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[3H]BQ-123 Binding to Native Endothelin ETA Receptors in Human Astrocytoma 1321N1 Cells and Screening of Potential Ligands

Endothelin ET<sub>A</sub> receptors on the human astrocytoma 1321N1 cell line were characterized in radioligand binding studies using the ET<sub>A</sub>-selective antagonist radioligand [<sup>3</sup>H]BQ-123. A membrane preparation of the cell line showed a high expression level of ET<sub>A</sub> receptors (B<sub>max</sub> = 128 fmol/mg protein). The K<sub>D</sub> value for [<sup>3</sup>H]BQ-123 (K<sub>D</sub> = 2.29 nmol/l) was comparable to that previously determined at the human neuroblastoma cell line SK-N-MC. The specific labeling of ET<sub>A</sub> receptors on astrocytoma 1321N1 cells by [<sup>3</sup>H]BQ-123 was confirmed by determining the rank order of potency of a series of agonists and antagonists. A competition assay for the screening of potential ET<sub>A</sub> ligands was established and a series of new disubstituted imidazo[1,2-c]pyrimidine and pyrimido[1,6-a]pyrimidine derivatives was investigated. Moderate ET<sub>A</sub> affinity was detected for several derivatives [30–40% inhibition of [<sup>3</sup>H]BQ-123 binding (3 nmol/l) at a concentration of 10 µmol/l]. Astrocytoma 1321N1 cells natively express relatively high levels of ET<sub>A</sub> receptors. They are useful for the screening of potential ET<sub>A</sub> ligands in radioligand binding assays with [<sup>3</sup>H]BQ-123.