The effect of prostacyclin on angiotensin II-induced placental vasoconstriction

Significant alterations in vascular responsiveness to angiotensin II have been documented during pregnancy. We have observed that prostacyclin, a potent vasodilating prostaglandin, does not dilate the ovine placental vasculature. However, we thought it might modulate the placental vasoconstriction produced by angiotensin II. Regional blood flows and resistances were measured by the radioactive microsphere technique in six near-term sheep. Blood flows were measured in the control condition and 15 minutes after beginning an infusion of angiotensin II at 5 micrograms/min (T1). Additional measurements were made 15 minutes after the addition of 50 micrograms/min of prostacyclin to the angiotensin II infusate (T2) and 15 minutes after withdrawing prostacyclin from the angiotensin II infusion (T3). Mean arterial pressure rose in response to angiotensin II and decreased significantly with prostacyclin administration. The renal and uterine nonplacental vascular beds showed the expected vasoconstriction in response to angiotensin II, which was then reversed to control levels by prostacyclin infusion. Unexpectedly, prostacyclin did not reverse the angiotensin II vasoconstriction in the placenta but further increased resistance (p less than 0.03). Placental resistance changed from 0.33 +/- 0.04 peripheral resistance units in the control condition to 0.42 +/- 0.06 peripheral resistance units for T1 (p less than 0.03), and prostacyclin infusion further increased placental resistance to 0.63 +/- 0.10 peripheral resistance units for T2 (p less than 0.03). We conclude that the placental vascular response to prostacyclin is different from that of other organs and that prostacyclin does not dilate, but further constricts the placenta in the near-term sheep with angiotensin II-induced systemic vasoconstriction.