Catecholamine Receptor Effects in Brief Summary

Beta in general– Gs protein coupled – activate adenylyl cyclase, increase cAMP levels

  •     relax ciliary muscle

Beta 1

  •     accelerates sinoatrial node
  •     accelerates ectopic pacemakers
  •     increases contractility of the heart
  •     increases rennin release by the kidney

Beta 2

  • accelerates sinoatrial node
  • accelerates ectopic pacemakers
  • increases contractility of the heart
  •  relaxes the smooth muscle of skeletal muscle arterioles
  • relaxes bronchiolar smooth muscle
  • relaxes gut wall smooth muscle
  • relaxes the bladder wall
  • relaxes the pregnant uterus
  • increases gluconeogenesis and glycogenolysis in the liver

Beta 3

  • all it does is increases the rate of lipolysis in fat cells

Alpha in general

  • contracts the blood vessels in skin and the various non-vital organs
  • contracts the pregnant uterus
  • causes ejaculation
  • cause piloerector contraction, raising little skin hairs
  • causes apocrine sweat glands to activate and produce sweat
  • stimulates glycogenolysis and glyconeogenesis by the liver

Alpha 1 : Gq protein coupled – second messenger is IP3, causing an increase of intracellular calcium

  • contracts the radial muscle of the iris (dilates pupil)
  • contracts the gut sphincters

Alpha 2– Gi protein coupled – inhibit adenylyl cyclase, decrease cAMP

  • relaxes the walls of the gut wall smooth muscles
  • Central effects are sympatholytic - presynaptic inhibition takes place

In greater detail:

Alpha-1 receptor intracellular signalling pathway

These are Gq-protein coupled receptors

These receptors are all heptaspanning membrane proteins.

The aim of acting them is to cause smooth muscle contraction, by stimulating the release of calcium from the sarcoplasmic reticulum.

These are the steps by means which this is achieved.

alpha-1 intracellular signalling pathway

Effects of activating alpha-1 receptors

  • Strongly contracts the radial muscle of the iris, dilating the pupil
  • Slightly increases lacrimation (weak effect, its mainly a parasympathetic thing)
  • Slightly vasoconstricts coronary arteries (but local effects still result in vasodilation)
  • Significantly vasoconstricts the arteries of the skin and mucosa
  • Slightly vasoconstricts the arteries of the skeletal muscle
  • Very slightly vasoconstricts the arteries of the cerebral circulation
  • Slightly vasoconstricts the arteries of the pulmonary circulation
  • Significantly vasoconstricts the arteries of the abdominal viscera
  • Significantly vasoconstricts the renal arteries
  • Significantly vasoconstricts the arteries of the salivary glands
  • Vasoconstricts the veins
  • Decreases secretions in the bronchial glands
  • Decreases motility and tone of the stomach
  • Contracts the gastric sphincters
  • Decreases motility and tone of the intestine
  • Contracts the intestinal sphincters
  • Slightly reduces renin secretion by the kidney
  • Contracts the trigone of the bladder, and the urinary sphincter
  • Increases the motility and tone of the ureter
  • Contracts a pregnant uterus
  • Causes ejaculation
  • Causes piloerection in the skin
  • Causes localised sweating
  • Causes contraction of the splenic capsule (in some animals, this causes an autotransfusion of a significant volume of blood)
  • Increases glycogenolysis and gluconeogenesis in the liver
  • Decreases pancreatic secretions (digestive enzymes)
  • Increases lipolysis and thermogenesis at the adipocyte
  • Increases salivation

If you can come up with a dirty mnemonic for this, you deserve a medal.

Alpha-2 receptor intracellular signalling pathway

These are Gi protein coupled receptors

alpha-2 intracellular signalling pathway.

 
  • Alpha-2 preceptors DEACTIVATE adenylyl cyclase.
    • Active adenylyl cyclase activates cyclic aMP, which is a widespread second messenger.
    • Thus, the effect of alpha-2 activation is a decrease in cAMP

    There is a greater pharmacological relevance for the alpha-2 receptor: in the CNS, presynaptic alpha-2 receptors inhibit the release of noradrenaline.

    This means alpha-2 agonists which penetrate the central nervous system act as sympathetic antagonists. Three examples of this are clonidine, methyldopa and dexmedetomidine.

Effects of activating alpha-2 receptors

  • Constricts the coronary arterioles
  • Constricts the cutaneous arterioles
  • Slightly increases lacrimation (weak effect, its mainly a parasympathetic thing)
  • Slightly vasoconstricts coronary arteries (but local effects still result in vasodilation)
  • Significantly vasoconstricts the arteries of the skin and mucosa
  • Significantly vasoconstricts the renal arteries
  • Significantly vasoconstricts the arteries of the salivary glands
  • Vasoconstricts the veins
  • Decreases motility and tone of the stomach
  • Inhibits gastric secretions
  • Decreases motility and tone of the intestine
  • Inhibits intestinal secretions
  • Decreases pancreatic secretions (digestive enzymes)
  • Significantly decreases the secretion of insulin
  • Inhibits lipolysis at the adipocyte
  • Presynaptically, inhibits the release of noradrenaline (mainly an alpha-2a effect)

Beta receptor intracellular signalling pathway

These are all  Gs-protein coupled receptors.

  • All the beta receptors increase the synthesis of cAMP.
    All excitable tissues become more excitable with extra cAMP.

beta intracellular signalling pathway

Effects of activating Beta-1 receptors

  • The ratio in the heart of beta-1 to beta-2 is 3:2 in the atria and 4:1 in the ventricles.

    • Significantly increases the sinoatrial node firing rate, increasing the heart rate
    • Significantly increases atrial contractility and conduction velocity
    • Significantly increases AV node automaticity and conduction velocity
    • Significantly increases His-Purkinje system automaticity and conduction velocity
    • Significantly increases ventricular contractility, conduction velocity, as well as automaticity and  rate of any random idioventricular pacemakers
    • Significantly vasodilates the renal arteries
    • Decreases motility and tone of the stomach
    • Decreases motility and tone of the intestine
    • Significantly increases renin secretion by the kidney
    • Increases lipolysis and thermogenesis at the adipocyte
    • Increases melatonin synthesis at the pineal gland
    • Increases ADH secretion at the posterior pituitary

Effects of activating Beta-2 receptors

  • Relaxes the ciliary muscle for far vision
  • Slightly increases the sinoatrial node firing rate, increasing the heart rate
  • Slightly increases atrial contractility and conduction velocity
  • Slightly increases AV node automaticity and conduction velocity
  • Slightly increases His-Purkinje system automaticity and conduction velocity
  • Slightly increases ventricular contractility, conduction velocity, as well as automaticity and  rate of any random idioventricular pacemakers
  • Significantly vasodilates coronary arteries
  • Significantly vasodilates the arteries of the skeletal muscle
  • Slightly vasodilates the arteries of the abdominal viscera
  • Significantly vasodilates the renal arteries
  • Vasodilates the veins
  • Bronchodilates
  • Increases secretions in the bronchial glands
  • Decreases motility and tone of the stomach
  • Decreases motility and tone of the intestine
  • Relaxes the gallbladder
  • Relaxes the urinary bladder (detrusor muscle)
  • Relaxes the uterus, pregnant or not
  • Causes the relaxation of the splenic capsule
  • Causes increased contractility of the skeletal muscle
  • Causes increased glycogenolysis in skeletal muscle
  • Causes increased potassium uptake into skeletal muscle (this is why salbutamol is useful in the treatment of hyperkalemia)
  • Increases glycogenolysis and gluconeogenesis in the liver
  • Increases the secretion of insulin
  • Increases lipolysis and thermogenesis at the adipocyte
  • Increases melatonin synthesis at the pineal gland

Effects of activating Beta- 3 receptors

  • Increases lipolysis and thermogenesis at the adipocyte

References

For this sort of really basic stuff, no matter where you look you will find essentially the same information.


I used chapters from "Goodman & Gilman's The Pharmacological Basis of Therapeutics" 11th ed by Brunton et al, and "Basic & Clinical Pharmacology" 11th ed. By Katzung et al.

I also perused Peck and Hill "Pharmacology for Anaesthesia and Intensive care" as well as the notoriously error-prone "Handbook of Pharmacology and Physiology in Anaesthetic Practice" by Stoelting and Hillier. Neither covered this subject in a depth I found satisfying.

Goodman and Gilman's remains a canonical text.