Bradykinin

What it is

Bradykinin is a naturally occurring peptide in the human body that acts as a powerful vasodilator and pain mediator. It plays a central role in hereditary angioedema, inflammation, and cardiovascular regulation — and is the reason ACE inhibitors can cause cough and swelling as side effects.

The scientific side

bradykinin is a nine-amino-acid peptide (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) generated locally at sites of tissue injury and inflammation through the kallikrein-kinin system. Circulating and tissue kallikreins cleave high-molecular-weight kininogen (HMWK) to release bradykinin, which then acts in a paracrine or autocrine fashion through two G-protein-coupled receptors: the constitutively expressed B2 receptor (B2R) and the inflammation-inducible B1 receptor (B1R). The B2 receptor is the primary mediator of bradykinin's acute effects and requires the intact nine-residue peptide; the B1 receptor is preferentially activated by the metabolite des-Arg9-bradykinin, which is generated when carboxypeptidase N (kininase I) cleaves the C-terminal arginine. B2R activation on vascular endothelium triggers intracellular signaling through phospholipase C, protein kinase C, ERK1/2, and p38 MAPK pathways, resulting in production of nitric oxide (NO) via endothelial nitric oxide synthase (eNOS), prostaglandins via cyclooxygenase-2 (COX-2), and NF-kB-driven cytokine release. These downstream mediators collectively produce the cardinal responses for which bradykinin is known: profound vasodilation, increased vascular permeability, smooth muscle contraction, and sensitization of peripheral nociceptors to pain. Bradykinin's inactivation is achieved primarily by angiotensin-converting enzyme (ACE/kininase II) and neutral endopeptidase; inhibition of ACE with ACE inhibitor drugs therefore potentiates bradykinin activity, explaining the bradykinin-mediated side effects of dry cough and angioedema seen in 10–20% and 0.1–0.7% of ACE inhibitor users, respectively. In the cardiovascular system, bradykinin exerts potentially cardioprotective effects: it attenuates cardiac fibrosis by inhibiting endothelial-to-mesenchymal transition (EndMT) via B2R activation and NO production following ischemia-reperfusion injury, and it contributes to ACE inhibitor-mediated benefits in heart failure by upregulating B2 receptors and enhancing signal transduction. In hereditary angioedema (HAE), C1-inhibitor deficiency allows uncontrolled contact-pathway activation and massive bradykinin surges, causing the characteristic non-pitting swelling attacks. The B1 receptor is induced by cytokines and bacterial endotoxins and plays a strategic role in chronic inflammatory states including diabetes, rheumatoid arthritis, and neuropathic pain, where its expression is upregulated in sensory neurons and peripheral tissues.

Class: Endogenous nonapeptide; vasoactive kinin; inflammatory mediator; kallikrein-kinin system component

What it's studied for

• Hereditary angioedema (HAE) pathophysiology — bradykinin as the central disease mediator Human observational
• ACE inhibitor-induced cough and angioedema — bradykinin accumulation as mechanism Human observational
• Cardiovascular protection — bradykinin-mediated cardioprotection in ischemia-reperfusion and heart failure Animal studies only
• Pain sensitization and inflammatory nociception — B1 and B2 receptor roles Mixed
• Sepsis and atherosclerotic cardiovascular disease — bradykinin as shared inflammatory mediator Human observational
• Respiratory disease and asthma — airway bradykinin receptor contributions Human observational
• Prostaglandin E2 release and NF-kB pathway activation in musculoskeletal inflammation Mechanistic only

Safety signals

• Profound hypotension following systemic or intravenous administration — bradykinin is among the most potent known vasodilators; even nanomolar concentrations can cause significant falls in blood pressure through NO and prostaglandin release from vascular endothelium
• Bronchoconstriction and airway hyperresponsiveness — bradykinin provokes bronchoconstriction in asthmatic patients more potently than in controls, via B2 receptor activation on airway smooth muscle, and may precipitate acute asthma attacks in susceptible individuals
• Angioedema — bradykinin excess (whether from HAE, ACE inhibitor use, or exogenous administration) causes vascular permeability leading to potentially life-threatening swelling of the face, tongue, oropharynx, larynx, or abdomen; laryngeal angioedema can cause fatal asphyxiation if untreated
• Pain and hyperalgesia at injection site and systemically — bradykinin is one of the most potent endogenous pain-producing substances; it directly activates and sensitizes nociceptors, activates TRPV1 and TRPA1 channels, and triggers prostaglandin-mediated peripheral sensitization, causing significant pain on local or systemic administration
• Uterine contraction and obstetric risk — bradykinin stimulates uterine smooth muscle contraction via B2 receptors; systemic bradykinin elevation or exogenous administration in pregnancy poses risk of uterine hypertonicity or premature labor
• Bradykinin-mediated ACE inhibitor cough — pharmacological potentiation of endogenous bradykinin by ACE inhibitors causes persistent dry cough in approximately 10–20% of treated patients; this is mechanistically distinct from allergy and does not respond to antihistamines or cough suppressants
• Vascular permeability and edema in inflammatory states — bradykinin released during sepsis, trauma, or surgery contributes to capillary leak syndrome, third-spacing of fluids, and hemodynamic instability; elevated kininogen-bradykinin levels correlate with sepsis severity
• Growth-stimulatory activity in small cell lung cancer — bradykinin acts as an autocrine growth factor for small cell lung cancer cells via B2 receptors; exogenous bradykinin administration in individuals with occult or established lung malignancy could theoretically promote tumor growth

No peer-reviewed studies indexed for this peptide yet.