Evidence for a functional renin-angiotensin system in full-term fetoplacental unit

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Abstract
This investigation was performed to study the renin-angiotensin system in the human fetoplacental circulation. Full-term placentas from uncomplicated pregnancies were studied within 30 min of delivery. The umbilical artery and vein to a single placental cotyledon were cannulated and the artery perfused with RPMI media (0.764 ml/min). Angiotensin II caused a dose-dependent increase in perfusion pressure that was blunted by the administration of the competitive angiotensin II receptor antagonist saralasin. The properties of human placental angiotensin II receptors were further defined in binding studies performed on a crude membrane fraction of placental cotyledons. In experiments performed at 22 degrees C, saturable binding reached steady state at 30 min and was linear with protein concentration. Scatchard analysis of binding data indicated a single class of high-affinity binding sites. The potency order to competitive binding of analogues and antagonists of angiotensin II was [Ile5]angiotensin II = [Sar1, Ala8]-angiotensin II greater than [Val5]angiotensin II greater than angiotensin III greater than angiotensin II-(3-8) hexapeptide. Further evidence for the physiological significance of angiotensin II binding sites was provided by measurements of the circulating components of the renin-angiotensin system in umbilical venous blood (n = 7). Plasma renin activity, angiotensin I, angiotensin-converting enzyme activity, angiotensin II, and aldosterone were each present in elevated amounts. These experiments provide evidence for an active renin-angiotensin system in the human fetal circulation that may modulate placental perfusion and function under physiological conditions.