Effect of Aquaporin 5 and Carbonic Anhydrase II on the Diffusion of CO2 in Xenopus Oocytes

Abstract

Transport of gases across the plasma membrane is not only dependent on the permeability, but also the concentration gradient. Carbon dioxide (CO2) is one of the most important physiological gases in the human body, but undergoes slow hydration/dehydration reactions. As a gas channel, human (h) AQP5 can accelerate the diffusion of CO2 by increasing membrane permeability to CO2. Carbonic anhydrases (CAs) also can accelerate the transport of CO2 across the membrane by maximizing the CO2 gradient via the consumption/production of CO2. Here, by measuring the maximal extracellular‐surface pH increase (DpHS) caused by the introduction of extracellular CO2/HCO3− at constant extracellular pH (pHo), we obtain a semiquantitative index of the CO2 influx into Xenopus oocytes. On Day 1, we injected oocytes with the cRNA encoding hAQP5 (or H2O as a control). On Day 4, we injected purified bovine CA II protein in Tris buffer (or Tris buffer as a control). In agreement with previous reports, both hAQP5 and CA II individually increase DpHS vs. H2O/Tris‐injected control oocytes. However, the effect of CA II alone in raising DpHS even at very high CA II concentrations (1000ng/oocyte), is not as effective as AQP5 alone. Most likely, the native CO2 permeability of Xenopus oocytes is so low that it limits transmembrane CO2 diffusion. Interestingly, injection of CA II—even at CA II levels so low that they barely increase DpHS in oocytes not expressing AQP5—into oocytes expressing AQP5 causes DpHS to increase substantially vs. AQP5 oocytes without CA II. Our data suggest that when membrane CO2 permeability is low, the effects of AQP5 and CA on CO2 fluxes are supra‐additive.Support or Funding InformationNIH U01‐GM111251, N00014‐15‐1‐2060, N00014‐16‐1‐2535This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.