Sodium reabsorption
Transporters involved in sodium reabsorption
Apical membrane of tubular cells
Proximal tubules
Symporters
- Na-nutrient
- Na-phosphate
- Na-sulfate
Antiporters
- Na-H
* Account for most of Na+ resorptions
Channels
Basolateral membrane of tubular cells
- Na-K-ATPase pump
- Na-HCO3 symporter
* Proximal tubule
* Ascending thick LOH
Proximal tubule
Na-K ATPase pump in basolateral membrane powers all of the actions
Na-H antiporter
Accounts for
- Most of the sodium reabsorption
- Bicarbonate reabsorption
- Coupling with Cl-base antiporter
--> Reabsorption of Cl-
Others
- Na-amino acid symporter
- Na-glucose symporter
* i.e. SGLT-1
- Na-phosphate
Loop of Henle
Descending limb
- No significant reabsorption of Na+
- But permeable to water
Effect of descending limb
- Reabsorption of water (10%)
- Increased filtrate osmolality
Ascending limb - thin
Passive reabsorption of Na+
- Probably via paracellular route
- Due to increased sodium concentration after descending limb
NB:
- Permeable to Na+
- Impermeable to water
Ascending limb - thick
Active reabsorption of Na+
- Impermeable to water
- Reabsorption of sodium and chloride
* via Na-K-2Cl symporter
- Large number of K+ channels in apical membrane
Other routes of Na+ reabsorption
- Na-H antiporter
* Some contribution
- Paracellular route
Na-K-2Cl symporter
- Blocked by loop diuretic
- K+ recycles back into lumen via apical K+ channels
--> Luminal [K+] does not limit Na+ reabsorption
Paracellular route
Due to
- High paracellular conductance for sodium, and
- Positive luminal potential
NB:
- Probably account for as much as 50% of Na+ reabsorption in thick ascending limb
- Also allow for K+ and Ca2+ reabsorption
- After LOH, very little leakage of Na+ through tight junction
Effect of ascending limb
- Reabsorption of Na+ (25%)
- Decreased filtrate osmolality
Overall effect of LOH
- Reabsorption of water (10%)
- Reabsorption of Na+ (25%)
--> More solute is reabsorbed than water
--> Dilution of filtrate
Thus,
- Iso-osmotic filtrate from proximal tubule becomes hypo-osmotic
Distal convoluted tubule
- Secondary active reabsorption, via
* via Na-Cl symporter (main)
* via apical sodium channel (ENaC)
* [AV6:p85]
Na-Cl symporter
- Blocked by thiazide, e.g. hydrochlorthiazide
Collecting ducts
Sodium reabsorption
* via apical Na+ channel (i.e. ENaC) in principle cells
* Not affect much by ADH level
Medullary collecting ducts
Some Na reabsorption
???? Via ENaC