What does the occupancy theory predict?
That for a full agonist, the concentration to produce 80% of the max response is 16x greater than the conc to produce 20% response.
The ability of the drug to produce an element of response when bound to each receptor.
Size of the stimulus was thought to depend on two factors?
- Agonist fractional occupancy
- Agonist efficacy
High efficacy agonist?
Produce maximal responses when occupying a low proportion of the receptors
Lower efficacy agonist?
Cannot activate receptors to the same degree.
They may not even porduec the same maximal response even when occupying all the receptors.
Name the 5 types of ligands?
- Super agonist: E>100
- Full agonist: E=100
Parital agonist: O
- Silent antagonist: E=0
- Inverse agonist: E<0
What is the equation to determine the size of the stimulus?
How to derive it?
See additional sheet 3
Graphical representation of the agonist concentration response curve for Stephenson's analysis
How is the number of stimulus units determined to give maximum response?
Obtained by stephenson graph.
Looking at the conc-response relationship for the agonist
Why does the number of stimulus unit vary for the same drug?
From one ileum to the next
Each have a slightly different proportions of spare receptors making up the total receptor population.
Additionally because of the different classes of receptors.
What is the universal number of stimulus units that give the maximum response?
10 units of stimulus
What did the Ariens model propose?
That all full agonists have the same intrinsic activity and need to bind to all receptors to produce a maximum response.
what does Stephenson's model propose?
That two drugs acting as full agonist do not need to have the same efficacy, because they can generate the same maximual response by having different combinations of efficacies and spare receptors.
Agonist 1- 10% receptors occupied- f.o= 0.1. Efficacy=100
S= e x fo = 100 x 0.1 = 10units.
Agonist 2- 50% receptors occupied. fo=0.5. Efficacy=20.
S= e x fo = 20 x 0.5 = 10 units
What is the efficacy in which an agonist would be a patrial agonist?
Would never deliver the full 10 units of stimulus.
S= e x fo = 10 (efficacy) x 1.0 (fo- 100% of receptors) = 10units.
How much efficacy does an antagonist have?
No stimulus produced when bound
Why is the EC50 and Kd concentrations not identical when spare receptors contribute to agonist responses?
Kd is the agonist concentration were 50% of the total receptor population is occupied.
EC50 is the agonist conc. for 50% of maximal response.
Therefore, the EC50 will be lower than Kd value as some drugs can produce a maximal response below 100% occupancy
Apparent potency is dependent on what two factors?
Kd and efficac.
What does an agonist do?
Combine with receptors to activate responses
What does an antagonist do?
Combine with receptors resulting in:
Prevention of agonist binding
No activation of responses by the antagonists themselves
Reversible competitive antagonism characteristic?
Characterised by the agonist and antagonist binding reversibly at the exactly the same site.
What is the Kant?
the dissociation constant for the antagonist
What does the antagonist introduces in the occupancy equation
See additional sheet 4
Graphical representation of the effects of competitive antagonism on occupancy-response graph?
Graphical representation of the effects of antagonist on the log concentration-response curve?
Graphical represnetaiton of the effects of antagonist on the double reciprocal plot?
Define dose ratio?
Graphical representation of this on the log concentration-response graph
Agonist concentrations needed to produce a given level or response in the presence or absence of antagonist
What does the schild plot represent?
Graphical representation of this?
plots the relationship between different dose ratios and the antagonist concentration.
How is the schild plot evaluated?
See additional sheet 5
How to extrapolate the pA2 value of competitive antagonist?
See additional sheet 6
Define the pA2 ?
indicates the antagonist dissociation constant
What is a reversible non-competitive antagonsim?
Binding to a different site from the agonist
Even if the agonist can bind- no response will be produced
Example of an agonist and a non-competitive antagonist on the glutamate receptor?
NMDA: agonist of the NMDA class of glutamate receptor.
Ketamine: non-competitive antagonist that binds to and blocks the ion channel.
What is noradrenaline?
Agonist at a1-adrenergic receptors on vascular smooth muscle cells.
activation of receptors causes calcium influx
Non-competitive antagonist that binds to and blocks voltage-gated calcium channels.
Characteristics of a reversible non-competitive antagonism?
Once bound it cannot be removed.
No response produced once bound, even if agonist binds afterwards.
Graphical representation on the effect of non-competitive antagonism on concentration-response curve?
Graphical representation on the effects of non-competitive antagonism on agonist occupancy?
Graphical representation on the effects of non-competitve antagonism on double reciprocal plot?
Why is it that even with the agonist bind no response is produced if antagonist is bound?
Name the 2 possible causes?
Binding often produces allosteric (conformational) changes in receptors.
- Antagonist has no effects on agonist binding, changes receptor conformation to prevent activation.
- Confirmational changes decreases strength of agonist binding
Some drugs can act as these
eg. batbiturate on GABA.
Change the shape allowing substrate to bind
Define agonist exclusion?
When antagonist site is unoccupied: agonist can bind with normal Kd.
When antagonist site is occupied: confirmational change- agonist site no longer available for binding. Increase antagonist concentration
Allosteric regulatory effects of drugs on many receptor types
Allosteric mechanisms can also regulate enzyme action
Give an example of an allosteric antagonist that regulates drug activity?
Gallamine is a non-competitive allosteric antagonist upon muscarinic ACh receptors
Give an example of an allosteric antagonist that regulate enzyme action?
d-tubocurarine inhibits acetylcholinesterase metabolism of ACh
Example of an irreversible competitive antagonism?
Irreversible μ opioid receptor antagonist
Selective for the μ1 receptor subtype.
What happens if the concentration of antagonist is in excess?
Eventaully all receptors will be inactivated.
No response will be possible
Define inverse agonist?
Drug that binds to the same site as the agonist but induces a response opposite to the agonist.
Enhances GABA-inudced chloride flux.
Produces sedative, anxiogenic and anticonvulsant proporties.
No effect by itself
Weak inverse agonist and prevent the actions of the aniolyti benzodiazepines.
Reduces GABA-induced chloride ion flux
Agonist of the GABAA receptor?
Inverse agonist of the GABAA receptor?
Antagonist of the GABAA receptor?
How is it possible for somes cells that have a large number of beta2Rs producing basal levels of cAMP in the absense of agonist?
Some receptors can spontaneously exist in the activation shape
What does transfection do the expression of beta2 Rs? Why?
Increase the expression of beta2 Rs to levels many times greater than normal
Easier for detection basal cAMP levels
beta 2 agonist?
Prefer to be in the activated state.
Increase in cAMP formation.
Inverse agonist of the beta2 Rs?
Prefer the unactivated state.
Pure antagonist of the beta2 Rs?
Bind to either confirmational with no preference.
No effect on pre-existing equilibrium
Different types of agonist and antagonist with their equilibrium.
Displayed by some receptors and enzymes
Multiple binding sites
Affinity of the binding sites for ligands is increased/decrease upon the binding of a ligand to a binding site.
Define homotropic cooperativity?
Molecules causing the cooperativity is the one that will be affected by it
Define heterotropic cooperativity?
Third substance causes the change in affinity.
What is the Hill coefficients?
Model of how simple occupancy theory might predict size of the agonist response if more than one drug molecule were needed.
How to derive the hill coefficent equation to have it as graphical representation?
Additional sheet 7
Graphic representation of the hill plot?
What are the 3 advantages of using allosteric modulators as therapeutic agents?
- GPCR allosteric binding sites have not faced the same evolutionary pressure as arthosteric sites. Greater GPCR selectivity.
- Decreases potential for toxic effect
- Aloosteric modulator does not possess appreciable efficacy.
Difference between EC50 and KD?
Drug concentration that produces 50% of maximal response
(EC50) is not equal to KD (saturation of 50% of receptors).
EC50 is not equal to KD when tissues have spare receptors.
Heart tissue has spare receptors (90%) which means that only 10
of 100 receptors have to be occupied to obtain maximal response.
Under these conditions EC50 is equal to 5 (50% of 10 receptors) and
KD is equal to 50 (50% of 100 receptors).