AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 3 670-R675, Copyright © 1993 by American Physiological Society
Asymmetrical transfer of inert hydrophilic solutes across rat placenta
J. Stulc and B. Stulcova
Department of Pharmacology, Second Faculty of Medicine, Charles University, Prague, Czech Republic.
Transfer of [14C]mannitol, 51Cr-labeled EDTA, and [14C]-inulin from mother
to fetus and from fetus to mother was measured in rats under pentobarbital
anesthesia. The clearance of the three substances from the mother to fetus
(Kmf) was 2.69 +/- 0.38, 1.93 +/- 0.73, and 0.47 +/- 0.14 microliter/min
(means +/- SE), respectively, and the clearance from fetus to mother (Kfm)
was 5.97, 6.66, and 4.95 microliters/min, respectively (the SE could not be
estimated). Kfm appears to be consistently higher than Kmf by an almost
constant value of approximately 4 microliters/min. To explain this a
hypothesis was proposed according to which volume flow circulates across
the placenta. Solute-free water is driven transcellularly across the
placental barrier from the maternal to the fetal side by a difference of
osmotic pressure created by active transport of some solutes (mainly Na+)
to the fetus. Water together with all solutes dissolved returns from fetus
to mother by filtration through wide extracellular channels in the placenta
down a hydrostatic pressure gradient.
