Role of anions and carbonic anhydrase in epithelia

The existence of carbonic anhydrase (carbonate dehydratase, EC 4.2.1.1) in blood was suspected and sought because the rates of spontaneous hydration and dehydration of CO2 and carbonic acid were slow compared with the rates of exchange of CO2 with blood. The existence of the enzyme in absorbing and secreting epithelial tissues has, in contrast, often been sought because its presence was required for the operations of theoretical models for the movements of H+ ions or HCO-3 into or out of epithelial cells. In addition to the HCl-secreting gastric mucosal epithelium, the enzyme was subsequently found in the rumen, in the kidney, especially those of species that produce acid urine, and salivary gland, the liver and biliary duct system, the mucosa of the small intestine, caecum and colon, the choroid plexuses and ciliary body of mammals, in toad urinary bladder and in the Cl-secreting cells of fish gill. The presence of carbonic anhydrase in exocrine pancreas does not seem to be well established. The enzyme, of molecular mass about 30kDa and containing one zinc atom, exists in three related forms: one of high specific activity and two of low specific activity, one of which is found in red skeletal muscle. Although most, but not all, types of erythrocyte contain both varieties, epithelia usually contain only the high-activity enzyme; however, ox rumen contains large quantities of the low-activity variety as do guinea-pig caecal and colonic mucosae. Salt transport in the intestinal tract is associated with movements of HCO-3 and H+ ions, yet although carbon dioxide stimulates solute and fluid transport in the gall bladder in jejunum, and inhibitors of carbonic anhydrase reduce fluid and ion transport across many epithelia the role of the enzyme in epithelial transport is not clearly understood. Knowledge of the rates of hydration and dehydration of CO2/HCO-3 in the fraction of the tissue water responsible for the H+-HCO-3 movements in many secretory epithelia is currently lacking.