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

• Published in: British journal of pharmacology Vol. 106; no. 1; pp. 101 - 108
• Main Authors: Diamant, Sophia, Eldar‐Geva, Talia, Atlas, Daphne
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.05.1992; Nature Publishing
• Subjects: 4-Aminopyridine - pharmacology; Adenosine Triphosphatases - metabolism; Amiloride - metabolism; Binding Sites; Biological and medical sciences; Cations, Monovalent - pharmacology; CDS‐clonidine displacing substance; Cells, Cultured; Dioxanes - metabolism; Female; Fundamental and applied biological sciences. Psychology; Human placenta; Humans; Idazoxan; imidazoline; Imidazoline Receptors; imidazoline sites; man; membrane proteins; Mother. Fetoplacental unit. Mammary gland. Milk; placenta; Placenta - enzymology; Placenta - metabolism; potassium; Pregnancy. Parturition. Lactation; receptors; Receptors, Adrenergic, alpha - metabolism; Receptors, Drug - metabolism; Temperature; transport; Trophoblasts - metabolism; Vertebrates: reproduction; Characterization; Human; Trophoblaste; Cell culture; Brush border; Platelet; Placenta; Binding site; In vitro; Biological receptor
• ISSN: 0007-1188; 1476-5381
• DOI: 10.1111/j.1476-5381.1992.tb14300.x

1 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; Kd 38.9 ± 3.4 nm). 2 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). 3 [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. 4 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. 5 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 IRa subtype (placenta, rabbit kidney, rabbit liver and rabbit adipose cells) as opposed to the IRb subtype which display low affinity for amiloride (> 2 μm, in all the other tissues). 6 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 (Kd = 18.9 ± 13.8 nm in human placenta), than the affinity for α2‐adrenoceptors (Kd = 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. 7 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.