Volume regulation and ion transport in renal cells.
Volume regulatory mechanisms are reviewed for both short- and long-term adaptation of renal cells to anisotonic media. Within minutes after exposure to hypotonic solutions a common feature of many renal cells is the increase in plasma membrane potassium and chloride conductances. Although extrusion of intracellular potassium certainly contributes to a regulatory volume decrease, the role of chloride efflux itself is probably modest, given the relatively low intracellular chloride concentration. Indeed, other intracellular osmolytes such as taurine and other amino acids are transported out of the cell to achieve a regulatory volume decrease. On a longer-term basis cells from the renal medulla have to adapt to an extracellular milieu which can become extremely hypertonic under certain conditions. In tissue culture models, cells exposed to hypertonic media react by actively taking up small molecules, such as betaine, taurine and myo-inositol, and by synthesizing more sorbitol and glycerophosphocholine. For each of these osmolytes hypertonicity was shown to activate the transcription of a specific enzyme or transporter gene, the expression of which reaches a peak within 18 h of hypertonicity.