FUNCTIONAL EXPRESSION OF NA-DEPENDENT NUCLEOSIDE TRANSPORT-SYSTEMS OFRAT INTESTINE IN ISOLATED OOCYTES OF XENOPUS-LAEVIS - DEMONSTRATION THAT RAT JEJUNUM EXPRESSES THE PURINE-SELECTIVE SYSTEM N1 (CIF) AND A 2ND, NOVEL SYSTEM N3 HAVING BROAD-SPECIFICITY FOR PURINE AND PYRIMIDINENUCLEOSIDES()

Isolated stage VI oocytes from Xenopus laevis expressed uridine transp ort activity after microinjection of mRNA from rat jejunum. Uridine up take during 30 min (10 muM, 20-degrees-C) by mRNA-injected oocytes rea ched 2.5 pmol/oocyte, compared with endogenous uptake by water-injecte d oocytes of about 0.05 pmol/oocyte. The expressed transport activity was 96% Na+-dependent, saturable (apparent K(m) = 15 muM) and inhibite d by phloridzin (IC50 = 100 muM). Nucleoside inhibition studies resolv ed the expressed transport activity into two components: 1) a novel Na +-dependent system of broad purine and pyrimidine specificity that was inhibited by low concentrations of guanosine, inosine, adenosine, uri dine, thymidine, and cytidine and 2) a Na+-dependent system of narrowe r specificity that was inhibited by low concentrations of guanosine, i nosine, adenosine, and uridine and by high concentrations of thymidine and cytidine. The characteristics of the latter system are consistent with those of the Na+-dependent nucleoside transport system N1 (cif), previously identified in a number of cell types and tissues, includin g intestinal epithelia and cultured cells of intestinal origin. The br oad specificity system, which was also detected in mRNA-injected oocyt es using thymidine as permeant, has been given the provisional designa tion N3 to distinguish it from the previously described N1 (purine-sel ective) and N2 (pyrimidine-selective) Na+-linked nucleoside transporte rs. Rat jejunal transporters N1 and N3 were both expressed maximally b y the same mRNA size fraction (1.6-3.0 kb, peak 2.3 kb).