Bruce J. Kelman

The Influence of Zinc on the Ontogeny of Hepatic Metallothionein in the Fetal Rat

Abstract The ontogeny of hepatic metallothioneins (Mt) in fetal tissue as related to dietary and hepatic Zn was investigated. Sixty 6-month-old female rats were divided into two groups and given either double-distilled water or water containing 700 μg of Zn per milliliter. Dams from each group were killed on 16, 19, or 21 days of gestation, and maternal and fetal livers were removed. Mt content of the tissue was estimated by Piotrowskis Hg-saturation method. Results established the presence of an endogenous hepatic Mt in the fetal rat as early as 16 days of gestation. We further demonstrated a marked progressive increase in fetal Mt from Day 16 through gestation accompanied by a decrease in maternal hepatic Mt. It is suggested that Zn increased fetal Mt by inducing fetal synthesis, redistributing fetal Mt, or increasing Mt transport to the fetus, because both fetal and maternal hepatic Mt were increased. Fetal hepatic Mt concentration was several times greater than maternal Mt at corresponding stages of gestation. Mt may serve to either ensure adequate storage of Zn or Cu for fetal development or protect the fetus against metal toxicity, but the significance of these high endogenous levels of fetal Mt are not clear at this time.

Transplacental Movements of Inorganic Lead from Mother to Fetus

Summary Placental transport of lead in the guinea pig was measured by perfusing the fetal circulation of placentas (at 60 days of gestation) in situ. The clearance of lead from mother to fetus was linearly related to umbilical flow rate and measured 0.047 ± 0.004 ml/min at an umbilical flow rate of 2.5 ml/min. Our data indicate that lead crosses the placenta from the maternal circulation to the fetal circulation at a lower clearance rate than either cadmium or methylmercury but at a greater rate than inorganic mercury.

Passage of Cadmium Across the Perfused Guinea Pig Placenta

Summary By perfusing the fetal circulation of placentas (61- to 63-days gestation) in situ, we were able to separate fetal uptake from placental transport of cadmium in the guinea pig. Our data indicate that cadmium crosses the placenta from the maternal circulation to the fetal circulation with relative ease. Since cadmium from an iv dose was rapidly removed from the maternal circulation, low levels of transfer to the fetus reported in the literature for many animals are more likely due to low levels of circulating maternal plasma cadmium than to the presence of a strong “placental barrier” to cadmium.

Absorption of mercury from ligated segments of the rat gastrointestinal tract.

Summary The ligated segment technique was used to compare the gastrointestinal absorption of methylmercury and mercury chloride in the rat. Methylmercury was more readily absorbed (15-35 times greater, depending on the absorption site) than inorganic mercury from all ligated segments. The relative order of methylmercury absorption from ligated segments was as follows: duodenum > stomach = ileum > jejunum. Differences in absorption of inorganic mercury between gastrointestinal segments were not observed. Endogenous excretion of both forms of mercury into intestinal tissue were equal. Our data indicate that the absorption of methylmercury from the stomach is significant relative to other parts of the gastrointestinal tract and that the stomach must be considered a major site of absorption.

The Concentration of No Toxicologic Concern (CoNTC) and Airborne Mycotoxins

The threshold of toxicologic concern (TTC) concept was developed as a method to identify a chemical intake level that is predicted to be without adverse human health effects assuming daily intake over the course of a 70-yr life span. The TTC values are based on known structure–activity relationships and do not require chemical-specific toxicity data. This allows safety assessment (or prioritization for testing) of chemicals with known molecular structure but little or no toxicity data. Recently, the TTC concept was extended to inhaled substances by converting a TTC expressed in micrograms per person per day to an airborne concentration (ng/m3), making allowance for intake by routes in addition to inhalation and implicitly assuming 100% bioavailability of inhaled toxicants. The resulting concentration of no toxicologic concern (CoNTC), 30 ng/m3, represents a generic airborne concentration that is expected to pose no hazard to humans exposed continuously throughout a 70-yr lifetime. Published data on the levels of mycotoxins in agricultural dusts or in fungal spores, along with measured levels of airborne mycotoxins, spores, or dust in various environments, were used to identify conditions under which mycotoxin exposures might reach the CoNTC. Data demonstrate that airborne concentrations of dusts and mold spores sometimes encountered in agricultural environments have the potential to produce mycotoxin concentrations greater than the CoNTC. On the other hand, these data suggest that common exposures to mycotoxins from airborne molds in daily life, including in the built indoor environment, are below the concentration of no toxicologic concern.

Methylmercury movements across the perfused guinea pig placenta in late gestation

Abstract Placentas from guinea pigs between 61 and 63 days gestation were perfused from the fetal side in situ at rates ranging from 0.35 to 4.54 ml/min. The dam was injected intravenously with tracer quantities of tritiated water and radiolabeled mercury in the form of methylmercuric chloride. Maternal plasma and the perfusate samples were analyzed for radiolabeled mercury and tritiated water, and corrections were made for net water flows by using radioiodinated serum albumin. Radiomercury and tritium clearances were therefore measured concurrently. The clearance of tritiated water was a linear function of perfusion rate. The relationship between tritium clearance and perfusion rate for all experiments combined was described by the equation: y = 0.16 + 0.54 x , where y is tritium clearance and x is perfusion rate. Clearance of radiolabeled mercury measured 0.15 ± 0.01 ml/min (mean ± SE). No significant relationship between radiomercury and tritium clearance was detected. It appears that calcium and low concentrations of methylmercury may move across the placenta by the same or similar mechanisms.

Fetal distribution of mercury following introduction of methylmercury into porcine maternal circulation

Tissue samples were obtained from 115 swine fetuses from 10 litters and analyzed for tissue-bound Hg 24 h after mothers were exposed to low levels of methylmercury by iv injection. Absorption of Hg by the fetus and placenta increased throughout gestation in concert with increasing fetal weight, as did fetal hepatic Hg. Fetal renal Hg increased throughout gestation, but the increase appeared to be much greater than would be expected on the basis of weight increase alone. Blood Hg concentrations did not change significantly. Fetal brain Hg content and concentration increased dramatically toward the end of pregnancy, the gestational period during which the rate of brain growth is greatest in swine. The finding that a period of increased Hg concentration in brain corresponded with the period of maximal brain growth velocity is particularly interesting because of the hypothesis that the brain is especially sensitive to nutritional and, presumably, toxicological perturbation while it is growing most rapidly.

Cadmium-binding in the pregnant and fetal rat

Abstract A cadmium-binding moiety with properties in common with adult cadmiumthionein was identified in the livers of fetal rats at day 18 of gestation. Pregnant rats (approx. 200 g) were subcutaneously injected with 50 μg of radiolabeled cadmium in the form of 115m CdCl 2 daily from days 11 through 15 of gestation. Maternal liver contained 14–18 μg Cd/g tissue; fetal liver contained 44–70 ng Cd/g tissue. A soluble (aqueous), heat stable, cadmium-binding molecule(s) was found which had an apparent molecular weight of approx. 10 000 in adult livers and 6000 in fetal livers by gel filtration chromatography. Both fetal and adult proteins absorbed maximally at 254 but not 280 nm.

Effect of dietary cadmium on calcium metabolism in the rat during late gestation.

Summary Sprague-Dawley rats were given access to water containing control, 10, or 25 ppm Cd beginning on day 0 of gestation. On day 21 of gestation, fetal and maternal tissues were collected from dams which had been dosed 24 hr previously with 16 μCi of 45CaCl2. Maternal and fetal hematocrits decreased, indicating a possible interference with iron metabolism. No changes in total or radiocalcium found in fetal body were detected as a function of Cd treatment. Fetal bone stable and radiocalcium increased by as much as 33% at 25 ppm while fetal liver and kidney stable Ca decreased compared to controls. It is evident that low Cd exposures lead to changes in fetal Ca metabolism rather than causing changes in the amount of Ca transported across the placenta.

Fetal to maternal cadmium movements across the perfused hemochorial placenta of the guinea pig

Guinea pig placentas (59 to 61 days gestation) were perfused from the fetal side in situ. Radiolabeled CdCl2 was added to the perfusing fluid, and the dam was injected with tracer quantities of tritiated water. Cadmium clearance from fetal to maternal circulations (CdF→M) for all samples pooled measured 0.063 ± 0.004 ml/min (mean ± SE), approximately 17% of the cadmium clearance from maternal to fetal circulations (CdM→F) previously reported. Unlike CdM→F, CdF→M was not correlated with perfusate flow through the fetal circulation of the placenta. Clearance of tritium from maternal to fetal circulations (TM→F) was measured concurrently with CdF→M. As previously reported by our laboratory, TM→F can be used as an indication of changes in maternal blood flow to the placenta. In most experiments, maternal blood flow to the placenta decreased after an initial stable period when placentas were perfused with a cadmium-containing medium. Even during initial periods, when blood flow appeared to be relatively stable, maternal blood flow to the placenta varied more widely when cadmium was present in the perfusing solution than when it was not. The data suggest that dose-related decreases in maternal blood flow to the placenta were due to the presence of cadmium in the perfusing solution. Differences in the effects of cadmium on maternal blood flows when administered from fetal or maternal sides of the placenta may be due to differences in binding of cadmium by fetal and maternal plasma proteins.