Na+- and K+-dependent uridine transport in rat renal brush-border membrane vesicles.
The transport of uridine into rat renal brush-border membrane vesicles was investigated using an inhibitor-stop filtration method. Uridine was not metabolized under these conditions. The rapid efflux of intravesicular uridine was prevented by adding 1 mM phloridzin to the ice-cold stop solution. In the presence of inwardly directed gradients of either Na+ or K+, zero-trans uridine uptake exhibited a transient overshoot phenomenon indicating active transport. The overshoot was much more pronounced with Na+ than K+ and it was not observed when either Na+ or K+ was at equilibrium across the membrane. The K+-induced overshoot was not due to the presence of a membrane potential alone, as an inwardly directed gradient of choline chloride failed to produce it. The amplitude of the overshoot was increased by raising either the Na+ or K+ concentration outside the membrane or by using more lipophilic anions (reactive order was NO3- greater than SCN- greater than Cl- greater than SO4(2-). Zero-trans efflux studies showed that the uridine transport is bidirectional. Li+ could substitute poorly for Na+ but not at all for K+. Stoichiometries of 1:1 and greater than 1:1 were observed for Na+: uridine and K+: uridine coupling, respectively. A preliminary analysis of the interactions between Na+ and K+ for uridine uptake showed complex interactions which can best be explained by the involvement of two different systems for nucleoside transport in the rat renal brush-border membrane, one requiring Na+ and the other K+ as transport coupler.
Lee
C W
CW
Department of Physiology, University of Alberta, Edmonton, Canada.
Cheeseman
C I
CI
Jarvis
S M
SM
D000818:Animals; D000838:Anions / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D066298:In Vitro Techniques; D007672:Kidney Cortex / Q000378:metabolism*; D007687:Kidney Tubules, Proximal / Q000378:metabolism; D007700:Kinetics; D008871:Microvilli / Q000378:metabolism; D011188:Potassium / Q000502:physiology; D051381:Rats; D012964:Sodium / Q000502:physiology; D014529:Uridine / Q000378:metabolism*
D000838:Anions; D012964:Sodium; D011188:Potassium; D014529:Uridine