0206 - Active Transport in the Gut Flashcards
What electrolytes are transported in the gut and where?
Na and Cl - absorbed by SI and LI
K - net absorbed by SI, net secreted by LI
HCO3- - secreted by LI and SI
SI - absorbs Na, Cl, K, secretes HCO3-
LI - Absorbs Na, Cl, secretes K, HCO3-
How is Na+ absorbed in the gut?
Absorbed in SI and LI
Na+/Nutrient cotransport J/I- primary mechanism after a meal. Electrogenic
Na/H exchange D/J - Stimulated by luminal alkalinity, electroneutral.
Na/H and Cl/HCO3- exchange I/pC - primary mechanism between meals.
Epithelial Na+ channels - dC
Briefly outline Na+/Nutrient cotransport
E.g. SGLT-1 or AA transporters. Primary mechanism of Na absorption postprandial.
Electrogenic. Occurs in jejunum and ilium.
Briefly outline Na/H exchange.
Occurs in duodenum and jejunum. Stimulated by luminal alkalinity.
Electroneutral NHE1 to cytoplasm, NHE2/3 to lumen, and are blocked by amiloride.
Briefly outline Na/H and Cl/HCO3- exchange.
Primary mechanism of Na+ absorption between meals.
Occurs in ileum and proximal colon. Na/H and Cl/HCO3 exchangers on luminal membrane. Carbonic anhydrase in the cell provides substrate, Na/K-ATPase maintains gradient.
Results in electroneutral absorption of NaCl. Unknown how Cl- exits the cell. Diminished activity results in diarrhea.
Briefly outline epithelial Na+ channels in the gut.
Occur in distal colon. Blocked by amiloride.
Electrogenic.
What are the three methods of Cl- absorption in the gut?
Passive - J, I dC - transcellular (channels) and paracellular.
Cl-/HCO3- exchange - I, pC, dC
Na/H and Cl/HCO3- exchange.
Briefly outline passive Cl- absorption in the gut.
Occurs in Jejunum, ileum, and distal colon.
Na+ transport from the lumen creates a ‘lumen-negative transepethilial potential’ dragging Cl out via chanells (transcellular) or paracellular.
Ultimately ends up in cytoplasm, like Na+ (due to Na/K-ATPase).
Briefly outline Cl-/HCO3- exchange in the gut.
Occurs in ileum, proximal, and distal colon.
Straight Cl-HCO3- exchanger on luminal membrane, relying on CA to provide substrate, and H+/Na+ exchanger and Na+/K+-ATPase to maintain cell pH.
Cl- cell exit mechanism unknown.
How does oral rehydration solution work?
Takes advantage of SGLT transport in SI. H2O osmotically follows Na, with K being absorbed by solvent drag (paracellular). Na concentration can be varied depending on degree of dehydration.
Briefly outline Cl- secretion in the gut.
Important in pathophysiology of many types of diarrhea. Occurs along full length of intestine.
cAMP or Ca++ insert Cl- channels from subapical vesicles into brush border.
K/Na/2Cl- cotransporter transports Cl- into cell, where it enters lumen via Cl- channels. Na+ follows transcellular, and H2O follows osmotically.
Outline passive K+ absorption and secretion in the intestines.
SI - net Absorption, LI net secretion.
Absorption - paracellular via solvent drag. (Jejunum, ileum)
Secretion - paracellular due to lumen potential of 25mV (NFI). (Proximal and distal colon)
Outline active K+ absorption and secretion in the intestines.
Occurs in the colon, not small intestine.
Absorption - K/H-ATPase, reliant on Na/K-ATPase for substrate. Mechanism for K+ cell exit unknown. Distal colon.
Secretion - K+ channels, reliant on Na/K/2Cl co-transporter for substrate. With Na/K-ATPase and leak channels as well, balance goes to lumen ‘pump-leak balance’. Proximal and distal colon.
Where are carbohydrates, proteins, and lipids absorbed in the gut?
Predominantly in Duodenum, then Jejunum, then ileum.
Where are calcium, iron, iron, and folate absorbed in the gut?
Iron and Folate - exclusively in duodenum.
Calcium, predominantly in duodenum, with low absorption in jejunum and ileum.
