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 (18)O 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).

We have determined CO2 permeabilities, P(CO2), of red cells of normal human blood and of blood deficient in various blood group proteins by a previously described mass spectrometric technique. While P(CO2) of normal red cells is approximately 0.15 cm/s, we find in red blood cells (RBCs) lacking the Rh protein complex (Rh(null)) a significantly reduced P(CO2) of 0.07 cm/s +/-0.02 cm/s (P<0.02). This value is similar to the value we have reported previously for RBCs lacking aquaporin-1 protein (AQP-1(null)), suggesting that each of the Rh and AQP-1 proteins is responsible for approximately 1/2 of the normal CO2 permeability of the RBC membrane. Four other blood group deficiencies tested lack diverse membrane proteins but exhibit normal CO2 permeability. The CO2 pathway constituted by Rh proteins was inhibitable at pH(e)= 7.4 by NH4Cl with an I50 of approximately 10 mM corresponding to an I50 for NH3 of approximately 0.3 mM. The pathway independent of Rh proteins, presumably that constituted by AQP-1, was not inhibitable by NH4Cl/NH3. However, both pathways were strongly inhibited by DIDS, which accounts for the marked inhibitory effect of DIDS on normal P(CO2), while in contrast another AE1 inhibitor, DiBAC, does not inhibit P(CO2), although it markedly reduces P(HCO3-). We conclude that Rh protein, presumably the Rh-associated glycoprotein RhAG, possesses a gas channel that allows passage of CO2 in addition to NH3.

Recent observations that some membrane proteins act as gas channels seem surprising in view of the classical concept that membranes generally are highly permeable to gases. Here, we study the gas permeability of membranes for the case of CO(2), using a previously established mass spectrometric technique. We first show that biological membranes lacking protein gas channels but containing normal amounts of cholesterol (30-50 mol% of total lipid), e.g., MDCK and tsA201 cells, in fact possess an unexpectedly low CO(2) permeability (P(CO2)) of ∼0.01 cm/s, which is 2 orders of magnitude lower than the P(CO2) of pure planar phospholipid bilayers (∼1 cm/s). Phospholipid vesicles enriched with similar amounts of cholesterol also exhibit P(CO2) ≈ 0.01 cm/s, identifying cholesterol as the major determinant of membrane P(CO2). This is confirmed by the demonstration that MDCK cells depleted of or enriched with membrane cholesterol show dramatic increases or decreases in P(CO2), respectively. We demonstrate, furthermore, that reconstitution of human AQP-1 into cholesterol-containing vesicles, as well as expression of human AQP-1 in MDCK cells, leads to drastic increases in P(CO2), indicating that gas channels are of high functional significance for gas transfer across membranes of low intrinsic gas permeability.