ABSTRACT
This chapter discusses the regulation of production, transport, and intracellular concentration of polyols and sugars, the major compatible osmolytes. The polyols include sorbitol, inositol, arabitol, mannitol, and heterosides, while the sugars include glucose, mannose, fructose, sucrose, and trehalose. Inner medullary osmolality varies with the state of hydration: increasing during antidiuresis and decreasing during diuresis. Sorbitol accumulates during antidiuresis in the inner medulla of all mammals studied to date, including rat, rabbit, vole, deer mouse, pocket mouse, sheep, and dog. The intrarenal distribution of aldose reductase has been determined by immunocytochemistry, enzyme activity, northern blotting, and reverse traoscription-polymcrase chain reaction. Cultured cells have been used widely to study the regulation and genetic control of sorbitol production. Sorbitol dehydrogenase catalyzes the degradation of sorbitol to d-fructose. In rats receiving food and water ad libitum, sorbitol dehydrogenase activity was found to be quite low in all medullary collecting duct subsegments, but was 20-fold higher in proximal tubule segments.
