DO ALPHA(2)-ADRENOCEPTORS AND IMIDAZOLINE BINDING-SITES COEXIST IN THE HUMAN TERM PLACENTA - EVIDENCE FROM DIRECT BINDING-STUDIES

alpha(2)-Adrenergic receptors and non-adrenergic imidazoline binding s ites (IBS) in human placental membranes were investigated by means of the radioligands [H-3]-RX 821002 and [H-3]-RX 781094 (idazoxan) respec tively. Human term placentae (38-40 weeks) were obtained immediately a fter vaginal delivery. The specific binding of the alpha(2)-subtype-se lective [H-3]-RX 821002 confirms the presence of alpha(2)-adrenoceptor s in the human placenta, while [H-3]-idazoxan binds to non-adrenergic IBS. The sites were characterized by displacement analyses with variou s imidazoline and non-imidazoline drugs. The presence of an endogenous ligand for IBS has not yet been demonstrated. Clonidine displacing su bstance (CDS) was recently identified as agmatine; it recognizes both alpha(2) and imidazoline receptors. This phenomenon was studied in cru de placental membranes. The studies revealed that: (i) alpha(2)-adreno ceptors coexist with non-adrenergic IBS in human placental membranes; (ii) there is a strong probability that alpha(2)-adrenoceptors and IBS are pharmacologically distinct; and (iii) agmatine binds to placental alpha(2) and imidazoline receptors with different affinities.