INTESTINAL UPTAKE OF URIDINE IN SUCKLING RATS - MECHANISM AND ONTOGENY

Nucleosides, essential substrates for a variety of intracellular metab olic reactions, are obtained from dietary and endogenous sources. Nucl eotides (which dephosphorylate to nucleosides prior to intestinal abso rption) are present in milk and have trophic effects on the developing gastrointestinal tract. The mechanism of transport of nucleosides in the developing intestine of suckling rats is unknown. To address this issue, we therefore examined uridine uptake in rat everted intestinal sacs. In suckling rats (15-17 days old), tissue uptake of low (5-mu M) and high (60 mu M) concentrations of [H-3]-uridine was linear for up to 2 min of incubation. Initial rate of uptake of [H-3]-uridine was (i ) not significantly different in the jejunum and the ileum; (ii) great er in the presence of Naf, than other cations; (iii) saturable as a fu nction of concentration with a V-max of 21,044 +/- 2,302 pmol/g tissue wet wt/30 sec and an apparent K-m of 33.8 +/- 10.1 mu M; (iv) inhibit ed by high concentration (500 mu M) of unlabeled uridine and other nuc leosides; (v) temperature-dependent; (vi) energy-dependent; and (vii) pH-sensitive. Developmental maturation was associated with a progressi ve decrease in the V-max of the uridine transport process (21,044 +/- 2,302, 14,651 +/- 1,679, and 8,461 +/- 1,369 pmol/g tissue wet wt/30 s ec for suckling, weanling, and adult rats, respectively) and a progres sive increase in the apparent K, of the uptake system (33.8 +/- 10.1, 55.6 +/- 13.1, and 61.7 +/- 14.5 mu M for suckling, weanling, and adul t rats, respectively). We concluded that uptake of uridine by the deve loping intestine of suckling rats involves a carrier-mediated system, which is energy- and temperature-dependent, and requires extracellular sodium. Furthermore, the uptake process was found to undergo clear on togenic changes with maturation.