Calcium transport in the early conceptus and associated maternal tissues in the rabbit

Effect of Ca2+ on structure and fluidity of microvillus membranes of human placenta

This investigation shows the effect of a Ca2+ addition on the structural and physiochemical properties of microvillus plasma membranes obtained from human placenta. Ca2+ addition induces an increase in microviscosity, as shown by the increase of order parameter and rotational correlation time of 5- and 16-doxylsterate derivatives and by the increase of fluorescence polarization of diphenylhexatriene. All the effects were obtained in a wide temperature range. The morphometric analysis of the ultrastructural images shows that the vesicle profiles of syncytiotrophoblast membranes decrease both area and form factor (FF) in the presence of Ca2+ with respect to the controls. The freeze-fracture results also show that Ca2+ induces an enhanced tendency to IMP clusterization. The Ca2+-induced changes were observed in both E and P faces. Our results underline the important role of Ca2+ in the cell membrane structure per se and in modulating interactions between cytoplasmic and extracellular microenvironments. The results of morphometric analysis of the ultrastructural images agree with biochemical data showing an increased stability induced by calcium on plasma membranes.

Identification and characterization of a calcium-binding protein in the mouse chorioallantoic placenta

Mouse chorioallantoic placenta contains a specific calcium-binding protein (MCaBP). A procedure involving gel filtration and ion-exchange chromatography was developed to purify the MCaBP. The MCaBP activity increased as a function of embryonic gestation and was highly specific for Ca2+. The MCaBP is a monomeric protein of Mr 57000, with pI 4.7. Specific antibodies were prepared against the MCaBP and were used to localize the MCaBP to syncytiotrophoblasts of the chorionic villi of mouse chorioallantoic placenta. These properties suggest that the MCaBP may be involved in transplacental calcium transport.

Ca2+-binding protein of the human placenta. Characterization, immunohistochemical localization and functional involvement in Ca2+ transport

The Ca2+-binding protein (HCaBP) of the human placenta was studied with respect to its biochemical properties, tissue and cellular distribution, and possible involvement in placental Ca2+ transport. Optimal Ca2+ binding by the HCaBP occurs at pH 7-8 and in 100 mM-Na+ and 3 mM-Ca2+. The HCaBP possesses at least 10 Ca2+-binding sites with a Kd of 5 X 10(-6) M ([Ca2+]). Highly specific rabbit-derived anti-HCaBP antibodies were used for HCaBP immunoquantification and immunohistochemistry, which revealed that the HCaBP is localized in the chorionic villi and is primarily associated with the trophoblastic cells of the placenta. In addition, an 'in vitro' cell-free assay system for Ca2+ uptake was constructed with microsomal membranes isolated from term placental tissues. Ca2+ uptake by the placental microsomal fraction exhibited characteristics indicative of active Ca2+ transport such as temperature-dependence, saturability and energetic requirement. In this system, preincubation of microsomal membranes with anti-HCaBP antibodies inhibited Ca2+ uptake, suggesting that the HCaBP is functionally involved in placental membrane Ca2+ uptake.

Identification and characterization of a calcium-binding protein from human placenta

The human placenta has been found to contain a specific calcium-binding protein (HCaBP) which increases in amount as a function of gestation. A procedure is described for the purification of the HCaBP to chromatographic and electrophoretic homogeneity. The HCaBP is (a) a high molecular weight protein with a native Mr of 150 000 and appears to be a dimer of subunits of Mr = 70 000; and (b) an acidic protein with a pI of 4.6. Amino acid analysis of the HCaBP revealed an abundance of acidic amino acid residues (27 per cent = Asp and Glu). A specific antiserum has been prepared against purified HCaBP. With Ouchterlony double immunodiffusion, the HCaBP appears to be specific for the human placenta, and is not cross-immunoreactive with the CaBP of the chick embryonic chorio-allantoic membrane.

Transport mechanisms in the preimplantation mammalian embryo

Compaction is associated with major changes in the transport processes in preimplantation embryos. Before this time the processes are homocellular, in which all the component cells exchange materials across their cell membranes with a common environment. After compaction the outer trophoblast cells become organized into a simple, squamous epithelium which is capable of transcellular vectorial transport, that selectively controls the movement of materials into the embryo. Measurements of the intracellular concentrations and membrane permeabilities of Na+, K+ and Cl- in the mouse oocyte and two-cell embryo have demonstrated that they undergo significant changes during this period of development. The development of transcellular transport across the trophectoderm is fundamental in the regionalization of the embryo. These physiological mechanisms are involved in the formation of the blastocoele fluid, and may be dependent on the development of regionally located Na+, K+-ATPase on the juxtacoelic surfaces of the trophoblast cells.