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From Citizendium, the Citizens' Compendium

Antidiuretic hormone also called vasopressin is "released by the neurohypophysis of all vertebrates (structure varies with species) to regulate water balance and osmolarity. In general, vasopressin is a nonapeptide consisting of a six-amino-acid ring with a cysteine 1 to cysteine 6 disulfide bridge or an octapeptide containing a cystine. All mammals have arginine vasopressin except the pig with a lysine at position 8. Vasopressin, a vasoconstrictor, acts on the kidney collecting ducts to increase water reabsorption, increase blood volume and blood pressure."^[1]

References

1. ↑ Anonymous, (2008) Vasopressins (English). Medical Subject Headings. U.S. National Library of Medicine.