Gastric acid secretion, serum gastrin and parietal cell histology in rat hyperthyroidism.

Experimental hyperthyroidism was produced in rats by thyroxin injection, and changes in gastric acid secretion and serum gastrin level were determined to analyze the relation between these changes and hyperthyroidism. Administration of thyroxin to two groups of rats-20 micrograms per 100 g of body weight for 20 days and 75 micrograms for 7 days-brought about significant increases in serum T3 level, gastric pH and serum gastrin level. An increase in gastric pH took place later than that in serum T3 level; this time lag implies that a decrease in acid secretion was not caused by an direct effect of thyroxin on the parietal cell but by its secondary effect. In the T4-injected rats with decreased gastric acid secretion, the parietal cell remained normal in form and succinic dehydrogenase activity was also normal. The electron microscopical observations showed nothing abnormal in the parietal cells. These findings suggest that a decrease in acid secretion was not due to any structural changes in the gastric mucosa but to functional suppression. The serum gastrin level rose in correlation with an increase in gastric pH and fell by HCl administration to the stomach. Feedback inhibition by pH remained in the G cell.