Adrenergic receptors according to Stedman’s Medical Dictionary are “reactive components of effector tissues, most of which are innervated by adrenergic postganglionic fibers of the sympathetic nervous system. Such receptors can be activated by norepinephrine and/or epinephrine and by various adrenergic drugs; receptor activation results in a change in effector tissue function, such as contraction of arteriolar muscles or relaxation of bronchial muscles; adrenergic receptors are divided into α-receptors and β-receptors, on the basis of their response to various adrenergic activating and blocking agents also called adrenoceptor, adrenoreceptors.
Some quick facts about adrenergic receptors:
All alpha adrenergic receptors use the cAMP secondary messenger system
All beta adrenergic receptors use the Ca²+ secondary messenger system
Postganglionic sympathetic neurons mostly use noradrenaline.
Adrenal medulla secretes mostly adrenaline; adrenaline gives the widespread effect of the sympathoadrenal system.
Alpha adrenergic receptors are more sensitive to noradrenaline than adrenaline.
Beta adrenergic receptors are more sensitive to adrenaline than noradrenaline, reason why beta adrenergic receptors are more involved during the widespread sympathetic activities.
* Difference between alpha & beta receptors-
1. Potency of the agonist:
alpha- Adrenaline > NorAdrenaline > Isoprenaline
beta- Isoprenaline > Adrenaline > NorAdrenaline
2. Most common Antagonist:
3. Effector pathway:
alpha- IP3/DAG increases, K+ channel increases, cAMP decreases agonistic activity
beta- cAMP increases, Ca2+ channel increases agonistic activity
* The alpha receptors are divided into- a1 (alpha 1) & a2 (alpha 2) receptors
a1- Postjunctional on the effector organs
a2- Prejunctional on nerve endings, postjunctional in brain & pancreatic b-cells
a1- Genito-urinary Smooth muscle contraction, Gland-secretion, GIT- relaxation, Liver- glycogenolysis, Heart- arrhythmia
a2- Inhibit the transmitter release, Vasoconstriction, Decreased central sympathetic flow, Decreased insulin release, Platelet aggregation
3. Selective agonist:
a1- Phenylephrine, Methoxamine
4. Selective antagonist:
a2- Yohimbine, Rauwolscine
* The beta receptors are divided into- b1, b2, b3 (Beta 1,2,3)
b1- Heart, JuxtaGlomerular cells in kidney
b2- Bronchus, blood vessels, uterus, liver, git, urinary tract, eye
2. Selective agonist:
b2- Salbutamol, terbutalin
3. Selective antagonist:
b1- Metoprolol, Atenolol
b2- ICI 118551 (also b3), a-methyl propranolol
4. Potency of NorAdrenaline as agonist: