Evidence that aquaporin 1 is a major pathway for CO₂ transport across the human erythrocyte membrane

We report here the application of a previously described method to directly determine the CO₂ 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 ¹⁸O between CO₂, HCO ₃^-, and H₂O in cell suspensions. In addition, we measure the alkaline surface pH (pHS) transients caused by the dominant effect of entry of CO₂ vs. HCO₃^- into oocytes exposed to step increases in [CO₂]. We report that 1) AQP1 constitutes the major pathway for molecular CO₂ 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 pHS measurements of oocytes indicate that DIDS inhibits the CO₂ pathway of AQP1 by half. 4) RBCs have at least one other DIDS-sensitive pathway for CO₂. We conclude that AQP1 is responsible for 60% of the high P_CO2 of red cells and that another, so far unidentified, CO₂ pathway is present in this membrane that may account for at least 30% of total P_CO2.