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 bar rier could account for the excess transfer of water. Six fetuses with indwelling catheters were infused with angiotensin-I and one with angi otensin-II; all, except one fetus in the first group, developed gross polyhydramnios. None of the transplacental concentration differences o f the small plasma solutes Na+, Cl-, HCO3-, K+, urea, or glucose showe d a demonstrable change and the same was true of the transplacental di fference in freezing point osmolality and for the transplacental diffe rence in plasma protein concentration. It is concluded that the infusi on of angiotensin at a low dose rate is a reliable protocol for produc ing polyhydramnios. However, the present findings lend no support to t he 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.