Evidence for a sorbitol transport system in immortalized human renal interstitial cells

Sorbitol plays a major role in the maintenance of cell volume and functiona l integrity of several renal cells. Sorbitol synthesis takes place in inner collecting duct cells, whereas sorbitol dehydrogenase activity, which cata lyzes the degradation of sorbiotol to fructose, could mainly be detected in renal inner medullary interstitial cells. Therefore, we supposed that inte rstitial cells would require a sorbitol transport into the cells. However, such a transport system has not yet been described. Therefore, we have char acterized the uptake of sorbitol in immortalized interstitial TK-173 cells, which were derived from human renal fibroblasts. Comparable to fresh isola ted renal fibroblasts of the rat, immortalized TK-173 cells have a high sor bitol dehydrogenase activity. In this report, a temperature-dependent sorbi tol uptake with saturation kinetics could be detected in immortalized TK-17 3 cells. The transport is characterized by a high velocity (V-max 84 mmol/l x h) and an apparent K-m of 10 mmol/l. The sorbitol uptake is independent of membrane potential, sodium, and chloride. Altogether, the physiological characteristics of this sorbitol transport are different from those of the osmotically regulated sorbitol efflux from epithelial cells. These results provide evidence that TK-173 cells derived from renal fibroblasts have a sp ecific sorbitol transport. Furthermore, these data suggest a cooperation be tween epithelial and interstitial cells concerning osmoregulation. Copyrigh t (C) 2001 S. Karger AG, Basel.