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Hormone Regulation of Digestion
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Hormone Regulation of Digestion

Hormonal Regulation of Digestion
Summary
Four Major Digestive Hormones
Gastrin
  • Stimuli: stomach expansion, protein and caffeine in the stomach, alkaline chyme in the stomach
– Secretion Site: enteroendocrine cells of the stomach mucosa
  • Major Actions:
– Stimulates gastric juice (HCl, mucus, pepsinogen) secretion – Stimulates gastric motility – Pyloric sphincter relaxation
Secretin
  • Stimuli: acidic chyme in the duodenum
– Secretion Site: enteroendocrine cells of the duodenal mucosa
  • Major Actions:
– Stimulates pancreatic enzyme secretions into duodenum via the sphincter of Oddi (Pancreatic secretions = bicarbonate-rich, neutralize acidic chyme.)
Cholecystokinin (CCK)
  • Stimulus: triglycerides, fatty acids, and amino acids (part of chyme) in the duodenum
– Secretion Site: enteroendocrine cells of the duodenal mucosa
  • Major Actions:
– Stimulates bile secretion from gallbladder – Stimulates pancreatic enzyme secretion – Promotes sphincter of Oddi relaxation.
CCK and secretin potentiation: together stimulate a much greater release of pancreatic enzymes than if either hormone acts alone.
Glucose-dependent insulinotropic peptide (GIP)
  • Stimulus: glucose (also: fatty acids and amino acids; all present in chyme) in the duodenum
– Secretion Site: enteroendocrine cells of the duodenal mucosa
  • Major Action:
– Stimulates pancreas insulin secretion – Inhibits gastric acid secretion.
Four Criteria of Gastrointestinal Hormone
A substance must meet four different criteria to be considered a gastrointestinal hormone.
  • The substance is secreted into the bloodstream in response to a physiologic stimulus (in this case, ingestion of a meal) to a target site, resulting in a physiologic action (to regulate digestive actions).
  • The function of the substance acts independently of neural activity.
  • The substance can be isolated and purified for chemical identification as well as synthesized again.
  • The isolated substance, upon intravenous injection, can induce the same physiologic response as when it receives the appropriate stimulus.
The four major digestive tract hormones meet all of these criteria.
Candidate Hormones
Candidate Hormone Description
  • Regulate digestion but fail to meet all four of the aforementioned criteria.
– Considered to have putative roles in GI regulation.
Key Candidate Hormones
  • Motilin – increases GI motility and specifically mediates the migrating myoelectric complex.
  • Pancreatic polypeptide – inhibits pancreatic bicarbonate and enzyme secretions.
  • Enteroglucagon – responds to low blood glucose concentration and stimulates glycogenolysis and glucagonogenesis by the liver.
  • Glucagon-like peptide-1 (GLP-1) – stimulates insulin secretion by the pancreas.
Additional Note:
There are many unique types of enteroendocrine cells, each of which secrete a specific hormone.* – For example, G cells are enteroendocrine cells that specifically secrete gastrin.

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