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.