ATP-dependent Ca2+ uptake in brush border membranes of human term placenta

Isolation of syncytial microvillous membrane vesicles from human term placenta and their application in drug-nutrient interaction studies

The initial step in placental uptake of nutrients occurs across the syncytial microvillous membrane of the trophoblast. This study was designed to isolate syncytial microvillous membrane vesicles (SMMV) of human term placenta, to validate their purity and viability, and to investigate the interaction of several commonly used drugs with the transport of two essential nutrients: alanine and choline. SMMV were isolated according to an established procedure, but instead of homogenization the initial preparation step was replaced by mincing of placental tissue followed by gently stirring to loosen the microvilli. These modifications doubled the protein recovery and increased the enrichment in alkaline phosphatase, whereas no substantial contamination with basal membranes nor interfering subcellular organelles was found. The functional viability of the vesicles was evaluated through the transport of alanine. In accordance with literature, uptake was sodium-dependent, inhibitable by structural analogues, and saturable. A number of cationic drugs were were able to able to inhibit choline uptake, whereas no effect on alanine transport was observed. Anionic drugs, drugs of abuse, and catecholamines did not interfere with alanine transport either. In conclusion, our isolated SMMV provide a suitable tool for screening drug-nutrient interactions at the level of membrane transport. In view of the very low susceptibility of the alanine transporter to drug inhibition and the relatively high drug concentrations necessary to inhibit choline transport, it seems unlikely that clinically important drug interactions may occur with these nutrients.

Na(+)-dependent amino acid uptake by human placental microvillous membrane vesicles: importance of storage conditions and preservation of cytoskeletal elements

Human placental microvillous membrane vesicles (MMV) were purified by precipitation of non-microvillous membrane with MgCl2. Two aspects of MMV preparation were found to be important to the interpretation of amino acid transport studies: (1) storage conditions, and (2) preservation of cytoskeletal elements. In non-frozen MMV, MeAIB uptake was stimulated by an inward Na+ gradient and showed 'overshoot'. The initial Na(+)-dependent uptake rate was concentration-dependent with a Vmax of 640 +/- 80 pmol/mg/30 sec and a Km of 0.44 +/- 0.77 mM. Na(+)-stimulated cysteine uptake (65 +/- 23 pmol/mg/30 sec), previously thought to be very low or absent in the human placenta, was comparable to MeAIB, although there was no 'overshoot'. Cysteine uptake was partially stimulated by Li+. In general, freezing and storage at either -80 degrees C or -196 degrees C markedly reduced Na(+)-dependent uptake of several amino acids, compared to vesicles stored at 4 degrees C. The greatest reduction was seen with storage at -80 degrees C, especially with cysteine. There was no effect of storage temperature on Na(+)-independent amino acid uptake. For frozen vesicles, there was no difference in uptake for 12 versus 60 h storage. Removal of cytoskeletal proteins with the chaotropic agent, KSCN, resulted in greater enrichment of MMV marker proteins, but the preparation lost the capacity for active MeAIB uptake. These data, especially with regard to storage conditions, highlight the importance of precise definition of preparation and storage conditions when interpreting results of amino acid uptake by human placental MMV.

Inhibitor action on placental calcium transport

Human term placental lobules were dually perfused with Krebs Ringer solution at 37 degrees C under open circuit conditions. Provided that perfusate Ca2+ concentrations were between 2.33 and 2.55 mM, there was a steady release of Ca2+ into the fetal circulation and uptake of Ca2+ from the maternal circulation. There was no significant calcium (Ca) protein binding in the perfusates. Addition of dinitrophenol altered the release of Ca2+ to an uptake on the fetal circuit and enhanced Ca2+ uptake on the maternal circuit. It also produced a release of potassium (K)+ and an uptake of Na+ on both sides of the placenta. Ouabain had no significant effect on Ca movements although it produced a marked release of K+ into the fetal perfusate. The effect of cooling on the fetal circuit was similar to that of dinitrophenol (DNP), although it did not produce significant changes in either Ca2+ or K+ movements on the maternal side of the lobule. Both DNP and cooling reduced the Ca concentration ratio between fetal and maternal outflows to unity. Replacement of Na+ by choline Ringer had only transient effect on the extraction of 45Ca from fetal perfusate. These observations indicate that a Ca2+/Na (sodium)+ exchanger does not make a major contribution to the transplacental movement of Ca2+ from mother to fetus and that this process is more probably associated with membrane-bound ATPases.

Expression of a cell surface antigen with potential Ca2+-sensor/receptor function in rat placenta and uterus

The reactivities of monoclonal anti-parathyroid antibodies identifying a cell surface 'calcium sensing' mechanism were investigated in the placenta and uterus of the pregnant rat. The rat placenta showed positively stained cells at the end of pregnancy. Staining was confined exclusively to the columnar epithelial cells lining the sinuses of Duval. In the uterus, positive staining of the epithelium lining the uterine lumen was obtained exclusively prior to and during implantation.