TRANSPLACENTAL CLEARANCES OF INERT HYDROPHILIC TRACERS IN RABBITS OF 18 DAYS GESTATION

Transplacental clearances were measured for radiolabelled Cl-, SO42-, mannitol, sucrose, raffinose, PEG-900 and inulin. Maternal placental b lood flows were measured with radiolabelled microspheres. At 18 days o f gestation (term 31 days), the fetuses and extra-fetal fluids were gr owing at 55 and 32 per cent per day, accounting for a net transplacent al filtration rate of 14.2 nl/sec per gram placental weight. Pore theo ry and a least squares fit of the Patlak equation yielded an equivalen t pore radius of 1.75 nm. It was demonstrated that the clearance of th e largest tracer, inulin, was 30 times higher than it would have been in the absence of net filtration. Comparison with literature data show ed that there was a small increase in placental permeability per gram placenta between 14 and 18 days of gestation but that the increase bet ween 18 and 28 days of gestation was about 14-fold for Cl- and 300-fol d for inulin. There was no evidence for a decreasing equivalent pore r adius in the course of gestation from 14-18 days.