Natriuretic hormones

3 types

1. Atrial natriuretic peptide
   * 28 amino acid residue
2. Brain natriuretic peptide
   * 32 amino acid residue
3. C-type natriuretic peptide
   * Mainly paracrinal

Production

Source --> Heart

BNP is also found is brain
* But level is higher in heart

Secretion is proportional to stretching of atria by central venous pressure

Regulation of production

ANP secretion increased when:

• ECF volume is increased
  * By high-salt diet, OR
  * Infusion of isotonic saline
• Immersion in water
  * Immersion also decrease renin and aldosterone

Decreased when:

• Rising from supine to standing
• IPPV, which causes decreased atrial distension
  --> Sodium retention in ventilated patients

Detector: stretching of atria

Effect of ANP and BNP

[WG21:p465]

Act on kidney to increase sodium excretion

Mechanism of natriuretic effect

• Vasodilation of afferent arterioles
  --> Increases GFR
• Relaxation of mesangial cells
  --> Increases GFR
• Directly inhibit sodium reabsorption by renal tubules
  * [AV6:p118] Acts on medullary collecting duct

Other effects

• Increase capillary permeability
  --> Extravasation of fluid
  --> Decrease in BP
• Relaxation of vascular smooth muscles in arterioles and venules
  * Counteract the pressor effect of catecholamines and angiotensin II
• Inhibit renin secretion
• Reduce responsiveness of zona glomerulosa to angiotensin II
  [WG21:p383]