Adrenergic Pharmacology

Correct Answer

B — Pure alpha-1 agonist with no beta-adrenergic activity

Rationale

Phenylephrine is a selective alpha-1 adrenergic agonist with no meaningful activity at beta-1 or beta-2 receptors. This pure alpha-1 profile causes vasoconstriction and raises blood pressure without directly increasing heart rate or cardiac contractility — producing predictable reflex bradycardia from baroreceptor activation. Norepinephrine is the mixed alpha-1 and beta-1 vasopressor used in shock. Clonidine and dexmedetomidine are the centrally acting alpha-2 agonists. Dobutamine is the selective beta-1 inotrope.

Correct Answer

C — Centrally acting alpha-2 agonist used for hypertension and opioid withdrawal

Rationale

Clonidine is an alpha-2 adrenergic receptor agonist that acts centrally in the brainstem to reduce sympathetic outflow, lowering blood pressure, heart rate, and plasma norepinephrine levels. Its clinical applications include hypertension, attention deficit hyperactivity disorder, and — through the same central sympatholytic mechanism — suppression of the autonomic symptoms of opioid withdrawal such as tachycardia, diaphoresis, and anxiety. Phenylephrine is the selective alpha-1 vasopressor. Albuterol is the selective beta-2 bronchodilator. Yohimbine is the alpha-2 antagonist that disinhibits norepinephrine release.

Correct Answer

A — Selective beta-2 agonist used as a short-acting rescue bronchodilator

Rationale

Albuterol is a selective beta-2 adrenergic receptor agonist with onset of action within 5 to 15 minutes and duration of 4 to 6 hours, making it the standard rescue inhaler for acute bronchospasm in asthma and chronic obstructive pulmonary disease. Its selectivity for beta-2 over beta-1 minimizes direct cardiac stimulation compared with non-selective beta agonists such as isoproterenol. Phenylephrine is the alpha-1 nasal decongestant. Isoproterenol and epinephrine are non-selective beta agonists. Dobutamine is the selective beta-1 positive inotrope.

Correct Answer

D — Salmeterol

Rationale

Salmeterol is classified as a long-acting beta-2 adrenergic receptor agonist (LABA) used as maintenance bronchodilator therapy for asthma and chronic obstructive pulmonary disease. Its prolonged duration of action (approximately 12 hours) results from its high lipophilicity and sustained receptor binding. Albuterol and terbutaline are short-acting beta-2 agonists used for rescue bronchodilation. Epinephrine is a non-selective alpha and beta agonist used primarily for anaphylaxis.

Correct Answer

B — Alpha-1 agonist prodrug used for orthostatic hypotension

Rationale

Midodrine is an oral prodrug converted in vivo to its active metabolite desglymidodrine, a selective alpha-1 agonist that constricts arterioles and veins, raising standing blood pressure in patients with orthostatic hypotension from autonomic insufficiency. It is used in combination with fludrocortisone, which expands plasma volume through mineralocorticoid-mediated sodium retention. Dobutamine is the beta-1 positive inotrope. Clonidine and dexmedetomidine are the centrally acting alpha-2 agonists. Terbutaline is the beta-2 tocolytic.

Correct Answer

A — Dexmedetomidine

Rationale

Dexmedetomidine is a highly selective alpha-2 adrenergic receptor agonist used for sedation in the intensive care unit and procedural settings. Its alpha-2A receptor activation in the locus coeruleus produces sedation from which patients can be easily aroused. Midodrine is an alpha-1 agonist prodrug used for orthostatic hypotension. Phenylephrine is a selective alpha-1 agonist used as a vasopressor. Dobutamine is a predominantly beta-1 selective agonist used for positive inotropy in cardiogenic shock.

Correct Answer

C — Alpha-1-mediated vasoconstriction raises blood pressure, activating baroreceptors and triggering compensatory vagal slowing of the sinoatrial node

Rationale

Phenylephrine is a pure alpha-1 agonist with no beta-receptor activity. Its only direct cardiovascular action is vasoconstriction, which raises peripheral vascular resistance and blood pressure. The baroreceptors in the carotid sinus and aortic arch detect this pressure rise and send afferent signals to the brainstem, which increases vagal (parasympathetic) outflow to the sinoatrial node, slowing the heart rate as a homeostatic response. This reflex bradycardia is predictable and distinguishes phenylephrine from epinephrine, which has direct beta-1 cardiac stimulation that offsets the baroreceptor reflex. Phenylephrine has no direct cardiac receptor activity (ruling out direct sinoatrial inhibition or beta-1 blockade) and no alpha-2 autoreceptor activity in this context.

Correct Answer

D — Chronic clonidine use upregulates alpha-2 receptors; abrupt discontinuation removes central sympatholysis, allowing a catecholamine surge that raises both blood pressure and heart rate

Rationale

Clonidine acts centrally to suppress sympathetic outflow. With chronic use, the central nervous system compensates by upregulating alpha-2 receptors and increasing sympathetic tone. When clonidine is abruptly discontinued, the suppression is suddenly removed but the compensatory upregulation remains — the result is a rebound excess of sympathetic activity, with markedly elevated plasma catecholamines that raise both blood pressure and heart rate. This withdrawal syndrome can be severe enough to cause hypertensive emergency with risk of stroke or myocardial infarction, and is managed by restarting clonidine and gradually tapering. Clonidine withdrawal involves excess catecholamines rather than renin inhibition or direct alpha-1 stimulation. The tachycardia indicates the rebound is not limited to beta-1 pathways alone.

Correct Answer

A — Beta-2 receptor activation stimulates sodium-potassium adenosine triphosphatase in skeletal muscle, driving potassium into cells and lowering serum levels

Rationale

Beta-2 adrenergic receptor activation in skeletal muscle raises intracellular cyclic adenosine monophosphate, which stimulates sodium-potassium adenosine triphosphatase (the sodium-potassium pump), accelerating the active transport of potassium into muscle cells. This intracellular shift reduces serum potassium without increasing renal excretion — it is a redistribution rather than a total body depletion. This effect is clinically relevant in high-dose albuterol use for both asthma emergencies and in the treatment of acute hyperkalemia (where albuterol is used as a temporizing measure to shift potassium intracellularly). Increased renal potassium excretion through collecting duct channels is the mechanism of aldosterone, not direct beta-2 agonism. Respiratory alkalosis can shift potassium intracellularly but is a secondary effect at best. Aldosterone is released from the adrenal cortex in response to angiotensin II and potassium, not from direct beta-2 adrenal stimulation.

Correct Answer

B — Long-acting beta-2 agonists provide sustained bronchodilation without treating airway inflammation, masking worsening disease and delaying appropriate management

Rationale

The SMART trial demonstrated that salmeterol monotherapy in asthma was associated with increased asthma-related deaths. The underlying pharmacological explanation is that salmeterol produces bronchodilation by relaxing airway smooth muscle, which relieves symptoms, but has no effect on the eosinophilic airway inflammation that constitutes the pathological substrate of asthma. Patients experience subjective symptomatic relief while the inflammatory disease progresses unchecked. When an acute exacerbation occurs in this setting, it can be more severe and more difficult to manage. Inhaled corticosteroids address the inflammatory component and are mandatory when long-acting beta-2 agonists are used in asthma. Salmeterol has no alpha-1 activity in the airway. Receptor desensitization from salmeterol can reduce rescue bronchodilator efficacy, though the mortality signal in the SMART trial was attributed to disease masking rather than impaired rescue response. Salmeterol and albuterol bind different beta-2 receptor populations because of their different lipophilicity profiles.

Correct Answer

C — Phenylephrine undergoes extensive first-pass metabolism in the intestinal wall and liver, leaving insufficient systemic concentrations to produce nasal vasoconstriction

Rationale

Phenylephrine is a substrate for sulfotransferase enzymes in the intestinal wall and undergoes extensive first-pass metabolism in the liver, such that oral bioavailability is very low — estimated at less than 40% in some studies and as low as 1 to 2% effective systemic delivery in others. The systemically available drug concentration after standard oral doses is insufficient to produce meaningful alpha-1-mediated vasoconstriction in the nasal mucosa. This is why intranasal phenylephrine (bypassing first-pass metabolism) remains an effective decongestant while oral formulations failed the Food and Drug Administration's 2023 efficacy review. Phenylephrine's lipophilicity is not the limiting factor — it does cross mucosal barriers. Monoamine oxidase plays a lesser role than sulfotransferase conjugation in the intestinal wall and liver in phenylephrine metabolism. Renal elimination plays a lesser role than hepatic first-pass metabolism in its pharmacokinetic limitations.

Correct Answer

D — Alpha-2A receptor activation in the locus coeruleus reduces noradrenergic arousal through a pathway that does not suppress medullary respiratory drive

Rationale

Dexmedetomidine produces sedation by activating alpha-2A receptors in the locus coeruleus, which is the brainstem's principal noradrenergic nucleus controlling arousal and wakefulness. Reduced noradrenergic output from the locus coeruleus shifts the patient into a sleep-like state. This pathway does not directly suppress the medullary respiratory centers that control breathing rhythm and carbon dioxide sensitivity — those centers respond to different neurotransmitter systems. By contrast, benzodiazepines potentiate gamma-aminobutyric acid at receptors throughout the brainstem, including in respiratory control centers, and opioids act on mu receptors in the medullary respiratory center — both producing dose-dependent respiratory depression. Dexmedetomidine does not act on peripheral respiratory muscles, gamma-aminobutyric acid receptors, or opioid receptors.

Correct Answer

A — Beta-2 receptor activation raises cyclic adenosine monophosphate, which activates protein kinase A to inhibit myosin light-chain kinase and reduce uterine smooth muscle contractility

Rationale

Uterine myometrium expresses beta-2 adrenergic receptors. Terbutaline activates these receptors, coupling to Gs proteins and activating adenylyl cyclase to increase intracellular cyclic adenosine monophosphate. Elevated cyclic adenosine monophosphate activates protein kinase A, which phosphorylates and inhibits myosin light-chain kinase — the enzyme responsible for initiating smooth muscle contraction. Inhibiting this enzyme reduces contractile force and frequency, producing tocolysis. The Food and Drug Administration black box warning reflects maternal cardiovascular adverse effects (tachycardia, pulmonary edema) and fetal tachycardia associated with prolonged systemic beta-2 agonist exposure. The tocolytic mechanism is specifically beta-2 agonism, not alpha-1 blockade, beta-1 stimulation, or central oxytocin inhibition.

Correct Answer

B — Active metabolite desglymidodrine produces alpha-1-mediated vasoconstriction that raises blood pressure in the supine position, risking supine hypertension during sleep

Rationale

Midodrine is a prodrug converted to desglymidodrine, which acts as a selective alpha-1 agonist on peripheral arterioles and veins. The therapeutic goal is to raise standing blood pressure by increasing venous return and arteriolar resistance when the patient is upright. However, when the patient lies supine, the normal gravitational redistribution of blood volume no longer causes orthostasis, and the same alpha-1 vasoconstriction that is beneficial while standing can produce supine hypertension. Taking midodrine close to bedtime risks having active desglymidodrine concentrations present during recumbency, when blood pressure is already higher than during standing. The bedtime restriction prevents this adverse effect. Midodrine does not act centrally, does not accumulate during sleep through altered elimination, and its vasopressor effect does not involve renal tubular mechanisms.

Correct Answer

D — Upregulated alpha-2 receptors lose central sympatholytic suppression, producing a catecholamine surge with excess sympathetic outflow to both the heart and peripheral vasculature

Rationale

Clonidine suppresses sympathetic outflow by activating alpha-2 receptors in the brainstem. During chronic treatment, compensatory upregulation of alpha-2 receptors and increased noradrenergic activity occurs to maintain homeostasis. When clonidine is abruptly discontinued, this compensatory upregulation is suddenly unmasked — the amplified central sympathetic circuitry fires without restraint, releasing excess catecholamines. The result is simultaneous tachycardia, hypertension, diaphoresis, and anxiety — a presentation resembling pheochromocytoma. Management is restoration of clonidine followed by gradual tapering. The combination of hypertension and tachycardia indicates the rebound involves the full sympathetic axis rather than isolated volume expansion or pure cardiac output increase.

Correct Answer

A — Add an inhaled corticosteroid as controller therapy, because frequent rescue inhaler use indicates persistent airway inflammation that bronchodilation alone does not address

Rationale

Albuterol use more than twice per week is an established clinical threshold indicating inadequate asthma control. The underlying problem is ongoing eosinophilic airway inflammation — the pathological substrate of asthma — for which albuterol provides no treatment. Albuterol relaxes airway smooth muscle through beta-2 agonism, providing symptomatic relief, but the inflammatory process continues unchecked. Adding an inhaled corticosteroid targets the inflammatory component, reducing airway hyperreactivity and the need for rescue bronchodilation. Substituting salmeterol without an inhaled corticosteroid is contraindicated in asthma based on the SMART trial mortality data. Increasing the albuterol dose treats symptoms rather than the underlying disease. Antihistamines address allergic rhinitis symptoms and are not primary asthma controllers.

Correct Answer

C — Beta-2 receptor activation increases cyclic adenosine monophosphate, which inhibits myosin light-chain kinase in uterine smooth muscle, reducing contractile force

Rationale

Terbutaline is a selective beta-2 adrenergic agonist. Uterine myometrium expresses beta-2 receptors coupled to Gs proteins. Terbutaline activates these receptors, stimulating adenylyl cyclase and raising intracellular cyclic adenosine monophosphate. Elevated cyclic adenosine monophosphate activates protein kinase A, which phosphorylates myosin light-chain kinase, reducing its activity. Because myosin light-chain kinase is required to initiate smooth muscle contraction, its inhibition reduces the force and frequency of uterine contractions. This mechanism is the same Gs-cyclic adenosine monophosphate-protein kinase A pathway that produces bronchodilation in airway smooth muscle. The tocolytic effect is achieved through beta-2 receptor-mediated smooth muscle relaxation, not through alpha receptor pathways, not through central oxytocin suppression, and not through myometrial pacemaker inhibition via beta-1 receptors.

Correct Answer

B — The active metabolite desglymidodrine causes alpha-1-mediated vasoconstriction that raises blood pressure in the supine position, and supine hypertension during sleep could cause stroke or myocardial ischemia

Rationale

Midodrine is converted to desglymidodrine, a selective alpha-1 agonist that raises standing blood pressure by increasing peripheral vascular resistance and venous return. This vasoconstriction is therapeutically beneficial while the patient is upright because gravity-dependent blood pooling would otherwise cause orthostatic hypotension. However, when the patient becomes supine, the normal hemodynamic distribution of blood volume shifts centrally, and the ongoing alpha-1 vasoconstriction from desglymidodrine adds a pharmacological pressure increase on top of the recumbent hemodynamics — producing supine hypertension. During sleep, undetected supine hypertension carries risk of hemorrhagic stroke, hypertensive encephalopathy, and myocardial ischemia. The bedtime and recumbency restrictions are designed to prevent active drug levels during sleep. Midodrine does not have central noradrenergic effects, does not alter renal clearance kinetics during sleep, and does not stimulate aldosterone secretion.