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      Investigation of the Ba 2+-Sensitive NH 4 + Transport Pathways in the Apical Cell Membrane of Primary Cultured Rabbit MTAL Cells

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          Background: Several apical ammonium (NH<sub>4</sub><sup>+</sup>/NH<sub>3</sub>) transport pathways have been described in medullary thick ascending limb (MTAL) cells. The exact nature and importance of some of these pathways remain controversial. Methods: Ammonium transport in primary cultured rabbit MTAL cells was investigated by measuring intracellular pH (pH<sub>i</sub>). Results: To create physiological conditions, experiments were performed in the symmetrical presence of NH<sub>4</sub>Cl, which acidified the cells to pH<sub>i</sub> 6.89. When blockers of apical NH<sub>4</sub><sup>+</sup> transport were used, the cells alkalinized due to a decreased NH<sub>4</sub><sup>+</sup> loading. The following values (pH units) were observed: bumetanide, +0.05; verapamil, +0.04; Ba<sup>2+</sup> and Cs<sup>+</sup>, +0.19; tertiapin, +0.09. Tetraethylammonium had no effect. Depolarizing the cells by increasing the K<sup>+</sup> concentration alkalinized the cells by 0.16 pH units. Because NH<sub>4</sub><sup>+</sup> might enter through nonspecific channels, ammonium pulse experiments were performed: an NH<sub>4</sub>Cl pulse acidified controls as well as depolarized cells. In contrast, when Ba<sup>2+</sup>, Cs<sup>+</sup> or tertiapin were present, an NH<sub>4</sub>Cl pulse alkalinized the cells. The pharmacological profile of this apical NH<sub>4</sub><sup>+</sup> transport pathway correlates with the renal outer medullary K<sup>+</sup> (ROMK) channel. Indirect immunofluorescence showed the presence of the ROMK protein. Conclusion: In these MTAL cells the Ba<sup>2+</sup>-sensitive component of NH<sub>4</sub><sup>+</sup> transport is predominant and consists of permeation of NH<sub>4</sub><sup>+</sup> through an apical ROMK-related channel.

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          Most cited references 14

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          Molecular diversity and regulation of renal potassium channels.

          K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.
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            Intracellular pH measurements in Ehrlich ascites tumor cells utilizing spectroscopic probes generated in situ

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              A novel high-affinity inhibitor for inward-rectifier K+ channels.

               Joseph Jin,  Z. Lu (1998)
              Inward-rectifier K+ channels are a group of highly specialized K+ channels that accomplish a variety of important biological tasks. Inward-rectifier K+ channels differ from voltage-activated K+ channels not only functionally but also structurally. Each of the four subunits of the inward-rectifier K+ channels has only two instead of six transmembrane segments compared to the voltage-activated K+ channels. Thus far, there are no high-affinity ligands that directly target any inward-rectifier K+ channel. In the present study, we identified, purified, and synthesized a protein inhibitor of the inward-rectifier K+ channels. The inhibitor, called tertiapin, blocks a G-protein-gated channel (GIRK1/4) and the ROMK1 channel with nanomolar affinities, but a closely related channel, IRK1, is insensitive to tertiapin. Mutagenesis studies show that teritapin inhibits the channel by binding to the external end of the ion conduction pore.

                Author and article information

                Nephron Physiol
                Nephron Physiology
                S. Karger AG
                July 2007
                15 June 2007
                : 106
                : 3
                : p45-p53
                Department of Physiology, Biomedical Research Institute, Universiteit Hasselt, Diepenbeek, Belgium
                103909 Nephron Physiol 2007;106:p45–p53
                © 2007 S. Karger AG, Basel

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                Page count
                Figures: 7, References: 34, Pages: 1
                Original Paper


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