PHARMACOLOGICAL CHARACTERIZATION OF ANGIOTENSIN-II AT(2) 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, AT(1) and AT(2). Radioligand receptor binding assays with I-125-sarcosine(1), isoleucine(8) angiotensin II (I-125-SI AII) in the presence of losartan, an AT(1) selective ligand, and PD123177 an AT(2) selective ligand, indicated that the AT(1) subtype was predominant in membrane homogenates of the rat adrenal cortex (AT(1) Bmax = 649 +/- 62 fmol/mg protein; AT(2) Bmax = 237 +/- 29 fmol/mg protein). In membrane homogenates of the adrenal medulla, the AT(2) subtype was predominant (AT(1) Bmax = 55 +/- 5 fmol/mg protein; AT(2) Bmax = 109 +/- 29 fmol/mg protein). Overall 58% of the I-125-SI AII binding in the rat adrenal was to the AT(1) subtypes, and 42% was to the AT(2) subtypes. The outer cortex contained 59% of the AII receptor binding sites in the adrenal, while the medulla accounted for the remaining 41%. The affinity of the AT(1) 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 for I-125-SII AII of AT(2) binding sites in membrane homogenates was higher than that of AT(1) binding sites. The affinity for I-125-SI AII of AT(2) 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 AT(2) 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 AT(2) binding sites may indicate existence of multiple AT(2) receptor subtypes in the rat adrenal.