We have previously described the synthesis of a novel fluorescent agonist for the adneosine-A1 receptor (A1-AR), ABEA-X-BY630 (Briddon et al, 2002). This compound, based on the known A1-AR agonist, N-ethyl carboxamidoadenosine (NECA), retains agonist activity and allows visualisation of membrane-bound ligand by confocal microscopy. Here, we have used fluorescence correlation spectroscopy (FCS) to quantify both the number and the diffusion characteristics of agonist-receptor complexes in small areas of the membrane of CHO cells expressing the human A1-AR.
Functional responses to A1-AR stimulation were assessed in CHO-A1 cells by labelling with either
ABEA-X-BY630 retained potent agonist activity in CHO-A1 cells as assessed by both inhibition of forskolin-stimulated cAMP production (pEC50=8.47±0.08 ; EMAX=99±1%, n=4) and inositol phosphate production (pEC50=7.42±0.04, n=3). In both cases, stimulation was through the A1-AR , since it was antagonised in a competitive manner by the antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 100nM; apparent pKD=7.85±0.01, n=4 and 8.14±0.16, n=3 for cAMP inhibition and inositol phosphate production, respectively). FCS analysis of ABEA-X-BY630 (50nM, in HEPES-buffered saline, Briddon et al. 2004) showed it had a diffusion time of D3 =71±2 µs (n=4). FCS measurements were then performed on the upper membrane of CHO-A1 cells following incubation with ABEA-X-BY630 (5nM, 10 min, 22°C). In addition to the fast-diffusing free ligand component, two slower diffusing components were found
FCS therefore allows both the diffusion properties and number of agonist-receptor complexes to be measured in small areas of cell membrane. Further investigation of these complexes will reveal more of the membrane organisation of receptor binding and activation.
Briddon, S.J. et al. (2002) Br. J. Pharmacol., 138, 126P.
We thank the Wellcome Trust for financial support.