ADH

Antidiuretic hormone (ADH) increases the permeability of the collecting duct walls to water, so more water is reabsorbed and less urine is produced ('antidiuresis' means producing less urine).How ADH acts on a collecting duct cell:1. ADH travels in the blood and binds to specific receptors on the cell surface membrane of collecting duct cells (the membrane facing the blood).
2. This activates the enzyme adenylyl cyclase, which makes cyclic AMP (cAMP) from ATP. cAMP acts as a second messenger.
3. cAMP activates a series of enzyme-controlled reactions (a phosphorylation cascade).
4. Vesicles in the cytoplasm, whose membranes contain water channel proteins called aquaporins, move to the membrane facing the lumen and fuse with it.
5. The number of aquaporins in the luminal membrane increases, so the wall becomes more permeable to water.
6. Water moves by osmosis from the filtrate, through the aquaporins, into the cells and then into the tissue fluid of the medulla, which has a low water potential. It is then carried away in the blood.The result is a small volume of concentrated urine.When ADH concentration falls, aquaporins are removed from the luminal membrane by endocytosis and stored again in vesicles. The collecting duct becomes less permeable, less water is reabsorbed and a large volume of dilute urine is produced.ADH also makes the lower collecting duct more permeable to urea, so more urea diffuses into the medulla and helps to keep its water potential low.ADH does not change how much water is filtered. It only controls how much is reabsorbed at the end of the nephron. The effect is graded: more ADH gives more aquaporins and more concentrated urine.