Abstract
An ATP-dependent calcium (Ca2+) sequestration activity was demonstrated in membrane vesicles prepared from the human term placenta. Microsomal and brush border membrane fractions accmulated Ca2+ and within a vesicular space by a saturable process requiring Mg2+ and ATP. The “uptake” activity was enriched sixfold in a microsomal membrane fraction and was only 1.5-fold enriched in purified brush border membranes compared to the activity present in the filtered homogenate. Mitochendrial inhibitors such as azide and oligomycin did not inhibit Ca2+ uptake in these preparations. The process was temperature dependent and displayed Michaelis-like kinetics with respect to free Ca2+ concentrations. At 30°C, the Vmax was 1.05 nmole/mg/min; Km = 74 nM for free Ca2+ in the microsomal fraction. Oxalate and phosphate enhanced uptake in both fractions. Ca2+ uptake activity was not associated with Ca2+ -stimulated ATPase, alkaline phosphatase, or other brush border markers during cell fractionation. The characteristics of the Ca2+ uptake process contrasted sharply with those of Ca2+-stimulated ATPase, and a Ca2+-stimulated, Mg2+-dependent ATPase activity could not be identified in these membrane vesicle preparations.
Speculation: The ATP-dependent Ca2+ uptake activity and Ca2+ binding by placental vesicles are likely to be involved in the regulation of intracellular Ca2+ within the placenta and in the transcellular transport of Ca2+ to the fetus while sewing to maintain low intracellular Ca2+ concentrations within the syncytium.
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Whitsett, J., Tsang, R. Calcium Uptake and Binding by Membrane Fractions of Human Placenta: ATP-dependent Calcium Accumulation. Pediatr Res 14, 769–775 (1980). https://doi.org/10.1203/00006450-198005000-00012
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DOI: https://doi.org/10.1203/00006450-198005000-00012
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