Chloride reabsorption
Two routes:
- Paracellular
* i.e. through tight junction
- Transceullar
In proximal tubule
Paracellular route
Reabsorption of water and sodium
--> INCREASED luminal [Cl-]
--> Greater than [Cl-] in peritubular capillaries
--> Diffusion via paracelluar route
NB:
- Occurs after the early portion of proximal tubule because water need to be reabsorbed first before [Cl-] rises.
Transcellular route
- via Cl-base antiporter
- Coupled with Na-H antiporter
Cl-base antiporter
????Powered by gradient of organic base
* [AV6:p81] Rather vague
- Cl- is reabsorbed from lumen
- Base is secreted into lumen
Na-H antiporter
Driven by Na+ gradient
- Na+ is reabsorbed from lumen
- H+ is secreted into lumen
NB:
In addition to coupling with Cl-base antiporter, Na-H is also responsible for
- Reabsorption of bicarbonate
- Most of the Na+ reabsorption in proximal tubule
Recycling
In the lumen, base + H+
--> neutral form of acid
--> diffuses out of lumen back into cells
Overall effect
A "virtual" Na-Cl SYMPORTER
i.e. Na+ and Cl- are cotransported out of lumen into cells
Loop of Henle
- Active reabsorption
* via Na-K-2Cl symporter
Distal convoluted tubule
- Active reabsorption
* via Na-Cl symporter
Collecting duct
In principle cells
- Cl- reabsorption
* via paracellular route
* Passive
In type B intercalated cells
- Cl- reabsorption
* via apical Cl-HCO3 antiporter
* Secondary active
In type A intercalated cells
- [AV6:p160]????Cl- secretion
