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1 Nephrology Section, Veterans Affairs Medical Center and New York University School of Medicine, New York, New York 10010; and 2 Department of Biochemistry and Molecular Biology, St. Louis University School of Medicine, St. Louis, Missouri 63104
To
determine the role of carbonic anhydrase (CA) in colonic electrolyte
transport, we studied Car-20 mice, mutants
deficient in cytosolic CA II. Ion fluxes were measured under
short-circuit conditions in an Ussing chamber. CA was analyzed by assay
and Western blots. In Car-20 mouse colonic mucosa,
total CA activity was reduced 80% and cytosolic CA I and
membrane-bound CA IV activities were not increased. Western blots
confirmed the absence of CA II in Car-20 mice.
Normal mouse distal colon exhibited net Na+ and
Cl
absorption, a serosa-positive PD, and was
specifically sensitive to pH. Decrease in pH stimulated active
Na+ and Cl
absorption whether it was
caused by increasing solution PCO2, reducing HCO
3 concentration, or
reducing pH in
CO2/HCO
3-free HEPES-Ringer
solution. Membrane-permeant methazolamide, but not impermeant
benzolamide, at 0.1 mM prevented the effects of pH.
Car-20 mice exhibited similar basal transport rates
and responses to pH and CA inhibitors. We conclude that basal and
pH-stimulated colonic electrolyte absorption in mice requires CA I. CA
II and IV may have accessory roles.
pH; carbon dioxide tension; carbonic anhydrase isoenzymes; methazolamide; benzolamide
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