An adrenergic antagonist is a

drug A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via insuffla ...

that inhibits the function of

adrenergic receptor The adrenergic receptors or adrenoceptors are a class of G protein-coupled receptors that are targets of many catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) produced by the body, but also many medications like beta ...

s. There are five adrenergic receptors, which are divided into two groups. The first group of receptors are the beta (β) adrenergic receptors. There are β₁, β₂, and β₃ receptors. The second group contains the alpha (α) adrenoreceptors. There are only α₁ and α₂ receptors. Adrenergic receptors are located near the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s, and

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...

. There are also α-adreno receptors that are located on

vascular smooth muscle Vascular smooth muscle is the type of smooth muscle that makes up most of the walls of blood vessels. Structure Vascular smooth muscle refers to the particular type of smooth muscle found within, and composing the majority of the wall of blood ...

. Antagonists reduce or block the signals of

agonists An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agon ...

. They can be drugs, which are added to the body for therapeutic reasons, or

endogenous ligand In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from ''ligare'', which means 'to bind'. In protein-ligand binding, the ligand is usually a m ...

s. The α-adrenergic antagonists have different effects from the β-adrenergic antagonists.

Pharmacology

Adrenergic ligands are endogenous proteins that modulate and evoke specific

cardiovascular The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

effects. Adrenergic antagonists reverse the natural cardiovascular effect, based on the type of adrenoreceptor being blocked. For example, if the natural activation of the α₁-adrenergic receptor leads to

vasoconstriction Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessel ...

, an α₁-adrenergic antagonist will result in

vasodilation Vasodilation is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstriction, ...

. Some adrenergic antagonists, mostly β antagonists, passively diffuse from the gastrointestinal tract. From there, they bind to

albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins ...

and α1-acid glycoprotein in the

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

, allowing for a wide spread through the body. From there, the

lipophilic Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar solvents are themselves lipop ...

antagonists are metabolized in the liver and eliminated with urine while the

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are no ...

ones are eliminated unchanged.Stereospecific Pharmacokinetics and Pharmacodynamics of Beta-Adrenergic Blockers in Humans
Mehvar, R., & Brocks, D. R. (2001). Stereospecific pharmacokinetics and pharmacodynamics of beta-adrenergic blockers in humans.

Mechanisms of action

There are three different types of antagonists.

Competitive

While only a few α-adrenergic antagonists are competitive, all β-adrenergic antagonists are

competitive antagonist A receptor antagonist is a type of receptor ligand or drug that blocks or dampens a biological response by binding to and blocking a receptor rather than activating it like an agonist. Antagonist drugs interfere in the natural operation of r ...

s. Competitive antagonists are a type of reversible antagonists. A competitive antagonist will attach itself to the same

binding site In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity. The binding partner of the macromolecule is often referred to as a ligand. Ligands may inclu ...

of the receptor that the agonist will bind to. Even though it is in activator region, the antagonist will not activate the receptor. This type of binding is reversible as increasing the concentration of agonist will outcompete the concentration of antagonist, resulting in receptor activation. Adrenergic competitive antagonists are shorter lasting than the other two types of antagonists. While the antagonists for alpha and beta receptors are usually different compounds, there has been recent drug development that effects both types of the adrenoreceptors. Phentolamine Structural Formulae

Examples

Two examples of competitive adrenergic antagonists are

propranolol Propranolol, sold under the brand name Inderal among others, is a medication of the beta blocker class. It is used to treat high blood pressure, a number of types of irregular heart rate, thyrotoxicosis, capillary hemangiomas, performance anx ...

and

phentolamine Phentolamine, sold under the brand name Regitine among others, is a reversible nonselective α- adrenergic antagonist. Mechanism Its primary action is vasodilation due to α1 blockade. Non-selective α-blockers can cause a much more pronounced ...

. Phentolamine is a competitive and nonselective α-adrenoreceptor antagonist. Propranolol is a β-adrenoreceptor antagonist.

Non-competitive

While competitive antagonists bind to the agonist or ligand binding site of the receptor reversibly, non-competitive antagonists can either bind to the ligand site or other site called the

allosteric site In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the ''allosteric site ...

. A receptor's agonist does not bind to its allosteric binding site. The binding of a non-competitive antagonist is irreversible. If the non-competitive antagonist binds to the allosteric site and an agonist binds to the ligand site, the receptor will remain unactivated. An example of an adrenergic non competitive antagonists is phenoxybenzamine. This drug is a non-selective α-adrenergic antagonist, which means it binds to both alpha receptors.

Uncompetitive

There were few if any adrenergic

uncompetitive antagonist A receptor antagonist is a type of receptor ligand or drug that blocks or dampens a biological response by binding to and blocking a receptor rather than activating it like an agonist. Antagonist drugs interfere in the natural operation of recep ...

s. An uncompetitive antagonist is slightly different from the other two types of antagonists. The action of an uncompetitive antagonist is dependent on the receptor's prior activation. This means only after the agonist binds to the receptor can the antagonist block the receptor's function.

Examples

Alpha blockers

Phentolamine Phentolamine, sold under the brand name Regitine among others, is a reversible nonselective α- adrenergic antagonist. Mechanism Its primary action is vasodilation due to α1 blockade. Non-selective α-blockers can cause a much more pronounced ...

phenoxybenzamine Phenoxybenzamine (marketed under the trade names Dibenzyline and Dibenyline) is a non-selective, irreversible alpha blocker. Uses It is used in the treatment of hypertension, and specifically that caused by pheochromocytoma. It has a slower on ...

Tamsulosin Tamsulosin, sold under the brand name Flomax among others, is a medication used to treat symptomatic benign prostatic hyperplasia (BPH) and chronic prostatitis and to help with the passage of kidney stones. The evidence for benefit with a ki ...

Beta blockers

Propranolol Propranolol, sold under the brand name Inderal among others, is a medication of the beta blocker class. It is used to treat high blood pressure, a number of types of irregular heart rate, thyrotoxicosis, capillary hemangiomas, performance anx ...

Nebivilol Nebivolol is a beta blocker used to treat high blood pressure and heart failure. As with other β-blockers, it is generally a less preferred treatment for high blood pressure. It may be used by itself or with other blood pressure medication. ...

Atenolol Atenolol is a beta blocker medication primarily used to treat high blood pressure and heart-associated chest pain. Atenolol, however, does not seem to improve mortality in those with high blood pressure. Other uses include the prevention of mi ...

Oxprenolol Oxprenolol (brand names Trasacor, Trasicor, Coretal, Laracor, Slow-Pren, Captol, Corbeton, Slow-Trasicor, Tevacor, Trasitensin, Trasidex) is a non-selective beta blocker with some intrinsic sympathomimetic activity. It is used for the treatment o ...

Metoprolol Metoprolol, sold under the brand name Lopressor, among others, is a selective β1 receptor blocker medication. It is used to treat high blood pressure, chest pain due to poor blood flow to the heart, and a number of conditions involving an a ...

Timolol Timolol is a beta blocker medication used either by mouth or as eye drops. As eye drops it is used to treat increased pressure inside the eye such as in ocular hypertension and glaucoma. By mouth it is used for high blood pressure, chest pai ...

Pindolol Pindolol, sold under the brand name Visken among others, is a nonselective beta blocker which is used in the treatment of hypertension.Drugs.coInternational brand names for pindolol Page accessed Sept 4, 2015 It is also an antagonist of the se ...

Nadolol Nadolol, sold under the brand name Corgard among others, is a medication used to treat high blood pressure, heart pain, atrial fibrillation, and some inherited arrhythmic syndromes. It has also been used to prevent migraine headaches and compl ...

Esmolol Esmolol, sold under the brand name Brevibloc, is a cardio selective beta1 receptor blocker with rapid onset, a very short duration of action, and no significant intrinsic sympathomimetic or membrane stabilising activity at therapeutic dosages. ...

Acebutolol Acebutolol, sold under the brand names Sectral among others, is a beta blocker for the treatment of hypertension and arrhythmias. Acebutolol is a cardioselective beta-1 blocker and has intrinsic sympathetic activity. It is commonly used in the ...

Sotalol Sotalol, sold under the brand name Betapace among others, is a medication used to treat and prevent abnormal heart rhythms. It is only recommended in those with significant abnormal heart rhythms due to potentially serious side effects. Evidence ...

Talinolol Talinolol is a beta blocker. Stereochemistry Talinolol contains a stereocenter and consists of two enantiomers. This is a racemate In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enan ...

* Betaxolol

Mixed action

Labetalol Labetalol is a medication used to treat high blood pressure and in long term management of angina. This includes essential hypertension, hypertensive emergencies, and hypertension of pregnancy. In essential hypertension it is generally less pref ...

Carvedilol Carvedilol, sold under the brand name Coreg among others, is a medication used to treat high blood pressure, congestive heart failure (CHF), and left ventricular dysfunction in people who are otherwise stable. For high blood pressure, it is gen ...

Major effects

Main complications of persistent high blood pressure

Adrenergic antagonists have inhibitory or opposing effects on the receptors in the adrenergic system. The adrenergic system modulates the

fight-or-flight response The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...

. Since this response, which is mostly seen as an increase in blood pressure, is produced by the release of the endogenous adrenergic ligands, administration of an adrenergic antagonist results a decrease in

blood pressure Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...

, which is controlled by both heart rate and vasculature tone. Administration of an adrenergic antagonist that specifically targets the beta receptors, results in this decrease in blood pressure by slowing or reducing cardiac output.

Medical uses

Adrenergic antagonists are mostly used for

cardiovascular disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, h ...

. The adrenergic antagonists are widely used for lowering blood pressure and relieving

hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...

. These antagonists have a been proven to relieve the pain caused by

myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may ...

, and also the infarction size, which correlates with heart rate. There are few non-cardiovascular uses for adrenergic antagonists. Alpha-adrenergic antagonists are also used for treatment of ureteric stones,

pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...

and

panic disorder Panic disorder is a mental disorder, mental and Abnormal behavior, behavioral disease#Disorder, disorder, specifically an anxiety disorder characterized by reoccurring unexpected panic attacks. Panic attacks are sudden periods of intense fear th ...

withdrawal Withdrawal means "an act of taking out" and may refer to: * Anchoresis (withdrawal from the world for religious or ethical reasons) * ''Coitus interruptus'' (the withdrawal method) * Drug withdrawal * Social withdrawal * Taking of money from a ban ...

, and

anesthesia Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), ...

Limitations

While these adrenergic antagonists are used for treating cardiovascular disease, mainly hypertension, they can evoke harmful cardiac events through prolongation of the

QT interval The QT interval is a measurement made on an electrocardiogram used to assess some of the electrical properties of the heart. It is calculated as the time from the start of the Q wave to the end of the T wave, and approximates to the time taken ...

. Some adrenergic antagonists have a diminished ability to reduce

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

compared to

placebo A placebo ( ) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like Saline (medicine), saline), sham surgery, and other procedures. In general ...

drugs.

Side effects and toxicity

While adrenergic antagonists have been used for years, there are multiple issues with using this class of drug. When overused, adrenergic antagonists can result in

bradycardia Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, a ...

hypotension Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dias ...

hyperglycemia Hyperglycemia is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a blood sugar level higher than 11.1 mmol/L (200 mg/dL), but symptoms may not start to become noticeable until even ...

and even hypodynamic shock. This is because adrenergic stimulation by agonists results in normal

calcium channel A calcium channel is an ion channel which shows selective permeability to calcium ions. It is sometimes synonymous with voltage-gated calcium channel, although there are also ligand-gated calcium channels. Comparison tables The following tables e ...

regulation. If these adrenergic receptors are blocked too often, there will be an excess in calcium channel inhibition, which causes most of these problems.

References

External links

* {{DEFAULTSORT:Adrenergic Antagonist Antihypertensive agents Parasympathomimetics

Pharmacology

Mechanisms of action

Competitive

Examples

Non-competitive

Uncompetitive

Examples

Alpha blockers

Beta blockers

Mixed action

Major effects

Medical uses

Limitations

Side effects and toxicity

See also

References

External links