Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology

()Map KIDNEY ELECTROLYTE BALANCE (Map)

What are Aquaporins

• Family of membrane channel proteins
• Over a dozen homologous water transporting proteins make up this family in mammals
• Allow rapid transport of substances accross membranes
• Found in all life forms
• Commonly abbreviated to AQP
• Ones found in the kidneys are:
  • AQP1 - Tubules
  • AQP2 - Collecting duct
  • AQP3 - Collecting duct
  • AQP4 - Collecting duct
  • AQP6 - Collecting duct (intercalated cells)
  • AQP7 - Proximal tubule
  • AQP11 - Kidney
• They are colour coded according to their function
  • Water channels
  • Glycerol channels
  • Intracellular chloride channels
  • Function Unknown

Function

• They allow the reabsorption of water, glycerol and chlorine from the filtrate
• Without them producing concentrated urine would be impossible

• AQP2 is stored in vesicle membranes within the principal cells of the collecting ducts
• ADH causes these vesicles to attach to the principal cell membranes.
• This is how ADH works.
  • The higher its concentration the more AQP2 bind to principal cell membranes therefore the more AQP2 available to water within the collecting duct
  • The more water is reabsorbed
  • Thanks to the gradient created by the hypertonic medulla which is present due to the countercurrent multiplier in the Loop of Henle
• This system allows both concentrated and dilute urine to be produced depending on ADH secretion and therefore the density of aquaporins in the principal cell membranes

Why Do We Need Aquaporins

• Contrary to popular belief diffusion of water accross lipid bilayers is too slow for most physiological processes
• Aquaporins solve this problem

Other Means of Water Homeostasis

As well as the aquaporins in the distal tubule and collecting duct significant water reabsorption occurs in the proximal tubule and the Loop of Henle and is linked to the absorption of sodium.