Antagonism

Question 1

What are the three types of antagonism?

Answer 1

Non competitive
Competitive
Irreversible competitive

Question 2

What is non competitive antagonism?

Answer 2

This stops agonists from working by binding some place downstream of agonist binding.
Agonist can be noradrenaline for example.

Important note: the antagonist does NOT compete with the agonist. The antagonist affects proteins downstream of where the agonist binds

Question 3

Describe the example of non competitive antagonism in smooth muscle by the action of antihypertensive drugs:

Answer 3

Calcium must enter the cells in smooth muscle via voltage gated L type channels for a muscle contraction.

Antihypertensive drugs can target these channels and block them. Examples include dihydropyridines and nifedipine. (The benefit of blocking these channels may that they stop muscle contraction in situations such as spasms)

Question 4

How does calcium in a cell cause muscle contraction?

Answer 4

Through entering the cell via L type receptors or through being released from intracellular stores -

These then work on a kinase to cause muscle contraction.

Question 5

What is Competitive antagonists? And what happens if you increase the concentration of agonist?

Answer 5

These bind to the same site as the agonist they are inhibiting

This type of antagonism REDUCES the number of places the agonist can bind to.

The effect of the antagonist can be overcome by increasing the concentration of agonists in comparison to agonists. If we want to limit an antagonists affect we should increase the concentration of agonist.

Question 6

What is the difference between competitive and non competitive antagonists?

Answer 6

Non competitive antagonists dont bind to the same location as the agonist does. It binds DOWNSTREAM

Competitive antagonists = compete with agonists and bind to the SAME location the agonist does.

Question 7

What is reversible competitive antagonism?

Answer 7

When an antagonist binds to the target, but can then unbind after some time

Question 8

What is an irreversible competitive antagonist?

Answer 8

These antagonists bind to receptors and make covalent bonds. They then can’t detach from the antagonist.

They bind to the SAME sight that agonists bind to. However unlike competitive antagonists they don’t unbind!

The effects of these antagonists are cumulative. They decrease the number of spare receptor reserves and agonist can bind to

Question 9

Receptors and maximum responses? And what do irreversible antagonists cause?

Answer 9

If we lose a large number of receptors we can still get a maximum response from receptors . I.e. you get their full action occuring.

Irreversible antagonists cause the maximum response of receptors to decrease overtime, due to the cumulative effects of the antagonist

Question 10

What does the dose reserve graph tell you?
This is the graph you did for liz with several lines and curves which shift right in response to increase in agonist concentration. This tells you maximum response of the agonist and how agonist concentration changes when you add an antagonist. Think of this an agonist vs antagonist graph.

Answer 10

On the graph you have max percentage on the left hand side and concentration of the agonist on the x axis

The lines shift right if the concentration of antagonist increase.

This will mean you need a higher concentration to reach the maximum response from a target. Note the max response stays the same!

Note the first line on the graph is always a control and tells you the normal concentration needed for a max response to occur. This allows for comparisons when finding the dose ratio

Question 11

On the dose reserve graph what type of antagonist does this concern?

Answer 11

Competitive antagonists which bind to the same place the agonist binds to. Increasing the antagonist concentration causes the graph to shift right

This doesnt work with non competitive antagonists as they can’t be outcompeted by agonists. So on the graph you who have a decrease in max response

Question 12

What does the dose ratio tell you?

Note you get this from a dose reserve graph

Answer 12

The dose ratio tells you how many more times an agonist is needed in the presence of an antagonist to get the maximum response on the dose reserve graph.

So note the dose ratio compares agonists and competitive antagonists

Question 13

How do we find the dose ratio?

Answer 13

So remember the dose ratio tells you how many more times an agonist is needed in the presence of an antagonist to get the maximum response on the dose reserve graph.

On the graph you take the ec50 (on the graph half the max response and draw a line across)
You draw a line to each line on the graph from the EC50 showing agonist action on the graph. You then look at the concentration needed for a 50% response from the agonist.

Note the first line on the graph is always a control and tells you the normal concentration needed for a max response to occur.

To find the dose ratio for each line you divide the concentration of agonist needed to produce the maximum response (or the EC50) whilst the competitive antagonist is present / concentration of agonist from the normal line when there is no antagonist presence.

Question 14

Dose ratio equation?

Answer 14

Concentration of agonist in presence of antagonist / concentration of agonist in absence of antagonist.

Question 15

What does the schild analysis tell you?

What does it look at?

Answer 15

It tells you antagonist affinity

It looks at the DOSE RATIO and concentration of antagonist added

Question 16

What does a schild plot tell you?

Note this is the graph you did with liz which has the log10 on the bottom and the log10 (dr-1) on the y axis.

Its the straight line on the graph which doesnt intercept zero.

Answer 16

So the plots tells you the affinity of an antagonist and its effects on a specific response

Note on the graph the plot is log10 (dr-1) and on the bottom is log10 (Antagonist conc)
Remember the schild plot equation = dose ratio = kb+1 / kd

The graph moves the +1 on the right hand side so dose ratio - 1 = kb / kd

Question 17

How to calculate the schild plot?

Answer 17

Here we need the dose ratio
The Kb which is the schild plot of antagonist (this is the x intercept of the graph)
And the KD - antagonist affinity constant

So dose ratio = Kb + 1 / KD

Note on the graph the plot is log10 (dr-1) and on the bottom is log10 (Antagonist conc)
Remember the schild plot equation = dose ratio = kb+1 / kd

The graph moves the +1 on the right hand side so dose ratio - 1 = kb / kd

Note when the dose ratio is one. The KD and the Kb are the same. Thus the x intercept tells of the schild plot tells you the kb (conc of agonist) and the KD antagonist affinity.

Question 18

Example question to work out the affinity of an antagonist using schild plot
We are told the dose ratio is 2, find the affinity of the antagonist which is KD

Answer 18

So the equation:
Dose ratio = kb +1 / KD

2= kb+1 / KD

So 2-1 = Kb/KD

1 / KD = Kb

Note when the dose ratio is one. The KD and the Kb are the same. Thus the x intercept tells of the schild plot tells you the kb (conc of agonist) and the KD antagonist affinity.

Question 19

What scale should the schild plot be?

And what type of line do we ALWAYS get?

Answer 19

You should have you values on the x axis in log form

But the graph should be linear not log!!

You always get a straight line

Question 20

What is pa2? And are they positive or negative?

Answer 20

This is another way of writing KD = antagonist affinity

It is just basically -1 x log KD

These are always positive!

Question 21

How to find KD on a graph from the schild plot?

Answer 21

You need to take the inverse log of the KD ( which is antagonist affinity)

Remember the KD is where the line intercepts the x axis!

Note when the dose ratio is one. The KD and the Kb are the same. Thus the x intercept tells of the schild plot tells you the kb (conc of agonist) and the KD antagonist affinity.

Question 22

Relationship of pa2 to affinity:

Answer 22

The higher the pa2 number is, the higher the affinity

Question 23

How do we see where antagonist activity is and if its working?
How can this help us to identify receptors?

Answer 23

Can look at regions of the brain. So we dont need a radioligand.

In brain experiments an antagonist was observed acting on m1 receptors. Receptors in the gut were also stimulated. This tells you that the gut has m1 receptors.

Question 24

What causes desensitisation and tachyphylaxis?

Answer 24

You see this after repeated stimulation of a receptor by agonists
We see a reduced effect of the agonist
Desenstitisation happens over minutes

Tolerance occurs over hours or days

Question 25

Issue of using an inhaler too often?

Answer 25

Causes receptors to become desensitised

Question 26

What does tolerance occur with?

Answer 26

Drugs such as opiates. Thats why people want more

Question 27

Responses for tolerance and desensitisation:

Answer 27

Receptor itself has changes to the degree that it no longer changes signals
- seen with spice - spices causes phosphorylation of receptors, and you get tolerant to it. The TRPV1 beta adrenoreceptors in particular

May get a loss of receptors:
They may be internalised by endocytosis
Morphine causes loss of receptor from cell surface
May happen with AMPA receptors which sense glutamate in the brain

Exhaustion of mediators is another reason
- we run out of agonists to stimulate the receptors. Seen with amphetamines which produce catecholamines

Changes in metabolic factors
- seen with degredation and exhaustion of drug

Physiological adaptation:
- can see this with thiazide diuretics - this causes homeostatic adaptation of renin - angiotensin system