Topic 44 - Transport in the Henle-loop and the distal tubule

Question 1

Words to include in transport in the Henle-loop

Answer 1

• Generation of hyperosmotic renal medulla
  
  • Concentrate urine
• Sodium
• Thick descending limb
  
  • Little secretory and absorptive ability
  • High permability
• Thick ascending limb
  
  • 25% reabsorbed
  • Na⁺/K⁺-ATPase pump
    
    • Na⁺​: Cell → intersitium
    • K⁺: Interstitium → cell
    • Against concentration gradient
  • Luminal side
    
    • Na⁺ reabsorbed due to decrease of IC Na⁺
    • K⁺ leaves passively
      
      • K⁺ channel
        
        • On basolateral side
• Furosemide sensitive Na⁺/K⁺/2Cl^- symporter
  
  • Protein
  • Luminal side
  • Electroneutral transport
  • Inhibited by:
    
    • Furosemide
    • Water
    • Salt loss
  • Diuresis
• Impermable for:
  
  • Water
    
    • Lumen becomes hypoosmotic

Question 2

Words to include in transport in the distal convoluted tubule

Answer 2

• Na⁺/Cl^- symport protein
• Electrocemically neutral way
• Tazide derivates
  
  • Diuresis
  • Inhibits
• Chloride
  
  • K⁺/Cl^- cotransporter protein
  • → interstitium
• Parathormone (PTH)

Question 3

Words to include additional

Answer 3

Hormonal and pharmalogical effects

• Distal convoluted tubule
  
  • PTH
    
    • Ca²⁺ reabsorption
  • Calcitonin
    
    • Ca²⁺ excretion
  • Both
    
    • Phosphate excretion
• Prox. and dist. convoluted tubule + collecting duct
  
  • Carbonic anhydrase inhibitors (actasolamide)
• Thick ascending limb
  
  • Frosamide
    
    • Na⁺/K⁺/Cl^- reabsorption (inhibits)
• Distal convoluted tubule
  
  • Thiazide
    
    • Na⁺/Cl^- symport

Question 4

Transport in the Henle-loop

Answer 4

• Most important function of the henle-loop: generation of a hyperosmotic renal medulla
  
  • ​A preprquest for the ability of the kidneys to concentrate the urine
• 30% of the Na⁺ content of the filtrate is reabsorbed here

Thick descneding limb

• Little secretory and absorptive ability
• No significant active transport in either direction
• Permeability is high
• As the paracellular transport is very limited on this section, and electrical potential gradient forms between the lumen and the interstitium

Thick ascending limb

• 25% of the filtered amount of substances are reabsorbed in this segment
• The main force is the Na⁺/K⁺-ATPase pump pumps against the concentration gradient:
  
  • Na⁺: from the cell → interstitium
  • K⁺: from the interstitium → cell
• On the luminal side Na⁺ can be reabsorbed due to the decrease of IC Na⁺
• The excess K⁺ leaves passively via the K⁺ channel of the basolateral side
• The most important symporter protein of the ascending limb:
  
  • Furosemide sensitive Na⁺/K⁺/2Cl^- symporter found on the luminal side.
  • This provides electroneutral transport increases withdrawal of the ions (further 20% of the filtered ion quantity is reabsorbed)
• Cycle of ammonia/ammonium:
  
  • NH₄ is taken up by the cells of the thick ascending limb with active K⁺/NH₄⁺ exchange
  • Ammonia formed in the cell can not leave on the luminal side, because that is impermable to ammonia. Therefor ammonia migrates to the intersitium and from there gets into the descending limb
    
    • This process works as a coutercurrent multipler and incereases the ammonia content in the medulla
    • Ammonia gets taken up actively by the collecting tubule cells and from there to the outside
• TAL is impermeable to:
  
  • Water and urea
    
    • ​Lumen becomes hypoosmoti

Question 5

Transport processes in distal convoluted tubule

Answer 5

• The Na⁺/Cl^- symport protein of this segment absorbs further 5% of the filtered Na⁺ on the luminal side in an electrochemically neutral way
• Can be inhibited by tiazide derivates causing diuresis
• The Cl^- is carried by the K⁺/Cl^- cotransporter protein into the interstitium
• The regulated transport of Ca²⁺ starts in this section, which is directed primarily by the Ca²⁺ saving effect of the parathormone (PTH)

Question 6

Hormonal and pharmalogical effect

Answer 6

• Distal convoluted tubule
  
  • PTH: Ca²⁺ reabsorption
  • Calcitonin: Ca²⁺ excretion
  • Both: phosphate excretion
• Proximal and distal convoluted tubule, and collecting ducts
  
  • Carbonic anhydrase inhibitors (acetasolamide)
• Thick ascending limb
  
  • Furosamide: Inhibitor of the Na⁺/K⁺/Cl^- reabsorption
• Distal convoluted tubule
  
  • Thiazide: Inhibitor of Na⁺/Cl^- symport