In antidiuresis, the intrarenal distribution of HSP25/27, alpha beta -crystallin, HSP72, OSP94 and HSP110 corresponds to the osmotic gradient between cortex and papilla: low amounts in the cortex and high values in the inner medulla and papilla. In addition, medullary HSP72 levels change appropriately with the diuretic state. Studies on MUCK cells suggest that, in the renal medulla in vivo, stressors, such as NaCl and low pH, may act in concert to induce HSP72 expression. Urea, added to the medium at high concentrations (600 mM), causes the majority of MUCK cells to die. Prior exposure of these cells to hypertonic media (NaCl addition), a maneuver that induces HSP72, protects the cells against the deleterious effects of high urea concentrations. Inhibition of HSP72 expression by stable antisense transfection or SB203580 treatment abolishes the beneficial effects of prior hypertonic stress. Conversely, overexpression of HSP72 under isotonic conditions by a dexamethasone-driven vector confers substantial resistance against subsequent exposure to high urea concentrations. Taken together these results suggest that also in the renal inner medulla, NaCl-induced enhancement of HSP72 expression may help counteract the detrimental effects of high urea concentrations. Copyright (C) 2000 S. Karger AG Basel.