Ammonium affects tight junctions and the cytoskeleton in MDCK cells

In the kidney medulla, tubule cells are exposed not only to elevated NaCl but also to high NH4Cl concentrations. Although it is well known that long-term exposure to high NaCl concentrations leads to reorganization of the actin-based cytoskeleton and to altered transport properties of renal epithelial cells, there have been no comparable studies on the effects of elevated extracellular NH4Cl concentrations. We therefore examined the effect of prolonged (up to 72 h) exposure of Madin-Darby canine kidney (MDCK) cells to increased NH4Cl concentrations on the actin-based cytoskeleton, the transepithelial electrical resistance (TER) and the expression and intracellular distribution of the tight junction protein occludin. NH4Cl exposure resulted in rarefaction of cytoplasmic stress fibres, formation of intense peripheral actin bands and reduced abundance of both F- and G-actin. While under control conditions occludin staining was restricted to the tight junction region, ample dot-like intracellular staining was apparent after NH4Cl exposure. These changes in cell structure were associated with an increase in TER and the enhanced expression of an additional putative, 40-kDa occludin isoform. Exposure to elevated extracellular NH4Cl concentrations thus leads to distinct alterations in the architecture and transepithelial transport properties of MDCK cells that may also be relevant for the tubule cells of the renal inner medulla.