Cellular localization of AT1 receptor mRNA and protein in normal placenta and its reduced expression in intrauterine growth restriction. Angiotensin II stimulates the release of vasorelaxants

Abstract

Angiotensin II (ANG II) is a potent vasoconstrictor and growth promoter. Quantitative receptor autoradiography using the nonselective radioligand [125I]ANG II and subtype-selective competing compounds demonstrated the presence of both ANG II receptor (AT)1 and AT2 receptor recognition sites. In addition, a relatively small population of apparently non-AT1/non-AT2 sites was identified that may represent a novel high affinity ANG II recognition site in human placenta. Using placental membrane preparations, the AT2 receptor antagonist PD123177 failed to compete for [3H]ANG II binding at relevant concentrations, whereas the AT1 receptor antagonist losartan competed in a monophasic manner for all the specific binding, suggesting that the non-AT1/non-AT2 recognition site identified using autoradiography may be a cytosolic binding site. AT1 receptor binding was significantly reduced (P < 0. 02) in intraeuterine growth restriction (IUGR) pregnancies. Western blot analysis confirmed this showing a reduction in AT1 receptor protein. In situ hybridization and immunocytochemistry revealed that AT1 receptor mRNA and protein were localized throughout pregnancy in the cytotrophoblast, syncytiotrophoblast, and extravillous trophoblast, as well as in or around the blood vessels of placental villi. The intensity of the hybridization signal for AT1 receptor mRNA over the syncytium was reduced in IUGR. ANG II evoked a rapid and concentration-dependent release of NO in first trimester cytotrophoblast-like cells that was abolished by the inclusion of the competitive NOS inhibitor NG-monomethyl-L-arginine. Neither losartan nor PD123177 alone significantly inhibited ANG II-evoked NO release, and when cells were stimulated with ANG II in the presence of losartan (10 microM) and PD123177 (10 microM) in combination, NO release was significantly inhibited (P < 0.05). These observations also suggest, for the first time, the existence of a cross-talk between AT1 or AT2 receptors in trophoblast and that the reduction in placental AT1 receptors in IUGR may, in part, account for poor placental function in this disorder.