Tolvaptan is a vasopressor receptor antagonist used to correct hyponatremia in the treatment of syndrome of inappropriate water excretion or SIADH :- Syndrome of inappropriate secretion of ADH is caused due to excesive secretion of antiduretic hormone. Antidiuertic hormone (ADH) or arginine vasopressin is synthesized by the hypothalamus. Its major effect is to prevent diuresis by promoting reabsorption of water in the kidneys.- ADH is the primary physiological determinant of the rate of free water excretion. - Its major renal affect is to augument the water permeability of the leminal membranes of the cortical and medullary collecting tubules thereby promoting water reabsorption.The antidiuretic effect of ADH is mediated by "V2  receptors". Tolvaptan is V2 receptor antagonist- It can be used to correct hyponatremia due to excesive water retnetion caused by increased ADH secretion.Randomized double blind trials have shown that oral or intravenous vasopressin antagonists are more effective than placebo in raising the serum sodium of hyponatremic patients. "Tolvaptan" and "conivaptan" but not lixivaptan are approved for use in the united states.​Vasopressor receptor antagonistThere are three receptor for vasopressin (ADH). - The V1 a, Vlb and V2 receptors. The V2 receptors primarily mediate the antidiuretic response while V1 a and Vlb primarily cause vasoconstriction and adrenocorticotropic hormone (ACTH) release.V2               →        Mediate antidiuretic response VlaVlb         →         Cause vasoconstriction and adrenocorticotropic hormone (ACTH) release.​The vasopressin receptor antagonist tolavaptan produces a selective water diuresis (aquaresis) without affecting sodium and potassium excretion.- The ensuing loss of electrolyte free water will tend to raise the serum sodium in patients with SIADH and may improve mental status in patients with serum sodium under 130 meq/L. ​