Transport of sodium and protons in the human placenta. Mechanisms in normal pregnancy and alterations in intrauterine growth restriction

Abstract

The syncytiotrophoblast cell of the human placenta represents the interface between maternal and fetal blood supplies. This polarized epithelium consists of a maternal facing microvillous membrane (MVM) opposing a fetal facing basal membrane (BM). This unique syncytial cell is responsible for transfer of all nutrients to the developing fetus. Intrauterine growth restriction (IUGR) is an important cause of perinatal morbidty and mortality, and is associated with a tendency toward fetal acidosis and decreased fetal plasma amino acid levels. Distinct alterations of Na+ dependent amino acid transporters in the MVM have been described in IUGR. The electrochemical gradient for Na+ is maintained by the Na+/K+ ATPase and is the driving force for secondary active transporters such as Na+ coupled amino acid transport, Na+/H+ exchanger (NHE) and Na+/Ca2+ exchanger. Despite the importance of Na+/K+ ATPase for energizing placental transport, information on tissue distribution and activity is limited. The aims of this thesis were therefore to establish the localization and activity of placental Na+/K+ ATPase and to investigate whether Na+/K+ ATPase is reduced in IUGR pregnancies, possibly contributing to decreased nutrient transfer. To investigate whether the function of the major placental pH regulator, the NHE is impaired in IUGR pregnancies and to study the hormonal regulation of NHE in placental cells. Finally, we identified an additional pH regulating mechanism in the human placenta. Using immunohistochemistry, Western Blotting and activity measurements, the Na+/K+ ATPase was shown to be expressed in both the MVM and BM, with the majority of activity and expression localized to the MVM. NHE isoforms 1-3 were shown to be expressed predominantely in the MVM. The expression and activity of NHE-1 were shown to be reduced by one third in preterm MVM isolated from IUGR pregnancies. In a simplified placental BeWo cell culture system, epidermal growth factor was shown to activate (one hour exposure) NHE, whereas insulin upregulated NHE-1 expression following 18 hours incubation. Finally, the non-gastric H+/K+ ATPase was shown to be expressed on the RNA and protein level in the placental MVM and BM. The distribution of Na+/K+ ATPase to the MVM may represent a route of early Na+ reexport from the fetal to the maternal compartment to reduce the fetal sodium load from secondary active transport systems. The decreased activity and expression of Na+/K+ ATPase in MVM from pregnancies complicated by IUGR possibly contributes to the decreased nutrient transfer capacity associated with this condition. The decrease in NHE activity and expression may explain the acidosis tendency of IUGR fetuses, especially in conjunction with the reduced Na+/K+ ATPase activity. The connection between the reduced NHE activity in placental MVM and IUGR may be hormonal, since we provide evidence that EGF and Insulin, both of which are known to be decreased during IUGR, regulate NHE in placental BeWo cells. Finally, the demonstration of the non gastric H+/K+ ATPase in the syncytiotrophoblast indicates the presence of a new placental pH regulating mechanism in MVM and BM of human placenta