Digestive System Hormones and Enzymes

GLUCAGON

PRODUCED / SECRETED BY: Alpha or A cells of pancreatic islets. STIMULATED BY: Decreased blood level of glucose, exercise, mainly protein meals; inhibited by somatostatin and insulin. MAJOR EFFECTS: Raises blood glucose level by acceleration breakdown of glycogen into glucose in the liver (glycogenolysis) and conversion of other nutrients into glucose in liver (gluconeogenesis) and releasing glucose into blood.

Insulin

PRODUCED / SECRETED BY: Beta or B cells of pancreatic islets. STIMULATED BY: Increased blood level of glucose, acetylcholine, arginine and leucine (two amino acids) glucagon, GIP, hGH, and ACTH; inhibited by somatostatin. MAJOR EFFECTS: Lowers blood glucose level by accelerating glucose uptake and utilization in cells; stimulating glycogen formation in skeletal muscle and liver cells (glycogenesis), also stimulates formation and storage of lipids (lipogenesis) and stimulates protein synthesis.

Somatostatin

PRODUCED / SECRETED BY: Delta or D cells of pancreatic islets. STIMULATED BY: Inhibited by pancreatic polypeptide. MAJOR EFFECTS: Inhibits secretion of insulin and glucagon and slows absorption of nutrients from the gastrointestinal tract.

Cholecystokinin (CCK)

PRODUCED / SECRETED BY: Enteroendocrine CCK cells in the mucosa of the small intestine. CCK is also released in the brain. STIMULATED BY: Partially digested proteins (amino acids) and triglycerides (fatty acids) that enter the small intestine. MAJOR EFFECTS: Stimulates secretion of pancreatic juice rich in digestive enzymes; stimulates contraction of the gall bladder, causes ejection of bile from the gall bladder and relaxes the sphincter of the hepatopancreatic ampulla; and induces satiety (feeling full to satisfaction). MINOR EFFECTS: Inhibits gastric emptying, promotes norma growth and maintenance of the pancreas, and enhances effects of secretin.

Gastrin

PRODUCED / SECRETED BY: Enteroendocrine G cells, located mainly in the mucosa of pyloric antrum. STIMULATED BY: Distension of stomach, partially digested proteins and caffeine in the stomach and high pH of stomach chyme. MAJOR EFFECTS: Stimulates secretion of HCl in stomach, stimulates secretion of gastric juice, increases gastric motility, promotes growth of gastric mucosa. MINOR EFFECTS: Constricts lower esophageal sphincter; relaxes pyloric sphincter and ileocecal sphincter.

Gastric Inhibitory Peptide (GIP)

PRODUCED / SECRETED BY: Enteroendocrine K cells in the mucosa of the small intestine. STIMULATED BY: Fatty acids and glucose that enter the small intestine. MAJOR EFFECTS: Stimulates release of insulin by beta cells in pancreatic islet, inhibits secretion of gastric juice, and slows gastric emptying, stimulates use of glucose by skeletal muscle, stimulates lipid synthesis.

Secretin

PRODUCED / SECRETED BY: Enteroendocrine S cells in the mucosa of the small intestine. STIMULATED BY: Acidic (high H+ level) chyme that enters the small intestine. MAJOR EFFECTS: stimulates secretion of pancreatic juice and bile that are rich in alkaline buffers HCO3 - (bicarbonate ions). Increases rate of biles secretion by liver cells. MINOR EFFECTS: Inhibits secretion of gastric juices; Promotes normal growth and maintenance of the pancreas; and enhances effects of CCK.

Pancreatic Polypeptide

PRODUCED / SECRETED BY: F cells of pancreatic islets STIMULATED BY: Meals containing protein, fasting, exercise, and acute hypoglycernia; inhibited by somatostatin and elevated blood glucose. MAJOR EFFECTS: Inhibits secretion of somatostatin, contraction of the Gall Bladder, and secretion of pancreatic digestive enzymes.