The transport of pyrimidines into tissue rings cut from rat small intestine.

1. At low concentrations (0.1 mM) the transport of uracil, 5-fluorouracil and thymine into jejunal tissue rings is an active process. 2. The transport of 5-fluorouracil into tissue rings cut from the duodenum and jejunum was greater than the transport into rings cut from the ileum. This difference was abolished by starving the rats for 48 h before the experiment. 3. The active transport can be abolished by replacing the Na+ in the incubation medium with either K+ or mannitol, or by increasing the concentration of the pyrimidine to 1.0 mM. 4. The accumulation of uracil or 5-fluorouracil into the jejunal rings was identical when determined by radioactive tracer or by high-performance liquid chromatography. 5. The apparent Michaelis constant (Km) for 5-fluorouracil transport into jejunal rings was 0.074 mM in the standard Na+ bicarbonate Krebs-Ringer solution and 0.394 mM in the K+-substituted bicarbonate Krebs-Ringer solution. 6. Both thymine and uracil inhibited the transport of 5-fluorouracil into jejunal tissue rings: however, cytosine and orotic acid did not.