Imidazoline binding sites in human placenta: evidence for heterogeneity and a search for physiological function

An α₂‐adrenoceptor antagonist, idazoxan, that binds to both α₂‐adrenoceptors and to imidazoline sites (IR), has been used to characterize human placental IR. Human placenta is shown to be the richest source of IR (1800 ± 100 fmol mg⁻¹ protein; K_d 38.9 ± 3.4 nm). Primary cells derived from human placenta and grown in monolayers, also displayed a high density of receptors (3209 ± 136 fmol mg⁻¹ in cytotrophoblasts and 3642 ± 144 fmol mg⁻¹ protein in syncytiotrophoblast enriched cell culture). [³H]‐idazoxan did not show binding characteristics of α₂‐adrenoceptors in human placental membranes or human trophoblastic cells, thus making it a ligand of choice to study the imidazoline site. The tissue appeared to be lacking α₂‐adrenoceptors in that other α₂‐adrenoceptor ligands, [³H]‐rauwolscine and [³H]‐clonidine, do not bind to α₂‐adrenoceptors in human placenta. IRs are localized on the cell surface, as determined by the release of bound [³H]‐idazoxan from cells, when washed with high ionic/acidic medium. Imidazoline receptors of human placenta display high affinity for amiloride (72 ± 27 nm). The high affinity was used as a criterion to classify IR to IR_a subtype (placenta, rabbit kidney, rabbit liver and rabbit adipose cells) as opposed to the IR_b subtype which display low affinity for amiloride (> 2 μm, in all the other tissues). Several novel ligands comprising a guanido functional group attached to an aromatic residue (e.g. benziliden‐amino‐guanidine (BAG), guanido pyrole) display pronounced selectivity for IR over the α₂‐adrenoceptors as the affinity of BAG is about 40 fold higher (K_d = 18.9 ± 13.8 nm in human placenta), than the affinity for α₂‐adrenoceptors (K_d = 768 ± 299 nm in human platelets). Imidazoline sites bind selectively BAG and other guanido ligands thus indicating a distinct structural requirement at its site of binding. K⁺ channel blockers and monovalent ions (e.g. Cs⁺ and NH₄⁺) interfere with idazoxan binding to IR, indicating a possible involvement of IR in K⁺ transport. 1992 British Pharmacological Society