Development of a high-affinity radioiodinated ligand for identification of imidazoline/guanidinium receptive sites (IGRS): intratissue distribution of IGRS in liver, forebrain, and kidney

B Ivkovic, V Bakthavachalam, W Zhang, A Parini, D Diz, S Bosch, JL Neumeyer and SM Lanier

Department of Pharmacology, Medical University of South Carolina, Charleston.

Imidazoline/guanidinium receptive sites (IGRS) are membrane proteins that exhibit high affinity for various compounds with an imidazoline or guanidinium moiety. The structure of these binding sites and their significance in the broad pharmacological action of such ligands are unclear. To address this issue, we developed selective high affinity compounds that could be radioiodinated and used as molecular probes for structural characterization of these proteins. This report describes the synthesis and characterization of such a molecule, 2-(3-amino-4- [125I]iodophenoxy)methylimidazoline ([125I]AMIPI). [125I]AMIPI is structurally related to cirazoline, an imidazoline exhibiting high affinity for IGRS and the family of related imidazoline binding sites. The phenyl-substituted analogue of cirazoline, 2-(3- aminophenoxy)methylimidazoline, was generated by alkylation of acetamidophenol with 2-chloromethylimidazoline. 2-(3- Aminophenoxy)methylimidazoline exhibited high affinity for IGRS in rabbit kidney membranes, as determined in competition binding studies with [3H]idazoxan (Ki = 12.5 +/- 7.5 nM), and was radioiodinated by chloramine-T oxidation to yield [125I]AMIPI. The binding properties of [125I]AMIPI were determined in membranes prepared from two representative tissues, rabbit kidney cortex and rat liver. Specific binding of [125I]AMIPI was saturable and of high affinity, as determined by Scatchard analysis of saturation binding isotherms (rabbit kidney, Kd = 2.0 +/- 0.9 nM, Bmax = 554 +/- 201 fmol/mg, five experiments; rat liver, Kd = 2.6 +/- 1.3 nM, Bmax = 73 +/- 10 fmol/mg, three experiments).(ABSTRACT TRUNCATED AT 250 WORDS)

Volume 46, Issue 1, pp. 15-23, 07/01/1994
Copyright © 1994 by American Society for Pharmacology and Experimental Therapeutics

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