Journal Article

3HXanthine Amine Congener of 1,3-dipropyl-8-phenylxanthine: An Antagonist Radioligand for Adenosine Receptors

Jacobson, Kenneth A., Kirk, Kenneth L., Ukena, Dieter, Daly, John W.

1986

Overview

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 [3XAC 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.

• Publisher: National Academy of Sciences of the United States of America,National Acad Sciences
• Subject: Agonists / Animals / Binding sites / Binding, Competitive / Biological and medical sciences / Brain / Brain - metabolism / Cattle / Cell Membrane - metabolism / Cell receptors / Cell structures and functions / Congeners / Fundamental and applied biological sciences. Psychology / Guinea Pigs / In Vitro Techniques / Ligands / Molecular and cellular biology / Purinergic P1 receptors / Radioligand Assay / Rats / Receptors / Receptors, Cell Surface - metabolism / Receptors, Purinergic / Xanthines / Xanthines - metabolism