CELLULAR-LOCALIZATION OF AT(1) RECEPTOR MESSENGER-RNA AND PROTEIN IN NORMAL PLACENTA AND ITS REDUCED EXPRESSION IN INTRAUTERINE GROWTH RESTRICTION - ANGIOTENSIN-II STIMULATES THE RELEASE OF VASORELAXANTS

Angiotensin II (ANG II) is a potent vasoconstrictor and growth promote r. Quantitative receptor autoradiography using the nonselective radiol igand [I-125]ANG II and subtype-selective competing compounds demonstr ated the presence of both ANG II receptor (AT)(1) and AT2( )receptor r ecognition sites. In addition, a relatively small population of appare ntly non-AT(1)/non-AT(2) sites was identified that may represent a nov el high affinity ANG II recognition site in human placenta. Using plac ental membrane preparations, the AT2( )receptor antagonist PD123177 fa iled to compete for [H-3]ANG II binding at relevant concentrations, wh ereas the AT(1) receptor antagonist losartan competed in a monophasic manner for all the specific binding, suggesting that the non-AT(1)/non -AT(2) recognition site identified using autoradiography may be a cyto solic binding site. AT(1) receptor binding was significantly reduced ( P < 0.02) in intraeuterine growth restriction (IUGR) pregnancies. West ern blot analysis confirmed this showing a reduction in AT(1) receptor protein. In situ hybridization and immunocytochemistry revealed that AT(1) receptor mRNA and protein were localized throughout pregnancy in the cytotrophoblast, syncytiotrophoblast, and extravillous trophoblas t, as well as in or around the blood vessels of placental villi. The i ntensity of the hybridization signal for AT(1) 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 N-G-monomethyl-L-arginine. Neither losartan nor PD123177 alone signifi cantly inhibited ANG II-evoked NO release, and when cells were stimula ted with ANG II in the presence of losartan (10 mu M) and PD123177 (10 mu M) in combination, NO release was significantly inhibited (P < 0.0 5). These observations also suggest, for the first time, the existence of a cross-talk between AT(1) or AT(2) receptors in trophoblast and t hat the reduction in placental AT(1) receptors in IUGR may, in part, a ccount for poor placental function in this disorder.