Adrenergic receptors are a type of receptor in which catecholamines are coupled. They are involved in various functions of the sympathetic nervous system, which involves fight-and-flight responses.
Below, we’ll take a closer look at the types and subtypes of these receptors, and explain what each of them is involved with.
What are adrenergic receptors?
Adrenergic receptors, also called adrenergic receptors, they are receptors coupled to G proteins. The two substances attached to it are norepinephrine and epinephrine, which are two catecholamines. They are also the site of certain beta blockers, β2 and α2 agonists, used to treat hypertension and asthma, among other medical conditions.
Many cells in the body contain adrenergic receptors and attach themselves to catecholamines which activate the receptor and induce 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, increase the heart rate and, essentially, mobilize the energy necessary to survive the potentially dangerous or stressful situation.
History of these receptors
In the 19th century, the idea was accepted that stimulation of the sympathetic nervous system could lead to several changes in the body, provided that there are one or more substances that induce this activation. But it was not until the following century that it was proposed how this phenomenon occurred:
One hypothesis maintained that they existed two different types of neurotransmitters that have had some effect on the sympathetic nerves. Another argued that instead of having two types of neurotransmitters, there should be two types of detection mechanisms for the same neurotransmitter, meaning that there would be two types of receptors for the same substance, this 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 be what would stimulate it, was called sympathin I (of “excitation”) and the other, which would be what inhibited it, was sympathin I (of “inhibition”).
The second proposal found support during the period 1906 to 1913. Henry Hallett Dale had explored the effects of adrenaline, then called adrenaline, injected into animals or into the human bloodstream. When injected, this substance increases blood pressure. When the animal was exposed to the ergotoxin, its blood pressure decreased.
Dale proposed the idea that ergotoxin-induced paralysis of mioneural motor junctionsIn other words, the parts of the body which 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. or the other system, involving 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 reinforced by the idea that muscles did indeed have two different types of mechanisms that could involve two different responses to the same compound. Responses were induced depending on the type of receptors in which adrenaline was placed, calling α and β.
Types of Receivers
There are two main groups of adrenoreceptors, Which are further subdivided into 9 subtypes in total:
Α is classified as α1 (a receptor coupled to a Gq protein) and α2 (a receptor coupled to a Gi protein)
- α1 has 3 subtypes: α1A, α1B and α1D
- α2 has 3 subtypes: α2A, α2B and α2C
Βs are divided into β1, β2 and β3. All three bind to Gs proteins, but the β2 receptor and β3 also bind to Gi proteins.
epinephrine reacts to α and β adrenergic receptors, Involving different types of responses made by the circulatory system. These effects include vasoconstriction, linked to α receptors, and vasodilation, linked to β receptors.
Although α-adrenergic receptors have been shown to be less sensitive to epinephrine, when activated with a pharmacological dose of this substance, they induce β-adrenergic mediated vasodilation. The reason is that the α1 receptors are more peripheral than β, and by this activation with a pharmacological dose, they receive the substance earlier in α than in β. High doses of epinephrine in the bloodstream induce vasoconstriction.
Depending on the location of the receptors, the muscle response to adrenaline is different. Smooth muscle contraction and relaxation is generally weak. Cyclic adenosine monophosphate has different effects on smooth muscle and heart muscle.
This substance, when found in large doses, contributes to the relaxation of smooth muscles, also increasing the contractility and heart rate of the heart muscles, 1 effect, at first glance, counterintuitive.
The different α receptor subtypes have actions in common. Among these common actions are, as main ones, the following::
- Reduced mobility of smooth tissues in the gastrointestinal tract.
Some α agonist substances can be used to treat rhinitis because they decrease the secretion of mucus. Α-antagonist substances can be used to treat pheochromocytomaAs they decrease the vasoconstriction caused by norepinephrine that occurs in this medical condition.
The main action exerted by the α1 receptors involves a contraction of smooth muscles. They produce vasoconstriction in many veins, including those found in the skin, gastrointestinal system, renal artery, and brain. Other areas where smooth muscle contraction can occur are:
- I rented a transmission.
- Hairy muscles.
- Enctero pregnant.
- Urethral sphincter.
- Veins of the ciliary body.
Α1 antagonists, i.e. substances which, when coupled, induce actions contrary to those exerted by agonists, they are used to treat hypertension, causing a decrease in blood pressure, And also benign prostatic hyperplasia.
The α2 receptor binds to Gi / o proteins. This receptor is presynaptic, inducing a negative feedback effect, i.e. control, on adrenergic substances such as norepinephrine.
For example, when norepinephrine is released into the synaptic space, it activates this receptor, causing a decrease in the release of norepinephrine from the presynaptic neuron and, thus, to prevent an overproduction from occurring which implies negative effects throughout the organism.
Among the actions of the α2 receptor are:
- Decreases 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 the release of norepinephrine in the central nervous system.
- Increase platelet aggregation.
- Decreases peripheral vascular resistance.
Α2 agonists can be used to treat high blood pressureAs they lower blood pressure by increasing the actions of the sympathetic nervous system.
Antagonists of these same receptors are used to treat impotence, relax the muscles of the penis and promote flow blood to the area; depression, as mood increases, increases the secretion of norepinephrine.
Β receptor agonists are used for heart failure, As they increase the cardiac response in an emergency. They are also used in cases of circulatory shock, redistributing blood volume.
Β Antagonists, called beta blockers, are used to treat cardiac arrhythmia because they decrease the sinoatrial nodule response, stabilizing heart function. As with agonists, antagonists can also be used in heart failure, preventing sudden death associated with this condition, usually due to ischemia and arrhythmias.
They are also used for hyperthyroidism, reducing the 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 the pressure inside the eyes.
Increases heart response by increasing heart rate, Conduction velocity and stroke volume.
The actions of the β2 receptor include:
- Relaxation of bronchial smooth muscle, gastrointestinal tract, veins and skeletal muscle.
- Lipolysis of adipose tissue (fat burning).
- Uterine relaxation in non-pregnant women.
- Glycogenolysis and gluconeogenesis.
- Stimulates the secretion of insulin.
- Sphincteric contraction of the gastrointestinal tract.
- Immune communication of the brain.
Β2 agonists are used to treat:
- Asthma: reduce the contraction of the bronchial muscle.
- Hyperkalaemia: increase cellular potassium intake.
- Premature birth: reduce the contraction of uterine smooth muscle.
Among the actions of β3, we find increase fat tissue lipolysis and bladder relaxation.
Β3 receptor agonists can be used as medication for weight loss, although their effect is still under investigation and has been linked to a worrying side effect: tremors in the limbs.
- Adam, A. and Prat, G. (2016). Psychopharmacology: mechanism of action, effect and therapeutic management. Barcelona, Spain. Books by Marge Medica.