High and low molecular weight kininogen and plasma prekallikrein/plasma kallikrein in villous capillaries of human term placenta.

This study examined the expression and presence of components of the kallikrein-kinin system in human term placenta. Immunohistochemical studies localized H-kininogen and plasma prekallikrein/plasma kallikrein to endothelial cells of placental villous capillaries. In larger placental blood vessels and umbilical cord, neither kininogens nor kallikreins were detected. High (H) and low (L) molecular weight kininogen, plasma prekallikrein and plasma kallikrein were detected by Western blot analysis in human term placenta and in maternal and fetal blood, whereas tissue kallikrein was not. Furthermore, mRNA of plasma prekallikrein was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) in placental homogenates, while mRNA of H-kininogen, L-kininogen and tissue kallikrein was not. Because H-kininogen and plasma prekallikrein circulate in a complexed form, we suggest that endothelial cells bind kininogen and plasma prekallikrein in which they are secreted by the fetal liver from fetal blood. The co-localization of kininogen and plasma prekallikrein/plasma kallikrein suggests that kinins could be generated locally in placental capillaries. When released, they may play a role in regulating placental blood flow and transplacental transport of substrates and metabolites.