Aquaporins: Mammals
There are thirteen known types of aquaporins in mammals, and six of these are located in the kidney. The most studied aquaporins are AQP1, AQP2, AQP3, and AQP4.

Water crosses the cell membrane by either diffusing through the phospholipid bilayer or by passing through special water channels called aquaporins. More than 10 mammalian aquaporins have so far been identified, but the existence of many more is suspected. Most aquaporins appear to be exclusive water channels that will not allow permeation of ions or other small molecules. Some aquaporins - known as aquaglyceroporins - transport water plus glycerol and a few other small molecules.

Aquaporin 1

AQP1 is a widely expressed water channel, whose physiological function has been most thoroughly characterized in the kidney. It is found in the basolateral and apical plasma membranes of the proximal tubules, the descending limb of the loop of Henle, and in the descending portion of the vasa recta. Additionally, it is found in red blood cells, vascular endothelium, the gastrointestinal tract, sweat glands, and lungs. It is not regulated by vasopressin (ADH).

Aquaporin 2

AQP2 is found in the apical cell membranes of the kidney's collecting duct principal cells and in intracellular vesicles located throughout the cell. This aquaporin is regulated in two ways by the peptide hormone vasopressin: short-term regulation (minutes) through trafficking of AQP2 vesicles to the apical region where they fuse with the apical plasma membrane, and long-term regulation (days) through an increase in AQP2 gene expression. Mutations in this channel are associated with nephrogenic diabetes insipidus, which can be either autosomal dominant or recessive. Lithium, which is often used to treat bipolar disorder, can cause acquired diabetes insipidus by decreasing the expression of the AQP2 gene. This can result in debilitating increases in the rate of urine production. The expression of the AQP2 gene is increased during conditions associated with water retention such as pregnancy and congestive heart failure.

Aquaporins 3 and 4

These aquaporins are found in the basolateral cell membrane of principal collecting duct cells and provide a pathway for water to exit these cells. In kidney, AQP3 gene expression is regulated by vasopressin (ADH), whereas AQP4 is constitutively expressed. AQP4 is expressed in astrocytes and are upregulated by direct insult to the central nervous system.

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