Expansion
Water retention through V2 receptors and vasoconstriction through V1 receptors
Expansion
Antidiuretic hormone (vasopressin) is synthesised in the hypothalamus and released from the posterior pituitary.
- V1 receptors - vascular smooth muscle: vasoconstriction
- V2 receptors - renal collecting duct: insertion of aquaporin-2 channels, so water reabsorption
Stimuli for release
- Rising plasma osmolality, the most sensitive trigger, detected by hypothalamic osmoreceptors and responding to changes of only 1 to 2 per cent
- Falling blood volume or pressure, detected by baroreceptors; less sensitive, needing a fall of around 10 per cent, but producing a far larger release
The second point matters clinically: in significant hypovolaemia, volume defence overrides osmotic control, so antidiuretic hormone is released even when the patient is hyponatraemic. This is why hypovolaemia perpetuates hyponatraemia until the volume deficit is corrected.
Vasopressin is used as a vasopressor in vasodilatory shock, where it acts through V1 receptors and remains effective when catecholamine responsiveness is impaired.