THE ANGIOTENSIN HEXAPEPTIDE 3-8 FRAGMENT POTENTLY INHIBITS [I-125] ANGIOTENSIN-II BINDING TO NON-AT1 OR -AT2 RECOGNITION SITES IN BOVINE ADRENAL-CORTEX

In the present studies, ligand competition experiments were conducted to examine the ability of angiotensin II peptide agonists and nonpeptide AT1- and AT2-Selective receptor antagonists to inhibit the binding of [I-125]angiotensin II to bovine adrenal cortical membranes. Angiotensin II, angiotensin III, the All-(3-8) hexapeptide fragment of angiotensin II, and the AT1-selective receptor antagonist L-158,809, inhibited [I-125]angiotensin II binding in a biphasic fashion indicative of a ligand interaction at more than one recognition site. Approximately 20% of low affinity [I-125]angiotensin II binding was inhibited only by high micromolar concentrations of L-158,809. RG 13647 (1(-1,4-benzodioxan-2-methyl)-5-diphenylacetyl-4,5,6,7-tetrahydro-1H-imidazo-[4,5,c]-pyridine-6-carboxylic acid) represents a potent and AT2-selective analog of PD 123177 and showed weak activity in competing for [I-125]angiotensin II binding with an IC50 value of 100-mu-M. When subsequent competition studies were conducted in the presence of 1-mu-M L-158,809 to block [I-125]angiotensin Il to the AT1 receptor subtype, the angiotensin Il agonists produced monophasic inhibition curves with All-(3-8) showing the greatest activity (IC50 = 6 nM) followed by angiotensin III (IC50 = 15 nM) much greater than angiotensin II (IC50 = 110 nM). RG 13647 was not found to significantly inhibit this portion of [I-125]angiotensin II binding. These data demonstrate that bovine adrenal cortex contains both the AT(1) receptor subtype, as well as, a novel class of [I-125]angiotensin II recognition sites which may be analogous to the recently described angiotensin IV (AT4) receptor.