Imidazoline binding sites in human placenta: evidence for heterogeneity and a search for physiological function.
1. An alpha 2-adrenoceptor antagonist, idazoxan, that binds to both alpha 2-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 alpha 2-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 alpha 2-adrenoceptors in that other alpha 2-adrenoceptor ligands, [3H]-rauwolscine and [3H]-clonidine, do not bind to alpha 2-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 (greater than 2 microM, 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 M2-adrenoceptors as the affinity of BAG is about 40 fold higher (Kd= 18.9 +/- 13.8 nM in human placenta), than the affinity for M2-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.
Diamant
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Department of Biological Chemistry, Hebrew University of Jerusalem, Israel.
Eldar-Geva
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Atlas
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D015761:4-Aminopyridine / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000584:Amiloride / Q000378:metabolism; D001665:Binding Sites; D002414:Cations, Monovalent / Q000494:pharmacology; D002478:Cells, Cultured; D004146:Dioxanes / Q000378:metabolism*; D005260:Female; D006801:Humans; D019329:Idazoxan; D054755:Imidazoline Receptors; D010920:Placenta / Q000201:enzymology / Q000378:metabolism*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism; D011955:Receptors, Drug / Q000378:metabolism*; D013696:Temperature; D014327:Trophoblasts / Q000378:metabolism
D002414:Cations, Monovalent; D004146:Dioxanes; D054755:Imidazoline Receptors; D011942:Receptors, Adrenergic, alpha; D011955:Receptors, Drug; D000584:Amiloride; D015761:4-Aminopyridine; D000251:Adenosine Triphosphatases; D019329:Idazoxan