Relative CO2/NH3 selectivities of mammalian aquaporins 0-9

Previous work showed that aquaporin 1 (AQP1), AQP4-M23, and AQP5 each has a characteristic CO2/NH3 and CO2/H2O permeability ratio. The goal of the present study is to characterize AQPs 0-9, which traffic to the plasma membrane when heterologously expressed in Xenopus oocytes. We use video microscopy to compute osmotic water permeability (Pf) and microelectrodes to record transient changes in surface pH (Delta pHS) caused by CO2 or NH3 influx. Subtracting respective values for day-matched, H2O-injected control oocytes yields the channel-specific values P-f* and Delta pHS*. We find that P-f* is significantly > 0 for all AQPs tested except AQP6. (Delta pHS*) CO2 is significantly > 0 for AQP0, AQP1, AQP4-M23, AQP5, AQP6, and AQP9. (Delta pHS*) NH3 is > 0 for AQP1, AQP3, AQP6, AQP7, AQP8, and AQP9. The ratio (Delta pHS*) CO2 / Pf* falls in the sequence AQP6 (infinity) > AQP5 > AQP4-M23 > AQP0 congruent to AQP1 congruent to AQP9 > others (0). The ratio (Delta pHS*) NH3/P-f* falls in the sequence AQP6 (infinity) > AQP3 congruent to AQP7 congruent to AQP8 congruent to AQP9 > AQP1 > others (0). Finally, the ratio (Delta pHS*) CO2/(-Delta pHS*) NH3 falls in the sequence AQP0 (infinity) congruent to AQP4-M23 congruent to AQP5 > AQP6 > AQP1 > AQP9 > AQP3 (0) congruent to AQP7 congruent to AQP8. The ratio (Delta pHS*) CO2 /(-Delta pHS*) NH3 is indeterminate for both AQP2 and AQP4-M1. In summary, we find that mammalian AQPs exhibit a diverse range of selectivities for CO2 vs. NH3 vs. H2O. As a consequence, by expressing specific combinations of AQPs, cells could exert considerable control over the movements of each of these three substances.