Nonadrenergic [³H]idazoxan binding sites are physically distinct from α₂-adrenergic receptors

We have recently demonstrated that the α₂-adrenergic radioligand [³H]idazoxan also labels additional sites that do not recognize catecholamines but bind with high affinity several chemically distinct drugs previously assumed to be highly selective for α-adrenergic receptors [Mol. Pharmacol. 35:324-330 (1989)]. We now have used three approaches to distinguish the nonadrenergic [³H]idazoxan sites from α₂-adrenergic receptors. (a) No nonadrenergic [³H]idazoxan binding sites were found in COS-7 cells transfected with the genes for the two known α₂-adrenergic receptor subtypes. (b) The ratio of α₂-adrenergic and nonadrenergic [³H]idazoxan sites in human platelet membranes varied considerably between various donors. (c) Highly purified platelet plasma membranes were enriched for α₂-adrenergic receptors but did not contain any nonadrenergic [³H]idazoxan binding sites. We conclude that the nonadrenergic [³H]idazoxan binding sites are not co-expressed with α₂-adrenergic receptor and at least in human platelets may be located in an intracellular compartment.