In: Anatomy and Physiology
There can be more than one right answer!
14. Aquaporins are always present (no control over them) in the
A. proximal convoluted tubule
B. descending limb of loop of Henle
C. ascending limb of loop of Henle
D. distal convoluted tubule
E. cortical collecting ducts
A. proximal convoluted tubule
B. descending limb of loop of Henle
Explanations:
The transport of water across the membrane of epithelial cells depends on the presence of aquaporins, the water channels made up of proteins. The aquaporins 1 and 2 are mainly involved in water reabsorption from the kidneys.
Most of the aquaporins are found in the kidney. The aquaporins one is mainly found on the proximal convoluted tubule and loop of Henle and aquaporins 2 is present on the collecting duct.
Types | distribution | functions |
Aquaporin 1 | Apical and basolateral membrane of the proximal convoluted tubule and thin descending limb of LOH |
67% of water reabsorption in the proximal convoluted tubule. imports free water permeability in the descending limb of LOH |
Aquaporin 2 | apical membrane of late distal convoluted tubule and vesicles of principal cells of cortical part of collecting duct | mediates ADH effects |
Aquaporins in the collecting duct:
Nephrogenic diabetes insipidus is characterized by a decrease in the expression of aquaporins in the collecting duct.
In the cortical part of the collecting duct, ADH increases the permeability of the collecting duct to water by inserting aquaporins 2 water channels. Antidiuretic hormone binds with the V-2 receptor on the basolateral membrane of the tubular cell, where it activates adenylate cyclase to increase the production of cyclic AMP. It activates protein kinase A which promotes the formation and insertion of vesicle containing aquaporins 2 on the luminal surface of epithelial cells that increase water permeability. Therefore, in the presence of ADH, water moves out of relatively hypotonic tubular fluid into the interstitium of the cortex, which accounts for the substantial reabsorption of water in the collecting duct in the presence of the hormone.