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

• Na+ channels (ENaC)

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