Characterization of calcium transport by basal plasma membranes from human placental syncytiotrophoblast.
We have studied the mechanisms involved in calcium (Ca2+) transport through the basal plasma membranes (BPM) of the syncytiotrophoblast cells from full-term human placenta. These purified membranes were enriched 25-fold in Na+/K(+)-adenosine triphosphate (ATPase), 37-fold in [3H] dihydroalprenolol binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg2+, a basal Ca2+ uptake was observed, which followed Michaelis-Menten kinetics, with a Km Ca2+ of 0.18 +/- 0.05 microM and Vmax of 0.93 +/- 0.11 nmol/mg/min. The addition of Mg2+ to the incubation medium significantly decreased this uptake in a concentration-dependent manner, with a maximal inhibition at 3 mM Mg2+ and above. The Lineweaver-Burk plots of Ca2+ uptake in the absence and in the presence of 1 mM Mg2+ suggest a noncompetitive type of inhibition. Preloading the BPM vesicles with 5 mM Mg2+ had no significant effect on Ca2+ uptake, eliminating the hypothesis of a Ca2+/Mg2+ exchange mechanism. This ATP-independent Ca2+ uptake was not sensitive to 10(-6) M nitrendipine nor to 10(-4) M verapamil. An ATP-dependent Ca2+ transport was also detected in these BPM, whose Km Ca2+ was 0.09 +/- 0.02 microM and Vmax 3.4 +/- 0.2 nmoles/mg/3 min. This Ca2+ transport requires Mg2+, the optimal concentration of Mg2+ being approximately 1 mM. Preincubation of the membrane with 10(-6) M calmodulin strongly enhanced the initial ATP-dependent Ca2+ uptake. Finally, no Na+/Ca2+ exchange process could be demonstrated.
Lafond
J
J
Maisonneuve-Rosemont Hospital, Montreal, Quebec, Canada.
Leclerc
M
M
Brunette
M G
MG
D000255:Adenosine Triphosphate / Q000502:physiology; D000469:Alkaline Phosphatase / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000493:pharmacokinetics*; D002352:Carrier Proteins / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism / Q000502:physiology; D004847:Epithelial Cells; D004848:Epithelium / Q000378:metabolism / Q000502:physiology; D005260:Female; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D008431:Maternal-Fetal Exchange / Q000502:physiology; D008565:Membrane Proteins / Q000502:physiology; D009568:Nitrendipine / Q000494:pharmacology; D010920:Placenta / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D011247:Pregnancy; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology; D014327:Trophoblasts / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D014700:Verapamil / Q000494:pharmacology
D002352:Carrier Proteins; D008565:Membrane Proteins; D019831:Sodium-Calcium Exchanger; D000255:Adenosine Triphosphate; D009568:Nitrendipine; D014700:Verapamil; D000469:Alkaline Phosphatase; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D002118:Calcium