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

Nazih L. Nakhoul, Bruce A. Davis, Michael F Romero, Walter F. Boron

Research output: Contribution to journalArticle

288 Citations (Scopus)

Abstract

It is generally accepted that gases such as CO2 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 CO2 permeability of Xenopus oocytes. We expressed AQP1 in Xenopus oocytes by injecting AQP1 cRNA, and we assessed CO2 permeability by using microelectrodes to monitor the changes in intracellular pH (pH(i)) produced by adding 1.5% CO2/10 mM HCO3/- to (or removing it from) the extracellular solution. Oocytes normally have an undetectably low level of carbonic anhydrase (CA), which eliminates the CO2 hydration reaction as a rate-limiting step. We found that expressing AQP1 (vs. injecting water) had no measurable effect on the rate of CO2-induced phi changes in such low-CA oocytes: adding CO2 caused pH(i) to fall at a mean initial rate of 11.3 x 10-4 pH units/s in control oocytes and 13.3 x 10-4 pH units/s in oocytes expressing AQP1. When we injected oocytes with water, and a few days later with CA, the CO2-induced pH(i) changes in these water/CA oocytes were more than fourfold faster than in water-injected oocytes (acidification rate, 53 x 10-4 pH units/s). Ethoxzolamide (ETX; 10 μM), a membrane-permeant CA inhibitor, greatly slowed the pH(i) changes (16.5 x 10-4 pH units/s). When we injected oocytes with AQP1 cRNA and then CA, the CO2-induced pH(i) changes in these AQP1/CA oocytes were ~40% faster than in the water/CA oocytes (75 x 10-4 pH units/s), and ETX reduced the rates substantially (14.7 x 10-4 pH units/s). Thus, in the presence of CA, AQP1 expression significantly increases the CO2 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 CO2 can permeate. Even if AQP1 should mediate a CO2 flux, it would remain to be determined whether this CO2 movement is quantitatively important.

Original languageEnglish (US)
JournalAmerican Journal of Physiology - Cell Physiology
Volume274
Issue number2 43-2
StatePublished - Feb 1998
Externally publishedYes

Fingerprint

Aquaporin 1
Aquaporins
Xenopus
Oocytes
Permeability
Carbonic Anhydrases
Water
Complementary RNA
Gases
Cell membranes
Membrane Lipids
Ethoxzolamide
Carbonic Anhydrase Inhibitors
Membranes
Acidification
Microelectrodes
Hydration

Keywords

  • Intracellular pH

ASJC Scopus subject areas

  • Clinical Biochemistry
  • Cell Biology
  • Physiology
  • Physiology (medical)

Cite this

Effect of expressing the water channel aquaporin-1 on the CO2 permeability of Xenopus oocytes. / Nakhoul, Nazih L.; Davis, Bruce A.; Romero, Michael F; Boron, Walter F.

In: American Journal of Physiology - Cell Physiology, Vol. 274, No. 2 43-2, 02.1998.

Research output: Contribution to journalArticle

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abstract = "It is generally accepted that gases such as CO2 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 CO2 permeability of Xenopus oocytes. We expressed AQP1 in Xenopus oocytes by injecting AQP1 cRNA, and we assessed CO2 permeability by using microelectrodes to monitor the changes in intracellular pH (pH(i)) produced by adding 1.5{\%} CO2/10 mM HCO3/- to (or removing it from) the extracellular solution. Oocytes normally have an undetectably low level of carbonic anhydrase (CA), which eliminates the CO2 hydration reaction as a rate-limiting step. We found that expressing AQP1 (vs. injecting water) had no measurable effect on the rate of CO2-induced phi changes in such low-CA oocytes: adding CO2 caused pH(i) to fall at a mean initial rate of 11.3 x 10-4 pH units/s in control oocytes and 13.3 x 10-4 pH units/s in oocytes expressing AQP1. When we injected oocytes with water, and a few days later with CA, the CO2-induced pH(i) changes in these water/CA oocytes were more than fourfold faster than in water-injected oocytes (acidification rate, 53 x 10-4 pH units/s). Ethoxzolamide (ETX; 10 μM), a membrane-permeant CA inhibitor, greatly slowed the pH(i) changes (16.5 x 10-4 pH units/s). When we injected oocytes with AQP1 cRNA and then CA, the CO2-induced pH(i) changes in these AQP1/CA oocytes were ~40{\%} faster than in the water/CA oocytes (75 x 10-4 pH units/s), and ETX reduced the rates substantially (14.7 x 10-4 pH units/s). Thus, in the presence of CA, AQP1 expression significantly increases the CO2 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 CO2 can permeate. Even if AQP1 should mediate a CO2 flux, it would remain to be determined whether this CO2 movement is quantitatively important.",
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N2 - It is generally accepted that gases such as CO2 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 CO2 permeability of Xenopus oocytes. We expressed AQP1 in Xenopus oocytes by injecting AQP1 cRNA, and we assessed CO2 permeability by using microelectrodes to monitor the changes in intracellular pH (pH(i)) produced by adding 1.5% CO2/10 mM HCO3/- to (or removing it from) the extracellular solution. Oocytes normally have an undetectably low level of carbonic anhydrase (CA), which eliminates the CO2 hydration reaction as a rate-limiting step. We found that expressing AQP1 (vs. injecting water) had no measurable effect on the rate of CO2-induced phi changes in such low-CA oocytes: adding CO2 caused pH(i) to fall at a mean initial rate of 11.3 x 10-4 pH units/s in control oocytes and 13.3 x 10-4 pH units/s in oocytes expressing AQP1. When we injected oocytes with water, and a few days later with CA, the CO2-induced pH(i) changes in these water/CA oocytes were more than fourfold faster than in water-injected oocytes (acidification rate, 53 x 10-4 pH units/s). Ethoxzolamide (ETX; 10 μM), a membrane-permeant CA inhibitor, greatly slowed the pH(i) changes (16.5 x 10-4 pH units/s). When we injected oocytes with AQP1 cRNA and then CA, the CO2-induced pH(i) changes in these AQP1/CA oocytes were ~40% faster than in the water/CA oocytes (75 x 10-4 pH units/s), and ETX reduced the rates substantially (14.7 x 10-4 pH units/s). Thus, in the presence of CA, AQP1 expression significantly increases the CO2 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 CO2 can permeate. Even if AQP1 should mediate a CO2 flux, it would remain to be determined whether this CO2 movement is quantitatively important.

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