Acid-base effects on electrolyte transport in CA II-deficient mouse colon

To determine the role of carbonic anhydrase (CA) in colonic electrolyte transport, we studied Car-2(0) 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 HCO3- concentration, or reducing pH in CO2/HCO3--free HEPES-Ringer solution. Membrane-permeant methazolamide, but not impermeant benzolamide, at 0.1 mM prevented the effects of pH. Car-2(0) mice exhibited similar basal transport rates and responses to pH and CA inhibitors. We conclude that basal and pHstimulated colonic electrolyte absorption in mice requires CA I. CAII and IV may have accessory roles.