Question

Describe how vasopressin changes the permeability of the collecting duct of the kidneys. Include the molecular mechanisms of the changes and the consequences for the composition of the urine with and without vasopressin.

Answer 1

Vasopressin aslo known as antidiuretic hormone is synthesized into the hypothalamus and is stored and released by posterior pituitary.

Vasopressin acts on the vasopressin receptors present on the distal tubule- and collectinh ducts. There are two types of receptors - V1 and V2.

Vasopressin has two functions - reabsorption of water and regulation of blood pressure by vasoconstriction.

Mechanism -

Vasopressin binds to the the V2 receptor and mediate its action through cAMP, and increases aquaporin-2 insertion through which water absorption is increased.

It is this insertion of aquaporin-2 into apical membrane of collecting duct which increases water reabsorption.

As more water is reabsorbed, more will be the formation of concentrated urine.

Vasopressin is released when there is decreased body fluid and increased tonicity of the body fluids. It acts on distal tubule and collecting duct to increase water reabsorption and returning tonicity of fluids to normal. The urine produced is thus highly concentrated.

In absence of vasopressin, dilute urine will be produced with excretion of excess water and frequent urination. Consequences will be low blood pressure due to dehydration.