Osmoregulation by slow changes in aldose reductase and rapid changes in sorbitol flux

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Abstract
Renal medullary extracellular NaCl concentration is high during antidiuresis. To compensate, the cells accumulate large amounts of nonperturbing, osmotically active solutes (organic “osmolytes”), including sorbitol. GRB-PAP1 is a continuous line of epithelial cells from rabbit inner medulla. These cells accumulate sorbitol when medium NaCl concentration is elevated. The accumulation involves increase in aldose reductase, which catalyzes production of sorbitol from glucose. The purpose of the present study was to investigate control of cell sorbitol once aldose reductase was induced. We measured cell sorbitol, cell-to-medium sorbitol flux, and aldose reductase in cells grown in medium made hyperosmotic (600 mosmol/kg) with added NaCl and at intervals after medium osmolality was reduced to 300 mosmol/kg. In the hyperosmotic medium, cell sorbitol averaged 990 mmol/kg protein (approximately 260 mM), and its flux into the medium was 740 mmol.kg cell protein-1.day-1 (permeability less than 2 X 10(-9) cm/s). Within 5 min after return to isosmotic medium, sorbitol efflux increased greater than 150-fold. By the end of 1 day, cell sorbitol fell 77% but aldose reductase decreased only 10%. Aldose reductase then fell slowly to low levels over 2 wk. Thus renal medullary cells, chronically adapted to high NaCl, reduced their sorbitol level on return to isosmotic conditions by at least two mechanisms: 1) rapid increase in sorbitol flux into the medium, and 2) slow changes in the amount of aldose reductase.