UBC Theses and Dissertations
Calcium related properties of plasma membranes from guinea pig placenta Shami, Yehezkel
Calcium transport across the placenta is asymmetrical and is believed to be an active transport. An essential step in such a transport is translocation of the ion across a single plasma membrane. The objective of this thesis was to study the Ca2+ -related properties of the placental plasma membranes and to gain some knowledge of their role in Ca2+ -transport. Three Ca2+ -related properties were studied: 1. Ca2+ -binding to the placental plasma membranes; 2. The membrane bound enzyme Ca -ATPase; and 3. Ca2+ -uptake by the placental plasma membrane vesicles. Ca2+ -binding properties of the membrane preparation were studied by the use of a new method, the flow dialysis system. Two types of sites for Ca were found: 1) high affinity, low capacity sites, and 2) low affinity, high capacity sites. The high affinity sites had 10-fold higher affinity for Ca2+ than for Mg2+ . A calcium-stimulated, membrane-bound enzyme, namely Ca2+ -ATPase, was located in the placental plasma membranes. This enzyme is distinct from the Na+, K+-ATPase and alkaline phosphatase. The enzyme can be activated by Mg2+ but with lower efficiency. Both Ca2+ and Mg2+ activate the enzyme at the same site. A formula was derived, enabling one to predict very precisely the velocity of the enzyme incubated under any combination of Ca2+ and Mg2+ ; this relationship is presented in a three dimensional model. The formula can be used for other enzymes or other substrates, as was demonstrated with ATP and ADP. The placental plasma membrane vesicles are capable of accumulating Ca2+ . Ca2+ -uptake was defined as the amount of Ca2+ which is not available for rapid exchange and cannot be displaced by a high concentration of competitor in the presence of ATP. This definition is different from and more accurate than the one which is widely used and cited in the literature. An intravesicular Ca2+ concentration of 190 mM was recorded, which was 24-fold higher than the external Ca2+ concentration (8 mM). Ca2+ -uptake was dependent on ATP hydrolysis by the placental Ca2+ -ATPase. This process was independent of Mg2+ . It is suggested that while the substrate for Ca2+ -ATPase is Ca-ATP, the substrate for Ca2+ -uptake is Ca2+. The overall Ca2+ -related properties of the placental plasma membranes are independent of Mg and the entire process from binding to membrane through activation of the enzyme and finally Ca2+ -uptake is dependent on Ca2+ alone. This situation is unique to the placental plasma membranes. It is tempting to speculate that the link between the maternal and the fetal circulation is achieved by forming vesicles loaded with Ca2+ on the maternal side and unloading them through fusion with the basal plasma membrane on the fetal side. The Ca2+ -related properties of placental plasma membranes described in this thesis, provide many answers regarding the first step in the asymmetrical transplacental Ca2+ -transport. Further investigation is required before a full understanding of the entire process is achieved.
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