Effect of expressing the water channel aquaporin-1 on the CO2 permeability of Xenopus oocytes

Abstract

It is generally accepted that gases such as CO 2 cross cell membranes by dissolving in the membrane lipid. No role for channels or pores in gas transport has ever been demonstrated. Here we ask whether expression of the water channel aquaporin-1 (AQP1) enhances the CO 2 permeability of Xenopus oocytes. We expressed AQP1 in Xenopus oocytes by injecting AQP1 cRNA, and we assessed CO 2 permeability by using microelectrodes to monitor the changes in intracellular pH (pH i) produced by adding 1.5% CO 2/10 mM[Formula: see text] to (or removing it from) the extracellular solution. Oocytes normally have an undetectably low level of carbonic anhydrase (CA), which eliminates the CO 2 hydration reaction as a rate-limiting step. We found that expressing AQP1 (vs. injecting water) had no measurable effect on the rate of CO 2-induced pH i changes in such low-CA oocytes: adding CO 2 caused pH i to fall at a mean initial rate of 11.3 × 10−4 pH units/s in control oocytes and 13.3 × 10−4 pH units/s in oocytes expressing AQP1. When we injected oocytes with water, and a few days later with CA, the CO 2-induced pH i changes in these water/CA oocytes were more than fourfold faster than in water-injected oocytes (acidification rate, 53 × 10−4 pH units/s). Ethoxzolamide (ETX; 10 μM), a membrane-permeant CA inhibitor, greatly slowed the pH i changes (16.5 × 10−4 pH units/s). When we injected oocytes with AQP1 cRNA and then CA, the CO 2-induced pH i changes in these AQP1/CA oocytes were ∼40% faster than in the water/CA oocytes (75 × 10−4 pH units/s), and ETX reduced the rates substantially (14.7 × 10−4 pH units/s). Thus, in the presence of CA, AQP1 expression significantly increases the CO 2 permeability of oocyte membranes. Possible explanations include 1) AQP1 expression alters the lipid composition of the cell membrane, 2) AQP1 expression causes overexpression of a native gas channel, and/or 3) AQP1 acts as a channel through which CO 2 can permeate. Even if AQP1 should mediate a CO 2 flux, it would remain to be determined whether this CO 2 movement is quantitatively important.