Institute of Physiology, Christian Albrechts University of Kiel , Kiel , Germany.
Am J Physiol Renal Physiol. 2018 Feb 1;314(2):F190-F195. doi: 10.1152/ajprenal.00419.2017. Epub 2017 Sep 27.
The nephron segments in the inner medulla are part of the urine concentrating mechanism. Depending on the diuretic state, they are facing a large range of extracellular osmolality. We investigated whether water homeostasis affects tubular transport and permeability properties in inner medullary descending thin limb (IMdTL) and ascending thin limb (IMaTL). Three experimental groups of rats under different diuretic states were investigated on metabolic cages: waterload, furosemide-induced diuresis, and control (antidiuresis). Urine production and osmolalities reflected the 3-day treatment. To functionally investigate tubular epithelial properties, we performed experiments in freshly isolated inner medullary thin limbs from these animals. Tubular segments were acutely dissected and investigated for trans- and paracellular properties by in vitro perfusion and electrophysiological analysis. IMdTL and IMaTL were distinguished by morphological criteria. We confirmed absence of transepithelial electrogenic transport in thin limbs. Although diffusion potential measurements showed no differences between treatments in IMdTLs, we observed increased paracellular cation selectivity under waterload in IMaTLs. NaCl diffusion potential was -5.64 ± 1.93 mV under waterload, -1.99 ± 1.72 mV under furosemide-induced diuresis, and 0.27 ± 0.40 mV under control. The corresponding permeability ratio P was 1.53 ± 0.21 (waterload), 1.22 ± 0.18 (furosemide-induced diuresis), and 0.99 ± 0.02 (control), respectively. Claudins are main constituents of the tight junction responsible for paracellular selectivity; however, immunofluorescence did not show qualitative differences in claudin 4, 10, and 16 localization. Our results show that IMaTLs change tight junction properties in response to diuretic state to allow adaptation of NaCl reabsorption.
内髓的肾单位段是尿液浓缩机制的一部分。根据利尿状态，它们面临着大范围的细胞外渗透压。我们研究了水稳态是否会影响内髓降支细段（IMdTL）和升支细段（IMaTL）的管状转运和通透性特性。在不同利尿状态下的三组实验大鼠在代谢笼中进行了研究：水负荷、速尿诱导的利尿和对照（抗利尿）。尿产量和渗透压反映了 3 天的治疗。为了功能研究管状上皮特性，我们从这些动物中分离了新鲜的内髓薄肢进行实验。管状段被急性解剖，并通过体外灌注和电生理分析研究跨细胞和旁细胞特性。通过形态学标准区分了 IMdTL 和 IMaTL。我们证实薄肢中不存在跨上皮电驱动转运。尽管扩散电位测量显示在 IMdTL 中处理之间没有差异，但我们观察到在水负荷下 IMaTL 的旁细胞阳离子选择性增加。在水负荷下，NaCl 扩散电位为-5.64±1.93 mV，在速尿诱导的利尿下为-1.99±1.72 mV，在对照下为 0.27±0.40 mV。相应的渗透比 P 分别为 1.53±0.21（水负荷）、1.22±0.18（速尿诱导的利尿）和 0.99±0.02（对照）。紧密连接是负责旁细胞选择性的主要成分；然而，免疫荧光没有显示 Claudin 4、10 和 16 定位的定性差异。我们的结果表明，IMaTL 会根据利尿状态改变紧密连接特性，以适应 NaCl 的重吸收。
