Characterization of an imidazoline/guanidinium receptive site distinct from the α₂-adrenergic receptor

α₂-Adrenergic receptors recognize a number of molecules with diverse chemical structures, including the yohimban diastereoisomers yohimbine and rauwolscine, catecholamines, guanidinium analogs, and imidazolines, such as clonidine. The affinity of the receptor protein for some of these ligands can vary by 10-100-fold among various tissues and species, suggesting a heterogeneous class of binding sites. Certain cellular effects elicited by the compounds possessing an imidazoline or guanidinium moiety may actually be mediated by a membrane receptor distinct from the α₂-adrenergic receptor. To determine whether this imidazoline/guanidinium receptive site (IGRS) and the α₂-adrenergic receptor represent distinct proteins, we solubilized and partially characterized the two binding sites in rabbit kidney. This tissue expresses both α₂-adrenergic receptors and high affinity imidazoline/guanidinium binding sites, the latter which are rauwolscine-insensitive but can be identified with the benzodioxan [³H]idazoxan. The IGRS and α₂-adrenergic receptor in rabbit kidney exhibit distinct ligand recognition properties, which are maintained after solubilization and partial purification. In addition, the two receptors can be physically separated by heparin-agarose or lectin affinity chromatography indicating that the two binding sites are distinct entities. [³H]Idazoxan binding is trypsin-sensitive, indicating that the IGRS is a protein rather than a lipid component of the plasma membrane. [³H]Idazoxan binding is not inhibited by endogenous agonists for known neurotransmitter receptors. However, the IGRS does not recognize clonidine-displacing substance, a small non-catechol compound isolated from calf brain, suggesting the existence of a previously uncharacterized hormonal/neurotransmitter receptor system.