Adrenergic Receptors: What They Are, Functions, and Types

Adrenergic receptors are a type of receptor to which catecholamines are coupled They are involved in several functions of the sympathetic nervous system, which involves fight and flight responses.

Below we will look in more depth at the types and subtypes of these receptors, in addition to explaining what each of them is involved in.

Table of Contents

What are adrenergic receptors?

Adrenergic receptors, also called adrenoceptors, They are receptors that couple to G proteins The two substances that bind to them are norepinephrine and adrenaline, which are two catecholamines. They are also the place where some beta-blocker type drugs, β2 and α2 agonists, used to treat hypertension and asthma, among other medical conditions, are placed.

Many cells in the body contain adrenergic receptors, and catecholamines are coupled to them, activating the receptor and inducing stimulation of the sympathetic nervous system. This system is responsible for preparing the body for a flight or fight situation, causing the pupils to dilate, the heartbeat to increase and, in essence, to mobilize the energy necessary to survive the potentially dangerous or stressful situation.

History of these receivers

In the 19th century, the idea was accepted that stimulation of the sympathetic nervous system could involve several changes in the body, as long as there were one or several substances that induced this activation. But it was not until the following century that it was proposed how this phenomenon occurred:

One hypothesis held that there were two different types of neurotransmitters that had some effect on the sympathetic nerves Another maintained that instead of there being two types of neurotransmitters there should be two types of detector mechanisms for the same neurotransmitter, that is, there would be two types of receptors for the same substance, which would imply two types of responses.

The first hypothesis was proposed by Walter Bradford Cannon and Arturo Rosenblueth, who proposed the existence of two neurotransmitters. One, which would stimulate, was called sympathin E (for “excitation”) and the other, which would inhibit, was sympathin I (for “inhibition”).

The second proposal found support during the period from 1906 to 1913. Henry Hallett Dale had explored the effects of adrenaline, called adrenin at the time, injected into animals or the human bloodstream. When injected, this substance increased blood pressure. When the animal was exposed to ergotoxin its blood pressure decreased.

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Dale proposed the idea that ergotoxin induced paralysis of the myoneural motor junctions, that is, those parts of the body that are responsible for controlling blood pressure. He indicated that, under normal conditions, there was a mixed mechanism that induced both paralysis and its activation, causing either contraction or relaxation depending on environmental demands and organic needs, and that these responses were made depending on whether a The same substance had affected one or the other system, implying two different types of responses.

Later, in the 1940s, it was discovered that substances chemically related to adrenaline could induce different types of responses in the body. This belief was strengthened by seeing that muscles had, in effect, two different types of mechanisms that could imply two different responses to the same compound. The responses were induced depending on the type of receptors in which the adrenaline was placed, calling them α and β.

Types of receivers

There are two main groups of adrenoceptors which are subdivided into 9 subtypes in total:

The α are classified into α1 (a receptor coupled to a Gq protein) and α2 (a receptor that couples to a Gi protein)

The β are divided into β1, β2 and β3. All three couple to Gs proteins, but the β2 and β3 receptor also couple to Gi proteins.

circulatory function

epinephrine reacts to both α and β adrenergic receptors, involving different types of responses carried out by the circulatory system. Among these effects are vasoconstriction, related to α receptors, and vasodilation, related to β receptors.

Although it has been seen that α-adrenergic receptors are less sensitive to epinephrine, when they are activated with a pharmacological dose of this substance, they induce vasodilation mediated by β-adrenergics. The reason for this is that α1 receptors are more peripheral than β receptors, and through this activation with pharmacological doses the α receptors receive the substance sooner than the β receptors. High doses of epinephrine in the bloodstream induce vasoconstriction

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Subtypes

Depending on the location of the receptors, the muscle response to adrenaline is different. Contraction and relaxation of smooth muscles is generally low Cyclic Adenosine Monophosphate has different effects on smooth muscle than on cardiac muscle.

This substance, when found in high doses, contributes to the relaxation of smooth muscle, also increasing contractility and heartbeat in the cardiac muscles, an effect, at first glance, counterintuitive.

α receptors

The different subtypes of α receptors have actions in common. Among these common actions are, as main ones, the following:

Some α agonist substances can be used to treat rhinitis, because they decrease mucus secretion. α-antagonist substances can be used to treat pheochromocytoma since they reduce the vasoconstriction caused by norepinephrine that occurs in this medical condition.

1. α1 receptor

The main action played by α1 receptors involves contraction of smooth muscle They cause vasoconstriction of many veins, including those found in the skin, the gastrointestinal system, the renal artery, and the brain. Other areas in which smooth muscle contraction can occur are:

Ureter
Different conductor.
Hairy muscles.
Pregnant uterus.
Urethral sphincter.
Bronchioles.
Veins of the ciliary body.

α1 antagonists, that is, those substances that when coupled induce actions contrary to those that agonists would perform, They are used to treat hypertension, inducing a decrease in blood pressure and also benign prostatic hyperplasia.

2. α2 receptor

The α2 receptor couples to Gi/o proteins. This receptor is presynaptic, inducing negative feedback effects, that is, control, on adrenergic substances such as norepinephrine.

For example, when norepinephrine is released into the synaptic cleft, it activates this receptor, causing the release of norepinephrine from the presynaptic neuron to decrease and, thus, preventing overproduction from occurring that would have negative effects on the entire organism.

Among the actions of the α2 receptor are:

Decrease the release of insulin in the pancreas.
Increase the release of glucagon in the pancreas.
Contraction of the sphincters of the gastrointestinal tract.
Control of norepinephrine release in the central nervous system.
Increase platelet aggregation.
Reduce peripheral vascular resistance.

α2 agonist substances can be used to treat hypertension since they reduce blood pressure by increasing the actions of the sympathetic nervous system.

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Antagonists for these same receptors are used to treat impotence, relaxing the muscles of the penis and promoting blood flow in the area; depression, since they raise the mood by increasing norepinephrine secretion.

β receptors

β-receptor agonists are used for heart failure, since they increase the cardiac response in case there is an emergency. They are also used in circulatory shock, redistributing blood volume.

β-antagonists, called beta-blockers, are used to treat cardiac arrhythmia, since they decrease the response of the sinoatrial node, stabilizing cardiac function. As with agonists, antagonists can also be used in heart failure, preventing sudden death related to this condition, which is usually due to ischemias and arrhythmias.

They are also used for hyperthyroidism, reducing excessive peripheral synaptic response In migraine they are used to reduce the number of attacks of this type of headache. In glaucoma they are used to reduce pressure inside the eyes.

1. β1 receptor

Increases cardiac response by increasing heart rate conduction velocity and stroke volume.

2. β2 receptor

Actions of the β2 receptor include:

Smooth muscle relaxation of bronchi, gastrointestinal tract, veins and skeletal muscle.
Lipolysis of adipose tissue (burning fat).
Uterus relaxation in non-pregnant women.
Glycogenolysis and gluconeogenesis.
Stimulates the secretion of insulin.
Contraction of sphincters of the gastrointestinal tract.
Immune communication of the brain.

β2 agonists are used to treat:

Asthma: reduce bronchial muscle contraction.
Hyperkalemia: increase cellular potassium intake.
Premature birth: they reduce the contraction of uterine smooth muscle.

3. β3 receptor

Among the actions of β3 are increase lipolysis of adipose tissue and relaxation of the bladder

β3 receptor agonists can be used as weight loss drugs, although their effect is still being studied and has been linked to a worrying side effect: tremors in the extremities.