A cloned angiotensin receptor isoform from the turkey adrenal gland is pharmacologically distinct from mammalian angiotensin receptors

A 2046-base pair cDNA clone, homologous to mammalian angiotensin (AT) AT₁ receptors, was isolated from a library prepared from adrenal glands of the domestic turkey (Meleagris gallopavo). Sequence analysis of the cDNA insert in clone pTAT2' reveals a 1077-base pair open reading frame predicting a 359- amino acid protein ~75% homologous to mammalian AT₁ receptors. Saturation radioligand binding studies performed in membranes of COS-7 cells transfected with pTAT2' show high affinity specific binding of ¹²⁵I-angiotensin II, with a K(d) of 172 pM. The rank order of affinities for a series of ligands determined by competition binding studies is angiotensin II ≥ [Sar¹,Ile⁸]- angiotensin II > angiotensin II ≃ [Sar¹,Ala⁸]-angiotensin II ≃ CGP42112A > angiotensin I> Dup753 > PD123177. This rank order of affinity series differs substantially from that for mammalian AT₁ receptors and AT₂ binding sites. Angiotensin II (100 nM) can stimulate inositol phosphate production similarily in COS-7 cells transfected with pTAT2' and in COS-7 cells transfected with the AT(1a) receptor cDNA pCa18b. This response in pTAT2'- transfected cells is not attenuated in the presence of 30 μM Dup753. In contrast, this concentration of antagonist attenuates >90% of the inositol phosphate response to angiotensin II in COS-7 cells transfected with the rat AT(1a) receptor cDNA. These results demonstrate an avian structural homologue of mammalian AT₁ receptors possessing distinct pharmacological properties with both peptide and nonpeptide AT receptor ligands.