The secretion of pancreatic juice is stimulated by both secretin and CCK. These two hormones, however, are secreted in response to different stimuli, and they have different effects on the composition of pancreatic juice. The release of secretin occurs in response to a fall in duodenal pH to below 4.5; this pH fall occurs for only a short time, however, because the acidic chyme is rapidly neutralized by alkaline pancreatic juice. The secretion of CCK, by contrast, occurs in response to the protein and fat content of chyme in the duodenum.
Secretin stimulates the production of bicarbonate by the pancreas. Since bicarbonate neutralizes the acidic chyme and since secretin is released in response to the low pH of chyme, a negative feedback loop is completed. CCK, by contrast, stimulates the production of pancreatic enzymes such as trypsin, lipase, and amylase. Partially digested protein and fat are the most potent stimulators of CCK secretion, and CCK secretion continues until the chyme has passed through the duodenum and early region of the jejunum.
Secretin and CCK can have different effects on the same cells (the pancreatic acinar cells) because their actions are mediated by different second messengers. The second messenger of secretin action is cyclic AMP, whereas the second messenger for CCK is Ca2+.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.