Renal regulation of potassium

K+ is freely filtered

 

Contribution by segments

• Proximal tubule = 60-80% reabsorbed
• Loop of Henle = about 5-25% reabsorbed
• About 10% is left at the start of DCT
• Some secretion and reabsorption in DCT

In cortical collecting duct

• Principle cells = secretion rate variable (>15% to almost nil)
• Type A intercalated cell = 10% reabsorption

In medullary collecting duct

• 5% reabsorption

NB:

• Cortical collecting duct has much larger effect on K+ reabsorption/secretion than DCT
• Reabsorption is continuous in the collecting ducts
• Secretion is variable, and is regulated

 

Mechanisms of K+ reabsorption

Proximal tubules

Paracellular diffusion (secondary to water reabsorption)

Thick ascending limb

Mainly by Na-K-2Cl symporter

Partially by paracellular diffusion

Cortical collecting duct

Principle cells

Secrete K+
* Active transport of K+ from interstitium into cell across basolateral membrane
* Then passive diffusion out of the cell (back into interstitium or into lumen by luminal K+ channels or K-Cl symporter)
* Diffusion into lumen predominates due to negative charged lumen and greater number of luminal K+ channel

Type A Intercalated cells

Reabsorb K+ by H-K antiporter
* Thus H+ is secreted in exchange for K+

 

Factors affecting K+ secretion

1. [K+] in blood perfusing kidney
2. Plasma level of aldosterone
3. Delivery of Na+ to distal nephrone
4. Flow rate in collecting duct

Aldosterone

Aldosterone stimulate secretion by

• Increasing luminal potassium channel (ROMK) in principle cells
• Increasing Na-K ATPase pump activities

When aldosterone decreases due to increased [Na+]

--> Reduced aldosterone reduces K+ secretion, which is offset by increased flow rate to the cortical collecting duct

Thus,

• Changes in aldosterone level due to changes in Na+ usually do not cause major changes in K+ balance
  * Due to changes in flow secondary to changes in sodium balance

NB:

• Increase in [K+] also stimulate aldosterone secretion
• Aldosterone also regulation insertion of ENaC channels into principle cells

Delivery of Na+

• Principle cells takes up Na+ from lumen through ENaC, and then into interstitium in exchange for K+
• K+ secretion is dependent on this process
  * Decreased Na+ delivery
  --> Decreased secretion

Flow rate in collecting duct

Remember K+ secretion is essentially a passive process

Thus, when flow rate is high

--> Less opportunity for [K+] in tubule to raise up to significant level

--> Increased [K+] secretion

Effects of diuretics

Reabsorption is reduced

However, increased K+ loss is due to increased excretion by cortical collecting ducts

Because:

• Increased flow rate
• Increased delivery of sodium