BIOL 2420 Lecture Notes - Lecture 11: Gastric Inhibitory Polypeptide, Enteric Nervous System, Chyme
26 Apr 2018
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Unit 9 – Lecture 11
The Intestine Absorbs Ions and Water
- most water absorption takes place in the small intestine – with an additional o.5L/day absorbed in
the colon
o absorption of nutrients moves solute from the lumen of the intestine to the ECF
▪ creates an osmotic gradient that allows water to follow
- Ion absorption into the body also creates the osmotic gradients needed for water movement
o Enterocytes in the small intestine and colonocytes
▪ Colonocytes – epithelial cells on the luminal surface of the colon
• Absorb Na+ using three membrane proteins
o Apical Na+ channels
▪ ENaC
o Na+-Cl- symporter
o Na+-H+ exchanger (NHE)
• in the small intestine, a significant fraction of Na+ absorption also takes
place through Na+-dependent organic solute uptake, such as the SGLT
and Na+-amino acid transporters
- on the basolateral side of both enterocytes and colonocytes, the primary transporter for Na+ is
Na+-K+-ATPase
- chloride uptake uses an apical Cl- -HCO3- exchanger and a basolateral Cl channel to move across
the cells
- potassium and water absorption in the intestine occur primarily by the paracellular pathway
Regulation of the Intestinal Phase
- regulation of intestinal digestion and absorption comes primarily from signals that control
motility and secretion
o sensors in the intestine trigger neural and endocrine reflexes that feed hack to regulate the
delivery rate of chyme from the stomach, and feed forward to promote digestion,
motility, and utilization of nutrients
- the control signals to the stomach and pancreas are both neural and hormonal
1. chyme entering the intestine activates the enteric nervous system
▪ which then decreases gastric motility and secretion and slows gastric emptying
▪ in addition, 3 hormones reinforce the “decrease motility” signal
• secretin
• cholecystokinin (CCK)
• gastric inhibitory peptide (GIP)
2. secretin
▪ released by the presence of acidic chyme in the duodenum
▪ inhibits acid production and decreases gastric motility
▪ stimulates production of pancreatic bicarbonate to neutralize the acidic chyme
that has entered the intestine
3. CCK
▪ Secreted into the bloodstream if a meal contains fats
▪ Slows gastric motility and acid secretion
▪ Fat digestion proceeds more slowly than either protein or carbohydrate digestion
• Crucial that the stomach allow only small amounts of fat into the
intestine at one time
4. Incretin hormones
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