Effect of ammonium on the expression of osmosensitive genes in Madin-Darby canine kidney cells.

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.