1. Duodenal mucosa was collected from control rats and from animals which had received cysteamine, cysteamine plus cimetidine or pentagastrin. Animals which received cysteamine with or without cimetidine developed acute duodenal ulcers. Cysteamine treatment resulted in gastric acid hypersecretion, which was largely abolished by concurrent cimetidine administration.

2. Activities of enzymes implicated in bicarbonate secretion, HCO3-activated ATPase, carbonic anhydrase and Na+ + K+-activated ATPase, were measured in the duodenal mucosa of control rats and animals 24 h after subcutaneous administration of cysteamine. Assays of these enzymes in duodenal mucosal homogenates from animals with cysteamine-induced ulcers showed significant decreases in HCO3-activated ATPase and carbonic anhydrase activities compared with controls.

3. Alkaline phosphatase activity also fell significantly in the cysteamine-treated animals, and possibly reflects the HCO3-activated ATPase activity in the brush-border membrane. in contrast, activities of other marker enzymes from the brush-border membrane and from several intracellular organelles were unchanged, indicating an absence of gross organelle pathology in this experimental model.

4. Similar changes in enzyme activity were not caused by treatment with pentagastrin. Administration of cimetidine with the cysteamine did not protect the animals against ulceration, and the activity of HCO3-activated ATPase was persistently decreased. However, the carbonic anhydrase activity was unaltered in this latter group, compared with controls.

5. These findings suggest that in cysteamine-induced duodenal ulceration both gastric acid hypersecretion and impaired duodenal resistance occurs. It is suggested that decreased activities of key enzymes implicated in HCO3 secretion may reflect the biochemical basis for the decreased mucosal resistance.

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