Angiotensin II receptor characteristics and subtype expression in uterine arteries and myometrium during pregnancy.

The uteroplacental vasculature is less sensitive to angiotensin II (ANG II)-induced vasoconstriction than the systemic vasculature. Although the mechanism(s) responsible is unclear, uterine arteries (Ua) may demonstrate ANG II receptor (AT receptors) down-regulation or expression of AT2 receptors, which do not mediate vasoconstriction. We determined AT receptor binding characteristics and subtype expression in Ua from normotensive pregnant (n = 14; 38 +/- 0.5 weeks gestation) and nonpregnant (n = 28) women. Comparative studies were performed with myometrium, a nonvascular smooth muscle. We measured binding density (Bmax) and affinity (Kd) in plasma membrane preparations employing radioligand binding. Receptor subtypes were assessed by inhibiting [125I]ANG II binding with specific antagonists. During pregnancy, the Ua Bmax and Kd were unchanged (P > 0.1; 221 +/- 36 vs. 159 +/- 27 fmol/mg protein and 0.8 +/- 0.1 vs. 0.9 +/- 0.1 nmol/L, respectively). However, myometrial Bmax decreased 92% (580 +/- 129 vs. 44 +/- 7 fmol/mg protein; P < 0.001), and Kd rose 4-fold (1.5 +/- 0.4 to 6.0 +/- 0.6 nmol/L; P < 0.001). AT1/AT2 expression averaged 15%/85% in Ua from nonpregnant and pregnant women, whereas in myometrium, values were 10%/90% and 60%/40%, respectively. In myometrium from laboring women (n = 8), force (1.38 +/- 0.14 to 1.59 +/- 0.12 x 10(4) N/m2; P < 0.04) and contractile frequency (0.038 +/- 0.05 to 0.116 +/- 0.014 contractions/min; P < 0.001) increased with 10(-5) mol/L ANG II and were abolished by AT1 receptor inhibition. Myometrium from nonpregnant women (n = 3) was unresponsive, and AT2 inhibition did not alter responses. In nonpregnant women, AT2 receptors predominate in Ua and myometrium. Although Ua AT receptors are unaltered during pregnancy, myometrial Bmax decreases, reflecting decreases in the expression of AT2 >> AT1 receptors and differential receptor regulation.