Bradykinin is a small peptide, just nine amino acids long, that acts as one of the body’s most potent inflammatory signals. When tissues are injured, infected, or stressed, bradykinin is released into the surrounding area and triggers a cascade of effects: blood vessels widen, fluid leaks into tissues, and pain nerves fire. Those responses sound unpleasant, but they are the body’s way of rushing immune cells and nutrients to a problem area. Bradykinin’s reach extends well beyond simple inflammation, though, touching blood pressure regulation, kidney function, heart protection, and even the permeability of the barrier that shields the brain.
How Bradykinin Is Made and Cleared
Bradykinin is carved from a larger precursor protein called kininogen, which circulates in the blood. When tissue damage or inflammation activates an enzyme called kallikrein, kallikrein clips bradykinin free from kininogen, and the peptide immediately goes to work on nearby cells. The system is designed for speed: bradykinin appears at the site of an injury almost as fast as the injury happens, which is part of why the first moments after a bee sting or a sprained ankle feel so intense so quickly.
Equally important is how fast the body gets rid of bradykinin once it has done its job. The primary enzyme responsible for breaking bradykinin down is angiotensin-converting enzyme, commonly known as ACE. This is the same enzyme that generates angiotensin II, a powerful blood-vessel constrictor. In other words, ACE sits at the crossroads of two opposing systems: one that raises blood pressure (by producing angiotensin II) and one that lowers it (by destroying bradykinin).1Circulation. Role of bradykinin in mediating vascular effects of angiotensin-converting enzyme inhibitors in humans That dual role becomes medically relevant when people take ACE-inhibitor drugs, a topic covered further below.
Two Receptor Types With Different Jobs
Bradykinin does not act directly on cells. It binds to specialized receptor proteins on cell surfaces, and those receptors then relay the signal inward. There are two types, called B1 and B2.2PubMed. Bradykinin receptors and their antagonists The B2 receptor is the one most cells already have on hand. It handles the immediate, acute responses: rapid vasodilation, a quick burst of pain, and the initial wave of swelling. In healthy tissue at rest, B2 receptors are the main players.
The B1 receptor works differently. Under normal conditions, very few cells express it. But when inflammation becomes prolonged or tissue is seriously damaged, B1 receptors get manufactured and installed on cell surfaces in increasing numbers. This shift matters because B1-mediated signaling tends to sustain and amplify the inflammatory response over time rather than just initiating it. In diseases where chronic inflammation is a problem, B1 receptor activity can become part of the cycle that keeps tissue damage going.
What Bradykinin Does to Blood Vessels
The most thoroughly studied effect of bradykinin is vasodilation, the widening of blood vessels. When bradykinin activates B2 receptors on the cells lining a blood vessel, those endothelial cells release nitric oxide, prostacyclin, and a substance called endothelium-derived hyperpolarizing factor. All three relax the smooth muscle wrapped around the vessel, letting it expand and increasing blood flow to the area.3PubMed Central. A Review of the Role of Bradykinin and Nitric Oxide in the Cardioprotective Action of Angiotensin-Converting Enzyme Inhibitors: Focus on Perindopril – Section: Bradykinin and the Endothelium This is why a bruise swells and turns warm: local bradykinin release is dilating the small blood vessels, flooding the area with blood.
Bradykinin also increases vascular permeability, meaning it makes vessel walls leakier. Fluid and proteins that normally stay inside the bloodstream seep out into the surrounding tissue, producing the puffiness associated with inflammation. Research on lymphatic endothelial cells shows that bradykinin-induced B2-receptor signaling raises internal calcium levels and lowers levels of a molecule called cAMP, which loosens the junctions between cells and lets more fluid pass through.4PubMed. Bradykinin signaling regulates solute permeability and cellular junction organization in lymphatic endothelial cells In a small cut, this leakiness is helpful because it delivers clotting factors and immune cells. When it goes unchecked, it can cause dangerous swelling.
Pain Signaling
If you have ever wondered what actually makes an injury hurt, bradykinin is a big part of the answer. Pain-sensing nerve endings called nociceptors carry B2 receptors, and when bradykinin binds to them, the nerve fires. Bradykinin both excites nociceptors directly and sensitizes them so that other stimuli, like heat or pressure, feel more painful than they normally would.5PubMed Central. Depolarizing Effectors of Bradykinin Signaling in Nociceptor Excitation in Pain Perception – Section: Abstract That sensitization explains why an inflamed area becomes tender to the touch even after the initial injury has stopped.
Bradykinin is not the only molecule that triggers pain during inflammation; prostaglandins, histamine, and other mediators contribute too. But bradykinin is one of the most potent of the group, and its ability to make nerves hypersensitive means it often amplifies the pain caused by everything else in the inflammatory mix. This is also why anti-inflammatory drugs that reduce bradykinin’s effects can provide pain relief beyond what simple nerve-blocking would achieve.
Kidney Function and Salt Balance
Inside the kidney, bradykinin plays a quieter but essential role in regulating how much sodium and water the body holds on to. When bradykinin is infused into the inner part of the kidney (the medulla) in animal studies, it increases blood flow to that region and roughly doubles both urine output and sodium excretion through a nitric oxide-dependent mechanism.6PubMed. Kinin actions on renal papillary blood flow and sodium excretion In plain terms, bradykinin helps the kidney flush out salt and water.
This is not just an effect that shows up when extra bradykinin is added from outside. When researchers blocked the B2 receptor inside the kidney medulla of rats eating a normal-salt diet, sodium excretion dropped by about 40%, showing that bradykinin is tonically active, meaning the kidney relies on a constant low-level bradykinin signal to keep sodium excretion at its normal rate.7PubMed Central. Blockade of renal medullary bradykinin B2 receptors increases tubular sodium reabsorption in rats fed a normal-salt diet This steady contribution to salt balance ties bradykinin to long-term blood pressure regulation in addition to its more dramatic acute effects on blood vessels.
Protecting the Heart
One of the more surprising things bradykinin does is help shield heart muscle from damage during a heart attack. Researchers studying a phenomenon called ischemic preconditioning, where brief episodes of reduced blood flow make the heart more resilient to a longer blockage, found that bradykinin is a key mediator. In rabbit heart experiments, blocking the B2 receptor completely abolished the protection that preconditioning normally provides.8PubMed. Role of bradykinin in protection of ischemic preconditioning in rabbit hearts And when bradykinin was infused directly, it mimicked the protective effect: infarct size dropped from about 41% of the at-risk zone in untreated hearts to roughly 16% in bradykinin-treated hearts, and this was similarly abolished by a B2-receptor blocker.9PubMed. Role of bradykinin in myocardial preconditioning
The cardioprotective pathway appears to work through protein kinase C, an intracellular signaling molecule, rather than through the nitric oxide or prostaglandin pathways that mediate vasodilation.8PubMed. Role of bradykinin in protection of ischemic preconditioning in rabbit hearts This distinction matters because it means the heart-protective effects of bradykinin are somewhat independent of its blood-vessel effects. It also contributes to the rationale for ACE inhibitors in cardiac medicine: by slowing the breakdown of bradykinin, these drugs may give the heart an extra layer of defense beyond simply lowering blood pressure.
The ACE Inhibitor Connection
ACE inhibitors are among the most widely prescribed drugs for high blood pressure and heart failure. They were designed to block the production of angiotensin II, the peptide that constricts blood vessels. But because the same enzyme also destroys bradykinin, taking an ACE inhibitor raises bradykinin levels as a side effect. This is increasingly seen as a feature rather than a bug, since higher bradykinin means more vasodilation and more of those cardioprotective benefits.3PubMed Central. A Review of the Role of Bradykinin and Nitric Oxide in the Cardioprotective Action of Angiotensin-Converting Enzyme Inhibitors: Focus on Perindopril – Section: Bradykinin and the Endothelium
The downside is the cough. Somewhere around 5 to 20 percent of people taking ACE inhibitors develop a persistent dry cough that can be annoying enough to stop the medication. Research in animal models has shown that elevated bradykinin sensitizes sensory nerves in the airways, making them hyper-reactive to irritants. In guinea pigs given the ACE inhibitor captopril, the cough response to inhaled irritants increased, and this increase was completely prevented by a bradykinin B2 receptor blocker.10PubMed. Bradykinin-evoked sensitization of airway sensory nerves: a mechanism for ACE-inhibitor cough The finding confirmed that the cough is not a direct irritation of the lungs but rather a bradykinin-driven sensitization of the cough reflex. When the cough becomes intolerable, doctors typically switch patients to a different class of blood pressure medication that does not affect bradykinin levels.
The Renin-Angiotensin Balancing Act
The relationship between bradykinin and the renin-angiotensin system is not limited to sharing an enzyme. The two systems appear to counterbalance each other at multiple levels. Angiotensin II constricts blood vessels and promotes sodium retention; bradykinin dilates vessels and promotes sodium excretion. An enzyme called prolylcarboxypeptidase, which breaks down angiotensin II, also activates the precursor to kallikrein, the enzyme that produces bradykinin. So a single enzyme simultaneously dials down the blood-pressure-raising system and ramps up the blood-pressure-lowering one.11PubMed. The kallikrein-kinin and the renin-angiotensin systems have a multilayered interaction Additional crosstalk occurs in the kidney, where components of both systems interact to regulate vascular resistance.12PubMed. Interaction of Angiotensin-(1-7) with kinins in the kidney circulation: Role of B(1) receptors This layered interplay means that changes in one system almost always affect the other, which has implications for how blood pressure drugs work and why combination therapies can sometimes have unexpected effects.
When Bradykinin Goes Wrong: Hereditary Angioedema
Hereditary angioedema (HAE) is the clearest example of what happens when the body loses control of bradykinin production. In the most common form, patients lack a functional version of C1-inhibitor, a protein that normally keeps the kallikrein-kinin cascade in check. Without adequate C1-inhibitor, kallikrein runs unchecked, generating excessive bradykinin. The result is unpredictable episodes of severe swelling in the skin, gut, or airway that can last days and, if the throat swells shut, can be life-threatening.
During HAE attacks, the expression of B1 receptors on immune cells roughly doubles or triples compared to healthy subjects, suggesting that the inflammatory response escalates through both receptor types.13PubMed Central. The Role of Bradykinin Receptors in Hereditary Angioedema Due to C1-Inhibitor Deficiency – Section: 2. Results / 2.2. Disease Activity Correlation with BR1 A rarer form of HAE involves a mutation in coagulation factor XII. In a series of 57 patients with this variant, attacks often worsened during pregnancy or estrogen-containing contraceptive use, and male carriers of the mutation were more frequently asymptomatic than females.14PubMed Central. Hereditary angioedema with normal C1 inhibitor and factor XII mutation: a series of 57 patients from the French National Center of Reference for Angioedema
Because bradykinin is the key mediator of HAE symptoms, drugs that block the B2 receptor can cut attacks short. Icatibant, a synthetic B2 receptor blocker, reached its primary endpoint of symptom relief in about two hours in clinical trials, compared to 12 hours for an older treatment.15New England Journal of Medicine. Icatibant, a new bradykinin-receptor antagonist, in hereditary angioedema This targeted approach has changed the management of HAE from a wait-and-hope situation to one where patients can self-administer an injection and see rapid improvement.
Bradykinin and the Blood-Brain Barrier
The blood-brain barrier is a tightly sealed layer of cells that prevents most blood-borne molecules from entering the brain. Bradykinin can loosen this barrier by acting on B2 receptors, raising internal calcium levels, and triggering a chain of events that ultimately weakens the junctions between cells.16PubMed Central. Inflammatory mediators and modulation of blood-brain barrier permeability This permeability increase can be harmful during a stroke or brain injury, where a leaky barrier lets fluid and inflammatory molecules flood into brain tissue, worsening swelling and damage.
Lab studies on brain microvascular endothelial cells have shown that bradykinin treatment reduces the expression of tight-junction proteins like claudin-5 and occludin while increasing the release of inflammatory molecules such as IL-6 and IL-8.17PubMed Central. Bradykinin/bradykinin 1 receptor promotes brain microvascular endothelial cell permeability and proinflammatory cytokine release by downregulating Wnt3a Under inflammatory conditions, the B1 receptor also gets involved, and tissue kallikrein (a bradykinin-generating enzyme) can further worsen barrier breakdown.18PubMed Central. Increase in Blood-Brain Barrier Permeability is Modulated by Tissue Kallikrein via Activation of Bradykinin B1 and B2 Receptor-Mediated Signaling Researchers are exploring whether blocking these receptors could reduce brain swelling after stroke or traumatic injury, though that work is still largely preclinical.
Respiratory Effects and the COVID-19 Bradykinin Storm
In the lungs, bradykinin can cause bronchoconstriction, the tightening of the airways. Under normal circumstances, ACE and another enzyme called neutral endopeptidase break down bradykinin in the lung tissue quickly enough that it rarely causes problems. But when those enzymes are inhibited, inhaled bradykinin triggers both airway constriction and an influx of inflammatory cells, mediated through B2 receptors.19The Journal of Pharmacology and Experimental Therapeutics. Bradykinin-Induced Lung Inflammation and Bronchoconstriction: Role in Parainfluenze-3 Virus-Induced Inflammation and Airway Hyperreactivity – Section: ABSTRACT In allergen-challenged animals, the bronchoconstrictor response to bradykinin becomes exaggerated, peaking about 24 hours after exposure.20PubMed. Airway hyperresponsiveness to bradykinin induced by allergen challenge in actively sensitised Brown Norway rats
Bradykinin gained widespread attention during the COVID-19 pandemic. A computational analysis of gene expression in lung cells infected with SARS-CoV-2 found that the virus decreased levels of ACE while increasing levels of ACE2, effectively tipping the balance toward excessive bradykinin. The researchers proposed a “bradykinin storm” model in which unchecked bradykinin causes massive leakage of fluid into the lungs. Compounding the problem, the virus also appeared to boost production of hyaluronic acid, a molecule that can absorb more than a thousand times its weight in water. The combination of leaky vessels and a gel-like substance accumulating in the air sacs could help explain why some severely ill patients developed such profound difficulty with oxygen exchange.21eLife. A mechanistic model and therapeutic interventions for COVID-19 involving a RAS-mediated bradykinin storm While the bradykinin storm hypothesis remains a model rather than a proven mechanism, it prompted clinical interest in bradykinin-receptor blockers as potential treatments for severe respiratory infections.
Bradykinin and Gut Pain
Bradykinin is also active in the gastrointestinal tract, especially during inflammatory conditions. In rat models, blocking the B2 receptor reduced the abdominal pain response caused by rectal distension during inflammation and also reduced the inhibition of gastric emptying caused by chemical irritation of the gut.22PubMed. Influence of bradykinin in gastrointestinal disorders and visceral pain induced by acute or chronic inflammation in rats The finding is consistent with bradykinin’s broader role as a pain amplifier: in inflamed gut tissue, it contributes to the visceral hypersensitivity that makes conditions like inflammatory bowel disease and irritable bowel syndrome so uncomfortable. Interestingly, the B2 receptor blocker had no effect on gut pain responses in non-inflamed tissue, suggesting that bradykinin’s contribution to gut pain specifically kicks in during active inflammation rather than under normal conditions.
Connections to Mast Cells and Urticaria
For a long time, bradykinin-driven swelling and histamine-driven swelling were thought of as separate phenomena. Hereditary angioedema was “the bradykinin disease,” while hives and allergic reactions were “the histamine disease.” That divide is starting to blur. Research has found that mast cell degranulation, the explosive release of histamine and other mediators from mast cells, can itself activate the kallikrein-kinin cascade and lead to bradykinin formation.23PubMed Central. Mast cell degranulation and bradykinin-induced angioedema – searching for the missing link
In chronic spontaneous urticaria, a condition marked by recurrent itchy welts and sometimes angioedema, patients showed elevated markers of bradykinin production at levels comparable to those seen in hereditary angioedema. When patients entered remission, those markers returned to normal.24PubMed. Evidence for bradykinin release in chronic spontaneous urticaria The overlap raises the possibility that some patients with urticaria who do not respond well to antihistamines might have a bradykinin component to their symptoms, though targeted therapies for this overlap are still being investigated.
Tumor Growth and Angiogenesis
Bradykinin’s ability to make blood vessels leaky and to stimulate the growth of new blood vessels has drawn attention in cancer research. Solid tumors need a blood supply to grow, and bradykinin appears to assist by increasing vascular permeability in the early stages of tumor development through B2 receptors on endothelial cells and by stimulating the production of vascular endothelial growth factor (VEGF) in surrounding tissue during later stages.25International Immunopharmacology. Roles of bradykinin in vascular permeability and angiogenesis in solid tumor In cervical cancer cells, bradykinin promoted proliferation, migration, and invasion, and blocking the B2 receptor reversed those effects.26PubMed Central. Bradykinin promotes proliferation, migration, and invasion of cervical cancer cells through STAT3 signaling pathways Whether bradykinin-receptor blockers could one day serve as an adjunct in cancer treatment remains an open question, but the fact that one molecule links inflammation, blood vessel growth, and tumor-cell behavior in this way is a reminder of how deeply embedded bradykinin is in the body’s signaling networks.
A Discovery Born From Snake Venom
Bradykinin was discovered in 1948 in São Paulo, Brazil, by the pharmacologist MaurÃcio Rocha e Silva and his colleagues. They were studying what happened when they added venom from the pit viper Bothrops jararaca to blood plasma. Instead of finding histamine, the molecule they expected, they identified a completely new substance that powerfully lowered blood pressure and contracted smooth muscle. They named it bradykinin, from the Greek words for “slow” and “movement,” because it caused a slow contraction of isolated gut tissue.27PubMed. MaurÃcio Rocha e Silva MD: snake venom, bradykinin and the rise of autopharmacology
The story has a remarkable sequel. Later work by Sérgio Ferreira, one of Rocha e Silva’s students, isolated peptides from the same snake venom that could enhance bradykinin’s effects by blocking the enzyme that degrades it. Those venom-derived peptides turned out to be natural ACE inhibitors, and they became the template for captopril, the first ACE-inhibitor drug, approved for human use in 1981.27PubMed. MaurÃcio Rocha e Silva MD: snake venom, bradykinin and the rise of autopharmacology A class of drugs now taken by tens of millions of people worldwide traces its origin directly to the venom of a Brazilian snake and to the discovery of the peptide it helped reveal.