Osmotic water permeabilities of human placental microvillous and basal membranes

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Abstract
Literature data suggest that water accumulation by the human fetus is driven by osmotic gradients of small solutes. However, the existence of such gradients has not been supported by prior measurements. Attempts to estimate the size of the gradient necessary to drive net water movement have been seriously hampered by the lack of permeability data for the syncytiotrophoblast membranes. Stopped-flow light scattering techniques were employed to measure the osmotic water permeability (P f )of microvillous (MVM) and basal membrane (BM) vesicles isolated from human term placenta. At 37°C, the P f was determined to be 1.9±0.06 × 10+−3 cm/sec for MVM and 3.1±0.20 × 10+−3 cm/sec for BM (mean ±SD, n = 6). At 23°C, P f was reduced to 0.7±0.04 × 10+−3 cm/sec in MVM and 1.6±0.05 × 10+−3 cm/sec in BM. These P f values are comparable to those observed in membranes where water has been shown to permeate via a lipid diffusive mechanism. Arrhenius plots of P f over the range 20–40°C were linear, with activation energies of 13.6 ± 0.6 kcal/mol for MVM and 12.9±1.0 kcal/mol for BM. Water permeation was not affected by mercurial sulfhydryl agents and glucose transport inhibitors. These data clearly suggest that water movement across human syncytiotrophoblast membranes occurs by a lipid diffusion pathway. As noted in several other epithelial tissues, the basal membrane has a higher water permeability than the microvillous membrane. It is speculated that water accumulation by the human fetus could be driven by a solute gradient small enough to be within the error of osmolarity measurements.