title: Lead enters Rcho-1 trophoblastic cells by calcium transport mechanisms and complexes with cytosolic calcium-binding proteins.
abstract: Within the placenta, a specialized Ca(2+) transport pathway develops in trophoblasts to promote growth of the fetus and hypothetically to enhance fetal uptake of Pb(2+). This hypothesis could not be tested until a method to monitor Pb(2+) influx by indo-1 fluorescence quench became available. We have applied this new method to cultured undifferentiated and differentiated Rcho-1 trophoblastic cells. Pb(2+) concentrations of 1 and 10 microM are equivalent to blood levels of 20 and 200 microg/dl in pregnant women. Over this range, Pb(2+) uptake increased with time and concentration in medium containing 1 mM Ca(2+) but was greater in Ca(2+)-omitted solutions. Activation of capacitative Ca(2+) entry (CCE) with thapsigargin, an endoplasmic reticulum (ER) Ca(2+) pump inhibitor, increased Pb(2+) uptake, while inhibition of CCE by La(3+) decreased influx. Parathyroid hormone-related peptide (PTHrP) stimulates the synthesis of Ca(2+)-binding proteins (CaBPs), as well as Ca(2+) transporters, during trophoblastic differentiation. Pretreatment for 72 h with PTHrP increased Pb(2+) uptake by undifferentiated Rcho-1 cells but had little effect on the quench in differentiated cells, probably due to their greater content of CaBPs which competed for Pb(2+)-binding with indo-1. This competition was most evident in differentiated cells when 1 microM Pb(2+) caused an initial quench, followed by a rise in fluorescence. This rise was not inhibited by thapsigargin, thereby ruling out sequestration into the ER and leaving complexation of Pb(2+) by CaBPs as the most plausible interpretation. We conclude that trophoblasts have the ability to clear Pb(2+) from the maternal circulation and deliver it to the fetus.
authors.affiliation: Department of Veterinary Pathobiology, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211, USA. EvansT@missouri.edu
authors.forename: Tim J
authors.initials: TJ
authors.lastname: Evans
authors.forename: Marilyn R
authors.initials: MR
authors.lastname: James-Kracke
authors.forename: Steven B
authors.initials: SB
authors.lastname: Kleiboeker
authors.forename: Stan W
authors.initials: SW
authors.lastname: Casteel
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002454:Cell Differentiation; D017136:Ion Transport; D007811:Lanthanum / Q000494:pharmacology; D007854:Lead / Q000378:metabolism*; D044162:Parathyroid Hormone-Related Protein; D036361:Peptide Hormones / Q000494:pharmacology; D051381:Rats; D019284:Thapsigargin / Q000494:pharmacology; D014327:Trophoblasts / Q000166:cytology / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D002135:Calcium-Binding Proteins; D044162:Parathyroid Hormone-Related Protein; D036361:Peptide Hormones; D007854:Lead; D019284:Thapsigargin; D007811:Lanthanum; D002118:Calcium