Ontogeny of imidazoline binding sites in the human placenta

Non-adrenergic imidazoline binding sites (IBS) were described as pharmacologically distinct from alpha2-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 alpha2-adrenoceptors in the human placenta, it was reported that no alpha2-receptors were detected in human placental membranes with the radiolabelled alpha2-adrenoceptor antagonist [3H]rauwolscine or the alpha2-adrenoceptor agonist [3H]clonidine. This scientific contradiction has been solved when the authors have recently demonstrated that IBS and alpha2-adrenoceptors coexist in human term placental membranes.

Scientific literature does not provide any information regarding the ontogeny of IBS. The present study intended to determine the density of IBS and alpha2-adrenoceptors in correlation with gestational age.

Human first and second trimester placentas (6-10 and 14-18 weeks of gestation, respectively) were obtained immediately following the interruption of gestation, third trimester placentas (38-40 weeks) were obtained after normal vaginal delivery. Human placental membrane fractions were prepared and radioligand binding assays were performed in duplicate, using [3H]RX 821002 and [3H]RX 781094 (idazoxan) as radioligands.

According to the results of the binding assays, the concentration of alpha2-adrenoceptors decreased with the advancing gestational age. In contrast with this pattern, the density of IBS significantly increased.

Our present results demonstrated that the density of IBS shows a significantly increasing tendency throughout gestation. The unique position of the placenta between maternal and fetal circulations determines its function as a mediator in transport mechanisms. The increasing expression of IBS in the growing placenta might suggest a role for these sites in the mediation of the transport of nutrients, ions, etc.