Interactions of Nonpeptide Angiotensin II Receptor Antagonists at Imidazoline/Guanidinium Receptor Sites in Rat Forebrain

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Abstract
Imidazoline/guanidinium receptive sites (IGRS) are shown to be present in the subfornical organ and hypothalamic arcuate nucleus by a derivative of cirazoline, 2-(3-amino-4-[125I]iodophenoxy)methylimidazoline ([125I]AMIPI). Because many of the nonpeptide angiotensin II (Ang II) receptor antagonists contain imidazole ring structures, they may interact with IGRS. Therefore, we studied competitive activity of Ang II and several nonpeptide Ang II receptor antagonists [DuP 753 (losartan), EXP 3174, CV11974, and PD123319] at IGRS in rat forebrain. The results showed specific binding of 944 +/- 169 fmol/mg protein in the subfornical organ (n = 11) and of 367 +/- 27 fmol/mg protein in the arcuate nucleus (n = 6) at 0.4 nM [125I]AMIPI, as defined by competition with 10 microM cirazoline. Specific [125I]AMIPI binding was competed for completely by 10 microM idazoxan or clonidine as further characterization of IGRS. Ang II and the nonpeptide AT1 and AT2 antagonists did not significantly compete for specific [125I]AMIPI binding in either brain region at concentrations of 10 microM (< 20% competition with each compound), which is 10- to 100-fold higher than the concentration necessary to compete completely for their respective Ang II receptor subtypes. Only at the highest concentration (100 microM) did losartan compete significantly for binding (56 +/- 8%). Therefore, Ang II receptor antagonists interact with IGRS in rat forebrain cardiovascular areas only at high concentrations.