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
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
- [K+] in blood perfusing kidney
- Plasma level of aldosterone
- Delivery of Na+ to distal nephrone
- 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