L. B. Sasser

Intestinal absorption and retention of cadmium in neonatal rat

Abstract Newborn rats were orally dosed at 2 or 24 hr of age with 1 μCi of 115mCd and killed at intervals between 1 and 21 days after dosing. Cadmium absorption (systemic absorption) was about six times greater in 2-hr-old pups as compared with 1-day-old pups, but the rate of absorption after Day 1 was similar in both age groups. Post weanling rats dosed at 6 weeks of age absorbed 0.5% of the Cd dose, whereas rats dosed at 2 and 24 hr of age absorbed approximately 12 and 5% of the dose, respectively. Cadmium was tenaciously retained in the gastrointestinal tract of newborn rats, with over 60% of the Cd dose remaining in the gastrointestinal tract 15 days after dosing. Autoradiographs showed considerable amounts of radioactivity not only in the intestinal surface epithelium and brush borders, but also in the crypts, lamina propia, and muscular layer. Because of the greater absorption and the prolonged intestinal retention of Cd in the newborn rat and the long biological half-life of Cd in the body, Cd exposure in infancy could result in greater lifetime Cd accumulation. The prolonged intestinal retention of Cd may have significant implications on the interpretation of short-term balance studies of metals.

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.

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.

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.

Intestinal absorption of Cd in vitro

Abstract Everted gut sacs from the rat were utilized to characterize the movement of Cd across the intestinal tract. Total uptake of Cd (tissue content plus serosal fluid content) after incubation for 30 min as a function of the Cd concentration in mucosal fluid (range 5 μ m to 3.4 m m ) exhibited first-order kinetics. However, tissue Cd concentrations approached a steady state (1–5 μ m Cd/g tissue) within 30-min incubation with mucosal concentrations greater than 1.2 × 10−4 m . Serosal fluid demonstrated an increased rate of accumulation of Cd at the higher mucosal fluid concentrations. This increased rate preceded the attainment of a steady state in tissue Cd by approximately 10 min. These results are consistent with the hypothesis that Cd diffuses into the intestinal tissue where part is bound and the remainder passes into the serosal fluid. As the tissue concentration of Cd approaches a steady state, increasing amounts of Cd move into the serosal fluid.

Effect of red-cell-bound mercury on measurements of tissue mercury distribution

Blood is the transport vehicle of methylmereury (MeHg) in the body (GIBLIN and MASSARO, 1974; NORDBERG and SKERFVING, 1972; PASSOW, 1970; WHITE and ROTHSTE!N, 1973), with almost complete binding to the hemoglobin molecule (GARCIA et al., 1974; ROTHSTEIN, 1973). Although some variation in the percentage of mercury contained in red cells has been reported among species, most data indicate that red cells contain over 90% of the total blood MeHg (LUNDGREN et al., 1967; NORDBERG and SKERFVING, 1972; SUZUKI et al., 1971; SWENNSON et al., 1959). Because of the high affinity of the red cell for MeHg, gross errors in tissue content of MeHg could result unless corrections are made for the MeHg in red cells retained in the tissues. This may be particularly true if tissues are examined within a few hours or days after dosing (before equilibration between red cells and tissues occurs). Results from most studies using isotope techniques to show distribution and tissue content of MeHg do not reflect residual red cell MeHg (ANSARI et al., 1973; IVERSON et al., 1973; REYNOLDS and PITKIN, 1975; ULFVARSON, 1969). Few studies (MOFFITT and CLARY, 1974; SOMJEN et al., 1973) have been performed in which such corrections were made for MeHg bound to the hemoglobin of red cells. The purpose of this experiment was to determine whether the relative contribution of red cell MeHg to total tissue MeHg is sufficiently great to-affect the measurement of total tissue MeHg.