Proximal Tubular Transport - Muster

Question 1

What is the contribution of the proximal tubule to the overall reabsorption of the GFR?

Answer 1

• 2/3 of all filtered solutes and water are reabsorbed in the proximal tubule
  
  • ~66% of Na⁺, Cl^-, H₂O
  • 100% of glucose
  • 80% HCO₃

Question 2

What forces are contributing to the reabsorptive nature of the peritubular capillaries compared to filtrative nature of glomeruli?

Answer 2

• Starling forces:
  
  • increased oncotic pressure in capillary due to filtration of solute and water
  • decreased hydrostatic pressure in capillary due to resistance
  • Net Result = large force towards reabsorption

Question 3

How is sodium reabsorbed?

Answer 3

• Na⁺ pumped out of proximal tubule cells against electrochemical gradient via Na⁺/K⁺ ATPase
  
  • higher concentration outside of capillary due to this pump
  • Na⁺ diffuses down concentration gradient into fenestrated capillary
• Some Na⁺ tags along with Cl^- during paracellular movement
• Higher [Na⁺] outside of capillary also due to:
  
  • basolateral Na⁺/HCO₃^- symporter

Question 4

How is Chloride reabsorbed?

Answer 4

• Increased [Cl^-] gradient in late sections of proximal tubule => favorable for transcellular movement
• Luminal side:
  
  • Formate/Cl^- antiporter
• Basolateral side:
  
  • K⁺/Cl^- symporter
  • Cl^- channel
• Para-cellular movement through tight junctions
• Diffuses down concentration gradient into fenestrated capillary

Question 5

How is glucoses reabsorbed?

Answer 5

• Na⁺-Glucose symporters AKA Sodium-Glucose Linked Transporters (SGLTs)
  
  • transport of glucose against gradient with help of Na⁺ going down its gradient
    
    • Secondary Active Transport
• Glucose leaves proximal tubule cell via carrier-mediated transport to interstitium
• Diffuses down concentration gradient into fenestrated capillary

Question 6

How are amino acids reabsorbed?

Answer 6

• Na⁺-AA symporters
  
  • transport of AA’s against gradient with help of Na⁺ going down its gradient
    
    • Secondary Active Transport
• AA’s leave proximal tubule cell via carrier-mediated transport to interstitium
• Diffuses down concentration gradient into fenestrated capillary

Question 7

How is bicarbonate reabsorbed?

Answer 7

• Bicarbonate is created and put back into the blood stream (reclaimed)
  
  • CO₂ + H₂O => H₂CO₃ => HCO₃^- + H⁺
    
    • rxn sped up by Carbonic anhydrase
    • H⁺ will leave via Na⁺/H⁺ antiporter
  • HCO₃^- then leaves proximal tubular cell via Na⁺/HCO₃^- symporter
• Diffuses down concentration gradient into fenestrated capillary

Question 8

How are weak organic acids and bases reabsorbed?

Answer 8

• Monocarboxylic acids are transported into the proximal tubular cell via MCA/Na⁺ symporter
  
  • MCA’s are pumped against concentration using energy from Na⁺ going down concentration gradient
  • Pump is saturable
  • MCA’s leaves proximal tubule cell via carrier-mediated transport to interstitium
  • Diffuses down concentration gradient into fenestrated capillary

Question 9

What are the three ways water is reasbsorbed?

Answer 9

1. Diffusion (minor amount)
2. Aquaporin
3. Paracellular transport

Question 10

What is primary active transport?

Answer 10

• moves solute against electrochemical gradient
  
  • requires ATP!
  • e.g. ATPases

Question 11

What is secondary active transport?

Answer 11

• One solute moves down electrochemical gradient to drive another solute against it’s gradient

Question 12

What is facilitated transport?

Answer 12

• Facilitated diffusion across lipid bi-layer
  
  • e.g. Channels, carrier-mediated

Question 13

What is diffusion?

Answer 13

• Movement of solute down a gradient
• Primary method across peri-tubular capillary and paracellular movement
  
  • better for non-charged solutes