Relative CO_2/NH_3 selectivities of AQP1, AQP4, AQP5, AmtB, and RhAG

摘要

The water channel aquaporin 1 (AQP1) and certain Rh-family members are permeable to CO_2 and NH_3. Here, we use changes in surface pH (pH_s) to assess relative CO_2 vs. NH_3 permeability of Xenopus oocytes expressing members of the AQP or Rh family. Exposed to CO_2 or NH_3, AQP1 oocytes exhibit a greater maximal magnitude of pH_s change (ApHs) compared with day-matched controls injected with H_2O or with RNA encoding SGLT1, NKCC2, or PepT1. With CO_2, AQP1 oocytes also have faster time constants for pH_s relaxation (τ_(phs)). Thus, AQP1, but not the other proteins, conduct CO_2 and NH_3. Oocytes expressing rat AQP4, rat AQP5, human RhAG, or the bacterial Rh homolog AmtB also exhibit greater △pH_s(CO_2) and faster τ_(phs) compared with controls. Oocytes expressing AmtB and RhAG, but not AQP4 or AQP5, exhibit greater △pH_s(NH_3) values. Only AQPs exhibited significant osmotic water permeability (P_f). We computed channel-dependent (~*) △pH_s or P_f by subtracting values for H_2O oocytes from those of channel-expressing oocytes. For the ratio △pH_s(CO_2)~*/P_f~* the sequence was AQP5 > AQP1 ≌ AQP4. For △pH_s(CO_2)~*/△pHs(NH_3)~*, the sequence was AQP4 ≌ AQP5 > AQP1 > AmtB > RhAG. Thus, each channel exhibits a characteristic ratio for indices of CO_2 vs. NH_3 permeability, demonstrating that, like ion channels, gas channels can exhibit selectivity.