Osmolytes.
The cells of the renal medulla osmotically adapt to chronic alterations in extracellular tonicity by appropriate changes in the intracellular contents of organic osmoeffectors. The major organic osmolytes are glycerophosphorylcholine, betaine, myo-inositol, sorbitol, and, possibly, taurine. When the concentrations of poorly permeant external solutes are acutely reduced, cells that have been adapted to high tonicities rapidly release organic osmolytes via specific transmembrane transport pathways. In contrast, when medullary cells are depleted of organic osmolytes, osmolyte accumulation on acute elevation of external tonicity is slow and involves stimulation of uptake, intracellular de novo synthesis, or inhibition of intracellular degradation, and is preceded by increased intracellular electrolyte concentrations. The available evidence suggests that this rise in intracellular ionic strength plays an important role in the initiation of those processes responsible for full adaptation of renal medullary cells to high tonicities. Recently, complementary DNAs encoding a myo-inositol and a betaine transporter have been isolated.