L5- measurements of drug action at receptors Flashcards
what is radioligand binding
a way to measure a drugs affinity
labelling a drug with a radioactive isotope then measuring/detecting radioactivity
what info can radioligand binding tell us
- no of binding sites in tissue
2. affinity of drug for these sites
what does ‘hot’ and ‘cold’ drug mean in radioligand binding
hot - radiolabelled drug
cold - drug not radiolabelled
what is the purpose of adding the cold drug in the second part of radioligand binding?
to bind to all the specific receptor sites, so that the hot drug can then bind to the non specific sites
this graph can then be subtracted from the first graph to show only the specific binding sites
define Bmax
max no. of specific receptors in a tissue
how can antagonist affinity be measured?
by quantifying the extent an agonist curve is shifted to the right in the presence of the antagonist
how is affinity of a reversible competitive antagonist quantified
using its PA2
-log of the molar conc of antagonist that produces a dose ratio 2
what does pA2 mean?
the -log of the molar [antagonist] that produces a dose ratio 2
describe what is meant by dose ratio
if you had to increase the [agonist] 5x to overcome the antagonist (produce the same response as before antagonist added) then the dose ratio is 5
what effect does increasing [reversible competitive antagonist] have on dose ratio?
increases LINEARLY
how is PA2 obtained?
- construt several log conc - response curves at different [antagonist]
- use shild equation
- plot this on graph
y: log(dose ratio - 1)
x: log([antagonist])
what will be the gradient of a shild graph for a reversible competitive antagonist?
y: log(dose ratio - 1)
x: log([antagonist])
1
what will the x intercept be on a shild plot?
y: log(dose ratio - 1)
x: log([antagonist])
log(Kb)
how is dose ratio calculated
EC50 (without antagonist)
what effect does changing the agonist (eg using NA instead of A) have on the PA2 of antagonist
no effect
PA2 independent of agonist used
