Receptor density and IC50/Vmax - (Jan/03/2011 )

I've done some very simple experiments just to check for functionality of some splice variants of a G-protein linked receptor. I used stably transfected cells expressing the different receptor variants and measured cAMP levels in response to different ligands. In this system, exposure to a ligand that binds the receptors will cause a reduction in the intracellular cAMP levels. However, the number of receptors have not been calculated (have not done any classical binding studies). And based on confocal studies of these cells, we know that the expression levels of these receptors varies - one receptor variant is expressed at very high levels, while the others are expressed at varying and somewhat lower levels.
So, I have different receptors at varying receptor density, tested with a battery of ligands.
My question is then: can I compare values for IC50 and/or Vmax between these receptor variants at all? Or can I only look at one single receptor at the time and comment upon how that receptor responds to the different ligands?
I guess this really is a question about how IC50, Vmax and receptor density is linked together (which I should know, of course, but I don't... ). Any input is greatly appreciated!

sonjaa on Mon Jan 3 12:36:18 2011 said:

Just my stab at an answer:
If the assumptions of ligand-receptor equilibrium and high ligand/receptor ratio are valid I don’t see why you can’t compare the IC50s. The Vmax on the other hand will be strongly influenced by receptor number.