AIM STEP 10
3rd Year MBBS
Pharmacology
3rd Year MBBS
Pharmacology
Student Memory Support
Pharmacodynamics, Drug Receptors and Dose–Response Relationships
High-yield memory reinforcement for rapid KMU exam revision.
1. High-Yield Flashcards
Tap each question to reveal the answer.
What is pharmacodynamics?
The study of the biochemical and physiological effects of drugs and their mechanisms of action.
How does a full agonist differ from a partial agonist?
A full agonist can produce the maximal response, whereas a partial agonist produces a lower maximal response even with full receptor occupancy.
What is the key action of a pharmacological antagonist at a receptor?
It binds to the receptor but does not activate it and reduces or prevents agonist action.
What distinguishes an inverse agonist from an antagonist?
An inverse agonist reduces constitutive receptor activity, whereas an antagonist blocks agonist action without producing this opposite receptor effect.
What are serpentine receptors?
Seven-transmembrane G-protein-coupled receptors that transmit extracellular signals through intracellular signaling pathways.
What are spare receptors?
Receptors present in excess of the number that must be occupied to produce the maximal response.
What is an orphan receptor?
A receptor for which the endogenous ligand has not yet been identified.
Which important intracellular molecules act as second messengers?
cAMP, cGMP, IP3, DAG and intracellular Ca2+.
What receptor change may follow prolonged agonist exposure?
Receptor down-regulation or desensitization, resulting in a reduced response.
What receptor change may follow prolonged receptor blockade?
Receptor up-regulation, which may increase responsiveness when the blockade is removed.
What does a graded dose–response curve measure?
The magnitude of response produced by increasing drug doses in an individual biological system.
What does a quantal dose–response curve show?
The proportion of a population showing a defined all-or-none response at each dose.
How are potency and efficacy distinguished?
Potency describes the dose required for an effect; efficacy describes the maximum effect a drug can produce.
What is the formula for therapeutic index using toxic dose?
Therapeutic index = TD50 ÷ ED50.
How does reversible competitive antagonism affect an agonist dose–response curve?
It shifts the curve to the right while the maximal response remains achievable.
How does non-competitive antagonism typically affect maximal agonist response?
It reduces the maximal response, which cannot be fully restored simply by increasing agonist concentration.
2. Mnemonics
Mnemonic Title: Major Second Messengers
CID-C
Meaning: CAMP · IP3 · DAG · Ca2+ · with cGMP remembered alongside the cyclic nucleotide messengers.
Mnemonic Title: Core Receptor Ligand Actions
APAI
Meaning: Agonist activates · Partial agonist activates with lower efficacy · Antagonist blocks · Inverse agonist reduces constitutive activity.
Mnemonic Title: Pharmacodynamic Drug Targets
REIT
Meaning: Receptors · Enzymes · Ion channels · Transporters
3. Memory Tables
Potency vs Efficacy
| Feature | Potency | Efficacy |
|---|---|---|
| Meaning | Dose needed for an effect | Maximum effect achievable |
| Curve clue | Horizontal position | Maximum height |
| Greater value | Curve lies further left | Higher Emax |
| Clinical importance | Influences required dose | Determines achievable benefit |
Competitive vs Non-Competitive Antagonism
| Feature | Competitive | Non-Competitive |
|---|---|---|
| Agonist competition | Competes for receptor binding | Not overcome by simple competition |
| More agonist | Can overcome reversible blockade | Cannot fully restore response |
| Curve effect | Rightward shift | Reduced Emax |
| Maximum response | Preserved | Reduced |
4. Rapid Revision Points — Last-Minute Revision
Must Remember:
- Bioassay estimates biological activity by comparing the response of an unknown preparation with a standard.
- Biological assay is particularly valuable when biological activity cannot be adequately predicted by physical or chemical measurement alone.
- Drug targets include receptors, enzymes, ion channels and transporters.
- Spare receptors allow maximal response without occupation of every available receptor.
- Log-dose plotting spreads the dose scale and makes the central portion of a dose–response relationship easier to compare.
- Graded curves compare response magnitude; quantal curves describe defined responses across a population.
- ED50 is the median effective dose; TD50 is the median toxic dose; LD50 is the median lethal dose.
- A narrow therapeutic window means smaller separation between effective and toxic concentrations and therefore requires greater therapeutic caution.
- Drug selectivity depends on preferential action at one target or tissue and may decrease as concentration rises.
- Efficacy is generally more important clinically than potency because it determines the maximum therapeutic response available.
KMU Trap: Do not equate a more potent drug with a more effective drug. A leftward curve indicates greater potency; a higher maximal response indicates greater efficacy.
5. Clinical Memory Hooks
Same maximal effect but a larger agonist dose is required
→
Think reversible competitive antagonism.
→
Think reversible competitive antagonism.
Increasing agonist concentration fails to restore the original maximum response
→
Think non-competitive antagonism.
→
Think non-competitive antagonism.
Repeated receptor stimulation produces progressively less response
→
Receptor desensitization or down-regulation.
→
Receptor desensitization or down-regulation.
Two drugs reach different maximum effects despite increasing doses
→
The difference reflects efficacy rather than potency.
→
The difference reflects efficacy rather than potency.
Effective and toxic dose ranges lie close together
→
Narrow therapeutic window and greater need for careful therapeutic use.
→
Narrow therapeutic window and greater need for careful therapeutic use.
6. Starred High-Yield Exam Points
- ⭐ A partial agonist has lower efficacy than a full agonist even when all available receptors are occupied.
- ⭐ Spare receptors explain how Emax may occur before complete receptor occupancy.
- ⭐ Potency is reflected by the dose required for a given response; efficacy is reflected by Emax.
- ⭐ Quantal dose–response data are used to determine population measures such as ED50 and TD50.
- ⭐ Therapeutic index = TD50/ED50; a wider separation indicates a greater margin between desired and toxic effects.
- ⭐ Reversible competitive antagonism shifts the agonist curve right without reducing Emax.
- ⭐ Non-competitive antagonism reduces Emax and cannot be fully overcome by increasing agonist concentration.