Aquaporin-1 (AQP1) water channel plays a critical role for water reabsorption in the urinary concentrating mechanism. AQP1 expression in renal cells is upregulated by hypertonicity, but not urea, suggesting the requirement of an osmotic gradient. To investigate whether AQP1 expression is regulated by apical and/or basolateral hypertonicity, murine renal medullary mIMCD-K2 cells grown on permeable support were exposed to hypertonic medium. When the medium on the apical or basolateral membrane side was switched to hypertonic, the transcellular osmotic gradient was dissipated within 8 It. Basolateral hypertonicity increased AQP1 expression more than apical hypertonicity. Comparable apical and basolateral hypertonicity without a transcellular hypertonic gradient, however, increased AQP1 expression. Cell surface biotinylation experiments revealed that hypertonicity promoted AQP1 trafficking to both plasma cell membranes. These results indicate that AQP1 expression is predominantly mediated by basolateral hypertonicity but a transcellular osmotic gradient is not necessary for its induction. (C) 2004 Elsevier Inc. All rights reserved.