Adrenergic Receptors And Their Distribution

Introduction

Adrenergic receptors are the glycoproteins present on cell surface which recognize and selectively binds to the catecholamines (norepinephrine, epinephrine, etc). These receptors are the mediators of sympathetic nervous system like smooth muscle contraction, cardiac contraction, etc. Adrenergic receptors produce its action by transducing external catecholamine stimulus into the intracellular signal. Activation or blockage of these receptors is the major therapeutic approach for the management of cardiovascular disorders like, hypertension, angina pectoris and cardiac arrhythmias. In this article we will see adrenergic receptors, types of adrenergic receptors and distribution of adrenergic receptors.

Table of Contents

Adrenergic receptors

Adrenergic receptors are the members of superfamily of receptors which includes about 30 distinct but related proteins. All adrenergic receptors are composed of single polypeptide chains containing 402 to 515 amino acids in length. Each receptors contain seven stretches of 20-28 hydrophobic amino acids. The structure of adrenergic receptors is highly homologous to rhodopsin (visual transduction mediator).

The amino acid terminal of each receptor is located extracellularly and contains several active sites at which the proteins are glycosylated. The carboxyl terminal is located intracellularly and contains active sites at which proteins are modified by phosphorylation or by thioesterification. The transmembrane spanning regions are linked by three intracellular and three extracellular loops of variable length. The first and second extracellular loop contain cysteine residues. The third intracellular loop interacts directly with the receptor coupled G-protein.

Types of adrenergic receptors

Adrenergic receptors can be broadly classified into two groups as α-adrenoreceptor and β-adrenoreceptor.
Both the α and β adrenoreceptor are G-protein coupled receptor. But the type of G-protein is different with which they couple. α-adrenoreceptor couple with G₀ and β-adrenoreceptor couple with G_S.
The α-adrenoreceptor consist of two sub types α₁ and α₂. Each of these subtype produces different type of secondary message. The α adrenoreceptors further sub categorised as α_1A, α_1B, α_1D, α_2A, α_2B, α_2C.
The α₁ receptor activate inositol triphosphate (IP3) and diacylglycerol (DG), whereas α₂ receptor inhibit the production of secondary messenger cyclic-AMP.
β adrenoreceptors consist of three sub types β₁, β₂, β₃. All of these sub types activate the formation of cyclic-AMP.

Distribution of adrenergic receptors

The adrenoreceptors present at various sites in body, also their physiological role is different. Certain tissues contain more of one type of adrenoreceptor than another. α receptors generally contracts smooth muscles (except in gut), whereas β receptor relaxes smooth muscles. The smooth muscle relaxation effect is mediated through β₂ receptors. In the heart β₁ adrenoreceptors predominates and activation results in contraction of muscles.

α receptor: α receptor have following common actions;

Vasoconstriction
Decreased flow of smooth muscles in GIT

Subtype of α receptor have individual effects as follows.

α₁ receptor: Action of α₁ receptor mainly involves smooth muscle contraction. It causes vasoconstriction in many blood vessels, including those of skin, GIT, kidney and brain. Other areas of smooth muscle contraction are,

Ureter
Vas deferens
Hair
Uterus (when pregnant)
Urethral sphincter
Urothelium
Bronchioles
Blood vessels of ciliary body (mydriasis)

α₂ receptor: Actions of α₂ receptor include,

Decrease insulin release from the pancreas
Increased glucagon release from the pancreas
Contraction sphincters of the GIT
decreased platelet aggregation

β receptor: Subtype unspecific β agonist can be used to treat,

heart failure
circulatory shock
anaphylaxis

Subtype unspecific β antagonist (β blocker) can be used to treat,

cardiac arrhythmia
coronary artery disease
heart failure
hyperthyroidism
migraine
glaucoma

β₁ receptor: Actions of β₁ receptor include,

increased cardiac output
increased renin secretion
increased ghrelin secretion from stomach

β₂ receptor: Actions of β₂ receptor include,

smooth muscle relaxation
lipolysis in adipose tissue
anabolism in skeletal muscle
uptake of potassium into cells
relax uterus of non-pregnant woman
relax detrusor urinae muscle of bladder wall
glycogenolysis and gluconeogenesis
stimulates insulin secretion
contracts sphincters of GIT
thickened secretions from salivary gland
inhibit histamine release from mast cells

β₃ receptor: Actions of β₃ receptor include,

increase of lipolysis in adipose tissue
relax the bladder

Types of adrenoreceptors, distribution of adrenoreceptors and effect of activating these receptors is given in below table.

Organ/ tissue	Predominant adrenoreceptor	Physiological effect
Heart muscle	β₁	Increased heart rate and force
Bronchial muscle	α₁	Closes airways
Bronchial muscle	β₂	Dilates and open airways
Arteriole smooth muscle (not supplying muscle)	α	Constrict arterioles and increases blood pressure (hypertension)
Arteriole smooth muscle (supplying muscle)	β₂	Dilates arterioles and increases blood supply to muscles
Veins	α	Constricts veins and increases blood pressure (hypertension)
Veins	β₂	Dilates vein
Liver	α₁ & β₂	Breakdown of glycogen to produce glucose
GIT smooth muscle	α₁, α₂ & β₂	Shuts down digestion
Kidney	β₂	Increases renin secretion
Fat cells	β₃	Fat breaks down

Summary

Adrenergic receptors are broadly classified as alpha and beta subtypes. These receptors are found throughout the body, including cardiovascular system, lungs and various organs. Understanding their distribution is important for comprehending the physiological responses mediated by adrenergic signaling, blood pressure, heart rate and smooth muscle contraction. This knowledge is fundamental for pharmacological interventions targeting adrenergic pathways in medical treatments.

Frequently asked questions

What are the adrenergic receptors?

Adrenergic receptors are the cell surface glycoproteins that recognize and selectively bind the catecholamines, norepinephrine and epinephrine, which are released from sympathetic nerve endings and the adrenal medulla.

What are the main 2 types of adrenergic receptors?

Adrenergic receptors are mainly categorized into 2 types as alpha receptors and beta receptors.

What is the function of alpha- and beta-adrenergic receptors?

Alpha receptors cause muscle contraction and vasoconstriction in certain body parts, while beta receptors cause muscle relaxation and vasodilation in other areas.

What is the main function of adrenergic receptors?

The important function of adrenergic receptors includes activating our fight-or-flight responses and helping to regulate cardiovascular functions like lowering blood pressure.

Where are adrenergic receptors located in the eye?

In the eye, adrenergic receptors (alpha-2) are found in iris epithelium and ciliary epithelium as well as in the ciliary muscle, and retina.

Adrenergic Receptors and their distribution