The unique phenotype of renal medullary cells allows them to survive and functionally adapt to changes of interstitial osmolality/tonicity. We investigated the effects of acute hypertonic challenge on AQP2 (aquaporin-2) water channel trafficking. In the absence of vasopressin, hypertonicity alone induced rapid (<10 min) plasma membrane accumulation of AQP2 in rat kidney collecting duct principal cells in situ, and in several kidney epithelial lines. Confocal microscopy revealed that AQP2 also accumulated in the trans-Golgi network (TGN) following hypertonic challenge. AQP2 mutants that mimic the Ser 256 -phosphorylated and -nonphosphorylated state accumulated at the cell surface and TGN, respectively. Hypertonicity did not induce a change in cytosolic cAMP concentration, but inhibition of either calmodulin or cAMP-dependent protein kinase A activity blunted the hypertonicity-induced increase of AQP2 cell surface expression. Hypertonicity increased p38, ERK1/2, and JNK MAPK activity. Inhibiting MAPK activity abolished hypertonicity-induced accumulation of AQP2 at the cell surface but did not affect either vasopressin-dependent AQP2 trafficking or hypertonicity-induced AQP2 accumulation in the TGN. Finally, increased AQP2 cell surface expression induced by hypertonicity largely resulted from a reduction in endocytosis but not from an increase in exocytosis. These data indicate that acute hypertonicity profoundly alters AQP2 trafficking and that hypertonicity-induced AQP2 accumulation at the cell surface depends on MAP kinase activity. This may have important implications on adaptational processes governing transcellular water flux and/or cell survival under extreme conditions of hypertonicity.Most mammalian cells are exposed to an extracellular environment that remains isotonic under normal physiological conditions, largely as a result of renal regulatory mechanisms that maintain water and electrolyte body fluid composition within a very narrow range. This process relies on the countercurrent concentration mechanism that occurs along the loop of Henle and which, in turn, depends on the generation of a NaCl and urea concentration gradient along the cortico-papillary axis. As a consequence, renal medullary cells are physiologically exposed to changing conditions of variable interstitial osmolality/tonicity that can reach up to 1200 mosmol/kg in human inner medulla and up to 4500 mosmol/kg in some remarkable rodent species that are adapted to life in arid desert conditions (1). The unique phenotype of these cells allows them to survive and function under these "hostile" conditions and to rapidly adapt to recurrent variations in their environmental tonicity that follow physiological and behavioral changes. In addition to the promotion of intracellular events that ensure cell survival, hypertonicity also promotes events that mediate changes in cell function in various physiological settings. Both rapid and slow responses mediate cell adaptation to increased extracellular tonicity. Rapid responses include actin and micr...