Hypotonicity-evoked release of organic osmolytes from distal renal cells: systems, signals, and sidedness.

After a detailed description of early cellular, membrane and intracellular events in rat renal medullary collecting duct cells when exposed to hypotonicity, a synopsis on organic osmolyte transport properties, possible trigger mechanisms, and the cellular location of transport pathways is given. From the data currently available on renal and nonrenal cells, it is concluded that hypotonicity-evoked efflux of all organic osmolytes appears to be mediated by transport proteins which share a variety of properties more typical for channels than for carriers. A large diversity seems to exist, however, for the signalling mechanisms. Such diversity allows the cells to regulate the intracellular concentration of different organic osmolytes independently of each other, giving flexibility to the spectrum of osmotic responses. The site of release also varies from cell to cell; here conservation of organic osmolytes for future reuptake or further metabolism appears to be the major determinant.