Natriuretic Peptides

The natriuretic peptide system is a family of structurally related endogenous peptide hormones that oppose the renin-angiotensin-aldosterone system (RAAS) to regulate blood pressure, fluid volume, and cardiac structure. Discovered in the 1980s when atrial tissue extracts were shown to cause natriuresis (sodium excretion in urine), natriuretic peptides have become among the most clinically important peptides in medicine — both as diagnostic biomarkers and as therapeutic agents.

The three natriuretic peptides

Atrial natriuretic peptide (ANP)

ANP is a 28-amino acid peptide synthesized and stored in atrial cardiomyocytes as a preprohormone (proANP). It is released in response to atrial wall stretch — the mechanical signal that blood volume is expanded and the heart is working harder than it should.

Primary stimulus: Atrial distension from volume overload

Source: Atrial cardiomyocytes (predominantly right atrium)

Half-life: Approximately 2 to 3 minutes (rapidly cleared by neprilysin and natriuretic peptide receptor C)

ANP is the acute responder. When you drink excess fluid or receive an IV saline bolus, the resulting atrial stretch triggers ANP release within minutes, promoting sodium and water excretion to restore normal volume.

B-type natriuretic peptide (BNP)

BNP is a 32-amino acid peptide originally identified in porcine brain tissue (hence "brain" natriuretic peptide), though its primary physiological source is the ventricles of the heart. BNP is synthesized as a preprohormone (preproBNP), which is cleaved to proBNP and then split into the active BNP and the inactive N-terminal fragment NT-proBNP.

Primary stimulus: Ventricular wall stress (pressure and volume overload)

Source: Ventricular cardiomyocytes

Half-life: BNP approximately 20 minutes; NT-proBNP approximately 120 minutes

BNP is the chronic responder. While ANP responds primarily to volume, BNP responds to sustained ventricular wall stress from conditions like heart failure, left ventricular hypertrophy, and valvular disease. This is why BNP and NT-proBNP are the preferred biomarkers for heart failure diagnosis.

C-type natriuretic peptide (CNP)

CNP is a 22-amino acid peptide with a distinct physiological profile from ANP and BNP. It is produced primarily by endothelial cells and chondrocytes rather than cardiomyocytes, and it signals through a different receptor.

Primary stimulus: Shear stress on endothelium, inflammatory cytokines

Source: Vascular endothelium, cartilage, brain

Half-life: Approximately 2 to 3 minutes

CNP is not primarily a cardiac hormone. Its main roles are in vascular tone regulation (local vasodilation), endochondral bone growth (critical for longitudinal bone development), and anti-fibrotic signaling. CNP deficiency causes dwarfism in animal models, underscoring its importance in skeletal growth.

Receptors and signaling

Natriuretic peptides signal through three receptors:

NPR-A (natriuretic peptide receptor A / guanylyl cyclase A)

• Ligands: ANP (high affinity), BNP (moderate affinity)
• Mechanism: Transmembrane guanylyl cyclase that generates cyclic GMP (cGMP) upon ligand binding
• Distribution: Kidney, adrenal glands, vasculature, heart
• Effects: Natriuresis, vasodilation, inhibition of aldosterone secretion, anti-hypertrophic cardiac signaling

NPR-B (natriuretic peptide receptor B / guanylyl cyclase B)

• Ligand: CNP (selective)
• Mechanism: Transmembrane guanylyl cyclase generating cGMP
• Distribution: Vasculature, brain, bone growth plates
• Effects: Vasodilation, bone growth, anti-fibrotic signaling

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

• Ligands: All natriuretic peptides (non-selective)
• Mechanism: Binds and internalizes natriuretic peptides for lysosomal degradation. No guanylyl cyclase activity
• Function: Clearance receptor that removes natriuretic peptides from circulation, serving as a buffer that modulates peptide availability

Physiological effects

Renal effects

Natriuretic peptides (primarily ANP and BNP via NPR-A) produce natriuresis and diuresis through several mechanisms:

• Increased glomerular filtration rate: Afferent arteriolar dilation and efferent arteriolar constriction increase filtration pressure
• Direct tubular effects: Inhibition of sodium reabsorption in the collecting duct
• Suppression of renin secretion: Reduces angiotensin II and aldosterone, further decreasing sodium retention
• Inhibition of ADH (vasopressin): Reduces water reabsorption

The net effect is increased excretion of sodium and water, reducing blood volume and preload.

Cardiovascular effects

• Vasodilation: cGMP-mediated smooth muscle relaxation reduces systemic vascular resistance and blood pressure
• Anti-hypertrophic: ANP and BNP inhibit pathological cardiac hypertrophy through cGMP-PKG signaling, which opposes the calcineurin-NFAT pathway that drives maladaptive cardiac growth
• Anti-fibrotic: Natriuretic peptides inhibit cardiac fibroblast proliferation and collagen synthesis, opposing the fibrotic remodeling that characterizes heart failure progression

Metabolic effects

An underappreciated role: natriuretic peptides promote lipolysis in adipose tissue via NPR-A/cGMP/PKG signaling, independent of catecholamine-mediated lipolysis. They also enhance mitochondrial biogenesis and fat oxidation in skeletal muscle. Individuals with genetically higher natriuretic peptide levels have lower rates of obesity and type 2 diabetes, suggesting a protective metabolic role.

Natriuretic peptides as biomarkers

NT-proBNP and BNP in heart failure

The clinical use of natriuretic peptide measurement has transformed heart failure diagnosis:

• BNP > 100 pg/mL or NT-proBNP > 300 pg/mL in a patient with dyspnea strongly suggests heart failure as the cause
• BNP < 100 pg/mL or NT-proBNP < 300 pg/mL effectively rules out heart failure (negative predictive value > 95%)

NT-proBNP is often preferred over BNP for testing because its longer half-life (120 minutes vs. 20 minutes) provides more stable plasma concentrations and wider analytical measurement windows. However, NT-proBNP is renally cleared, so levels are elevated in kidney disease independent of heart failure.

Prognostic value

Serial natriuretic peptide measurements predict outcomes in established heart failure. Declining BNP/NT-proBNP levels during treatment indicate favorable response, while rising levels predict hospitalization and mortality. Natriuretic peptide-guided therapy — adjusting heart failure medications to achieve target BNP/NT-proBNP levels — has shown benefit in some clinical trials.

Beyond heart failure

Elevated natriuretic peptides are seen in atrial fibrillation, pulmonary embolism, acute coronary syndromes, pulmonary hypertension, sepsis, and any condition causing myocardial stress. Context matters: an elevated NT-proBNP does not automatically mean heart failure.

Therapeutic applications

Nesiritide (recombinant BNP)

Nesiritide is recombinant human BNP approved for acute decompensated heart failure. Administered intravenously, it produces vasodilation and natriuresis, reducing preload and afterload. However, clinical trials (ASCEND-HF) showed symptom improvement without mortality benefit, and concerns about renal toxicity limited its adoption. It is now used infrequently.

Sacubitril/valsartan (neprilysin inhibition)

Rather than administering exogenous natriuretic peptides, a more successful therapeutic strategy is preventing the degradation of endogenous natriuretic peptides. Neprilysin is the enzyme that cleaves and inactivates ANP and BNP. Sacubitril inhibits neprilysin, raising endogenous natriuretic peptide levels.

Combined with the angiotensin receptor blocker valsartan (to prevent the hypertensive effects of neprilysin inhibition on angiotensin II, which neprilysin also degrades), sacubitril/valsartan (Entresto) demonstrated a 20% reduction in cardiovascular death or heart failure hospitalization compared to enalapril in the PARADIGM-HF trial. It is now a cornerstone of heart failure with reduced ejection fraction (HFrEF) therapy.

Vosoritide (CNP analog)

Vosoritide is a CNP analog approved for achondroplasia, a genetic form of dwarfism caused by activating mutations in FGFR3 that suppress bone growth. CNP signaling through NPR-B opposes FGFR3's growth-inhibiting effects on chondrocytes. Daily subcutaneous vosoritide injections increase growth velocity in children with achondroplasia — a striking example of a natriuretic peptide therapeutic application entirely outside cardiology.

Degradation and clearance

Natriuretic peptides are cleared through two mechanisms:

1. NPR-C receptor-mediated internalization and lysosomal degradation
2. Enzymatic cleavage by neprilysin (neutral endopeptidase, NEP)

The short half-lives of ANP and BNP (minutes) mean that therapeutic use requires either continuous infusion (nesiritide) or enzymatic inhibition (sacubitril). This pharmacokinetic challenge is a recurring theme in endogenous peptide therapeutics.

Bottom line

Natriuretic peptides (ANP, BNP, CNP) form the body's endogenous counter-regulatory system against sodium retention, volume overload, and pathological cardiac remodeling. Their clinical impact spans diagnostics (NT-proBNP as the standard biomarker for heart failure) and therapeutics (neprilysin inhibition with sacubitril/valsartan, CNP analogs for skeletal growth). The natriuretic peptide system illustrates a broader principle in peptide biology: sometimes the most effective therapeutic strategy is not administering exogenous peptides but rather preventing the degradation of the body's own.