Topic 43 - Transport processes in the proximal tubule

Question 1

Words to include

Answer 1

• Not permable for:
  
  • Creatinine
  • Urea
  • HCO₃^-
  • Proteins
• Reabsorption
  
  • Actively to interstitium
  • Passively to the peritubular capillaries
• Lumen
• ISF
• Sodium (Na⁺)
  
  • Na⁺/K⁺-ATPase pump
    
    • 3Na out/2K in
• Hydrogen (H⁺)
  
  • Na⁺ entry = H⁺ secretion
  • Amiloride
  • Carbonic anhydrase
• Bicarbonate (HCO₃^-)
  
  • Impermable cells for HCO₃^-
    
    • ^​Indirect transport of HCO₃^-
  • CO₂ diffusing into cell
    
    • H⁺
    • HCO₃^-
    • IC carbonic anhydrase
• Chloride (Cl^-)
  
  • Luminal membrane
    
    • Chloride-acidic anion antiporter protein
      
      • Cl^- → cell
      • Acidic anion → lumen
        
        • Binds H⁺
        • Free acid → diffuses back to cell
• Water (H₂O)
  
  • Type 1:
    
    • Increased peritubular oncotic pressure
    • Paracellulary migrates
      
      • Lumen → interstitium
  • Type 2:
    
    • Aquaphorin-1
• Glucose, amino acids
  
  • Secondary acitve symport
    
    • Na⁺/K⁺-ATPase pump
    • Glucose + Na⁺
    • Amino acid + Na⁺
  • Specific carrier
• Urea
  
  • Passively reabsorbed through cells
  • Paracellular pathways
  • Stays in interstitium
    
    • Formation of osmotic layering
• Proteins
  
  • Lume
  • Back to tubular cells by pinocytosis
    
    • From tubular cells as amino acids to interstitium
• Organic anions and cations
  
  • Secretion
    
    • Na⁺/K⁺-ATPase pump

Question 2

Topics to include in the essay

Answer 2

1. Characteristic of the proximal tubule
2. Sodium
3. Hydrogen
4. Bicarbonate ion
5. Chloride
6. Water
7. Glucose & amino acids
8. Urea
9. Proteins
10. Organic anions & cations

Question 3

Give the general characteristics of the transport processes in the proximal tubule

Give the substances transported in the proximal tubule

Answer 3

• High transport capacity
• No high gradient will be created
• 2/3 of water
• 95% of the filtered bicarbonate reabsorbed
• 70% of the filtrate is reabsorbed in the proximal tubule
• Hormonal regulation is not significant
• The wall of the tubule is not permeable for:
  
  • Creatinine
  • Urea
  • HCO₃^-
  • Proteins
• Reabsorption takes place in two phases:
  
  1. Actively to the intersitium
  2. Passively to the peritubular capillaries
• Substances transported in the proximal tubule:
  
  1. Sodium (Na⁺)
  2. Hydrogen (H⁺)
  3. Bicarbonate ion (HCO₃^-)
  4. Chloride (Cl^-)
  5. Water (H₂O)
  6. Glucose
  7. Amino acids
  8. Urea
  9. Proteins
  10. Organic cations & anions

Question 4

Transport of sodium in the proximal tubule

Answer 4

• Main motor: Na⁺/K⁺-ATPase pump
  
  • ​3 Na⁺ out / 2 K⁺ in

Question 5

Transport of hyrdogen in the proximal tubule

Answer 5

• Na⁺ entry = H⁺ secretion
• Inhibited by amilloride

Question 6

Transport of bicarbonate ion in the proximal tubule

Answer 6

• The cell is impermable for HCO₃^-
  
  • ^​Indirect HCO₃^- transport
• CO₂ diffusing into the cell transforms into H⁺ and HCO₃^- with the help of IC carbonic anhydrase

Question 7

Transport of chloride in the proximal tubule

Answer 7

1. The prerequisite for the transcellular chloride transport is the lower pH in the lumen than in the cell
2. Then the chloride-acidic anion antiporter protein of the luminal membrane transports chloride to the cell, and acidic anion from the cell to the lumen
3. In the lumen the acidic anion binds H⁺ and as free acid easily diffuses back to the cell
4. Here it dissociated and another chloride transport can be carried out

Question 8

Transport of water in the proximal tubule

Answer 8

• Water can be transportet from the lume to the ISF in two different ways:
  
  1. Increased peritubular oncotic pressure makes the water paracellularly migrate from the lume → interstitium
  2. Facilitated by aquaporin-1, a special water channel

Question 9

Transport of glucose & amino acids in the proximal tubule

Answer 9

• Glucose and amino acids are 100% withdrawn from the proximal tubule together with Na⁺, via secondary active symport maintained by the Na⁺/K⁺-ATPase pump
• Glucose and amino acids have their own specific carriers

Question 10

Transport of urea in the proximal tubule

Answer 10

• Half of the urea are either:
  
  • Passively resorbed through the cells
  • Paracellular pathways
• Rest of the urea stays in the interstitium
  
  • ​Contribues to the formation of osmotic layering in the kidney

Question 11

Transport of proteins in the proximal tubule

Answer 11

• Some proteins can get into the lumen
  
  • ​The majority of these get back to the tubular cells by pinocytosis, then form here as amino acids to the interstitium

Question 12

Transport of organic anions & cations in the proximal tubule

Answer 12

• The removal of organic ions are not yet understood propperly
• The basis of the secretion is presence of normal Na⁺/K⁺-ATPase pump