Differential development of umbilical and systemic arteries. I. ANG II receptor subtype expression

In fetal sheep umbilical responses to angiotensin II (ANG II) exceed those by systemic vasculature. Two ANG II receptors (AT) exist, AT₁ and AT₂, but only AT₁ mediates vasoconstriction in adult tissues. Thus differences in reactivity could reflect differences in subtype expression. Using competitive radioligand binding assays, we demonstrated AT₁ predominance in umbilical arteries and AT₂ in femoral arteries. Steady- state responses to intravenous ANG II (0.229-1.72 μg/min) were studied in 16 fetuses with umbilical and/or femoral artery flow probes without and with local AT₁ (L-158,809) or AT₂ (PD-123319) blockade. ANG II dose dependently (P < 0.001) increased umbilical resistance more than arterial pressure (MAP) while decreasing umbilical blood flow. Femoral vascular resistance also increased dose dependently (P = 0.02), but responses were less than umbilical (P = 0.0001) and paralleled increases in MAP; blood flow was unaffected. Cumulative local doses of L-158,809 (125 μg) inhibited all responses (P < 0.001); however, 1,000 μg of the AT₂ antagonist had no effect. Plasma renin activity (PRA) was unaltered by local AT₁ blockade, whereas PRA doubled (P = 0.001) after systemic infusion of only 50 μg of the AT₁ antagonist and remained elevated. Differences in umbilical and femoral vascular responses to ANG II are in large part due to differences in AT subtype expression. Furthermore, in fetal sheep the ANG II negative feedback on PRA is mediated by AT₁ receptors, and it is substantially more sensitive to receptor blockade than the vasculature.