Adrenergic Pharmacology

Correct Answer

C — Indirect sympathomimetic that causes reverse transport at the dopamine and norepinephrine transporters, disrupts vesicular storage, and inhibits monoamine oxidase

Rationale

Amphetamine increases synaptic monoamine concentrations through three complementary mechanisms. First, it enters the presynaptic terminal via the dopamine transporter and norepinephrine transporter and causes these carriers to run in reverse, pumping dopamine and norepinephrine out of the cytosol into the synapse. Second, it disrupts vesicular monoamine transporter-2 function, releasing stored dopamine and norepinephrine from vesicles into the cytosol where they become available for reverse transport. Third, it inhibits monoamine oxidase intraneuronally, preventing degradation of the cytosolic monoamines. The combined result is a large, action potential-independent efflux of catecholamines. Phenylephrine and epinephrine are direct receptor agonists. Methylphenidate and cocaine are reuptake inhibitors without reverse transport. Clonidine and dexmedetomidine are the centrally acting alpha-2 agonists.

Correct Answer

A — Indirect sympathomimetic that blocks the dopamine transporter and norepinephrine transporter without causing reverse transport, vesicular depletion, or monoamine oxidase inhibition

Rationale

Methylphenidate increases synaptic dopamine and norepinephrine by blocking the dopamine transporter and norepinephrine transporter, preventing reuptake from the synapse back into the presynaptic terminal. This is its only mechanism — methylphenidate does not cause reverse transport, does not disrupt vesicular monoamine transporter-2 to release stored monoamines from vesicles, and does not inhibit monoamine oxidase. The result is a smaller, more physiologically regulated increase in synaptic monoamines compared with amphetamine, which contributes to its lower cardiovascular adverse effect burden and reduced abuse potential at therapeutic doses. Amphetamine uses reverse transport and vesicular disruption as its primary mechanisms. Dopaminergic direct agonists include drugs such as pramipexole used in Parkinson disease. Lisdexamfetamine is the prodrug converted by red blood cell hydrolases to dextroamphetamine.

Correct Answer

D — Reuptake inhibitor at the dopamine, norepinephrine, and serotonin transporters that also blocks voltage-gated sodium channels

Rationale

Cocaine blocks all three monoamine reuptake transporters — the dopamine transporter (producing euphoria through mesolimbic pathway activation), the norepinephrine transporter (producing sympathomimetic cardiovascular effects), and the serotonin transporter. Unlike amphetamine, cocaine does not cause reverse transport or vesicular depletion; it simply prevents existing synaptic monoamines from being cleared. Cocaine also blocks voltage-gated sodium channels in a use-dependent manner, producing local anesthetic properties and — at toxic doses — wide-complex cardiac arrhythmias similar to class I antiarrhythmic toxicity. This dual mechanism makes cocaine pharmacologically unique and explains why sodium bicarbonate is used for its arrhythmic complications. Amphetamine causes reverse transport and vesicular disruption. Reserpine is the irreversible vesicular monoamine transporter-2 inhibitor. Dexmedetomidine is the centrally acting alpha-2 agonist.

Correct Answer

B — Irreversible vesicular monoamine transporter-2 inhibitor that depletes dopamine, norepinephrine, and serotonin in both the central nervous system and peripheral sympathetic neurons

Rationale

Reserpine permanently blocks vesicular monoamine transporter-2, the protein responsible for packaging dopamine, norepinephrine, and serotonin from the cytosol into synaptic vesicles. Without vesicular protection, these monoamines are exposed to intraneuronal monoamine oxidase and are progressively degraded. Reserpine crosses the blood-brain barrier and depletes monoamines in both central nervous system neurons and peripheral sympathetic neurons. Peripheral depletion produces antihypertensive effects through reduced sympathetic tone; central depletion causes the profound depression, sedation, and extrapyramidal effects that led to reserpine's abandonment as an antihypertensive. Because reserpine acts covalently, recovery requires new vesicular monoamine transporter-2 protein synthesis, taking days to weeks. Guanethidine (not reserpine) requires norepinephrine transporter uptake and acts peripherally without central nervous system effects. Prazosin and doxazosin are the selective alpha-1 antagonists. Phenelzine and tranylcypromine are the monoamine oxidase inhibitors.

Correct Answer

C — Peripheral adrenergic neuron blocker that requires norepinephrine transporter uptake into the neuron to deplete peripheral norepinephrine stores without central nervous system effects

Rationale

Guanethidine must be actively transported into peripheral adrenergic nerve terminals by the norepinephrine transporter to exert its pharmacological effect. Once inside the terminal, it accumulates in vesicles and progressively depletes norepinephrine stores, blocking adrenergic neurotransmission at peripheral sympathetic synapses. Because guanethidine does not cross the blood-brain barrier, it produces no central nervous system monoamine depletion and therefore no central nervous system adverse effects such as depression or sedation. Its requirement for norepinephrine transporter-mediated uptake creates a critical drug interaction: any drug that blocks the norepinephrine transporter — tricyclic antidepressants, cocaine, ephedrine — prevents guanethidine from entering the neuron, completely abolishing its antihypertensive effect. Reserpine is the vesicular monoamine transporter-2 inhibitor with both central and peripheral depletion. Clonidine and dexmedetomidine are the centrally acting alpha-2 agonists. Prazosin is the direct alpha-1 postsynaptic antagonist.

Correct Answer

A — Amphetamine prodrug covalently linked to lysine and cleaved by red blood cell hydrolases to release active dextroamphetamine, producing a smoother pharmacokinetic profile with reduced abuse potential

Rationale

Lisdexamfetamine is dextroamphetamine covalently attached to the amino acid lysine, rendering it pharmacologically inert until cleaved. Cleavage occurs by hydrolase enzymes in circulating red blood cells, releasing active dextroamphetamine gradually into the plasma. This prodrug design produces a slower rise in plasma concentration — and consequently in brain drug levels — compared with immediate-release amphetamine formulations. The slower onset dramatically reduces the rewarding pharmacological response that drives abuse because the subjective euphoria associated with stimulant drugs is closely linked to the rate of dopamine rise in the nucleus accumbens rather than to the total amount of dopamine released. Snorting or injecting lisdexamfetamine does not bypass the enzymatic cleavage step and does not produce the rapid high associated with misuse of immediate-release formulations. Methylphenidate prodrugs use different mechanisms. Lisdexamfetamine does not act on dopamine receptors directly and does not inhibit monoamine oxidase enzymes.

Correct Answer

D — Amphetamine causes reverse transport and vesicular depletion in addition to reuptake inhibition, producing a larger and more abrupt non-vesicular monoamine surge than methylphenidate's reuptake block alone

Rationale

The greater adverse effect burden of amphetamine reflects its more extensive mechanism. Amphetamine causes reverse transport — forcing the dopamine transporter and norepinephrine transporter to pump monoamines outward rather than inward — and disrupts vesicular monoamine transporter-2 to release stored vesicular reserves into the cytosol, where they are available for reverse transport. This produces a large, action potential-independent flood of dopamine and norepinephrine into the synapse. Methylphenidate only blocks transporter-mediated reuptake, allowing a smaller, more physiologically regulated increase in synaptic monoamines that depends on normal neuronal firing rather than forced efflux. The magnitude and abruptness of the monoamine surge determines cardiovascular stimulation (tachycardia, hypertension) and central effects (psychosis at high doses). Half-life differences do not explain the adverse effect profile at equivalent therapeutic exposure. Methylphenidate has no meaningful serotonin transporter activity compared with cocaine. Amphetamine does not act through alpha-2 autoreceptor blockade.

Correct Answer

B — Voltage-gated sodium channel blockade by cocaine at toxic concentrations slowing cardiac conduction; treated with intravenous sodium bicarbonate to overcome the channel blockade

Rationale

Cocaine blocks voltage-gated sodium channels in a use-dependent manner — the same mechanism responsible for its local anesthetic properties. At toxic concentrations, this sodium channel blockade affects cardiac myocytes, slowing the rate of phase 0 depolarization and widening the QRS complex, producing wide-complex tachycardia similar to class I antiarrhythmic toxicity or tricyclic antidepressant overdose. Sodium bicarbonate reverses this toxicity through two mechanisms: alkalinization reduces cocaine binding to sodium channels, and the sodium load overcomes the blockade by providing excess sodium ions to compete for channel access — the same mechanism used in tricyclic antidepressant-induced wide-complex arrhythmias. Non-selective beta-blockers are contraindicated in cocaine toxicity because blocking beta-2-mediated vasodilation leaves alpha-1 vasoconstriction unopposed. Serotonin transporter blockade by cocaine does not cause wide-complex tachycardia. Alpha-1-mediated coronary vasospasm produces ischemia and ventricular fibrillation rather than wide-complex tachycardia through sinoatrial node hypoperfusion.

Correct Answer

A — Non-selective beta-2 blockade removes vasodilatory tone while alpha-1 vasoconstriction from excess catecholamines remains active, potentially worsening coronary vasospasm and hypertension

Rationale

Cocaine blocks norepinephrine transporter reuptake, amplifying the sympathomimetic effects of endogenous catecholamines at all receptor subtypes. Circulating catecholamines simultaneously activate alpha-1 receptors (vasoconstriction) and beta-2 receptors (vasodilation in skeletal muscle and coronary arteries), with these effects partially counterbalancing each other. When a non-selective beta-blocker such as propranolol is given, it eliminates beta-2-mediated vasodilation throughout the systemic and coronary vasculature while alpha-1 receptor-mediated vasoconstriction remains fully active and unopposed. The net result is potentially worsened hypertension, increased coronary vascular resistance, and exacerbated coronary vasospasm — directly counterproductive in a patient with cocaine-related acute coronary symptoms. Benzodiazepines and phentolamine are the preferred agents for addressing the adrenergic excess in this setting. Beta-blockers do not interact with cocaine at the dopamine transporter, and propranolol's membrane-stabilizing sodium channel effect is pharmacologically distinct from cocaine's use-dependent blockade.

Correct Answer

C — Profound depression and sedation from central nervous system depletion of dopamine, norepinephrine, and serotonin

Rationale

Reserpine crosses the blood-brain barrier and depletes monoamines in central neurons as effectively as in peripheral sympathetic neurons. Central depletion of dopamine produces extrapyramidal effects from nigrostriatal pathway disruption; depletion of norepinephrine and serotonin in limbic and cortical circuits produces profound depression and sedation. Reserpine-induced depression was historically one of the most common causes of drug-induced major depressive disorder, occurring in a substantial minority of treated patients. This central nervous system toxicity, rather than cardiovascular adverse effects, was the primary reason reserpine was abandoned in modern antihypertensive practice. Guanethidine — not reserpine — causes severe orthostatic hypotension through peripheral adrenergic neuron blockade without crossing the blood-brain barrier. Reserpine withdrawal does not produce rebound hypertensive crisis comparable to clonidine withdrawal because its mechanism is depletion rather than receptor upregulation. Reserpine does not cause direct bradycardia through cardiac ganglia blockade.

Correct Answer

D — Imipramine blocks the norepinephrine transporter, preventing guanethidine from entering adrenergic nerve terminals and abolishing its antihypertensive mechanism

Rationale

Guanethidine's entire mechanism depends on active uptake into adrenergic nerve terminals via the norepinephrine transporter. Once inside the terminal, it accumulates in vesicles and depletes norepinephrine stores. Imipramine is a tricyclic antidepressant that blocks the norepinephrine transporter as part of its antidepressant mechanism — the same transporter that guanethidine requires for neuronal entry. When imipramine occupies the norepinephrine transporter, guanethidine can no longer be taken up into the terminal; it remains in plasma and tissues where it has no pharmacological effect on adrenergic neurotransmission. The result is complete loss of guanethidine's antihypertensive action. This interaction also occurs with cocaine, ephedrine, and amphetamines — all of which block or reverse the norepinephrine transporter. Imipramine does not stimulate norepinephrine release, inhibit guanethidine metabolism through cytochrome P450, or activate postsynaptic alpha-1 receptors.

Correct Answer

B — Phenelzine inhibited intestinal and hepatic monoamine oxidase A, eliminating the first-pass enzymatic barrier that normally prevents absorbed tyramine from reaching systemic circulation

Rationale

Normally, dietary tyramine is extensively metabolized by monoamine oxidase A in the intestinal wall and liver before reaching the systemic circulation — a first-pass protective barrier. Phenelzine, a non-selective irreversible monoamine oxidase inhibitor, eliminates this barrier. Tyramine that would ordinarily be degraded instead reaches adrenergic nerve terminals, where it is taken up by the norepinephrine transporter and triggers massive norepinephrine release, causing the hypertensive crisis. The mechanism is loss of first-pass enzymatic clearance — a single step that explains the drug-food interaction.

Correct Answer

A — Meperidine blocks serotonin reuptake; combined with monoamine oxidase inhibition preventing serotonin degradation, serotonin accumulates in synapses and overstimulates serotonin receptors throughout the nervous system

Rationale

This is serotonin syndrome — a dangerous drug interaction between monoamine oxidase inhibitors and meperidine. Meperidine has serotonin reuptake inhibiting properties in addition to its opioid activity. When combined with a monoamine oxidase inhibitor that prevents serotonin degradation, serotonin accumulates to toxic levels in synapses. The result is overstimulation of serotonin receptors producing the classic triad of altered mental status, neuromuscular hyperactivity (rigidity, clonus), and autonomic instability (hyperthermia, tachycardia). Meperidine is contraindicated in patients taking monoamine oxidase inhibitors for this reason.

Correct Answer

C — Each dose of ephedrine depletes the releasable norepinephrine pool from adrenergic nerve terminals faster than new synthesis can replenish it, progressively attenuating the indirect sympathomimetic response

Rationale

Ephedrine is a mixed-acting sympathomimetic that produces pressor effects through both direct receptor agonism and indirect norepinephrine release from presynaptic terminals. The indirect component depends on the availability of releasable norepinephrine in vesicular stores. Each dose of ephedrine triggers norepinephrine release, and if doses are administered in rapid succession, the released norepinephrine is not replaced by new synthesis fast enough to maintain adequate vesicular stores. With each successive dose, less norepinephrine is available for release, and the indirect pressor response diminishes — a phenomenon called tachyphylaxis. This is a defining property of indirect sympathomimetics and does not occur with direct agonists such as phenylephrine or norepinephrine, which bind receptors directly without depending on presynaptic stores. Ephedrine is given intravenously in anesthesia and does not undergo meaningful first-pass metabolism in this route. Postsynaptic receptor downregulation occurs over hours to days, not minutes. The direct agonist component of ephedrine does not show rapid G-protein desensitization at the clinically relevant timescale.

Correct Answer

B — Phenelzine eliminated the intestinal and hepatic monoamine oxidase A first-pass barrier, allowing dietary tyramine to reach adrenergic nerve terminals, enter via the norepinephrine transporter, and trigger massive norepinephrine release that could not be inactivated by the inhibited monoamine oxidase

Rationale

Aged cheese contains high concentrations of tyramine, formed by bacterial decarboxylation of tyrosine during the aging process. In healthy individuals, monoamine oxidase type A in the intestinal mucosa and liver destroys nearly all ingested tyramine before it reaches the systemic circulation. Phenelzine is a non-selective, irreversible monoamine oxidase inhibitor that eliminates this protective barrier. With monoamine oxidase A inhibited, tyramine is absorbed intact and circulates to peripheral adrenergic nerve terminals, where it enters via the norepinephrine transporter and displaces vesicular norepinephrine stores into the synapse. The released norepinephrine cannot be degraded because intraneuronal monoamine oxidase is also inhibited, and the transporter-mediated reuptake is overwhelmed. The result is a sustained, massive elevation of synaptic norepinephrine producing the described hypertensive crisis. Management involves intravenous phentolamine or nicardipine; non-selective beta-blockers are avoided because they would remove beta-2 vasodilation and worsen hypertension through unopposed alpha-1 vasoconstriction.

Correct Answer

D — Nortriptyline blocks the norepinephrine transporter, preventing guanethidine from being taken up into adrenergic nerve terminals and thereby eliminating the mechanism through which guanethidine depletes norepinephrine stores

Rationale

Guanethidine must enter peripheral adrenergic nerve terminals via the norepinephrine transporter to exert its antihypertensive effect. Once inside, it accumulates in vesicles and progressively depletes norepinephrine stores. Nortriptyline is a tricyclic antidepressant whose antidepressant mechanism involves blocking the norepinephrine transporter to increase synaptic norepinephrine. When nortriptyline occupies the norepinephrine transporter, guanethidine can no longer gain entry to the nerve terminal; it remains extraneuronal where it has no effect on adrenergic neurotransmission. The norepinephrine stores are no longer being depleted, sympathetic transmission to the vasculature is restored, and blood pressure rises back toward pretreatment levels. This interaction applies to all tricyclic antidepressants and explains why guanethidine cannot be combined with this drug class. Nortriptyline does not directly activate alpha-1 receptors as an agonist, does not inhibit cytochrome P450 enzymes to reduce guanethidine concentrations, and does not activate monoamine oxidase.

Correct Answer

A — Propranolol blocked beta-2-mediated vasodilation in skeletal muscle and other vascular beds, leaving alpha-1-mediated vasoconstriction from cocaine-amplified catecholamines unopposed and markedly increasing peripheral vascular resistance

Rationale

Cocaine blocks the norepinephrine transporter, amplifying the effects of endogenous catecholamines at all adrenergic receptor subtypes. In this setting, alpha-1 receptor-mediated vasoconstriction and beta-2 receptor-mediated vasodilation coexist, with the two effects partially balancing each other. Propranolol is a non-selective beta-blocker that eliminates beta-2-mediated vasodilation throughout the peripheral vasculature while leaving the amplified alpha-1 vasoconstriction from cocaine-augmented catecholamines fully active. The loss of the vasodilatory counterbalance produces a sharp rise in peripheral vascular resistance — the same mechanism that makes beta-blockers dangerous in pheochromocytoma when given before alpha blockade. Benzodiazepines are the preferred first-line agents in cocaine toxicity because they reduce central sympathetic outflow without creating this unopposed vasoconstriction. The mechanism is direct receptor pharmacodynamics rather than renin-angiotensin activation, sodium channel interaction, or adrenal catecholamine surges.

Correct Answer

C — Reserpine depleted vesicular norepinephrine stores, abolishing the indirect sympathomimetic mechanism of ephedrine, while phenylephrine acts directly on postsynaptic alpha-1 receptors and is unaffected by presynaptic store depletion; postsynaptic supersensitivity from chronic denervation amplified the phenylephrine response

Rationale

Reserpine's irreversible vesicular monoamine transporter-2 blockade progressively depletes all releasable norepinephrine from presynaptic vesicles. Ephedrine is an indirect sympathomimetic — its pressor effect depends on releasing stored norepinephrine from vesicles into the synapse. With vesicular stores depleted by reserpine, ephedrine has no releasable norepinephrine to displace, and its indirect mechanism produces no pressor response. Phenylephrine, by contrast, is a pure direct alpha-1 agonist that bypasses the presynaptic terminal entirely and binds directly to postsynaptic alpha-1 receptors in the vascular smooth muscle. Direct agonists remain fully effective regardless of presynaptic store status. The exaggerated response to phenylephrine reflects postsynaptic supersensitivity: chronic loss of normal sympathetic norepinephrine input (analogous to denervation) causes upregulation of postsynaptic adrenergic receptors, amplifying the response to direct agonists. This case illustrates the fundamental clinical importance of distinguishing direct from indirect sympathomimetics when choosing vasopressors in patients with depleted catecholamine stores.