IDENTIFICATION OF ANGIOTENSIN-II RECEPTOR AND DETERMINATION OF ITS SUBTYPES IN RABBIT AND GUINEA-PIG PLACENTA, CHORION AND AMNION

Membrane angiotensin II receptors were measured in trophoblastic tissues using a 2-step procedure. The first step consisted of the relative measurement performed at a fixed I-125[Sar1 IIe8]All concentration of 0.15 nM in order to determine which tissues had a sufficient number of binding sites for studying the competition curves. The second consisted of determining the maximal binding (B(max)) and the dissociation constant (K(d)) for [Sar1 IIe8] All and the receptor subtypes in these tissues. The relative binding measurement revealed a significant number of occupied sites in rabbit fetal placenta and chorion (159 +/- 17 and 51 +/- 10 fmol/mg proteins) and in guinea pig chorion (132 +/- 12). The mean values of the other trophoblastic tissues were 3-10-fold lower in the 2 species. The competition curves obtained from tissues with high angiotensin II binding receptors showed the predominance of the AT2 subtype in rabbit fetal placenta (AT1/AT2 = 25/75) and of the AT1 receptor in guinea pig chorion (97/3) and in rabbit chorion (90/10). The [SAR1 IIe8] All affinity (K(d)) obtained from Scatchard plot analysis was 1.2 +/- 0.2 nM (n = 5) in fetal placenta and 1.2 (n = 1) in rabbit chorion and 0.5 +/- 0.1 (n = 3) in guinea pig chorion. In these tissues, the respective B(max) values were 1 281 +/- 115 (n = 5), 263 (n = 1) and 1 188 +/- 134 fmol/mg proteins (n = 3). These findings indicate that rabbit fetal placenta and chorion and guinea pig chorion are the most important sites of action for the renin-angiotensin system present in trophoblastic tissues.