Dose response curves. Affinity and efficacy.

Question 1

What are examples of functional responses that can be measured in vivo ?

Answer 1

• Change in heart rate
• Change in blood pressure
• Hormone secretion – e.g. Insulin
• Nerve AP frequency

Question 2

What are examples of functional responses that can be measured in vitro ?

Answer 2

Isolated tissues:
- Smooth muscle contraction
- Brain slice transmitter release
- Nerve compound action potential
Single cells:
- Membrane potential change
- Membrane ion channel current [cAMP], [Ca2+], [cGMP], [H+]
Multicellular assays:
- Cell proliferation
- Reporter gene product

Question 3

What is a dose-response curve ?

Answer 3

A plot of the % MAX response against log [drug]

Question 4

What is the relationship between [drug] and receptor occupancy ?

Answer 4

A + R –> AR

Question 5

What law in particular applies to the reaction between drug and receptor ?

Answer 5

The Law of Mass Action applies to this reaction :

“The rate of a reaction is proportional to the product of the concentrations of the reactants.”

Question 6

In a drug-receptor reaction, in what state will the system be ?

Answer 6

The system is at equilibrium.

“The rates of the forward and backward reactions will be equal.” –> k+1 [A][R] = k-1 [AR]

Question 7

How are receptor proportions defined in pharmacology ?

Answer 7

Proportion of receptor in R state : pR = [R]/[R]T
Proportion of receptors occupied : pAR = [AR]/[R]T
At equilibrium : k+1 [A][R]/[R]T = k-1 [AR]/[R]T
<=> k+1 [A]pR = k-1 pAR <=> [A{pR = KapAR
<=> pR= (KA/[A]) pAR

Question 8

Define d=the dissociation equilibrium constant.

Answer 8

Ka = k-1 / k+1

Question 9

Given that pR+pAR = 1 and Ka/[A] pAR + pAR = 1, sttae the Hill-Langmuir equation.
What happens when Ka = [A] ? - when [A]&raquo_space; Ka ?

Answer 9

pAR = [A] / (Ka + [A])
When [A] =Ka, pAR = 0.5
When [A]&raquo_space; Ka, pAR ~ 1

Question 10

What assumptions di AJ Clark and AC Hill make while establishing the relationship between receptor occupancy and response ?

Answer 10

– Law of mass action applies to the drug-receptor reaction
– The drug-receptor reaction is at equilibrium
– Response of the tissue is linearly related to receptor occupancy response = constant x occupancy
Clark tentatively considered whether the response might be directly proportional to occupancy (constant =1) although it was clear he knew this to be unlikely.

Question 11

How do we refer to the agonist concentration giving half maximal response (y = 50%) ?

Answer 11

[A]50

Question 12

What is the Hill plot ?

What crucial oversimplification is made in here ?

Answer 12

log(y/(100-y)) = log[A] - log{a}50

No distinction is made here between occupied and activated receptors: it is assumed all occupied receptors are active.

Question 13

What is the difference between a full agonist and a partial agonist ?

Answer 13

full agonists : elicit a maximal tissue response
partial agonists : cannot elicit this maximal response
The difference has to do with the efficacy of the agonist (and not the potency/affinity).

Question 14

Give examples of full and partial agonists.

Include the receptors at which they act.

Answer 14

beta-AR :
- PA = prenalterol, terbutaline
- FA = AD, isoprenaline
Musc receptors :
- PA = pilocarpine
- FA = ACh
Histamine H2 receptors :
- PA = impromidine
- FA = histamine

Question 15

Why does the maximal response of the partial agonist, and the [A]50 of both full and partial agonist depend on the tissue ?

Answer 15

The answer lies in the density of receptors in each tissue, and the ability of the agonist to activate the receptor – the agonist efficacy.

Question 16

Why is there concept of “spare receptors” important for the distinction of full and partial agonists ?

Answer 16

• The Law of Mass action tells us that at high enough agonist concentration, all receptors will be occupied
• Full agonists need to activate less receptors than partial
  agonists in order to elicit a maximal response
• Thus in some tissues there is a receptor reserve, often referred to as “spare receptors”

Question 17

What is the purpose of spare receptors ?

Answer 17

The presence of a receptor reserve increases the sensitivity of the cell to hormone or drug action at the receptor.
Thus control of the density of functional receptors on the cell surface allows the cell to regulate hormone sensitivity.

Question 18

Why is it important to draw a distinction between receptor binding and receptor activation ?
Support your answer with a mechanism.

Answer 18

If we consider the del-Castillo-Katz mechanism :
A + R –> AR –> AR*
Ka E
We can see that some receptors can be bound to receptor and sill be in an inactive state.
Thus, the dissociation equilibirum constant Ka is a measure of the affinity of the ligand for the receptor, and the constant E is a measure of the efficacy of the ligand, i.e. its ability to activate the receptor.