Role of anion exchanger 1 in red cell CO2 permeation Objectives: It has been suggested in literature that the erythrocytic HCO3- / Cl- transporter (AE1) may be involved in the transport of molecular CO2 across biological membranes. In previous studies we and others had shown that DIDS, a potent inhibitor of AE1, reduces not only the bicarbonate permeability (P_HCO3) of red blood cells (RBC), but also their CO2 permeability (P_CO2). We wanted to study if AE1 either acts a direct CO2 pathway like a CO2 channel, or if alternatively CO2 entry into the RBC is based on HCO3- transfer via AE1, equivalent transfer of H+ via a proton (or OH-) pathway, and subsequent carbonic anhydrase (CA)-catalysed recombination of both ions to yield CO2 right after membrane passage. Methods: To test these hypothetical mechanisms we studied in RBC experiments i) the effects of several other inhibitors of AE1 on P_CO2 , and ii) the effect of lack of CAII, which is widely believed to be entirely associated with AE1 on the internal side of the membrane. In another experimental series, we expressed fusion proteins of AE1 and yellow fluorescent protein (YFP) in tsA201 cells and measured P_HCO3 and P_CO2 of control and AE1-YFP transfected cells. Results: In the RBC experiments we found that two other inihibitors of AE1, DiBAC and phloretin, while effectively reducing HCO3- transport, in contrast to DIDS do not affect P_CO2 . Similarly, CAII-deficient human red cells showed no major reduction in P_CO2 compared to normal RBCs. In the tsA201 cells experiments, expression of AE1 caused a significant increase of P_HCO3, but had no effect on P_CO2. Conclusion: The RBC experiments with inhibitors and CA II deficiency and, more clearly, the expression of AE1 in tsA201 cells do not support either of the proposed mechanisms of CO2 transfer across cell membranes involving AE1.