Development of passive permeability characteristics of rat placenta during the last third of gestation

To investigate how the passive permeability of the rat placenta changes during the last third of gestation we measured the unidirectional maternofetal clearance (K(mf)) of three polar nonelectrolytes. We found that K(mf) (μl·min-1·g placenta-1) for all three tracers increased between days 15 and 22 (term is 23 days). The increase was less for the largest tracer, [14C]inulin (3.5-fold increase from 0.5 ± 0.1 to 1.6 ± 0.1 μl·min-1·g placenta-1), than for the smaller tracers, [14C]mannitol (7-fold increase from 1.6 ± 0.1 to 10.9 ± 0.4 μl·min-1·g placenta-1) and 51Cr-labeled EDTA (9.5-fold increase from 0.8 ± 0.04 to 7.6 ± 0.7 μl·min-1·g placenta-1). These data suggest that whilst the overall permeability of the rat placenta increases during this period of pregnancy, restriction to the diffusion of large (≥1.5 nm molecular radius) polar nonelectrolytes also increases.