Aquaporin 6 is permeable to glycerol and urea

Abstract

The passive water permeability (L _p) of AQP6 is activated by Hg²⁺. Our aim was to test if L _p was associated with a permeability to small hydrophilic molecules such as glycerol and urea. AQP6 was expressed in Xenopus laevis oocytes and activated by 0.3 mM of HgCl₂ in the bathing solution. HgCl₂ caused the oocytes to swell at a rate of about 0.3% min⁻¹. The L _p of AQP6 was determined from brief additions or removals of mannitol from the bathing solution, and compared to the L _p obtained from adding glycerol or urea. In paired experiments, L _{p (mannitol)} was 2.4±0.1, L _{p (glycerol)} 1.5±0.2, and L _{p (urea)} 0.7±0.1 (units: 10⁻⁵ cm s⁻¹ osmol⁻¹; 14 oocytes); the latter was not different from L _ps obtained for native oocytes. The L _ps were independent of the Hg²⁺-induced swelling and of the magnitude of the osmotic challenge (−75 to +100 mosmol). Hg²⁺ stimulated the uptake of [¹⁴C]glycerol fivefold and [¹⁴C]urea twofold in AQP6-expressing oocytes. There were no significant uptakes in native oocytes nor in AQP1-expressing oocytes whether these were treated with HgCl₂ or not. We conclude that water, glycerol and urea share an aqueous pathway in AQP6.