ABSTRACT
Sorbitol, the sugar alcohol or polyol of D-glucose, is a compatible organic osmolyte. The cells of the renal medulla cope with this milieu by regulating the accumulation and dissipation of five principal organic osmolytes: sorbitol, inositol, betaine, glycerophosphocholine, and taurine. The intracellular signal for induction of aldose reductase activity and subsequent sorbitol accumulation has been investigated by comparing the effects of using different solutes to raise medium osmolality. When PAP-HT25 cells adapted to hypertonic medium are switched to isotonic medium, sorbitol exits rapidly from the cells. The PAP-HT25 cells accumulate sorbitol in response to osmotic induction because aldose reductase activity increases. The PAP-HT25 cells maintained at a constant osmolality in either hypertonic or isotonic medium have a very low permeability to sorbitol. The results are similar when the cells are grown under conditions in which they also contain large amounts of the other organic osmolytes in addition to sorbitol.
