ADRENERGIC BLOCKERS

Adrenergic receptor blockers may be considered in two groups:

1. Drugs blocking the ą adrenergic receptor
2. Drugs blocking the b Adrenergic receptor

These drugs prevent the response of effectors organs to adrenaline, noradrenaline and other sympathomimetic amines whether released in the body or injected. Circulating catecholamines are antagonized more readily than are the effects of sympathetic nerve stimulation. The drugs act by competing with the catechoamines for a or b receptors on the effectors organs. They don’t alter the production or release of the substances.

Adrenergic blockers

Alpha adrenergic receptor antagonists may be reversible or irreversible. Reversible antagonists dissociate from the receptors e.g. phentolamine, tolazoline, prazosin,yohimbine, etc. Irreversible antagonists tightly bind to the receptor so that their effects may persist long after the drug has been cleared from the plasma e.g. phenoxybenzamine

Pharmacologic Effects:

Alpha receptor antagonist drugs lower peripheral vascular resistance and blood pressure. Hence, postural hypotension and reflex tachycardia are common during the use of these drugs. Other minor effects include miosis, nasal stuffiness, etc.

Prazosin

This is an effective drug for the management of hypertension. It has high affinity for alpha₁ receptor and relatively low affinity for the alpha₂ receptor. Prazosin leads to relaxation of both arterial and venous smooth muscles due to the blockage of alpha₁ receptors. Thus, it lowers blood pressure, reduces venous return and cardiac output. It also reduces the tone of internal sphincter of urinary bladder.

Indications:

• Essential hypertension
• Raynaud’s syndrome Benign prostatic hyperplasia