Anti-diuretic hormone (ADH) - essential facts (ii)

• the generation of action potentials in supra-optic and paraventricular neurones cause release of ADH from nerve endings into the bloodstream.
• ADH acts on target cells in the kidney (mainly cells lining collecting ducts) to conserve water.
• ADH deficiency results in diabetes insipdus.
• ADH also causes vasoconstriction of blood vessels in the kidney, liver and brain and increases ACTH secretion by an action on the anterior pituitary gland.

Additional Information

Diabetes insipdus
Lack of ADH causes collecting ducts and distal tubules to be impermeable to water and leads to excretion of copious amounts of urine (15 litres / day) and thirst. This can be treated by administration of ADH (nasal spray or injection).

Water Conservation
ADH acts on vasopressin V2 receptors on luminal membranes of cells mainly in collecting ducts but also in distal convoluted tubules to increase permeability to water (and to a lesser extent urea). Activation of the receptor increases intracellular cAMP levels by a G-protein coupled mechanism. Water moves passively from collecting ducts into peritubular spaces and diffuses into blood lowering blood osmolality and reducing stimulation of osmoreceptors (negative feedback).

When an individual drinks a large quantity of fluid, blood becomes hypoosmotic and ADH secretion ceases. Collecting ducts remain impermeable to water and a dilute urine is excreted.