Long-term adaptation of renal cells to hypertonicity: role of MAP kinases and Na-K-ATPase

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Abstract
Renal cells in culture have low viability when exposed to hypertonicity. We developed cell lines of inner medullary collecting duct cells adapted to live at 600 and 900 mosmol/kgH2O. We studied the three modules of the mitogen-activated protein (MAP) kinase family in the adapted cells. These cells had no increase in either extracellular signal-regulated kinase, c-Jun NH2-terminal kinase, or p38 MAP kinase protein or basal activity. When acutely challenged with further increments in tonicity, they had blunted activation of these kinases, which was not due to enhanced phosphatase activity. In contrast, the cells adapted to the hypertonicity displayed a marked increment in Na-K-ATPase expression (5-fold) and ouabain-sensitive Na-K-ATPase activity (10-fold). The changes were reversible on return to isotonic conditions. Replacement of 300 mosmol/kgH2O of NaCl by urea in cells adapted to 600 mosmol/kgH2O resulted in marked decrement in Na-K-ATPase and failure to maintain the cell line. Replacement of NaCl for urea in cells adapted to 900 mosmol/kgH2O did not alter either Na-K-ATPase expression, or the viability of the cells. The in vivo modulation of Na-K-ATPase was studied in the renal papilla of water-deprived mice (urinary osmolality 2,900 mosmol/kgH2O), compared with that of mice drinking dextrose in water (550 mosmol/kgH2O). Increased water intake was associated with a ∼30% decrement in Na-K-ATPase expression ( P < 0.02, n = 6), suggesting that this enzyme is osmoregulated in vivo. We conclude that whereas MAP kinases play a role in the response to acute changes in tonicity, they are not central to the chronic adaptive response. Rather, in this setting there is upregulation of other osmoprotective proteins, among which Na-K-ATPase appears to be an important component of the adaptive process.