1.6 receptors Flashcards
(31 cards)
What is the purpose of Schild analysis?
To determine which receptor subtypes mediate agonist-induced effects using receptor-selective, competitive antagonists.
Why can’t binding studies alone identify receptor function?
They indicate presence and density but not whether the receptors mediate specific cellular responses.
What kind of study is Schild analysis classified as?
A functional study that uses competitive antagonists to assess receptor-mediated responses.
What defines competitive (orthosteric) antagonism?
Both agonist and antagonist compete for the same receptor binding site.
What factors influence which ligand binds at any time?
The relative concentrations and affinities of agonist and antagonist.
How does a competitive antagonist affect agonist binding?
It reduces the fractional receptor occupancy of the agonist in a concentration-dependent manner.
How do competitive antagonists affect dose-response curves (DRCs)?
They shift the DRC to the right without changing maximum effect.
What is a “dose ratio” in this context?
The ratio of agonist concentrations that produce the same effect with and without antagonist: [A]+ / [A].
What does a larger dose ratio indicate?
A stronger shift and likely higher antagonist affinity.
What is the Schild equation?
log(DR – 1) = log[B] – logKB
How can KB be calculated from a single concentration?
KB = [B] / (DR – 1)
What is pA₂?
The –log of the antagonist concentration that causes a 2-fold shift in the DRC (i.e., when DR = 2).
When does pA₂ equal –logKB?
When the Schild plot is linear with a slope of 1.0.
What data are needed to construct a Schild plot?
Dose ratios for agonist DRCs in the presence of multiple antagonist concentrations.
What goes on each axis of a Schild plot?
x-axis: log[antagonist]; y-axis: log(DR – 1)
What does the x-intercept of a Schild plot represent?
The pA₂ value.
What does a Schild plot slope of 1.0 indicate?
The antagonist is acting as a competitive antagonist with reversible binding.
What does the SPIKES acronym stand for?
Shape, Position, Inclination, KB value, Elimination, Summation.
What does ‘Shape’ refer to?
Whether the Schild plot is linear (essential for valid interpretation).
What does ‘Inclination’ refer to?
The slope, which should ideally be 1.0; deviations may indicate non-competitive mechanisms.
What does ‘Elimination’ involve?
Comparing –logKB values with known –logKi values to exclude receptor subtypes.
When can KB values identify receptor subtypes?
When pA₂ is derived from a Schild plot that is linear and has slope = 1.0.
What should be done if multiple selective antagonists are tested?
Use SPIKES analysis to identify the receptor subtype by elimination.
What is an example of this approach?
If pirenzepine shows a pA₂ of 8.2 matching M1 affinity, and other antagonists don’t match, M1 is implicated.
