In vivo placental permeability to hydrophilic solutes as a function of fetal weight in the guinea pig.

The permeability of the placenta was measured with unlabelled inulin in unanaesthetized guinea pigs. Inulin was injected into the sows and the plasma concentrations were recorded at regular intervals. The inulin contents of the conceptuses were calculated from fetal weights, fetal plasma inulin concentrations and a measured inulin distribution volume of 180 ml/kg fetus. From fetal inulin contents and the time integral of the difference in maternal and fetal plasma concentrations, we calculated a mean inulin permeability of 29 +/- 9 (s.e.m.) nl/s per gram placental weight. The inulin permeability per gram placental weight rose statistically significantly with increasing fetal weight. Comparison with a similar rise observed for the smaller cyanocobalamin molecule studied previously led to the conclusions that in the last ten days of gestation the transplacental passages narrow while at the same time there are apparent changes in the numbers and/or lengths of these passages that suffice to increase the unrestricted permeability fourfold.