Vasopressin, also known as antidiuretic hormone, is a nonapeptide, and found to be essential for cardiovascular homeostasis. The antidiuretic effect of vasopressin has been exploited clinically for over half a century to treat diabetes insipidus. Three receptor subtypes are known to date (V1A-B, and V2 (original designation) or V1-3 (where V1A=V1 and V1B=V3)). Although all three of these proteins are G-protein coupled receptors (GPCRs, subfamily A6), activation of V1A (V1) and V1B (V3) stimulate phospholipase C, while activation of V2 stimulates adenylate cyclase[1].