Multiple Sodium-Dependent Nucleoside Transport-Systems in Bovine Renal Brush-Border Membrane-Vesicles

Na+-dependent nucleoside transport was examined in bovine renal brush-border membrane vesicles. Two separate Na+/nucleoside cotransporters were shown to be present: (1) a system specific for purine nucleosides and uridine, designated as the N1 carrier, and (2) an Na+-dependent nucleoside transporter that accepts pyrimidine nucleosides, adenosine and analogues of adenosine, designated as the N2 system. Both systems exhibit a high affinity for nucleosides (apparent K(m) values approximately 10-mu-M), are insensitive to inhibition by facilitated-diffusion nucleoside transport inhibitors, are rheogenic and exhibit a high specificity for Na+. Na+ increases the affinity of the influx of guanosine and thymidine, nucleosides that serve as model permeants for the N1 and N2 nucleoside transporters respectively. The Na+/nucleoside coupling stoichiometry is consistent with 1:1 for both carriers.