ANP

Overview

Atrial natriuretic peptide (ANP) is a 28-amino-acid peptide hormone secreted primarily by atrial cardiomyocytes in response to atrial wall stretch caused by volume expansion or increased atrial pressure. Discovered independently by Adolfo de Bold and colleagues in 1981, ANP was the first member of the natriuretic peptide family to be identified and established the concept of the heart as an endocrine organ.

De Bold's landmark experiment demonstrated that injection of atrial (but not ventricular) tissue extracts into rats produced rapid and pronounced natriuresis (sodium excretion) and diuresis (water excretion), accompanied by a drop in blood pressure. The responsible peptide was subsequently isolated, sequenced, and named atrial natriuretic factor (later atrial natriuretic peptide).

ANP is a founding member of the natriuretic peptide family, which also includes B-type natriuretic peptide (BNP, primarily ventricular) and C-type natriuretic peptide (CNP, primarily endothelial and neural). Together, these peptides constitute a hormonal system that counterbalances the vasoconstrictive and fluid-retaining effects of the renin-angiotensin-aldosterone system (RAAS), the sympathetic nervous system, and vasopressin.

Structure and Biosynthesis

Molecular characteristics:

• Mature peptide: 28 amino acids (ANP(99-126) of the prepro-ANP precursor)
• Molecular weight: 3,080 Da
• Key features: 17-amino-acid ring structure formed by a single Disulfide bond between Cys105 and Cys121, with N-terminal and C-terminal tails
• Precursor: Pre-proANP (151 amino acids) processed to proANP (126 amino acids), stored in atrial granules, then cleaved by the transmembrane serine protease corin to release mature ANP upon secretion

Storage and release:

• ProANP is stored in electron-dense secretory granules within atrial cardiomyocytes
• Atrial granules are visible on electron microscopy and were noted long before ANP's discovery
• Atrial wall stretch is the primary stimulus for ANP release; additional stimuli include endothelin-1, sympathetic activation, angiotensin II, and hypoxia
• Upon secretion, corin cleaves proANP at Arg98-Ser99, releasing the mature 28-amino-acid ANP and the N-terminal pro-ANP fragment (NT-proANP, amino acids 1-98)

Natriuretic Peptide Receptor Signaling

ANP signals through two receptors:

NPR-A (natriuretic peptide receptor A / GC-A):

• Transmembrane guanylate cyclase receptor (the signaling receptor)
• Binding activates intracellular cGMP production
• Expressed on vascular smooth muscle, renal tubular cells, adrenal glomerulosa cells, and adipocytes
• Mediates all major physiological effects of ANP

NPR-C (natriuretic peptide receptor C / clearance receptor):

• Lacks guanylate cyclase activity
• Functions primarily as a clearance receptor, internalizing and degrading bound natriuretic peptides
• Also couples to Gi to modulate adenylyl cyclase in some cell types
• Regulates local natriuretic peptide concentrations

Additionally, ANP is degraded by neutral endopeptidase (NEP/neprilysin), a membrane-bound metalloprotease. The neprilysin inhibitor sacubitril (combined with valsartan as Entresto) exploits this degradation pathway to therapeutically elevate endogenous natriuretic peptide levels in heart failure.

Physiological Functions

Cardiovascular Effects

Vasodilation:

• cGMP-mediated relaxation of vascular smooth muscle reduces systemic vascular resistance
• Both arterial and venous vasodilation decrease afterload and preload
• ANP also reduces vascular smooth muscle cell proliferation, contributing to long-term vascular remodeling effects

Cardiac effects:

• Direct anti-hypertrophic effects on cardiomyocytes via cGMP/PKG signaling
• Anti-fibrotic effects by inhibiting cardiac fibroblast proliferation and collagen synthesis
• Modulates cardiac contractility and heart rate through autonomic effects

Renal Effects

Natriuresis and diuresis:

• Increases glomerular filtration rate (GFR) by dilating the afferent arteriole and constricting the efferent arteriole
• Inhibits sodium reabsorption in the inner medullary collecting duct via cGMP-mediated suppression of epithelial sodium channels (ENaC)
• Inhibits Vasopressin-stimulated water reabsorption in the collecting duct
• Net effect: increased urinary sodium and water excretion, reducing blood volume

Neurohormonal Suppression

• RAAS inhibition: Directly inhibits renin secretion from juxtaglomerular cells and aldosterone secretion from adrenal glomerulosa cells
• Sympathetic suppression: Reduces sympathetic nerve activity through effects on central and peripheral baroreflexes
• Vasopressin suppression: Inhibits vasopressin release from the posterior pituitary

Metabolic Effects

• Promotes lipolysis in adipocytes via NPR-A/cGMP signaling
• Stimulates fatty acid oxidation in skeletal muscle
• Enhances thermogenesis in brown adipose tissue
• These metabolic effects have generated interest in the natriuretic peptide system as a potential therapeutic target in metabolic disease

Clinical Significance

Heart Failure Biomarker

While BNP and NT-proBNP are the natriuretic peptide Biomarkers most widely used in clinical practice, NT-proANP (and the mid-regional fragment, MR-proANP) also serves as a diagnostic and prognostic marker for heart failure. MR-proANP is measured using the BRAHMS MR-proANP assay and has demonstrated utility comparable to BNP/NT-proBNP in heart failure diagnosis.

Therapeutic Applications

Carperitide (recombinant ANP):

• Approved in Japan (since 1995) for the treatment of acute heart failure
• Administered as a continuous intravenous infusion
• Not approved in the US or Europe, where Nesiritide (recombinant BNP) has been the available natriuretic peptide therapeutic

Neprilysin inhibition:

• Sacubitril/valsartan (Entresto) indirectly enhances ANP, BNP, and CNP levels by inhibiting their enzymatic degradation, representing the most successful clinical application of natriuretic peptide biology to date
• This approach elevates endogenous natriuretic peptide tone rather than administering exogenous peptide

Deficiency States

• Obesity: Natriuretic peptide levels are paradoxically lower in obese individuals (the "natriuretic peptide handicap"), possibly due to increased NPR-C-mediated clearance in expanded adipose tissue
• Corin and NPPA mutations: Rare genetic variants in the corin gene or the ANP gene (NPPA) have been associated with hypertension and atrial fibrillation

Pharmacokinetics

Parameter	Value
Half-life	~2-4 minutes
Clearance	NPR-C receptor-mediated internalization + neprilysin degradation + renal filtration
Volume of distribution	~0.2 L/kg
Onset of natriuresis	Minutes after IV infusion

The very short half-life reflects efficient clearance mechanisms and is consistent with ANP's role as a rapid-response hormone for acute hemodynamic perturbations.

Dosing Protocols

The following dosing information is compiled from published research for educational purposes only. Always consult a qualified healthcare professional.

As an endogenous cardiac hormone, ANP is not routinely administered exogenously in Western clinical practice. However, a recombinant form (carperitide, marketed as Hanp) is approved in Japan for acute heart failure.

Compound	Indication	Dose	Route
Carperitide (Japan only)	Acute heart failure	0.1 mcg/kg/min (range 0.0125-0.2 mcg/kg/min)	Continuous IV infusion

Native ANP has a plasma half-life of only 2-5 minutes, requiring continuous IV infusion. In most countries, the therapeutic exploitation of natriuretic peptide biology occurs through nesiritide (recombinant BNP) or neprilysin inhibitors (sacubitril) that enhance endogenous natriuretic peptide levels.

Scientific Legacy

ANP's discovery transformed cardiovascular physiology by establishing the heart as an endocrine organ and revealing the natriuretic peptide system as a fundamental counter-regulatory axis in fluid and blood pressure homeostasis. This discovery laid the groundwork for natriuretic peptide-based diagnostics (BNP/NT-proBNP assays in heart failure), therapeutics (nesiritide, carperitide, sacubitril/valsartan), and continues to inspire research into natriuretic peptide-based approaches for heart failure, hypertension, and metabolic disease. The cardiovascular protocol landscape increasingly incorporates natriuretic peptide biology.