QUANTITATIVE AUTORADIOGRAPHY OF ANGIOTENSIN-II AT(2), RECEPTORS WITH [I-125] CGP 42112

Most radiolabeled ligands for angiotensin II (Ang II) receptors do not discriminate between the AT(1) and AT(2) receptor subtypes, which must be distinguished by displacement with selective AT(1) or AT(2) ligands. We compared [I-125]CGP 42112 with the non-selective agonist [I-125]Sar(1)Angiotensin II. We studied the inferior olive, medial geniculate nucleus and the adrenal medulla, areas rich in AT(2) receptors, using both ligands with quantitative autoradiography and membrane binding techniques. [I-125]CGP 42112 bound with high affinity (K-d = 0.07-0.3 nM, depending on the area studied). [I-125]CGP 42112 binding was selective for AT(2) receptors, as determined by lack of competition with the AT(1) ligand losartan, and competition by the AT(2) ligands PD 123177 and unlabeled CGP 42112 and the non-selective peptides Ang II and angiotensin III (Ang III). Using [I-125]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 [I-125]CGP 42112 is the most selective, highest affinity ligand available for AT(2) receptors. Because of these characteristics, and low non-specific binding, quantitative autoradiography with [I-125]CGp 42112 is the method of choice to selectively characterize AT(2) receptors, especially in tissues like the brain, with a highly heterogeneous distribution of receptor subtypes.