Evidence that aquaporin 1 is a major pathway for CO 2 transport across the human erythrocyte membrane

We report here the application of a previously described method to directly determine the CO 2 permeability (P CO2 ) of the cell membranes of normal human red blood cells (RBCs) vs. those deficient in aquaporin 1 (AQP1), as well as AQP1‐expressing Xenopus laevis oocytes. This method measures the exchange of 18 O between CO 2 , HCO 3 – , and H 2 O in cell suspensions. In addition, we measure the alkaline surface pH (pH S ) transients caused by the dominant effect of entry of CO 2 vs. HCO 3 – into oocytes exposed to step increases in [CO 2 ]. We report that 1) AQP1 constitutes the major pathway for molecular CO 2 in human RBCs; lack of AQP1 reduces P CO2 from the normal value of 0.15 ± 0.08 (SD; n 85) cm/s by 60% to 0.06 cm/s. Expression of AQP1 in oocytes increases P CO2 2‐fold and doubles the alkaline pH S gradient. 2) pCMBS, an inhibitor of the AQP1 water channel, reduces P CO2 of RBCs solely by action on AQP1 as it has no effect in AQP1‐deficient RBCs. 3) P CO2 determinations of RBCs and pH S measurements of oocytes indicate that DIDS inhibits the CO 2 pathway of AQP1 by half. 4) RBCs have at least one other DIDS‐sensitive pathway for CO 2 . We conclude that AQP1 is responsible for 60% of the high P CO2 of red cells and that another, so far unidentified, CO 2 pathway is present in this membrane that may account for at least 30% of total P CO2 .—Endeward, V., Musa‐Aziz, R., Cooper, G. J., Chen, L., Pelletier, M. F., Virkki, L. V., Supuran, C. T., King, L. S., Boron, W. F., Gros, G. Evidence that aquaporin 1 is a major pathway for CO 2 transport across the human erythrocyte membrane. FASEB J. 20, 1974–1981 (2006)