Dual Effects of Hypertonicity on Aquaporin-2 Expression in Cultured Renal Collecting Duct Principal Cells

Abstract

The driving force for renal water reabsorption is provided by the osmolarity gradient between the interstitium and the tubular lumen, which is subject to rapid physiologic variations as a consequence of water intake fluctuations. The effect of increased extracellular tonicity/osmolarity on vasopressin-inducible aquaporin-2 (AQP2) expression in immortalized mouse collecting duct principal cells (mpkCCD_cl4) is investigated in this report. Increasing the osmolarity of the medium either by the addition of NaCl, sucrose, or urea first decreased AQP2 expression after 3 h. AQP2 expression then increased in cells exposed to NaCl- or sucrose-supplemented hypertonic medium after longer periods of time (24 h), while urea-supplemented hyperosmotic medium had no effect. Altered AQP2 expression induced by both short-term (3 h) and long-term (24 h) exposure of cells to hypertonicity arose from changes in AQP2 gene transcription because hypertonicity did not modify AQP2 mRNA stability nor AQP2 protein turnover. On the long-term, vasopressin (AVP) and hypertonicity increased AQP2 expression in a synergistic manner. Hypertonicity altered neither the dose-responsiveness of AVP-induced AQP2 expression nor cAMP-protein kinase (PKA) activity, while PKA inhibition did not reduce the extent of the hypertonicity-induced increase of AQP2 expression. These results indicate that in collecting duct principal cells: (1) a short-term increase of extracellular osmolarity decreases AQP2 expression through inhibition of AQP2 gene transcription; (2) a long-term increase of extracellular tonicity, but not osmolarity, enhances AQP2 expression via stimulation of AQP2 gene transcription; and (3) long-term hypertonicity and PKA increases AQP2 expression through synergistic but independent mechanisms.