Relative affinity of angiotensin peptides and novel ligands at AT₁ and AT₂ receptors

AT₁R (angiotensin type 1 receptor) and AT₂R (angiotensin type 2 receptor) are well known to be involved in the complex cardiovascular actions of AngII (angiotensin II). However, shorter peptide fragments of AngII are thought to have biological activity in their own right and elicit effects that oppose those mediated by AngII. In the present study, we have used HEK (human embryonic kidney)-293 cells stably transfected with either AT₁R or AT₂R to perform a systematic analysis of binding affinities of all the major angiotensin peptides. Additionally, we tested the novel AT₂R agonist Compound 21, as well as the MasR (Mas receptor) agonist and antagonist AVE0991 and A-779 respectively, for their ability to bind to AT₁R or AT₂R. Candesartan, CGP42214 and PD123319 were used as reference compounds. Binding studies using ¹²⁵I-[Sar¹Ile⁸]AngII on the AT₁R-transfected HEK-293 cells revealed only AngII, AngIII [angiotensin III; angiotensin-(2–8)] and candesartan to have high affinity for AT₁R. In the AT₂R-transfected HEK-293 cells, competition for ¹²⁵I-[Sar¹Ile⁸]AngII binding was observed for all ligands except candesartan, AVE0991 and A-779, the latter two compounds having negligible affinity at either AT₁R or AT₂R. The rank order of affinity of ligands at AT₂R was CGP42112>AngII≥AngIII>Compound 21≥PD123319≫AngIV [angiotensin IV; angiotensin-(3–8)]>Ang-(1–7) [angiotensin-(1–7)]. Of note, although AngIV and Ang-(1–7) exhibited only modest affinity at AT₂R compared with AngII, these two angiotensin peptides, together with AngIII, had substantial AT₂R selectivity over AT₁R. Collectively, our results suggest that shorter angiotensin peptides can act as endogenous ligands at AT₂R.