Block A Lecture 2 - Antagonists and Receptors

Question 1

What are the 3 types of antagonism?

Answer 1

Chemical antagonism
Physiological antagonism
Pharmacological antagonism
(Slide 3)

Question 2

What is chemical antagonism?

Answer 2

When one drug counteracts the action of another by chemically combining with it

Note: I don’t think this needs to be done by an antagonist?
(Slide 3)

Question 3

What is physiological antagonism?

Answer 3

When 2 drugs counteract each other by producing opposing effects on different receptors

Note: This is the work of 2 seperate agonists, and isn’t done by an antagonist
(Slide 3)

Question 4

What is pharmacological antagonism?

Answer 4

When an antagonist binds to a receptor and stops an agonist binding to the same receptor, blocking the agonist from producing its effect
(Slide 3)

Question 5

Does an antagonist have affinity or efficacy?

Answer 5

They have affinity for their receptor but no efficacy
(Slide 3)

Question 6

Antagonists don’t express efficacy and therefore don’t produce a measureable affect. Therefore how do we measure antagonist affinity in the lab?

Answer 6

Usually by constructing an agonist concentration response curve and then adding a concentration of the antagonist and seeing how the effects the curve
(Slide 5)

Question 7

What does a competitive antagonist do to the conentration response curve of a given agonist?

Answer 7

It shifts it to the right in a parallel fashion as antagonist concentration increases.
(Slide 6)

Question 8

How is the agonist response restored after adding a competitive antagonist?

Answer 8

By simply increasing the concentration of the agonist
(Slide 6)

Question 9

Do antagonists obey the law of mass action?

Answer 9

Yes
(Slide 7)

Question 10

Since competitive antagonism is reversible by increasing the concentration of the agonist, what is the effect competitive antagonism said to be?

Answer 10

Surmountable
(Slide 7)

Question 11

What is the gaddum equation?

Answer 11

An extension of the Hill-Langmuir equation used to describe a reaction involving 2 or more competing drugs binding to a single site on a receptor:

Pa = XA /
XA + Ka(1+ XB/KB)

With KA being the dissociation constant for the agonist
KB being the dissociation constant for the antagonist
XA being agonist concentration
XB being antagonist concentration
(Slide 8)

Question 12

What is the rightward shift of an agonist concentration response curve caused by an antagonist denoted by?

Answer 12

ratio (r)
(Slide 9)

Question 13

How is the ratio calculated?

Answer 13

r = A1 / A
Where A is the EC50 value of the original agonist response without the antagonist and
A1 is the EC50 value of the increased concentration of agonist, paired with the antagonist which produces the same level of response as the original response
(Slide 9)

Question 14

If you work through the Gaddum equation what equation do you end up with, that can be used to work out the dissociation constant of the agonist?

Answer 14

The Schild (AKA Gaddum-Schild) equation:

KB = XB / r-1
Where XB is the concentration of the antagonist and r is the ratio that the antagonist shifts the agonist concentration response curve to the right
(Slide 10)

Question 15

What happens in the Gaddum-Schild equation when the ratio equals 2 and what does this mean?

Answer 15

KB = XB / r-1
KB = XB / 2-1
KB = XB
Meaning that the concentration of antagonist which moves the concentration response curve of an agonist to the right by 2 fold gives the KD (the dissociation constant) for the antagonist
(Slides 11 and 12)

Question 16

Do antagonists have the same KD value for every receptor they bind to?

Answer 16

No, they can have different KD values for different receptors, but their KD value will always be the same for a given receptor (e.g Atropines KD value for an M3 receptor is 10^-9M)
(Slide 13)

Question 17

Does the antagonist KD value depend on the agonist used?

Answer 17

No, it is an interaction between the antagonist and receptor and it doesn’t matter which agonist is used as long as it activates the receptor
(Slid 14)

Question 18

What are the 2 things is an antagonist’s rightward shift of an agonist’s concentration response curve dependant on?

Answer 18

The affinity and the concentration of the antagonist
(Slide 16)

Question 19

What is a Schild plot used for?

Answer 19

To plot what happens when you test several, increasing concentrations of an antagonist, also used to determine the KB of an antagonist
(Slides 17 and 18)

Question 20

How can the Schild / Gaddum-Schild equation be expressed logarithmically?

Answer 20

Rearranged to r = XB / KB + 1
(the one is NOT part of the bottom of the fraction, it’s really hard to show this on the flashcards without images)
then:
Log (r-1) = log [XB] - log [KB]
(Slide 18)

Question 21

What is a graph of Log(r-1) plotted against Log[XB] commonly called?

Answer 21

A Schild plot
(Slide 18)

Question 22

How can KB be estimated from a Schild plot?

Answer 22

Arunlakshana and Schild showed that plotting log (R-1) against log [XB] for increasing concentrations of antagonist resulted in a straight line.

Therefore by setting the left side of the Log(r-1) = log XB - log KB to 0 KB can be estimated as the point at which the line crosses the X axis

(as 0 = log XB - log KB simplifies to log[KB] = log[XB])
(Slide 19)

Question 23

Why can KB be estimated from XB at any point of the line for an antagonist which produces a ratio (r) value of 2?

Answer 23

As given the equation
log(r-1) = log[XB] - log[KB]
a value of r=2
gives log(1) = log[XB] - log[KB]
and since log(1) = 0 this is the same as manually setting the left side of the equation to 0, except in this case it applies to any point of the line, as the r value will always be 2 and thus the left side will always equal 0.
(Slide 19)

Question 24

How can the KB value be numerically simplified?

Answer 24

It an be simplified to the term pA2 by expressing the value as a negative log :
pA2 = -log(KB)
Note: This is log base 10 so a kb of 10^-6 M will give a pA2 of 6, a 10^-8 Kb gives a pA2 of 8.

(Slide 20)

Question 25

pA2 values are on a logarithmic scale, why is this important?

Answer 25

As a pA2 value of 8 indicates a 100 x bigger potency of an antagonist which has a pA2 value of 6

Note: A higher pA2 value indicates a MORE potent antagonist
(Slide 20)

Question 26

What is irreversible competitive antagonist?

Answer 26

Where the bond between the antagonist and the receptor is so strong that even increasing concentrations of agonists cannot displace the antagonist
(Slide 21)

Question 27

What is irreversible competitive antagonism usually a result of?

Answer 27

A covalent bond between the antagonist and the receptor
(Slide 21)

Question 28

What is the difference between a parallel and a non-parallel shift?

Answer 28

A parallel shift is when the line (or curve) in a graph shifts to the right (or left) without changing it’s shape due to for example, an antagonist being added.
A non-parallel shift is when this line (or curve) is shifted to the right (or left) but it’s shape also changes.

(Slide 21)

Question 29

Why does irreversible competitive antagonism result in a non-parallel shift of the concentration response curve but competitive antagonism results in a parallel shift?

Answer 29

Irreversible competitive antagonism results in a non-parallel shift as it decreases the maximal efficacy of the agonist, resulting in a permanently decreased maximal response, whereas reversible competitive antagonism doesn’t decrease the maximal efficacy of the agonist as the agonist can still reach its maximum response if a high enough concentration of it is added

(Slide 21)

Question 30

What is an example of irreversible competitive antagonism?

Answer 30

Phenoxybenzamine blocking the α-adrenoreceptor
(Slide 21)

Question 31

Why can irreversible competitive antagonists sometimes produce a substantial righthand shift before maximal response being permanently lowered, resulting in a anti-parallel shift?

Answer 31

As less than 100% of receptors need to be occupied to produce a maximal tissue response due to tissues having “spare receptors”
(Slide 22)

Question 32

What is non-competitive antagonism?

Answer 32

When an antagonist binds to a site other than the active site (known as an allosteric site) and permeantly alters the shape of the receptor, resulting in the agonist being unable to bind to it
(Slide 23)

Question 33

What kind of shift do non-competitive antagonists cause on the concentration response curve of the agonist and why do they do this?

Answer 33

They produce a downwards shift rather than a right hand shift as they only affect efficacy and not potency, only lowering the maximal response the agonist can produce but not the affinity of the agonist.

Note: Don’t look at the graph on this slide, they said in the lecture it was shit / wrong

(Slide 23)

Question 34

What are the 2 types of allosteric modulators and what do they do?

Answer 34

Positive allosteric modulators (PAMs) enhance the effect of the agonist, effectively increasing efficacy or potency

Negative allosteric modulators( NAMs) decrease the effect of the agonist, effectively decreasing efficacy or potency

Both bind to a allosteric site.
(Slide 24)

Question 35

Do positive allosteric modulators activate a receptor on their own?

Answer 35

No, they still need an agonist to bind to the receptor to produce their effect, they only enhance the agonist and do not activate the receptor.
(Slide 24)

Question 36

Are non-competitive antagonists allosteric modulators?

Answer 36

Yes, they are a type of negative allosteric modulators (NAMs)
(Slide 24)

Question 37

Non-competitive antagonists bind to an allosteric site; what site of an enzyme do competitive antagonists bind to?

Answer 37

An orthosteric site (AKA active site, they just seemed to really want us to know it’s also called that)
(Slide 25)

Question 38

What are 2 reasons which allosteric modulators make good drugs?

Answer 38

There are lots of variations in allosteric sites in receptors, so it’s easier to make a selective modulator

They also don’t tend to block the receptor completely - meaning that there is a less likely chance of the patient experiencing a severe side effect - (enough modulation of the receptor to produce the desired therapeutic effect but not too much)
(Slide 27)