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¹ Physiologisches Institut der Universität, 80336 München, Germany

The cells of the kidney medulla are exposed routinely to high extracellular concentrations of various solutes including NaCl, urea and ammonium (NH₄⁺). Although it is well established that the expression of a variety of osmosensitive genes and proteins, which confer cytoprotection on renal medullary cells, is induced by high NaCl concentrations, the role of NH₄⁺ in these cellular responses is unclear. This study thus addressed the effect of NH₄⁺ on the expression of the betaine/GABA transporter (BGT-1), the sodium/myo-inositol cotransporter (SMIT), aldose reductase (AR), and heat shock protein 70 (HSP70) in Madin–Darby canine kidney (MDCK) cells, using Northern and Western blot analyses and enzyme-linked immunosorbent assay (ELISA). The incidence of apoptosis was monitored by determining caspase-3 activity and annexin V binding. Addition of NH₄Cl (50 mM; total osmolality 400 mosmol (kg H₂O)^–1 to the medium was more effective than equiosmolar NaCl in increasing BGT-1 and HSP70 mRNA abundance, but less effective in enhancing BGT-1 and HSP70 expression at the protein level. Qualitatively similar results were obtained for SMIT and AR mRNAs. Exposure to both isotonic and hypertonic, NH₄Cl-containing medium enhanced apoptosis compared with equiosmolar, NaCl-containing media. These results suggest that, in addition to NaCl, NH₄Cl may play a role in regulating the intracellular accumulation of organic osmolytes, the abundance of HSP70 and cell turnover in the renal medulla in vivo.

(Received 20 October 2004; accepted after revision 21 December 2004; first published online 21 December 2004)
Corresponding author F. X. Beck: Physiologisches Institut der Universität, Pettenkoferstrasse 12, 80336 München, Germany. Email: fx.beck{at}physiol.med.uni-muenchen.de

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