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

Br J Pharmacol. 1992 May;106(1):101-8. doi: 10.1111/j.1476-5381.1992.tb14300.x.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 4-Aminopyridine / pharmacology
  • Adenosine Triphosphatases / metabolism
  • Amiloride / metabolism
  • Binding Sites
  • Cations, Monovalent / pharmacology
  • Cells, Cultured
  • Dioxanes / metabolism*
  • Female
  • Humans
  • Idazoxan
  • Imidazoline Receptors
  • Placenta / enzymology
  • Placenta / metabolism*
  • Receptors, Adrenergic, alpha / metabolism
  • Receptors, Drug / metabolism*
  • Temperature
  • Trophoblasts / metabolism


  • Cations, Monovalent
  • Dioxanes
  • Imidazoline Receptors
  • Receptors, Adrenergic, alpha
  • Receptors, Drug
  • Amiloride
  • 4-Aminopyridine
  • Adenosine Triphosphatases
  • Idazoxan