title: Effects of Na+ and H+ on steady-state and presteady-state currents of the human concentrative nucleoside transporter 3 (hCNT3).
abstract: Human concentrative nucleoside transporter 3 (hCNT3) uses the electrochemical gradient of Na(+) and H(+) to drive the transport of nucleosides and therapeutic nucleoside analogs into the cells. We employed the two-electrode voltage clamp technique to compare the steady-state and presteady-state kinetics of hCNT3 in the presence of Na(+) and H(+). We found that H(+) supported a higher maximal rate of uridine transport than Na(+), but the efficiency of transport was lower. For both cations, maximal uridine-induced current increased with hyperpolarizing potentials and did not saturate within the voltage range tested. Apparent affinity of hCNT3 for uridine in H(+) was insensitive to membrane voltage at negative potentials, and decreased with depolarization. In contrast, apparent affinity for uridine in Na(+) decreased with hyperpolarization and was independent of voltage at depolarizing potentials. H(+)-coupled hCNT3 exhibited lower affinity for all natural nucleosides and different substrate selectivity compared to Na(+)-coupled hCNT3. In H(+), lack of the hydroxyl groups at 2' and 5' decreased the affinity, while lack of the nitrogen N-7 or inversion of the configuration of the hydroxyl group at 2' prevented transport. Presteady-state charge movements of hCNT3 did not decrease when extracellular cation concentration (Na(+) or H(+)) was reduced, but the tau(ON)-V and Q-V curves were shifted to more negative potentials. The different effects of uridine and inosine on presteady-state currents in H(+) indicated a change in rate-limiting step for the transport of these substrates by H(+)-coupled hCNT3.
authors.affiliation: Department of Food Science, Physiology and Toxicology, University of Navarra, 31008, Pamplona, Spain.
authors.forename: Edurne
authors.initials: E
authors.lastname: Gorraitz
authors.forename: Marçal
authors.initials: M
authors.lastname: Pastor-Anglada
authors.forename: Maria Pilar
authors.initials: MP
authors.lastname: Lostao
mesh_terms: D000241:Adenosine / Q000378:metabolism; D000818:Animals; D005260:Female; D006801:Humans; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011522:Protons; D012964:Sodium / Q000378:metabolism*; D014529:Uridine / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D026901:Membrane Transport Proteins; D011522:Protons; C065171:cif nucleoside transporter; D012964:Sodium; D000241:Adenosine; D014529:Uridine