Plosone study

[Cotcha Yankinov]

Thanks for the explanations as always!

If the test ligand is an agonist and activates the receptor, that wouldn't have any effect on the subsequent binding of an antagonist radioligand because antagonists are equally happy to bind to the active or the inactive conformation.

Imagine we have an agonist test ligand and antagonist radioligand mixture in two different homogenates - one featuring a cell line with lots of low-affinity state receptors and the other with lots of high-affinity state receptors.

If the agonist test ligand is better at displacing the antagonist radioligand in a high-affinity state homogenate compared to a low-affinity state homogenate, could a situation like this cause an appreciable difference in measured affinity between those two assays?

Thanks for any thoughts.

 

[serotonin2A]

Thanks for the explanations as always!


Imagine we have an agonist test ligand and antagonist radioligand mixture in two different homogenates - one featuring a cell line with lots of low-affinity state receptors and the other with lots of high-affinity state receptors.

If the agonist test ligand is better at displacing the antagonist radioligand in a high-affinity state homogenate compared to a low-affinity state homogenate, could a situation like this cause an appreciable difference in measured affinity between those two assays?

Thanks for any thoughts.

That type of difference typically wouldn't happen because the equilibrium should remain relatively constant. The one exception is that with high levels of receptor expression, there may be a receptor reserve, meaning that there is an excess of receptors compared to G proteins. There will always be affinity differences across preparations, but not necessarily large differences. You can look at the data in the PDSP database to see what kinds of differences are common.

 

[Charles_Voynich]

When I was being taught GPCR signaling, I was taught it using a model like this:

https://openi.nlm.nih.gov/detailedresult.php?img=PMC1769407_pcbi.0030006.g001&req=4

I think it helps to visualize all of the different equilibrium that are happening simultaneously.

 

[markosheehan]

How do you know it is not reliable when there is so little to compare it to.
Not for all drugs and receptors but for some there is very little other data to go off so it is hard to tell.

 

[Charles_Voynich]

How do you know it is not reliable when there is so little to compare it to.
Not for all drugs and receptors but for some there is very little other data to go off so it is hard to tell.

A lot of it is just the nature of assembling information from many different experiments conducted many different ways and then drawing conclusions from that. Saying "X is 10 times stronger than Y at Z receptor" might not be the most accurate comparison when the values you're comparing were gathered in completely different experiments, even if that fact may ultimately be true. It helps when you find a paper that compares a series of test ligands against a receptor in one, or a series of experiments.