Free calcium concentrations in Krebs-Ringer bicarbonate buffer: effects on 45Ca- and 32P-transport across the perfused guinea pig placenta

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Placental transfer of phosphate in anaesthetized rats

Mechanisms of transfer of inorganic phosphate, Pi, across the placenta of rats at 21 days of gestation were studied using 32Pi. In one group of experiments the unidirectional transfer of Pi from mother to fetus was estimated from radioactivity in the fetus at various intervals after the tracer injection into the mother. At 15 min after tracer injection, the transfer rate was only slightly higher than the estimated rate of fetal accretion of Pi, and it decreased rapidly with the length of the experiment suggesting deterioration of the transfer mechanism under conditions of an acute experiment. In other experiments, transfer of 32Pi and 51Cr-EDTA (a marker of paracellular transfer) were measured across the dually-perfused placenta in the maternal-fetal direction. The transfer rate of 32Pi was an order of magnitude higher than the transfer of 51Cr-EDTA indicating that most of the Pi transfer is transcellular. The transfer of 32P decreased when the concentration of Na+ in the maternal perfusate was reduced, it was related inversely to the concentration of Pi on the fetal side of the placenta, and it was related directly to the concentration of Ca2+ on the fetal side. The maternal-fetal transfer of Pi exhibited saturation kinetics with a K(m) of about 0.4 mM suggesting that at a physiological concentration of Pi in maternal plasma the transfer mechanism is nearly saturated. The present observations are consistent with Pi being transferred in contransport with Na+. The maternal-fetal transport of Pi appears to be stabilized by the high affinity of the transport system to Pi, and controlled by a negative feedback between fetal concentration of Pi and the Pi transfer rate. It may also be controlled, to some degree, by the fetal plasma Ca2+.

Sodium gradient-dependent phosphate transport in placental brush border membrane vesicles

We recently described a sodium gradient-dependent transport of phosphate through the brush border membrane vesicles from human placenta. In order to characterize this transport carrier further, we studied the influence of temperature and membrane potential on the transport of this electrolyte, the stoichiometry of the sodium-phosphate interaction, and the interrelationship between phosphate uptake and other sodium-dependent systems. Temperature influenced phosphate uptake by changing the maximal velocity and the affinity of the carrier for the substrate. The Arrhenius plot for uptake velocity exhibited an abrupt breakpoint at 28.6 degrees C, suggesting that membrane fluidity is a factor in phosphate uptake. Increasing the sodium concentration in the incubation medium augmented the phosphate uptake according to a sigmoid curve, and the Hill plot analysis of these data indicates that at least two sodium ions are transported with each phosphate radical. The effect of membrane potential on phosphate uptake was studied by inducing potassium diffusion with valinomycin and by using various sodium salts with different anion conductance in the incubation medium. In both series of experiments, the inside-negative potential significantly enhanced phosphate uptake. We concluded that the phosphate-sodium cotransport is an electrogenic process, a conclusion which is compatible with the observation that at least two sodium ions accompany each phosphate radical. Glycine, alanine and proline all inhibited phosphate uptake according to an uncompetitive type of inhibition. In contrast, the addition of glucose to the incubation medium had no effect.

Ionized calcium measurement in vitro: is it necessary?

Experimental data obtained from a study on the inhibition of placental Ca-ATPase by ethacrynic acid were graphically analyzed using either total calcium or ionized calcium as the independent variable. Correct interpretation of the results required actual measurement of ionized calcium in the incubation medium. The assumption that ionized calcium is equal to the total calcium resulted in artifacts and led to erroneous conclusions. These observations emphasize the necessity for accurate measurement of ionized calcium in test systems where calcium effects are being examined.

Ca2+-binding protein of the human placenta. Characterization, immunohistochemical localization and functional involvement in Ca2+ transport

The Ca2+-binding protein (HCaBP) of the human placenta was studied with respect to its biochemical properties, tissue and cellular distribution, and possible involvement in placental Ca2+ transport. Optimal Ca2+ binding by the HCaBP occurs at pH 7-8 and in 100 mM-Na+ and 3 mM-Ca2+. The HCaBP possesses at least 10 Ca2+-binding sites with a Kd of 5 X 10(-6) M ([Ca2+]). Highly specific rabbit-derived anti-HCaBP antibodies were used for HCaBP immunoquantification and immunohistochemistry, which revealed that the HCaBP is localized in the chorionic villi and is primarily associated with the trophoblastic cells of the placenta. In addition, an 'in vitro' cell-free assay system for Ca2+ uptake was constructed with microsomal membranes isolated from term placental tissues. Ca2+ uptake by the placental microsomal fraction exhibited characteristics indicative of active Ca2+ transport such as temperature-dependence, saturability and energetic requirement. In this system, preincubation of microsomal membranes with anti-HCaBP antibodies inhibited Ca2+ uptake, suggesting that the HCaBP is functionally involved in placental membrane Ca2+ uptake.

Transport of inorganic phosphate by the fetal border of the guinea-pig placenta perfused in situ

Unidirectional flux of inorganic phosphate, Pi, from the umbilical vascular bed perfused in situ into the cell compartment of the guinea-pig placenta, Jfc , was estimated using the indicator dilution method, with 32P-phosphate and 14C-sucrose as the tracers. The flux did not depend on fetal weight; its mean value was 0.25 +/- 0.02 mumol/min (s.e., n = 19). Jfc decreased when the placenta was perfused with CN (10(-3)M) or with Na-free fluid. Elevation of the Pi concentration in the perfusate caused an increase in Jfc which, however, was not proportional. These properties of Jfc were taken to be compatible with a Na-dependent active transport mechanism. The flux in the opposite direction, Jcf , correlated with the fetal weight. It decreased when the umbilical concentration of Pi was elevated. The efflux of 32P from placentae preloaded with the tracer changed inversely with the umbilical concentration of Pi. It is concluded that there is a bidirectional transcellular transfer of Pi between the two sides of the placenta, the flux in the maternal-fetal direction being the prevailing one. The unidirectional transfer to the fetus seems to be regulated directly by the umbilical concentration of Pi.

Calcium and phosphate fluxes across the fetal membranes of the guinea pig: in vitro measurement

Bidirectional calcium (45Ca) and phosphate (32P) fluxes across the amnion and visceral yolk sac of the guinea pig were measured in vitro in modified Ussing chambers. The net flux of these ions across both fetal membranes was in the maternal-to-fetal direction. The net flux of 45Ca and 32P across the yolk sac was significantly greater than that across the amnion. This difference was due to a greater maternal-to-fetal flux across the yolk sac. In addition, net 32P flux was greater than net 45Ca flux across the yolk sac, while in the amnion, there was no significant difference in the net flux of the two ions. It is suggested that the fetal membranes, especially the visceral yolk sac, contribute significantly to fetal acquisition of calcium and phosphate in mammals possessing a functional yolk sac placenta.