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Adrenergic Agonists and
Antagonists: Pharmacology and
Clinical Applications
Adrenergic Agonists and Antagonists
Indirect Agonists
Indirect agonists elevate catecholamine levels at the synapse without directly binding to receptors. Examples include amphetamine, cocaine, and tyramine.
Sympathomimetics
These drugs mimic the actions of epinephrine or norepinephrine, producing "fight or flight" responses.
Adrenoreceptor Types and Subtypes
Adrenoreceptors are classified into alpha (α) and beta (β) types, each with subtypes. α1 receptors cause smooth muscle contraction and mydriasis. α receptors have mixed smooth muscle effects. β1 receptors increase cardiac chronotropic and ionotropic effects. β2 receptors cause bronchodilation. β receptors increase lipolysis.
Adrenergic Receptors: Inducers vs Inhibitors
Dobutamine is an agonist that induces positive ionotropic effects, increasing the force of contraction. It is used clinically for acute heart failure and cardiogenic shock. Adverse effects include tachycardia, arrhythmias, hypertension, chest pain, and shortness of breath.
Beta-2 Agonists
Albuterol is a short-acting beta2-agonist (SABA) that causes bronchial smooth muscle dilation, decreasing bronchospasms. It is a bronchodilator used for asthma and COPD, administered via inhaler or nebulizer. Adverse effects include tachycardia and tremor. Terbutaline causes systemic smooth muscle relaxation, leading to bronchial dilation for asthma and COPD, and uterine relaxation to slow preterm labor contractions.
Direct Acting NON-Selective Adrenergic Agonists
Oxymetazoline is an alpha-1 and alpha-2 agonist. Its main effect is α1- mediated vasoconstriction, with partial α2 activity sustaining the vasoconstriction longer. It is used topically for nasal congestion, epistaxis,
and eye redness. Epinephrine is an alpha-1, alpha-2, beta-1, and beta- agonist. It causes α1-mediated vasoconstriction, increasing BP; α2-mediated inhibition of NE release; β1-mediated increases in HR, contractility, and conduction velocity; and β2-mediated bronchodilation. It is used for anaphylaxis, cardiac arrest, and shock. Adverse effects include tachycardia, arrhythmias, hypertension, stroke, and tremor. Norepinephrine is an alpha-1, alpha-2, and beta-1 agonist. It causes strong α1-mediated vasoconstriction, increasing BP; α2-mediated inhibition of NE release; and β1-mediated increases in HR and contractility. It has minimal β2 effects. It is used for septic shock, neurogenic shock, and hypotension during anesthesia. Adverse effects include severe hypertension, reflex bradycardia, arrhythmias, and peripheral ischemia.
Mixed Acting Adrenergic Agonists
Ephedrine is an alpha-1, alpha-2, beta-1, and beta-2 agonist and releasing agent. It was the first orally active sympathomimetic drug, found in ma huang. It has CNS activity, acting as a mild stimulant. Pseudoephedrine is used as a nasal decongestant. OTC purchase restrictions are in place due to its use in methamphetamine production.
Indirect Acting Adrenergic Agonists
Amphetamine promotes the release of NE, DA, and serotonin, and also inhibits reuptake. Tyramine displaces NE from storage vesicles in nerve terminals, especially in MAO-inhibited patients. Cocaine blocks NE/DA reuptake via transporter inhibition, increasing NE, DA, and serotonin. Imipramine increases NE and serotonin. It is used for depression and anxiety. Adverse effects include increased BP, arrhythmias, CNS stimulation, and seizures (cocaine). Monoamine oxidase inhibitors (MAO-Is) prevent the breakdown of catecholamines inside the neuron, increasing NE, DA, and serotonin levels. Nonselective MAO-A and B inhibitors include phenelzine and tranylcypromine. Selegiline is a MAO-B selective inhibitor. They are used for atypical depression and treatment-resistant major depressive disorder. Adverse effects include the "cheese effect" and serotonin syndrome.
Adrenergic Antagonists
Alpha receptor antagonists include nonselective agents like phentolamine, α1-selective agents like terazosin and tamsulosin, and α2-selective agents like yohimbe. Beta receptor antagonists include nonselective agents like propranolol and β1-selective agents like metoprolol. Labetalol is a nonselective beta and alpha-1 blocking agent.
Alpha Receptor Antagonists
Phenoxybenzamine irreversibly blocks α1 and α2 receptors, causing vasodilation and lowering BP. HR rises due to baroreflex activation. Phentolamine reversibly blocks α1 and α2 receptors, causing vasodilation and lowering BP. It has a shorter duration of tachycardia. It is used
