Gas channels

While studying pH_i regulation in gastric parietal cells we found that the apical membrane has no detectable permeability to either CO₂ or NH₃—the first documented gas-impermeable membrane. This led us to rethink “Overton’s rule” and to describe the first gas channel aquaporin 1 (AQP1), which is not only permeable to H₂O but also CO₂. Others later showed that AQP1 is permeable to NH₃, and that the rhesus (Rh) proteins are permeable to CO₂ and NH₃.

We developed an approach for using a large pH microelectrode to monitor the extracellular surface pH (pH_S) on Xenopus oocytes exposed to CO₂ or NH₃. Initial pH_S transients are indices of CO₂ or NH₃ permeability. Studying a wide range of AQPs and Rh proteins, we see that each has a characteristic CO₂/NH₃ permeability ratio—the first example of gas selectivity. Preliminary work suggests that NH₃ moves only through AQP or Rh monomers, whereas CO₂ generally prefers the central pore of the oligomer. Preliminary stopped-flow work with erythrocytes (RBCs) indicates that certain inhibitor combinations can block 75% of O₂ efflux from RBCs—the first evidence for O₂ channels.