It has been long known that water must cross biological membranes by means other than simple diffusion. However, it was not until recently, with the discovery of the Aquaporin 1 water channel that this question was answered. The  importance of this discovery was underlined by the awarding, in 2003, the Nobel Prize in Chemistry to Peter Agre for the discovery of water channels. 

Today, eleven mammalian proteins that belong to the Aquaporin family have been identified. The proteins present a conserved structure of six transmembrane domains with intracellular N- and C-termini.

Aquaporin 6 (AQP6) appears to be exclusively expressed in the kidney, particularly in intracellular vesicles in collecting duct intercalated cells. Unlike the other aquaporins, AQP6 was found to allow permeation of anions following activation with acidic pH or Hg²⁺ ions. Increased expression of AQP6 was shown in models of chronic alkalosis and lithium-induced nephrogenic diabetes insipidus.