MECHANISMS OF GASTROINTESTINAL ABSORPTION
Learning Objectives
1.
2.
3.
4.
5.
How is the intestinal villus designed to facilitate absorption?
Distinguish between paracellular and transcellular transport in the enterocyte.
Describe the absorption of sodium and chloride in the small intestine.
Distinguish between passive and active absorption of potassum and where do they occur?
How does the absorption of calcium and magnesium differ in regions of the small
intestine and what role does vitamin D play in this absorption?
6. Contrast the absorption of heme iron with that of non-heme (ionized) iron.
7. How do binding proteins affect the absorption of cobalamin?
8. What substances are primarily if not exclusively absorbed in the ileum?
9. Describe the role of apical and basolateral membranes of the enterocyte in the absorption
of polysaccharides and oligosaccharides.
10. What is the contribution made by brush-border peptidases and cytosolic peptidases to the
hydrolysis of proteins?
11. Contrast the secondary active transport of amino acids with that of di and tri-peptides
across enterocyte apical and basolateral membranes.
12. Clarify the role of Na-K ATPase in sodium-dependent transport of glucose, amino acids,
vitamin C and bile acids.
13. What is the composition of a mixed micelle and how do these components cross the
unstirred layer?
14. Distinguish between absorption of medium-chain fatty acids and that of long-chain fatty
acids.
15. Describe the formation of chylomicrons and the role they and VLDL’s play in the
absorption of fat.
16. What transport processes are uniquely present or absent in the colon?
17. How do enteric organisms affect normal gut physiology and how do pathogenic species
affect gut function?
18. Describe the controls over food intake with particular emphasis on the signaling and
the role of the hypothalamus.
____________________________________________________________________________
I. Stomach
A. Solubility
1. Fat soluble substances are absorbed across the gastric mucosa. Ionized substances are
water soluble and affected by pH. There is very little nutrient absorption in the
stomach.
2. The rate of absorption of nutrients, minerals and vitamins primarily depends upon the
rate of gastric emptying.
34
II. Small intestine
A. Villus (Figure 2-1)
1. The functional unit of intestinal absorption is the villus. Villus motion occurs.
Three regions are defined.
a) The crypt is the site of mitosis and secretion.
b) The tip (upper 1/3 of villus) is the region of absorption. Enterocytes at the
villus tip slough away after 4-5 days as a result of apoptosis.
c) The zone of maturation is where cell differentiation occurs.
2. The mucosal epithelium possesses microvilli (brush border) on the luminal side and
tight junctions between cells. Mucosal transport mechanisms are transcellular and
paracellular. Enzymes and transporters are present in the brush border that differ
from those present in basolateral membranes.
3. The unstirred layer is a layer of fluid 200-500 μm in thickness. It separates the
well-mixed bulk phase of intestinal contents in the lumen from the mucosal cell surface.
A concentration gradient exists across the unstirred layer. Diffusing substances
cross the unstirred layer to reach the apical membranes of the enterocytes.
41
4
442141144144141
Figure 2-1. Components of the intestinal villus essential to absorption processes.
35
B. Absorption of water
1. The daily water load is 8-10 L coming from food and liquids, 2 L; saliva, 1.5 L;
gastric secretions, 2 L; pancreatic secretions, 1.5 L; bile, 0.5 L; small intestine, 1 L.
2. About 99% of the water that enters the GI tract is absorbed. This absorption occurs by
both transcellular and paracellular processes primarily in the jejunum and duodenum.
3. The tight junctions between enterocytes consist of transmembrane proteins called
claudins. Aqueous pores in the tight junctions are lined by fixed charged loops.
a. Claudins determine the selective size, charge and conductance properties of the
paracellular pathway. Transport is passive across tight junctions but this is
subject to conditions.
b) The accumulation of solutes in the interstitial spaces between enterocytes pulls
water into these spaces by osmotic forces.
4. Transcellular absorption of water is linked to the absorption of nutrients through the
SGLT-1 transporter. SGLT-1 transports 1 Na+ and 1 glucose and water follows.
C. Absorption of ions
1. Sodium (Figure 2-2)
a. Na-H exchange results in net absorption of Na+ during the interdigestive period.
This transport occurs across apical membranes and affects the pH of the lumen.
b. Cotransport with glucose and amino acids is the major mechanism for the
post-prandial absorption of sodium.
c. Na+ is actively transported into the intercellular spaces by basolateral Na+, K+
ATPase. This ATPase activity is essential to the transport of Na+ in the gut lumen
across apical membranes and is the only primary active transport mechanism for
Na+.
2. Chloride (Figure 2-2)
a. Voltage-dependent absorption includes passive diffusion across tight junctions
(paracellular) and through Cl- channels.
b. Cl-HCO3 exchange during the inter-digestive period results in net Clabsorption.
36
LUMEN
PLASMA
K+
3. Potassium
ENTEROCYTE
a. Sources of potassium include food,
gastric, pancreatic and biliary
secretions.
Na+
3Na+
H+
2K+
Na+
K+
Glucose
b. Absorption is passive except for the
distal colon.
K+
HCO3-
Cl-
Cl-
c. Postprandial removal from the blood is
stimulated by insulin, aldosterone and
epinephrine. Renal excretion into the
urine occurs. These activities prevent
hyperkalemia.
HCO3Na+
Na+
Amino
acids
Cl-
4. Calcium (Figure 2-3)
Fig. 2-2. Ion transport in the small intestine.
a.
Passive absorption occurs
paracellularly throughout the small intestine and exceeds active absorption.
b.
Active absorption occurs via
calcium channels in apical
membranes of the enterocytes.
c.
Free Ca2+ in the cytosol binds to
calbindin. Free Ca2+ and bound
Ca2+are in dynamic exchange
with Ca2+ in organelles.
LUMEN
PLASMA
ENTEROCYTE
d.
e.
Export proteins in the basolateral
membranes of enterocytes include
Ca2+ ATPase and the Na+- Ca2+
exchanger.
Vitamin D stimulates the active
absorption of Ca2+.
Vit. D
Ca2+
+
3Na+
NUCLEUS
Protein
synthesis
Ca2+
Ca2+-calbindin
Ca2+
Ca2+
5. Magnesium
Figure 2-3. Absorption of calcium in the
a. Most absorption is active and
small intestine.
occurs in the ileum. Passive absorption
occurs in the rest of the small intestine. Active absorption is not tightly linked to
vitamin D nor is it adjusted to the dietary load of magnesium.
37
LUMEN
PLASMA
6. Iron (Figure 2-4)
ENTEROCYTE
a. Most non-heme iron in the diet
is present as Fe3+. An enterocyte
iron reductase reduces Fe3+ to
Fe2+ for transport by DCT1. This
transporter functions best when
the pH of the intestinal lumen is
less than intracellular pH.
Storage
sites
Fe3+
Iron reductase
Fe2+
H+
Fe2+
DCT1
Fe3+
Mf
IREG 1
b. Heme iron enters the enterocyte
via a heme carrier protein,
HCP1. The heme is cleaved by
heme oxygenase to release the
iron. Intracellular Fe3+ can be
stored bound to ferritin or
iron transport protein, IREG1,
is transported in plasma bound to
transferrin.
Iron
Transferrin
Ferritin
Heme
Heme
CO
Biliverdin
HCP 1
Utilization
sites
.
Figure 2-4. Intestinal transport of iron.
c. The rate of absorption of iron increases with demand. When transferrin-iron
levels in plasma are low, enterocyte cytosol levels of iron decrease and ironregulatory proteins (IRP) in the cytosol bind to mRNAs that code for the
synthesis of iron transport proteins. Iron absorption increases. When transferrinFe3+ levels are high, Fe3+ binds to IRP. Translation of iron transport proteins is
inhibited and iron absorption declines.
D. Absorption of water-soluble vitamins
1. Not all of the water-soluble vitamins are absorbed at the same site or by the same
mechanism. Vitamin C and B12 are primarily absorbed in the ileum by co-transport
with Na+.
2. Cobalamin (Vit B12) is bound to ingested animal protein. At low pH, protein-bound
cobalamin is released and rapidly binds to haptocorrin present in saliva and gastric
secretions. Pancreatic proteases hydrolyze the cobalamin-haptocorrin complex and
cobalamin binds to intrinsic factor (IF). Brush border receptors for IF-cobalamin
are present in the ileum. Within the enterocytes, cobalamin binds to transcobalamin
II and enters portal blood. The complex is cleared from portal blood by hepatocyte
membrane receptor-mediated endocytosis and stored or secreted in bile.
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E. Absorption of dietary carbohydrate
1. Oligosaccharides and polysaccharides are hydrolyzed to monosaccharides prior to
absorption. Starch composes 45 to 60% of dietary carbohydrate and consists of
amylose and amylopectin, both of which contain multiple glucose residues.
2. Dietary fiber provides bulk. Hydrolases expressed by certain colon bacteria act on
dietary fiber that escaped digestion in the small intestine.
3. α-amylase in saliva and pancreatic secretions hydrolyzes starch.
4. Enterocyte brush border disaccharidases facilitate complete hydrolysis of oligosaccharides, figure 2-5. Lactase deficiency causes lactose intolerance.
5. Glucose and galactose absorption via SGLT1 is sodium-dependent at the apical
brush border. This requires basolateral Na-K exchange.
6. Fructose crosses the enterocyte apical membrane via GLUT5. All three
monoglycerides are transported across the basolateral membrane by GLUT 2, figure
2-6.
7. Mutations in
SGLT1 cause
glucosegalactose
malabsorption
resulting in
diarrhea.
LUMEN
LUMEN
ENTEROCYTE
a-Dextrin
Isomaltase
ENTEROCYTE
Salivary amylase
Pancreatic amylase
GLUCOSE
STARCH
3Na+
Salivary amylase
Pancreatic amylase
Maltose
2K+
Maltase
Na+
GLUCOSE
GALACTOSE
GALACTOSE
FRUCTOSE
Figure 2-5 (left)
Hydrolysis of
dietary carbohydrates in the
mall intestine.
Figure 2-6.
Monosacclharide
transport in the
small intestine.
PLASMA
PLASMA
Glucose
Galactose
SGLT 1
Lactase
Lactose
Glucose
Galactose
Fructose
GLUCOSE
Fructose
Sucrose
Glut 5
Sucrase
FRUCTOSE
F. Absorption of protein (Figures 2-7, 2-8)
1. Proteins are hydrolyzed to oligopeptides or single amino acids by pancreatic and
apical membrane enzymes. Pepsin plays a relatively minor role in the hydrolysis of
dietary protein.
39
GLUT 2
a. Five pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidase A
and B) contribute to protein digestion. They are secreted as proenzymes. Active
hydrolysis converts about 70% of the luminal proteins to oligopeptides and 30%
to free amino acids.
b. Apical brush border peptidases convert some oligopeptides to free amino acids.
Cytosolic peptidases hydrolyze oligopeptides to free amino acids.
LUMEN
LUMEN
PLASMA
PLASMA
Protein
Pepsin +
pancreatic
proteases
ENTEROCYTE
ENTEROCYTE
Na+
3Na+
Oligopeptides
H+
2K+
H+
Di and tripeptides
Di and tripeptides
Di and tripeptides
PepT1
Cytosol
peptidases
Cytosol
peptidases
Amino
acids
Di and tripeptides
Na+
Amino
acids
Amino
acids
Figure 2-7. Protease activity essential to
hydrolysis and absorption of dietary protein.
B
Amino
acids
Figure 2-8. Absorption of protein
hydrolysis products.
2. Multiple apical membrane amino acid transporters exist.
3. Amino acids exit the basolateral membrane of enterocytes by both Na-dependent
and Na-independent transport .
G. Absorption of Lipid
1. Emulsification of dietary lipid is accomplished by food preparation, chewing and
gastric mixing. Lingual and gastric lipase digest about 15% of the dietary fat to
a single fatty acid and diglyceride.
2. Pancreatic lipase, colipase and esterases aided by bile salts complete lipid
hydrolysis in the upper intestine.
40
3. By-products of lipid hydrolysis consisting of long-chain fatty acids and 2monoglyceride are solubilized in mixed micelles Micelles are cylindrical in shape
with the bile salts on the outside.
4. The pH of the unstirred layer facilitates the shift of lipid soluble products from
mixed micelles to a free state for absorption across the brush-border of the
enterocytes.
5. Short-chain and medium-chain free fatty acids are absent from micelles and
diffuse across brush-border membranes with glycerol into villus blood capillaries.
6. Products of fat hydrolysis in the enterocyte
make their way to the smooth endoplasmic
reticulum (SER) via a fatty acid-binding
protein. Re-esterified lipids are packaged
in chylomicrons, figure 2-10.
a. During fasting, endogenous lipids
are processed and secreted in
VLDLs.
b. VLDLs and chylomicrons
permeate villus lacteal
capillaries, not villus blood
capillaries.
LUMEN
Dietary
triglycerides
Lipase
ENTEROCYTE
Triglyceride
2-monoglyceride
LCFAs
2-monoglyceride
FATP4 LCFAs
FABP
Mixed
micelle
Chylomicron
Lymph
Fat-soluble vitamins
Lysolecithin
Cholesterol
Phospholipids
Plasma
Bile salts
SCFAs
MCFAs
Glycerol
H. Absorption of bile salts
FATP4 : fatty acid transport protein 4; FABP: fatty acid binding
1. De-conjugated and de-hydroxylated
protein
bile acids are absorbed passively
because they are more lipid soluble.
Figure 2-10. Luminal and intracellular
Conjugated bile acids are actively
events in the digestion and absorption of fat.
absorbed in the terminal ileum by a
sodium-dependent bile acid transporter and by an organic salt transporter (OATP)
2. ~95% of the bile salts secreted by the liver are reabsorbed on each pass through the
intestine and return to the liver via portal circulation. Hepatocytes extract bile acids
with high efficiency where they are reprocessed and resecreted into bile. Bile acids
that escape first-pass clearance by the liver are filtered by renal glomeruli and
reabsorbed in the proximal tubules.
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I. Absorption of fat-soluble vitamins
1. A, D, E and K are lipid soluble and are present in mixed micelles in the intestinal
lumen.
2. Vitamins are incorporated into chylomicrons and VLDLs prior to entering lymph.
III. Absorption in the Colon
A. Water transport (Figure 3-1)
1.5 L
4L
6L
1.4L
3.9L
4.4L
0.1L
Normal
stool water
0.1L
Normal
stool water
1.6L
Diarrhea
1.5 L
0.5L
2.0L
Diarrhea
Figure 3-1. Fluid flow, water absorption and stool water in the colon.
1. One to two liters of fluid normally pass through the ileocecal valve each day. Only
about 100-200 ml of water are lost in stools. The maximum absorptive capacity is
about 4.5 liters per day.
2. Active transport of electrolytes into the intercellular space creates an osmotic
gradient that draws water into that space. Tight junctions are less“leaky” than those in
the small intestine.
3. Various agents including bacterial and viral toxins can alter water absorption
mechanisms and thereby cause diarrhea. Secretory diarrhea differs from osmotic
diarrhea in both cause and treatment.
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IV. Intestinal Bacterial Flora
A. Normal enteric microbes
1. See Table 4-1. Note the relative low numbers in the stomach and jejunum.
2. Effects of these bacterial populations are shown in Table 4-2.
Table 4-1. Enteric bacterial populations.
___________________________________________________________________________
Stomach
Jejunum
Ileum
Colon
___________________________________________________________________________
0-103
Total bacteria/gm
0-104
104-108
1010-1012
(aerobes and anaerobes)
pH
~3
6-7
7.5
6.8-7.3
___________________________________________________________________________
Table 4-2. Effects of enteric bacteria
___________________________________________________________________________
Substrate
Enzymes
Products
Disposition
___________________________________________________________________________
Endogenous substrates
Urea
Urease
Bilirubin
Reductases
Primary bile acids Dehydroxylases
Conjugated bile Deconjugases
acids
Exogenous substrates
Carbohydrate,
Fermentation
fat residues
enzymes
Ammonia
Passive absorption or
secretion as NH4+
Urobilinogen
Passive reabsorption
Stercobilins
Excreted
Secondary bile acids Passive reabsorption
Unconjugated bile
Passive reabsorption
acids
SCFAs, H2, CO2,
CH4
Active absorption, expired
in breath or excreted
in flatus
Decarboxylases
Ammonia, HCO3Reabsorbed or excreted
and deaminases
Sulfatases
Hydrogen sulfide
Excreted in flatus
_________________________________________________________________
3. Consequences of bacterial overgrowth
a. Fermentation that results in intestinal gas and bloatiness
Amino acids
b. Competition for uptake of vitamin B-12.
43
c. Deconjugation of bile acids resulting in poor digestion and absorption of fat.
d. Immune and inflammatory responses
that result in dysfunction.
LUMEN
ENTEROCYTE
H+
B. Gut pathogens
1. Salmonella sp. and E. coli directly bind to
tight conjunction proteins and alter
paracellular transport of water and ions.
2. Secreted toxins alter second messenger
signaling.
3 Na+
Na+
_
Cl-
+
2 K+
cAMP
HCO3Na+
S
H2 O
ENTEROCYTE
.
ENAC
a. V. cholera stimulates c-AMP dependent
cAMP
secretion of Cl-. Rotavirus stimulates Ca2
Ca2+
ClCFTR
activated secretion of Cl and inhibits the
K+
cotransport of glucose and amino acids
Cl
with Na+.
3. Cytokines secreted during inflammation (e.g.
nitric oxide and TNFα ) alter ion
transport from the gut lumen to blood.
Figure 4-1. Targets for pathogen
toxins and effects on gut function.
V. Energy Balance: Mind-Gut Interactions
Na+
Na+
2 ClK+
A. Control Centers in the Brain (Figure 5-1)
1. The hypothalamus receives neural signals from the GI tract (e.g. gastric distention);
chemical signals from nutrients in the blood; signals from GI hormones; signals from
adipose tissue and neural signals from the cerebral cortex relating to sight, taste,
smell.
2. Activation of POMC neurons in the arcuate nuclei stimulates secretion of α-MSH
which stimulates synthesis of cocaine/amphetamine-related transcripts. α-MSH acts
on melanocortin receptors 3 and 4 present in the paraventricular nuclei of the
hypothalamus. Activation of these receptors decreases food intake and increases
energy expenditure.
3. Activation of NPY/AGRP neurons stimulates food intake by inhibiting the effects of
α-MSH on MCR 3 and 4.
44
Figure 5-1. Appetite control centers in the hypothalamus. From Guyton and
Hall, Textbook of Medical Physiology, 12th ed., fig. 71-2, p. 847.
B. Short term control
1. Vagal sensory signals from gastric filling and oral factors of chewing, tasting
and swallowing have a “meter” effect on the regulation of food intake.
2. CCK suppresses hunger. PYY primarily from the ileum and colon is secreted in
proportion to calories ingested. Glucose-dependent insulinotropic peptide and
glucagon-like peptides suppress hunger.
3. Ghrelin, a product of parietal cells primarily, rises in blood during a fast. Ghrelin
stimulates food intake.
C. Intermediate and long-term control
1. The concentrations of glucose, amino acids and lipids in blood modulate food intake.
Elevated blood glucose stimulates increased firing rates of glucoreceptor neurons in
the satiety center in ventromedial and paraventricular nuclei of the hypothalamus.
2. Leptin from adipocytes stimulates leptin receptors in the hypothalamus. The effect is
to inhibit NPY/AGRP neurons; stimulate POMC neurons; increase production of
CRH; stimulate sympathetic nerve activity and decrease secretion of insulin.
45
3. Table 5-1. Summary of neurotransmitters and hormones affecting food intake.
Substance
Effect on Food Intake
_______________________________________________
CCK
Inhibitory
Insulin
Inhibitory
Glucagon-like peptides
Inhibitory
5-HT (serotonin)
Inhibitory
Leptin
Inhibitory
α-MSH
Inhibitory
PYY
Inhibitory
Neuropeptide Y
Stimulatory
Endogenous opioids
Stimulatory
Glucocorticoids
Stimulatory
Ghrelin
Stimulatory
Orexins, A and B
Stimulatory
_______________________________________________
QUESTIONS FOR STUDY AND REVIEW
1. Which of the following does not appropriately characterize mixed micelles?
.
A.
B.
C.
D.
E.
They contain bile acids.
They are unstable in the unstirred layer.
They contain LCFAs
They are removed from portal blood by hepatocytes.
They contain cholesterol.
2. What do vitamin C and iron reductase have in common?
A.
B.
C.
D.
E.
Both stimulate the synthesis of transferrin.
Both stimulate the synthesis of mobilferrin.
Both reduce Fe3+ to Fe2+.
Both increase enterocyte membrane permeability to heme molecules.
Both stimulate the endocytosis of heme molecules by enterocytes.
3. Haptocorrin and intrinsic factor are critical to the absorption of
A.
B.
C.
D.
E.
vitamin B12.
vitamin D.
vitamin E.
bile acids.
cholesterol.
46
4. Glucose and galactose are end-products of lactase activity. Where is lactase found?
A.
B.
C.
D.
E.
In the basolateral enterocyte membranes.
In the brush border of enterocytes.
In the cytosol of enterocytes.
In hepatocytes.
In the vascular endothelium.
5. What does the recycling of bile acids increase?
A.
B.
C.
D.
E.
Bile acid synthesis.
7 α-hydroxylase activity.
The concentration of bilirubin in the blood.
Bile flow.
Motility in the duodenum.
6. What does the cotransport of Na+ and glucose across the enterocyte apical membrane facilitate
the absorption of?
A.
B.
C.
D.
E.
Cholesterol.
Vitamin B12.
Calcium.
Glycerol.
Water.
7. What will a sample of normal hepatic portal blood least likely contain?
A.
B.
C.
D.
E.
Free amino acids.
Deconjugated bile acids.
Cholesterol.
Cobalamin.
Intact dietary protein.
8. What is the transport of a substance with sodium in the small bowel highly dependent upon?
A.
B.
C.
D.
E.
Na+ -K+ ATPase activity.
The paracellular transport of water.
The paracellular secretion of K+.
The transcellular movement of Ca2+.
The transcellular movement of Fe2+.
47
9. Mutated CFTR channel proteins affect transport in epithelial cells by altering
A.
B.
C.
D.
E.
ENaC activity.
Cl- cycling in enterocytes.
Cl- secretion by various epithelial cells.
paracellular transport of water.
each of the above.
10. Maintaining a hydrated state in the body is highly dependent upon the intestinal absorption of
water. What is the major absorption of water most closely linked to?
A.
B.
C.
D.
E.
Flow across tight junctions in the colon.
Transcellular movement in the colon.
The paracellular transport of nutrients in the small bowel.
The transcellular transport of nutrients in the small bowel.
The recycling of bile acids.
ANSWERS TO STUDY QUESTIONS
Organization
1. A
2. D
3. A
4. B
5. C
6. B
Motility
1.
2.
3.
4.
B
D
B
B
5.
6.
7.
8.
E
B
D
D
9. C
10. E
11. C
12. B
5.
6.
7.
8.
E
C
D
E
9.
10.
11.
12.
Secretion
1.
2.
3.
4.
C
D
B
B
E
C
B
E
13. A
14. A
Absorption
1.
2.
3.
4.
5.
D
C
A
B
D
6. E
7. E
8. A
9. E
10. D
48
______________________________________________________________________________
49