Adrenergic Pharmacology

Correct Answer

C — Irreversible non-selective alpha-1 and alpha-2 antagonist used for pheochromocytoma preoperative preparation

Rationale

Phenoxybenzamine forms a covalent bond with alpha-adrenergic receptors, producing irreversible blockade of both alpha-1 and alpha-2 subtypes. Its long duration of action — 24 to 48 hours or more, lasting until new receptors are synthesized — makes it the preferred agent for preoperative preparation in pheochromocytoma, where stable alpha blockade over 1 to 2 weeks is required before surgery. Phentolamine is the reversible competitive alpha-1 and alpha-2 antagonist for acute emergencies. Tamsulosin and silodosin are the selective alpha-1A uroselective agents for benign prostatic hyperplasia. Clonidine and dexmedetomidine are the centrally acting alpha-2 agonists.

Correct Answer

A — Prazosin

Rationale

Prazosin is a selective alpha-1 adrenergic receptor antagonist used for both hypertension and benign prostatic hyperplasia. Its selectivity for alpha-1 receptors over alpha-2 receptors preserves presynaptic alpha-2 autoreceptor function. Phenoxybenzamine is an irreversible non-selective alpha-1 and alpha-2 antagonist. Yohimbine is an alpha-2 antagonist. Tamsulosin is a highly selective alpha-1A antagonist with greater uroselectivity than prazosin, used primarily for benign prostatic hyperplasia.

Correct Answer

D — Selective alpha-1A antagonist used for benign prostatic hyperplasia with minimal blood pressure lowering

Rationale

Tamsulosin has preferential selectivity for alpha-1A receptors, which predominate in the prostate, bladder neck, and urethra. By blocking alpha-1A-mediated smooth muscle contraction in these structures, tamsulosin relieves urinary outflow obstruction in benign prostatic hyperplasia. Its relative sparing of alpha-1B receptors in the vasculature means less blood pressure lowering and less orthostatic hypotension compared with non-selective alpha-1 antagonists — an advantage in older patients who are at risk for falls. Phenoxybenzamine is the irreversible pheochromocytoma agent. Phentolamine is the acute non-selective antagonist for hypertensive crises. Clonidine is the centrally acting alpha-2 agonist.

Correct Answer

B — Phentolamine

Rationale

Phentolamine is a reversible competitive antagonist at both alpha-1 and alpha-2 adrenergic receptors. Its competitive (reversible) binding distinguishes it from phenoxybenzamine, which forms an irreversible covalent bond with alpha receptors. Prazosin is a selective alpha-1 antagonist with no significant alpha-2 activity. Tamsulosin is a highly selective alpha-1A antagonist. Phentolamine's non-selective alpha blockade and reversible kinetics make it useful for acute situations requiring titratable alpha receptor blockade.

Correct Answer

A — Selective alpha-1 antagonist used for hypertension and benign prostatic hyperplasia

Rationale

Doxazosin is a selective alpha-1 antagonist with once-daily dosing that is used for both hypertension and benign prostatic hyperplasia. Like prazosin and terazosin, it blocks all three alpha-1 subtypes without uroselective preference, producing both vascular smooth muscle relaxation (lowering blood pressure) and prostatic smooth muscle relaxation (improving urinary flow). The ALLHAT trial demonstrated that doxazosin was associated with a higher rate of combined cardiovascular events than chlorthalidone, particularly heart failure, which has reduced its use as a primary antihypertensive. Tamsulosin and silodosin are the alpha-1A uroselective agents with minimal vascular effects. Carvedilol is the non-selective alpha and beta antagonist for heart failure. Metoprolol and atenolol are the beta-1 selective agents for atrial fibrillation rate control.

Correct Answer

C — Highly selective alpha-1A antagonist for benign prostatic hyperplasia with the highest degree of uroselectivity among available agents

Rationale

Silodosin has the highest alpha-1A to alpha-1B selectivity ratio of currently available alpha antagonists, producing pronounced urological effects with minimal vascular blood pressure lowering. This uroselective profile is achieved through preferential binding to alpha-1A receptors in the prostate and bladder neck over alpha-1B receptors in vascular smooth muscle. The trade-off for extreme uroselectivity is a high rate of retrograde ejaculation — reported in 28% of patients with silodosin compared with 4 to 11% with tamsulosin — reflecting alpha-1A receptor blockade in the vas deferens and seminal vesicles. Phenoxybenzamine is the irreversible non-selective agent. Prazosin and doxazosin are the non-uroselective alpha-1 antagonists. No alpha-1 and beta-1 combination antagonist exists; carvedilol and labetalol combine alpha-1 and non-selective beta blockade.

Correct Answer

D — Acute alpha-1 blockade causes venous pooling and reduced venous return, lowering blood pressure; alpha-2 autoreceptors remain intact and blunt norepinephrine release, attenuating the expected compensatory tachycardia

Rationale

Selective alpha-1 antagonists such as prazosin spare alpha-2 autoreceptors on presynaptic adrenergic terminals. When alpha-1 blockade causes venous pooling and orthostatic hypotension, the baroreceptor reflex would normally trigger a norepinephrine surge to increase heart rate and cardiac output. However, the preserved alpha-2 autoreceptors provide negative feedback that blunts this norepinephrine release — the compensatory tachycardia is attenuated. This is in contrast to non-selective agents such as phentolamine, which block both alpha-1 and alpha-2 receptors; with alpha-2 blocked, norepinephrine release is disinhibited and reflex tachycardia is pronounced. Prazosin has no beta-1 blocking activity and does not cause vasoconstriction. It does not directly depress the sinoatrial node.

Correct Answer

B — Beta-2-mediated vasodilation in skeletal muscle is blocked, leaving alpha-1-mediated vasoconstriction from excess tumor catecholamines unopposed

Rationale

Pheochromocytoma secretes excess catecholamines, predominantly epinephrine and norepinephrine. Circulating epinephrine activates both alpha-1 receptors (vasoconstriction) and beta-2 receptors (vasodilation in skeletal muscle beds) simultaneously, with the vasodilatory beta-2 component partially counterbalancing the vasoconstrictive alpha-1 component. When a non-selective beta-blocker is given first, it eliminates beta-2-mediated vasodilation while alpha-1-mediated vasoconstriction continues unopposed. The result is a sharp increase in peripheral vascular resistance and a hypertensive crisis. Establishing alpha blockade first prevents this by blocking the alpha-1 vasoconstriction before any beta-blocker is added. Beta-blockers do not stimulate catecholamine release, acutely activate the renin-angiotensin system to levels producing hypertensive crisis, or activate central alpha-2 receptors.

Correct Answer

A — Phenoxybenzamine forms an irreversible covalent bond with the receptor; blockade persists until new receptors are synthesized, regardless of drug plasma concentration

Rationale

Phenoxybenzamine is an alkylating agent that forms a stable covalent bond with the alpha-adrenergic receptor, permanently inactivating it. Recovery of adrenergic responsiveness requires new receptor protein synthesis, which takes 24 to 48 hours or longer. This means the duration of blockade is determined by receptor turnover rate, not by drug plasma levels — phenoxybenzamine can be cleared from plasma while the receptor remains irreversibly blocked. Phentolamine, by contrast, binds competitively and reversibly; its blockade dissipates as the drug is cleared and catecholamines compete for receptor access. The key distinction is the covalent versus non-covalent nature of receptor binding, not differences in plasma half-life or lipid distribution.

Correct Answer

C — Alpha-1A blockade causes atrophy and fibrosis of the iris dilator muscle that persists indefinitely after drug discontinuation, preventing adequate pupil dilation during surgery

Rationale

The iris dilator muscle depends on alpha-1A receptor-mediated sympathetic stimulation to maintain its tone and structural integrity. Prolonged alpha-1A blockade from tamsulosin or silodosin leads to smooth muscle atrophy and fibrosis within the dilator muscle itself. This structural change — not ongoing pharmacological blockade — is why the risk of intraoperative floppy iris syndrome persists indefinitely after drug discontinuation. During cataract surgery, the floppy iris billows and prolapses in response to irrigation fluid, and the pupil constricts progressively despite preoperative mydriatic drops. Surgeons who know about prior alpha-1A antagonist use can prepare with pharmacological and mechanical pupil expansion strategies. The mechanism involves muscle atrophy rather than lens storage, direct damage, or metabolite accumulation.

Correct Answer

D — Phentolamine competitively blocks alpha-1 receptors in local arteriolar smooth muscle, reversing the vasoconstriction caused by extravasated norepinephrine

Rationale

Norepinephrine extravasation causes intense local alpha-1 receptor-mediated vasoconstriction in the surrounding tissue, reducing blood flow and producing ischemia that can lead to skin necrosis. Subcutaneous infiltration of phentolamine into the affected area competitively blocks alpha-1 receptors in local arteriolar smooth muscle, reversing the vasoconstriction and restoring tissue perfusion. Phentolamine must be administered within 12 hours for maximum efficacy. The mechanism is direct receptor competition at the local tissue level, not chemical neutralization, beta-2 receptor stimulation, or presynaptic autoreceptor activation.

Correct Answer

B — Doxazosin was associated with a higher rate of combined cardiovascular events, particularly heart failure, compared with chlorthalidone, despite similar blood pressure reduction

Rationale

The ALLHAT trial found that patients randomized to doxazosin had a 25% higher rate of combined cardiovascular events compared with those on chlorthalidone, driven largely by a doubling of the rate of heart failure. Blood pressure reduction was similar between the two arms, indicating the difference was not explained by antihypertensive efficacy alone. The mechanism likely relates to the favorable neurohormonal and hemodynamic properties of thiazide diuretics beyond blood pressure lowering. As a result, alpha-1 blockers such as doxazosin are no longer recommended as first-line monotherapy for hypertension in patients without other specific indications such as benign prostatic hyperplasia. The ALLHAT finding was specifically related to heart failure events, not myocardial infarction rates, stroke, or nephrotoxicity.

Correct Answer

A — Take the first dose at bedtime, start at 1 mg, and avoid phosphodiesterase type 5 inhibitors within 4 to 6 hours of dosing

Rationale

The first-dose phenomenon occurs because sudden alpha-1 receptor blockade causes venous pooling in the upright position, reducing venous return and cardiac output, and producing orthostatic hypotension. Three strategies mitigate this risk. Taking the first dose at bedtime means the patient remains supine during the period of peak drug effect, eliminating the orthostatic component. Starting at the lowest available dose (1 mg) limits the magnitude of alpha-1 blockade during the vulnerable first-dose period. Avoiding phosphodiesterase type 5 inhibitors such as sildenafil and tadalafil within 4 to 6 hours prevents additive vasodilation that could compound the hypotensive effect. Morning dosing while upright risks syncope. Higher starting doses increase the probability of a severe hypotensive event. Combination with calcium channel blockers adds vasodilatory risk without specific first-dose protection.

Correct Answer

C — Alpha-1 blockade in the brain reduces noradrenergic signaling during rapid eye movement sleep, attenuating the central autonomic arousal that contributes to trauma nightmares

Rationale

Post-traumatic stress disorder nightmares are associated with heightened central noradrenergic activity during rapid eye movement sleep, when the locus coeruleus is active and trauma memories undergo fearful replay. Prazosin, which is sufficiently lipophilic to cross the blood-brain barrier, blocks alpha-1 receptors in the prefrontal cortex, limbic system, and other brain regions that mediate the noradrenergic arousal response. By attenuating central alpha-1-mediated noradrenergic signaling during rapid eye movement sleep, prazosin reduces the intensity and frequency of nightmares. This is a central nervous system mechanism rather than a peripheral vascular one. Prazosin does not potentiate gamma-aminobutyric acid, does not block beta-1 receptors in the locus coeruleus, and its nightmare-reducing effect is not explained by peripheral blood pressure lowering below an arousal threshold.

Correct Answer

B — Begin phenoxybenzamine to establish irreversible alpha blockade first, then add a beta-blocker only after adequate alpha blockade is confirmed, to prevent unopposed alpha-1 vasoconstriction

Rationale

The alpha-before-beta rule is absolute in pheochromocytoma preoperative management. Starting phenoxybenzamine first blocks alpha-1 receptors irreversibly, preventing the catecholamine-mediated vasoconstriction that causes hypertensive crises during tumor manipulation. Once adequate alpha blockade is established over one to two weeks — confirmed by resolution of hypertension and development of mild orthostatic hypotension indicating effective receptor blockade — a beta-blocker is added to control the reflex tachycardia that follows alpha-1 blockade. If a beta-blocker is given first, beta-2-mediated vasodilation in skeletal muscle beds is removed while alpha-1 vasoconstriction from excess catecholamines remains fully active, precipitating a hypertensive crisis. Simultaneous initiation risks this same complication if alpha blockade is inadequate when beta blockade begins. Phentolamine has too short a duration for weeks-long preoperative preparation.

Correct Answer

D — Selective alpha-1 blockade by prazosin preserves presynaptic alpha-2 autoreceptors, which limit the norepinephrine release that would otherwise produce compensatory tachycardia

Rationale

Prazosin selectively blocks postsynaptic alpha-1 receptors while leaving presynaptic alpha-2 autoreceptors intact. When the baroreceptor reflex is activated by the orthostatic hypotension, it signals for increased sympathetic norepinephrine release. However, as norepinephrine is released into the synapse, it binds the intact presynaptic alpha-2 autoreceptors, which provide negative feedback that limits further norepinephrine release. This preserved autoreceptor inhibition attenuates the catecholamine surge that would produce compensatory tachycardia. In contrast, non-selective alpha-1 and alpha-2 antagonists block the autoreceptor and disinhibit norepinephrine release, producing pronounced reflex tachycardia. Prazosin has no beta-1 receptor blocking or muscarinic receptor activity, and the scenario does not suggest underlying conduction disease.

Correct Answer

A — Prolonged alpha-1A blockade caused atrophy of the iris dilator muscle that persists indefinitely, predisposing to intraoperative floppy iris syndrome regardless of when the drug was discontinued

Rationale

The iris dilator muscle requires tonic alpha-1A-mediated sympathetic stimulation for both function and structural maintenance. Prolonged pharmacological blockade of alpha-1A receptors causes the dilator muscle to undergo smooth muscle atrophy and fibrosis — a structural rather than pharmacological change. This structural deterioration persists indefinitely after drug discontinuation because the muscle atrophy does not reverse when alpha-1A tone is restored. During cataract surgery, the atrophied iris dilator cannot maintain pupil dilation and the flaccid iris billows in response to irrigation currents, prolapses toward the phacoemulsification probe, and constricts progressively — the triad defining intraoperative floppy iris syndrome. Surgeons must be informed regardless of how long ago tamsulosin was stopped. The mechanism is structural muscle atrophy, not metabolite storage, receptor upregulation, or inflammation.

Correct Answer

C — Alpha-1 blockade relaxes smooth muscle in both peripheral arteriolar walls (reducing vascular resistance and blood pressure) and the prostate and bladder neck (relieving urinary outflow obstruction)

Rationale

Alpha-1 adrenergic receptors are expressed on smooth muscle in multiple locations. In peripheral arterioles, alpha-1 receptor activation maintains vascular tone; blockade reduces peripheral vascular resistance and lowers blood pressure. In the prostate, bladder neck, and urethra, alpha-1 receptor activation contributes to smooth muscle contraction that obstructs urinary outflow; blockade relaxes this smooth muscle and improves urinary flow. A non-uroselective alpha-1 antagonist such as doxazosin or terazosin exploits both tissue locations simultaneously, making it an efficient choice when both conditions require treatment. This single receptor mechanism at two anatomical sites is what distinguishes alpha-1 antagonists from agents that address only one of the two conditions. Beta-1 blockade does not relax prostatic smooth muscle. Alpha-2 agonism reduces sympathetic outflow but does not act directly on prostatic alpha-1 receptors at therapeutic doses. Finasteride reduces prostate volume gradually through androgen suppression but has no antihypertensive mechanism.