Relative CO 2 /NH 3 permeabilities of several members of the mammalian Aquaporin family: bAQP0, hAQP1, hAQP2, rAQP3, rAQP4‐M1, rAQP4‐M23, and hAQP8

Abstract

The mammalian aquaporins (AQPs) have 13 members in two major families, the orthodox AQPs and the aquaglyceroporins, which also transport glycerol and other small solutes. Previously, we showed that human (h) AQP1, rat (r) AQP4-M23, and rAQP5—abundantly expressed in cells with high rates of gas transport—exhibit selectivity for CO2 over NH3 (AQP4-M23 ≅ AQP5 > AQP1). Here we extend this analysis to: bAQP0, hAQP1 (repeated), hAQP2, rAQP3, rAQP4-M1, rAQP4-M23 (repeated), and hAQP8. We expressed the AQPs in Xenopus oocytes and used microelectrodes to record the maximum transient change in surface pH (ΔpHS) caused by exposure to extracellular CO2 (an increase) or NH3 (a decrease). We computed osmotic water permeability (Pf) from rates of cell swelling under hypotonic conditions. Subtracting the respective values for day-matched, H2O-injected control oocytes yielded channel-specific values (*). (ΔpHS*)CO2 was significantly different from 0 only for: AQP0, AQP1, and AQP4-M23. (−ΔpHS*)NH3 was significantly different from 0 only for: AQP1, AQP3, and AQP8. Pf* was significantly different from 0 for all tested AQPs. The ratio (ΔpHS*)CO2/Pf* fell in the sequence: hAQP1 > AQP4-M23 > AQP0 > [AQP2 ≅ AQP3 ≅ AQP4-M1 ≅ AQP8 = 0]. The ratio (ΔpHS*)CO2/(−ΔpHS*)NH3 fell in the sequence: [AQP0 ≅ AQP4-M23 = infinity] > AQP1 > [AQP3 ≅ AQP8 = 0]. Thus, this group of AQPs exhibits a diverse range of selectivities for CO2 vs NH3 vs H2O.