Expression of the Na⁺ Dependent Uridine Transport System of Rabbit Small Intestine : Studies with mRNA-Injected Xenopus laevis Oocytes

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Xenopus laevis oocytes were used as an expression system to prove and characterize the carrier-mediated transport of uridine in the small intestine. Significant Na⁺ dependency was observed for the uptake of [³H]uridine by Xenopus laevis oocytes injected with poly(A)⁺RNA prepared from rabbit small intestinal mucosa. By contrast, the uptake of [³H]uridine was negligible in water-injected oocytes. There was no significant difference in the Na⁺ dependent uptake rates of [³H]uridine among oocytes expressed by using mRNA prepared by three different methods. The uptake of [³H]uridine by mRNA-injected oocytes was enhanced by increasing the culturing time after mRNA injection. Concentration dependency for uridine transport was observed with the Michaelis constant of 8.27 μM, which was comparable to that reported in the study using the brush-border membrane vesicles from rabbit small intestine (6.4 μM). Furthermore, the uptake of [³H]uridine was significantly inhibited by adenosine and thymidine, but not by adenine and uracil. Consequently, the transport system of uridine expressed in mRNA-injected oocytes is clarified to be similar to that functioning in the brush-border membrane of the small intestine.