Active secretions of organic anions and cations

Active secretion of organic anions in proximal tubules

Transport from ISF into tubular epithelial cells
* By active transporters in basolateal membrane
* Rate-limiting step

--> Exit into lumen
* By facilitated diffusion
* Via uniporters or sodium-dependent antiporters

Also a Tm system.

Examples

• Bile salts
  * i.e. glucuronate or sulfate conjugates
• Hydroxybenzoates
• Fatty acids
• PAH
• Oxalate
• Prostaglandins
• Urate

Drugs

• Acetazolamide
• Chlorothiazide
• Ethacrynate
• Furosemide
• Penicillin
• Probenecid
• Saccharin
• Salicylates
• Sulfonamides

 

Urate

• Not protein-bound
  --> Freely filterable
• Almost all filtered urate are reabsorbed early in the proximal tubule
• Then later on actively secreted in proximal tubules, then reabsorbed again later
• Overall --> reabsorption
• Controlled to maintain relatively constant plasma level

 

Active secretion of organic cations in proximal tubules

Transport from ISF into tubular epithelial cells
* By uniporters

--> Exit into lumen
* By antiporter (exchange with H+)

Also a Tm system

Examples

Acetylcholine

Choline

Creatinine

Dopamine

Epinephrine

Guanidine

Histamine

Serotonin

Norepinephrine

Thiamine

Drugs

Atropine

Isoproterenol

Cimetidine

Meperidine

Morphine

Procaine

Quinine

Tetraethyl ammonium