The transport and metabolism of naturally occurring pyrimidine nucleosides by isolated rat jejunum.

1. Uridine perfused through the lumen of isolated loops of rat jejunum over a concentration range of 0.1-1.0 mM gave rise to higher serosal concentrations of uracil than the equivalent luminal concentration of uracil (P less than 0.001). No serosal uridine could be detected. 2. Luminal thymidine over a concentration range of 0.1-0.5 mM gave rise to the same serosal concentration of thymine as the equivalent luminal concentration of thymine (P greater than 0.1). Low concentrations of serosal thymidine were detected. Both luminal thymidine and thymine gave rise to elevated levels of serosal uracil. 3. Luminal cytidine at concentrations of 0.1-0.5 mM was poorly transported and yielded low serosal concentrations of cytidine. No serosal cytosine was detected, although elevated levels of uracil were found in the serosal secretions. 4. Cytosine over a luminal concentration range of 0.1-0.5 mM gave rise to low concentrations of cytosine in the serosal secretions. These results were consistent with a passive diffusion model for cytosine transport. No increase in serosal uracil was detected. 5. The cleavage of uridine and thymidine to their respective pyrimidine bases occurred via a cytoplasmic nucleoside phosphorylase, which had a similar Michaelis constant (Km), (61.0 +/- 4.4 and 97.1 +/- 5.7 microM for uridine and thymidine, respectively) but a maximal velocity (Vmax) for uridine cleavage (320 +/- 32 nmol min-1 (mg protein)-1) 13 times that for thymidine cleavage (24.7 +/- 1.4 nmol min-1 (mg protein)-1). 6. The differences between the three pyrimidine nucleosides are discussed with reference to the interactions between their epithelial transport and metabolism.