Placental transfer of aldosterone in the sheep.

To determine the percentage of the maternal secretion of aldosterone which crosses the placenta the blood clearance rate (BCR) of aldosterone was measured in pregnant sheep and in chronically cannulated fetuses by the constant infusion of [3H]aldosterone alternately into the maternal and fetal compartments. When equilibrium had been reached the concentration of [3H]aldosterone was measured in both maternal and fetal compartments. Aldosterone BCR in eight pregnant ewes was 98 +/- 5 (S.E.M.) litres/h which was not significantly different from that of ten non-pregnant ewes at 95 +/- litres/h. The BCR of aldoterone in seven fetuses was 24 +/- litres/h. A small percentage (4.4 +/- 0.3; n = 7) of the maternal production rate was transferred to the fetus, whilst 29 +/- 4% (n = 8) of the fetal production rate was transferred to the maternal compartment. Whjen aldosterone was measured in maternal and fetal blood samples collected simultaneously from sodium-replete sheep more than 80% of the aldosterone in fetal blood was of fetal origin if the actual fetal concentration of aldosterone was greater than 1.5 ng/dl.