Atrial natriuretic factor, also called atrial natriuretic peptide (ANP), is a hormone produced in the atria of the heart in most mammals. It reduces the amount of sodium in circulation and lowers the blood pressure. Like the other hormones that control sodium and water balance, atrial natriuretic factor is an important component of circulatory regulation. High levels of natriuretic peptides are a clinical indicator of heart failure, since the reduced circulation that characterizes the disease is partly due to their effects on blood pressure.
Released by heart cells that sense stretching of the atrial walls, ANP responds to high sodium in circulation and to angiotensin II, a chemical that raises the blood pressure. ANP is part of a family of peptide hormones that regulate blood volume and pressure as well as sodium and hydration levels. Brain natriuretic peptide is another member of this group. Despite having been discovered in the brains of pigs, this hormone is found in the ventricles of the human heart, where it has the same effects as atrial natriuretic factor has in the atria.
When it binds to receptors in the heart, kidneys, and blood vessels, atrial natriuretic factor lowers blood pressure. It does this by dilating veins to reduce the volume of blood that reaches the heart. ANP also dilates arteries, reducing the cardiac output. In the kidneys, ANP regulates the filtration and excretion of sodium from the bloodstream, and decreases the release of the hormone renin. Renin causes blood pressure to increase.
The natriuretic peptides are hormones that bind to peptide receptors located in the membranes of their target cells. They are produced from natriuretic peptide precursor C, are degraded by the enzyme neutral peptidase, and share the same natriuretic and vasodilating effects. The clearance of sodium through the kidneys increases urinary output, making these hormones diuretics. Some metabolic disorders can disrupt the feedback loop between the natriuretic peptides that lower blood pressure and the hormones like angiotensin II that raise it.
Clinically, high levels of atrial natriuretic factor are an important sign of congestive heart failure. Not long after the discovery of ANP in 1981, natriuretic peptide levels became a standard marker to assess in patients presenting with cardiac emergencies. In heart failure, brain and atrial natriuretic factor are over-expressed, seriously impairing the normal feedback loop that regulates the volume of blood pumped through the heart. Reducing ANP levels is a goal for the treatment of heart disease. Since neutral endopeptidase removes ANP from circulation, several drugs have been developed based on the actions of this enzyme to treat heart failure patients.