Pharmacology 2: Agonists and Antagonists Flashcards
What is an antagonist?
An antagonist is a drug that has affinity for a receptor but no efficacy. Agonists bind to a receptor and inhibit the action of an agonist.
What are the types of antagonism?
There are several types of antagonism:
- Competitive (or surmountable) antagonism
- Non-competitive (or irreversible) antagonism
- Physiological antagonism
What is competitive antagonism?
Competitive antagonism occurs where agonists and antagonists compete for the same receptor sites. Maximal effect remains unchanged (!!!!!) because the antagonist is surmountable (if a high enough dose of agonist is used). Results in a parallel shift of Log-Dose Response Curve to the right,
What is non-competitive antagonism?
Non-competitive antagonism occurs where antagonists bind covalently and irreversibly to the receptor, causing a change in the receptor so that the agonist can no longer bind, effectively removing receptors from the system. As maximum effect can no longer be produced, there is a downward shift of Log-Dose Response Curve (REDUCED MAXIMUM AND SLOPE EXCEPT WHEN THERE ARE SPARE RECEPTORS)
What are spare receptors?
For instance, when you only need 80% of receptors for a maximal response, the remainder are termed spare receptors and can be used to oversome non-competitive antagonism
What is physiological antagonism?
Physiological antagonism occurs where two agonists act on different receptors to produce opposite physiological effects. The drugs have different mechanisms of action. E.g. Caffeine + alcohol, or Histamine (bronchoconstriction) and adrenanline (vasodilator)
Answer the following :)
The effect of an antagonist relies solely on _____?
Blocking the action of an agonist that is already producting a certain effect.
What would happen if you inject atropine into someone who had a slow heart rate?
It would counteract/block the effect of acetylcholine, thus bringing the heart rate back to normality. It wouldn’t do anything if not for the agonist (acetylcholine).
What are partial agonists?
- Full agonists bind to receptors and very efficiently produce a response. Partial agonists are less efficacious than full agonists.
- Never achieve maximal effect
- Hence also act as a competitive antagonist.
When a partial agonist (only) is used, and compared to a full agonist. The maximum is reduced when using the partial agonist (see diagram).
When a partial agonist and a full agonist are added together, the Log-DR curve looks like the one for competitive antagonism
What are inverse antagonists?
Some receptors are constitutionally active (they do stuff) even in the absence of an agonist. Hence, an inverse agonist restores the receptor to its inactive state.
What is potentiation?
Potentiation - describes increasing a drug’s effectiveness. Potentiation usually occurs due to decreased inactivation of an agonist.
What is quantative response?
Quantitative response – is a response that is measure numerically in gradual steps. E.g. blood pressure
What is quantal response?
Quantal response – is a response that is “all or nothing”. E.g. responders vs non-responders
Observe the following quantal dose response curve! Do you understand it?
What is the Therapeutic Ratio?
Therapeutic ratio – LD50/ED50
The dose that kills 50% of a population over the dose that produces the specific desired effect in 50% of a population.
No longer used:
- Unethical
- Doesn’t give information about sub-lethal side effects.
Paracetamol poising 3 times higher than Therapeutic dose.
