Organic osmolyte channels in the renal medulla: Their properties and regulation

In the mammalian kidney renal medullary cells use organic osmolytes such as sorbitol, myo-inositol, glycerophosphorylcholine, betaine, and taurine to adjust their intracellular osmolarity (and thereby their volume) to rapid and drastic changes in extracellular osmolarity. Using an immortalized cell line derived from rabbit thick ascending limb of Henle's loop (TALH cells) and primary cultures of rat inner medullary collecting duct (IMCD cells) the membrane transport systems activated during exposure to hypotonicity were investigated. In TALH cells an increase in sorbitol permeability of the (luminal) plasma membrane occurs by activation of a channel-like transporter involving a calcium/calmodulin-dependent protein kinase. A similar system seems to operate in IMCD cells. In addition, the latter cells possess a swelling-activated anion channel that is also permeable for taurine and myo-inositol and inhibited by "anion channel" blockers, such as NPPB and DIDS. The sorbitol permeability of the plasma membrane appears to be furthermore regulated by a transient insertion of active transporters into the basolateral cell surface by a membrane recycling mechanism