19. Adrenergic receptor antagonists

Last updated on January 10, 2019 at 20:15

α-receptor antagonists

Non-selective

α1-selective

α2-selective

Phenoxybenzamine

Non-subtype selective

α1A-selective

α1B-selective

Yohimbine

Phentolamine

Urapidil

Alfuzosin

Doxazosin

Tamsulosin

Prazosin

Terazosin

β-receptor antagonists

Non-selective

β1-selective

α + β antagonists

Acebutolol

Atenolol Carvedilol

Oxprenolol

Betaxolol

Labetalol

Penbutolol

Bisoprolol

Pindolol

Celiprolol

Propranolol

Esmolol

Timolol Metoprolol
Nebivolol
α-receptor antagonists

Alpha receptor antagonists decrease the effects of noradrenaline and adrenaline on α receptors. α1 and α2 subtypes exist, and for α1 both A and B subtypes exist. I don’t think it’s important to know which drugs act on which α1 subtypes.

Hypertensive crisis occurs in case of pheochromocytoma because there is a sudden large release of noradrenaline and adrenaline, which binds to α1 receptors, causing vasoconstriction. By giving α-blockers can we inhibit this effect.

Phentolamine are used to treat hypertensive crisis, especially due to pheochromocytoma or another hyperadrenergic state. It acts on both α1 and α2 receptors. Phentolamine has a short duration of action.

Phenoxybenzamine also acts on both α receptors. It is an irreversible antagonist, meaning that is has a very long duration of action. It’s used for prophylactic treatment of pheochromocytomas, especially before surgical removal of the tumor.

Urapidil is an antagonist for all types of α1 receptors. It is used in hypertensive crises, especially if there is acute cerebral haemorrhage.

Alfuzosin and tamsulosin are α1A-selective antagonists. They’re used to relax the urethral sphincter in prostate hypertrophy.

Doxazosin, prazosin and terazosin are α1B-selective antagonists. They have antihypertensive effects. They also relax the urethral sphincter in prostate hypertrophy.

Yohombine is an α2-selective antagonist. It has no clinical use.

β-receptor antagonists

Beta receptor antagonists, or beta blockers are used to treat the following:

  • Angina pectoris
  • Myocardial infarction
  • Arrhythmias
  • Heart failure
  • Hypertension
  • Palpitations, tremors
  • Prophylaxis for migraine
  • Glaucoma
  • Thyrotoxicosis

Beta-blockers treat hypertension not by causing vasodilation, but by causing decreased chronotropic and inotropic effects on the heart, reducing renin release and decreasing sympathetic activity centrally, which indirectly decreases blood pressure. This effect comes from blocking β1-receptors. This also decreases strain on the heart, which is why they’re used in heart failure.

Propranolol (not propanolol) is a non-selective beta blocker. It is lipid-soluble, so it enters the brain to give the patient nightmares and other weird CNS effects.

Acebutolol, oxprenolol, penbutolol, pindolol and timolol are partial non-selective agonists. You can read more about how partial agonists can be antagonists here. These drugs are recommended for elderly people.

Atenolol, betaxolol, bisoprolol, celiprolol, metoprolol and nebivolol are β1-selective antagonists. These drugs lower blood pressure by complex mechanisms, including reducing cardiac output. The fact that they don’t target β2-receptors is an advantage which gives them fewer side effects.

Esmolol is also a β1-selective antagonist. It’s special because it’s inactivated by enzymatic hydrolysis.

Carvedilol and labetalol are both α and β blockers. They’re used to treat hypertension in hypertensive crisis and hypertension in pregnancy.

Side effects of beta blockers
  • Bronchoconstriction
    • Can be a problem for asthmatics, COPD patients
  • Cardiac depression
    • Bad for elderly people
  • Bradycardia
  • Hypoglycaemia
    • Adrenaline releases blood glucose
    • Especially in diabetics
  • Fatigue
    • Due to reduced cardiac output and reduced muscle perfusion
  • Cold extremities
    • Due to loss of β-receptor mediated vasodilation in skin
  • CNS effect like nightmares
    • Mainly in very lipophilic drugs like propranolol

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4 thoughts on “19. Adrenergic receptor antagonists”

    1. I didn’t think people were interested in that, so I wasn’t going to write a post about it, but since you’re asking will I do it.

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