Use of multiple radioligands to characterize adenosine receptors in human placenta

To identify and characterize fully the adenosine receptor of human placenta, particulate preparations of placental parenchyma were examined using several adenosine receptor ligands, [3H]2-chloradenosine ([3H]2-Cl-ADO), [3H]N6-ethylcarboxamideadenosine ([3H]NECA), and [3H]N6-(L-phenylisopropyl)-adenosine ([3H]PIA). Each of the three ligands bound to the particulate preparations with unique binding characteristics. Binding of [3H]PIA characterized a high-affinity low-capacity receptor while the [3H]NECA displayed lower-affinity, higher-capacity binding. Saturation isotherms for [3H]2-Cl-ADO appeared intermediate in terms of affinity and binding capacity. Saturation isotherms of binding data also disclosed displaceable non-receptor binding at high labelled ligand concentrations. Competitive binding studies supported the observation that PIA and NECA bind to sites of high and low affinity, respectively. However, the multicomponent competition curves for [3H]2-Cl-ADO binding suggest binding to the receptors with differential affinity. These studies confirm the presence of adenosine receptors in the human placenta and suggest that these receptors are of both the high-and low-affinity subtypes.