McCloud E, Mathis R K, Grant K E, Said H M
Department of Pediatrics, University of California-School of Medicine, Irvine 92668.
Proc Soc Exp Biol Med. 1994 Sep;206(4):425-30. doi: 10.3181/00379727-206-43782.
Nucleosides, essential substrates for a variety of intracellular metabolic reactions, are obtained from dietary and endogenous sources. Nucleotides (which dephosphorylate to nucleosides prior to intestinal absorption) 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-microM) and high (60 microM) concentrations of [3H]-uridine was linear for up to 2 min of incubation. Initial rate of uptake of [3H]-uridine was (i) not significantly different in the jejunum and the ileum; (ii) greater in the presence of Na+, than other cations; (iii) saturable as a function of concentration with a Vmax of 21,044 +/- 2,302 pmol/g tissue wet wt/30 sec and an apparent Km of 33.8 +/- 10.1 microM; (iv) inhibited by high concentration (500 microM) of unlabeled uridine and other nucleosides; (v) temperature-dependent; (vi) energy-dependent; and (vii) pH-sensitive. Developmental maturation was associated with a progressive decrease in the Vmax of the uridine transport process (21,044 +/- 2,302, 14,651 +/- 1,679, and 8,461 +/- 1,369 pmol/g tissue wet wt/30 sec for suckling, weanling, and adult rats, respectively) and a progressive increase in the apparent Km of the uptake system (33.8 +/- 10.1, 55.6 +/- 13.1, and 61.7 +/- 14.5 microM for suckling, weanling, and adult rats, respectively). We concluded that uptake of uridine by the developing 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 ontogenic changes with maturation.
核苷是多种细胞内代谢反应的必需底物，可从饮食和内源性来源获得。核苷酸（在肠道吸收前会脱磷酸化为核苷）存在于牛奶中，对发育中的胃肠道具有营养作用。乳鼠发育中的肠道中核苷的转运机制尚不清楚。因此，为了解决这个问题，我们检测了大鼠外翻肠囊对尿苷的摄取。在乳鼠（15 - 17日龄）中，低浓度（5 μM）和高浓度（60 μM）的[³H] - 尿苷在长达2分钟的孵育过程中，组织摄取呈线性。[³H] - 尿苷的初始摄取速率：（i）空肠和回肠中无显著差异；（ii）在有Na⁺存在时比其他阳离子存在时更高；（iii）作为浓度的函数是可饱和的，Vmax为21,044 ± 2,302 pmol/g组织湿重/30秒，表观Km为33.8 ± 10.1 μM；（iv）受到高浓度（500 μM）未标记尿苷和其他核苷的抑制；（v）依赖温度；（vi）依赖能量；（vii）对pH敏感。发育成熟与尿苷转运过程的Vmax逐渐降低（乳鼠、断奶幼鼠和成年大鼠分别为21,044 ± 2,302、14,651 ± 1,679和8,461 ± 1,369 pmol/g组织湿重/30秒）以及摄取系统的表观Km逐渐增加（乳鼠、断奶幼鼠和成年大鼠分别为33.8 ± 10.1、55.6 ± 13.1和61.7 ± 14.5 μM）相关。我们得出结论，乳鼠发育中的肠道对尿苷的摄取涉及一个载体介导的系统，该系统依赖能量和温度，且需要细胞外钠。此外，发现摄取过程随着成熟会发生明显的个体发育变化。
