ABSTRACT
Hypertonicity(hypernatremia) of extracellular fluid causes water movement out of cells, while hypotonicity(hyponatremia) causes water movement into cells, resulting in cellular shrinkage or cellular swelling, respectively. In most part of the body, the osmolality of extracellular fluid is maintained within narrow range(285-295 mOsm/kgH2O) and some deviations from this range are not problematic in most tissue of the body except brain. On the other hand, the osmolality in the human renal medulla fluctuates between 50 and 1,200 mOsm/kgH2O in the process of urine dilution and concentration. The adaptation of renal medullary cells to the wide fluctuations in extracellular tonicity is crucial for the cell survival. This review will summarize the mechanisms of urine concentration and the adaptation of renal medullary cells to the hypertonicity, which is mediated by TonEBP transcription factor and its target gene products(UT- A1 urea transporter etc.).