Pharmacological characterization of angiotensin II AT₂ receptor subtype heterogeneity in the rat adrenal cortex and medulla

Adrenal angiotensin II (AII) receptors have been pharmacologically and structurally divided into two main subtypes, AT₁ and AT₂. Radioligand receptor binding assays with¹²⁵I-sarcosine¹, isoleucine⁸ angiotensin II (¹²⁵I-SI AII) in the presence of losartan, an AT₁ selective ligand, and PD123177 an AT₂ selective ligand, indicated that the AT₁ subtype was predominant in membrane homogenates of the rat adrenal cortex (AT₁ Bmax=649 ± 62 fmol/mg protein; AT₂ Bmax=237 ± 29 fmol/mg protein). In membrane homogenates of the adrenal medulla, the AT₂ subtype was predominant (AT₁ Bmax=55 ± 5 fmol/mg protein; AT₂ Bmax=109 ± 29 fmol/mg protein). Overall 58% of the¹²⁵I-SI AII binding in the rat adrenal was to the AT₁ subtypes, and 42% was to the AT₂ subtypes. The outer cortex contained 59% of the AH receptor binding sites in the adrenal, while the medulla accounted for the remaining 41%. The affinity of the AT₁ binding sites in membrane homogenates of the cortex and medulla (K _D =672 ± 123 pM and 573 ± 85 pM, respectively) was not significantly different. The affinity for¹²⁵I-SH AII of AT₂ binding sites in membrane homogenates was higher than that of AT, binding sites. The affinity for¹²⁵I-SI All of AT₂ binding sites in membrane homogenates of the outer cortex (K _D =265 ± 35 pM) was significantly less than that in the medulla (K _D =133 ± 11 pM). In vitro receptor autoradiography also demonstrated that the AT₂ subtype in frozen sections of the cortex had a lower affinity (K _D =1512 ± 191 pM) than that in the medulla (K _D =867 ± 72 pM). The heterogeneous affinity of adrenal AT₂ binding sites may indicate existence of multiple AT₂ receptor subtypes in the rat adrenal.