Drug-Receptor Interactions

Question 1

Give an example of a group of drugs that act on ion channels.

Answer 1

Local anaesthetics – they block the voltage gated sodium channels of nociceptor neurons to prevent the conduction of pain signals to the CNS

Question 2

Give an example of a drug that acts on transport systems.

Answer 2

Tricyclic antidepressants
Cardiac glycosides – it slows down the Na+/K+ pump thereby increasing the intracellular calcium ion concentration, which leads to an increased force of contraction

Question 3

What are the three ways in which drugs can interact with enzymes?

Answer 3

Enzyme inhibitors (e.g. anticholinesterases) 
False transmitter (e.g. methyldopa) 
Prodrugs (e.g. chloral hydrate)

Question 4

What is a common example of the unwanted effects of drug interaction with enzymes?

Answer 4

Paracetamol overdose – this will saturate the microsomal enzymes in the liver so the paracetamol is then broken down by another set of enzymes (P450) which generates toxic metabolites

Question 5

Name three groups of drugs that are exceptions to the four target site rule.

Answer 5

General anaesthetics – reduce synaptic transmission without interacting with transport systems or receptors

Antacids – these are basic so they simply neutralize some of the stomach acid

Osmotic purgatives – draw water into the bowel due to its physicochemical properties

Question 6

What is selectivity?

Answer 6

Drugs have a preference for binding to certain receptors (it is rarely specific – they normally bind to a few different receptors)

Question 7

What is the difference between full agonists with a high affinity and full agonists with a lower affinity?

Answer 7

Full agonists with a lower affinity can still generate a maximum response but requires a higher dose than the full agonist with lower affinity

Question 8

Describe antagonists in terms of affinity and efficacy.

Answer 8

Antagonists have affinity but NO efficacy

Question 9

What are the two types of antagonist?

Answer 9

Competitive – they bind to the same site as the agonist on the receptor – they are surmountable
Irreversible – could bind to the same site as the agonist but will bind more tightly with covalent forces so that they can’t be moved – some irreversible antagonists will bind to sites different to the site that the agonist binds to – insurmountable

Question 10

What effect do these two types of antagonist have on dose-response curves?

Answer 10

Competitive – shifts the D-R curve to the RIGHT
Irreversible – shifts the D-R curve to the RIGHT and LOWERS the response elicited (it can no longer generate a full response)

Question 11

What is receptor reserve?

Answer 11

In some tissues, not all the receptors need to be stimulated to generate a maximum response (sometimes as little as 1% of receptors may need to be activated)
This increases the sensitivity of the tissue to the agonist

Question 12

True or false: full agonists that are selective for a given receptor will have the same efficacy.

Answer 12

True

They are full agonists so they all elicit a maximum response hence they have the same efficacy