibited Na+-uridine uptake into BBMV. Previous studies have suggested that Na+-nucleoside co-transport occurs via two major systems (Vijayalakshmi et al. (1988) J. Biol. Chem. 263, 19417-19423). One system (cit) is generally pyrimidine-selective; thymidine serves as a model substrate. The other system (cif) is generally purine-selective; formycin B serves as a model substrate. Uridine and adenosine are substrates of both systems. Thymidine and cytidine (100-mu-M), but not formycin B (100-mu-M) inhibited Na+-uridine uptake. In addition, [H-3]thymidine exhibited an Na+-driven overshoot phenomenon whereas [H-3]formycin B did not. Na+-thymidine uptake was inhibited by (100-mu-M) adenosine, uridine, guanosine, but not by formycin B and inosine. Further studies demonstrated that guanosine trans-stimulated thymidine uptake suggesting that guanosine and thymidine share a common transporter in the human renal BBMV. A different pattern was identified in BBMV from the rabbit kidney where both [H-3]thymidine and [H-3]formycin B as well as [H-3]uridine exhibited a transient Na+-driven overshoot phenomenon. Collectively, these data suggest that in rabbit renal BBMV both cif and cit systems are present whereas in human renal BBMV, there appears to be a single concentrative Na+-nucleoside cotransport system that interacts with uridine, cytidine, thymidine, adenosine and guanosine but not with formycin B and inosine. The system is similar to the previously described cit system except that guanosine is also a substrate.