Relative CO 2 /NH 3 selectivities of AQP1, AQP4, AQP5, AmtB, and RhAG

Abstract

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 (ΔpH S ) compared with day-matched controls injected with H 2 O 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 ΔpH S (CO 2 ) and faster τ pHs compared with controls. Oocytes expressing AmtB and RhAG, but not AQP4 or AQP5, exhibit greater ΔpH S (NH 3 ) values. Only AQPs exhibited significant osmotic water permeability (P f ). We computed channel-dependent (*) ΔpH S or P f by subtracting values for H 2 O oocytes from those of channel-expressing oocytes. For the ratio ΔpH S (CO 2 )*/P f *, the sequence was AQP5 > AQP1 ≅ AQP4. For ΔpH S (CO 2 )*/ΔpH S (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.