Adrenergic Antagonists Overview
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Questions and Answers

What is a significant contraindication for the use of phentolamine?

  • Elevated blood pressure
  • Increased heart rate
  • Decreased coronary perfusion (correct)
  • Chronic kidney disease
  • Which therapeutic indication is NOT associated with phentolamine?

  • Direct intracavernous injection for male sexual dysfunction
  • Short term control of hypertension in pheochromocytoma
  • Long-term management of chronic hypertension (correct)
  • Treatment of hypertensive crisis from tyramine ingestion
  • What is the primary receptor blocked by prazosin?

  • α~2~-AR
  • 5-HT receptor
  • β-AR
  • α~1~-AR (correct)
  • Which statement about prazosin is accurate?

    <p>It has a bioavailability of 50 to 70%.</p> Signup and view all the answers

    Which agent is noted to be less potent than phentolamine?

    <p>Tolazoline</p> Signup and view all the answers

    What is a major potential adverse effect of certain antihypertensive agents?

    <p>Marked postural hypotension</p> Signup and view all the answers

    Which condition is NOT an established therapeutic indication for prazosin?

    <p>Chronic obstructive pulmonary disease</p> Signup and view all the answers

    Which of the following is a clinical application of selective α2-AR blockers?

    <p>Improvement of male sexual function</p> Signup and view all the answers

    What type of drug are all clinically available β-blockers classified as?

    <p>Competitive antagonists</p> Signup and view all the answers

    What effect can occur 30 to 90 minutes after the first dose of certain antihypertensive agents?

    <p>Postural hypotension</p> Signup and view all the answers

    Which of the following statements about labetalol is true?

    <p>It can be used for pregnancy-induced hypertension.</p> Signup and view all the answers

    What is the mechanism of action of reserpine?

    <p>It blocks the storage of biogenic amines in vesicles.</p> Signup and view all the answers

    Which adverse effect is associated with the use of alpha-1 blockers like labetalol?

    <p>Orthostatic hypotension</p> Signup and view all the answers

    How does guanethidine lower blood pressure?

    <p>By blocking the release of stored norepinephrine.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of labetalol?

    <p>It solely acts on beta-adrenergic receptors.</p> Signup and view all the answers

    What is the primary action of α-adrenergic blockers on blood pressure?

    <p>Prevent vasoconstriction of peripheral blood vessels</p> Signup and view all the answers

    What is a characteristic of phenoxybenzamine as an adrenergic antagonist?

    <p>It binds covalently to adrenoreceptors</p> Signup and view all the answers

    Which of the following drugs is considered a reversible α-adrenergic antagonist?

    <p>Both B and C</p> Signup and view all the answers

    What occurs as a reflex action due to reduced blood pressure from α-adrenergic blockade?

    <p>Reflex tachycardia</p> Signup and view all the answers

    What is the effect of α-adrenergic antagonists on the sympathetic nervous system?

    <p>They interfere with its normal functions</p> Signup and view all the answers

    How long do the actions of phenoxybenzamine typically last after a single dose?

    <p>24 hours</p> Signup and view all the answers

    Which type of adrenergic antagonist targets only β1-adrenoreceptors?

    <p>Relatively selective β<del>1</del>-blockers</p> Signup and view all the answers

    What physiological effect results from using α-adrenergic blockers in cardiovascular medicine?

    <p>Decreased peripheral vascular resistance</p> Signup and view all the answers

    What is the primary therapeutic use of phenoxybenzamine?

    <p>Long-term treatment of hypertension in pheochromocytoma</p> Signup and view all the answers

    What adverse effect may be caused by phenoxybenzamine due to impaired smooth muscle contraction?

    <p>Reversible inhibition of ejaculation</p> Signup and view all the answers

    Which condition is phenoxybenzamine used to preempt during surgical removal of a tumor?

    <p>Hypertensive crisis</p> Signup and view all the answers

    What is a common adverse effect of both phenoxybenzamine and phentolamine?

    <p>Postural hypotension</p> Signup and view all the answers

    Which of the following is true regarding phentolamine's action?

    <p>It produces a competitive block of α<del>1</del> and α<del>2</del> adrenergic receptors</p> Signup and view all the answers

    What is one of the physiological effects of decreased peripheral resistance due to phenoxybenzamine?

    <p>Reflex tachycardia</p> Signup and view all the answers

    For which condition is phenoxybenzamine NOT typically indicated?

    <p>Chronic migraines</p> Signup and view all the answers

    What effect can phenoxybenzamine have on the heart rate and cardiac output?

    <p>Increases heart rate and may contribute to increased cardiac output</p> Signup and view all the answers

    Study Notes

    Adrenergic Antagonists:

    • Adrenergic antagonists, also known as adrenergic blockers or sympatholytic agents, bind to adrenoreceptors without triggering their usual effects.
    • These drugs can bind reversibly or irreversibly, preventing activation by catecholamines like adrenaline, noradrenaline, and dopamine.
    • They are categorized based on their affinity for α or β-adrenoreceptors in the periphery.
    • They interfere with functions of the sympathetic nervous system, affecting organs innervated by it.
    • Many of these agents are crucial in clinical medicine, particularly for treating cardiovascular diseases.

    α-adrenergic Blockers:

    • α-AR blockers are either reversible (competitive) or irreversible (non-competitive) in their interaction with receptors.
    • They significantly affect blood pressure by lowering peripheral vascular resistance due to α1-AR blockade.
    • This can trigger reflex tachycardia caused by the lowered BP because β-ARs, especially β1-ARs on the heart, are unaffected by α blockade.

    Phenoxybenzamine:

    • Phenoxybenzamine, a haloalkylamine drug, is nonselective, binding to both postsynaptic α1-ARs and presynaptic α2-ARs irreversibly.
    • The block is noncompetitive, requiring synthesis of new ARs, which takes at least a day, meaning the effects last about 24 hours after a single dose.
    • It is a pro-drug, activated via biotransformation, leading to a delayed onset of blockade.
    • Cardiovascular effects: Phenoxybenzamine prevents vasoconstriction of peripheral blood vessels by blocking α1-ARs. The decreased peripheral resistance leads to reflex tachycardia. It also affects presynaptic α2-ARs, increasing cardiac output.
    • It is less effective in maintaining lowered BP in hypertension.

    Therapeutic Uses of Phenoxybenzamine:

    • Long-term treatment of hypertension in patients with pheochromocytoma (tumor of the adrenal medulla).
    • Used prior to surgical removal of the tumor to prevent hypertensive crises.
    • Also used for chronic management, especially if the tumor is inoperable.
    • Treatment of Raynaud's disease (spasm of arteries causing finger & toe pallor).
    • Used for benign prostatic hyperplasia (enlarged prostate causing urination difficulty) to decrease obstructive symptoms and urinary frequency.

    Adverse Effects of Phenoxybenzamine:

    • Postural hypotension (low BP upon standing) often accompanied by reflex tachycardia and other arrhythmias.
    • Inhibition of ejaculation due to impaired smooth muscle contraction of the vas deferens and ejaculatory duct.
    • Nasal stuffiness, nausea, and vomiting.

    Phentolamine and Tolazoline:

    • Phentolamine produces a competitive block of α1- and α2-ARs.
    • Cardiovascular effects are similar to those of phenoxybenzamine, referred to as "classical" α-AR blockers.
    • Single administration lasts approximately 4 hours, also producing postural hypotension and adrenaline reversal.
    • It can trigger arrhythmias & anginal pain and is contraindicated in patients with decreased coronary perfusion.
    • It blocks 5-HT receptors, triggers histamine release from mast cells, and blocks K+ conductance
    • Tolazoline is less potent than phentolamine, not marketed but stimulates secretions by salivary, lacrimal, and sweat glands.

    Therapeutic Indications of Phentolamine:

    • Short-term control of hypertension in patients with pheochromocytoma.
    • Treatment of hypertensive crisis induced by sudden clonidine withdrawal or tyramine-containing food ingestion during non-selective MAO inhibitor use.
    • Direct intracavernous injection of phentolamine is investigated for male sexual dysfunction.

    Prazosin, Terazosin, Doxazosin, Tamsulosin, and Alfuzosin:

    • Prazosin is the prototype, containing the piperazinyl quinazoline nucleus.
    • It is a potent, selective, and competitive α1-AR antagonist with over 1000 times more affinity for these receptors than α2-ARs.
    • These agents are more clinically useful and have replaced nonselective α-AR antagonists like phenoxybenzamine and phentolamine.

    Prazosin:

    • The main effects of prazosin result from α1-AR blockade in arterioles and veins, causing decreased peripheral resistance and venous return to the heart.
    • Unlike other vasodilating drugs, prazosin does not increase heart rate or promote NA release (has little α2-AR-blocking effects).
    • It is well absorbed orally with bioavailability between 50-70%.
    • Plasma half-life is approximately 2-3 hours and the duration of action is 7-10 hours for hypertension treatment.
    • It is extensively metabolized and has a half-life of 3-5 hours.

    Therapeutic Indications of Prazosin:

    • Essential hypertension.
    • Congestive heart failure (ACE inhibitors are the preferred treatment).
    • BPH.
    • Other indications with inconclusive evidence:
      • Variant angina due to coronary vasospasms.
      • Aortic valvular insufficiency.
      • Vasospasms in patients with Raynaud’s disease.

    Adverse Effects of Prazosin:

    • The major adverse effect is the first-dose effect: marked postural hypotension, and first-dose syncope, occurring 30 to 90 minutes after the initial dose.
    • Syncope can also occur with a rapid increase in dosage or with the addition of another antihypertensive drug.
    • Non-specific adverse effects include headache, dizziness, and asthenia, which rarely limit treatment.
    • Tamsulosin impairs ejaculation.

    Selective α2-AR blockers:

    • Examples include rauwolscine, yohimbine, idazoxan, and tolazoline.
    • Their clinical role is not established, mostly used in experimental pharmacology like treating type-2 diabetes (α2-AR stimulation inhibits insulin release).
    • Theoretically, they could be useful in autonomic insufficiency by blocking presynaptic α2-ARs.
    • Yohimbine is suggested to improve male sexual function (insufficient data), but PDE5 inhibitors (sildenafil) are preferred.
    • Yohimbine readily crosses the BBB, where it acts to increase BP and heart rate, opposing the actions of clonidine.

    β-Adrenergic Blocking Agents:

    • All clinically available β-blockers are competitive antagonists.
    • Divided into selective and non-selective blockers.
    • Nonselective β-blockers act at both β1- and β2-ARs, while cardioselective blockers primarily block β1-ARs.
    • Valuable in diabetics because carbohydrate metabolism is less affected.

    Labetalol, Bucindolol, Nebivolol, Celiprolol, and Carvedilol:

    • Antagonists of both α- and β-ARs.
    • They reversibly block β- and α1-ARs, producing peripheral vasodilation and lowering BP.
    • Unlike other β-blockers (which cause peripheral vasoconstriction), they are useful in treating hypertensive patients where increased peripheral vascular resistance is undesirable.
    • They do not alter serum lipid or blood glucose levels.

    Clinical Uses of Labetalol:

    • Treating elderly or Black hypertensive patients with undesirable increased peripheral vascular resistance.
    • Alternative to hydralazine for pregnancy-induced hypertension treatment.
    • Used to treat hypertensive emergencies due to its rapid blood pressure reduction.

    Adverse Effects of Labetalol:

    • Orthostatic hypotension and dizziness (associated with α1-blockade).

    Drugs Affecting Neurotransmitter Release or Uptake:

    Reserpine:

    • It blocks the Mg2+/ATP-dependent transport of biogenic amines (NA, D, and serotonin) from the cytoplasm into storage vesicles.
    • This depletes NA levels in the adrenergic neuron because MAO degrades NA in the cytoplasm.
    • Has a slow onset and long duration of action, lasting many days after discontinuation.
    • It is only used for hypertension that doesn’t respond to other treatments.

    Guanethidine:

    • It blocks the release of stored NA, leading to gradual lowering of BP in hypertensive patients and a decrease in cardiac rate.

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