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VASOPRESSIN (ANTIDIURETIC HORMONE, ADH)
Vasopressin is a peptide hormone released by the posterior pituitary in response to rising plasma tonicity or falling blood pressure. Vasopressin possesses antidiuretic and vasopressor properties. A deficiency of this hormone results in diabetes insipidus.
Vasopressin is a nonapeptide with a 6-amino-acid ring and a 3-amino-acid side chain. The residue at position 8 is arginine in humans and in most other mammals except pigs and related spe-cies, whose vasopressin contains lysine at position 8 (Figure 37–5). Desmopressin acetate (DDAVP, 1-desamino-8-D-arginine vaso-pressin) is a long-acting synthetic analog of vasopressin with mini-mal pressor activity and an antidiuretic-to-pressor ratio 4000 times that of vasopressin. Desmopressin is modified at position 1 and contains a D-amino acid at position 8. Like vasopressin and oxytocin, desmopressin has a disulfide linkage between positions 1 and 6.
Vasopressin is administered by intravenous or intramuscular injection. The half-life of circulating vasopressin is approximately 15 minutes, with renal and hepatic metabolism via reduction of the disulfide bond and peptide cleavage.
Desmopressin can be administered intravenously, subcutane-ously, intranasally, or orally. The half-life of circulating desmopres-sin is 1.5–2.5 hours. Nasal desmopressin is available as a unit dose spray that delivers 0.1 mL per spray; it is also available with a calibrated nasal tube that can be used to deliver a more precise dose. Nasal bioavailability of desmopressin is 3–4%, whereas oral bioavailability is less than 1%.
Vasopressin activates two subtypes of G protein-coupled receptors . V1 receptors are found on vascular smooth muscle cells and mediate vasoconstriction via the coupling protein Gq. V2 receptors are found on renal tubule cells and reduce diure-sis through increased water permeability and water resorption in the collecting tubules via Gs. Extrarenal V2-like receptors regulate the release of coagulation factor VIII and von Willebrand factor.
Vasopressin and desmopressin are treatments of choice for pitu-itary diabetes insipidus. The dosage of desmopressin is 10–40 mcg(0.1–0.4 mL) in two to three divided doses as a nasal spray or, as an oral tablet, 0.1–0.2 mg two to three times daily. The dosage by injection is 1–4 mcg (0.25–1 mL) every 12–24 hours as needed for polyuria, polydipsia, or hypernatremia. Bedtime desmopressin therapy, by intranasal or oral administration, ameliorates noctur-nal enuresis by decreasing nocturnal urine production. Vasopressin infusion is effective in some cases of esophageal variceal bleeding and colonic diverticular bleeding.Desmopressin is also used for the treatment of coagulopathy in hemophilia A and von Willebrand’s disease .
Headache, nausea, abdominal cramps, agitation, and allergic reactions occur rarely. Overdosage can result in hyponatremia and seizures.Vasopressin (but not desmopressin) can cause vasoconstriction and should be used cautiously in patients with coronary artery disease. Nasal insufflation of desmopressin may be less effective when nasal congestion is present.
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