Antagonism

Question 1

What is the definition of an antagonist?

Answer 1

A drug that prevents the response of an agonist

Question 2

What is a chemical antagonist and give an example?

Answer 2

Drug that interacts/ binds to the active component to inhibit its action e.g. adding a chelating such as dimercaprol to lead, mercury and cadmium to reduce toxicity

Question 3

What is a pharmacokinetic antagonist? Give examples

Answer 3

Acts to reduce the concentration of the agonist at the site of action

1) reduce absorbance in Gi tract or increase renal secretion
2) increase metabolism of the drug e.g. some anti-biotics

Question 4

What is a physiological antagonist and give an example?

Answer 4

When 2 drugs work in opposing manners e.g. histamine causes vasodilation but noradrenaline causes increase in blood pressure

Question 5

What is a non-competitive antagonist?

Answer 5

Inhibits the response of an agonist via inhibition of the signalling cascade- is non-competitive as it doesn’t act to compete for receptor occupation

Question 6

What are competitive antagonists and what are the types?

Answer 6

They compete for receptor occupancy against the agonist. They can be reversible (surmountable) with there being a parallel shift in the graph(no change in max response) or irreversible where the drug forms a covalent bond with the receptor- decreases the max response e.g. histamine and dibenamine

Question 7

Why is the schild analysis used and how can a dose response curve help make it?

Answer 7

Is used to measure antagonist affinity. x= log antagonist, y= log(DR-1), X intercept= PA2= the concentration of the antagonist to give a DR of 2= -logKd
DR= Ec50 of drug with antagonist/ Ec50 without

Question 8

What are the 5 things that could cause desensitisation/tachyphylaxis?

Answer 8

1) loss of receptor via endocytosis
2) phosphorylation of receptor
3) increased metabolism or excretion of the drug
4) exhaustion of the mediators
5) physiological adaptation