General pharmacology and pharmacokinetics: receptor reserve: 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 · General pharmacology and pharmacokinetics: receptor reserve

Sample item

In an isolated smooth muscle preparation, an agonist produces the full maximal contraction while occupying only about 10 percent of the available receptors. The concentration giving half-maximal contraction is far lower than the agonist's dissociation constant. Which concept explains this?

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

The answer and the reasoning

B. Spare receptors, so a maximal response needs only partial occupancy

When a tissue has a receptor reserve, a full response is reached before all receptors are occupied, so the EC50 lies to the left of the Kd. An irreversible antagonist can then remove receptors without lowering the maximum until the reserve is used up. Partial agonists may act as full agonists in tissues with large reserve.

Option by option

  • A. Receptor down-regulation after prolonged exposure

    Down-regulation reduces the number of receptors over time and would raise the EC50, not lower it.

  • B. Spare receptors, so a maximal response needs only partial occupancy (correct)

    Correct. Receptor reserve makes the EC50 lower than the Kd and a maximal effect possible at low occupancy.

  • C. Allosteric potentiation by a second ligand

    Allosteric potentiation needs a second ligand, which the stem does not describe.

  • D. Inverse agonism at constitutively active receptors

    Inverse agonism lowers activity below the baseline level, which is not what is described.

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.

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Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is B, Spare receptors, so a maximal response needs only partial occupancy. When a tissue has a receptor reserve, a full response is reached before all receptors are occupied, so the EC50 lies to the left of the Kd.

Why is "Receptor down-regulation after prolonged exposure" not the answer?

Down-regulation reduces the number of receptors over time and would raise the EC50, not lower it.

Why is "Allosteric potentiation by a second ligand" not the answer?

Allosteric potentiation needs a second ligand, which the stem does not describe.

Why is "Inverse agonism at constitutively active receptors" not the answer?

Inverse agonism lowers activity below the baseline level, which is not what is described.

What concept does this question test?

It tests General pharmacology and pharmacokinetics: receptor reserve in Pharmacology. An irreversible antagonist can then remove receptors without lowering the maximum until the reserve is used up. Partial agonists may act as full agonists in tissues with large reserve.

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 General pharmacology and pharmacokinetics: receptor reserve?

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.