Prunuske: Intestinal Absorption Flashcards
What are the duodenal/jejunum features that facilitate digestion and absorption?
- Plicae circularis (SI slows transit of food)
- Villi increase SA
- Segmenting contractions facilitate mixing
Where does enzymatic hydrolysis of carbohydrates and proteins occurs?
in the lumen through pancreatic enzymes
at the microvillous membrane
in the enterocytes for peptides
Intraluminal hydrolysis of polymers (starch/glycogen) by a-amylase (salivary and pancreatic) leads to…
small di and tri-saccharides
branched dextrins
Are amylase blocker supplements (Phaseolus vulgaris- white bean protein extract) an effective way to promote weight loss?
NO
What drug has been used to treat type II diabetes?
Acarbose
Can amylase products be immediately absorbed?
NO
They require further cleavage into the 3 absorbable monosaccharaides at the brush border
What is end product inhibition?
glucose can inhibit lactase activity
How is the expression of brush border hydrolases regulated?
In the intestinal villi
Higher in the proximal than distal intestine
What protects brush border hydrolases from proteases?
Heavily glycosylated
What is sucrase isomaltase?
single polypeptide
contains 2 active sites that are cleaved into two pieces by trypsin
What is hypolactasia?
lactose intolerance
What causes hypolactasia?
Injury to absorptive cells:
Chron’s disease
Celiac disease
alcohol
A person w/ lactose intolerance often presents w/ diarrhea. Why?
W/out lactase> Bacteria in the gut ferment lactose> GAS ACCUMULATION/PAIN> increase in lactic acid> increases osmotic load> pulls water into lumen> increased peristalsis/diarrhea
How to you tx hypolactasia?
avoid lactose
lactase (lactaid)
What may sequester monosaccharides form bacteria?
Uptake in the duodenum and jejunum through:
SGLT1
GLUT5
SGLT1
Na monosaccharide symporter
Actively takes up Glucose and galactose
What type of transport process does SGLT1 use?
Secondary active transport process (2Na/1sugar)
Depends on low Na generated by the Na/K pump
What facilitates fructose uptake?
Glut 5
How do monsaccharides leave the basolateral membrane and go to the liver?
Glut 2
What limits digestion of proteins in the stomach?
pepsin
When is pepsin inactive?
pH > 4.5
Once proteins reach the duodenum, what enzymes are secreted?
Pancreatic proteases
Pancreatic proteases, like endopeptidases and carboxypeptidases require what brush border enzymes for activation?
enteropeptidase
What type of proteins are resistant to digestion?
glycoslyated and PROLINE rich
Where are AA absorbed?
villi of duodenum and jejunum
How are AA transported across the apical membrane?
by Na dependent symporters that are dependent on the basolateral Na/K pump (secondary active transport
What is PEPT1?
The peptide cotransporter!
Located on the lumenal side, it cotransports peptides w/ protons
What supplies the luminal H for PEPT1?
Apical Na/H exchanger (NHE) that supplies H nad is suppored by hte basolateral Na/K/ATPase
Peptides that are absorbed by PEPT1 are then…
digested by cytosolic proteases
What happens to surplus AA?
transported into the blood
In summary, how are peptides and AA transported out of the lumen?
Peptides cotransported w/ H
AA cotransported w/ Na
What is the mechanism for passive immunity in infants?
for the first 6 mos, intact proteins are absorbed by endocytosis
M cells take up proteins which are transferred to lymphocytes as antigens
Why do genetic disorders of apical AA transporters not result in AA def?
?
What peptide transporter facilitates the uptake of many drugs?
PEPT1
The majority of dietary lipids are…
TGs
What is emulsification?
chewing breaks down particles and increases surface area for particles to be coated w/ bile salts and phospholipids
Where does gastric lipase come from? What does it do? Optimal pH?
Chief cells
not required but facilitaes 10-30% of lipolysis
5.4
Why does gastric lipase play a prominent role in neonates?
developmental delay of pancreatic enzyme expression in ppl w/ pancreatic insufficiency
What triggers the release of CCK?
Free FA in the duodenum
What does CCK do?
activates pancreatic acinar secretion of LIPASE and PROCOLIPASE
What inhibits pancreatic lipase activity?
low pH and bile acids prevents lipase from binding to fat droplets
What activates colipase and what does it do?
activated by Trypsin
binds bile acids and recruits lipase to cleave FA
What enzymes from pancreas acinar cells that become active in the duodenum (higher levels of Ca) may contirbute to lipid digestion?
Cholesterol esterase
Phospholipase A2
What is cholesterol esterase?
Hydrolyze hte 2 position of FA left untouched by lipase
What does phospholipase A2 do?
converts phospholipids into FA and lysophospholipids
Are inhibitors of fat assimilation effective for weight loss?
?
What is olestra?
undigestable fat, too big to be absorbed
What is a pancreatic lipase inhibitor used to tx obesity?
orlistat/xenical
What effect does orlistat have?
Effect is modest and many patients regain weight if taken off drug
Reduces risk of type II diabetes and lowers blood pressure
What are SE of orlistat?
Fatty stool
Causes abdominal cramping and diarrhea
Inhibit absorption of vitamins A, D, E, K
What is critical micelle concentration?
the minimum level of bile salts that must be present before micelles will form and fat soluble vitamins can be packaged for uptake
How do lipids enter enterycytes?
- nonionic diffusion
- collision w/ hte membrane
- carrier mediated transport
What happens to most absorbed FA?
reassembed into triglycerides and coated with apoproteins to form chylomicrons
What supports the absorption of vitamin B12?
IF
Removal of the ILEUM or METFORMIN can interfere w/ the absorption of what vitamin?
B12
Pts should be screened for B12 def especially if htey’re…
anemic
Colipase/lipase def
chronic pancreatitis or congenital mutations (triglycerides only need 10-15% of normal enzyme levels).
Treat with pancreatic enzyme supplements
Zolinger Ellision
excess H+ overwhelms the buffering capacity of bicarbonate in the duodenum. Leading to inactivation of pancreatic lipase (more sensitive to pH than proteases)
What can cause poor lipid absorption?
Inadequate bile salt concentrations due to liver dysfunction or outflow of bile leads to emulsification problems.
Bile uptake defects in the terminal ileum (Crohn’s disease) leads to inadequate bile concentrations.
Abnormal chylomicron formation
(abetalipoproteinemia)
Abnormal lymphatic transport processes
Abnormal intestinal motility, short bowel syndrome, or rapid gastric emptying may lead to inadequate time for fat digestion and/or absorption.
