title: [3H]xanthine amine congener of 1,3-dipropyl-8-phenylxanthine: an antagonist radioligand for adenosine receptors.
abstract: An amine-functionalized derivative of 1,3-dipropyl-8-phenylxanthine has been prepared in tritiated form as a xanthine amine congener ([3H]XAC) for use as an antagonist radioligand for adenosine receptors. [3H]XAC has higher receptor affinity, higher specific activity, lower nonspecific membrane binding, and more favorable hydrophilicity than 1,3-diethyl-8-[3H]phenylxanthine, the xanthine commonly used for adenosine receptor binding. In rat cerebral cortical membranes, [3H]XAC exhibits saturable, specific binding with a Kd of 1.23 nM and a Bmax of 580 fmol/mg of protein at 37 degrees C. N6-(R-Phenylisopropyl)adenosine is a more potent inhibitor of [3H]XAC binding than is 5'-N-ethylcarboxamidoadenosine, indicating that binding is to an A1-adenosine receptor. In the absence of GTP, the inhibition curves for adenosine agonists versus [3H]XAC binding are biphasic, indicating that [3H]XAC is binding to low- and high-affinity agonist states of the A1 receptor. In the presence of GTP, adenosine analogs exhibit monophasic, low-affinity inhibition of binding of [3H]XAC. Inhibition of [3H]XAC binding by theophylline or by various 8-phenylxanthines is monophasic, and the potencies are commensurate with the potencies of these xanthines as adenosine receptor antagonists. The receptor sites in calf brain membranes exhibit a higher affinity (Kd = 0.17 nM) for [3H]XAC, whereas sites in guinea pig exhibit a slightly lower affinity (Kd = 3.0 nM). Densities of [3H]XAC binding sites are similar in brain membranes from all species.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D006168:Guinea Pigs; D066298:In Vitro Techniques; D011869:Radioligand Assay; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011983:Receptors, Purinergic; D014970:Xanthines / Q000378:metabolism*
chemical_list: D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D014970:Xanthines; C049140:1,3-dipropyl-8-phenylxanthine