Beta adrenergic receptors antagonists

Table of Contents

Beta (β) adrenergic receptors antagonists are grouped as Non-selective β adrenergic receptor antagonists and Selective β adrenergic receptor antagonists.

Non-selective β adrenergic receptor antagonists (beta 1 and 2)- Propranolol, Nadolol, Pindolol, Timolol, Sotalol
Selective β adrenergic receptor antagonists
- Selective β₁-adrenergic receptor antagonists- Metoprolol, Atenolol, and Esmolol
- Selective β₂-adrenergic receptor antagonist- Butoxamine.

1. Non-selective β adrenergic receptor antagonists

Propranolol is a popular Non-selective β adrenergic receptor antagonist drug

Propranolol

Propranolol interacts with both β₁ and β₂ receptors and blocks them competitively. It has powerful local anesthetic effect also.

Cardiovascular effects of Propranolol
- Blockade of β₁ –receptors produces decrease in heart rate, force of contraction and cardiac output. These are more evident during stress or exercise.
- Cardiac work and oxygen demand are reduced. The AV conduction time is slowed, ectopic pacemaker activity is reduced and automaticity is suppressed.
- Propranolol blocks cardiac stimulant action of adrenergic agonistic drugs, but not that of digitalis, Ca⁺⁺ or methylxanthines.

Propranolol increases peripheral vascular resistance as a result of blockade of vascular β₂– receptors. Hence, blood flow to all organs, except heart decreases. Propranolol blocks vasodilatation and fall in blood pressure caused by adrenaline. Pressor response of noradrenaline is slightly reduced due to blockade of cardiac β₁response.

Propranolol blocks β₂– adrenergic receptors in bronchial smooth muscle leading to increased airway resistance. This bronchoconstriction is more marked in asthmatic patients.

Propranolol blocks adrenaline induced lipolysis and glycogenolysis. Clinically used as an anti arrhythmic drug.

2. Selective β adrenergic receptor antagonists

Metoprolol

Metoprolol is the prototype of cardio-selective β₁ receptor blockers. Its potency to block β₁– receptors equals that of propranolol, but about 50-100 times higher dose is required to block β₂-receptors.

Cardiovascular effects secondary to metoprolol’s negative inotropic and chronotropic actions include decreased sinus heart rate, slow AV conduction, decreased cardiac output, myocardial oxygen demand and reduced blood pressure. It reduces plasma renin activity in hypertensive patients. Unlike propranolol, metaprolol as relatively little effect on pulmonary function, peripheral resistance and carbohydrate metabolism.

Metoprolol is used in arrhythmias, systemic hypertension, ventricular hypertrophy.

Atenolol

Unlike propranolol and metoprolol, atenolol has very low lipid solubility, so it penetrates the brain only to a very limited extent.

Esmolol

Esmolol is an ultra short acting β₁-rceptor blocker. It is given IV when β₁– receptor blockade is required for short duration. It is very useful for the investigation and immediate therapy of tachycardia.

Butoxamine

Butoxamine is relatively β₂-adrenergic receptor antagonist. It is not used clinically as blockade of β2- adrenergic receptors has no therapeutic applications and on the other hand causes bronchoconstriction.