Aquaporin 2

AQP2 is found in the apical cell membranes of the is_associated_with::kidney's is_associated_with::collecting duct principal cells and in is_associated_with::intracellular vesicles located throughout the cell.

Regulation
It is the only aquaporin regulated by is_associated_with::vasopressin. The basic job of aquaporin 2 is to reabsorb water from the urine while its being removed from the blood by the kidney. Aquaporin 2 is in kidney epithelial cells and usually lies dormant in intracellular vesicle membranes, but when it is needed vasopressin binds to the cell surface vasopressin receptor, activating a signaling pathway that causes the aquaporin 2 containing vesicles to fuse with the plasma membrane so the aquaporin 2 can be used by the cell.

This is_associated_with::aquaporin is regulated in two ways by the is_associated_with::peptide hormone is_associated_with::vasopressin:
 * short-term regulation (minutes) through trafficking of AQP2 vesicles to the apical region where they fuse with the apical plasma membrane
 * long-term regulation (days) through an increase in AQP2 gene expression.

This is_associated_with::aquaporin is also regulated by food intake. Fasting reduces expression of this aquaporin independently of vasopressin.

Clinical significance
Mutations in this channel are associated with is_associated_with::nephrogenic diabetes insipidus, which can be autosomal dominant or recessive. Mutations in the vasopressin receptor cause a similar X-linked phenotype.

is_associated_with::Lithium, which is often used to treat is_associated_with::bipolar disorder, can cause acquired is_associated_with::diabetes insipidus (characterized by the excretion of large volumes of dilute urine) by decreasing the expression of the AQP2 gene.

The expression of the AQP2 gene is increased during conditions associated with water retention such as is_associated_with::pregnancy and is_associated_with::congestive heart failure.