Transport of volatile solutes through AQP1

For almost a century it was generally assumed that the lipid phases of all biological membranes are freely permeable to gases. However, recent observations challenge this dogma. The apical membranes of epithelial cells exposed to hostile environments, such as gastric glands, have no demonstrable permeability to the gases CO 2 and NH 3 . Additionally, the water channel protein aquaporin 1 (AQP1), expressed at high levels in erythrocytes, can increase membrane CO 2 permeability when expressed in Xenopus oocytes. Similarly, nodulin-26, which is closely related to AQP1, can act as a conduit for NH 3 . A key question is whether aquaporins, which are abundant in virtually every tissue that transports O 2 and CO 2 at high levels, ever play a physiologically significant role in the transport of small volatile molecules. Preliminary data are consistent with the hypothesis that AQP1 enhances the reabsorption of HCO 3 − by the renal proximal tubule by increasing the CO 2 permeability of the apical membrane. Other preliminary data on Xenopus oocytes heterologously expressing the electrogenic Na + -HCO 3 − cotransporter (NBC), AQP1 and carbonic anhydrases are consistent with the hypothesis that the macroscopic cotransport of Na + plus two HCO 3 − occurs as NBC transports Na + plus CO 3 2- and AQP1 transports CO 2 and H 2 O. Although data – obtained on AQP1 reconstituted into liposomes or on materials from AQP1 knockout mice – appear inconsistent with the model that AQP1 mediates substantial CO 2 transport in certain preparations, the existence of unstirred layers or perfusion-limited conditions may have masked the contribution of AQP1 to CO 2 permeability.