Pharmacology 2: Agonists and Antagonists Flashcards Preview

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Flashcards in Pharmacology 2: Agonists and Antagonists Deck (31):

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?


With respect to the Quantal dose response curve, the bigger the gap _______?

The safer the drug is.


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.


What is the Toxic Ratio?

The Toxic ratio – TD50/ED50

The does that produces a particular side effect in 50% of a population over the dose that produces the specific desired effected in 50% of a population.



What is tolerance?

Tolerance – the same dose of the drug, on repeated administration, produces less effect


What is aspirin?

A non-steroidal anti-inflammatory drug.


What is Tachyphylaxis?

Tachyphylaxis – tolerance which develops very rapidly


What is desensitisation?

Desensitisation – less effect is produced the longer the agonist remains in contact with the receptor.


What are the causes of Desensitisation?

  • Change in receptors (phosphorylation)
  • Down regulation of receptors (number of receptors decreasing in the tissue) due to:
    • Internalization (receptors no longer available)
    • Reduced expression (synthesizing fewer receptors)
  • Depletion of mediators (stores of mediators run out, have to wait for the body to synthesize more)
  • Increased metabolic breakdown – often via adaption of system involved in metabolic breakdown (e.g. alcohol)


Consider the following. Can you answer them all?

  • Define agonist and antagonist
  • Distinguish between affinity, efficacy and potency
  • Define ED50 (quantal vs quantatative)
  • What are the main features of comperative antagonism
  • What is partial antagonism
  • What is a therapeutic ratio



What is the role of pharmacodynamics in pharmacology

  • Pharmacodynamics provides information regarding dosage regimen vs response.
  • Factors affecting pharmacodynamics together with pharmacokinetics are considered when a dose is individualized for special populations such as the elderly.
  • It is a useful tool for new indications, dosages or treatments.


What are things to consider when prescribing drugs?

Prescription considerations include:


few drugs as possible for as short a time as possible (to reduce side effects),

regularly reviewable drug therapy,

provide medication charts,

consider non-compliance.