SODIUM-DEPENDENT NUCLEOSIDE TRANSPORT IN RABBIT INTESTINAL EPITHELIUM

Cellular uptake of formycin B, a poorly metabolized analog of inosine, by the isolated epithelium of rabbit jejunum is three times higher in the presence of Na+ than without this cation. The Na+-dependent nucleoside transport system is located in the apical membrane of the enterocytes and is capable of uphill transport, as shown for formycin B and adenosine with brush border membrane vesicles. According to present and earlier evidence, nucleoside transport across the basolateral membrane appears to have the properties of facilitated diffusion. Na+-dependent formycin B transport activity in intestinal epithelium decreases from jejunum to ileum and is absent in descending colon. As with Na+ -coupled cotransport systems for other organic solutes, apical entry of formycin B is driven by the electrochemical Na+ gradient into the cell. In contrast to the facilitated diffusion system for nucleosides, Na+ -dependent formycin B transport is not inhibited by nitrobenzylthioinosine, but both carrier systems are sensitive to inhibitors of d-glucose transport. Natural purine nucleosides and uridine are strong inhibitors of Na+-dependent formycin B transport. Trans-epithelial flux measurements substantiated that the Na+ -dependent transport mechanism for formycin B functions as an absorptive system. © 1991.