Potassium Regulation

Question 1

Glomerular Capillary role in regulation of K+ Balance

Answer 1

• Filtration occurs freely across glomerular capillaries

- TF/Pk+ in Bowman’s space in 1.0

Question 2

Proximal Tubule regulation of K+ Balance

Answer 2

-Reabsorbs 67% of filtered K+ along w/ H2O

Question 3

Thick ascending limb of loop of Henle regulation of K+ Balance

Answer 3

• Reabsorbs 20% of filtered K+

- Involves Na+/K+/2Cl- cotransporter in luminal membrane

Question 4

Distal Tubule and Collecting Duct Regulation of K+ Balance

Answer 4

• Reabsorbs or secretes K+ depending on dietary K+ intake
• Reabsorption of K+
  
  • H+/K+ ATPase in luminal alpha intercalated cells
  • Occurs only in low K+ diet
  • Excretion can be as low as 1% as kidney conserves
• Secretion of K+
  
  • Occurs in principal cells
  • Variable & accounts for range of urinary K+ excretion
  • Depends on diet K+, aldosterone, acid/base, and urine flow

Question 5

Mechanism of Distal Tubule K+ Secretion

Answer 5

• K+ actively transported into cell at basolateral membrane by Na+/K+ pump
  
  • Maintains high intracellular K+
• K+ passively secreted at luminal membrane thru K+ channels
  
  • magnitude determined by electrochemical forces
  • High intracellular K+ increases K+ secretion
  • Low intracellular K+ decreases secretion

Question 6

Factors that change distal tubule K+ secretion

Answer 6

• Dietary K+
  
  • High K+ diet increases secretion
  • Low K+ diet decreases secretion
• Aldosterone increases K+ secretion
  
  • Involves increased Na+ entry via Na+/K+ ATPase
  • Pumps Na+ out and K+ in
  • Increased intracellular K+ drives more secretion
  • Also increases # of luminal K+ channels
  • Hyperaldosteronism increases K+ secretion (Hypokalemia)
  • Hypoaldosteronism decreases K+ secretion (Hyperkalemia)
• Acid/Base balance
  
  • H+ and K+ exchange for each other across basolateral mem
  • Acidosis decreases K+ secretion: H+ enter and K+ leaves
    
    • Intracellular K+ decreases and so does secretion
  • Alkalosis increases K+ secretion: H+ leaves and K+ enters
    
    • Intracellular K+ increases and so does secretion
• Thiazide and Loop Diuretics
  
  • Increase K+ secretion
  • Increase flow rate thru distal tubule
  • Cause dilution of luminal K+, increasing secretion force
• K+ Sparing Diuretics
  
  • Decrease K+ secretion
  • If alone, can cause hyperkalemia
  • Spironolactone antagonist of aldosterone
  • Triamterene & amiloride act directly on principal cells
  • Combo w/ thiazide/loop diuretics to reduce K+ loss
• Luminal Anions
  
  • Anions increase K+ secretion by increasing negativity