Placental permeability in the sheep.

The transplacental flux of different radiolabelled substances was measured, according to Fick's principle, in chronically catheterised pregnant sheep (124 to 143 days of gestation). Knowing the uterine blood flow, it was possible to calculate the net flux and derive a permeability coefficient, K. With 14C erythritol, K soon reaches its ultimate value, thus suggesting there is only one rate-limiting step to transfer and no significant middle compartment in the sheep placenta. The comparison of feto-maternal fluxes of different metabolically inert, lipid insoluble molecules (urea, erythritol, mannitol and Cr-EDTA) demonstrates a very sharp decrease in permeability as molecular size increases. The membrane behaves like a tight epithelium, with pores of of an approximate radius of 0.45 nm which allow but a poor passive diffusion. On the contrary, permeability to 3 methylglucose and 2 deoxyglucose is much too high for passive diffusion. Transfer competition between D-glucose and 3 methylglucose affords other evidence for a stereospecific monosaccharide carrier. The relationship between the presumably "active" sodium flux out of the fetus and the electrical potential difference across the placenta is discussed. The fetus is usually negative to the mother but the potential difference changes from day to day, and in twin gestation, the respective fetomaternal potentials seem to be independent. Acid infusion to the fetus increases the potential difference. The maintenance of this electrical or chemical gradients is taken as a consequence of the low passive permeability of the placenta.