Atrial-natriuretic factor

Atrial natriuretic factor (ANF), atrial natriuretic peptide (ANP) or atriopeptin, is a peptide hormone involved in the homeostatic control of body water and sodium.

Structure
ANF is a 28 amino acid peptide with a 17 AA ring, and is closely related to BNP (Brain Natriuretic Peptide, but produced largely in the heart) and CNP (C-type Natriuretic Peptide) which all share the same amino acid ring. ANF was discovered in 1981 by a team in Ottawa led by Mercedes Kuroski de Bold after they made the seminal observation that injection of atrial (but not ventricular) tissue extracts into rats caused copious natriuresis.

ANF is released by walls of the cardiac atrium in response to high sodium concentration or stretching of the atria. Causes of stretching include high extracellular fluid volume, high blood volume, and atrial fibrillation. Notably, it is secreted in response to immersion of the body in water, which causes an atrial stretch due to altered distribution of intravascular fluid. It has been shown that in horses, it is also released in response to exercise. Once released, ANF then acts via various mechanisms to excrete Na, and to cause vasodilation in the circulatory system.

Renal

 * Dilates the afferent glomerular arteriole, constricts the efferent glomerular arteriole, and relaxes the mesangial cells. This increases the glomerular filtration rate, resulting in greater excretion of Na+ and water.


 * Decreases Na resorption in the renal distal convoluted tubule and cortical collecting duct.


 * Inhibits renin secretion.

Vascular

 * Relaxes vascular smooth muscle in arterioles and venules and opposes the pressor effects of catecholamines and Angiotensin-II

Receptors
There are three distinct receptors identified so far in mammals, natriuretic peptide receptors A, B and C (NPRA, NPRB and NPRC).

NPRA and NPRB are linked to guanylyl cyclases, while NPRAC is G-protein linked and furthermore is a "clearance receptor" which acts to internalise and destroy the ligand.

Other natriuretic factors
In addition to the mammalian natriuretic factors (ANP, BNP, CNP), two other peptides have been isolated. Tervonen (1998) described a salmon natriuretic factor (Salmon cardiac peptide) with a similar structure and properties and Dendroaspis Natriuretic Peptide (DNP) was discovered in the venom of the green mamba by Schweitz et al. (1992).