Pharmacology Sheet 7: Adrenoceptor Antagonists

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Questions and Answers

What is a potential danger of β-blockers for patients with congestive heart failure?

They can increase cardiac output.

They can be life-threatening. (correct)

They can prevent an increase in heart rate.

They may cause severe hypertension.

Why should β-blockers be used cautiously in patients with obstructive airway disease?

They cause increased cardiac output.

They are known to induce hallucinations.

They can reduce glycogen mobilization.

They promote further bronchoconstriction. (correct)

What central effects may occur due to high doses of β-blockers?

Hallucinations, nightmares, and insomnia. (correct)

Enhanced cognitive function.

Improved mood and energy levels.

Increased appetite and weight gain.

What is the potency ratio of β-blocking to α-blocking activity in labetalol when administered orally?

3:1 Signup and view all the answers

What adverse effect can be potentiated by β-blockers due to their action on glycogen?

Hypoglycemia. Signup and view all the answers

What is a common side effect of topical application of timolol to the eye?

Dryness and burning of the eyes. Signup and view all the answers

Which drug possesses both α- blocking and β-blocking activity?

Labetalol. Signup and view all the answers

What percentage range of side effects is reported for timolol's topical administration?

5% to 10%. Signup and view all the answers

Which group of α-blockers is considered to have the greatest clinical utility?

Quinazoline derivatives Signup and view all the answers

What type of α-antagonism is produced by quinazoline derivatives like prazosin?

Equilibrium-competitive Signup and view all the answers

Which of the following is a characteristic effect of high doses of prazosin?

Vasodilation independent of α-receptors Signup and view all the answers

What physiological changes are less prominent following chronic treatment with prazosin?

Increases in heart rate Signup and view all the answers

Why has the use of haloalkylamines and imidazolines diminished in recent years?

They lack selectivity for α-receptors Signup and view all the answers

When considering the pharmacological actions of α-blockers, what do they primarily antagonize?

Vascular smooth muscle contraction Signup and view all the answers

In which condition can prazosin be safely used due to its minimal impact on certain blood levels?

Hypertension in patients with diabetes mellitus Signup and view all the answers

Which of the following is a clinical use of prazosin?

Management of benign prostatic obstruction Signup and view all the answers

Which of the following is NOT a characteristic class of α-receptor blocking agents?

Quinoline derivatives Signup and view all the answers

What is the primary therapeutic target of quinazoline derivatives?

α1-adrenoceptors Signup and view all the answers

How does prazosin affect the presynaptic α2-receptors?

It has no effect on them Signup and view all the answers

Which statement describes the antihypertensive effects of prazosin?

It is effective in reducing all grades of hypertension. Signup and view all the answers

Which of the following statements about α-blockers is true?

They have a negligible effect on α2-adrenoceptors at therapeutic doses. Signup and view all the answers

What urinary complications does prazosin help alleviate?

Symptoms of benign prostatic obstruction Signup and view all the answers

What effect does prazosin have on plasma renin activity during chronic treatment?

It has no impact on plasma renin activity Signup and view all the answers

Which other antihypertensive medications enhance the action of prazosin?

Thiazides Signup and view all the answers

What is the primary mechanism of action of labetalol?

It exhibits selective α1-receptor antagonism. Signup and view all the answers

What happens to peripheral vascular resistance after acute administration of labetalol?

It decreases. Signup and view all the answers

How does labetalol differ from traditional β-blockers in its hemodynamic effects?

It lowers blood pressure without affecting heart rate. Signup and view all the answers

In which condition is labetalol NOT typically used?

Heart failure. Signup and view all the answers

What is a unique characteristic of the action of labetalol at β2-receptors?

It exerts a partial agonist effect. Signup and view all the answers

How does labetalol’s α-blocking action relate to prazosin?

Both show selectivity for α1-receptors. Signup and view all the answers

What is the effect of traditional α-blockers compared to labetalol on heart rate and cardiac output?

They increase heart rate reflexively. Signup and view all the answers

What main clinical usage of labetalol is highlighted in the text?

Chronic treatment of primary hypertension. Signup and view all the answers

What is the primary action of β-blockers on the cardiovascular system?

Decrease heart rate and myocardial contractility Signup and view all the answers

How does chronic administration of β-blockers affect blood pressure?

It reduces blood pressure Signup and view all the answers

In what situation should β-blockers be used with caution?

In patients susceptible to hypoglycemia Signup and view all the answers

What effect do β-blockers have on β2-receptor mediated actions?

Antagonism leading to bronchoconstriction Signup and view all the answers

What is a therapeutic use of β-blockers mentioned in the content?

Prophylactic treatment of angina pectoris Signup and view all the answers

Which type of β-receptor is primarily found in the heart?

β1 receptors Signup and view all the answers

What is a common misconception about the effect of β-blockers on blood pressure after acute administration?

It only slightly alters blood pressure Signup and view all the answers

Which of the following best describes cardioselective β-blockers?

They selectively antagonize β1 receptors in the heart Signup and view all the answers

Study Notes

Classification of Blocking Drugs

α-Receptor blockers include Haloalkylamines (phenoxybenzamine), Imidazolines (phentolamine), and Quinazoline derivatives (e.g., prazosin, terazosin).
Quinazoline compounds are the most clinically useful among α-adrenoceptor antagonists.
Haloalkylamines and imidazolines have decreased clinical use due to lack of selectivity for α1- and α2-receptors.

Mechanism of Action (Quinazolines)

Prazosin and quinazoline derivatives exhibit equilibrium-competitive antagonism, selectively targeting α1-adrenoceptors.
At therapeutic doses, minimal antagonism of α2-adrenoceptors is observed.
High doses promote vasodilation via a direct effect on smooth muscles, possibly related to phosphodiesterase inhibition.

Pharmacological Actions

Antagonize vascular smooth muscle contraction induced by sympathetic activity or adrenomimetics.
Chronic treatment minimizes postural hypotension and lessens the expected increases in heart rate and contractility.

Clinical Uses of Prazosin

Effective in managing all grades of hypertension, especially when combined with thiazides or other antihypertensives.
Safe for patients with diabetes or gout, as it does not significantly affect blood glucose or uric acid levels.
Used for benign prostatic obstruction management, alleviating urinary urgency, particularly at night.

Cardiovascular Effects of β-Blockers

β-Blockers predominantly decrease heart rate, myocardial contractility, and cardiac output.
Chronic administration results in reduced blood pressure, beneficial in primary hypertension.
Provide prophylaxis for angina pectoris, but acute administration may only have slight blood pressure effects.

Metabolic Effects of β-Blockers

Blockade of β-receptors mitigates glycogenolytic and lipolytic actions of catecholamines.
Caution is needed in diabetic patients at risk of hypoglycemia due to potential exacerbation of symptoms.

Respiratory Effects and Contraindications of β-Blockers

Propranolol may increase airway resistance by blocking β2-receptor-mediated bronchodilation, posing risks for asthmatic patients.
Caution in congestive heart failure patients, as β-blockers can reduce heart rate and cardiac output.
Central effects (e.g., hallucinations, depression) may occur especially at high doses.

Combined α- and β-Blocking Activity (Labetalol)

Labetalol has both α and β-blocking capabilities, with a β-to-α activity ratio of approximately 3:1 (oral) and 7:1 (intravenous).
Exhibits equilibrium-competitive antagonism at both receptor types, lacking selectivity for β1 or β2 receptors.

Pharmacological Actions of Labetalol

Causes decreased peripheral vascular resistance and blood pressure without significantly altering heart rate or cardiac output, differing from standard α or β-blockers.

Clinical Uses of Labetalol

Effective for chronic treatment of primary hypertension, often combined with other antihypertensives.
Used intravenously in hypertensive emergencies, showcasing its versatility in hypertension management.

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Description

Explore the classification and mechanisms of adrenoceptor antagonists in this pharmacology quiz. Learn about important α-receptor blocking agents and their chemical groups, essential for understanding adrenergic pharmacology. Test your knowledge on drugs like phenoxybenzamine and phentolamine.