Excess extrafetal fluid without demonstrable changes in placental concentration gradients after week-long infusions of angiotensin into fetal lambs

It is known that a week-long infusion of angiotensin into fetal sheep produces polyhydramnios. The purpose of the present experiments was to determine whether an increased osmotic force across the placental barrier could account for the excess transfer of water. Six fetuses with indwelling catheters were infused with angiotensin-I and one with angiotensin-II; all, except one fetus in the first group, developed gross polyhydramnios. None of the transplacental concentration differences of the small plasma solutes Na+, Cl-, HCO3-, K+, urea, or glucose showed a demonstrable change and the same was true of the transplacental difference in freezing point osmolality and for the transplacental difference in plasma protein concentration. It is concluded that the infusion of angiotensin at a low dose rate is a reliable protocol for producing polyhydramnios. However, the present findings lend no support to the hypothesis that a primary change in transplacental osmotic force is the cause of the increased transplacental water transfer in this form of polyhydramnios. Alternative hypotheses are discussed in the light of recent discoveries.