Main Difference – Alpha vs Beta Receptors
Alpha and beta receptors are the two types of adrenergic receptors involved in the regulation of fight-or-flight mechanisms in the body. The fight-or-flight mechanism is a response to the stress. Both alpha and beta receptors occur postsynaptically at the sympathetic junctions of some organs such as the heart, blood vessels, lungs, uterus, and fatty tissues. The main difference between alpha and beta receptors is that alpha receptors stimulate the effector cells whereas beta receptors relax the effector cells. The two main types of alpha receptors are alpha 1 and alpha 2 receptors. On the other hand, the three main types of beta receptors are beta 1, beta 2, and beta 3 receptors. The agonist that stimulates alpha, beta 1 and beta 2 receptors are the two adrenergic hormones: epinephrine and norepinephrine.
Key Areas Covered
1. What are Alpha Receptors
– Definition, Mechanism, Role
2. What are Beta Receptors
– Definition, Mechanism, Role
3. What are the Similarities Between Alpha and Beta Receptors
– Outline of Common Features
4. What is the Difference Between Alpha and Beta Receptors
– Comparison of Key Differences
Key Terms: Alpha-adrenergic Receptors, Alpha Receptors, Beta Receptors, Effector Cells, Epinephrine, Norepinephrine, Postsynaptic Membrane, Sympathetic Junctions
What are Alpha Receptors
Alpha receptors are the cell receptors that control physiological processes like vasoconstriction, intestinal relaxation, and pupil dilation upon interaction with epinephrine and norepinephrine. These receptors are also called the alpha-adrenergic receptors. The two types of alpha receptors are alpha 1 receptors and alpha 2 receptors. Epinephrine and norepinephrine serve as the alpha-adrenoreceptor agonists. Epinephrine and norepinephrine are shown in figure 1.
Figure 1: Epinephrine and Norepinephrine
Vascular smooth muscles consist of alpha receptors linked to the Gq-proteins. The alpha 1 receptors stimulate the contraction of vascular smooth muscles through IP3 signal transduction pathway. Some of the vessels consist of alpha 2 receptors as well. These receptors are linked to Gi-proteins. The binding of the agonist to an alpha 2 receptor decreases the intracellular cAMP levels, causing the contraction of smooth muscles. Though alpha receptors constrict both arteries and veins, the effect on arteries is more pronounced. The alpha receptors constrict smooth muscles in the ureter, urethral sphincter, urothelium, bronchioles, hair, and uterus during pregnancy.
What are Beta Receptors
Beta receptors refer to a group of receptors that control vasodilation, relaxation of the bronchial and uterine smooth muscles, and increase heart rate. Beta receptors are also stimulated by epinephrine and norepinephrine. These receptors are found in cardiac and smooth muscles. The stimulation of beta receptors also induces glycogenolysis in liver and renin secretion in the kidney. The mechanism of adrenergic receptors is shown in figure 2.
Figure 2: Mechanism of Adrenergic Receptors
The three types of beta receptors are beta 1, beta 2, and beta 3. The stimulation of beta receptors occurs through beta-adrenoreceptor-adenylyl cyclase-protein kinase A cascade. The activated cascade increases the levels of cAMP inside the cell. This causes the relaxation of the effector cells by inhibiting myosin light chain kinase. However, the increased cAMP levels induce cardiac myocyte contraction. Typically, beta 1 receptors are involved in the heart muscle contraction while beta 2 receptors are involved in smooth muscle relaxation, serving as a bronchodilator.
Similarities Between Alpha and Beta Receptors
- Both alpha and beta receptors are adrenergic receptors.
- Both alpha and beta receptors are located on the postsynaptic membrane at the sympathetic junctions.
- Both alpha and beta receptors are innervated by the sympathetic nervous system.
- Both alpha and beta receptors are stimulated by epinephrine and norepinephrine.
- Both alpha and beta receptors stimulate or relax effector cells of several organs in the body.
- There are several types of alpha and beta receptors
Difference Between Alpha and Beta Receptors
Definition
Alpha Receptors: Alpha receptors are the cell receptors that control physiological processes like vasoconstriction, intestinal relaxation, pupil dilation upon interaction with epinephrine and norepinephrine.
Beta Receptors: Beta receptors are a group of receptors that control vasodilation, relaxation of the bronchial and uterine smooth muscles, and increased heart rate.
Effect
Alpha Receptors: Alpha receptors stimulate effector cells.
Beta Receptors: Beta receptors relax effector cells.
Types
Alpha Receptors: The two types of alpha receptors are alpha 1 and alpha 2.
Beta Receptors: The three types of beta receptors are beta 1, beta 2, and beta 3.
Occurrence
Alpha Receptors: Alpha receptors mainly occur in vascular smooth muscles and effector tissues.
Beta Receptors: Beta receptors mainly occur in bronchi muscles, heart muscles, and uterine muscles.
Muscle Stimulation
Alpha Receptors: Alpha receptors stimulate smooth muscles.
Beta Receptors: Beta receptors stimulate both cardiac and smooth muscles.
Examples
Alpha Receptors: The stimulation of alpha receptors in the heart constricts blood vessels.
Beta Receptors: The stimulation of beta receptors in the heart increases the heart rate and the strength of contraction.
Medication
Alpha Receptors: Methoxamine is an example of alpha 1 agonist while clonidine is an example of alpha 2 agonists.
Beta Receptors: Albuterol is an example of beta 1 agonist while metoprolol is an example of beta 2 agonists.
Conclusion
Alpha and beta receptors are two types of adrenergic receptors stimulated by the sympathetic nervous system. Alpha receptors stimulate effector cells while beta receptors relax effector cells. Alpha receptors stimulate vasoconstriction while beta receptors stimulate vasodilation. The main difference between alpha and beta receptors is the effect of each type of receptor on the effector cells.
Reference:
1.“Alpha-Adrenoceptor Agonists (α-Agonists).” Image for Cardiovascular Pharmacology Concepts, Richard E Klabunde PhD, Available here.
2.Wallukat, G. “The beta-Adrenergic receptors.” Herz., U.S. National Library of Medicine, Nov. 2002, Available here.
3.“Beta-Adrenoceptor Agonists (β-Agonists).” Image for Cardiovascular Pharmacology Concepts, Richard E Klabunde PhD, Available here.
Image Courtesy:
1. “Epinephrine” By originally by Chepry – Own work (Public Domain) via Commons Wikimedia
2. “Norepinephrine structure” By Acdx – Own work, based on Image:Noradrenaline chemical structure.png (CC BY-SA 3.0) via Commons Wikimedia
3. “Adrenoceptor-Signal transduktion” By Sven Jähnichen. Partially translated by Mikael Häggström – Image:Adrenozeptor-Signaltransduktion.jpg (CC BY-SA 3.0) via Commons Wikimedia
Leave a Reply