Adenosine A1 receptors in human brain and transfected CHO cells: Inhibition of [3H]CPFPX binding by adenosine and caffeine

Abstract

In vivo imaging of adenosine function has become feasible with the specific A1 adenosine receptor ligand [18F]CPFPX and positron emission tomography (PET). It is, however, still an open question whether [18F]CPFPX is displaceable by endogenous adenosine, which would allow to detect activity-dependent adenosine release in vivo. We used the tritiated analog of [18F]CPFPX, [3H]CPFPX, to quantify A1 adenosine receptors (A1AR) in grey matter tissue homogenates of four human brains and A1AR transfected Chinese hamster ovary cells, respectively. Saturation binding experiments in the presence of a stable GTP analog revealed a dissociation constant (KD) of 2.4±0.5nM. The unselective endogenous A1AR agonist adenosine and the antagonist caffeine displaced specific [3H]CPFPX binding completely at high doses. Concentrations sufficient to inhibit 50% of binding (IC50) were 6.9±2.7μM for adenosine and 148±15.4μM for caffeine. Respective inhibition constants (Ki) were 2.8±0.9μM and 61.4±11.2μM.The present report supports the possibility of studying acute effects of adenosine and caffeine in vivo with [18F]CPFPX and PET. Pathophysiological conditions like hypoxia which increase endogenous adenosine concentrations several folds might interfere with in vivo [18F]CPFPX binding. Caffeine intake previous to the investigation should be considered as a confounding factor regarding the determination of receptor densities with [18F]CPFPX and PET.