Signal for induction of aldose reductase in renal medullary cells by high external NaCl.

GRB-PAP1 is a continuous line of epithelial cells derived from rabbit renal inner medulla. These cells accumulate large amounts of sorbitol when extracellular NaCl concentration is elevated. The accumulation involves an increase in aldose reductase, an enzyme that catalyzes production of sorbitol from glucose. The purpose of the present studies was to investigate the mechanism by which high NaCl triggers an increase in aldose reductase activity. When NaCl was added to increase medium osmolality from 300 to 500 mosmol/kg, enzyme activity began to increase after 12-24 h, was half-maximal in 2 days, and was maximal by 4 days. To test the effect of other solutes, medium osmolality was increased to 500 mosmol/kg by adding raffinose, urea, or glycerol. Raffinose increased aldose reductase activity as much as NaCl did. In contrast, neither urea nor glycerol affected the enzyme activity. NaCl and raffinose caused a sustained decrease in cell water content and an increase in cell sodium and potassium concentration, but urea did not. In some experiments, ouabain was added either with or without an increase in osmolality. Taking into account all of the experiments, with and without ouabain, we found that aldose reductase activity did not correlate with either cell sodium concentration or cell water content alone. It did correlate, however, with cell potassium concentration and even more strongly with the sum of cell sodium plus potassium concentration. We conclude that the signal by which hyperosmolality triggers an increase in aldose reductase activity most likely involves an increase in intracellular ionic strength.