Characterization of angiotensin AT1A receptor isoform by its ligand binding signature

The objective of this study was to determine whether the binding signature of the cloned rat AT1A receptor transfected into Chinese hamster ovary cells could be distinguished from that of the endogenous AT1B receptor expressed in rat adrenal cortex. An extensive series of peptide and nonpeptide Ang II analogs was used for the characterization. The binding of [125I]Ang II to the recombinant AT1A receptors was quite sensitive to inhibition by GTP gamma S. Scatchard analysis of the competition of Ang II revealed two populations of binding sites (site 1: KD = 3.05 +/- 0.27 nM and a maximum binding (Bmax) of 134 +/- 26 fmol/mg protein; site 2: KD = 253 +/- 77 nM and Bmax = 1.05 +/- 0.19 pmol/mg protein). The ligand binding signature of the AT1A receptor is defined by the affinity (Ki = nM) and order of potency of the following ligands: saralasin (2.07) > Ang II (3.35) > losartan (14) > Ang III (20) > GR 117289C (28) > EXP6803 (160) > Ang I (281) > PD123177 (> 10,000). This binding signature of the cloned AT1A receptors appears to be similar to that displayed by rat adrenal cortical cells where AT1B is predominantly expressed. These findings suggest that AT1A and AT1B receptors may not be easily distinguishable by the currently available ligand agonists or antagonists. Consequently, AT1A or AT1B may be considered as isoforms rather than subtypes of the AT1 receptors.