Kamath S G, Haider N, Smith C H
Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, St Louis Children's Hospital, Missouri 63110.
Placenta. 1994 Feb-Mar;15(2):147-55. doi: 10.1016/s0143-4004(05)80451-5.
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.
人胎盘合体滋养层细胞膜对大量Ca2+的转运对于胎儿正在生长的骨骼矿化至关重要。我们研究了基底（面向胎儿）质膜（BPM）的转运情况。纯化的BPM囊泡以时间依赖性方式摄取Ca2+，约60分钟达到平衡。表观平衡空间比使用其他底物（如亮氨酸）测定的平衡空间高许多倍，这表明Ca2+在囊泡内被浓缩或结合。在A23187存在下摄取和排出更快，这表明膜转运是限速步骤，且Ca2+被内化到膜空间内。摄取的初始速率约为孵育最初2秒内测量得到的速率。浓度依赖性数据符合具有一个饱和位点和扩散的米氏模型（Km = 12 microM；Vmax = 4 nmol/min/mg；KD = 39 nmol/min/mg/mM）。可饱和的Ca2+结合（Kd = 16 microM；Bmax = 3.4 nmol/mg）能力低于先前在微绒毛膜中观察到的情况。
