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GI Week 2 Physio
Williams lectures: Liver/Integration; Digestion/Absorption; Motility
124
Medical
Graduate
01/12/2010

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Term
What is the difference between the function of the portal vein and hepatic veins? Where do each drain?
Definition
Portal v - drains gut and delivers nutrient-rich, O2 poor blood to liver to get "cleaned"
hepatic v- 3 veins which drain O2 poor blood from the liver to the IVC
Term
what is the contribution of blood supply to the liver from the portal v and hepatic a?
Definition
portal v - 75%
hepatic a - 25%
Term
describe the organization of a classic lobule
Definition
hexagonal shape with a draining central vein in the center and portal triads at each vertice
Term
what structures make up the portal triad?
Definition
portal vein, hepatic artery, bile duct
Term
describe the flow of bile and blood within a lobule
Definition
blood flows thru the sinusoids and drains into the central vein while bile flows in the opposite direction, thru canaliculi to the bile duct
Term
where are bile canaliculi located?
Definition
between adjacent hepatocytes, sealed off by tight junctions on either side
Term
which surface of a hepatocyte is the apical surface and which is the basolateral?
Definition
apical - facing other hepatocytes (bile canaliculi located b/w apical faces of 2 adjacent hepatocytes)
basolateral - facing blood sinusoid and the space of Disse
Term
what are the major components of bile?
Definition
water ~ 1 L/day
bile acids
phospholipids
cholesterol
bile pigments (bilirubin)
metabolites of drugs, hormones
HCO3-
Term
there are many different transporters in hepatocyte membranes to transport the various components of bile in and out of the cell. what regulates gene expression of these transporters?
Definition
bile salts, via Bile acid receptor/farnesoid X receptor (FXR)
Term
what types of cells line bile ducts?
Definition
cholangiocytes - columnar epithelial cells
Term
what kind of modifications of bile occur in the ducts?
Definition
movement of water (bile becomes isotonic); absorption of glucose and amino acids; secretion of HCO3- in response to secretin; secretion of IgA molecules
Term
what is the precursor of bile acid?
Definition
cholesterol
Term
what is the first and regulated step of bile acid synthesis?
Definition
7-alpha-hydroxylation
Term
what is the difference b/w primary and secondary bile acids?
Definition
7-dehydroxylation by gut bacteria cause bile acids to lose one -OH group
Term
how do bile acids in hepatocytes increase their solubility in water?
Definition
conjugation with glycine or taurine lowers their pKa, increase solubility
Term
what happens to bile acids above the critical micellar concentration (CMC)?
Definition
spontaneously self-associate into micelles with hydrophobic areas facing away from the aqueous environment
Term
what is a mixed micelle?
Definition
in bile, micelles can associate with phospholipids and cholesterol, forming mixed micelles
Term
T/F: while stored in the gallbladder, bile is concentrated and becomes hypertonic.
Definition
F: bile IS concentrated 5-20 fold, but isotonicity is maintained b/c micelles have little osmotic activity
Term
what is CCK's effect on the gallbladder and the sphincter of Oddi? what is the mechanism for these responses?
Definition
gallbladder - contraction due to CCK receptors on smooth muscle in gall bladder
sphincter - CCK activates inhibitory neurons that release VIP and NO
Term
what is the difference in mechanism b/w VIP and secretin receptors and Bombesin, CCK, and ACh receptors? (pancreas lecture)
Definition
VIP, secretin - act via cAMP messenger
Bombesin, ACh, CCK - act via Ca2+ and calmodulin
Term
what are two bile acid modifications that increase their solubility? where do these modifications take place?
Definition
conjugation with glycine or taurine in hepatocyte
deconjugation and dehydroxylation by bacteria in ileum and colon, producing secondary bile acids
Term
what happens to bile salts that are not excreted in feces?
Definition
Na+ dependent transporters on the sinusoidal membrane of the hepatocyte reabsorb 80-90% of bile salts that reach the liver via hepatic portal blood
Term
what is lecithin?
Definition
phosphatidylcholine - major form of phospholipid in bile
Term
what is the function of phospholipids in bile?
Definition
bile salts + phospholipids solubilize other lipids better than bile salts alone
Term
how is lecithin secreted into bile?
Definition
MDR3 transporter
Term
what is the liver's source of cholesterol?
Definition
synthesizes cholesterol and takes it up from plasma lipoproteins
Term
what is the function of ABC5/8?
Definition
secretes free cholesterol into bile
Term
what makes urine yellow and poop brown?
Definition
bilirubin
Term
what is bilirubin?
Definition
bile pigment derived from the breakdown of RBC's in the spleen
Term
what are some common hormone metabolites excreted in bile?
Definition
CONJUGATED thyroid and steroid hormones
Term
which of the following proteins are synthesized by the liver:
albumin
fibrinogen
angiotensinogen
Apo D
Definition
all
Term
in order for bile salts to be recycled, they must be absorbed from the blood and secreted into the canaliculi. what transporters are used in these processes?
Definition
basolateral: cotransport - Na+ and bile salts, use NTCP transport protein.
canaliculi/apical: secreted via bile salt export protein (BSEP)
Term
which transporter moves phospholipids into the canaliculi?
Definition
MDR3
Term
which protein transports bilirubin glucuronide into the canaliculi?
Definition
MRP2
Term
which protein transports cholesterol into the into the canaliculi?
Definition
ABC 5/8
Term
name the molecule transported by each protein below, found on the apical membrane of hepatocytes:
MRP2
MDR3
ABC 5/8
BSEP
Definition

MRP2 - biliRubin glucuronide

MDR3 - phospholipiD

ABC 5/8 - cholesterol

BSEP - bile salts

Term
where do substances secreted into the canaliculi end up?
Definition
canaliculi drain into bile ducts and end up in the gallbladder or eventually in the gut (enters at 2nd part of duodenum via greater duodenal papilla)
Term
what important characteristic of bile acids allows them to sit at oil-water interfaces?
Definition
they are amphipathic, with hydrophobic and hydrophilic regions
Term
[wk 1]
what stimulates CCK secretion?
what are its actions?
Definition
presence of protein and/or fat in intestine
actions: 1. gallbladder contraction; 2. HCO3- pancreatic enzyme secretion; 3. inhibition of gastric emptying
Term
how are bile salt levels regulated?
Definition
bile salts in portal blood stimulate bile secretion and inhibit bile acid synthesis
Term
what is enterohepatic circulation?
Definition
it refers to the circulation of bile salts from liver to intestines back to liver due to mechanisms for reabsorption and re-secretion of bile salts
Term
is bilirubin conjugated or unconjugated in the plasma? in the canaliculi?
Definition

plasma - unconjugated

canaliculi - conjugated (get conjugated in hepatocyte to make it more soluble in bile, then transported into canaliculi)

Term
what are xenobiotics?
Definition
exogenous organic compounds with no nutritive value or enzyme-cofactor function
Term
what is the liver's role in carbohydrate metabolism?
Definition
gluconeogenesis, glycogenolysis: produce glucose
stores glucose as glycogen after meals
Term
what is the liver's role in protein metabolism?
Definition
can produce acetyl CoA and ketone bodies from a.a.'s and a-keto acids
Term
what is the liver's role in fat metabolism?
Definition
take up chylomicron remnants and long chain fatty acids.
metabolizes fatty acids to acetyl CoA and acetoacetate
synthesizes and secretes VLDLs
Term
what is the liver's role in cholesterol metabolism?
Definition

-synthesizes cholesterol from acetyl CoA

-utilizes cholesterol from bile salts

-excretes cholesterol in bile

Term
what are the main functions of the small intestine?
Definition
digestion
absortion
secretion
motility
Term
what are 3 ways the small intestine increases its surface from a simple cylinder?
Definition
1. circular folds
2. villi
3. microvilli
Term
where does the bulk of digestion take place within the small intestines?
Definition
upper part - duodenum, proximal jejunum
Term
what is the average total daily intake of:
carbohydrates
protein
fat
cholesterol
Definition
carbs - 200-300g
protein - 70-90g
fat - 100g
cholesterol - 1200-1700 mg
Term
what types of carbohydrates can be absorbed?
Definition
only monosaccharides
Term
how many phases are in carbohydrate digestion? what are the phases called?
Definition
luminal phase - when food is in lumen of gut
brush border phase - when food is interacting with enzymes in the brush border (microvilli) of the intestines
Term
what are the luminal phase enzymes involved in carb digestion? what is their optimum pH? what are the end products of the luminal phase?
Definition
pancreatic and salivary a-amylase
pH 7
cleave a-1,4 internal linkages only
end products: di- (maltose)and tri- saccharides (maltotriose) and small branched chain glucose polymers (a-limit dextrin)
Term
what are the brush border phase enzymes involved in carb digestion? what are the end products?
Definition
sucrase-isomaltase, lactase, maltase
end products: monosaccharides (glucose, fructose, galactose)
**now can be absorbed
Term
how do glucose and galactose get absorbed from lumen into blood across the hepatocyte?
Definition
apical/luminal side: SGLT1 transporter -co-transports Na+, glucose/galactose
basolatera/blood side: GLUT2 - facilitated diffusion of glucose/ galactose down concentration gradient
Term
how does fructose get absorbed from lumen into blood across the hepatocyte?
Definition
lumen side: GLUT5 - facil diffusion
blood side: GLUT 2 - facil diffusion
Term
describe the overall driving force for glucose, galactose, fructose transport across intestinal epithelium
Definition
Na+/K+ ATPase on basolateral membrane keeps intracellular [Na+] low. So Na+ flows down its conc. gradient, pulling glucose/galactose with it thru SGLT1. Fructose diffuses down its conc. gradient thru GLUT5. Once in enterocyte, glucose, galatose, fructose diffuse out via GLUT2.
Term
what kinds of proteins can be absorbed?
Definition
di- and tri- peptides, via specific carriers
Term
what are the phases of protein digestion in the intestine? what happens in each phase?
Definition
-luminal phase: hydrolysis of proteins by gastric (pepsin) and pancreatic proteases
-brush border phase: further digestion of large proteins to a.a., di- and tri- peptides
Term
how do the pancreatic enzymes in the luminal phase get activated?
Definition
in the enterocyte trypsinogen is cleaved by enterokinase and activated to trypsin. Trypsin then cleaves other pancreatic zymogens (chymotrypsin, elastase, carboxypeptidase a, carboxypeptidase b)
Term
what are the end products of the luminal phase of protein digestion?
Definition
40% amino acids
60% small peptides
Term
what is that major di- and tri- peptide transporter on the luminal enterocyte membrane? how does it function?
Definition
PepT1 - is a H+ linked electrogenic proton/peptide cotransporter
a Na+/H+ transporter pumps Na+ in and keeps intracellular H+ low (pumps it out). PepT1 carries H+ and a peptide back in
Term
what is the effect of the Na+/H+ in the luminal enterocyte membrane?
Definition
creates a region of high [H+] in the brush border - "unstirred layer" with pH 6; creates H+ gradient to "power" the H+, peptide cotransport thru PepT1
Term
what happens to peptides once inside the enterocyte?
Definition
further digestion to a.a.'s by intracellular peptidases
Term
how do breakdown products of protein get into the blood? what other substances exit the cell in the same manner?
Definition
-a.a.'s exit via basolateral a.a. transporters
-peptides can also exit via transporters; peptide like drugs (ACE inhibitors, etc) are also transported
Term
what genetic defects might affect protein absorption? name 2 diseases of protein absorption
Definition
altered expression or function of a.a. or peptide transport proteins
-cystinuria: increased excretion of cation a.a.'s and cystine cause renal stones
-Hartnup - impaired absorption of neutral a.a.'s. resulting in niacin deficiency (synth. from Tryptophan, a neutral a.a.
Term
do patients with cystinuria or Hartnup's disease show symptoms of protein deficiency? why?
Definition
usually no, due to absorption of di- and tri- peptides
Term
what are the physical and chemical processes that occurs in the gastric phase of lipid digestion?
Definition
physical: breakdown and emulsification of proteins and phospholipids to small lipid particles
chemical: breakdown by gastric lipase and in newborns, by lipase from milk
Term
what is gastric lipases optimum pH and its end products?
Definition
pH 4-5
breakdowns triglycerides to make diglycerides and free fatty acids
Term
what are the 3 enzymes for lipid digestion and where do they act? what is the role of colipase?
Definition
intestinal lipid digestion occurs by pancreatic lipases in lumen
-pancreatic lipase: specific for 1, 3 fatty acids in triglyceride
-cholesterol esterase: cleaves a variety of ester bonds
-phospholipase A2: cleaves lecithin to lysolecithin (good emulsifying agent)
-colipase: anchors lipase to the fat droplet
Term
what is the optimum pH for the intestinal lipases? what does it suggest about the conditions needed for their activity?
Definition
pancreatic lipase, cholesterol esterase, phopholipase A2 all have pH optimum of 6-7
-depend on gastric acid neutralization to be effective
Term
how do digestion products of the lipases interact with bile acis?
Definition
digestion products (free fatty acids (FFA), monoglycerides, glycerol) and bile acids form mixed micelles and diffuse thru unstirred layer to the brush border
Term
how do FFA and glycerol in mixed micelles get absorbed?
Definition
fatty acids are in equilibrium b/w being in micelle and being free. FFA can get taken into cell via fatty acid transport protein
Term
what happens to FFA's once absorbed?
what happen to bile salts?
Definition
FFA are bound to intracellular fatty acid binding protein (FABP). Resynthesis of triglycerides occurs in enterocytes and they are packaged into chylomicrons.
-bile salts travel to terminal ileum where they get reabsorbed
Term
which digestion products of lipids require transport by micelle and which can freely diffuse w/o micelle solubilization?
Definition
monoglycerides, long chain fatty acids need to be incorporated into micelles
glycerol, short- and medium- chain fatty acids can diffuse thru unstirred layer without micelle solubilization
Term
what is a chylomicron?
what is the eventual fate of a chylomicron?
Definition
lipid digestion products are re-synthesized into triglycerides and packaged into chylomicrons. These are released by exocytosis and taken into lacteals (terminal lymph vessels in intestine)
Term
what is the role of Apo proteins in chylomicrons transport?
Definition
Apo B-48, Apo A-1 and A-4 form coats for chylomicron secretion and aid in their uptake and metabolism by peripheral tissues.
recall that Apo proteins are synthesized in the liver
Term
what are sources of cholesterol?
Definition
diet, bile, shed mucosal cells (small amt)
Term
what kind of digestive processes must cholesterol undergo for absorption?
Definition
dietary cholesterol is sometimes esterified, must be hydrolyzed by cholesterol esterase
Term
what accounts for the selective absorption of cholesterol vs. plant sterols across the brush border?
Definition
two ABC transporters pump plant sterols out of cells into lumen; other ABC transporters transport cholesterol in (mechanism not well understood)
Term
what is the fate of cholesterol after absorption into the enterocyte?
Definition
esterification by acyl coA cholesterol acetyltransferase (ACAT) in ER and incorporated into chylomicrons
Term
what happens to cholesterol in chylomicron remnants after passage thru the lymph?
Definition
chylomicron remnants are taken up by liver, cholesterol is excreted into bile or secreted into plasma as VLDL or HDLs
Term
what is the relationship b/w Ca2+ absorption and Vit D3?
Definition
1,25(OH)2-D3 binds to nuclear vit D receptor and regulated gene transcription of the proteins needed for transcellular Ca2+ absorption.
Term
what are the two types of Ca2+ absorption? which are vit D3 dependent?
Definition
paracellular - thru tight junctions, vit D independent
transcellular - thru enterocyte, vit D dependent
Term
where in gut is transcellular Ca2+ absorption most active? what are the proteins involved in absorption?
Definition
-most active in duodenum
-brush border transport channel: CaT1
-inside enterocyte: calbindin (Ca2+ binding protein) binds and transports Ca2+
-basolateral: PMCA1, Ca2+ ATPase
Term
how is 1,25(OH)2-D3 made? where is it made?
Definition
1. Skin: vit D3 made by acting on 7-dehydrocholesterol; 2. Liver: hydroxylation on 25 position; 3. Kidney: hydroxylation on 1 position (regulated by parathyroid hormone)
Term
what substances are primarily absorbed in the stomach?
Definition
-lipophilic molecules (ethanol)
-weak acids (aspirin, gets protonated in acidic environment and diffuses across mucosa - link to pharm: ion-trapping)
Term
what substances are primarily absorbed in the duodenum and proximal jejunum?
Definition
iron, calcium, fat, sugars, amino acids
Term
what substances are primarily absorbed in the terminal jejunum?
Definition
fatty acids and sugars
Term
what substances are primarily absorbed in the ileum?
Definition
bile salts
vit B12 (remember Vit B12 is bound to intrinsic factor in the stomach and carried thru intestines, finally absorbed in terminal ileum)
Term
what substances are primarily absorbed in the colon?
Definition

 short chain fatty acids from fermentation of indigestible carbs (i.e. fiber, etc.)

water, Na+ Cl-

Term
how does water absorption occur in the duodenum?
Definition
via leaky tight junctions; water rushes into lateral intercellular spaces, making fluid isotonic. it then drains into capillaries and lymph
Term
what are the 2 primary countertranporters involved in ion uptake in the intestine? what is their net effect?
Definition
Na+/H+ countertransport
Cl-/HCO3- countertransport
net: NaCl absorption and H+, HCO3- secretion
Term
what are the 3 regions of the GI tract that secrete HCO3-?
Definition
bile duct cells (cholangiocytes); pancreatic duct cells; duodenum (from Brunner's glands)
Term
which cells of the small intestine are mostly involved in secretion, and which are involved in absorption?
Definition
secretion: crypt cells
absorption: villus cells
Term
what is the method of Cl- secretion in the crypt cells of the small intestine? what regulates it?
Definition
Na+/Cl- cotransport via basal side; Cl- transport out on apical side via CFTR chloride channel. channel is regulated (activated) by cAMP
Term
how do bacterial endotoxins (cholera) cause diarrhea?
Definition
activates adenylcyclase, makes cAMP, overstimulates secretions
Term
what is the frequency of slow waves in the:
stomach
duodenum
ileum
Definition
3 wave/mins
12 waves/ min
9 waves/min
Term
what determines whether or not a slow wave in intestinal smooth muscle causes a contraction?
Definition
whether it surpasses the electrical threshold and stimulates action potentials
Term
what is the sources of slow waves in the intestine? how can these cells be identified?
Definition
interstitial cells of cajal (ICC cells). have spontaneous electrical activity, and are couples to smooth muscle cells via gap junctions. identified by presence of cKit receptor
Term
what are kinds of motility patterns of the small intestine occur during the absorptive?
Definition
absorptive: local contractions to mix up chyme; contractions of muscularis mucosa cause villi to contract, mixing the unstirred layer and compressing lacteals. Near end of absorptive phase, short peristallic movements propel food aborally
Term
what kind of reflex loops are active in carrying out peristalsis (starting with distention as the stimulus)?
Definition
distension is sensed by mechanoreceptors-> passed to cells UPstream that release activating neurotransmitters (ACh, substance P); passed to cells DOWNstream that release inhibitory neurotransmitters (VIP). net result: contraction upstream of bolus, relaxation downstream
Term
what is the predominant motility pattern in the interdigestive/postabsorptive phase?
Definition
migrating motility complex (MMC) - wave of motion that starts in gastric antrum and sweeps indigestible material and bacteria out of the small intestine
Term
what is role of motilin in the MMC? what inhibits the MMC?
Definition
-initiation of MMC occurs when motilin levels are at their peak
-when food enters the small intestine the MMC is inhibited
Term
if you swallow a foreign object (i.e. a dime), how will it get out of your GI tract?
Definition
the MMC will sweep it thru your intestine and facilitate its excretion
Term
how does the autonomic system regulate small intestinal motility?
Definition
excitatory and inhibitory neurotransmitters modulate slow wave amplitude and determine whether electrical threshold is reached and action potentions (i.e. contractions) are generated
Term
what is the response of the ileocecal sphincter to distension of the ileum or cecum?
Definition
increased pressure/distension of the cecum or ileum causes relaxation of the sphincter; utilizes reflex circuits of the myenteric plexus
Term
what is the response to the anal sphincters to rectal distention? what type of muscle is each sphincter made of? what causes these responses?
Definition
increased pressure in the rectum causes relaxation of the internal anal sphincter (smooth muscle); contractions of external sphincter (skeletal muscle).
-acts via myenteric reflexes
Term
what is the pathology of Hirschprungs disease?
Definition
denervation of part of the myenteric plexus of the colon (usually exerts inhibitory influence), so colon gets distended --> congenital megacolon
Term
why is the composition of gases in the stomach and intestines different?
Definition
in stomach acid kills most bacteria; gas composition is similar to that of air. in intestines, gut bacteria produce gases and alter composition
Term
what are some of the roles of intestinal bacteria?
Definition
fermentation of indigestible carbohydrates, metabolize bile acids, synthesize vit K
Term
what are the main ions absorbed in the colon?
Definition
Na+ and Cl-
this results in absorption of water
Term
what is the main route of Na+ and Cl- absorption in the proximal colon?
Definition
same as intestinal mechanism:
Na+/H+ countertransport and HCO3-/Cl- countertransport are coupled
Term
what role does the cecum have in breakdown of indigestible carbohydrates (i.e. fiber)? how are the digestion products absorbed?
Definition
undigested carbohydrates in the cecum get broken down to short chain fatty acids
-absorption is facilitated by Na+/H+ exchange
Term
what is main route of Na+ absorption in distal colon? what regulates this?
Definition
Na+ moves into cell, down its electrochemical gradient, thru sodium channels
-aldosterone regulates number of channels; aldo release increases number
Term
how does Na+ that is transported into the cells of the colon get across the basolateral membrane?
Definition
via Na+/K+ ATPase
Term
what is the colon's role in maintaining K+ levels?
Definition
colon can absorb or secrete K+ as needed
Term
what is the likely pathway for water absorption in the colon? why?
Definition
transcellular (not paracellular) thru aquaporins because it has tight epithelia
Term
how can you distinguish between small intestinal disease and colon disease in a person with diarrhea?
Definition
if you have usual amount of water (1.5L/day) entering colon, most likely colon disease (are getting net secretion from colon rather than absorption). greater than usual flow into ileocecal junction suggests small intestinal disease
Term
in which part of the gut would you find the following enzymes: maltase, sucrase, isomaltase, lactase?
Definition
small intestine
Term
which organ produces the following enzymes: amylase, trypsin, chymotrypsin, carboxypeptidase, elastase, lipase, phospholipase A2, cholesterol esterase?
Definition
pancreas - gets released into duodenum
Term
what are two parts of the GI tract that contain skeletal muscle?
Definition
upper 1/3 of esophagus
external anal sphincter
Term
does the colon mucosa have crypts?
does it have villi?
Definition
crypts - yes
villi - no
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