Receptor theory

Question 1

Examples of what the ligand can be radioligand binding assays

Answer 1

Neurotransmitters, hormones, growth factors, cytokines, drugs, toxins

Question 2

Radioligand binding assays

Answer 2

Prepare cells e.g. with detergent treatment and centrifugation, aliquot out membranes onto filters, add radiojlabel at different concentrations to each tube, add an excess of non-radiolabelled to one tube

Question 3

How can you reduce non-specific binding?

Answer 3

Anti-absorbents

Question 4

What must the ligand be in a radioligand binding assay?

Answer 4

Biologically active

Question 5

Advantages of using I125 in a radioligand binding assay

Answer 5

If the compound has an aromatic hydroxyl group it can e incorporated at very high specific activities, ionisation easy in most labs and cheap

Question 6

Disadvantages of using I125

Answer 6

More readily degraded, short half life (67 days), function of compound can be altered

Question 7

What can the tissue be in a radioligand binding assay?

Answer 7

Isolated membrane, slices, synaptosomes, cultured cells or solubilised/ purified receptors from total brain or specific brain region

Question 8

What concentrations of protein and assay volumes do you need in incubation?

Answer 8

Protein conc- 0.1-1mg membrane protein/ ml. Assay volumes - 0.25-1ml

Question 9

Scatchard equation

Answer 9

B/F=Bmax-B/ Kd Bmax= max number of binding sites, B- specifically bound F-free Kd-affinity

Question 10

Kd

Answer 10

The drug concentration when 50% of the receptors are occupied by drug

Question 11

Langmuir equation

Answer 11

Bound= Bmax+Xa/Xa+Kd

Question 12

Units of Bmax and KD

Answer 12

Bmax- pmole/mg protein. Kd=mol/litre

Question 13

Potency depends on

Answer 13

Affinity, efficacy and spare receptors

Question 14

Equation for response

Answer 14

Response= max x [Xa]n / [Xa]n = [EC50]n n= slope factor (number of agonists that need to bind to each receptor)

Question 15

What are partial agonists useful for?

Answer 15

Less overdose, less desensitisation of the receptors, opiates as pain killers, alleviate addicts

Question 16

Example of chemical antagonism

Answer 16

Inactivation of heavy metals whose activity is reduced with the addition of a chelating agent

Question 17

Example of pharmacokinetic antagonism

Answer 17

Patients taking Warfarin have to be careful when taking antibiotics as they may stimulate the metabolism of warfarin so reducing its effective concentration in the blood

Question 18

Example of non-competitive antagonist

Answer 18

Smooth muscle histamine receptor, activates GPCR, generates IP3, causes calcium release from store, activates CaM, phosphorylation of myosin, Cav L type open which contributes to calcium signalling and causes contraction. Dihydoperidines block Cav L type for people with high blood pressure so reduces calcium influx therefore contraction

Question 19

What is dose ratio?

Answer 19

How much more time agonist is needed in the presence of an antagonist

Question 20

How do you find pA2 from a schild plot?

Answer 20

Negative of the value where the line crosses the x axis

Question 21

Why might the effect of a drug decrease over time?

Answer 21

Enocytosis of receptors, change in receptor itself, exhaustion of mediators, increased metabolic degradation of extrusion of the drug, physiological adaptation

Question 22

Tachyphlaxis

Answer 22

Rapid and short term onset of drug tolerance