CHARACTERIZATION AND ONTOGENY OF ENDOTHELIN RECEPTORS IN HUMAN PLACENTA

Because of the potent mitogenic and vasoactive properties of endothelin-1 (ET-1) and the presence of its receptor in third trimester placenta, we postulated that ET-1 might be involved in human placental growth and vascularization during development. As an initial approach to test this hypothesis, placental ET receptors were characterized and quantified in each trimester of pregnancy. Membrane-rich particulates were prepared from first-, second-, and third-trimester villous human placenta obtained immediately after pregnancy termination or delivery. ET receptors were characterized by radioligand saturation analysis, ligand competition, and reverse transcription-polymerase chain reaction (RT-PCR) to determine the concentration, affinity, and specificity of ET binding sites, and to document the presence of specific ET-receptor subtype mRNA transcripts in placentas from each trimester. Kinetic determinations of I-125-labeled ET-1 binding yielded a K(d) = 61 pM, consistent with the equilibrium determinations of 34 +/- 6 pM (n = 11). The concentration of ET receptors decreased significantly from 682 +/- 94 fmol/mg protein (n = 4) in the first trimester to 266 +/- 89 fmol/mg protein (n = 4) in the third trimester. Competition studies with unlabeled ET-1 indicated a single class of binding sites with a K(i) = 49 +/- 5 pM (n = 9), whereas competition with ET-3 demonstrated binding sites with two affinities. The predominant sites had a K(i) = 84 +/- 14 pM, similar to that for ET-1. The RT-PCR data confirmed that both ETA and ETB receptors mRNA transcripts are expressed in human placenta. These data demonstrate that ET receptor concentrations are highest in first-trimester placenta and decrease progressively toward term. The similar affinity for ET-1 and ET-3 indicates that the predominant ET receptor in placenta belongs to the ETB or ''nonselective receptor'' subtype, characteristic of the ET receptor subtype in endothelial cells.