Adrenergic Antagonists: Pharmacological Actions of ɑ-Receptor Blockers

Recall that α-receptor antagonists are reversible or irreversible adrenoceptor inhibitors.

As reversible antagonists are dissociable, the presence of high agonist concentrations can overcome their pharmacological action.

However, irreversible antagonists form a reactive intermediate that irreversibly binds to α-adrenoceptors and cannot be displaced by additional agonists.

α₁ -adrenoceptor blockers lower blood pressure by causing vasodilation.

α₁-antagonists are also effective in treating urinary obstruction caused by benign prostatic hyperplasia. They act by reducing muscle tone in the prostate and bladder neck.

α₂-adrenoceptor blockers enhance sympathetic neurotransmitter release from presynaptic neurons and activate the adrenoceptors of target organs.

Nonselective α-blockers are not clinically used due to serious side effects.

However, they are prescribed for pheochromocytoma to prevent blood pressure elevation. Pheochromocytoma is a tumor of the catecholamine-secreting chromaffin cells of the adrenal medulla. The excess catecholamine release leads to sustained hypertension.