Genetic regulation of placental function: a quantitative in situ hybridization study of calcium binding protein (calbindin-D9k) and calcium ATPase mRNAs in sheep placenta.

The calcium requirement of the ovine fetus increases progressively throughout pregnancy. The 9-kDa calcium binding protein (calbindin-D9k; 9CBP) is considered to be a reliable marker for epithelia mediating calcium transport. This quantitative in situ hybridization study shows that the levels of 9CBP mRNA show a pregnancy stage-related increase which correlates with fetal calcium demand only in maternal endometrial gland and fetal interplacentomal trophoblast epithelia. Levels of 9CBP mRNA in the placentome, which has by far the greater area of maternofetal contact, show no changes during pregnancy. mRNA for the CaATPase enzyme, a second requirement for calcium transport, is shown to be present in epithelia in interplacentomal and placentomal regions but shows no change in concentration as pregnancy progresses. Results with the 9CBP and CaATPase mRNAs confirm our recent immunocytochemical results with ruminant placenta and indicate the basis for a cellular calcium transport system analogous to that in the enterocyte. The interplacentomal trophoblast system appears to be eminently suitable for investigations of details of the cellular mechanism and control of epithelial calcium transport.