Gastric Carbonic Anhydrase In

Davenport and Fisher (1938) reported the discovery DOGS P School of Medicine of large amounts of carbonic anhydrase in the gastric mucosa of cats, rats and rabbits. Davenport (1939) demonstrated that carbonic anhydrase is present in high concentration in the parietal cells and in lower concentration in the cells of the surface epithelium of the gastric mucosa of cats and rats. When those investigations were carried out dogs were not available. At present work on the mechanism of the secretion of hydrochloric acid is being done using dogs as experimental animals. It is believed that carbon dioxide is formed in the parietal cells by the oxidation of a metabolite. This carbon dioxide is rapidly hydrated to carbonic acid with carbonic anhydrase catalysing the reaction. The carbonic acid then ionizes to give hydrogen ions and bicarbonate ions. By some means not at present understood the hydrogen ions are concentrated and secreted as the cations of hydrochloric acid while the bicarbonate ions are returned to the blood to replace the chloride ions removed in the acid secretion. Since the presence of carbonic anhydrase in the parietal cells is an important part of this hypothesis, it was thought desirable to demonstrate the occurrence of carbonic anhydrase in the gastric mucosa of dogs. METHODS. The dogs were freed of blood by the viviperfusion method of Whipple (1926). The dogs were anesthetized with ether, and a cannula was inserted into the jugular vein. Ringer's solution containing 5 per cent glucose was injected by gravity, and the circulating fluid was allowed to flow out of the carotid artery at the same rate. By means of adrenalin injections the dogs' hearts were kept beating as long as possible. The hematocrit reading of the effluent fluid fell from about 38 per cent to below 1 per cent. At death the animals' tissues were not absolutely free of blood. Carbonic anhydrase was determined by the method of Meldrum and Roughton (1933). The apparatus was used at OOC. and at atmospheric pressure. The activity was calculated and expressed as enzyme units (E) according to the method of Meldrum and Roughton, but no correction was applied to bring the calculated activity to 15OC. The enzyme unit used in this work is therefore 2 to 3 times smaller than that of Meldrum and Roughton.