Wattle Pharmacy Prep

Receptor pharmacodynamics
Practice questions with explanations

Receptor pharmacodynamics belongs to Pharmacokinetics and pharmacodynamics, which makes up about 10% of the Overseas Pharmacist Readiness Assessment (OPRA®). Answer each question yourself before opening the explanation.

1. Which mechanism explains tolerance to a drug developing over days to weeks?

  1. Receptor downregulation or desensitisation, and sometimes induction of the enzymes that clear the drug
  2. Improved absorption over time
  3. Increased receptor numbers with repeated exposure
  4. The drug becoming chemically inactive in the bottle
Show answer

A. Receptor downregulation or desensitisation, and sometimes induction of the enzymes that clear the drug

Persistent stimulation leads cells to internalise receptors, uncouple them from their signalling machinery, or reduce their expression, so the same concentration produces less effect. Pharmacokinetic tolerance from enzyme induction, as with carbamazepine's autoinduction, adds to it. The mirror image is upregulation during chronic blockade, which produces rebound when a beta-blocker or clonidine is stopped abruptly.

2. How does an allosteric modulator differ from an orthosteric agonist?

  1. It permanently destroys the receptor
  2. It acts only on nuclear receptors
  3. It binds the same site as the natural ligand
  4. It binds a site distinct from the natural ligand's and changes the receptor's response to that ligand rather than activating it directly
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D. It binds a site distinct from the natural ligand's and changes the receptor's response to that ligand rather than activating it directly

Benzodiazepines are the classic positive allosteric modulators: they cannot open the GABA-A channel themselves but increase how often GABA does, so their effect has a ceiling set by endogenous transmitter. That dependence is precisely what makes them safer in overdose than barbiturates, which at high concentration open the channel directly.

3. Which statement about EC50 is correct?

  1. It is the concentration producing half the maximal effect and is an index of potency
  2. It is the concentration producing a toxic effect in half of patients
  3. It is the maximum effect achievable
  4. It measures how long the effect lasts
Show answer

A. It is the concentration producing half the maximal effect and is an index of potency

EC50 locates the curve on the concentration axis: a lower value means greater potency, meaning less drug is needed for the same effect. It says nothing about the height of the curve, which is efficacy. Potency matters for choosing a dose; efficacy matters for choosing a drug, which is why a highly potent drug with a low ceiling may be the wrong choice for severe symptoms.

These are 3 of the 20 questions on receptor pharmacodynamics in the app.

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More topics in Pharmacokinetics and pharmacodynamics

Absorption and bioavailability · Distribution and protein binding · Excretion and clearance · Metabolism and cytochrome P450 · Pharmacokinetic calculations · Dosing in special populations