NONELECTROLYTE SOLUTE PERMEABILITIES OF HUMAN PLACENTAL MICROVILLOUS AND BASAL MEMBRANES

1. Permeability to non-electrolytes of isolated microvillous and basal membranes from human term placenta was measured using stopped-flow li ght-scattering techniques. The studied solutes were urea, ethylene gly col, glycerol, creatinine, erythritol, arabitol and mannitol. 2. At 37 -degrees-C, permeability of the microvillous membrane to mannitol and urea was 0.30 +/- 0.02 x 10(-6) cm/s (mean +/- S.E.M.) and 3.2 +/- 0.2 x 10(-6) cm/s, respectively. The corresponding permeabilities for the basal membrane were 1.2 +/- 0.1 x 10(-6) cm/s (mannitol) and 4.4 +/- 0.3 x 10(-6) cm/s (urea). The basal membrane was substantially more pe rmeable to hydrophilic solutes than the microvillous membrane. This is probably due to differences in lipid composition, as illustrated by m embrane cholesterol content, which was found to be approximately 50 % lower in the basal as compared to the microvillous membrane. 3. Simila rities between permeabilities in placental membranes and lipid bilayer s and the linear relationship noted between solute hydrophobicity and placental permeability suggested that solutes permeate both human sync ytiotrophoblast membranes by a solubility/diffusion mechanism. In the microvillous membrane this was supported by data obtained for activati on energies (> 10 kcal/mol) and reflection coefficients (close to 1). In the basal membrane, low activation energies for glycerol and urea a nd a low reflection coefficient for urea indicated that these solutes may, in part, share a common pathway with water. 4. It was estimated t hat the placental permeability to molecules with a molecular weight un der 200 observed in vivo can, to a great extent, be accounted for by t ranscellular permeation.