133b - Water and Electrolyte Absorption

Question 1

Which transporter do we use to absorb fructose?

Answer 1

GLUT5 (apical)

GLUT2 (basolateral)

• Fructose cannot use the SGLT1 transporter
  
  • => Cannot pull in Na+
  • => Less hydrating

Question 2

Why does gatorade hydrate you more than water?

(From an ion transport perspective)

Answer 2

The SGLT1 transporter is only active in the presence of both sodium and glucose (or galactose)

• More sodium and sugar absorption = stronger gradient pulling water in
• Cl- also plays a role
  
  • Na+ pulls Cl- in too
  • More ions = more gradient for water

Question 3

Which protein drives water absorption in the intestine?

Answer 3

Na+/K+ ATPase

• Pumps Na+ out through the basolateral membrane
• Sets up a gradient for Na+ absorption
  
  • Via Na+/H+ exchange, SGLT1, ENaC, etc
• H2O follows

Question 4

Where in the GI tract are most fatty acids absorbed?

Answer 4

Distal ileum

Question 5

How does K+ homeostasis differ in different parts of the colon?

Answer 5

• Proximal colon = mostly secretion
• Distal colon = mostly absorption

[Think about the logic of resorption, it needs to be further down to catch it as it is secreted]

Question 6

How is chloride secretion regulated?

What is the significance?

Answer 6

CFTR channel opening is regulated by cAMP

• Cl- is secreted through the CFTR channel
• More cAMP = more Cl- secretion
• Excess Cl- secretion = watery diarrhea
  
  • ​Cholera toxin -> increased cAMP -> watery diarrhea

[Cl- is high inside the cell, there is always a gradient to leave

Question 7

In general, which ions are abosrbed in the small intestine and colon?

Which are secreted?

Answer 7

• Absorbed
  
  • Na+, H2O
• Secreted
  
  • HCO3-, H+
• Cl- depends on action of CFTR
  
  • Regulated by cAMP

Question 8

In the gut lumen, vitamin D3 upregulates the absorption of which two ions?

Answer 8

Calcium

Phosphorous

• Vitamin D3 upregulates the transcription of proteins required for absorption

Question 9

What protein is required for us to absorbe Ca2+ from the intestinal lumen?

Answer 9

Calcium Binding Protein (CBP)

• Intracellular Ca2+ is low so there is a gradient for absorption
• CBP helps neutralize the charge
  
  • CBP transcription is upregulated by vitamin D3
• ATPase required to absorbe Ca2+ through the basolateral membrane