Pharmacology: α-Antagonists Overview

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary use of phenoxybenzamine?

Treatment of hypertension

Relief of angina symptoms

Management of inoperable or metastatic pheochromocytoma (correct)

Control of acute asthma attacks

What type of receptor blocker is phenoxybenzamine?

Non-selective and irreversible α-receptor blocker (correct)

Selective and reversible α-receptor blocker

Selectively blocks α1 receptors

Competitive β-receptor blocker

What is a major limitation of phenoxybenzamine in clinical applications?

It cannot be used with beta-blockers.

It has short-lasting effects.

It is highly selective and has numerous side effects.

It is not successful in maintaining lowered blood pressure. (correct)

What condition is phentolamine primarily used to manage?

Short-term management of pheochromocytoma Signup and view all the answers

What is the duration of action for a single dose of phentolamine?

4 hours Signup and view all the answers

Why are selective α1 antagonists able to allow NE to exert unopposed negative feedback?

They selectively block presynaptic α2 receptors. Signup and view all the answers

What risk is associated with phentolamine in patients with decreased coronary perfusion?

Potential induction of reflex cardiac stimulation Signup and view all the answers

Which of the following drug classes does NOT represent selective α1 antagonists?

Phenoxybenzamine Signup and view all the answers

What is the primary therapeutic action of selective α2 antagonists?

They increase sympathetic outflow to the periphery. Signup and view all the answers

Which of the following drugs is a selective α2 antagonist?

Yohimbine Signup and view all the answers

What is a common therapeutic use of Yohimbine?

Treatment of impotence Signup and view all the answers

Which class of β-blockers selectively inhibits β1 receptors?

Selective β-blockers Signup and view all the answers

What is a characteristic of nonselective β-blockers?

They can cause bronchoconstriction. Signup and view all the answers

Which statement about β-blockers is accurate?

Most β-blockers end in 'olol'. Signup and view all the answers

Which drug is an example of a nonselective β-blocker?

Propranolol Signup and view all the answers

What is the contraindication for using Yohimbine?

CNS and cardiovascular disorders Signup and view all the answers

What is a primary effect of blocking α1 receptors in relation to hypertension?

Dilates both arterioles and venules Signup and view all the answers

In which position is blood pressure reduction from α1 receptor antagonists more pronounced?

Upright Signup and view all the answers

Which condition is NOT a therapeutic application of selective α1 antagonists?

Hypotension Signup and view all the answers

What is the first-dose effect associated with selective α1 antagonists?

Orthostatic hypotension leading to potential loss of consciousness Signup and view all the answers

What common adverse effect results from dilation of nasal mucosa blood vessels due to α1 antagonists?

Nasal congestion Signup and view all the answers

How do selective α1 antagonists affect renal blood flow and GFR?

They reduce them Signup and view all the answers

Tamsulosin selectively inhibits which type of α1 receptors?

α1A receptors Signup and view all the answers

Which of the following is a consequence of sodium and water retention associated with selective α1 antagonists?

Orthostatic hypotension Signup and view all the answers

What is the primary action of indirect-acting sympatholytic agents?

Interfering with NE release to reduce sympathetic stimulation. Signup and view all the answers

Which of the following drugs are classified as adrenergic neuron-blocking agents?

Reserpine and Guanethidine Signup and view all the answers

What is a common clinical use of centrally acting adrenergic drugs?

To treat hypertension that is resistant to other treatments. Signup and view all the answers

What is the mechanism of action of Clonidine?

It acts as a selective α2-receptor agonist. Signup and view all the answers

What is the primary effect of β1-blockers on heart rate?

Reduce heart rate Signup and view all the answers

Which of the following is NOT a side effect of Clonidine?

Increased cardiac output Signup and view all the answers

How is Clonidine primarily excreted from the body?

By the kidney. Signup and view all the answers

Which condition can β-blockers help manage by decreasing cardiac workload?

Angina pectoris Signup and view all the answers

What is the duration of action for Clonidine in renal disease patients?

Up to 40 hours. Signup and view all the answers

What effect does propranolol have on vascular resistance?

No effect on vascular resistance Signup and view all the answers

What physiological effect does Clonidine primarily have on blood pressure?

Decreases blood pressure gradually over several hours. Signup and view all the answers

What is a potential risk of using propranolol in type-1 diabetic patients?

Decreased glucagon secretion Signup and view all the answers

How do β-blockers help in severe hypertension?

By preventing reflex tachycardia Signup and view all the answers

What is the effect of β2-receptor blockade in patients with asthma or COPD?

Bronchoconstriction Signup and view all the answers

What distinguishes cardioselective β1-blockers from nonselective blocks like propranolol?

Cardioselective β1-blockers primarily reduce heart rate Signup and view all the answers

Which of the following is NOT a consequence of β1-blocking?

Increased heart rate Signup and view all the answers

Study Notes

Non-Selective α-Antagonists

Phenoxybenzamine is a non-selective and irreversible α-receptor blocker used with a beta-blocker to treat inoperable or metastatic pheochromocytoma.
Phenoxybenzamine blocks α1 receptors, reversing the vasoconstriction of epinephrine.
Phenoxybenzamine blocks presynaptic α2 receptors, leading to an increase in cardiac output.
Phentolamine is used for the short-term management of pheochromocytoma and a hypertensive crisis.
Phentolamine is a competitive blocker of both α1 and α2 receptors with an action lasting approximately four hours.
Phentolamine is contraindicated in patients with decreased coronary perfusion because it may induce reflex cardiac stimulation.

Selective α1 Antagonists

Selective α1 antagonists are competitive blockers of the α1 receptor, such as Prazosin, Terazosin, Doxazosin, Tamsulosin, and Alfuzosin.
α1-receptor selectivity allows NE to exert unopposed negative feedback mediated by the presynaptic α2 receptors.
Blocking α1 receptors reduces hypertension by dilating both the resistance (arterioles) and capacitance (venules) blood vessels.
Blood pressure is reduced more in the upright than in the supine position.
Selective α1 antagonists cause minimal effects on cardiac output, renal blood flow, and GFR.
Therapeutic applications of α1 receptor blocking include essential hypertension, benign prostatic hyperplasia, congestive heart failure, pheochromocytoma, and Raynaud’s disease.
Adverse effects include orthostatic hypotension, first-dose effect, nasal congestion, sodium and water retention, and reflex tachycardia.

Selective α2 Antagonists

Selective α2 antagonists do not have much utility in clinical practice.
Yohimbine, a selective, competitive blocker of α2 receptors found in the bark of the Yohimbe tree, is the only drug commonly used.
Yohimbine increases sympathetic outflow to the periphery.
Yohimbine is sometimes used as a sexual stimulant for the treatment of impotence.
Yohimbine is used to relieve vasoconstriction associated with Raynaud’s disease.

β-Adrenergic Antagonists (β-blockers)

β-blockers inhibit β receptors, ending in "-olol" except for labetalol and carvedilol.
Nonselective β-blockers inhibit both β1 and β2 receptors.
Selective (cardioselective) β-blockers inhibit β1 receptors only.
β-blockers are used to treat angina pectoris, cardiac dysrhythmias, myocardial infarction and heart failure, hyperthyroidism, neurologic disorders, migraine headaches, and glaucoma.
β1-blocking reduces heart rate, force of contraction, and velocity of impulse conduction at the AV node.
Propranolol, the first nonselective β-blocker, is effective in the treatment of hypertension and ischemic heart disease.
Cardioselective β1-blockers such as metoprolol and atenolol are now more commonly used.
β-blockers are useful for the treatment of mild to moderate hypertension without prominent postural hypotension.
β-blockers are useful in preventing reflex tachycardia that often results from vasodilators.

Propranolol

Propranolol is a prototype β-adrenergic antagonist that blocks both β1 and β2 receptors with equal affinity.
Propranolol blocks β1-receptors, reducing heart rate and cardiac output.
Propranolol blocks β2-receptors, preventing vasodilation, which reduces blood pressure.
Blocking β2-receptors in COPD or asthmatic patients induces bronchoconstriction.
Propranolol blocks β2-receptors, decreasing glycogenolysis and glucagon secretion.
Propranolol should be used with caution in type-1 diabetic patients.

Indirect-Acting Sympatholytic Agents

These agents interfere with NE release, reducing sympathetic stimulation of peripheral receptors.
Adrenergic neuron-blocking agents, such as Reserpine and Guanethidine, deplete NE in sympathetic neurons, impairing sympathetic functions.
These drugs are no longer available clinically due to intolerable toxicity.
Centrally acting adrenergic drugs, such as Clonidine and α-Methyldopa, act within the CNS as selective α2-receptor agonists, decreasing NE release and reducing sympathetic outflow.
Centrally acting drugs are primarily used for the treatment of hypertension.

Clonidine

Clonidine is a selective α2-receptor agonist causing bradycardia, decreasing cardiac output, and promoting vasodilation.
Clonidine is primarily used for the treatment of hypertension that has not responded to other therapies.
Clonidine does not reduce renal blood flow or glomerular filtration and is useful in the treatment of patients with renal disease.
Blood pressure declines within 30 to 60 minutes after an oral dose with the greatest decrease occurring within 2 to 4 hours.
Clonidine is well absorbed orally (95%) and is excreted by the kidney.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers the mechanisms and clinical uses of non-selective and selective α-antagonists in pharmacology. It discusses drugs such as Phenoxybenzamine and Phentolamine, along with their effects on α1 and α2 receptors. Test your knowledge on how these medications manage conditions like pheochromocytoma and hypertensive crises.