Effect of PCMBS on CO₂permeability of <i>Xenopus</i> oocytes expressing aquaporin 1 or its C189S mutant

<jats:p> A recent study on Xenopus oocytes [N. L. Nakhoul, M. F. Romero, B. A. Davis, and W. F. Boron. Am. J. Physiol. 274 ( Cell Physiol. 43): C543–548, 1998] injected with carbonic anhydrase showed that expressing aquaporin 1 (AQP1) increases by ∼40% the rate at which exposing the cell to CO<jats:sub>2</jats:sub> causes intracellular pH to fall. This observation is consistent with several interpretations. Overexpressing AQP1 might increase apparent CO<jats:sub>2</jats:sub> permeability by 1) allowing CO<jats:sub>2</jats:sub> to pass through AQP1, 2) stimulating injected carbonic anhydrase, 3) enhancing the CO<jats:sub>2</jats:sub> solubility of the membrane’s lipid, or 4) increasing the expression of a native “gas channel.” The purpose of the present study was to distinguish among these possibilities. We found that expressing the H<jats:sub>2</jats:sub>O channel AQP1 in Xenopus oocytes increases the CO<jats:sub>2</jats:sub> permeability of oocytes in an expression-dependent fashion, whereas expressing the K<jats:sup>+</jats:sup> channel ROMK1 has no effect. The mercury derivative p-chloromercuriphenylsulfonic acid (PCMBS), which inhibits the H<jats:sub>2</jats:sub>O movement through AQP1, also blocks the AQP1-dependent increase in CO<jats:sub>2</jats:sub> permeability. The mercury-insensitive C189S mutant of AQP1 increases the CO<jats:sub>2</jats:sub> permeability of the oocyte to the same extent as does the wild-type channel. However, the C189S-dependent increase in CO<jats:sub>2</jats:sub>permeability is unaffected by treatment with PCMBS. These data rule out options 2–4 listed above. Thus our results suggest that CO<jats:sub>2</jats:sub>passes through the pore of AQP1 and are the first data to demonstrate that a gas can enter a cell by a means other than diffusing through the membrane lipid. </jats:p>