ATP independent calcium transport and binding by basal plasma membrane of human placenta.
The transport of large amounts of Ca2+ by the plasma membranes of human placental syncytiotrophoblast is essential to the mineralization of the growing fetal skeleton. We have investigated transport by the basal (fetal-facing) plasma membrane (BPM). Ca2+ was taken up by purified BPM vesicles in a time-dependent manner and equilibrium attained in approximately 60 min. The apparent equilibrium space was many fold higher than that determined using other substrates (e.g. leucine), suggesting that Ca2+ is concentrated or bound within the vesicles. The more rapid uptake and exit in the presence of A23187 indicates that membrane transport is rate limiting and that Ca2+ is internalized within the membrane space. The initial rate of uptake was approximately by measurement during the first 2 s of incubation. Concentration dependence data were fit to a Michaelis-Menten model with one saturable site and diffusion (Km = 12 microM; Vmax = 4 nmol/min/mg; KD = 39 nmol/min/mg/mM). Saturable Ca2+ binding (Kd = 16 microM; Bmax = 3.4 nmol/mg) was of lower capacity than previously observed for microvillous membrane.
Kamath
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SG
Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, St Louis Children's Hospital, Missouri 63110.
Haider
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N
Smith
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CH
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D001693:Biological Transport, Active / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism*; D005260:Female; D006801:Humans; D007700:Kinetics; D010920:Placenta / Q000648:ultrastructure*; D011247:Pregnancy; D014327:Trophoblasts / Q000648:ultrastructure
D002121:Calcium Channel Blockers; D000001:Calcimycin; D000255:Adenosine Triphosphate; D002118:Calcium