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3002 > 20 Gastrointestinal absorption and secretion > Flashcards

20 Gastrointestinal absorption and secretion Flashcards

1
Q

Carbohydrate digestion summary

A
  • Only monosaccharides can be absorbed

* Begins with salivary alpha-amylase (minor role)

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2
Q

Products of carbohydrate digestion and their absorption and mechanism?

A
  1. Glucose:
    • Absorption - SI
    • Mechanism - Na+-dependent cotransport
  2. Galactose:
    • Absorption - SI
    • Mechanism - Na+-dependent cotransport
  3. Fructose:
    • Absorption - SI
    • Mechansim - Facilitated diffusion
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3
Q

Absorption of carbohydrates

A
  1. Secondary active transport:
    • SGLT 1 (Na+ dependent glucose transporter 1)
    • Located on apical membrane and transports glucose and galactose
  2. Facilitated diffusion:
    • GLUT 5 (glucose transporter 5)
    • Transports fructose across apical membrane
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4
Q

Protein digestion summary

A
  • Begins in stomach with pepsin
  • Completed in small intestine with pancreatic and brush-border proteases
  • Endopeptidases -hydrolyse interior peptide bonds of proteins
  • Exopeptidases - hydrolyse one amino acid at a time from
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5
Q

Protein digestion in small intestine

A

• Pancreatic proteases secreted as inactive precursors:
- Trypsinogen
- Proelastase
- Procarboxypeptidase A
- Procarboxypeptidase B
• Initial step-activation of trypsinogen –> trypsin (active form) by brush-border enzyme enterokinase
• Trypsin catalyses conversion of other inactive precursors to active enzymes

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6
Q

Activation of proteases

A
  1. Stomach:
    • Pepsinogen —> Pepsin (low pH)
  2. Small intestine:
    • Trypsinogen —> Trypsin (enterokinase - brush border)
  • Trypsinogen –> Trypsin (trypsin)
  • Chymotrypsinogen –> chymotrypsin (trypsin
  • Proelastase –> Elastase (trypsin)
  • Procarboxypeptidase A –> Carboxypeptidase (trypsin)
  • Procarboxypeptidase B –> Carboxypeptidase (trypsin
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7
Q

Products of protein digestion and their absorption and mechanism?

A
  1. Amino acids:
    • Absorption - SI
    • Mechanism - Na+-dependent cotransport
  2. Dipeptides:
    • Absorption - SI
    • Mechanism - H+-depeptide cotransport
  3. Tripeptides:
    • Absorption: SI
    • Mechansim - H+-tripeptide cotransport
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8
Q

Digestion and absorption of lipids?

A
  1. Dietary lipids are hydrophobic (insoluble in water)
  2. They must be solubilised before digestion and absorption can occur
  3. Digestion begins in stomach with action of lingual and gastric lipases
  4. Digestion is completed in small intestine with action of pancreatic enzymes
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9
Q

Digestion of lipids in stomach?

A
  • Stomach churns and mixes lipids to initiate enzymatic digestion
  • Lingual and gastric lipases hydrolyse 10% of ingested triglycerides –> glycerol + free fatty acids
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10
Q

What is the key role of gastric lipase?

A

To slow the rate of gastric emptying so that the pancreatic enzymes are able to digest lipid

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11
Q

What is secreted in response to the presence of monoglycerides and fatty acids and small peptides and amino acids?

A

Cholecystokinin secreted from the I cells of the duodenal and jejunal mucosa

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12
Q

Digestion of lipids in the small intestine?

A
  1. Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids
  2. Emulsification produces small droplets of lipids dispersed in an aqueous solution - creating a large SA for pancreatic enzyme digestion
  3. Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phospholipase A2) and the protein, colipase are secreted to complete digestion
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13
Q

Stages of digestion and absorption of lipids?

A
  1. Bile salts from liver coat fat droplets
  2. Pancreatic lipase and colipase break down fats –> monoglycerides + fatty acids which are stored in micelles
  3. Monoglycerides + fatty acids move out of micelles and enter cells by diffusion
  4. Cholesterol is transported into cells by a membrane transporter
  5. Absorbed fats combine with cholesterol and proteins in the intestinal cells to from chylomicrons
  6. Chylomicrons are released into the lymphatic system
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14
Q

Chylomicrons:
• Structure?
• Location?
• Transport?

A

Structure:
• 100nm diameter
• Core of triglycerides and cholesterol ester (80%)
• Outside: phopholipids and apoproteins (20%)

Location:
• Packaged into secretory vesicle on Golgi membrane
• Exocytosed across basolateral membrane

Transport:
• Lymphatic circulation carries chylomicrons –> thoracic duct which empties –> blood stream

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15
Q

Products of lipid digestion and their absorption and mechanism?

A
  1. Products: fatty acids, monoglycerides, cholesterol
  2. Absorption: SI
  3. Mechanism:
    • Biles salts from micelles
    • Diffusion of products into intestinal cells
    • Re-esterification within the cell to triglycerides and cholesterol
    • Chylomicrons form in the cell and transfer to lymph
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16
Q

Describe trans-epithelial transport in intestines

A

• Intestinal fluid and electrolyte absorption and secretion involve trans-cellular and paracellular routes across the epithelium

  • Tight junctional resistance increases towards the colon
  • Tight junctions in SI are ‘leaky’ (permeable via the paracellular route)
  • Tight junctions in the colon are ‘tight’ (impermeable via the paracellular route)
17
Q

What structures in the small and large intestine are responsible for absorption and secretion?

A
  1. Small intestine:
    • Villi - absorption
    • Crypts of Lieberkuhn - secretion
  2. Large intestine:
    • Surface epithelial cells - absorption
    • Colonic glands - secretion
18
Q

How are villous cells replaced and how often?

A

Zone of cell proliferation in the base of the crypts replaces villous cells every 48-96 hours

19
Q

Describe intestinal absorption and their regional variations

A
  • H2O - majority in small intestine (6.5 L)
  • Na+ - jejunum, ileum and colon

• Electrolyte uptake - Na+, Cl-, HCO3- and K+ are absorbed isosmotically with water by the villi (as in the renal proximal tubule)

20
Q

Describe intestinal secretion in small intestine

A

Epithelial cells lining the crypts of Lieberkuhn:
• Cl- enters the cell via (Na+)+(K+)+(2Cl-) basal transporter
• Cl- diffuses across the apical membrane through apical Cl- channels (regulated by secretagogues)
• Na+ follows Cl- secretion passively via the paracellular pathway
• H2O follows NaCl secretion

21
Q

What is cholera caused by?

A
  • Acute infectious disease caused by a bacterium, Vibrio cholerae - results in watery diarrhoea
  • Ingesting organism through food or water sources contaminated with V. cholerae
22
Q

Some affected individuals of cholera have copious amounts of diarrhoea
• Result in?
• Treatment for severe form of cholera?

A
  • Develop severe dehydration which can lead to death

* Treatment: Hydration (IV for very ill) and antibiotics

23
Q

What are the symptoms and signs of cholera?

A
  • Watery diarrhoea containing whitish material (mucus and some GI epithelial cells)
  • “Rice-water stool” and smells “fishy”
  • Volume of diarrhoea: as high as 10-18L over 24 hours
24
Q

Mode of action of secretagogues: Hormones and neurotransmitters

A
  1. VIP - cAMP
  2. Guanylin - cGMP
  3. ACh - Ca2+
  4. Bradykinin - Ca2+
  5. Serotonin (5HT) - Ca2+
25
Q

Mode of action of secretagogues: Bacterial enterotoxins

A
  1. Cholera toxin - cAMP
  2. E coli toxins: heat labile - cAMP
  3. E coli toxins: heat stable - cGMP
  4. Yersinia toxin - cGMP
  5. Clostridium difficile - Ca2+
26
Q

End effect of second messengers

A
  1. cAMP:
    • Increased anion secretion +++
    • Inhibited NaCl absorption +++
  2. cGMP:
    • Increased anion secretion +
    • Inhibited NaCl absorption +++
  3. Ca2+:
    • Increased anion secretion +++
    • Inhibited NaCl absorption +++
27
Q

Absorption and mechanism of fat soluble vitamins?

A

Absorption:
• Small intestine

Mechanism:
• Bile salts form micelles
• Diffusion into intestinal cells

28
Q

Absorption and mechanism of water soluble vitamins?

A

Absorption:
• Small intestine

Mechanism:
• Na+-dependent cotransport

29
Q

Absorption and mechanism of vitamin B12?

A

Absorption:
• Ileum

Mechanism:
• Intrinsic factor

30
Q

Absorption and mechanism of bile salts?

A

Absorption:
• Ileum

Mechanism:
• Na+-bile salt cotransport

31
Q

Absorption and mechanism of Ca2+?

A

Absorption:
• Small intestine

Mechanism:
• Vitamin D - dependent Ca2+ binding protein - Calbindin

32
Q

Absorption and mechanism of Fe2+?

A

Absorption:
• Small intestine

Mechanism:
• Binds to apoferritin in intestinal cell
• Binds to transferrin in blood

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