Novel Transporter Responsible for Intestinal Glycerol Absorption : Characterization of Its Function and Physiological Role

大鼠小肠外翻囊对甘油的摄取是高度饱和的，表明载体介导的转运参与了甘油的转运。虽然也观察到了适度的被动传输成分，但发现载流子介导的传输在远低于米氏常数的浓度下占优势，此时载流子介导的传输处于线性阶段，并且最有效。进一步认为甘油载体具有钠离子依赖性和二次活性，是甘油及其类似化合物的专一性载体。还应该注意的是，载体介导的甘油转运是高效的，与以非常高的效率而闻名的SGLT1(钠依赖葡萄糖转运体1)的D-葡萄糖转运相当。载体介导甘油转运的这些特点与大鼠在体肠闭合环段和灌流段的甘油转运特征一致。因此，我们成功地证明了在大鼠肠道组织中存在甘油及其类似物的载体介导的转运系统，并对其进行了动力学表征。HCT-15是一种人结肠癌细胞系，在本研究项目中发现它能够进行钠离子依赖的甘油摄取。尽管HCT-15细胞中的甘油载体与小肠中的甘油载体有50倍的米氏常数差异，但不同化合物对甘油转运的抑制曲线非常相似，表明在识别底物和/或抑制剂方面具有相似的特征。因此，有可能利用HCT-15细胞来筛选抑制剂和潜在的底物。HCT-15可能仍是一种有用的模型细胞系，用于鉴定一组依赖Na~+的甘油载体并阐明它们的转运机制。这类载体作为药物输送的可能途径和药物开发的目标将引起人们的兴趣。

The uptake of glycerol in the everted sacs of the rat small intestine was highly saturable, suggesting the involvement of carrier-mediated transport. Although moderate passive transport component was also observed, it was found that carrier-mediated transport is prevailing at concentrations far lower than the Michaelis constant, where carrier-mediated transport is in the linear phase and most efficient. The glycerol carrier was further suggested to be of Na^+-dependent and secondary active, and specific to glycerol and some analogous compounds. It should also be noted that carrier-mediated glycerol transport is highly efficient, being comparable to D-glucose transport by SGLT1(sodium-dependent glucose transporter 1), which is well known for its very high efficiency. These characteristics of carrier-mediated glycerol transport were consistent with those in the closed loop and the perfused segment of the rat small intestine in situ. Thus, we successfully demonstrated the presence of a carrier-mediated transport system specific to glycerol and some analogous compounds in the rat intestinal tissue, and characterized it kinetically.HCT-15 is a human colon cancer cell line, which was found to be able to perform Na^+-dependent glycerol uptake in the present research project. Although the glycerol carrier in HCT-15 cells was suggested to be different from the one in the small intestine as there was a 50-fold difference in the Michaelis constants, the profiles of inhibition of glycerol transport by various compounds were quite similar, indicating similar characteristics in recognition of substrates and/or inhibitors. Therefore, it maybe possible to use HCT-15 cells for screening inhibitors and potential substrates. HCT-15 could still be a useful model cell line for studies to identify a group of Na^+-dependent glycerol carriers and to elucidate their transport mechanism. Such carriers would be of interest as possible pathways of drug delivery and targets of drug development.