title: Properties of Na(+)-dependent nucleoside transport in the proximal and distal small intestine of cows.
abstract: Large amounts of nucleic acids associated with rumen microorganisms are digested in the proximal part of the small intestine of ruminants. We studied how the proximal-distal gradient in nucleic acid digestion is related to activity of Na(+)-nucleoside transporters in brush border membrane vesicles isolated from the proximal and distal small intestine of cows. Two Na(+)-dependent nucleoside transporters with overlapping substrate specificity were shown to be present at the two intestinal sites, one for pyrimidine nucleosides and one for purine nucleosides. Affinity constants (K(m)-values) for both thymidine and guanosine transport were similar at the two intestinal sites, while transport capacity (V(max)) was 2-3 times higher in the proximal than in the distal small intestine. Glucose and alpha-methyl-D-glucoside (0.1 mmol/l or 2 mmol/l) inhibited transport of thymidine and guanosine markedly only in the proximal small intestine. It is concluded that absorption of nucleosides by the two Na(+)-nucleoside transporters reflects the proximal-distal gradient in nucleic acid digestion.
authors.affiliation: Institut für Veterinärphysiologie, Universität Zürich, Winterthurerstrasse 260, 8057 Zürich, Switzerland. scharrer@vetphys.unizh.ch
authors.forename: E
authors.initials: E
authors.lastname: Scharrer
authors.forename: B
authors.initials: B
authors.lastname: Grenacher
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002417:Cattle; D005260:Female; D006151:Guanosine / Q000493:pharmacokinetics; D007421:Intestine, Small / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008871:Microvilli / Q000378:metabolism; D009005:Monosaccharides / Q000378:metabolism; D013936:Thymidine / Q000493:pharmacokinetics; D014316:Tritium
chemical_list: D026901:Membrane Transport Proteins; D009005:Monosaccharides; C065171:cif nucleoside transporter; D014316:Tritium; D006151:Guanosine; D013936:Thymidine