Adenosine A(1) receptors in human brain and transfected CHO cells: Inhibition of [(3)H]CPFPX binding by adenosine and caffeine.
In vivo imaging of adenosine function has become feasible with the specific A(1) adenosine receptor ligand [(18)F]CPFPX and positron emission tomography (PET). It is, however, still an open question whether [(18)F]CPFPX is displaceable by endogenous adenosine, which would allow to detect activity-dependent adenosine release in vivo. We used the tritiated analog of [(18)F]CPFPX, [(3)H]CPFPX, to quantify A(1) adenosine receptors (A(1)AR) in grey matter tissue homogenates of four human brains and A(1)AR transfected Chinese hamster ovary cells, respectively. Saturation binding experiments in the presence of a stable GTP analog revealed a dissociation constant (K(D)) of 2.4±0.5nM. The unselective endogenous A(1)AR agonist adenosine and the antagonist caffeine displaced specific [(3)H]CPFPX binding completely at high doses. Concentrations sufficient to inhibit 50% of binding (IC(50)) were 6.9±2.7μM for adenosine and 148±15.4μM for caffeine. Respective inhibition constants (K(i)) 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 [(18)F]CPFPX and PET. Pathophysiological conditions like hypoxia which increase endogenous adenosine concentrations several folds might interfere with in vivo [(18)F]CPFPX binding. Caffeine intake previous to the investigation should be considered as a confounding factor regarding the determination of receptor densities with [(18)F]CPFPX and PET.
Institute of Neuroscience and Medicine, INM-2, Research Center Juelich, Leo-Brandt-Str., 52425 Juelich, Germany. d.elmenhorst@fz-juelich.de
David
D
Elmenhorst
Valentina
V
Garibotto
Andreas
A
Prescher
Andreas
A
Bauer
D000241:Adenosine / Q000493:pharmacokinetics*; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000000981:diagnostic imaging*; D016466:CHO Cells; D002110:Caffeine / Q000493:pharmacokinetics*; D006224:Cricetinae; D003412:Cricetulus; D006801:Humans; D049268:Positron-Emission Tomography / Q000379:methods; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D043682:Receptor, Adenosine A1 / Q000378:metabolism*; D014162:Transfection; D014316:Tritium; D014970:Xanthines / Q000493:pharmacokinetics*
C468796:8-cyclopenta-3-(3-fluoropropyl)-1-propylxanthine; D019275:Radiopharmaceuticals; D043682:Receptor, Adenosine A1; D014970:Xanthines; D014316:Tritium; D002110:Caffeine; D000241:Adenosine