133. Water, Electrolyte Absorption

Question 1

How is NaCl absorbed in small intestine and colon?

Answer 1

1. Na/K ATPase pumps Na out of cell basolateral
2. Na/H exchanger (apical) brings Na into cell
3. Cl passively absorbed via Cl/bicarb exchange

Question 2

How are nutrients, like carbs and AA, absorbed?

Answer 2

Na-dependent indirectly active transport
driver: basolateral Na/K ATPase
apical side: SGLT1 (2 Na with 1 glu/gal) - used to replenish electrolytes with gatorade/oral rehydration (contains Na/K/Cl/glucose)
GLUT5 passive fru apical channel
basolateral side: GLUT2 - passive glu/gal/fru channel

AA absorbed coupled with Na

Question 3

How is Na absorbed in the Ileum + Proximal colon? How does this differ from distal colon?

Answer 3

Ileum/Proximal colon
Apical Na/H + Cl/bicarb exchangers driven by Na/K ATPase drive Na/Cl absorption

Distal Colon
Epithelial Na Channel (ENaC) driven by Na/K ATPase (cause Cl/H2O obligate absorption)

Question 4

How is Cl secreted in the small intestine and colon? How is this secretion regulated?

Answer 4

Na/K ATPase basolateral drives basolateral NKCC1 cotransport (KEY CHANNEL)
Cl secreted via passive CFTR

VIP and PGE2 cause rise in cAMP = open CFTR = more Cl secretion
(pathway can be activated by bacterial toxins like cholera = high chloride diarrhea)

Question 5

How and where is K absorbed and secreted throughout the small and large intestine?

Answer 5

Jejunum/Ileum: Passive K Absorption: more electrolyte absorption raises GI lumen K which drives more K diffusion across enterocytes

Proximal + Distal Colon:
Passive K Secretion: lumen has more negative potential, more K intercellular diffusion
Active K Secretion: basolateral Na/K ATPase drives basolateral NKCC = more K in cell = K secreted via apical channels

Only distal colon:
Active K Absorption: minor apical H/K ATPase drives K absorption

Question 6

How is phosphate absorbed? Where are its reservoirs?

How is Mg absorbed?

How is Ca absorbed? What is its biggest reservoir? What regulates its absorption?

Answer 6

P: reservoirs in soft tissues + skeletal system, transported with Na

Mg: transepithelial channels or intercellular transport

Ca: biggest reservoir is bone
when low Ca blood = more production of vit D3 = production of Ca ATPases and calbindin carrier = more active absorption from enterocytes via basolateral Ca/Na exchanger and Ca ATPase
(active transport required because Ca concentrations same in lumen as plasma)