Characterization of imidazoline binding protein(s) solubilized from human brainstem: Studies with [³H]idazoxan and [³H]clonidine

Abstract

Imidazoline binding sites from the human brainstem were solubilized with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane-sulfonate (CHAPS). [³H]idazoxan and [³H]clonidine were used as ligands to characterize the solubilized binding sites. In both the soluble and membrane fractions, [³H]idazoxan binding was saturable, stereoselective, sensitive to imidazolines and insensitive to (-)norepinephrine and to amiloride. The affinities of [³H]idazoxan for the soluble and membrane sites were similar (K_D = 25 ± 11 nM and 20 ± 3 nM). In both soluble and membrane fractions, the α₂-adrenoceptor binding being masked with (-)norepinephrine, [³H]clonidine bound to a low affinity site which was insensitive to (-)norepinephrine and which exhibited the same selectivity for various drugs as the [³H]idazoxan binding site. α₂-adrenoceptor binding was present in the membrane and the soluble fractions although it was difficult to detect in the soluble fraction because of inhibition of [³H]rauwolscine binding by the CHAPS detergent.