1. In dogs with chronic gastric and
pancreatic fistulas, a liver extract meal adjusted to various pH levels ranging from 7.0 to 2.0, was introduced into the stomach and the increments in plasma
secretin levels were correlated with the pH of the liver extract meal and pancreatic
bicarbonate outputs. 2. The pH threshold for both
bicarbonate secretion and
secretin release was found to be about 4.5. With a stepwise decrease in the pH of the meal below pH 4.5, there were stepwise increments in the plasma
secretin concentrations and pancreatic
bicarbonate outputs. 3. Exogenous
secretin, given in graded doses ranging from 0.03 to 2.0 clinical unit/kg per hr, increased the plasma
secretin concentrations and
bicarbonate secretion in a dose-dependent fashion. 4. These results indicate that the pH threshold for release of endogenous
secretin is 4.5 and suggests that, at pH levels below 4.5, pancreatic
bicarbonate secretion depends upon the duodenal
acid load and is linearly correlated to an increment in plasma
secretin concentrations. 5. It is concluded that endogenous
secretin is a major determinant of pancreatic
bicarbonate secretion after a meal. 6. Pancreatic
protein secretion by intragastric liver extract meal was greatly increased both in experiments with liver extract meal, pH 4.0 or below, and I.V. infusion of
secretin at a dose of 0.12 u./kg per hr. It is questioned, however, whether this effect of
secretin is physiological.