Chapter 6: Drugs Acting on Autonomic Nervous System: Adrenergic Agonists and Antagonists Agents

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6.12:

Adrenergic Antagonists: Pharmacological Actions of ɑ-Receptor Blockers

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JoVE Core Pharmacology

Adrenergic Antagonists: Pharmacological Actions of ɑ-Receptor Blockers

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6.11: Adrenergic Antagonists: Chemistry and Classification of ɑ-Receptor Blockers

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6.13: Adrenergic Antagonists: Chemistry and Classification of β-Receptor Blockers

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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.

6.12:

Adrenergic Antagonists: Pharmacological Actions of ɑ-Receptor Blockers

α-Adrenergic antagonists, known as α-blockers, exert their effects by inhibiting α-adrenoceptors, leading to specific physiological actions. α₁-blockers and α₂-blockers have distinct pharmacological actions and therapeutic applications.

α₁-blockers: These drugs inhibit α₁-adrenoceptors on smooth muscle cells, resulting in vasodilation. This vasodilation lowers blood pressure, making α₁-blockers valuable in treating hypertension. Additionally, α₁-blockers effectively address urinary obstruction caused by benign prostatic hyperplasia (BPH). These drugs reduce urine flow resistance and alleviate BPH-related symptoms by relaxing smooth muscles in the prostate and bladder neck.

α₂-blockers: These antagonists act on presynaptic α₂-adrenoceptors, increasing sympathetic neurotransmitters' release. This promotes adrenoceptor activation in target organs. However, non-selective α-blockers interacting with both α₁ and α₂ receptors are generally not used for hypertension management due to their severe side effects.

Overall, adrenergic antagonists, particularly α₁-blockers, play a crucial role in modulating the sympathetic nervous system and offer therapeutic benefits for conditions such as hypertension and urinary obstruction associated with BPH.

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Adrenergic Antagonists α-receptor Blockers β-adrenergic Antagonists α1-blockers α2-blockers Vasodilation Blood Pressure Hypertension Urinary Obstruction Benign Prostatic Hyperplasia (BPH)Smooth Muscle Cells Sympathetic Neurotransmitters Adrenoceptor Activation Side Effects Sympathetic Nervous System