Ontogeny of imidazoline binding sites in the human placenta

Introduction. Non-adrenergic imidazoline binding sites (IBS) were described as pharmacologically distinct from alpha(2)-adrenergic receptors. Recently it was shown that the human placenta is the richest source of IBS, however , no function has been assigned to this new putative receptor. As concerns the presence of alpha(2)-adrenoceptors in the human placenta, it was report ed that no alpha(2)-receptors were detected in human placental membranes wi th the radiolabelled alpha(2)-adrenoceptor antagonist [H-3]rauwolscine or t he alpha(2)-adrenoceptor agonist [H-3]clonidine. This scientific contradict ion has been solved when the authors have recently demonstrated that IBS an d alpha(2)-adrenoceptors coexist in human term placental membranes. Study objective. Scientific literature does not provide any information reg arding the ontogeny of IBS. The present study intended to determine the den sity of IBS and alpha(2)-adrenoceptors in correlation with gestational age. Materials and methods. Human first and second trimester placentas (6-10 and 14-18 weeks of gestation, respectively) were obtained immediately followin g the interruption of gestation, third trimester placentas (38-40 weeks) we re obtained after normal vaginal delivery. Human placental membrane fractio ns were prepared and radioligand binding assays were performed in duplicate , using [H-3]RX 821002 and [H-3]RX 781094 (idazoxan) as radioligands. Results. According to the results of the binding assays, the concentration of alpha(2)-adrenoceptors decreased with the advancing gestational age. In contrast with this pattern, the density of IBS significantly increased. Conclusion. Our present results demonstrated that the density of IBS shows a significantly increasing tendency throughout gestation. The unique positi on of the placenta between maternal and fetal circulations determines its f unction as a mediator in transport mechanisms. The increasing expression of IBS in the growing placenta might suggest a role for these sites in the me diation of the transport of nutrients, ions, etc.