Atrial natriuretic peptide
Overview
Atrial natriuretic peptide (ANP) is a hormone secreted from the cardiac atria that regulates blood pressure, sodium and lipid metabolism, and long-term regulation of sodium balance. It is one of the key players in the body's cardiovascular homeostasis mechanism.
Structure
ANP is a 28-amino acid peptide with a 17-amino acid ring in the middle of the molecule. The ring is formed by a disulfide bond between two cysteine residues at positions 7 and 23. The amino acid sequence of ANP is: Ser-Leu-Arg-Arg-Ser-Ser-Cys-Phe-Gly-Gly-Arg-Met-Asp-Arg-Ile-Gly-Ala-Gln-Ser-Gly-Leu-Gly-Cys-Asn-Ser-Phe-Arg-Tyr.
Synthesis and Secretion
ANP is synthesized as a preprohormone (prepro-ANP) in the cardiac myocytes of the atria. This preprohormone is then cleaved to form pro-ANP, which is further cleaved to form the active hormone, ANP. The secretion of ANP is stimulated by atrial stretch, which can occur due to increased blood volume, increased atrial pressure, or increased sodium concentration.
Function
The primary function of ANP is to reduce the water, sodium and adipose loads on the circulatory system, thereby reducing blood pressure. It does this by promoting natriuresis (the excretion of sodium in the urine), diuresis (increased urine production), and vasodilation (widening of blood vessels). ANP also inhibits the release of renin, a hormone that promotes the production of angiotensin, a potent vasoconstrictor.
Receptors and Signal Transduction
ANP exerts its effects by binding to a specific receptor, known as the natriuretic peptide receptor A (NPR-A). This receptor is a transmembrane guanylyl cyclase that, when activated by ANP, increases the intracellular concentration of cyclic guanosine monophosphate (cGMP). cGMP serves as a second messenger in cells, mediating the effects of ANP.
Clinical Significance
Abnormal levels of ANP are associated with various cardiovascular disorders. Elevated levels of ANP are found in conditions such as congestive heart failure, atrial fibrillation, and myocardial infarction. On the other hand, low levels of ANP are associated with conditions such as hypertension and obesity. ANP and its related peptides are therefore potential therapeutic targets for these conditions.
