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

V. Endeward, R. Musa-Aziz^*, G. J. Cooper^*,1, L.-M. Chen^*, M. F. Pelletier^*, L. V. Virkki^*,2, C. T. Supuran, L. S. King, W. F. Boron^* and G. Gros³

Zentrum Physiologie, Abt. Vegetative Physiologie, Medizinische Hochschule Hannover, Hannover, Germany;
^* Department of Cellular and Molecular Physiology, Yale School of Medicine, New Haven, Connecticut, USA;

Dipartimento di Chimica, Laboratorio di Chimica Bioinorganica, University of Florence, Sesto Fiorentino, Firenze, Italy; and

Department of Medicine, The Johns Hopkins University, Baltimore, Maryland, USA

³Correspondence: Zentrum Physiologie, Abt. Vegetative Physiologie, Medizinische Hochschule Hannover, 30625 Hannover, Germany. E-mail: gros.gerolf{at}mh-hannover.de

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, HCO₃^–, and H₂O in cell suspensions. In addition, we measure the alkaline surface pH (pH_S) 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 pH_S 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.—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 CO₂ transport across the human erythrocyte membrane.

Key Words: human red cell membrane • CO₂ permeability • pCMBS • DIDS

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