Intracellular sorbitol content in isolated rat inner medullary collecting duct cells. Regulation by extracellular osmolarity.

In order to study the mechanisms involved in the regulation of renal inner medullary sorbitol content, collecting duct cells were isolated from rat inner medulla and the effect of extracellular osmolarity on sorbitol synthesis and sorbitol content was investigated. Cells isolated at 300 mosmol/l and incubated up to 24 h as primary cultures in 300 mosmol/l media or in media made 600 mosmol/l by the addition of 150 mM NaCl showed no difference in total synthesis. Intracellular sorbitol content was, however, 2.3-fold higher in the cells kept in the higher osmotic medium. Cells isolated at 600 mosmol/l released sorbitol about 8 times faster when transferred into hypoosmotic medium (300 mosmol/l) than when transferred into isoosmotic (600 mosmol/l) media. Cells exposed to hyperosmotic media (900 mosmol/l with NaCl) maintained a higher intracellular sorbitol content than cells incubated in isoosmotic media. Changes of intracellular sorbitol content could not be attributed entirely to cell lysis--as demonstrated by determination of cellular content of lactate and lactate dehydrogenase. The alteration in sorbitol membrane permeability was reversible and was only observed when poorly permeable solutes (such as NaCl and sucrose) were used for the experiments, changes in urea elicited no effect. It is proposed that rapid changes in membrane permeability to sorbitol play an important role in the adjustment of intracellular sorbitol concentration in inner medullary collecting duct cells to changes in extracellular osmolarity.