Laboratory of Physiology School of Food and Nutritional Sciences, University of Shizuoka, Suruga-ku, Shizuoka , Japan.
Laboratory of Biochemistry, Department of Biopharmaceutical Sciences, Gifu Pharmaceutical University , Gifu , Japan.
Am J Physiol Gastrointest Liver Physiol. 2018 Nov 1;315(5):G799-G809. doi: 10.1152/ajpgi.00099.2018. Epub 2018 Aug 23.
Intestinal cell line studies indicated luminal Na homeostasis is essential for proton-coupled peptide absorption, because the driving force of PepT1 activity is supported by the apical Na/H exchanger NHE3. However, there is no direct evidence demonstrating the importance of in vivo luminal Na for peptide absorption in animal experiments. To investigate the relationship between luminal Na homeostasis and peptide absorption, we took advantage of claudin 15-deficient (cldn15) mice, whereby Na homeostasis is disrupted. We quantitatively assessed the intestinal segment responsible for peptide absorption using radiolabeled nonhydrolyzable dipeptide (glycylsarcosine, Gly-Sar) and nonabsorbable fluid phase marker polyethylene glycol (PEG) 4000 in vivo. In wild-type (WT) mice, the concentration ratio of Gly-Sar to PEG 4000 decreased in the upper jejunum, suggesting the upper jejunum is responsible for peptide absorption. Gly-Sar absorption was decreased in the jejunum of cldn15 mice. To elucidate the mechanism underlining these impairments, a Gly-Sar-induced short-circuit ( I) current was measured. In WT mice, increments of Gly-Sar-induced I were inhibited by the luminal application of a NHE3-specific inhibitor S3226 in a dose-dependent fashion. In contrast to in vivo experiments, robust Gly-Sar-induced I increments were observed in the jejunal mucosa of cldn15 mice. Gly-Sar-induced I was inhibited by S3226 or a reduction of luminal Na concentration, which mimics low luminal Na concentrations in vivo . Our study demonstrates that luminal Na homeostasis is important for peptide absorption in native epithelia and that there is a cooperative functional relationship between PepT1 and NHE3. NEW & NOTEWORTHY Our study is the first to demonstrate that luminal Na homeostasis is important for proton-coupled peptide absorption in in vivo animal experiments.
肠细胞系研究表明，腔内钠稳态对于质子偶联肽吸收是必需的，因为 PepT1 活性的驱动力由顶端 Na+/H+交换器 NHE3 提供。然而，在动物实验中，没有直接证据表明体内腔内 Na+对肽吸收的重要性。为了研究腔内钠稳态与肽吸收之间的关系，我们利用 Claudin 15 缺陷型（cldn15）小鼠，其中钠稳态被破坏。我们利用放射性标记的不可水解二肽（甘氨酰肌氨酸，Gly-Sar）和不可吸收的流体相标记物聚乙二醇（PEG）4000 在体内定量评估负责肽吸收的肠段。在野生型（WT）小鼠中，Gly-Sar 与 PEG 4000 的浓度比在上段空肠中降低，表明上段空肠负责肽吸收。Gly-Sar 的吸收在 cldn15 小鼠的空肠中减少。为了阐明这些损伤的潜在机制，我们测量了 Gly-Sar 诱导的短路（I）电流。在 WT 小鼠中，Gly-Sar 诱导的 I 增加被 NHE3 特异性抑制剂 S3226 以剂量依赖性方式抑制。与体内实验相反，在 cldn15 小鼠的空肠黏膜中观察到强大的 Gly-Sar 诱导的 I 增加。Gly-Sar 诱导的 I 被 S3226 或腔内 Na+浓度降低抑制，这模拟了体内低腔内 Na+浓度。我们的研究表明，腔内钠稳态对于天然上皮中的肽吸收很重要，并且 PepT1 和 NHE3 之间存在协同功能关系。
新的和值得注意的是，我们的研究首次证明，在体内动物实验中，腔内钠稳态对于质子偶联肽吸收很重要。
