Figure

Intracellular action of antidiuretic hormone. The multiple actions of vasopressin can be accounted for by its interaction with the V2 receptor found in the kidney. After stimulation, vasopressin binds to the V2 receptor on the basolateral membrane of the collecting duct cell. This interaction of vasopressin with the V2 receptor leads to increased adenylate cyclase activity via the stimulatory G protein (Gs), which catalyzes the formation of cyclic adenosine 3', 5'-monophosphate (cAMP) from adenosine triphosphate (ATP). In turn, cAMP activates a serine threonine kinase, protein kinase A (PKA). Cytoplasmic vesicles carrying the water channel proteins migrate through the cell in response to this phosphorylation process and fuse with the apical membrane in response to increasing vasopressin binding, thus increasing water permeability of the collecting duct cells. These water channels are recyled by endocyto-sis once the vasopressin is removed. The water channel responsible for the high water permeability of the luminal membrane in response to vasopressin has recently been cloned and designated as aquaporin-2 (AQP-2) [8]. The other members of the aquaporin family, AQP-3 and AQP-4 are located on the basolateral membranes and are probably involved in water exit from the cell. The molecular biology of these channels and of receptors responsible for vasopressin action have contributed to the understanding of the syndromes of genetically transmitted and acquired forms of vaso-pressin resistance. AVP—arginine vasopressin.

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