Application of fluorescence correlation spectroscopy to the measurement of agonist binding to a G-protein coupled receptor at the single cell level

The A₁-adenosine receptor (A₁-AR) is a member of the G-protein coupled receptor superfamily, which has significant pathophysiological importance in disorders such as heart arrhythmias, asthma and stroke. Here, we have used fluorescence correlation spectroscopy (FCS) to facilitate the study of A₁-AR pharmacology at the subcellular level. To this end, we have successfully designed and synthesised a fluorescently labelled A₁-AR agonist, ABA-BY630. ABA-BY630 is an N⁶- derivative of adenosine conjugated to the red-excited fluorophore, BODIPY® 630/650. In CHO cells expressing the human A₁-AR, ABA-BY630 shows strong and potent agonist activity at this receptor. Specific binding of ABA-BY630 to the A₁-AR in cell membranes of living CHO cells can also be visualised using confocal microscopy. Moreover, using FCS, we can detect and quantify the binding of ABA-BY630 to the A₁-AR in a small area (0.2 μm²) of the upper cell membrane. FCS measurements indicate the presence of at least two populations of receptor–ABA-BY630 complexes with diffusion times of 8 and 233 ms. The quantity of both of these complexes was significantly reduced by pre-incubation with the A₁-AR antagonist DPCPX. Application of FCS in conjunction with ABA-BY630 will allow the comparison of A₁-AR pharmacology in single cells from healthy and diseased tissue.