Antidiuretic Hormone (Vasopressin)

• retention of water by kidney and vasoconstriction (in high concentrations)

Release

• controlled by plasma osmolarity, blood volume and arterial pressure
• osmoreceptors trigger firing of AVP neurons through cholinergic synapse
• action potentiated by angiotensin II

Site of action

• binds to V₂ receptor on cells of distal convoluted tubule and medullary collecting ducts

Mechanism of action

• V₁ receptors activate the PI signaling system in vascular smooth muscle
  • intense peripheral vasoconstriction
• V₂ receptors, which are coupled to Adenylate Cyclase, on the proximal collecting duct
  • inserts aquaporin into luminal  membrane
  • thereby increasing their permeability to water
  •  water leaves the collecting ducts down its osmotic gradient into the interstium
  • They also result in the release of factor VIII and von Willebrand factor

Disorders

• deficiency in vasopressin action leads to diabetes insipidus
  • either through failure to secrete (CNS problems) or failure to respond (renal problems)
  • unable to concentrate urine, so frequently urinate and constantly thirsty
• overproduction leads to Syndrome of Inappropriate AntiDiuretic Hormone (SIADH)
  • hyponatraemia
  • continued renal sodium excretion
  • no appreciable fluid volume loss
  • inappropriately high urine osmolality

14.2.2.1 antidiuretic hormone = vasopressin

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