GI absorption: Nutrients/Water/Electrolytes Flashcards
What are the factors that influence the rate of fluid absorption?
Where does the most/least fluid absoprtion occur?
Luminal Osmolality
Rate of active solute transport
Bowel segment (Most fluid absorption - duodenum + proximal small intestine- jejunum; least fluid absorption - colon)
If you ingest hypotonic food, fluid is __ to make things isotonic. Fluid is added to __ food to make things isotonic. All this happens in the __ prior to entering the small intestine.
Fluid is removed to make hypotonic food isotonic
Fluid is added to hypertonic food to make things isotonic
Happens in the duodenum
With increasing distance down the GI tract, what is the permeability of water?
The farther down the GI tract you go, the lower the permeability to water.
Most permeable: duodenum and jejunum
Least permeable: rectum
Describe the process of neutral sodium absorption in the gut. During which phase in what part if the gut is this type of transport most important?
T/F: This is a major route of salt reabsorption in the colon.
Describe the effect of cAMP on this type of transport
Uses apical NCC, NHE3, and AE1 to drive salt inside the cell
True
Increased cAMP transport blocks these channels
Describe how linked transport leads to Na reabsorption
Coupled transport of sodium with other things (either SGLT2 or other things like AAs, peptides, fat soluble vitamins etc)
Describe how linked transport is used to treat cholera
Na+/Glucose therapy: lots of glucose/little salt >> SGLT2 >> causes solvent drag (water passively follows absorption of solute) >> rehydrate cholera pts
How is calcium absorbed in the gut (hint: there’s 2 ways)? What is the primary absorption site of calcium?
What is the role of VitD in calcium reasborption?
calcium enters thru whatever calcium channel>> calbindin traps calcium inside cell (regulates intracellular calcium levels) >> release of Ca2+ to efflux channels
Also absorbed paracellularly
Duodenum = primary absorption site of calcium
Vit D regulates the expression of Ca2+ channels
What is the primary site of absorption for Iron? Describe the process of iron absorption at this site.
What are the roles of ferritin, ferroportin, hepcidin, hephaestin and transferrin? What are the two iron channels involved and what do they do?
How do we get rid of excess iron?
Primary site of absorption of iron = proximal small instestine
Ferritin: binds intracellular iron; bound iron is for storage
Ferroportin: Transports iron out of the cell to send it to the bloodstream (via hephaestin ; changes Fe2+ back to Fe3+)
Transferrin: binds Fe3+ and transports it in the bloodstream
Hepcidin: blocks ferroportin which decreases iron reabsorption (prevents iron release)
DMT1 and Heme transporter; DMT1 transports non heme iron/Heme transporter transports heme iron
Shedding of epithelial cells (which will have iron bound to ferritin in them)
What is the body’s response to iron deficiency/iron overload (i.e. of the iron transporters/proteins discussed, which one is upregulated/downregulated and how does that affect iron retention/excretion)
Iron deficiency: increased iron uptake (increased transferrin), less iron lost in shedding epithelial cells
Iron overload: decreased uptake (decreased transferrin), more iron lost in shedding epithelial cells
Describe the process of chloride absorption and secretion in the gut.
What is the significance of the CFTR channel? How is it regulated by cAMP levels?
Absorption: apical AE1 and NCC involved in chloride reaborption
Secretion: Apical CFTR kicks Cl- out. Increased cAMP levels overstimulate CFTR channel
Describe the changes in chloride absorption in Cystic Fibrosis. How is meconium ileus demonstrative of CF?
CFTR channel defect
Loss of cAMP dependent chloride transport
Decreased chloride secretion (water secretion) >> surface membrane drier
Meconium = much drier, which forms pebbles that cause bowel obstruction
What is the mechanism by which Cholera toxin causes secretory diarrhea?
How do you treat cholera?
Cholera Tx increases Cl- secretion and inhibits sodium reabsorption
Cholera tx >> increased cAMP levels >> upregulated CFTR >> increased Cl- secretion (water follows) = secretory diarrhea
Treatment: anti-biotics or salt/sugar solution (utilizes SGLT2 mechanism)
What are the differences between the small bowel and the colon with regards to the following?:
permeability gradient
presence of nutrient transporters
effect of aldosterone
glucocorticoids
Permeability gradient: small bowel has permability gradient throughout; colon has low permeability throughout
Nutrient transporters: yes in small instestine; not in colon
Aldosterone: no effect in small intestine; increases Na+ absorption via ENaC channels in colon
Glucocorticoids: increase Na uptake in both
Describe the differences in ion absorption in proximal and distal colon
What is the role of amiloride in regulating Na absorption?
What’s the difference between secretory and osmotic diarrhea?
Proximal colon: Na+ reabsorption also coupled to short chain fatty acid transport
Distal colon: aldosterone increases # ENaC channels >. increases Na+ reabsorption
Amiloride - diuretic; blocks Na+ absorption via ENaC
Secretory diarrhea: happens when there’s increased ion secretion; Osmotic diarrhea: happens when there’s malabsorption of solutes, which creates high osmotic pressure
Explain the general process of intraluminal digestion. What is the role of the PepT protein?
Pancreatic proteases breakdown proteins into di/tri peptides >> transported via PepT into cell >> intracellular di/tri peptidases break down di/tripeptides into amino acids >> aa’s absorbed into portal blood
Explain the general process of luminal digestion
(begins in the stomach)
Protein broken down by pepsin >> oligopeptide >> broken down by various enzymes (trypsin/chymotrypsin/carboxypeptidases A,B/elastase)
Intraluminal digestion of carbohydrates begins in the mouth via the __ enzyme and continues in the ___. T/F: The process also happens in the stomach.
The a-1-4 linkages in amylose are cleaved by __, turning amylose
into __, and __
Salivary amylase
Proximal duodenum
Falsehood. The stomach doesn’t have salivary amylase.
Amylase, maltotriose and maltose
Amylopectin is broken down into __, __ and __ by amylase. __ is formed b/c amylose can’t break down a-1-6 bonds. Isomaltase further breaks it down to __ and __.
Describe the process of brush border digestion of carbs
Maltotriose, maltose, alpha-limit dextran
Maltose and sucrose
Brush border digestion:
Maltose broken down into 2x glucose
Sucrose broken down into glucose and fructose
Lactose broken down into glucose and galactose
Draw and describe the transport of glucose and fructose across an enterocyte (name the transporters and what side they’re on)
Apical SGLT1/2: Na+/Glucose or galactose
Apical GLUT5: Fructose
Basolateral: Glucose
Explain how insulin regulates the activity of GLUT2
What happens with GLUT2 in between meals vs when glucose conc is high
Insulin stops GLUT2 from translocating to the basolateral membrane >> stops glucose absorption into bloodstream
In between meals: GLUT2 translocates to BLM per usual
High glucose conc: GLUT2 translocates to both apical and basolateral membranes to increases glucose uptake
Describe the functions of the following lipid enzymes:
Lipase-colipase complex
Phospholipase 2
Cholesterol esterase
Lipase-colipase complex: triglycerides >> FAs and monoglycerides
PhosphoLipase 2: lecithin >> FAs and Lysolecithin
Cholesterol esterase: Cholesterol ester >> FAs and Free cholesterol
Describe the role of colipase in facilitating lipid break down
Colipase frees up the active site of lipase, which can be inhibited by bile acids
What is the role of chylomicrons in fat digestion (i.e. what two things make it different from micelles)?
Chylomicrons are composed of proteins and fats, so they can package both. They also package and transport things to be transported in lymph vessels, not in the blood stream
Fill in the table below the role of colonic bacteria in carbohydrate digestion
