Sodium-dependent nucleoside transport in the human intestinal brush-border membrane

Am J Physiol. 1997 Jun;272(6 Pt 1):G1314-20. doi: 10.1152/ajpgi.1997.272.6.G1314.

Abstract

The objective of the study was to determine the identity and kinetic characteristics of nucleoside transporters present in the brush-border membrane of the human jejunum. With use of brush-border membrane vesicles, uptake of [3H]uridine was stimulated two- to threefold by an inwardly directed Na+ gradient and was inhibited by both 100 microM thymidine and 100 microM guanosine nucleosides, which serve as model substrates for purine (N1, cif) and pyrimidine (N2, cit) transporters, respectively. [3H]thymidine and [3H]guanosine transport exhibited an overshoot phenomenon only in the presence of a Na+ gradient. Na(+)-thymidine uptake was inhibited by 100 microM cytidine or thymidine but not by guanosine, inosine, formycin B, or hypoxanthine. [3H]guanosine uptake was inhibited by 100 microM inosine, guanosine, or formycin B but not by thymidine or cytidine. Both adenosine and uridine inhibited uptake of [3H]thymidine and [3H]guanosine to a similar extent, indicating that both N1, cif and N2, cit Na(+)-nucleoside transporters are expressed in human jejunum. Enhanced uptake of Na(+)-thymidine by an inside-negative potential difference generated by K+ and valinomycin provides evidence that nucleoside transport is rheogenic, involving net transfer of a positive charge. The Hill coefficient was unity for all three substrates, indicating a Na(+)-nucleoside coupling stoichiometry of 1:1. At saturating Na+ concentration (150 mM) the kinetic parameters (n = 3-4) Michaelis-Menten constant and maximum velocity for uridine, thymidine, and guanosine uptake were 4.15 +/- 1.79, 2.74 +/- 0.58, 12.02 +/- 1.34 microM and 25.93 +/- 7.38, 16.10 +/- 3.64, 63.92 +/- 10.23 pmol.mg-1.10 s-1, respectively. These results suggest that, in contrast to the human kidney that expresses the N4 nucleoside transporter, the human jejunum expresses both N1 and N2 Na(+)-nucleoside transporters.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Aged
  • Biological Transport / drug effects
  • Cations, Monovalent / pharmacology
  • Female
  • Guanosine / metabolism
  • Humans
  • Intestinal Mucosa / metabolism*
  • Intestine, Small
  • Kinetics
  • Male
  • Microvilli / metabolism*
  • Middle Aged
  • Nucleosides / metabolism*
  • Nucleosides / pharmacology*
  • Sodium / pharmacology*
  • Thymidine / metabolism
  • Tissue Donors
  • Uridine / metabolism

Substances

  • Cations, Monovalent
  • Nucleosides
  • Guanosine
  • Sodium
  • Thymidine
  • Uridine