Adrenergic Pharmacology

Correct Answer

B — Phenylephrine

Rationale

Phenylephrine is a selective alpha-1 adrenergic receptor agonist. It acts on alpha-1 receptors in vascular smooth muscle to produce vasoconstriction and raise blood pressure. Clonidine is an alpha-2 agonist that acts centrally to reduce sympathetic outflow. Propranolol is a non-selective beta-adrenergic antagonist. Prazosin is an alpha-1 antagonist used for hypertension and benign prostatic hyperplasia.

Correct Answer

D — Prazosin

Rationale

Prazosin is a selective alpha-1 adrenergic receptor antagonist used for hypertension and benign prostatic hyperplasia. It blocks alpha-1 receptors in vascular smooth muscle and the prostate, reducing peripheral vascular resistance and urinary outflow obstruction. Dobutamine is a beta-1 selective agonist. Yohimbine is an alpha-2 antagonist that disinhibits norepinephrine release. Clonidine is an alpha-2 agonist that reduces central sympathetic outflow.

Correct Answer

A — Clonidine

Rationale

Clonidine is an alpha-2 adrenergic receptor agonist. It acts on alpha-2 receptors in the locus coeruleus and presynaptic adrenergic terminals to reduce central sympathetic outflow and inhibit norepinephrine release. Phenylephrine is an alpha-1 agonist. Albuterol is a beta-2 agonist used as a bronchodilator. Metoprolol is a selective beta-1 antagonist.

Correct Answer

C — Dobutamine

Rationale

Dobutamine is a predominantly selective beta-1 adrenergic receptor agonist used for its positive inotropic effect in cardiogenic shock and acute decompensated heart failure. It increases cardiac contractility with relatively modest effects on heart rate and vascular tone. Propranolol is a non-selective beta antagonist. Albuterol is a selective beta-2 agonist used for bronchodilation. Phenylephrine is a selective alpha-1 agonist.

Correct Answer

A — Propranolol

Rationale

Propranolol is a non-selective beta-adrenergic receptor antagonist, blocking both beta-1 and beta-2 receptors with equal affinity. This distinguishes it from cardioselective agents such as metoprolol, which preferentially block beta-1 receptors at standard doses. Prazosin is an alpha-1 antagonist. Clonidine is an alpha-2 agonist.

Correct Answer

D — Yohimbine

Rationale

Yohimbine is an alpha-2 adrenergic receptor antagonist. By blocking presynaptic alpha-2 autoreceptors, it removes the inhibitory feedback on norepinephrine release, increasing sympathetic tone and raising blood pressure. Prazosin is an alpha-1 antagonist. Phenylephrine is an alpha-1 agonist. Propranolol is a non-selective beta antagonist.

Correct Answer

C — Gq protein activation of phospholipase C, generating inositol trisphosphate and diacylglycerol

Rationale

Alpha-1 receptors signal through Gq proteins, which activate phospholipase C. This enzyme cleaves phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate and diacylglycerol. Inositol trisphosphate triggers calcium release from the endoplasmic reticulum, and diacylglycerol activates protein kinase C — together producing smooth muscle contraction and vasoconstriction. Gs-coupled signaling (option A) is used by beta-1 and beta-2 receptors. Gi-coupled inhibition of adenylyl cyclase (option B) is the pathway for alpha-2 receptors. Potassium channel activation (option D) is not a primary adrenergic receptor mechanism.

Correct Answer

A — Increased cyclic adenosine monophosphate activating protein kinase A, which phosphorylates calcium channels and contractile proteins

Rationale

Beta-1 receptors couple to Gs proteins, which activate adenylyl cyclase to increase cyclic adenosine monophosphate. Elevated cyclic adenosine monophosphate activates protein kinase A, which phosphorylates L-type calcium channels (increasing calcium influx and contractility) and phospholamban (increasing sarcoplasmic reticulum calcium uptake and release speed), producing the positive chronotropic and inotropic effects. Inositol trisphosphate signaling (option B) is the alpha-1 pathway. Direct myosin light-chain kinase activation (option C) occurs through smooth muscle calcium-calmodulin signaling, not beta-1 receptor activation. Decreased cyclic adenosine monophosphate (option D) is the consequence of Gi-coupled alpha-2 receptor activation, which has the opposite effect.

Correct Answer

B — Inhibit further norepinephrine release when synaptic concentrations are elevated

Rationale

Presynaptic alpha-2 receptors serve as autoreceptors — they sense the concentration of norepinephrine in the synapse and, when activated, inhibit further release. This is a negative feedback mechanism: as norepinephrine builds up in the cleft, it binds presynaptic alpha-2 receptors, reducing vesicle exocytosis and preventing excessive sympathetic stimulation. The signaling pathway is Gi-mediated inhibition of adenylyl cyclase, decreasing cyclic adenosine monophosphate. Stimulating further release (option A) is the opposite of the autoreceptor function. Cyclic adenosine monophosphate elevation (option C) would increase, not decrease, vesicle fusion. Norepinephrine reuptake (option D) is the function of the norepinephrine transporter, not the alpha-2 receptor.

Correct Answer

D — Gs-mediated adenylyl cyclase activation, increasing cyclic adenosine monophosphate and reducing smooth muscle tone

Rationale

Beta-2 receptors couple to Gs proteins, which activate adenylyl cyclase, raising intracellular cyclic adenosine monophosphate. Elevated cyclic adenosine monophosphate activates protein kinase A, which phosphorylates myosin light-chain kinase (reducing its activity) and promotes calcium reuptake, resulting in smooth muscle relaxation and bronchodilation. Phospholipase C activation with increased calcium (option A) causes smooth muscle contraction — the opposite effect and the alpha-1 pathway. Inhibition of adenylyl cyclase (option B) is the alpha-2 Gi pathway, not the beta-2 pathway. Direct muscarinic receptor blockade (option C) is the mechanism of anticholinergic bronchodilators such as ipratropium, not beta-2 agonists.

Correct Answer

B — Predominant beta-2 receptor activation in skeletal muscle vasculature causing vasodilation, while beta-1 activation increases cardiac output

Rationale

At low doses, epinephrine preferentially activates the higher-affinity beta receptors before alpha-1 receptors are meaningfully engaged. Beta-2 activation in skeletal muscle vasculature causes vasodilation, lowering peripheral vascular resistance and diastolic blood pressure. Simultaneously, beta-1 activation increases heart rate and contractility, raising systolic blood pressure. The net result is a widened pulse pressure with a decreased or unchanged mean arterial pressure. At high doses, alpha-1 receptor activation dominates, overriding beta-2 vasodilation and raising both systolic and diastolic pressure. Option A describes the high-dose pattern. Option C describes the response to pure alpha-1 agonists such as phenylephrine, not low-dose epinephrine. Option D is not a primary mechanism of the hemodynamic response to epinephrine.

Correct Answer

A — Norepinephrine produces greater alpha-1-mediated blood pressure elevation, triggering baroreceptor-mediated vagal bradycardia, whereas epinephrine's beta-1 cardiac stimulation and beta-2 vasodilation prevent this reflex

Rationale

Norepinephrine has strong alpha-1 activity but minimal beta-2 activity. The resulting vasoconstriction markedly raises blood pressure, activating baroreceptors that trigger compensatory vagal (parasympathetic) slowing of the sinoatrial node — reflex bradycardia. Epinephrine acts on all three receptor subtypes: its beta-1 activity directly increases heart rate, and its beta-2 activity vasodilates skeletal muscle beds, blunting the blood pressure rise and preventing the baroreceptor reflex from overriding the direct cardiac stimulation. Norepinephrine does not directly activate muscarinic receptors (option B). Half-life comparisons (option C) do not explain the opposite direction of heart rate responses. Norepinephrine has weaker, not stronger, beta-2 affinity compared with epinephrine (option D).

Correct Answer

C — Downregulation of beta-2 receptors in response to sustained agonist exposure

Rationale

Prolonged exposure to a receptor agonist triggers receptor downregulation: the cell reduces the number of surface receptors available for activation, either through internalization or decreased synthesis. This is a general principle of adrenergic pharmacology — chronic agonist stimulation leads to reduced receptor density and diminished response, a form of pharmacological tolerance. This explains why continuous or high-frequency beta-2 agonist use can lead to loss of bronchodilatory efficacy over time, underscoring the importance of using short-acting beta agonists only as needed rather than continuously. Competitive metabolite blockade (option A) is not a recognized mechanism for albuterol. Adenylyl cyclase upregulation (option B) does not accurately describe cellular adaptation to chronic beta agonism. Albuterol is not a substrate for monoamine oxidase (option D).

Correct Answer

D — Detrusor relaxation, increasing bladder capacity and reducing urgency

Rationale

Beta-3 receptors in the bladder detrusor muscle, when activated, produce smooth muscle relaxation through Gs-mediated cyclic adenosine monophosphate elevation. This increases bladder storage capacity and reduces urinary urgency. Mirabegron is a selective beta-3 agonist approved for overactive bladder that exploits this mechanism. Detrusor contraction (option A) is produced by muscarinic receptor activation, not beta-3 stimulation. Internal urethral sphincter contraction (option B) is mediated by alpha-1 receptor activation. Beta-3 receptors are not linked to parasympathetic tone (option C); they are part of the sympathetic system.

Correct Answer

C — Beta-2-mediated vasodilation was blocked, leaving alpha-1-mediated vasoconstriction from excess catecholamines unopposed

Rationale

In pheochromocytoma, the tumor secretes large amounts of catecholamines continuously. Under normal circumstances, circulating epinephrine activates both alpha-1 receptors (vasoconstriction) and beta-2 receptors (vasodilation in skeletal muscle beds), with the two effects partially counterbalancing each other. When a non-selective beta-blocker is given first, it eliminates beta-2-mediated vasodilation while alpha-1 vasoconstriction remains fully active and unopposed, causing a sharp rise in peripheral vascular resistance and a hypertensive crisis. This is why alpha blockade must always be established before beta blockade in pheochromocytoma. Beta-blockers do not directly stimulate alpha receptors (option A) or inhibit norepinephrine reuptake (option B). Beta-1 blockade in the kidney actually reduces renin release (option D), which would lower, not raise, blood pressure.

Correct Answer

A — Alpha-1 receptor blockade relaxes smooth muscle in both arteriolar walls and the prostate and bladder neck

Rationale

Alpha-1 receptors are expressed on smooth muscle in both peripheral arterioles and the prostate and bladder neck. A selective alpha-1 blocker such as doxazosin or terazosin relaxes arteriolar smooth muscle, reducing peripheral vascular resistance and lowering blood pressure, while simultaneously relaxing prostatic and bladder neck smooth muscle, relieving outflow obstruction and improving urinary flow. This dual benefit from a single receptor makes alpha-1 antagonists particularly useful in older men with both conditions. Beta-1 blockade (option B) reduces cardiac output but does not relax the prostate. Alpha-2 agonism (option C) reduces central sympathetic drive but does not directly act on prostatic smooth muscle at therapeutic doses. Finasteride inhibits 5-alpha-reductase (option D) to shrink the prostate over months but has no direct antihypertensive effect.

Correct Answer

D — Alpha-1-mediated vasoconstriction raised blood pressure, triggering baroreceptor-mediated vagal slowing of the sinoatrial node

Rationale

Phenylephrine is a pure alpha-1 agonist with no beta-receptor activity. By constricting peripheral arterioles, it raises blood pressure, which is sensed by baroreceptors in the carotid sinus and aortic arch. These baroreceptors send signals through the nucleus tractus solitarius to increase parasympathetic (vagal) tone to the sinoatrial node, slowing heart rate as a reflex response to the elevated pressure. This reflex bradycardia is predictable and expected with pure alpha-1 agonists — unlike epinephrine, phenylephrine has no direct beta-1 cardiac stimulation to offset the baroreceptor reflex. Phenylephrine has no beta-1 activity (option A) and does not block muscarinic receptors (option B). Alpha-2 autoreceptor stimulation (option C) reduces norepinephrine release but does not explain the acute heart rate drop seen with a pure alpha-1 agonist.

Correct Answer

B — Activation of alpha-2 receptors in the brainstem, reducing sympathetic outflow to the heart and vasculature

Rationale

Clonidine is a centrally acting alpha-2 agonist. It crosses the blood-brain barrier and acts on alpha-2 receptors in the locus coeruleus and nucleus tractus solitarius in the brainstem. Activation of these receptors reduces the firing of sympathetic preganglionic neurons, decreasing the efferent sympathetic drive to both the heart (lowering heart rate and cardiac output) and the peripheral vasculature (reducing vascular tone and blood pressure). This central mechanism distinguishes clonidine from peripheral alpha-1 antagonists such as prazosin. Alpha-1 receptor blockade (option A) is the mechanism of prazosin and doxazosin, not clonidine. Beta-1 blockade (option C) is the mechanism of beta-adrenergic antagonists. Clonidine acts on alpha-2, not alpha-1 receptors (option D), and the antihypertensive mechanism is central, not peripheral.