i have just done a competitve radioligand receptor binding assay with the radioactive inverse agonist ([3H]-SR141716A and agonist (CP55940) in the presense absense of GTPgammaS.
The results showed that in the presense of GTPgammaS, there is a higher concentration needed to displace ([3H]-SR141716A by CP55940. why? whats the mechanism for this? thanks for your help

May be due to the mode of action of your ligand. In the presence of GTP gamma S. It may act metabotrophically so that it the ligand binds with other protein messengers downstream. It may need higher concentration of your agonist for effective replacement.