Amniotic fluid concentrations of renin and aldosterone during development in the fetal sheep

Simultaneous plasma renin activity and aldosterone levels were measured in the chronically catheterized fetal lamb and pregnant ewe between 100 days gestation and term (140 to 150 days). Amniotic fluid renin levels were not statistically different after the addition of sheep renin substrate [0.76 +/- 0.18 ng/ml/hr (mean and S.E.) under 120 days and 1.33 +/- 0.29 after 125 days]. Amniotic fluid renin levels were unmeasurable without the addition of renin substrate. Basal plasma renin activity levels increased from 4.27 +/- 0.66 ng/ml/hr (P less than 0.01) (mean and S.E.) between 95 to 120 days of gestation in the fetal lamb to 10.14 +/- 1.46 between 125 to 150 days; these levels were greater than the amniotic fluid levels (P less than 0.001). Basal plasma renin activity levels in the pregnant ewe increased from 2.61 +/- 0.40 ng/ml/hr under 120 days of gestation to 7.13 +/- 1.41 (P less than 0.01) after 125 days; these levels were greater than the simultaneous amniotic fluid levels (P less than 0.001). The basal amniotic fluid, fetal, and maternal aldosterone levels of 5.0 +/- 0.53, and 5.83 +/- 0.76 ng/dl (mean and S.E.) increased to 8.97 +/- 0.86, 6.67 +/- 1.21, and 12.28 +/- 2.87 ng/dl, respectively, (P less than 0.05). Neither the fetal nor the maternal levels were statistically different from the amniotic fluid aldosterone levels during development. These data suggest that active renin and renin substrate pass poorly into the amniotic fluid; however, aldosterone from both the fetus and mother pass easily and contribute to the level of this steroid in amniotic fluid.