Adrenergic drugs: vasomotor reversal: 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.

Try it

Pharmacology · Adrenergic drugs: vasomotor reversal

Sample item

In an anaesthetised animal pretreated with phenoxybenzamine, an intravenous bolus of adrenaline is given. Compared with the response before pretreatment, what happens to mean arterial pressure?

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

The answer and the reasoning

C. It falls, because unopposed beta2-mediated vasodilatation in skeletal muscle is unmasked

Adrenaline normally raises blood pressure because alpha1-mediated vasoconstriction outweighs beta2 vasodilatation. After an alpha-blocker the vasoconstrictor component is removed and the beta2 effect dominates, so pressure falls (Dale's vasomotor reversal). Noradrenaline has little beta2 action and so is not reversed.

Option by option

  • A. It rises as before, because adrenaline acts mainly on alpha2 receptors

    The pressor effect of adrenaline is mainly alpha1-mediated, and that is exactly what phenoxybenzamine blocks.

  • B. It rises further, because beta1 receptors are sensitised by alpha blockade

    Beta1 receptors are not sensitised by alpha blockade, and the pressor component is lost, not enhanced.

  • C. It falls, because unopposed beta2-mediated vasodilatation in skeletal muscle is unmasked (correct)

    Correct. With alpha1 receptors blocked, only the beta2 vasodilator action remains.

  • D. It is unchanged, because phenoxybenzamine inactivates circulating adrenaline

    Phenoxybenzamine is a receptor antagonist and does not inactivate adrenaline in plasma.

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.

More Pharmacology questions

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is C, It falls, because unopposed beta2-mediated vasodilatation in skeletal muscle is unmasked. Adrenaline normally raises blood pressure because alpha1-mediated vasoconstriction outweighs beta2 vasodilatation.

Why is "It rises as before, because adrenaline acts mainly on alpha2 receptors" not the answer?

The pressor effect of adrenaline is mainly alpha1-mediated, and that is exactly what phenoxybenzamine blocks.

Why is "It rises further, because beta1 receptors are sensitised by alpha blo…" not the answer?

Beta1 receptors are not sensitised by alpha blockade, and the pressor component is lost, not enhanced.

Why is "It is unchanged, because phenoxybenzamine inactivates circulating adr…" not the answer?

Phenoxybenzamine is a receptor antagonist and does not inactivate adrenaline in plasma.

What concept does this question test?

It tests Adrenergic drugs: vasomotor reversal in Pharmacology. After an alpha-blocker the vasoconstrictor component is removed and the beta2 effect dominates, so pressure falls (Dale's vasomotor reversal). Noradrenaline has little beta2 action and so is not reversed.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: vasomotor reversal?

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.