HUMAN PLACENTAL PERMEABILITY TO HYDROPHILIC MATERIALS

Previous work from this laboratory has shown that the hemochorial placentas
of the rabbit and the guinea pig do not strongly discriminate molecular
size. It is also known that the epitheliochorial placenta of the sheep so
strongly discriminates size that the placenta is essentially opaque to
hydrophilic materials of greater than 200 molecular weight. It is the
purpose of this proposal to prove our hypothesis that the permeability
characteristics of the human placenta will be like those of the other
hemochorial placentas and not like those of the placenta of the sheep.
Nontoxic, nonradioactive, and metabolically inert tracers will be given
i.v. to pregnant women shortly before their scheduled elective cesarean
sections. At the time of birth, fetal tracer contents will be estimated
from fetal plasma concentrations and an approximation of the distribution
volume of the tracer and/or from the integral collection of fetal urine
until all tracer has been excreted. The timing of the injections will be
such that at birth fetal plasma concentrations will be less than 10% of the
final maternal plasma concentrations. The transplacental concentration
difference is therefore essentially equal to the maternal plasma
concentration. The quantitative results from this study should be of help
in calculating fetal drug exposures when hydrophilic drugs are present in
maternal plasma in known concentrations. They should also allow the
construction of a more accurate theoretic model for prenatal water
transfer, and thus help elucidate the pathophysiology of hydrops fetalis.