Posterior pituitary hormones and analogues

Vasopressin: antidiuretic hormone (ADH)

Vasopressin is a nonapeptide (tV2 20 min) with two separate G-protein coupled target receptors responsible for its two roles. The Vj receptor on vascular smooth muscle cells is coupled to calcium-ion entry. This receptor is not usually stimulated by physiological concentrations of the hormone. The V2 receptor is coupled to adenylyl cyclase, and regulates opening of the water channel, aquaporin, in cells of the renal collecting duct.

Secretion of the antidiuretic hormone is stimulated by any increase in the osmotic pressure of the blood supplying the hypothalamus and by a variety of drugs, notably nicotine. Secretion is inhibited by a fall in blood osmotic pressure and by alcohol.

In large nonphysiological doses (pharmacotherapy) vasopressin causes contraction of all smooth muscle, raising the blood pressure and causing intestinal colic. The smooth-muscle stimulant effect provides an example of tachyphylaxis (frequently repeated doses give progressively less effect). It is not only inefficient when used to raise the blood pressure, but is also dangerous, since it causes constriction of the coronary arteries and sudden death has occurred following its use.

For replacement therapy of pituitary diabetes insipidus the longer acting analogue desmopressin is used.

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