Jans Frank, Balut Corina, Ameloot Marcel, Wouters Patrick, Steels Paul
Department of Physiology, Biomedical Research Institute, Universiteit Hasselt, Diepenbeek, Belgium.
Nephron Physiol. 2007;106(3):p45-53. doi: 10.1159/000103909. Epub 2007 Jun 15.
Several apical ammonium (NH(4)(+)/NH(3)) transport pathways have been described in medullary thick ascending limb (MTAL) cells. The exact nature and importance of some of these pathways remain controversial.
Ammonium transport in primary cultured rabbit MTAL cells was investigated by measuring intracellular pH (pH(i)).
To create physiological conditions, experiments were performed in the symmetrical presence of NH(4)Cl, which acidified the cells to pH(i) 6.89. When blockers of apical NH(4)(+) transport were used, the cells alkalinized due to a decreased NH(4)(+) loading. The following values (pH units) were observed: bumetanide, +0.05; verapamil, +0.04; Ba(2+) and Cs(+), +0.19; tertiapin, +0.09. Tetraethylammonium had no effect. Depolarizing the cells by increasing the K(+) concentration alkalinized the cells by 0.16 pH units. Because NH(4)(+) might enter through nonspecific channels, ammonium pulse experiments were performed: an NH(4)Cl pulse acidified controls as well as depolarized cells. In contrast, when Ba(2+), Cs(+) or tertiapin were present, an NH(4)Cl pulse alkalinized the cells. The pharmacological profile of this apical NH(4)(+) transport pathway correlates with the renal outer medullary K(+) (ROMK) channel. Indirect immunofluorescence showed the presence of the ROMK protein.
In these MTAL cells the Ba(2+)-sensitive component of NH(4)(+) transport is predominant and consists of permeation of NH(4)(+) through an apical ROMK-related channel.
髓袢升支粗段（MTAL）细胞中已描述了几种顶端铵（NH₄⁺/NH₃）转运途径。其中一些途径的确切性质和重要性仍存在争议。
通过测量细胞内pH值（pH(i)）研究原代培养兔MTAL细胞中的铵转运。
为创造生理条件，在对称存在NH₄Cl的情况下进行实验，这使细胞酸化至pH(i) 6.89。当使用顶端NH₄⁺转运阻滞剂时，由于NH₄⁺负载减少，细胞碱化。观察到以下值（pH单位）：布美他尼，+0.05；维拉帕米，+0.04；Ba²⁺和Cs⁺，+0.19；替地品，+0.09。四乙铵无作用。通过增加K⁺浓度使细胞去极化使细胞碱化0.16个pH单位。由于NH₄⁺可能通过非特异性通道进入，进行了铵脉冲实验：NH₄Cl脉冲使对照细胞以及去极化细胞酸化。相反，当存在Ba²⁺、Cs⁺或替地品时，NH₄Cl脉冲使细胞碱化。这种顶端NH₄⁺转运途径的药理学特征与肾外髓质K⁺（ROMK）通道相关。间接免疫荧光显示存在ROMK蛋白。
在这些MTAL细胞中，NH₄⁺转运的Ba²⁺敏感成分占主导，由NH₄⁺通过顶端ROMK相关通道的通透组成。
