Quantitative autoradiography of angiotensin II AT2 receptors with [125I]CGP 42112

Most radiolabeled ligands for angiotensin II (Ang II) receptors do not discriminate between the AT1 and AT2 receptor subtypes, which must be distinguished by displacement with selective AT1 or AT2 ligands. We compared [125I]CGP 42112 with the non-selective agonist [125I]Sar1 Angiotensin II. We studied the inferior olive, medial geniculate nucleus and the adrenal medulla, areas rich in AT2 receptors, using both ligands with quantitative autoradiography and membrane binding techniques. [125I]CGP 42112 bound with high affinity (Kd = 0.07-0.3 nM, depending on the area studied). [125I]CGP 42112 binding was selective for AT2 receptors, as determined by lack of competition with the AT1 ligand losartan, and competition by the AT2 ligands PD 123177 and unlabeled CGP 42112 and the non-selective peptides Ang II and angiotensin III (Ang III). Using [125I]CGP 42112 binding, we found the same order of potency: CGP 42112 > Ang II = Ang III > PD 123177 using both quantitative autoradiography or membrane binding methods. Our results demonstrate that [125I]CGP 42112 is the most selective, highest affinity ligand available for AT2 receptors. Because of these characteristics, and low non-specific binding, quantitative autoradiography with [125I]CGP 42112 is the method of choice to selectively characterize AT2 receptors, especially in tissues like the brain, with a highly heterogeneous distribution of receptor subtypes.