Adrenergic drugs: noradrenaline versus adrenaline on heart rate: practice question

A Pharmacology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Pharmacology · Adrenergic drugs: noradrenaline versus adrenaline on heart rate

Sample item

In a healthy volunteer, an intravenous infusion of noradrenaline raises mean arterial pressure markedly but the heart rate falls, whereas an adrenaline infusion at a similar dose raises the heart rate. What best explains the fall in heart rate with noradrenaline?

ABlockade of cardiac beta-1 receptors by noradrenaline at high dosesBBeta-2 mediated vasodilation reducing venous returnCBaroreceptor-mediated vagal reflex overriding its direct cardiac beta-1 stimulationDDirect stimulation of cardiac M2 receptors by noradrenaline

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Baroreceptor-mediated vagal reflex overriding its direct cardiac beta-1 stimulation

Noradrenaline strongly stimulates alpha-1 receptors and raises total peripheral resistance, so blood pressure rises sharply. The baroreceptor reflex then increases vagal tone and reduces sympathetic outflow, which outweighs its direct beta-1 chronotropic effect. Adrenaline also stimulates beta-2 receptors, causing vasodilation, so the pressure rise is smaller and tachycardia persists.

Option by option

A. Blockade of cardiac beta-1 receptors by noradrenaline at high doses
Noradrenaline is a beta-1 agonist, not an antagonist, at all doses.
B. Beta-2 mediated vasodilation reducing venous return
Noradrenaline has little beta-2 activity, so beta-2 vasodilation does not explain the bradycardia.
C. Baroreceptor-mediated vagal reflex overriding its direct cardiac beta-1 stimulation (correct)
The reflex vagal slowing driven by the pressure rise overrides the weak direct beta-1 effect.
D. Direct stimulation of cardiac M2 receptors by noradrenaline
Noradrenaline has no agonist action at muscarinic receptors; the slowing is reflex vagal firing, not a direct effect.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is C, Baroreceptor-mediated vagal reflex overriding its direct cardiac beta-1 stimulation. Noradrenaline strongly stimulates alpha-1 receptors and raises total peripheral resistance, so blood pressure rises sharply.

Why is "Blockade of cardiac beta-1 receptors by noradrenaline at high doses" not the answer?

Noradrenaline is a beta-1 agonist, not an antagonist, at all doses.

Why is "Beta-2 mediated vasodilation reducing venous return" not the answer?

Noradrenaline has little beta-2 activity, so beta-2 vasodilation does not explain the bradycardia.

Why is "Direct stimulation of cardiac M2 receptors by noradrenaline" not the answer?

Noradrenaline has no agonist action at muscarinic receptors; the slowing is reflex vagal firing, not a direct effect.

What concept does this question test?

It tests Adrenergic drugs: noradrenaline versus adrenaline on heart rate in Pharmacology. The baroreceptor reflex then increases vagal tone and reduces sympathetic outflow, which outweighs its direct beta-1 chronotropic effect. Adrenaline also stimulates beta-2 receptors, causing vasodilation, so the pressure rise is smaller and tachycardia persists.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Adrenergic drugs: noradrenaline versus adrenaline on heart rate?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.