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Evidence against aquaporin-1-dependent CO₂ permeability in lung and kidney

Xiaohui Fang, Baoxue Yang, Michael A. Matthay and A. S. Verkman

AQP1-dependent CO₂ transport has been suggested from the increased CO₂ permeability in Xenopus oocytes expressing AQP1. Potential implications of this finding include AQP1-facilitated CO₂ exchange in mammalian lung and HCO₃^-/CO₂ transport in kidney proximal tubule. We reported previously that: (a) CO₂ permeability in erythrocytes was not affected by AQP1 deletion, (b) CO₂ permeability in liposomes was not affected by AQP1 reconstitution despite a 100-fold increased water permeability, and (c) CO₂ blow-off by the lung in living mice was not impaired by AQP1 deletion. We extend these observations by direct measurement of CO₂ permeabilities in lung and kidney. CO₂ transport across the air-space-capillary barrier in isolated perfused lungs was measured from changes in air-space fluid pH in response to addition/removal of HCO₃^-/CO₂ from the pulmonary artery perfusate. The pH was measured by pleural surface fluorescence of a pH indicator (BCECF-dextran) in the air-space fluid. Air-space fluid pH equilibrated rapidly (t_1/2 6 s) in response to addition/removal of HCO₃^-/CO₂. However, the kinetics of pH change was not different in lungs of mice lacking AQP1, AQP5 or AQP1/AQP5 together, despite an up to 30-fold reduction in water permeability. CO₂ transport across BCECF-loaded apical membrane vesicles from kidney proximal tubule was measured from the kinetics of intravesicular acidification in response to rapid mixing with a HCO₃^-/CO₂ solution. Vesicles rapidly acidified (t_1/2 10 ms) in response to HCO₃^-/CO₂ addition. However the acidification rate was not different in kidney vesicles from AQP1-null mice despite a 20-fold reduction in water permeability. The results provide direct evidence against physiologically significant transport of CO₂ by AQP1 in mammalian lung and kidney.

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