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Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520
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 (pHi) produced
by adding 1.5% CO2/10 mM
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
pHi to fall at a mean initial rate
of 11.3 × 10
4 pH
units/s in control oocytes and 13.3 × 104 pH units/s in oocytes
expressing AQP1. When we injected oocytes with water, and a few days
later with CA, the CO2-induced
pHi changes in these water/CA
oocytes were more than fourfold faster than in water-injected oocytes
(acidification rate, 53 × 104 pH units/s).
Ethoxzolamide (ETX; 10 µM), a membrane-permeant CA inhibitor, greatly
slowed the pHi changes (16.5 × 104 pH
units/s). When we injected oocytes with AQP1 cRNA and then CA, the
CO2-induced
pHi changes in these AQP1/CA
oocytes were ~40% faster than in the water/CA oocytes (75 × 104 pH units/s), and ETX
reduced the rates substantially (14.7 × 104 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.
intracellular pH
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