Vasopressin as a regulator of tonicity and body water volume

The main stimulus for vasopressin release is an increase in plasma tonicity.

In the simplest way, vasopressin adjusts renal water excretion in response to changes in extracellular fluid tonicity.

The sensitivity threshold is a 1% increase in plasma tonicity.

Even though vasopressin concentration continues to rise, at a serum concentration of 5pM (pico-Moles) the maximal urinary concentration is achieved, and no greater water retention is possible.

After this point, defence of osmolality must be accomplished by increasing water intake, rather than preventing water loss.

However, vasopressin concentration continues to rise; it is no longer merely an antidiuretic hormone. At such concentrations, it exerts a vasoconstrictor effect.

Thirst is the response to osmolality over 290 mOsm/Kg; It increases water intake once maximal renal vasopressin effect is achieved, thereby diluting the ECF.

This is the conscious sensation of the physiological urge to drink. It originates in the thirst centre of the hypothalamus.

People's normal daily water intake is not motivated by this sort of thirst; rather, most of your daily water is consumed socially, not because your osmolality is deranged. This is so-called hedonistic drinking.

In summary:

• • Vasopressin release occurs at osmolality over 280 mOsm/Kg
  • 1% changes in osmolality cause a change in vasopressin release
  • The scale of vasopressin response is linear, in proportion to the osmolality
  • Beyond an osmolality of 290, increasing vasopressin levels do not increase water reabsorption
  • Beyond an osmolality of 290, water intake via thirst is the only way to defend against hyperosmolarity.

Vasopressin response occurs when plasma volume decreases by 8-10% .

Unlike the linear response to changes in osmolality, in response to hypovolemia the rate of vasopressin secretion increases exponentially

What happens if you are hypovolemic AND hypotonic?

The defence of tonicity is sacrificed in the defence of volume.

The greater the volume loss, the lower the osmolality homeostatic set point, and the steeper the linear response of vasopressin release.

This way, when blood is lost, a lower serum osmolality is tolerated.

The baroreceptor response is more potent that the osmoreceptor response.

If there happens to be hypotonicity, you would expect water retention to decrease (after all, you would want less dilution of the solutes in the ECF). However, if there also happens to be hypovolemia, the loss of volume overrides the loss of tonicity, and water will be retained.

Tonicity will actually drop as the result. This is because the osmolar threshold for vasopressin release is set lower.

The hypothalamus will tolerate a more dilute ECF if it helps maintain euvolemia.

Drugs which increase vasopressin release

• Vincristine
• Cyclophosphamide
• Tricyclics
• Barbiturates
• Nicotine
• Adrenaline
• High dose opiates

Drugs which decrease vasopressin release

• Ethanol
• Phenytoin
• Corticosteroids
• Haloperidol
• Phenergan (promethazine)
• Low dose opiates