Abstrakt
In order to investigate mechanisms of fetal-maternal (F-M) transfer of Na+, clearance of Na-22(+) and Cr-51-EDTA was measured simultaneously across the dually perfused placenta of the rat. In eight experiments clearance was measured successively in the F-M and in the maternal-fetal (M-F; K-mf) directions.
Clearance of Na-22(+) in the two directions was approximately equal (K-mf = 11.6 +/- 2.0 mu l/min; K-fm = 11.1 +/- 1.7 mu l/min: mean +/- s.d.) while K-fm Of Cr-51-EDTA (4.4 +/- 0.7 mu l/min) was nearly double K-mf(2.4 +/- 0.8 mu l/min) for this tracer. Even greater asymmetry in the transfer of Cr-51-EDTA was found when measured across intact (non-perfused) placenta.
It is suggested that this asymmetry is caused by volume flow in the F-M direction. In other experiments transfer was measured in the F-M direction only.
Ouabain (0.1 mM) on the maternal side and reduced concentration of Na+ (25 m/M) on the fetal side had no effect on the F-M transfer of the tracers. Reducing the temperature of the preparation by 5 degrees C significantly decreased transfer of Na+.
The transfer of Na-22 was inversely related to the concentration of K+ on the fetal side. These observations suggest that the F-M transfer of Na+ has three components. diffusion through paracellular routes; convective flow by filtration through wide placental pores, and transcellular transport by cc mechanism which is uncertain at present.