Laboratorio de Biología Professor E. Caviedes Codelia, Facultad de Ciencias Humanas, Universidad Nacional de San Luis, 5700 San Luis, Argentina.
Comp Biochem Physiol A Mol Integr Physiol. 2013 Jan;164(1):71-6. doi: 10.1016/j.cbpa.2012.09.008. Epub 2012 Sep 19.
Water-soluble nutrients are absorbed by the small intestine via transcellular and paracellular processes. The capacity for paracellular absorption seems lower in nonfliers than in fliers, although that conclusion rests largely on a comparison of relatively larger nonflying mammals (>155g) and relatively smaller flying birds (<155g). We report on paracellular absorption in laboratory mice, the smallest nonflying mammal species studied to date. Using a standard pharmacokinetic technique, we measured the extent of absorption (fractional absorption=f) of inert carbohydrate probes: L-arabinose (M(r)=150.13Da) and cellobiose (342.3) that are absorbed exclusively by the paracellular route, and 3-O-methyl D-glucose (3OMD-glucose) (M(r)=194) absorbed both paracellularly and transcellularly. f was measured accurately in urine collection trials of 5-10h duration. Absorption of 3OMD-glucose by mice was essentially complete (f=0.95±0.07) and much higher than that for L-arabinose (f=0.21±0.02), indicating that in mice, like other nonflying mammals, >80% of glucose is absorbed by mediated process(es) rather than the passive, paracellular route. As in all other vertebrates, absorption of cellobiose (f=0.13±0.02) was even lower than that for L-arabinose, suggesting an equivalent molecular size cut-off for flying and nonflying animals and thus a comparable effective TJ aperture. An important ecological implication is that smaller water-soluble plant secondary metabolites that have been shown to be absorbed by the paracellular path in cell culture, such as phenolics and alkaloids, might be absorbed in substantial amounts by bats and small birds relative to nonflying mammals such as mice.
水溶性营养素通过细胞旁和细胞内过程被小肠吸收。尽管这一结论主要基于对相对较大的非飞行哺乳动物（>155g）和相对较小的飞行鸟类（<155g）的比较，但非飞行动物的细胞旁吸收能力似乎低于飞行动物。我们报告了实验室中小鼠的细胞旁吸收情况，这是迄今为止研究过的最小的非飞行哺乳动物物种。我们使用标准的药代动力学技术，测量了惰性碳水化合物探针的吸收程度（分数吸收=f）：仅通过细胞旁途径吸收的 L-阿拉伯糖（M(r)=150.13Da）和纤维二糖（342.3），以及 3-O-甲基 D-葡萄糖（3OMD-葡萄糖）（M(r)=194），这两种物质都可以通过细胞旁途径和细胞内途径吸收。在 5-10 小时的尿液收集试验中准确测量 f。3OMD-葡萄糖在小鼠中的吸收几乎是完全的（f=0.95±0.07），高于 L-阿拉伯糖（f=0.21±0.02），表明在小鼠中，与其他非飞行哺乳动物一样，>80%的葡萄糖是通过介导的过程（而非被动的细胞旁途径）吸收的。与所有其他脊椎动物一样，纤维二糖的吸收（f=0.13±0.02）甚至低于 L-阿拉伯糖，这表明飞行动物和非飞行动物的有效 TJ 孔径相当，因此具有相同的分子大小截止值。一个重要的生态意义是，已经在细胞培养中证明通过细胞旁途径吸收的较小的水溶性植物次生代谢物，如酚类和生物碱，可能会被蝙蝠和小鸟大量吸收，而不是像小鼠这样的非飞行哺乳动物。
