Angiotensin II: Effector Peptide of the Renin-Angiotensin System

Chemical Identity

Angiotensin II (Ang II) is the principal effector peptide of the renin-angiotensin-aldosterone system (RAAS), the primary hormonal axis regulating blood pressure and electrolyte homeostasis. It is generated from angiotensin I by angiotensin-converting enzyme (ACE).

Property	Value
Chemical Formula	C₄₉H₇₁N₁₃O₁₂
Molecular Weight	1046.18 g/mol
CAS Number	4474-91-3
Isoelectric Point (pI)	7.8
Amino Acid Count	8
Disulfide Bridge	None (linear)

Primary Structure

Angiotensin II is a linear octapeptide with the sequence:

Asp–Arg–Val–Tyr–Ile–His–Pro–Phe

Key structural features:

N-terminal aspartic acid (Asp1) is essential for activity; removal produces Ang III (Asp-Arg-Val-Tyr-Ile-His-Pro), which has reduced potency
C-terminal Phe8 critical for AT1 receptor binding and activation
Arg2 contributes to electrostatic interactions with the receptor
The peptide is resistant to peptidase degradation at neutral pH

Receptor Interactions

Angiotensin II signals through two principal G-protein coupled receptors with opposing physiological roles:

Receptor	G-Protein	Primary Effects	Expression
AT1	Gq/11, Gi	Vasoconstriction, aldosterone release, fibrosis	Ubiquitous, particularly kidney, heart, vessels
AT2	Gi, Gαi-3	Vasodilation, apoptosis, anti-proliferation	Brain, adrenal, fetal tissues

AT1 receptor signaling:

Gq/11 → PLC → IP₃ → Ca²⁺ → smooth muscle contraction
Gi → adenylyl cyclase inhibition → reduced cAMP
NADPH oxidase activation → reactive oxygen species (ROS) generation
MAPK/ERK pathway → cell growth and hypertrophy

AT2 receptor signaling:

Bradykinin/NO pathway → vasodilation
PP2A activation → growth inhibition
Counterbalances AT1-mediated effects in most tissues

Role in the Renin-Angiotensin-Aldosterone System

The RAAS cascade proceeds through well-characterized steps:

Angiotensinogen (liver) → Angiotensin I (via renin from juxtaglomerular cells)
Angiotensin I → Angiotensin II (via ACE in pulmonary endothelium)
Angiotensin II → effects via AT1 and AT2 receptors

Ang II also serves as a substrate for angiotensin-converting enzyme 2 (ACE2), which generates Ang-(1-7), a heptapeptide with vasodilatory and anti-fibrotic properties acting through the Mas receptor.

Physiological Functions

Angiotensin II exerts broad effects on cardiovascular and renal physiology:

Vascular Tone: Potent direct vasoconstriction of arterioles via AT1-mediated smooth muscle contraction.
Sodium Retivation: Stimulates aldosterone secretion from the adrenal zona glomerulosa, promoting distal tubular Na⁺ reabsorption.
Sympathetic Activation: Facilitates norepinephrine release and enhances central sympathetic outflow.
Cardiac Remodeling: Promotes left ventricular hypertrophy, myocardial fibrosis, and pathological remodeling.
Glomerular Filtration: Preferentially constricts efferent arterioles, maintaining glomerular filtration pressure.

Clinical Significance

Dysregulation of Ang II signaling is central to the pathogenesis of hypertension, heart failure, and chronic kidney disease. Pharmacological targeting of the RAAS occurs at multiple levels:

ACE Inhibitors (e.g., enalapril, ramipril): Block conversion of Ang I to Ang II
ARBs (e.g., losartan, valsartan): Selectively antagonize AT1 receptors
Direct Renin Inhibitors (e.g., aliskiren): Block the first step of the cascade
ACE2/Ang-(1-7) axis: Emerging therapeutic target for counter-regulatory effects

References

Bader, M., & Ganten, D. (2024). Angiotensin II and cardiovascular disease. European Journal of Pharmacology, 958, 175928.
Paul, M., et al. (2006). The renin-angiotensin system. Physiological Reviews, 86, 1–205.